Development of New Advanced Ti-Mo Alloys for Medical Applications

*Petrică Vizureanu, Mădălina Simona Bălțatu and Andrei Victor Sandu*

### **Abstract**

The use of titanium and titanium-based alloys with applications in implantology and dentistry has made remarkable progress in the promotion of new technologies and new materials that have been developed in recent years. This is justified thanks to their excellent mechanical, physical, and biological performance. Today's generation promotes new titanium alloys, with nontoxic elements and long-term performance and without rejection of the human body. This book chapter describes new original compositions of Ti-based alloys for medical applications, with improved properties compared to existing classical alloys (C.p. Ti, Ti6Al4V, CoCrMo, etc.). The addition of nontoxic elements such as Mo, Si, Zr, and Ta brings benefits as reduced modulus of elasticity, increased corrosion resistance, and improved biocompatibility.

**Keywords:** Ti-Mo alloys, microstructural characterization, corrosion resistance, low elastic modulus

### **1. Introduction**

Materials with the possibility of performing a biological function are increasingly sought. In the medical field, implants require a high compatibility with the hard tissue for osteointegration and bone formation and a compatibility with the soft tissue for the adhesion of the epithelium to them and the acquisition of antibacterial properties for inhibiting or forming the biofilm at the interface. These biofunctional characteristics have two contradictory properties: inhibition and enhancement of protein adsorption, respectively, and cell adhesion [1, 2].

The usual classification of synthetic biomaterials is carried out structurally, according to the classes of materials used. The main types of synthetic biomaterials are metallic, ceramic, polymeric, composite, and of natural origin, but they can also be divided into several categories, as can be seen in **Figure 1**.

Biocompatible materials are intended to "work under biological constraint" and thereby become adapted to various medical applications.

When a metallic material is implanted in a human body, immediate reactions occur between their surface and the living tissues. In other words, an immediate reaction during the introduction period is determined and defines the biofunctionality of the metallic material [3].

**Figure 1.** *The main types of biomaterials [4].*

The quality of a material used in the construction of an implant must respect the following two criteria: the biochemical criterion and the biomechanical criterion. According to the biochemical criterion, the applicability of a material is determined by its biocompatibility, and from the biomechanical point of view of fatigue resistance, it is the most important parameter but not the only one.

The most used metallic biomaterials are stainless steels, Co-Cr alloys, titanium alloys, and magnesium-based alloys. Each class of biomaterials has its advantages and disadvantages (**Figure 2**), their use in the execution of different implants being influenced by both the properties of the biomaterials and the functional requirements imposed to the implants [5].

Among the most important factors that intervene on a biomaterial successfully integrated in the human body, we mention the physical-chemical properties, the design, the biocompatibility, the surgical technique applied to the implantation, and last but not least, the patient's health.

The selection of materials used in contact with living cells or tissues for implantation in the human body, as biomaterials, is determined primarily by their acceptance by the human tissues with which they interact (biocompatibility) and by the ability to perform their functional role for which they were implanted (biofunctionality) [6].

Out of the metallic biomaterials, a special interest is for those with osteotropic structure, of which the titanium belongs. These biomaterials, thanks to the chemical and micromorphological biocompatibility with the bone tissue, achieve with this physical-chemical connection, the interface phenomenon being assimilated with the linking osteogenesis.

Titanium alloys are frequently used, due to the need of replacing stainless steels and cobalt-based alloys that have limitations in use, causing some deficiencies of biocompatibility with human tissues. These deficiencies are caused by some elements present in their chemical composition (e.g., nickel), which have a toxic effect

**79**

reactions [7].

**Figure 2.**

risk at 300°;

supportability;

conductivity; and

special mechanical quality;

*Development of New Advanced Ti-Mo Alloys for Medical Applications*

on human tissues, causing inflammatory allergic reactions or implant rejection

• melting point—the titanium melts at 1660°, and it can be sterilized without

• resistance—the implants are made from a single pure titanium bar by mechani-

• hardness—the titanium has a hardness comparable to that of steel, giving it

• rigidity—the implants do not deform when applying, mounting, or milling

• nonmagnetic—the titanium has no magnetic effect, resulting in good tissue

• regenerative and therapeutic action—research and practical experience have

• neutral pH—titanium dioxide, TiO2, which is formed immediately around the

• biological immunity—the implant can be stimulated in contact with the bone,

• excellent resistance to electric shock—the titanium has a very low thermal

• light weight—the density of titanium is close to that of light alloys [4, 8].

The properties of the titanium are as follows:

*Main characteristics of metallic orthopedic implants [6].*

cal processing, giving them maximum resistance;

forces nor in the biomechanics of chewing;

highlighted the healing qualities of titanium oxide;

metal molecules, has a pH of 7, completely neutral;

surrounding tissues and the oral cavity environment;

*DOI: http://dx.doi.org/10.5772/intechopen.91906*

*Development of New Advanced Ti-Mo Alloys for Medical Applications DOI: http://dx.doi.org/10.5772/intechopen.91906*

#### **Figure 2.**

*Biomaterials*

**Figure 1.**

The quality of a material used in the construction of an implant must respect the following two criteria: the biochemical criterion and the biomechanical criterion. According to the biochemical criterion, the applicability of a material is determined by its biocompatibility, and from the biomechanical point of view of fatigue resis-

The most used metallic biomaterials are stainless steels, Co-Cr alloys, titanium alloys, and magnesium-based alloys. Each class of biomaterials has its advantages and disadvantages (**Figure 2**), their use in the execution of different implants being influenced by both the properties of the biomaterials and the functional require-

Among the most important factors that intervene on a biomaterial successfully integrated in the human body, we mention the physical-chemical properties, the design, the biocompatibility, the surgical technique applied to the implantation,

The selection of materials used in contact with living cells or tissues for implantation in the human body, as biomaterials, is determined primarily by their acceptance by the human tissues with which they interact (biocompatibility) and by the ability to perform their functional role for which they were implanted (biofunctionality) [6]. Out of the metallic biomaterials, a special interest is for those with osteotropic structure, of which the titanium belongs. These biomaterials, thanks to the chemical and micromorphological biocompatibility with the bone tissue, achieve with this physical-chemical connection, the interface phenomenon being assimilated with

Titanium alloys are frequently used, due to the need of replacing stainless steels and cobalt-based alloys that have limitations in use, causing some deficiencies of biocompatibility with human tissues. These deficiencies are caused by some elements present in their chemical composition (e.g., nickel), which have a toxic effect

tance, it is the most important parameter but not the only one.

ments imposed to the implants [5].

*The main types of biomaterials [4].*

the linking osteogenesis.

and last but not least, the patient's health.

**78**

*Main characteristics of metallic orthopedic implants [6].*

on human tissues, causing inflammatory allergic reactions or implant rejection reactions [7].

The properties of the titanium are as follows:


#### *Biomaterials*

The biocompatibility of titanium is a consequence of the presence of the superficial oxide layer. The chemical properties and therefore the chemical processes on the interface are determined precisely by this layer of oxide and not by the metal itself. This feature is applicable to all metal materials used in the manufacturing of implants and prosthetic parts. Among the metal materials used for hard tissue repair in human body, the elastic modulus of titanium (about 80–110 GPa) is the closest to hard human tissue, which can reduce the mechanical incompatibility between metal implants and bone tissue [9].

Titanium alloys are used for medical applications in multiple fields in human body and became the first choice for orthopedic products. **Figure 3** shows the main applications of titanium alloys used in orthopedic applications [2–8].

In conclusion, it can be said that titanium biomaterials, by its properties, respond to almost all the requirements necessary for the achievement of osteogenesis, osteointegration, and durability over time. The pure titanium implant offers perfect compatibility, correct and concrete osteogenesis, and demonstrable timelapse viability.

Adding the alloying elements gives titanium a wide range of properties through different microstructures and properties.

After microstructure, the alloys are grouped into three categories depending on the type of stabilizing elements added to the titanium alloy. The mechanical properties and corrosion resistance of the alloys depend on the morphology and structure of the α or β phase particles in the alloy matrix.

Thus, the alloying elements are divided into three categories as follows:


Over the years, many titanium alloys have been developed and investigated for the implantation of implants for medical applications, of which few have been accepted by the human body, namely those that have certain properties necessary for long-term success.

The biocompatibility of an alloy depends on the alloying elements. Alloying elements such as Zr, Ta, Nb, and Sn do not affect cell viability and have shown a reduced amount of ions released into the body, but Al and V contribute to reducing cell viability. Other elements such as Ag, Co, Cr, and Cu have moderate cytotoxic behavior, but their presence in these alloys significantly reduces their toxicity [1–5]. By analyzing the current research, these alloys were studied in order to develop

#### **Figure 3.**

*Orthopedic products made by titanium and titanium alloys: (a) endoprosthesis for joint replacement; (b) system plate screws for bone fracture repair; (c) screws for bone repair; and (d) intramedullary nail [2, 9].*

**81**

electrode.

tungsten.

temperatures.

*Development of New Advanced Ti-Mo Alloys for Medical Applications*

**2. Characterization of the obtained titanium alloys**

lation for the elaboration of homogeneous alloys.

alloys by chemical, structural, surface, and mechanical analyses.

new recipes of titanium-based alloys with elements with high biocompatibility on

The experimental tests aim at a characterization of new developed titanium

This chapter describes the following investigations for the new alloys developed:

• **Development of alloys** was carried out using a Vacuum Arc Remelting instal-

• **Structural characterization** is necessary for the study of the microstructure, the crystallographic orientation, the texture, and the identification of the

• **Mechanical characterization** highlights the mechanical properties of the

• **Corrosion resistance** determines the stability of the proposed alloys in the

• **Surface characterization** takes into account the measurement of the contact angle of the surface of the alloys for achieving/optimizing the adhesion and cell

In order to obtain the titanium alloys, the MRF ABJ 900 Vacuum Arc Remelting has been used. Vacuum arc remelting is a commonly used process in the development of alloys. The process itself is used to refill the ingots and refine the structure by using nonconsumable mobile electrode of thorium tungsten. The process itself

In principle, the process of remelting with a vacuum arc is a process based on continuous melting with the use of the electric arc and nonconsumable mobile

• It ensures the possibility of melting the metallic vacuum samples under a protective atmosphere by means of a nonconsumable mobile electrode of thorium

• It creates alloys with uniform composition, through repeated remeltings.

• It ensures the possibility of mixing elements with different melting

can also be used to obtain special alloys, superalloys, and titanium alloys.

Advantages of using this equipment are as follows:

• It can achieve very high melting temperatures.

• **Elemental composition** is necessary to determine the percentages of the

chemical elements that make up the elaborated titanium alloys.

developed titanium alloys: hardness and elasticity module.

*DOI: http://dx.doi.org/10.5772/intechopen.91906*

constituent phases.

simulated body fluids.

proliferation.

**2.1 Vacuum arc remelting**

human tissue such as Mo, Ta, Zr, and Si [10, 11].

*Biomaterials*

lapse viability.

between metal implants and bone tissue [9].

different microstructures and properties.

• α stabilizers: C, N2, O2, and Al;

for long-term success.

of the α or β phase particles in the alloy matrix.

• neutral elements: Zr, Sn, Hf, Ge, and Th [1, 5, 8].

The biocompatibility of titanium is a consequence of the presence of the superficial oxide layer. The chemical properties and therefore the chemical processes on the interface are determined precisely by this layer of oxide and not by the metal itself. This feature is applicable to all metal materials used in the manufacturing of implants and prosthetic parts. Among the metal materials used for hard tissue repair in human body, the elastic modulus of titanium (about 80–110 GPa) is the closest to hard human tissue, which can reduce the mechanical incompatibility

Titanium alloys are used for medical applications in multiple fields in human body and became the first choice for orthopedic products. **Figure 3** shows the main

Adding the alloying elements gives titanium a wide range of properties through

After microstructure, the alloys are grouped into three categories depending on the type of stabilizing elements added to the titanium alloy. The mechanical properties and corrosion resistance of the alloys depend on the morphology and structure

Thus, the alloying elements are divided into three categories as follows:

• β stabilizers: V, Nb, Mo, Ta, Fe, Mn, Cr, Co, W, Ni, Cu, Si, and H2; and

Over the years, many titanium alloys have been developed and investigated for the implantation of implants for medical applications, of which few have been accepted by the human body, namely those that have certain properties necessary

The biocompatibility of an alloy depends on the alloying elements. Alloying elements such as Zr, Ta, Nb, and Sn do not affect cell viability and have shown a reduced amount of ions released into the body, but Al and V contribute to reducing cell viability. Other elements such as Ag, Co, Cr, and Cu have moderate cytotoxic behavior, but their presence in these alloys significantly reduces their toxicity [1–5]. By analyzing the current research, these alloys were studied in order to develop

*Orthopedic products made by titanium and titanium alloys: (a) endoprosthesis for joint replacement; (b) system plate screws for bone fracture repair; (c) screws for bone repair; and (d) intramedullary nail [2, 9].*

In conclusion, it can be said that titanium biomaterials, by its properties, respond to almost all the requirements necessary for the achievement of osteogenesis, osteointegration, and durability over time. The pure titanium implant offers perfect compatibility, correct and concrete osteogenesis, and demonstrable time-

applications of titanium alloys used in orthopedic applications [2–8].

**80**

**Figure 3.**

new recipes of titanium-based alloys with elements with high biocompatibility on human tissue such as Mo, Ta, Zr, and Si [10, 11].

## **2. Characterization of the obtained titanium alloys**

The experimental tests aim at a characterization of new developed titanium alloys by chemical, structural, surface, and mechanical analyses.

This chapter describes the following investigations for the new alloys developed:


### **2.1 Vacuum arc remelting**

In order to obtain the titanium alloys, the MRF ABJ 900 Vacuum Arc Remelting has been used. Vacuum arc remelting is a commonly used process in the development of alloys. The process itself is used to refill the ingots and refine the structure by using nonconsumable mobile electrode of thorium tungsten. The process itself can also be used to obtain special alloys, superalloys, and titanium alloys.

In principle, the process of remelting with a vacuum arc is a process based on continuous melting with the use of the electric arc and nonconsumable mobile electrode.

Advantages of using this equipment are as follows:


**Figure 4** shows all stages of titanium alloying, which includes the weighing of the raw material, the loading of the alloying elements, and the final semi-finished obtained products.

The load calculation has considered the characteristics of the different alloying elements and their physical-chemical properties.

Elaboration of the alloys was carried out in two charges to obtain two alloys in each charge. **Table 1** shows alloys proposed the cavities used for each alloy.

Elaboration of the titanium alloys made with a vacuum arc melting system, took place by the melting of the elements, and followed by the remeltings of alloys for six times, a necessary operation for the refining and homogenization of the alloys. The melting of the elements took place uniformly, resulting alloys with a precise and homogeneous chemical composition. The samples had a homogeneous structure, which means that the installation, the elaboration protocol, and the elements were chosen correctly.

After the solidification, two samples of each alloy were obtained in the form of ingots, shapes, and different masses but with sufficient quantity for taking the specimens required for all proposed laboratory tests.

### **2.2 Determination of the chemical composition of titanium alloys by EDAX analysis**

A complete characterization of a metallic material consists in knowing its composition, the concentration of the various elements, or the impurities in the mass of the alloy. An extremely important aspect is the determination as precisely as possible of the chemical composition of the titanium alloys obtained after elaboration.

The EDAX system is a microanalysis detector, equipped with an electron microscope, which uses the resulting X-ray energy on the surface of the samples.

Determination of chemical composition can be performed, both punctually and in a well-defined region on the surface of the analyzed sample.

This method is a variant of X-ray fluorescence spectroscopy, in which the sample investigation is based on the interactions between the electromagnetic radiation and the sample, analyzing the X radiation emitted by the sample as a response to the charging of particles loaded with electric charges. The characterization possibilities are largely according to the fundamental principle that each chemical element has a unique atomic structure that allows the characteristic X-rays of the atomic structure of an element to characterize it uniquely from another.

In order to achieve the structural and thermal characterization, it is necessary to identify the chemical composition of the alloys obtained. EDAX microanalysis with energy dispersion of X radiation was used to determine the chemical composition of the TiMo alloys developed. Determination of the chemical composition by EDAX microanalysis is the first laboratory investigation required to highlight the proportions obtained between the pure chemical elements and was performed on titanium alloys obtained.

**83**

alloys.

**Table 1.**

**Figure 4.**

*Development of New Advanced Ti-Mo Alloys for Medical Applications*

In order to validate the results regarding the concentration, for each sample, 10

**Alloy element Ti Mo Si Zr Ta**

*Stages of titanium alloying obtaining process: (a) weighing of raw materials and gravimetric dosing; (b) loading of the raw material; and (c) titanium semi-products obtained after solidification [12].*

Ti15Mo0.5Si 84.50 15.00 0.50 — — Ti20Mo0.5Si 79.50 20.00 0.50 — — Ti15Mo7Zr10Ta 68.00 15.00 — 7.00 10.00 Ti20Mo7Zr10Ta 63.00 20.00 — 7.00 10.00

**(% weight)**

To determine the chemical composition of the obtained titanium alloys, the Vega Tescan LMH II equipment was performed using the EDAX by Bruker attached to

For the determination of the chemical composition of alloys obtained from the TiMo system, samples having dimensions of 10 mm × 10 mm × 5 mm were used. Before being examined, the samples were ground on abrasive paper to remove

**Table 2** shows the mass percentages of the elements identified in the alloy composition, the percentages of the elements varying slightly with the theoretical

**Figures 5**–**8** highlight EDX spectrum and element mapping of titanium

measurements on five different areas were done.

*Chemical composition proposed of the new titanium alloys.*

impurities and titanium oxide film on the surface of the alloy.

the SEM equipment.

batch calculation.

*DOI: http://dx.doi.org/10.5772/intechopen.91906*

*Development of New Advanced Ti-Mo Alloys for Medical Applications DOI: http://dx.doi.org/10.5772/intechopen.91906*

#### **Figure 4.**

*Biomaterials*

shapes and sizes.

obtained products.

chosen correctly.

**analysis**

• It can use various crucibles for elaboration and ensure the possibility of obtaining the samples under specific conditions in the form of a pill of different

• It is illuminated with a halogen lamp, thus helping to control the melting of the

**Figure 4** shows all stages of titanium alloying, which includes the weighing of the raw material, the loading of the alloying elements, and the final semi-finished

The load calculation has considered the characteristics of the different alloying

Elaboration of the alloys was carried out in two charges to obtain two alloys in

After the solidification, two samples of each alloy were obtained in the form of ingots, shapes, and different masses but with sufficient quantity for taking the

**2.2 Determination of the chemical composition of titanium alloys by EDAX** 

scope, which uses the resulting X-ray energy on the surface of the samples.

in a well-defined region on the surface of the analyzed sample.

of an element to characterize it uniquely from another.

A complete characterization of a metallic material consists in knowing its composition, the concentration of the various elements, or the impurities in the mass of the alloy. An extremely important aspect is the determination as precisely as possible of the chemical composition of the titanium alloys obtained after elaboration. The EDAX system is a microanalysis detector, equipped with an electron micro-

Determination of chemical composition can be performed, both punctually and

This method is a variant of X-ray fluorescence spectroscopy, in which the sample

In order to achieve the structural and thermal characterization, it is necessary to identify the chemical composition of the alloys obtained. EDAX microanalysis with energy dispersion of X radiation was used to determine the chemical composition of the TiMo alloys developed. Determination of the chemical composition by EDAX microanalysis is the first laboratory investigation required to highlight the proportions obtained between the pure chemical elements and was performed on titanium

investigation is based on the interactions between the electromagnetic radiation and the sample, analyzing the X radiation emitted by the sample as a response to the charging of particles loaded with electric charges. The characterization possibilities are largely according to the fundamental principle that each chemical element has a unique atomic structure that allows the characteristic X-rays of the atomic structure

Elaboration of the titanium alloys made with a vacuum arc melting system, took place by the melting of the elements, and followed by the remeltings of alloys for six times, a necessary operation for the refining and homogenization of the alloys. The melting of the elements took place uniformly, resulting alloys with a precise and homogeneous chemical composition. The samples had a homogeneous structure, which means that the installation, the elaboration protocol, and the elements were

each charge. **Table 1** shows alloys proposed the cavities used for each alloy.

• Loading and unloading is done in a simple way by lifting the cover that is

caught in the hinge to the rest of the camera.

alloying elements in the process [12].

elements and their physical-chemical properties.

specimens required for all proposed laboratory tests.

**82**

alloys obtained.

*Stages of titanium alloying obtaining process: (a) weighing of raw materials and gravimetric dosing; (b) loading of the raw material; and (c) titanium semi-products obtained after solidification [12].*


#### **Table 1.**

*Chemical composition proposed of the new titanium alloys.*

In order to validate the results regarding the concentration, for each sample, 10 measurements on five different areas were done.

To determine the chemical composition of the obtained titanium alloys, the Vega Tescan LMH II equipment was performed using the EDAX by Bruker attached to the SEM equipment.

For the determination of the chemical composition of alloys obtained from the TiMo system, samples having dimensions of 10 mm × 10 mm × 5 mm were used. Before being examined, the samples were ground on abrasive paper to remove impurities and titanium oxide film on the surface of the alloy.

**Table 2** shows the mass percentages of the elements identified in the alloy composition, the percentages of the elements varying slightly with the theoretical batch calculation.

**Figures 5**–**8** highlight EDX spectrum and element mapping of titanium alloys.

#### *Biomaterials*


**Table 2.**

*Chemical compositions of titanium alloys, expressed as a mass percentage, according to the EDX measurements.*

The analysis of the chemical composition obtained revealed that the main elements identified in the alloys elaborated are Ti, Mo, Zr, Ta, and Si, without the presence of other inclusions.

#### **2.3 Structural characterization of titanium alloys by optical microscopy**

Microscopic methods of structural analysis are used to characterize the materials based on their structure, constituents and phases present (nature, shape, dimensions, and distribution), and possible structural defects (pores, cracks, structural inhomogeneities, etc.). Structural analysis was performed using the OPTIKA XDS-3 MET microscope.

In order to investigate the metallographic structure, the preparation of the metallographic samples of the experimental titanium alloys included a sequence of steps: cutting to appropriate dimensions (e.g., 10 mm × 10 mm × 5 mm), incorporation in epoxy resin, grinding and polishing, and chemical attack with specific reagents (a solution with 10 mL of HF, 5 mL of HNO3, and 85 mL of H2O) for 30 s. After the preparation of the samples, this was analyzed at the optical microscope at various magnification powers in order to obtain detailed images on the microstructure.

**Figure 9** highlights images obtained by optical microscopy for titanium alloys at 100× magnification.

In **Figure 9**, the structure of titanium alloys with aspects of the specific grains of titanium is presented. The images obtained by optical microscopy for the elaborated alloys show a dendritic structure with irregular grain boundaries. These coarse structures are specific to β alloys.

The variation of the α, α + β, and β type phases consists of the differences in chemical composition of the constituent elements. The high percentage of β-stabilizing elements (Mo, Ta, and Si) led to the formation of a β-type structure, very well highlighted in the elaborated TiMo alloys.

#### **2.4 Mechanical characterization**

#### *2.4.1 Microindentation method*

The measurement of the longitudinal elastic modulus for the obtained titanium alloys was achieved by the microindentation method. This method consists of penetrating the surface of the sample with a conical palpate at a certain force.

From a practical point of view, the indentation characterization presents a major advantage over the standard methods of testing on standardized tests, namely, the testing can be done directly on the finished pieces.

**85**

**Figure 5.**

**Figure 6.**

*Development of New Advanced Ti-Mo Alloys for Medical Applications*

During the microindentation test, the values of the loading forces are recorded

**Figure 10** shows the response of the alloys during the indentation tests in the form of force-depth dependencies. The values of the modulus of elasticity for the

Among the mechanical properties that are considered when evaluating a biomaterial is the longitudinal elasticity module. If the biomaterial is used for orthopedic implants, it must have a modulus of longitudinal elasticity equivalent to that of the bone, which varies between 4 and 30 GPa, depending on the type of bone and the

A low modulus is reliable in inhibiting the bone resorption and enhancing the remodeling of bones, which may be due to the excellent stress transmission between the bone and the implant. A biomedical orthopedic implant should have a Young modulus matching or closer to that of human bone to avoid the stress shielding effect.

relative to the penetration depth of the indenter in the material. Based on the loading-unloading curve, a number of sizes can be determined that allow the

titanium alloys resulting from the indentation test are shown in **Table 3**.

characterization of the materials.

*EDX spectrum and mapping for Ti20Mo0.50Si alloy.*

direction of measurement [13–15].

*DOI: http://dx.doi.org/10.5772/intechopen.91906*

*EDX spectrum and mapping for Ti15Mo0.50Si alloy.*

*Development of New Advanced Ti-Mo Alloys for Medical Applications DOI: http://dx.doi.org/10.5772/intechopen.91906*

**Figure 5.** *EDX spectrum and mapping for Ti15Mo0.50Si alloy.*

*Biomaterials*

**Table 2.**

presence of other inclusions.

XDS-3 MET microscope.

microstructure.

100× magnification.

structures are specific to β alloys.

**2.4 Mechanical characterization**

*2.4.1 Microindentation method*

very well highlighted in the elaborated TiMo alloys.

testing can be done directly on the finished pieces.

The analysis of the chemical composition obtained revealed that the main elements identified in the alloys elaborated are Ti, Mo, Zr, Ta, and Si, without the

*Chemical compositions of titanium alloys, expressed as a mass percentage, according to the EDX measurements.*

**Alloy element Ti Mo Si Zr Ta**

Ti15Mo0.5Si 79.28 19.95 0.77 — — Ti20Mo0.5Si 78.98 20.06 0.96 — — Ti15Mo7Zr10Ta 75.40 10.41 — 7.69 6.50 Ti20Mo7Zr10Ta 71.51 14.05 — 7.04 7.40

**(% weight)**

Microscopic methods of structural analysis are used to characterize the materials based on their structure, constituents and phases present (nature, shape, dimensions, and distribution), and possible structural defects (pores, cracks, structural inhomogeneities, etc.). Structural analysis was performed using the OPTIKA

In order to investigate the metallographic structure, the preparation of the metallographic samples of the experimental titanium alloys included a sequence of steps: cutting to appropriate dimensions (e.g., 10 mm × 10 mm × 5 mm), incorporation in epoxy resin, grinding and polishing, and chemical attack with specific reagents (a solution with 10 mL of HF, 5 mL of HNO3, and 85 mL of H2O) for 30 s. After the preparation of the samples, this was analyzed at the optical microscope at various magnification powers in order to obtain detailed images on the

**Figure 9** highlights images obtained by optical microscopy for titanium alloys at

In **Figure 9**, the structure of titanium alloys with aspects of the specific grains of titanium is presented. The images obtained by optical microscopy for the elaborated alloys show a dendritic structure with irregular grain boundaries. These coarse

The measurement of the longitudinal elastic modulus for the obtained titanium

From a practical point of view, the indentation characterization presents a major advantage over the standard methods of testing on standardized tests, namely, the

alloys was achieved by the microindentation method. This method consists of penetrating the surface of the sample with a conical palpate at a certain force.

The variation of the α, α + β, and β type phases consists of the differences in chemical composition of the constituent elements. The high percentage of β-stabilizing elements (Mo, Ta, and Si) led to the formation of a β-type structure,

**2.3 Structural characterization of titanium alloys by optical microscopy**

**84**

**Figure 6.** *EDX spectrum and mapping for Ti20Mo0.50Si alloy.*

During the microindentation test, the values of the loading forces are recorded relative to the penetration depth of the indenter in the material. Based on the loading-unloading curve, a number of sizes can be determined that allow the characterization of the materials.

**Figure 10** shows the response of the alloys during the indentation tests in the form of force-depth dependencies. The values of the modulus of elasticity for the titanium alloys resulting from the indentation test are shown in **Table 3**.

Among the mechanical properties that are considered when evaluating a biomaterial is the longitudinal elasticity module. If the biomaterial is used for orthopedic implants, it must have a modulus of longitudinal elasticity equivalent to that of the bone, which varies between 4 and 30 GPa, depending on the type of bone and the direction of measurement [13–15].

A low modulus is reliable in inhibiting the bone resorption and enhancing the remodeling of bones, which may be due to the excellent stress transmission between the bone and the implant. A biomedical orthopedic implant should have a Young modulus matching or closer to that of human bone to avoid the stress shielding effect.

**Figure 7.** *EDX spectrum and mapping for Ti15Mo7Zr10Ta alloy.*

**Figure 8.** *EDX spectrum and mapping for Ti20Mo7Zr10Ta alloy.*

The developed titanium alloys have a low modulus of elasticity, close to that of the bone, with the exception of the Ti15Mo7Zr10Ta alloy and significantly lower values than CoCr alloys.

If the balance between mechanical properties and biocompatibility is achieved by both the implant and the bone tissue, the risk of negative effects is very small. The use of titanium materials with a low modulus of elasticity seems to be a good solution, and the chances of using the material for medical purposes are increasing.

#### *2.4.2 Determination of hardness for titanium alloys*

Hardness is a property of materials that express their ability to resist the action of mechanically penetrating a tougher body into its surface. When determining the hardness of the materials, the size of the traces produced by a penetration body, characterized by a certain shape and size, and the force acting on it is taken into account.

**87**

**Figure 10.**

**Figure 9.**

*Development of New Advanced Ti-Mo Alloys for Medical Applications*

*Optical microstructure of alloys investigated at 100× magnification power: (a) Ti15Mo0.5Si,* 

*The force-depth curve of the micro-indentation test for the investigated alloys: (a) Ti15Mo0.5Si,* 

*(b) Ti20Mo0.5Si, (c) Ti15Mo7Zr10Ta, and (d) Ti20Mo7Zr10Ta.*

*(b) Ti20Mo0.5Si, (c) Ti15Mo7Zr10Ta, and (d) Ti20Mo7Zr10Ta.*

*DOI: http://dx.doi.org/10.5772/intechopen.91906*

*Development of New Advanced Ti-Mo Alloys for Medical Applications DOI: http://dx.doi.org/10.5772/intechopen.91906*

#### **Figure 9.**

*Biomaterials*

**Figure 7.**

*EDX spectrum and mapping for Ti15Mo7Zr10Ta alloy.*

**86**

account.

values than CoCr alloys.

*2.4.2 Determination of hardness for titanium alloys*

*EDX spectrum and mapping for Ti20Mo7Zr10Ta alloy.*

increasing.

**Figure 8.**

The developed titanium alloys have a low modulus of elasticity, close to that of the bone, with the exception of the Ti15Mo7Zr10Ta alloy and significantly lower

If the balance between mechanical properties and biocompatibility is achieved by both the implant and the bone tissue, the risk of negative effects is very small. The use of titanium materials with a low modulus of elasticity seems to be a good solution, and the chances of using the material for medical purposes are

Hardness is a property of materials that express their ability to resist the action of mechanically penetrating a tougher body into its surface. When determining the hardness of the materials, the size of the traces produced by a penetration body, characterized by a certain shape and size, and the force acting on it is taken into

*Optical microstructure of alloys investigated at 100× magnification power: (a) Ti15Mo0.5Si, (b) Ti20Mo0.5Si, (c) Ti15Mo7Zr10Ta, and (d) Ti20Mo7Zr10Ta.*

#### **Figure 10.**

*The force-depth curve of the micro-indentation test for the investigated alloys: (a) Ti15Mo0.5Si, (b) Ti20Mo0.5Si, (c) Ti15Mo7Zr10Ta, and (d) Ti20Mo7Zr10Ta.*


#### **Table 3.**

**89**

**Table 4.**

*Development of New Advanced Ti-Mo Alloys for Medical Applications*

The methods for determining the hardness, depending on the speed of the force on the penetrator, are classified into static methods, where the drive speed is below

The Vickers hardness determination method uses a diamond penetrator in the form of a pyramid with a square base and consists in pressing it at a reduced speed and with a certain predetermined force F on the surface of the test material. The Vickers hardness, symbolized by HV, is expressed by the ratio of the applied force F to the area of the lateral surface of the residual trace produced by the penetrator. The trace is considered to be a straight pyramid with a square base, with diagonal d,

For the Vickers hardness determination method, at least three attempts are made on the test material. For each trace, the average diagonal value is calculated based on the magnitude of the two diagonals measured. It is recognized that the difference in

The hardness measurements highlight resistance and provide information on the behavior of the studied materials. In this way, we can analyze titanium alloys devel

oped for the purpose of fitting them into a specific medical application (**Table**

Both systems studied have different hardness results. Compared to other titanium biomaterials, TMZT alloys have a higher hardness, but close to the Ti6Al4V alloy, which are most commonly used in implantology. An important aspect that might have contributed to the increased hardness is the amount of stabilizing

Corrosion represents the physical-chemical, spontaneous, reversible, and undesirable destruction of metals and alloys under the chemical, electrochemical,

Corrosion monitoring is the practice of qualitative assessment and quantitative measurement of the corrosivity of an environment on a metal or an alloy immersed in this environment. Monitoring tests can be performed using mechanical, electrical, electrochemical, or chemical methods [18–20]. The nature of the monitoring sensor depends on the technique chosen for the study, the purpose pursued, and the particular characteristics of the sample used. In the older methods, the electrical measurements were often used, the monitoring technique and the methods of processing the experi

mental data being generally very laborious. The advances in the field of microelectron

ics have allowed the signals of the electrochemical sensors to be strictly conditioned, appropriately amplified, and processed based on complex data processing programs. Some techniques and methods of measurement allow continuous monitoring of corrosion—the sample is permanently exposed in the corrosion environment, while

**Alloy Ti15Mo0.5Si Ti20Mo0.5Si Ti15Mo7Zr10Ta Ti20Mo7Zr10Ta C.P.Ti Ti6Al4V CoCr alloys** HV 233.37 165.18 462.33 321.31 **128 381 600** *The bold values represent the values of the classical alloys used in implantology, values that do not belong to us and* 

the discontinuous methods are done only in specialized laboratories.

*are for a comparative points. They were bold to see the good results of our alloys.*

*The hardness values of titanium alloys measured by the Vickers method [5, 13, 14].*

HV hardness measurements on titanium alloys were performed with Wilson


**4**).

β



β elements increases

1 mm/s, and dynamic methods for which the drive speed exceeds this value.

diagonal dimensions is within an error margin of not more than 2%.

elements. It can be observed that as the amount of stabilizing

(Mo and Ta), it decreases the hardness values.

or biological action of the environment.

*DOI: http://dx.doi.org/10.5772/intechopen.91906*

having the same angle as the penetrator at the top.

Wolpert 751N.

**2.5 Corrosion resistance**

*Elastic modulus values for titanium alloys measured by indentation test [13, 14, 16, 17].*

#### *Development of New Advanced Ti-Mo Alloys for Medical Applications DOI: http://dx.doi.org/10.5772/intechopen.91906*

The methods for determining the hardness, depending on the speed of the force on the penetrator, are classified into static methods, where the drive speed is below 1 mm/s, and dynamic methods for which the drive speed exceeds this value.

The Vickers hardness determination method uses a diamond penetrator in the form of a pyramid with a square base and consists in pressing it at a reduced speed and with a certain predetermined force F on the surface of the test material. The Vickers hardness, symbolized by HV, is expressed by the ratio of the applied force F to the area of the lateral surface of the residual trace produced by the penetrator. The trace is considered to be a straight pyramid with a square base, with diagonal d, having the same angle as the penetrator at the top.

For the Vickers hardness determination method, at least three attempts are made on the test material. For each trace, the average diagonal value is calculated based on the magnitude of the two diagonals measured. It is recognized that the difference in diagonal dimensions is within an error margin of not more than 2%.

The hardness measurements highlight resistance and provide information on the behavior of the studied materials. In this way, we can analyze titanium alloys developed for the purpose of fitting them into a specific medical application (**Table 4**).

HV hardness measurements on titanium alloys were performed with Wilson Wolpert 751N.

Both systems studied have different hardness results. Compared to other titanium biomaterials, TMZT alloys have a higher hardness, but close to the Ti6Al4V alloy, which are most commonly used in implantology. An important aspect that might have contributed to the increased hardness is the amount of stabilizing β elements. It can be observed that as the amount of stabilizing β elements increases (Mo and Ta), it decreases the hardness values.

#### **2.5 Corrosion resistance**

*Biomaterials*

**88**

**Alloy** Elastic modulus (GPa)

**Table 3.** *Elastic modulus values for titanium alloys measured by indentation test [13, 14, 16, 17].*

19.81

37.53

76.88 *The bold values represent the values of the classical alloys used in implantology, values that do not belong to us and are for a comparative points. They were bold to see the good results of our alloys.*

**Ti15Mo0.5Si**

**Ti20Mo0.5Si**

**Ti15Mo7Zr10Ta**

**Ti20Mo7Zr10Ta**

43.41

**105**

**110**

**240**

**17**

**C.p. Ti**

**Ti6Al4V**

**CoCr alloys**

**Human bone**

Corrosion represents the physical-chemical, spontaneous, reversible, and undesirable destruction of metals and alloys under the chemical, electrochemical, or biological action of the environment.

Corrosion monitoring is the practice of qualitative assessment and quantitative measurement of the corrosivity of an environment on a metal or an alloy immersed in this environment. Monitoring tests can be performed using mechanical, electrical, electrochemical, or chemical methods [18–20]. The nature of the monitoring sensor depends on the technique chosen for the study, the purpose pursued, and the particular characteristics of the sample used. In the older methods, the electrical measurements were often used, the monitoring technique and the methods of processing the experimental data being generally very laborious. The advances in the field of microelectronics have allowed the signals of the electrochemical sensors to be strictly conditioned, appropriately amplified, and processed based on complex data processing programs.

Some techniques and methods of measurement allow continuous monitoring of corrosion—the sample is permanently exposed in the corrosion environment, while the discontinuous methods are done only in specialized laboratories.


**Table 4.**

*The hardness values of titanium alloys measured by the Vickers method [5, 13, 14].*

#### *Biomaterials*

Some techniques give direct information on material degradation or corrosion rate, while others are used to determine if a corrosive environment may exist. Also, some techniques are "destructive" altering more or less the surface of the metal, while others are nondestructive. The true methods of monitoring the corrosion are considered very sensitive measurements, which give a practical instantaneous signal, simultaneously with the change of the corrosion speed.

To obtain a more complete picture of the corrosion process, it is often necessary to obtain complementary data, from other sources or sensors, which are purchased simultaneously with those obtained from the corrosion sensor.

Three main aspects are pursued in the study of corrosion of alloys in various environments: (1) the type of corrosion involved in the process; (2) the corrosion rate; and (3) the nature of the corrosion products and their properties (chemical, structural, and protective). For this, numerous study methods can be used, which can be divided into three main classes: analytical methods, electrochemical methods, and optical methods. But in special cases, other methods are used (acoustic, nuclear, etc.).

Electrochemical impedance spectroscopy (SIE) data, were processed with the ZSimpWin software [8], in which the spectra are interpreted by the fit procedure developed by Boukamp - by the smallest squares method. In order to process with this software the data acquired by the VoltaMaster 4 program, this were converted by using the EIS file converter program.

The polarization resistance method was used to evaluate the corrosion rate. This method serves to determine the corrosion current, at the corrosion potential of the metal or alloy, from the linear polarization curve obtained for relatively small overvoltages. The corrosion current determined by this method therefore represents the current that appears at the metal/corrosive medium interface when the metal is immersed in the solution and represents the instantaneous corrosion current.

All measurements were made on freshly cleaned surfaces. Each sample was polished on SiC abrasive paper until granulation 2000, degreased with acetone, washed with distilled water, and kept in bidistilled water until introduced into the electrochemical cell.

**Figure 11** shows the linear polarization curves in semi-logarithmic coordinates for the samples studied in the Ringer solution, and in **Table 5**, the parameters of instantaneous corrosion in the same physiological environment are presented.

The corrosion potential, Ecor, measured in relation to the potential of the saturated calomel electrode, is the potential at which the oxidation-reduction reactions on the surface of the alloy are at equilibrium; the speed of the oxidation reaction is equal to the rate of the reduction reaction, and the total current intensity is zero. As the potential increases toward more positive values, the speed of the oxidation reaction increases, while the movement of the potential toward negative values, the oxidation process is reduced and the metal is passivized. As a qualitative aspect, the TiMoSi alloy series has a higher corrosion tendency than the TiMoZrTa alloys. The differences are significant, and the presence of zirconia and tantalum seems to cause a decrease in the corrosion rate.

The polarization resistors have high values, which are reflected in very low corrosion rates. The product of "corrosion" in the case of these alloys is mainly titanium oxide, TiO2, which is insoluble and adherent to the surface of the alloy. The oxide layer on the surface protects the alloy from the ages of the electrolytic media. In view of this, it can be admitted that in the artificial physiological environment, Ti-based alloys do not corrode but in fact undergo a passivation process. Under these conditions, the parameter Vcor—called corrosion rate—is actually passivation speed.

**91**

**2.6 Surface characterization**

**Alloy element Ecor [mV] Rp [kΩ/cm2**

**Figure 11.**

**Table 5.**

studied material [21, 22].

between 90 and 180°, material is hydrophobic.

One of the requirements of biomaterials is cellular adhesion on the surface of the material, depending on surface energy. The contact angle between a drop of liquid and a solid surface is a sensitive indicator of changes in surface energy and of the chemical and supramolecular structure on the surface. Specialty studies in domain indicated that contact angle measurement is important for the study of cell adhesion to the surface, being the one that characterizes the hydrophobicity of the

*Linear polarization curves in semi-logarithmic coordinates for titanium alloys developed in Ringer's solution:* 

**] Jcor [μA/cm2**

Ti15Mo0.5Si −266 14.91 2.131 20.59 200 −192 Ti20Mo0.5Si −227 17.71 2.089 20.19 310 −142 Ti15Mo7Zr10Ta −400.10 46.22 0.37 4.31 92.10 −91.20 Ti20Mo7Zr10Ta −425.50 50.33 0.38 4.47 130.20 −10.430

**] Vcor [μm /an] βa [mV] βc [mV]**

*(a) Ti15Mo0.5Si, (b) Ti20Mo0.5Si, (c) Ti15Mo7Zr10Ta, and (d) Ti20Mo7Zr10Ta.*

*Instantaneous corrosion parameters for titanium alloys developed in Ringer's solution.*

Measurement of the contact angle (**Figure 12**) is an experimental technique used to evaluate the hydrophilic or hydrophobic character of the surfaces. Surfaces can be classified as hydrophilic or hydrophobic reported at 90°. If the angle of contact is between 0 and 90°, the material is hydrophilic, and if the angle of contact is

*Development of New Advanced Ti-Mo Alloys for Medical Applications*

*DOI: http://dx.doi.org/10.5772/intechopen.91906*

*Development of New Advanced Ti-Mo Alloys for Medical Applications DOI: http://dx.doi.org/10.5772/intechopen.91906*

#### **Figure 11.**

*Biomaterials*

nuclear, etc.).

electrochemical cell.

by using the EIS file converter program.

cause a decrease in the corrosion rate.

Some techniques give direct information on material degradation or corrosion rate, while others are used to determine if a corrosive environment may exist. Also, some techniques are "destructive" altering more or less the surface of the metal, while others are nondestructive. The true methods of monitoring the corrosion are considered very sensitive measurements, which give a practical instantaneous

To obtain a more complete picture of the corrosion process, it is often necessary to obtain complementary data, from other sources or sensors, which are purchased

Three main aspects are pursued in the study of corrosion of alloys in various environments: (1) the type of corrosion involved in the process; (2) the corrosion rate; and (3) the nature of the corrosion products and their properties (chemical, structural, and protective). For this, numerous study methods can be used, which can be divided into three main classes: analytical methods, electrochemical methods, and optical methods. But in special cases, other methods are used (acoustic,

Electrochemical impedance spectroscopy (SIE) data, were processed with the ZSimpWin software [8], in which the spectra are interpreted by the fit procedure developed by Boukamp - by the smallest squares method. In order to process with this software the data acquired by the VoltaMaster 4 program, this were converted

The polarization resistance method was used to evaluate the corrosion rate. This

**Figure 11** shows the linear polarization curves in semi-logarithmic coordinates for the samples studied in the Ringer solution, and in **Table 5**, the parameters of instantaneous corrosion in the same physiological environment are presented. The corrosion potential, Ecor, measured in relation to the potential of the saturated calomel electrode, is the potential at which the oxidation-reduction reactions on the surface of the alloy are at equilibrium; the speed of the oxidation reaction is equal to the rate of the reduction reaction, and the total current intensity is zero. As the potential increases toward more positive values, the speed of the oxidation reaction increases, while the movement of the potential toward negative values, the oxidation process is reduced and the metal is passivized. As a qualitative aspect, the TiMoSi alloy series has a higher corrosion tendency than the TiMoZrTa alloys. The differences are significant, and the presence of zirconia and tantalum seems to

The polarization resistors have high values, which are reflected in very low corrosion rates. The product of "corrosion" in the case of these alloys is mainly titanium oxide, TiO2, which is insoluble and adherent to the surface of the alloy. The oxide layer on the surface protects the alloy from the ages of the electrolytic media. In view of this, it can be admitted that in the artificial physiological environment, Ti-based alloys do not corrode but in fact undergo a passivation process. Under these conditions, the parameter Vcor—called corrosion rate—is actually

method serves to determine the corrosion current, at the corrosion potential of the metal or alloy, from the linear polarization curve obtained for relatively small overvoltages. The corrosion current determined by this method therefore represents the current that appears at the metal/corrosive medium interface when the metal is immersed in the solution and represents the instantaneous corrosion current. All measurements were made on freshly cleaned surfaces. Each sample was polished on SiC abrasive paper until granulation 2000, degreased with acetone, washed with distilled water, and kept in bidistilled water until introduced into the

signal, simultaneously with the change of the corrosion speed.

simultaneously with those obtained from the corrosion sensor.

**90**

passivation speed.

*Linear polarization curves in semi-logarithmic coordinates for titanium alloys developed in Ringer's solution: (a) Ti15Mo0.5Si, (b) Ti20Mo0.5Si, (c) Ti15Mo7Zr10Ta, and (d) Ti20Mo7Zr10Ta.*


#### **Table 5.**

*Instantaneous corrosion parameters for titanium alloys developed in Ringer's solution.*

#### **2.6 Surface characterization**

One of the requirements of biomaterials is cellular adhesion on the surface of the material, depending on surface energy. The contact angle between a drop of liquid and a solid surface is a sensitive indicator of changes in surface energy and of the chemical and supramolecular structure on the surface. Specialty studies in domain indicated that contact angle measurement is important for the study of cell adhesion to the surface, being the one that characterizes the hydrophobicity of the studied material [21, 22].

Measurement of the contact angle (**Figure 12**) is an experimental technique used to evaluate the hydrophilic or hydrophobic character of the surfaces. Surfaces can be classified as hydrophilic or hydrophobic reported at 90°. If the angle of contact is between 0 and 90°, the material is hydrophilic, and if the angle of contact is between 90 and 180°, material is hydrophobic.

#### **Figure 12.**

*Images of water droplet on the surface of the elaborated alloys: (a) Ti15Mo0.5Si, (b) Ti20Mo0.5Si, (c) Ti15Mo7Zr10Ta, and (d) Ti20Mo7Zr10Ta.*


#### **Table 6.**

*Water contact angle values on the surface of elaborated titanium alloys.*

The equipment used allows the determination of the surface tension of the liquids and of the free surface energy of the solid. The principle of measuring the angle of contact consists in placing a drop of water with a microsurgery syringe with the drop volume of 4 μl. Drop lighting is made from behind and recorded from the opposite side with a digital camera. The image obtained is further analyzed through the FAMAS program, a KYOWA integrated goniometer software.

Ten measurements of the contact angle (θ) for each experimental alloy were performed, and the value presented is the average of the measurements made, with a maximum error of ±1°. The average value of the contact angle for each alloy is shown in **Table 6**.

All investigated alloys have a contact angle of less than 90°, thus having a hydrophilic character, which means a high adhesion of the cells to the surface of the alloys.

From the data obtained for the analyzed titanium alloy surfaces, it follows that the value of the highest arithmetic mean of the alloys is recorded at the level of contact angle with water on the surface of the Ti15Mo7Zr10Ta alloy, and the smallest level was Ti20Mo7Zr10Ta alloy, this alloy having a more pronounced hydrophilic character.

**93**

*Development of New Advanced Ti-Mo Alloys for Medical Applications*

Metals have traditionally been used to make implants subjected to high loads in the human body, used in various applications. They are known for their high

For a biomaterial to be functional for an extended period of time in the body, it should be nontoxic and engage in an adequate response with the body, so that it can

The alloys developed by the proposed method have the advantage of a modulus of elasticity close to that of the human bone and a good corrosion resistance in the simulated biological fluids. According to the obtained values for corrosion and the mechanical properties, the newly developed alloys, for a Young modulus, the value is the closest to the bone (from 19 to 77 GPa our alloys, C.p. Ti is 105 GPa, and the rest are higher, where the bone is 17 GPa) from all the commercial known alloys, and TMZT systems have the lowest corrosion rate. Also, according to the contact angle, the surfaces of the obtained alloys are susceptible for cell development. Because improving the properties of biomaterials is a necessity to reduce the failure rate of implants in human tissue, we can say that the alloys developed in this chapter can be successful candidates for orthopedic implants, thanks to the stabiliz-

The preliminary investigations presented in this chapter for the elaborated titanium alloys revealed the beneficial influence of some stabilizing β elements

resistance to wear, ductility, hardness, corrosion, and biocompatibility.

Petrică Vizureanu\*, Mădălina Simona Bălțatu and Andrei Victor Sandu Faculty of Materials Science and Engineering, "Gheorghe Asachi" Technical

© 2020 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium,

\*Address all correspondence to: peviz2002@yahoo.com

*DOI: http://dx.doi.org/10.5772/intechopen.91906*

**3. Conclusions**

fulfill its purpose.

(Mo, Ta, and Si).

ing β elements.

**Author details**

University of Iasi, Iasi, Romania

provided the original work is properly cited.

## **3. Conclusions**

*Biomaterials*

**92**

shown in **Table 6**.

the alloys.

**Table 6.**

**Figure 12.**

character.

The equipment used allows the determination of the surface tension of the liquids and of the free surface energy of the solid. The principle of measuring the angle of contact consists in placing a drop of water with a microsurgery syringe with the drop volume of 4 μl. Drop lighting is made from behind and recorded from the opposite side with a digital camera. The image obtained is further analyzed through

**Alloy Ti15Mo0.5Si Ti20Mo0.5Si Ti15Mo7Zr10Ta Ti20Mo7Zr10Ta** Liquid used water water water water Contact angle (°) 64.40 50.00 45.64 70.72

*Images of water droplet on the surface of the elaborated alloys: (a) Ti15Mo0.5Si, (b) Ti20Mo0.5Si,* 

Ten measurements of the contact angle (θ) for each experimental alloy were performed, and the value presented is the average of the measurements made, with a maximum error of ±1°. The average value of the contact angle for each alloy is

All investigated alloys have a contact angle of less than 90°, thus having a hydrophilic character, which means a high adhesion of the cells to the surface of

From the data obtained for the analyzed titanium alloy surfaces, it follows that the value of the highest arithmetic mean of the alloys is recorded at the level of contact angle with water on the surface of the Ti15Mo7Zr10Ta alloy, and the smallest level was Ti20Mo7Zr10Ta alloy, this alloy having a more pronounced hydrophilic

the FAMAS program, a KYOWA integrated goniometer software.

*Water contact angle values on the surface of elaborated titanium alloys.*

*(c) Ti15Mo7Zr10Ta, and (d) Ti20Mo7Zr10Ta.*

Metals have traditionally been used to make implants subjected to high loads in the human body, used in various applications. They are known for their high resistance to wear, ductility, hardness, corrosion, and biocompatibility.

For a biomaterial to be functional for an extended period of time in the body, it should be nontoxic and engage in an adequate response with the body, so that it can fulfill its purpose.

The preliminary investigations presented in this chapter for the elaborated titanium alloys revealed the beneficial influence of some stabilizing β elements (Mo, Ta, and Si).

The alloys developed by the proposed method have the advantage of a modulus of elasticity close to that of the human bone and a good corrosion resistance in the simulated biological fluids. According to the obtained values for corrosion and the mechanical properties, the newly developed alloys, for a Young modulus, the value is the closest to the bone (from 19 to 77 GPa our alloys, C.p. Ti is 105 GPa, and the rest are higher, where the bone is 17 GPa) from all the commercial known alloys, and TMZT systems have the lowest corrosion rate. Also, according to the contact angle, the surfaces of the obtained alloys are susceptible for cell development.

Because improving the properties of biomaterials is a necessity to reduce the failure rate of implants in human tissue, we can say that the alloys developed in this chapter can be successful candidates for orthopedic implants, thanks to the stabilizing β elements.

### **Author details**

Petrică Vizureanu\*, Mădălina Simona Bălțatu and Andrei Victor Sandu Faculty of Materials Science and Engineering, "Gheorghe Asachi" Technical University of Iasi, Iasi, Romania

\*Address all correspondence to: peviz2002@yahoo.com

© 2020 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

### **References**

[1] Elias CN, Lima JHC, Valiev R, Meyers MA. Biomedical applications of titanium and its alloys. Biological Materials Science. 2008;**60**(3):46-49

[2] Antoniac I, editor. Handbook of Bioceramics and Biocomposites. Springer. 2016. ISBN: 978-3-319-12459- 9. Available from: https://www.springer. com/gp/book/9783319124599

[3] Chen Q, Thouas GA. Metallic implant biomaterials. Materials Science and Engineering R. 2015;**87**:1-57

[4] Baltatu MS, Tugui CA, Perju MC, Benchea M, Spataru MC, Sandu AV, et al. Biocompatible titanium alloys used in medical applications. Revista de Chimie. 2019;**70**(4):1302-1306

[5] Geetha M, Singh AK, Asokamani R, Gogia AK. Ti based biomaterials, the ultimate choice for orthopaedic implants—A review. Materials Science. 2009;**54**:397-425

[6] Singh MKK. Review on titanium and titanium based alloys as biomaterials for orthopaedic applications. Materials Science and Engineering: C. 2019;**102**:844-862

[7] Sandu AV, Baltatu MS, Nabialek M, Savin A, Vizureanu P. Characterization and mechanical proprieties of new TiMo alloys used for medical applications. Materials. 2019;**12**(18):2973

[8] Niinomi M. Titanium Alloys. Encyclopedia of Biomedical Engineering. Elsevier. 2019:213-224. DOI: https://doi.org/10.1016/ B978-0-12-801238-3.99864-7

[9] Kokubo T, Yamaguchi S. Novel bioactive materials developed by simulated body fluid evaluation: Surface-modifified Ti metal and its alloys. Acta Biomaterialia. 2016;**44**:16-30

[10] Terpiłowska S, Siwicka-Gieroba D, Siwicki AK. Cytotoxicity of Iron (III), Molybdenum (III), and their mixtures in BALB/3T3 and HepG2 cells. Journal of Veterinary Research. 2018;**62**(4):527-533

[11] Correa DRN, Kuroda PAB, Lourenço ML, Fernandes CJC, Buzalaf MAR, Zambuzzi WF, et al. Development of Ti-15Zr-Mo alloys for applying as implantable. Journal of Alloys and Compounds. 2018;**749**:163-171

[12] Bălțatu MS, Vizureanu P, Geantă V, Nejneru C, Țugui CA, Focșăneanu SC. Obtaining and mechanical properties of Ti-Mo-Zr-Ta alloys. IOP Conference Series: Materials Science and Engineering. 2017;**209**(1):012019

[13] Bombac DM, Brojan M, Fajfar P, Kosel F, Turk R. Review of materials in medical applications. Materials and Geoenvironment. 2007;**54**(4):471-499

[14] Shah FA, Thomsen P, Palmquist A. A review of the impact of implant biomaterials on osteocytes. Journal of Dental Research. 2018;**97**(9):977-986

[15] Baltatu MS, Vizureanu P, Balan T, Lohan M, Tugui CA. Preliminary tests for Ti-Mo-Zr-Ta alloys as potential biomaterials, book series. IOP Conference Series: Materials Science and Engineering. 2018;**374**:012023

[16] Niinomi M. Mechanical properties of biomedical titanium alloys. Materials Science and Engineering A. 1998;**243**:231-236

[17] Minciună MG, Vizureanu P, Geantă V, Voiculescu I, Sandu AV, Achiței DC, et al. Effect of Si on the mechanical properties of biomedical CoCrMo alloys. Revista de Chimie. 2015;**66**(6):891-894

**95**

*Development of New Advanced Ti-Mo Alloys for Medical Applications*

*DOI: http://dx.doi.org/10.5772/intechopen.91906*

[18] Bălțatu MS, Vizureanu P, Mareci D, Burtan LC, Chiruță C, Trincă LC. Effect

of Ta on the electrochemical behavior of new TiMoZrTa alloys in artificial physiological solution simulating in vitro inflammatory conditions. Materials and Corrosion.

[19] Oliveira NTC, Guastaldi AC. Electrochemical stability and corrosion resistance of Ti–Mo alloys for biomedical applications. Acta Biomaterialia. 2009;**5**:399-405

[20] Bălțatu MS, Vizureanu P, Cimpoeșu R, Abdullah MMAB, Sandu AV. The corrosion behavior of TiMoZrTa alloys used for medical applications. Revista de Chimie.

[21] Vogler EA. Structure and reactivity

of water at biomaterial surface. Advances in Colloid and Interface

[22] Baier RE. Surface behavior of biomaterials: The theta surface for biocompatibility. Journal of Materials Science. Materials in Medicine.

2016;**67**(10):2100-2102

Science. 1998;**74**:69-117

2006;**17**(11):1057-1062

2016;**67**(12):1314-1320

*Development of New Advanced Ti-Mo Alloys for Medical Applications DOI: http://dx.doi.org/10.5772/intechopen.91906*

[18] Bălțatu MS, Vizureanu P, Mareci D, Burtan LC, Chiruță C, Trincă LC. Effect of Ta on the electrochemical behavior of new TiMoZrTa alloys in artificial physiological solution simulating in vitro inflammatory conditions. Materials and Corrosion. 2016;**67**(12):1314-1320

[19] Oliveira NTC, Guastaldi AC. Electrochemical stability and corrosion resistance of Ti–Mo alloys for biomedical applications. Acta Biomaterialia. 2009;**5**:399-405

[20] Bălțatu MS, Vizureanu P, Cimpoeșu R, Abdullah MMAB, Sandu AV. The corrosion behavior of TiMoZrTa alloys used for medical applications. Revista de Chimie. 2016;**67**(10):2100-2102

[21] Vogler EA. Structure and reactivity of water at biomaterial surface. Advances in Colloid and Interface Science. 1998;**74**:69-117

[22] Baier RE. Surface behavior of biomaterials: The theta surface for biocompatibility. Journal of Materials Science. Materials in Medicine. 2006;**17**(11):1057-1062

**94**

2016;**44**:16-30

*Biomaterials*

**References**

[1] Elias CN, Lima JHC, Valiev R, Meyers MA. Biomedical applications of titanium and its alloys. Biological Materials Science. 2008;**60**(3):46-49

[10] Terpiłowska S, Siwicka-Gieroba D, Siwicki AK. Cytotoxicity of Iron (III), Molybdenum (III), and their mixtures in BALB/3T3 and HepG2 cells. Journal of Veterinary Research.

2018;**62**(4):527-533

2018;**749**:163-171

2017;**209**(1):012019

[11] Correa DRN, Kuroda PAB, Lourenço ML, Fernandes CJC, Buzalaf MAR, Zambuzzi WF, et al. Development of Ti-15Zr-Mo alloys for applying as implantable. Journal of Alloys and Compounds.

[12] Bălțatu MS, Vizureanu P, Geantă V, Nejneru C, Țugui CA, Focșăneanu SC. Obtaining and mechanical properties of Ti-Mo-Zr-Ta alloys. IOP Conference Series: Materials Science and Engineering.

[13] Bombac DM, Brojan M, Fajfar P, Kosel F, Turk R. Review of materials in medical applications. Materials and Geoenvironment. 2007;**54**(4):471-499

[14] Shah FA, Thomsen P, Palmquist A. A review of the impact of implant biomaterials on osteocytes. Journal of Dental Research. 2018;**97**(9):977-986

[15] Baltatu MS, Vizureanu P, Balan T, Lohan M, Tugui CA. Preliminary tests for Ti-Mo-Zr-Ta alloys as potential biomaterials, book series. IOP Conference Series: Materials Science and Engineering. 2018;**374**:012023

[16] Niinomi M. Mechanical properties

Materials Science and Engineering A.

of biomedical titanium alloys.

[17] Minciună MG, Vizureanu P, Geantă V, Voiculescu I, Sandu AV, Achiței DC, et al. Effect of Si on the mechanical properties of biomedical CoCrMo alloys. Revista de Chimie.

1998;**243**:231-236

2015;**66**(6):891-894

[2] Antoniac I, editor. Handbook of Bioceramics and Biocomposites. Springer. 2016. ISBN: 978-3-319-12459- 9. Available from: https://www.springer.

com/gp/book/9783319124599

[3] Chen Q, Thouas GA. Metallic implant biomaterials. Materials Science

and Engineering R. 2015;**87**:1-57

Chimie. 2019;**70**(4):1302-1306

for orthopaedic applications.

Materials. 2019;**12**(18):2973

[8] Niinomi M. Titanium Alloys. Encyclopedia of Biomedical

Engineering. Elsevier. 2019:213-224. DOI: https://doi.org/10.1016/ B978-0-12-801238-3.99864-7

[9] Kokubo T, Yamaguchi S. Novel bioactive materials developed by simulated body fluid evaluation: Surface-modifified Ti metal and its alloys. Acta Biomaterialia.

2009;**54**:397-425

2019;**102**:844-862

[4] Baltatu MS, Tugui CA, Perju MC, Benchea M, Spataru MC, Sandu AV, et al. Biocompatible titanium alloys used in medical applications. Revista de

[5] Geetha M, Singh AK, Asokamani R, Gogia AK. Ti based biomaterials, the ultimate choice for orthopaedic

implants—A review. Materials Science.

[6] Singh MKK. Review on titanium and titanium based alloys as biomaterials

Materials Science and Engineering: C.

[7] Sandu AV, Baltatu MS, Nabialek M, Savin A, Vizureanu P. Characterization and mechanical proprieties of new TiMo alloys used for medical applications.

**97**

**Chapter 5**

**Abstract**

**1. Introduction**

Impact of Dopants on the

of Hydroxyapatite

*and Subbaraya Narayana Kalkura*

the next generation biomaterials are elucidated.

**Keywords:** doped hydroxyapatite, electrical, optical, luminescence

Electrical and Optical Properties

*Kumaravelu Thanigai Arul, Jayapalan Ramana Ramya* 

This chapter deals with the effect of alternating electrical current on hydroxyapatite [HAp, Ca10(PO4)6(OH)2] and doped HAp along with their optical response and the processes involved. The dielectric constant, permittivity and ac conductivity were analyzed to have an insight into the surface charge polarization phenomenon. Further, the magnitude and the polarity of the surface charges, microstructure, and phases also play significant role in the cell proliferation and growth on the implants. Besides, the mechanism behind the electrical properties and the healing of bone fracture are discussed. The influence of various dopants on the optical properties of HAp viz., absorbance, transmission, band gaps and defects energy levels are analyzed along with the photoluminescence and excitation independent emission. In the future outlook, the analysis of effect of doping is summarized and its impact on

Hydroxyapatite (HAp) is one of the phases of calcium phosphate having excellent biological properties. Human bone contains 75% of inorganic materials (HAp) and remaining organic contents (predominantly collagen) and water. The HAp is analogous to the inorganic compositions of bone. The drawbacks of HAp to be used as an implant (bone and dental replacement material) are its weak mechanical strength, resorbability and to an extent this could be overcome by doping with metal ions. However, there were no adequate new bone formation between living bone and the implants due to the slow osteoconductivity [1]. Even variations in shape, roughness of the implants did not enhance osteoconduction [2]. Some other routes say tissue engineering, even growth factors [bone morphogenic protein (BMP)] [3] etc., also possess some treatment issues. Further, these parameters could not improve the osteoconduction. So, the application of the electric field on the HAp nanoparticles to induce strongly or weakly oriented dipoles, depending on the polarization of the dopants and band gap of the HAp nanoparticles to improve its biocompatibility. Besides, the dopants modify the ionic and space charge polarizations which vary with the ionic size of the dopants.

Recently, many studies have been reported on the electrical properties of HAp/doped HAp. Das and Pamu suggested that HAp could be a suitable candidate

### **Chapter 5**

## Impact of Dopants on the Electrical and Optical Properties of Hydroxyapatite

*Kumaravelu Thanigai Arul, Jayapalan Ramana Ramya and Subbaraya Narayana Kalkura*

### **Abstract**

This chapter deals with the effect of alternating electrical current on hydroxyapatite [HAp, Ca10(PO4)6(OH)2] and doped HAp along with their optical response and the processes involved. The dielectric constant, permittivity and ac conductivity were analyzed to have an insight into the surface charge polarization phenomenon. Further, the magnitude and the polarity of the surface charges, microstructure, and phases also play significant role in the cell proliferation and growth on the implants. Besides, the mechanism behind the electrical properties and the healing of bone fracture are discussed. The influence of various dopants on the optical properties of HAp viz., absorbance, transmission, band gaps and defects energy levels are analyzed along with the photoluminescence and excitation independent emission. In the future outlook, the analysis of effect of doping is summarized and its impact on the next generation biomaterials are elucidated.

**Keywords:** doped hydroxyapatite, electrical, optical, luminescence

### **1. Introduction**

Hydroxyapatite (HAp) is one of the phases of calcium phosphate having excellent biological properties. Human bone contains 75% of inorganic materials (HAp) and remaining organic contents (predominantly collagen) and water. The HAp is analogous to the inorganic compositions of bone. The drawbacks of HAp to be used as an implant (bone and dental replacement material) are its weak mechanical strength, resorbability and to an extent this could be overcome by doping with metal ions. However, there were no adequate new bone formation between living bone and the implants due to the slow osteoconductivity [1]. Even variations in shape, roughness of the implants did not enhance osteoconduction [2]. Some other routes say tissue engineering, even growth factors [bone morphogenic protein (BMP)] [3] etc., also possess some treatment issues. Further, these parameters could not improve the osteoconduction. So, the application of the electric field on the HAp nanoparticles to induce strongly or weakly oriented dipoles, depending on the polarization of the dopants and band gap of the HAp nanoparticles to improve its biocompatibility. Besides, the dopants modify the ionic and space charge polarizations which vary with the ionic size of the dopants.

Recently, many studies have been reported on the electrical properties of HAp/doped HAp. Das and Pamu suggested that HAp could be a suitable candidate for biosensing and micro-electromechanical system applications [4]. The ferroelectric properties of graphene-doped HAp samples were studied by Hendi and Yakuphanoglu [5]. Iron-doped calcium phosphate demonstrated hyperthermia (42°C) within 4 minutes [6]. Study on various metals ions (iron, manganese, and cobalt ions)-doped HAp revealed an increase in the ac conductivity. Further, the annealed iron-doped HAp revealed ferromagnetism, whereas the manganese and cobalt ions doped samples exhibited super-paramagnetic property [7]. The dielectric properties of the chromium-doped HAp were also studied [8]. Various anions such as nitrate, acetate, chloride, and egg shell precursors have been used to prepare HAp particles and their corresponding dielectric constant were 9.96, 13.22, 9.92, and 10.86 at 5 MHz [9]. Porous HAp was prepared from *Pila globosa* shells which possessed high dielectric constant with low dielectric loss [10]. HApbarium titanate (BT) composite scaffolds having improved electric, compressive strength, toughness, density, and hardness were developed using cold isostatic pressing and sintering by Tavangar et al. [11]. Bismuth ions (10, 30 and 50 wt%) were substituted in the HAp matrix by the conventional solid-state reaction at 1300°C and their ac conductivity was reduced with an increase in Bi content [12]. Space charge and dipolar polarization were the dominant polarization mechanisms in Na0.5K0.5NbO3 (NKN)-HAp as reported by Verma et al. [13]. La, Ba, Fe, and Zn ions doped HAp synthesized by sol-gel route displayed a space charge polarization at low frequency with negative temperature coefficient of resistance [14]. Different contents (e.g., 0, 1, 3, 6, 19, 12, and 15% [wt.]) of gallium ion-doped HAp by microwave-assisted sol-gel technique, illustrated a higher inhibition of bacteria and fungi. Further, gallium ions influence the dielectric properties of HAp [15]. Dielectric properties of chlorinated ethylene propylene diene monomer/hydroxyapatite nanocomposites were robustly distorted at lower frequencies whereas above 103 Hz, it was frequency independent [16]. Horiuchi et al. reported that the polycrystalline HAp possesses conductive grains and insulating grain boundaries further, interfacial polarization was confirmed to be an electret [17]. In the case of chitosan/HAp composite, the concentration of chitosan plays a predominant role in regulating its hardness, conductivity and dielectric constant [18]. HAp particles were permanently polarized by both electric potential (500 V direct current) and thermal treatment (at 1000°C) leading to the high adsorption of inorganic bioadsorbates compared to the as prepared HAp [19]. Electric field assisted sintering such as spark plasma sintering (SPS) and flash sintering (FS) performed on HAp revealed nanovoids within HA grains. Further, in situ TEM heating produced nanovoids which remained stable up to 900°C and were not present at 1100°C [20]. Using sol-gel route, yttrium and strontium co-substituted nano-hydroxyapatite was prepared with a minor phase of β tricalcium phosphate, however the ac conductivity was enhanced with an increase in frequency [21]. Tungsten-doped hydroxyapatite (W-HAp) nanoparticles were prepared by chemical precipitation followed by thermal treatment (800, 1000 and 1200°C) leading to an enhancement in the mechanical strength, Young's modulus and dielectric constant [22]. The synthesis routes, concentration of doping, and doping characteristics affect the behavior of dielectrics as well as other correlated properties such as mechanical and biocompatibility. In the case of doping of semiconductors, the dielectric properties are robustly influenced due to the presence of holes and electrons. When the electric field is applied, the polarization varied in the matrix owing to the occurrence of both holes and electrons. The physical (pressure), chemical (electronic), and thermal treatment significantly alter the properties of HAp leading to the creation of porous, dense and bonded matrix. When electric field is applied to such systems, the electronic polarization, oxygen vacancies, dipole displacement, and atomic orientation are interrelated which alter the

**99**

*Impact of Dopants on the Electrical and Optical Properties of Hydroxyapatite*

charge that would aid the cell growth and cell proliferation.

The interaction of light with the HAp nanoparticles is another interesting phenomenon, though an insulating material, the presence of phosphate ions aids the emission of photons of different wavelengths. Addition of metal ions in the lattice of HAp tunes the defect energy levels and alters the chemical potential. The dopants can affect the optical and photoluminescence (PL) of HAp. These properties depend on the absorption, transmission, and band gap as well. Further, these properties favor the use of the HAp nanostructure for bio-imaging and biosensing applications. The doped HAp facilitates the trapping of photons at defect sites or ease transmission which enables them for opto-electronic device applications. Popa and Ciobanu reported the enhanced PL by the cerium ions doped HAp without any structural change [42]. The band gap of erbium-doped HAp was reduced displaying red and green emissions [43]. The various emissions were possible due to the formation of defect energy levels coupled with the HAp energy levels which alters the rate of recombination of electron and hole pairs. Further, the doping concentration varies the active centers of recombination. Feng et al. reported that the Eu3+/Gd3+dualdoped HAp nanorods show enhanced PL with a sustained ibuprofen (IBU) release. Further, the nanorods were injected into mice and demonstrated that these nanorods could be used for *in vivo* imaging [44]. Europium-doped HAp revealed high photoluminescence intensity at 900°C due to the migration of europium from Ca1 to Ca2 site [45]. Fluorescent europium-doped HAp nanowires were prepared by hydrothermal technique and in situ dispersion of them for dental applications [46]. Chang et al.

permittivity and ac conductivity of HAp. In the case of polymer/HAp composites, the dielectric behavior is distinct due to the presence of carbon and hydrogen chains and the type of bonding. On annealing, the dielectric response of the composites changes significantly due to the formation of voids and porous structures. Yamashita et al. reported the chemical effects of the electrically polarized HAp in simulated body fluid [23] and in addition enhanced the *in vitro* bioactivity and *in vivo* osteoconductivity [24, 25]. Similarly, the bioactivity of HAp has been improved by doping of metal ions viz., iron [26, 27], silver [28, 29], magnesium [30, 31], strontium [32, 33], etc., in HAp. Co-dopants like magnesium-silver ions [34], ironzinc [35], etc., have also been used for augmenting physical and biological properties. Electrical and dielectric properties of HAp are crucial for understanding the dipole polarization and surface charge. Depending on the radius of metal ions, substitution or doping takes place in the matrix of HAp which in turn alters the local dielectric polarization leading to micro and macro polarization. Further, these types of polarization modify the surface charges to either positive or negative. In the case of co/tri doping, the basis of lattice has higher atomic and electronic polarization which alter local bonding of the atoms. When electromagnetic field is applied to the co-doped lattice, the wave dispersion is modified when compared to the pristine dielectric material. The ex situ electromagnetic fields enhance the bone fracture healing and bone mineralization [36]. Bowen et al. reported that an enhancement in the barium titanate incorporation in HAp lattice increases the permittivity and ac conductivity [37]. Nakamura et al. revealed the migration of protons along the columnar structure of HAp under electric field at high temperature [38]. Enhanced bone growth was noticed on the negatively charged surface whereas it was reduced on positively charged surface [39]. Osteogenesis was promoted in the dog's jawbones using HA-BaTiO3 composites without any change in phase [40]. Nakamura et al. augmented the osteoconduction of the canine bone owing to the bioactive HAp surface as well as accelerated surface charge by electrical polarization [41]. In the lattice of HAp, the hydroxyl atoms were projected along c-axis and charges are modified on application of the electric field and with an increase in temperature. The projection of hydroxyl groups alters to either abundant negative or positive

*DOI: http://dx.doi.org/10.5772/intechopen.93092*

#### *Impact of Dopants on the Electrical and Optical Properties of Hydroxyapatite DOI: http://dx.doi.org/10.5772/intechopen.93092*

permittivity and ac conductivity of HAp. In the case of polymer/HAp composites, the dielectric behavior is distinct due to the presence of carbon and hydrogen chains and the type of bonding. On annealing, the dielectric response of the composites changes significantly due to the formation of voids and porous structures.

Yamashita et al. reported the chemical effects of the electrically polarized HAp in simulated body fluid [23] and in addition enhanced the *in vitro* bioactivity and *in vivo* osteoconductivity [24, 25]. Similarly, the bioactivity of HAp has been improved by doping of metal ions viz., iron [26, 27], silver [28, 29], magnesium [30, 31], strontium [32, 33], etc., in HAp. Co-dopants like magnesium-silver ions [34], ironzinc [35], etc., have also been used for augmenting physical and biological properties. Electrical and dielectric properties of HAp are crucial for understanding the dipole polarization and surface charge. Depending on the radius of metal ions, substitution or doping takes place in the matrix of HAp which in turn alters the local dielectric polarization leading to micro and macro polarization. Further, these types of polarization modify the surface charges to either positive or negative. In the case of co/tri doping, the basis of lattice has higher atomic and electronic polarization which alter local bonding of the atoms. When electromagnetic field is applied to the co-doped lattice, the wave dispersion is modified when compared to the pristine dielectric material. The ex situ electromagnetic fields enhance the bone fracture healing and bone mineralization [36]. Bowen et al. reported that an enhancement in the barium titanate incorporation in HAp lattice increases the permittivity and ac conductivity [37]. Nakamura et al. revealed the migration of protons along the columnar structure of HAp under electric field at high temperature [38]. Enhanced bone growth was noticed on the negatively charged surface whereas it was reduced on positively charged surface [39]. Osteogenesis was promoted in the dog's jawbones using HA-BaTiO3 composites without any change in phase [40]. Nakamura et al. augmented the osteoconduction of the canine bone owing to the bioactive HAp surface as well as accelerated surface charge by electrical polarization [41]. In the lattice of HAp, the hydroxyl atoms were projected along c-axis and charges are modified on application of the electric field and with an increase in temperature. The projection of hydroxyl groups alters to either abundant negative or positive charge that would aid the cell growth and cell proliferation.

The interaction of light with the HAp nanoparticles is another interesting phenomenon, though an insulating material, the presence of phosphate ions aids the emission of photons of different wavelengths. Addition of metal ions in the lattice of HAp tunes the defect energy levels and alters the chemical potential. The dopants can affect the optical and photoluminescence (PL) of HAp. These properties depend on the absorption, transmission, and band gap as well. Further, these properties favor the use of the HAp nanostructure for bio-imaging and biosensing applications. The doped HAp facilitates the trapping of photons at defect sites or ease transmission which enables them for opto-electronic device applications. Popa and Ciobanu reported the enhanced PL by the cerium ions doped HAp without any structural change [42]. The band gap of erbium-doped HAp was reduced displaying red and green emissions [43]. The various emissions were possible due to the formation of defect energy levels coupled with the HAp energy levels which alters the rate of recombination of electron and hole pairs. Further, the doping concentration varies the active centers of recombination. Feng et al. reported that the Eu3+/Gd3+dualdoped HAp nanorods show enhanced PL with a sustained ibuprofen (IBU) release. Further, the nanorods were injected into mice and demonstrated that these nanorods could be used for *in vivo* imaging [44]. Europium-doped HAp revealed high photoluminescence intensity at 900°C due to the migration of europium from Ca1 to Ca2 site [45]. Fluorescent europium-doped HAp nanowires were prepared by hydrothermal technique and in situ dispersion of them for dental applications [46]. Chang et al.

*Biomaterials*

for biosensing and micro-electromechanical system applications [4]. The ferroelectric properties of graphene-doped HAp samples were studied by Hendi and Yakuphanoglu [5]. Iron-doped calcium phosphate demonstrated hyperthermia (42°C) within 4 minutes [6]. Study on various metals ions (iron, manganese, and cobalt ions)-doped HAp revealed an increase in the ac conductivity. Further, the annealed iron-doped HAp revealed ferromagnetism, whereas the manganese and cobalt ions doped samples exhibited super-paramagnetic property [7]. The dielectric properties of the chromium-doped HAp were also studied [8]. Various anions such as nitrate, acetate, chloride, and egg shell precursors have been used to prepare HAp particles and their corresponding dielectric constant were 9.96, 13.22, 9.92, and 10.86 at 5 MHz [9]. Porous HAp was prepared from *Pila globosa* shells which possessed high dielectric constant with low dielectric loss [10]. HApbarium titanate (BT) composite scaffolds having improved electric, compressive strength, toughness, density, and hardness were developed using cold isostatic pressing and sintering by Tavangar et al. [11]. Bismuth ions (10, 30 and 50 wt%) were substituted in the HAp matrix by the conventional solid-state reaction at 1300°C and their ac conductivity was reduced with an increase in Bi content [12]. Space charge and dipolar polarization were the dominant polarization mechanisms in Na0.5K0.5NbO3 (NKN)-HAp as reported by Verma et al. [13]. La, Ba, Fe, and Zn ions doped HAp synthesized by sol-gel route displayed a space charge polarization at low frequency with negative temperature coefficient of resistance [14]. Different contents (e.g., 0, 1, 3, 6, 19, 12, and 15% [wt.]) of gallium ion-doped HAp by microwave-assisted sol-gel technique, illustrated a higher inhibition of bacteria and fungi. Further, gallium ions influence the dielectric properties of HAp [15]. Dielectric properties of chlorinated ethylene propylene diene monomer/hydroxyapatite nanocomposites were robustly distorted at lower frequencies whereas above 103 Hz, it was frequency independent [16]. Horiuchi et al. reported that the polycrystalline HAp possesses conductive grains and insulating grain boundaries further, interfacial polarization was confirmed to be an electret [17]. In the case of chitosan/HAp composite, the concentration of chitosan plays a predominant role in regulating its hardness, conductivity and dielectric constant [18]. HAp particles were permanently polarized by both electric potential (500 V direct current) and thermal treatment (at 1000°C) leading to the high adsorption of inorganic bioadsorbates compared to the as prepared HAp [19]. Electric field assisted sintering such as spark plasma sintering (SPS) and flash sintering (FS) performed on HAp revealed nanovoids within HA grains. Further, in situ TEM heating produced nanovoids which remained stable up to 900°C and were not present at 1100°C [20]. Using sol-gel route, yttrium and strontium co-substituted nano-hydroxyapatite was prepared with a minor phase of β tricalcium phosphate, however the ac conductivity was enhanced with an increase in frequency [21]. Tungsten-doped hydroxyapatite (W-HAp) nanoparticles were prepared by chemical precipitation followed by thermal treatment (800, 1000 and 1200°C) leading to an enhancement in the mechanical strength, Young's modulus and dielectric constant [22]. The synthesis routes, concentration of doping, and doping characteristics affect the behavior of dielectrics as well as other correlated properties such as mechanical and biocompatibility. In the case of doping of semiconductors, the dielectric properties are robustly influenced due to the presence of holes and electrons. When the electric field is applied, the polarization varied in the matrix owing to the occurrence of both holes and electrons. The physical (pressure), chemical (electronic), and thermal treatment significantly alter the properties of HAp leading to the creation of porous, dense and bonded matrix. When electric field is applied to such systems, the electronic polarization, oxygen vacancies, dipole displacement, and atomic orientation are interrelated which alter the

**98**

reported dual emission from nitrogen-doped carbon dots (N-CDs)/HAp:europium, gadolinium composite due to aggregation of N-CDs [47]. The nanostructures either one or zero dimensions significantly affect the electronic motion forming many energy levels due to the quantum confinement effect. Moreover, the optical properties of HAp vary on doping of quantum dots, nanowires, etc. When light is shone on the materials, various local emissions occurred that would be absorbed in the matrix subsequently leading to the partial emission of photons. In the case of cerium-doped HAp, the luminescence quenching occurred between the nearest cerium ions in the matrix as reported by Kolesnikov et al. [48]. Various fluoridated HAp doped with Eu3+ ion nanoparticles were prepared by hydrothermal technique and the fluorine ions could influence the crystal field environment for luminescence conversion [49]. Reddy et al. observed a strong green emission from Ni2+-doped Ca-Li HAp (CLHA) [50]. Erbium-ytterbium-molybdenum tri-doped HA/β-TCP phosphor was synthesized using solid-state reaction by Van et al. They demonstrated 650 times higher green emission in the presence of molybdenum [51]. In the case of tri-doped HAp, each dopant generates its energy levels and active centers for recombination leading to different emissions. Further, the emission depends on the dopant concentration as well. The praseodymium-doped fluorapatite illustrated green emission at 545 nm and orange emission at 600 nm [52]. Europium/barium co-doped and F-substituted nHAp (HA@nFAp:Eu/Ba) showed high sensitivity in both computed tomography and fluorescence imaging [53]. Carbon dots/HAp/PVA (CDs/HA/PVA) dual-network (DN) hydrogel scaffold showed an excellent fluorescence in non-invasive monitoring field for the in vivo evaluation [54]. In vivo investigation is a challenging task; these advanced materials help in monitoring the cell viability and cell proliferation on implants without following supplementary way to understand cell interaction. Xing et al. developed heparin-coated Eu3+-doped HAp nanoparticles (SH-Eu:nHAP) for bioimaging [55]. HAp nanorods tune color from blue to red and also emit white colors depending on the temperature, excitation wavelengths, etc. [56]. Optical responses, band gap, and emission behavior of materials vary on thermal treatments; further, thermal defects and production of active centers were responsible for the rapid/slow recombination of the electron and hole pairs. Hence, thermal-based luminescence is a fascinating process of emission with respect to various temperatures. However, thermally induced defects are not stable and returned to the ground state by emitting photons. Further, doping modifies the interatomic and intra-atomic transition states of basis during thermoluminescence. For instance, thermoluminescence of HAp doped with different percentages of lanthanum (La), Eu, gadolinium (Gd), and dysprosium (Dy) has been examined under the gamma radiation and (1 m%) Dy-doped samples demonstrated its capability to be used for gamma radiation dosimetry [57]. Cathodoluminescence and thermoluminescence nature of calcium phosphates were also investigated in the UV-IR range with the wavebands due to hydroxyl groups, nonbridging oxygen, etc. [58]. HAp and YAG:Ce ceramics were synthesized by Huang et al. at 850°C to fabricate white light-emitting diodes with phosphor in ceramics (PiCs) color converters in transmission mode [59]. So, these green diodes are useful in reducing the consumption of toxic materials (europium, cadmium etc.) and will guide researchers to develop various green-based diodes. Gd-dependent blue emission was observed on selenium (Se) and gadolinium (Gd) dual ions doped HAp nanoparticles under ultraviolet (UV) irradiation [60]. Copper-doped Sr-HAp was prepared by solid-state reaction Sr10(VO4)6-x(PO4)xO2CuyH2-y-δ (x = 0–6; y = 0–0.24) as the x value decreases; copper-doped samples show intense blue emission [61]. In most of the cases, the PL emission was reduced due to the concentration quenching and non-radiative recombination of electron and hole pairs which evidently showed the variation of active centers in the matrix.

**101**

**Table 1.**

*Phases of calcium phosphate [65].*

*Impact of Dopants on the Electrical and Optical Properties of Hydroxyapatite*

tion advanced biomedical materials will also be examined.

**2. Types of calcium phosphates**

adsorption and resorption capacities.

Europium-doped calcium phosphate apatite-based colloids possess narrow emission with long luminescence lifetimes suitable for nanoprobe as reported by Kattan et al. [62]. These colloids were stable over time and excite near visible or visible light domains. The excited states in the colloids were highly stable and electron decay time was slow. Further, non-radiative recombination centers might be far away from each other to reduce the bleaching effect. Mesoporous strontium ions doped HAp samples exhibited blue emission and sustained drug release [63]. The PL intensity of doped HAp was correlated to the amount of ibuprofen released [63]. Similarly, Yang et al. reported the red emission of europium-doped HAp and its PL intensity altered with the release of drug molecules [64]. This idea has many practical difficulties at *in vivo* level and could be addressed by developing advanced analysis. Thus, doped HAp nanostructures can be an ideal candidate for both bio-imaging and drug delivery. Still, there is a lack of fundamental and basic understanding of electrical and optical nature of doped HAp. The main focus of this chapter is to discuss few instances of metal ions doped HAp and its electrical and optical behavior. Further, the path that lead to the development of next genera-

Calcium phosphates are found in the bone and teeth as the major inorganic constituent. Depending on the Ca/P ratio and structure, the phases of calcium phosphate are classified as tabulated in **Table 1**. Although these phases of calcium phosphate are different from each other by their chemical composition, but they are biocompatible, non-toxic, and osteoconductive in nature which have ability to stimulate tissue regeneration. The HAp and tricalcium phosphate (TCP) are the mostly used ceramics in biomedical applications among the phases of calcium phosphate. The combination of these two phases result in a new phase formation which was recognized as a biphasic calcium phosphate used in bone repair and replacement applications. The phases of calcium phosphate have their individual

**S. No. Phase Abbreviation Empirical formula Ca/P** 

1. Calcium phosphate monohydrate MCPM Ca(H2PO4)2.H2O 0.5 2. Monocalcium phosphate MCP Ca(H2PO4)2 0.5 3. Dicalcium phosphate DCP CaHPO4 1.0 4. Dicalcium phosphate dihydrate DCPD CaHPO4.2H2O 1.0 5. Octacalcium phosphate OCP Ca8H2(PO4)6.5H2O 1.33 6. Tricalcium phosphate TCP Ca3(PO4)2 1.5 7. Amorphous calcium phosphate ACP Ca10−xH2x(PO4)6(OH)2 1.2–2.2 8. Calcium-deficient hydroxyapatite CDHA Ca9(HPO4)(PO4)5(OH) 1.5–1.67 9. Hydroxyapatite HAp Ca10(PO4)6(OH)2 1.67 10. Oxyapatite OXA Ca10(PO4)6O 1.67 11. Tetracalcium phosphate TTCP Ca4O(PO4)2 2.0

**ratio**

*DOI: http://dx.doi.org/10.5772/intechopen.93092*

#### *Impact of Dopants on the Electrical and Optical Properties of Hydroxyapatite DOI: http://dx.doi.org/10.5772/intechopen.93092*

Europium-doped calcium phosphate apatite-based colloids possess narrow emission with long luminescence lifetimes suitable for nanoprobe as reported by Kattan et al. [62]. These colloids were stable over time and excite near visible or visible light domains. The excited states in the colloids were highly stable and electron decay time was slow. Further, non-radiative recombination centers might be far away from each other to reduce the bleaching effect. Mesoporous strontium ions doped HAp samples exhibited blue emission and sustained drug release [63]. The PL intensity of doped HAp was correlated to the amount of ibuprofen released [63]. Similarly, Yang et al. reported the red emission of europium-doped HAp and its PL intensity altered with the release of drug molecules [64]. This idea has many practical difficulties at *in vivo* level and could be addressed by developing advanced analysis. Thus, doped HAp nanostructures can be an ideal candidate for both bio-imaging and drug delivery. Still, there is a lack of fundamental and basic understanding of electrical and optical nature of doped HAp. The main focus of this chapter is to discuss few instances of metal ions doped HAp and its electrical and optical behavior. Further, the path that lead to the development of next generation advanced biomedical materials will also be examined.

### **2. Types of calcium phosphates**

*Biomaterials*

reported dual emission from nitrogen-doped carbon dots (N-CDs)/HAp:europium, gadolinium composite due to aggregation of N-CDs [47]. The nanostructures either one or zero dimensions significantly affect the electronic motion forming many energy levels due to the quantum confinement effect. Moreover, the optical properties of HAp vary on doping of quantum dots, nanowires, etc. When light is shone on the materials, various local emissions occurred that would be absorbed in the matrix subsequently leading to the partial emission of photons. In the case of cerium-doped HAp, the luminescence quenching occurred between the nearest cerium ions in the matrix as reported by Kolesnikov et al. [48]. Various fluoridated HAp doped with Eu3+ ion nanoparticles were prepared by hydrothermal technique and the fluorine ions could influence the crystal field environment for luminescence conversion [49]. Reddy et al. observed a strong green emission from Ni2+-doped Ca-Li HAp (CLHA) [50]. Erbium-ytterbium-molybdenum tri-doped HA/β-TCP phosphor was synthesized using solid-state reaction by Van et al. They demonstrated 650 times higher green emission in the presence of molybdenum [51]. In the case of tri-doped HAp, each dopant generates its energy levels and active centers for recombination leading to different emissions. Further, the emission depends on the dopant concentration as well. The praseodymium-doped fluorapatite illustrated green emission at 545 nm and orange emission at 600 nm [52]. Europium/barium co-doped and F-substituted nHAp (HA@nFAp:Eu/Ba) showed high sensitivity in both computed tomography and fluorescence imaging [53]. Carbon dots/HAp/PVA (CDs/HA/PVA) dual-network (DN) hydrogel scaffold showed an excellent fluorescence in non-invasive monitoring field for the in vivo evaluation [54]. In vivo investigation is a challenging task; these advanced materials help in monitoring the cell viability and cell proliferation on implants without following supplementary way to understand cell interaction. Xing et al. developed heparin-coated Eu3+-doped HAp nanoparticles (SH-Eu:nHAP) for bioimaging [55]. HAp nanorods tune color from blue to red and also emit white colors depending on the temperature, excitation wavelengths, etc. [56]. Optical responses, band gap, and emission behavior of materials vary on thermal treatments; further, thermal defects and production of active centers were responsible for the rapid/slow recombination of the electron and hole pairs. Hence, thermal-based luminescence is a fascinating process of emission with respect to various temperatures. However, thermally induced defects are not stable and returned to the ground state by emitting photons. Further, doping modifies the interatomic and intra-atomic transition states of basis during thermoluminescence. For instance, thermoluminescence of HAp doped with different percentages of lanthanum (La), Eu, gadolinium (Gd), and dysprosium (Dy) has been examined under the gamma radiation and (1 m%) Dy-doped samples demonstrated its capability to be used for gamma radiation dosimetry [57]. Cathodoluminescence and thermoluminescence nature of calcium phosphates were also investigated in the UV-IR range with the wavebands due to hydroxyl groups, nonbridging oxygen, etc. [58]. HAp and YAG:Ce ceramics were synthesized by Huang et al. at 850°C to fabricate white light-emitting diodes with phosphor in ceramics (PiCs) color converters in transmission mode [59]. So, these green diodes are useful in reducing the consumption of toxic materials (europium, cadmium etc.) and will guide researchers to develop various green-based diodes. Gd-dependent blue emission was observed on selenium (Se) and gadolinium (Gd) dual ions doped HAp nanoparticles under ultraviolet (UV) irradiation [60]. Copper-doped Sr-HAp was prepared by solid-state reaction Sr10(VO4)6-x(PO4)xO2CuyH2-y-δ (x = 0–6; y = 0–0.24) as the x value decreases; copper-doped samples show intense blue emission [61]. In most of the cases, the PL emission was reduced due to the concentration quenching and non-radiative recombination of electron and hole pairs which evidently showed

**100**

the variation of active centers in the matrix.

Calcium phosphates are found in the bone and teeth as the major inorganic constituent. Depending on the Ca/P ratio and structure, the phases of calcium phosphate are classified as tabulated in **Table 1**. Although these phases of calcium phosphate are different from each other by their chemical composition, but they are biocompatible, non-toxic, and osteoconductive in nature which have ability to stimulate tissue regeneration. The HAp and tricalcium phosphate (TCP) are the mostly used ceramics in biomedical applications among the phases of calcium phosphate. The combination of these two phases result in a new phase formation which was recognized as a biphasic calcium phosphate used in bone repair and replacement applications. The phases of calcium phosphate have their individual adsorption and resorption capacities.


#### **Table 1.** *Phases of calcium phosphate [65].*

The solubility of calcium phosphate (CaPs) varies from each other and the *in vivo* degradation was foreseen in the order MCPM>TTCP>α-TCP > DCPD>DCP > OCP > β-TCP > CDHA>HAp [29]. Monocalcium phosphate monohydrate (MCPM) is soluble in water and is acidic in nature. Therefore, it cannot be used in the bone repair applications. The MCPM combined with α-TCP or β-TCP, makes it less soluble [65, 66]. Usually the MCPM has been used as fertilizer and commonly known as superphosphate fertilizer. The Monocalcium phosphate (MCP) was prepared by heating the MCPM at 100–110°C and is similar to the MCPM. The MCP has been rarely used due to its highly hygroscopic nature [67].

Dicalcium phosphate (DCP) is biocompatible and bioresorbable and has been used as bone cement [68, 69]. Dicalcium phosphate dihydrate (DCPD) also known as brushite can be easily synthesized and has osteoconductive property. It is a metastable material and can be converted to other phases DCP, OCP, CDHA, and TCP by varying the pH of the reactant solutions. The octacalcium phosphate (OCP) is responsible for the formation of teeth and bone. It crystallizes very slowly and is not suitable for bone cement application. The α-TCP and β-TCP are polymorphs having same chemical composition with different structures [70, 71]. The α-TCP's solubility is better than that of β-TCP but both are biocompatible, resorbable, and have been used as bone cement. Among the phases of CaPs, the HAp is more stable and highly crystalline in nature. When the HAp is heated above 900°C, it is converted to oxyapatite (OXA) in an inert atmosphere. The tetracalcium phosphate (TTCP) was prepared by heating the DCP and calcium carbonate at 1400°C with rapid cooling [72].

### **3. Electrical properties**

HAp is a bioceramic having a high dielectric constant. Researchers have studied the impact of electric field on the HAp to understand the real part and imaginary part of the permittivity [37, 38]. These properties provide us an insight into the dielectric polarization of electric dipoles with respect to electric field. Further, the dielectric studies aid to understand *in vitro* electrical polarization. The respective real and imaginary part of permittivity grants dispersion of dipoles and energy dissipation. Some other parameters say temperature, experimental condition and dopants can influence and alter the dielectric properties. Doping sites and dopant's radius alter the electronic and orientation polarization, whereas co or tri-doping significantly affects the structure and phase, further, the dielectric properties of these materials might vary due to the rate of dipole relaxation. Generally, at low frequency alternating field, the dielectric constant is high and varies with increase in frequency. Nakamura et al. studied the polarization of HAp using direct current and showed that proton migrates along the columnar OH channel [38]. The polarized charge was high enough to improve biological activity. Usage of the electrical fields enabled the accelerated fracture healing in bones; however, it was slow in the case of long bone [72–74]. There are some reports on electrical stimulation which has been used to enhance the bone growth in spinal fusion [75, 76]. Further, it has been employed to treat osteonecrosis as well as osteoarthritis [77]. The dielectric constant of fluids and tissues in the bone and HAp is crucial for healing. The porosity of HAp plays a vital role for the local electrical field strength [78, 79]. Human bones also contain many elements such as magnesium, zinc, and strontium etc., so, the impact of the elements in the bone also play a vital role in the dielectric constant of bone. Further, it as well depends on the elemental size, concentration, and interrelated porosity.

**103**

**Figure 1.**

*(d) 3HAp (1×1017 ions/cm2*

*Impact of Dopants on the Electrical and Optical Properties of Hydroxyapatite*

substitution due to the change in orientation of hydroxyl groups [82].

In the case of tungsten (W)-doped HAp, the relative permittivity was reduced up to 1 MHz and from 1 to 5 MHz the permittivity was constant [83]. The dielectric constant of W-doped HAp was high at low frequency due to the polarization of electronic, ionic, dipolar, and space charge. As the doping concentration increases, the dielectric constant was enhanced [83]. Thus, the dopants enhance spreading of the electromagnetic fields in the damaged bone sites or fractured area for rapid healing. Similarly, the other metal ions strontium [21], iron [26] etc., have been used as dopants in HAp to enhance dielectric property. A crucial and impressive part is how dielectric constant varies with concentration and with various dopants and the mechanism involved. The co-doping in HAp leads to the creation of many defects and alters the ionic polarization with respect to electric field. For instance, iron and zinc ions were co-doped in HAp at various concentrations (0.01, 0.05,

*(A) Real part of relative permittivity of HAp and Mg-doped HAp (reproduced from Ref. [30] with permission from Elsevier). (B) Dielectric constant of (a) FZH0, (b) FZH1 (0.01 M), (c) FZH2 (0.05 M) and (d) FZH3 (0.1 M) (reproduced from [35] with permission from Elsevier) and (C) The real part of relative permittivity* 

*) (reproduced from [84] with permission from Elsevier).*

*), (c) 2HAp (1×1016 ions/cm2*

*) and* 

*as a function of frequency for (a) Pristine, (b) 1HAp (1×1015 ions/cm2*

For instance, doping of magnesium ions on HAp enhances the dielectric constant

(**Figure 1A**) [30]. At low frequency, the dielectric constant was high due to ionic polarization; however, it was reduced by lagging of dipoles with respect to electric field. At low magnesium ions doping, the dielectric constant was low and almost equal to the dielectric constant of HAp. So, the low level doping did not highly polarize the ions in the direction of electric field and difficult to locate the dopant site position either at calcium site 1 or calcium site 2. As the Mg concentration increases, both the sites might be occupied and at low frequency the dipoles were strongly oriented, whereas it was disturbed at high frequency. Similarly, the cadmium-doped HAp revealed a higher dielectric constant in comparison with pure HAp. However, 40% of cadmium-doped HAp shows a decrease in the dielectric property. At 1 kHz frequency, the dielectric constant of HAp, doped samples 10 at.% Cd, 20 at.% Cd, 25 at.% Cd, 30 at.% Cd, and 40 at.% Cd were 6.75, 7.12, 8.16, 7.13, 7.26, and 6.24 respectively. At 40 at.% Cd, the structural change from hexagonal to monoclinic with varying crystallinity was responsible for the reduction in the dielectric constant [80]. In the case of silver-doped HAp, the dielectric constant was enhanced due to a high dipole polarization [81]. The dopant size varies the phase of HAp leading to amorphous or partial amorphization which completely modifies the local coordination and chemical potential. Horiuchi et al. revealed the two step relaxations of fluorine substituted HAp (F-HAp) [82]. They depicted that one step from 600 to 60 Hz and other 60 to 20 Hz in HAp which were analogous to the F-HAp. At low frequency, the electric dipoles are largely (L) relaxed, whereas at high frequency, the relaxation was small (S). Further, they calculated the activation energy of HAp and F-HAp as 0.63 and 0.62 eV, respectively, for one type of relaxation and it was independent of fluorine substitution. In case of other relaxation, the activation energy was significantly influenced by fluorine

*DOI: http://dx.doi.org/10.5772/intechopen.93092*

#### *Impact of Dopants on the Electrical and Optical Properties of Hydroxyapatite DOI: http://dx.doi.org/10.5772/intechopen.93092*

For instance, doping of magnesium ions on HAp enhances the dielectric constant (**Figure 1A**) [30]. At low frequency, the dielectric constant was high due to ionic polarization; however, it was reduced by lagging of dipoles with respect to electric field. At low magnesium ions doping, the dielectric constant was low and almost equal to the dielectric constant of HAp. So, the low level doping did not highly polarize the ions in the direction of electric field and difficult to locate the dopant site position either at calcium site 1 or calcium site 2. As the Mg concentration increases, both the sites might be occupied and at low frequency the dipoles were strongly oriented, whereas it was disturbed at high frequency. Similarly, the cadmium-doped HAp revealed a higher dielectric constant in comparison with pure HAp. However, 40% of cadmium-doped HAp shows a decrease in the dielectric property. At 1 kHz frequency, the dielectric constant of HAp, doped samples 10 at.% Cd, 20 at.% Cd, 25 at.% Cd, 30 at.% Cd, and 40 at.% Cd were 6.75, 7.12, 8.16, 7.13, 7.26, and 6.24 respectively. At 40 at.% Cd, the structural change from hexagonal to monoclinic with varying crystallinity was responsible for the reduction in the dielectric constant [80]. In the case of silver-doped HAp, the dielectric constant was enhanced due to a high dipole polarization [81]. The dopant size varies the phase of HAp leading to amorphous or partial amorphization which completely modifies the local coordination and chemical potential. Horiuchi et al. revealed the two step relaxations of fluorine substituted HAp (F-HAp) [82]. They depicted that one step from 600 to 60 Hz and other 60 to 20 Hz in HAp which were analogous to the F-HAp. At low frequency, the electric dipoles are largely (L) relaxed, whereas at high frequency, the relaxation was small (S). Further, they calculated the activation energy of HAp and F-HAp as 0.63 and 0.62 eV, respectively, for one type of relaxation and it was independent of fluorine substitution. In case of other relaxation, the activation energy was significantly influenced by fluorine substitution due to the change in orientation of hydroxyl groups [82].

In the case of tungsten (W)-doped HAp, the relative permittivity was reduced up to 1 MHz and from 1 to 5 MHz the permittivity was constant [83]. The dielectric constant of W-doped HAp was high at low frequency due to the polarization of electronic, ionic, dipolar, and space charge. As the doping concentration increases, the dielectric constant was enhanced [83]. Thus, the dopants enhance spreading of the electromagnetic fields in the damaged bone sites or fractured area for rapid healing. Similarly, the other metal ions strontium [21], iron [26] etc., have been used as dopants in HAp to enhance dielectric property. A crucial and impressive part is how dielectric constant varies with concentration and with various dopants and the mechanism involved. The co-doping in HAp leads to the creation of many defects and alters the ionic polarization with respect to electric field. For instance, iron and zinc ions were co-doped in HAp at various concentrations (0.01, 0.05,

#### **Figure 1.**

*Biomaterials*

rapid cooling [72].

**3. Electrical properties**

The solubility of calcium phosphate (CaPs) varies from each other and the *in vivo* degradation was foreseen in the order MCPM>TTCP>α-

TCP > DCPD>DCP > OCP > β-TCP > CDHA>HAp [29]. Monocalcium phosphate monohydrate (MCPM) is soluble in water and is acidic in nature. Therefore, it cannot be used in the bone repair applications. The MCPM combined with α-TCP or β-TCP, makes it less soluble [65, 66]. Usually the MCPM has been used as fertilizer and commonly known as superphosphate fertilizer. The Monocalcium phosphate (MCP) was prepared by heating the MCPM at 100–110°C and is similar to the MCPM. The MCP has been rarely used due to its highly hygroscopic nature [67]. Dicalcium phosphate (DCP) is biocompatible and bioresorbable and has been used as bone cement [68, 69]. Dicalcium phosphate dihydrate (DCPD) also known as brushite can be easily synthesized and has osteoconductive property. It is a metastable material and can be converted to other phases DCP, OCP, CDHA, and TCP by varying the pH of the reactant solutions. The octacalcium phosphate (OCP) is responsible for the formation of teeth and bone. It crystallizes very slowly and is not suitable for bone cement application. The α-TCP and β-TCP are polymorphs having same chemical composition with different structures [70, 71]. The α-TCP's solubility is better than that of β-TCP but both are biocompatible, resorbable, and have been used as bone cement. Among the phases of CaPs, the HAp is more stable and highly crystalline in nature. When the HAp is heated above 900°C, it is converted to oxyapatite (OXA) in an inert atmosphere. The tetracalcium phosphate (TTCP) was prepared by heating the DCP and calcium carbonate at 1400°C with

HAp is a bioceramic having a high dielectric constant. Researchers have studied the impact of electric field on the HAp to understand the real part and imaginary part of the permittivity [37, 38]. These properties provide us an insight into the dielectric polarization of electric dipoles with respect to electric field. Further, the dielectric studies aid to understand *in vitro* electrical polarization. The respective real and imaginary part of permittivity grants dispersion of dipoles and energy dissipation. Some other parameters say temperature, experimental condition and dopants can influence and alter the dielectric properties. Doping sites and dopant's radius alter the electronic and orientation polarization, whereas co or tri-doping significantly affects the structure and phase, further, the dielectric properties of these materials might vary due to the rate of dipole relaxation. Generally, at low frequency alternating field, the dielectric constant is high and varies with increase in frequency. Nakamura et al. studied the polarization of HAp using direct current and showed that proton migrates along the columnar OH channel [38]. The polarized charge was high enough to improve biological activity. Usage of the electrical fields enabled the accelerated fracture healing in bones; however, it was slow in the case of long bone [72–74]. There are some reports on electrical stimulation which has been used to enhance the bone growth in spinal fusion [75, 76]. Further, it has been employed to treat osteonecrosis as well as osteoarthritis [77]. The dielectric constant of fluids and tissues in the bone and HAp is crucial for healing. The porosity of HAp plays a vital role for the local electrical field strength [78, 79]. Human bones also contain many elements such as magnesium, zinc, and strontium etc., so, the impact of the elements in the bone also play a vital role in the dielectric constant of bone. Further, it as well depends on the elemental size, concentration, and inter-

**102**

related porosity.

*(A) Real part of relative permittivity of HAp and Mg-doped HAp (reproduced from Ref. [30] with permission from Elsevier). (B) Dielectric constant of (a) FZH0, (b) FZH1 (0.01 M), (c) FZH2 (0.05 M) and (d) FZH3 (0.1 M) (reproduced from [35] with permission from Elsevier) and (C) The real part of relative permittivity as a function of frequency for (a) Pristine, (b) 1HAp (1×1015 ions/cm2 ), (c) 2HAp (1×1016 ions/cm2 ) and (d) 3HAp (1×1017 ions/cm2 ) (reproduced from [84] with permission from Elsevier).*

and 0.1 M) by ultrasonication. At higher concentration (FZH3) (0.1M), dielectric constant was greater (**Figure 1B**) compared to the other samples owing to the enhanced ionic polarization and strong orientation of dipoles at low frequency. As the frequency increases, the dielectric constant was reduced in all the samples due to lagging of the dipoles [35]. Generally, at low frequency, the value of dielectric is high due to slow relaxation of dipoles with high orientation. At high frequency, the dielectric constant was reduced due to strong re-alignment of dipoles with field [30, 83]. Further, it depends on the size of dopants, crystal structure, and concentration providing atomic site variations which enable the variations in the unit cell and crystal structure. Moreover, the dopants provide vacancies which affect space charge localization/delocalization. This facilitates the alterations in the dielectric constant. In case of the HAp structure, the hydroxyl ions aligned along c-axis parallel to calcium and phosphate(PO4) 3− ions favor high polarization of electric dipoles thereby increasing the ionic conduction [38]. Surface charge of HAp is another important parameter which assists the bone cell growth. The polarized surface of HAp had improved the osteobonding in canine bone tissues [39]. So, the electric properties have been employed to understand the cellular behavior on bone and to develop bone prostheses [38]. The surface charge of HAp depends on the porosity and dielectric strength. It can vary on low energy ion implantation which are capable of altering the surface properties without affecting the bulk properties of the materials. Nitrogen ions (N+1) were implanted in HAp matrix with varying ion fluences 1 × 1015 (1HAp), 1 × 1016 (2HAp), and 1 × 1017 (3HAp) ions/cm2 [84]. At higher ion fluence (3HAp), relative permittivity was significantly enhanced (**Figure 1C**) in comparison with other samples because of the higher ionic and space charge polarizations. The surface and subsurface of HAp were modulated on ion implantation altering the dipole polarization thereby leading to the variation in the surface charge of the samples. The polarization varies locally leading to dielectric strength modification at the surface, subsurface and at macro level. Hence, the surface charge can be tuned by different dopants, temperature, and frequency.

The variation of dielectric constant with frequency of pure and Fe-doped HAp, Mn-doped HAp, and Co-doped HAP sample at room temperature was examined by Panneerselvam et.al. [7]. The dielectric constant of the doped samples decreased sharply up to 10 kHz, and then reduced gradually with further increase in frequency. This difference in dielectric constant is due to the four types of polarization such as electronic polarization (frequencies up to 1016 Hz), ionic polarization (1013 Hz), orientation polarization (up to 1010 Hz), and space charge polarization or interfacial (up to 103 Hz) [7]. In all the samples, as the doping concentration increases, the dielectric constant was enhanced due to ionic polarization. The highest dielectric constant was exhibited by 5% MnHAp and the lowest by 5% CoHAp [7]. The dopants such as Fe, Mn, and Co possess different ionic radii and electronic cloud density. The variation of dielectric constant of the different metal ions doped HAp is due to the change in dipole strength, rate of dipole relaxation, and space charge polarization apart from ionic polarization. Tavangar et al. fabricated HApbarium titanate (BT) scaffolds by cold isostatic pressing and sintering. As the BT concentration increases, the dielectric loss (D) of the composite was reduced due to the low porosities and higher densities of composite when compared with pure HAp. The 60 wt% BT had the lowest dielectric loss at 1 kHz. The dielectric constant of the 60 wt% BT–40 wt% HA composite was 46.50 which was the highest compared to the other composites due to the coupling of low-dielectric constant of HA with BT leading to a parallel polarization in the presence of phases with different weight percentages and electrical conductivity [11]. The ac frequency facilitated the coupling of the phases of materials leading to a synergic effect on the dipole polarization and orientation. Verma et al. studied the effect of concentration of

**105**

**Figure 2.**

*Impact of Dopants on the Electrical and Optical Properties of Hydroxyapatite*

piezoelectric Na0.5K0.5NbO3 (NKN) on dielectric and electrical properties of HAp in the range of temperature (30–500°C) at frequency 1 Hz to 1 MHz. The composite was prepared by solid state ceramic method and sintered at 1075°C for 2 h [13]. The experimental values of the dielectric constant are lower than those of calculated using theoretical models [13] due to the connectivity between HAp and NKN phases at the lower concentration of NKN in HAp matrix. The connectivity of matrix and secondary phase affects the electrical as well as mechanical properties of the composites. At low content of NKN, the HAp phases were connected in three-dimension whereas, no connectivity between NKN phases and 0–3 connectivity between NKN and HAp phases were observed. With enhancing content of NKN in the HAp matrix, the interaction between piezoelectric phases was increased. The calculated dielectric values of HAp-10 NKN, HAp-20 NKN, and HAp-30 NKN were 30.54, 38.71 and 50.29 respectively. The effective dielectric constant of HAp-10 NKN, HAp-20 NKN and HAp-30 NKN at 10 kHz (room temperature) were calculated to be 29.81, 35.32 and 42.046 respectively. The calculated and experimental values were significantly varied at high concentrations due to the combined involvement of complex parameters such as orientation and electronic The microstructure, densification, undetectable phases etc. play a vital role in determining the efficient

The ac conductivity measurements are used to study the mechanism of hopping and it is a complex phenomenon. Further, it depends on frequency and temperature [30]. The ac conductivity of Mg-doped HAp was enhanced compared to HAp (**Figure 2A**). There was low ac conductivity at low frequency due to the weak turnaround of ions with the electric field. However, it was increased at higher frequency on Mg-doped samples due to the complex array of ions and proton segregation along c-axis [30, 85]. Similar trend also noticed in W-doped HAp samples which linearly depend on the frequency range. Further, it obeyed the universal frequency power law. Towards understanding the mechanism of ac conductivity of

B-constant, ω-angular frequency and s-frequency exponent. Using the slope of lnσ vs. lnωac and the s values were calculated as 1.0622, 1.0600, 1.0623, 1.0501, 1.0518, 1.0629, and 1.0534 for pure and 1, 5, 10, 20, 30, and 40% of W in HAp, respectively. Here, s is almost ≤1, revealing no measureable direct current conductivity and displays prompt hopping with a translational motion [83]. In the case of the Cd-doped HAp samples, the alternating current conductivity was enhanced from 10−10 to 10−5 s cm−1 with an increase in frequency. The frequency exponent(s) was

*(A) Ac conductivity as a function of frequency of HAp and Mg-doped HAp (reproduced from Ref. [30] with permission from Elsevier); and (B) The ac conductivity as a function of frequency for (a) Pristine, (b) 1HAp, (c) 2HAp and (d) 3HAp (reproduced from [84] with permission from Elsevier).*

) was used [86], where

*DOI: http://dx.doi.org/10.5772/intechopen.93092*

dielectric constant of the composite system [13].

the samples, the proverbial Jonscher equation (σac = σdc + Bω<sup>s</sup>

#### *Impact of Dopants on the Electrical and Optical Properties of Hydroxyapatite DOI: http://dx.doi.org/10.5772/intechopen.93092*

piezoelectric Na0.5K0.5NbO3 (NKN) on dielectric and electrical properties of HAp in the range of temperature (30–500°C) at frequency 1 Hz to 1 MHz. The composite was prepared by solid state ceramic method and sintered at 1075°C for 2 h [13]. The experimental values of the dielectric constant are lower than those of calculated using theoretical models [13] due to the connectivity between HAp and NKN phases at the lower concentration of NKN in HAp matrix. The connectivity of matrix and secondary phase affects the electrical as well as mechanical properties of the composites. At low content of NKN, the HAp phases were connected in three-dimension whereas, no connectivity between NKN phases and 0–3 connectivity between NKN and HAp phases were observed. With enhancing content of NKN in the HAp matrix, the interaction between piezoelectric phases was increased. The calculated dielectric values of HAp-10 NKN, HAp-20 NKN, and HAp-30 NKN were 30.54, 38.71 and 50.29 respectively. The effective dielectric constant of HAp-10 NKN, HAp-20 NKN and HAp-30 NKN at 10 kHz (room temperature) were calculated to be 29.81, 35.32 and 42.046 respectively. The calculated and experimental values were significantly varied at high concentrations due to the combined involvement of complex parameters such as orientation and electronic The microstructure, densification, undetectable phases etc. play a vital role in determining the efficient dielectric constant of the composite system [13].

The ac conductivity measurements are used to study the mechanism of hopping and it is a complex phenomenon. Further, it depends on frequency and temperature [30]. The ac conductivity of Mg-doped HAp was enhanced compared to HAp (**Figure 2A**). There was low ac conductivity at low frequency due to the weak turnaround of ions with the electric field. However, it was increased at higher frequency on Mg-doped samples due to the complex array of ions and proton segregation along c-axis [30, 85]. Similar trend also noticed in W-doped HAp samples which linearly depend on the frequency range. Further, it obeyed the universal frequency power law. Towards understanding the mechanism of ac conductivity of the samples, the proverbial Jonscher equation (σac = σdc + Bω<sup>s</sup> ) was used [86], where B-constant, ω-angular frequency and s-frequency exponent. Using the slope of lnσ vs. lnωac and the s values were calculated as 1.0622, 1.0600, 1.0623, 1.0501, 1.0518, 1.0629, and 1.0534 for pure and 1, 5, 10, 20, 30, and 40% of W in HAp, respectively. Here, s is almost ≤1, revealing no measureable direct current conductivity and displays prompt hopping with a translational motion [83]. In the case of the Cd-doped HAp samples, the alternating current conductivity was enhanced from 10−10 to 10−5 s cm−1 with an increase in frequency. The frequency exponent(s) was

#### **Figure 2.**

*Biomaterials*

lel to calcium and phosphate(PO4)

or interfacial (up to 103

and 0.1 M) by ultrasonication. At higher concentration (FZH3) (0.1M), dielectric constant was greater (**Figure 1B**) compared to the other samples owing to the enhanced ionic polarization and strong orientation of dipoles at low frequency. As the frequency increases, the dielectric constant was reduced in all the samples due to lagging of the dipoles [35]. Generally, at low frequency, the value of dielectric is high due to slow relaxation of dipoles with high orientation. At high frequency, the dielectric constant was reduced due to strong re-alignment of dipoles with field [30, 83]. Further, it depends on the size of dopants, crystal structure, and concentration providing atomic site variations which enable the variations in the unit cell and crystal structure. Moreover, the dopants provide vacancies which affect space charge localization/delocalization. This facilitates the alterations in the dielectric constant. In case of the HAp structure, the hydroxyl ions aligned along c-axis paral-

thereby increasing the ionic conduction [38]. Surface charge of HAp is another important parameter which assists the bone cell growth. The polarized surface of HAp had improved the osteobonding in canine bone tissues [39]. So, the electric properties have been employed to understand the cellular behavior on bone and to develop bone prostheses [38]. The surface charge of HAp depends on the porosity and dielectric strength. It can vary on low energy ion implantation which are capable of altering the surface properties without affecting the bulk properties of the materials. Nitrogen ions (N+1) were implanted in HAp matrix with varying ion

fluences 1 × 1015 (1HAp), 1 × 1016 (2HAp), and 1 × 1017 (3HAp) ions/cm2

At higher ion fluence (3HAp), relative permittivity was significantly enhanced (**Figure 1C**) in comparison with other samples because of the higher ionic and space charge polarizations. The surface and subsurface of HAp were modulated on ion implantation altering the dipole polarization thereby leading to the variation in the surface charge of the samples. The polarization varies locally leading to dielectric strength modification at the surface, subsurface and at macro level. Hence, the surface charge can be tuned by different dopants, temperature, and frequency.

The variation of dielectric constant with frequency of pure and Fe-doped HAp, Mn-doped HAp, and Co-doped HAP sample at room temperature was examined by Panneerselvam et.al. [7]. The dielectric constant of the doped samples decreased sharply up to 10 kHz, and then reduced gradually with further increase in frequency. This difference in dielectric constant is due to the four types of polarization such as electronic polarization (frequencies up to 1016 Hz), ionic polarization (1013 Hz), orientation polarization (up to 1010 Hz), and space charge polarization

increases, the dielectric constant was enhanced due to ionic polarization. The highest dielectric constant was exhibited by 5% MnHAp and the lowest by 5% CoHAp [7]. The dopants such as Fe, Mn, and Co possess different ionic radii and electronic cloud density. The variation of dielectric constant of the different metal ions doped HAp is due to the change in dipole strength, rate of dipole relaxation, and space charge polarization apart from ionic polarization. Tavangar et al. fabricated HApbarium titanate (BT) scaffolds by cold isostatic pressing and sintering. As the BT concentration increases, the dielectric loss (D) of the composite was reduced due to the low porosities and higher densities of composite when compared with pure HAp. The 60 wt% BT had the lowest dielectric loss at 1 kHz. The dielectric constant of the 60 wt% BT–40 wt% HA composite was 46.50 which was the highest compared to the other composites due to the coupling of low-dielectric constant of HA with BT leading to a parallel polarization in the presence of phases with different weight percentages and electrical conductivity [11]. The ac frequency facilitated the coupling of the phases of materials leading to a synergic effect on the dipole polarization and orientation. Verma et al. studied the effect of concentration of

Hz) [7]. In all the samples, as the doping concentration

3− ions favor high polarization of electric dipoles

[84].

**104**

*(A) Ac conductivity as a function of frequency of HAp and Mg-doped HAp (reproduced from Ref. [30] with permission from Elsevier); and (B) The ac conductivity as a function of frequency for (a) Pristine, (b) 1HAp, (c) 2HAp and (d) 3HAp (reproduced from [84] with permission from Elsevier).*

determined from the slope of logσ vs. logω and its value equal to 1 for the samples [80]. Eventually, it displays a lack of direct current conductivity [87]. Similar, s value was realized for the Te-doped HAp samples however, their ac conductivity enhanced at low concentration due to s ≥ 1 leading to localized hopping. At higher concentration, it was reduced due to s ≤ 1 leading to translation motion with a rapid hopping motion [88]. Ion implantation could also be employed to alter the ac conductivity, without using expensive and toxic chemicals. It can precisely modulate the ac conductivity up to a particular depth. Nitrogen ions implanted samples demonstrated higher ac conductivity at higher frequency owing to the strong arrangement of complex ions compared to pristine (**Figure 2B**). However, it was less at low frequency due to the weak turnover of ions [84].

### **4. Optical properties**

The interaction of light on materials leads to transmission, absorptions and reflection. These parameters depend on refractive index, wavelength, dielectric constant, and dopants. Dopants play a vital role in creating the abundant defects/vacancies in the lattice of materials. When light interacts on the bound charges of the materials, they either transmit or reflect back. The structure and phase of HAp varied depending on the type of dopant [80] which also alters the optical properties of HAp. The optical properties of HAp were enhanced by developing smaller grains and low porosity in the HAp matrix. It is an optically anisotropic material and possesses a refractive index in the range 1.644 and 1.651 depending on light polarization and direction of propagation [89]. A highly transparent HAp nanosized grains were used to observe *in vivo* interactions with proteins/cells [90, 91]. For a proper insight of the optical properties, intrinsic correlated parameters/quantities say crystalline structure, dispersion of phonon, band structure, dielectric response, band gap, etc. are required. Density functional theory displays a local or semi local exchange-correlation interaction of HAp providing a band gap between 4.5 and 5.4 eV depending on the type of valence states [92, 93]. However, the (semi) local exchange-correlation underestimates the band gap of HAp, and thus its value is above 5.5 eV [94]. So, the defect free HAp is transparent to visible light under middle or far ultraviolet illumination having a band gap >6 eV. Interesting point to note is that the dopants produce defects level in between conduction band and valence band.

For instance, the thenoyltrifluoroacetonate (TTA)- and europium (Eu)-doped HAp were used to enhance transmission spectra by 20% compared to PMMA. The TTA shows a red shift and the transmission edge from 275 to 375 nm. Both the TTA and HAp mixture restrict the formation of PMMA clusters improving the transparency. In this case, the absorption and scattering losses were introduced by the TTA and the HAp nanoparticles [95]. The other example, Cd-doped HAp showed enhanced reflectivity at the Cd (40 at.%) due to the structural transformation to the monoclinic phase. Further, it alters the refractive index of the samples as well [80]. At a particular atomic percentage, the structural transformation occurred affecting the electronic polarization thereby, altering the reflectivity. In the case of erbium doping in HAp, seven absorption bands were noticed [43]. The bands observed at 1520, 980, 803, 657, 524, 490, and 448 nm corresponding to the electronic transitions from ground state 4 I15/2 → 4 I13/2, 4 I15/2 → 4 I11/2, 4 I15/2 → 4 I9/2, 4 I15/2 → 4 F9/2, 4 I15/2 → 2 H11/2, 4 I15/2 → 4 F7/2, and 4 I15/2 → 4 F3/2, respectively [92]. Among all the transitions, the 4 I15/2 → 2 H11/2 transition had the highest intensity due to the hypersensitive transition [92]. Further, the peaks were broadened inhomogeneously due to the f-f

**107**

*Impact of Dopants on the Electrical and Optical Properties of Hydroxyapatite*

interactions of Er3+ ions [96]. As the erbium doping concentration increases, the intensity of the peaks increased due to the replacement of Ca2+ by Er3+ ions. Further, there was a blue shift observed on the doped HAp due to the reduced particle size [97]. The Er-HAp samples possess direct band gap (n = 1/2) so band to band transitions was allowed. As the erbium doping concentration increased, the band gap was enhanced to 4.46 eV from 4.02 eV. Hence the structure, phase, defects and particle size of HAp are the key parameters to alter either the absorption or reflection. Further, the symmetry of the HAp also plays a predominant role in modifying the

Apurba et al. studied the optical response of HAp films of various thicknesses on amorphous SiO2 substrates [98]. All the films showed 75–96% transparency in the visible region. There are two regions noticed, one is absorption and other a transparent oscillating region. Below 300 nm, the transmittance of the HAp films displayed robust absorption. When the film thickness enhances, the absorption edge was shifted to higher wavelength in turn altering electronic polarization Thus, these films revealed a strong absorption in UV region and transparency in the visible regions as well. The band gap of the varying thickness of HAp thin films was in the range 5.25–4.67 eV due to an increase in crystallinity and grain size. The band gap of thin films is similar to the band gap of bulk HAp (in the range of 5.4 to 4.51) [99]. For annealed films, the refractive indices were augmented with an increase in the film thickness [98]. Flores et al. reported that using the Kubelka-Munk function through the maximum of the first derivative, the absorption edge values were 5.62, 4.74 and 4.6 eV which correspond respectively to TbW0 [calcium-deficient (CD) HAp], TbW10 (terbium 10%-doped CDHA), and TbW12 (terbium 20%-doped CDHA) [100]. The optical band gap values of the TbW0, TbW10 and TbW12 samples were 5.41, 4.49 and 4.38 eV respectively. This demonstrates that as the Tb increases, the band gap was reduced. Here the host matrix having calcium vacancies was considered as n-type, whereas the Tb acts as a p-type dopant. When the Tb ions substitute at that calcium site then the electrons were delocalized to conduction band. Further, in the case of the TbW10 and TbW12 samples, the band gap was reduced. From the UV photoelectron spectra, the valence band energies of Ev0 (TbW0), Ev1 (TbW10) and Ev2 (TbW12) were 3.33, 2.55 and 2.40 eV, respectively [100]. It exhibits a decrease in the valence band

at the higher concentration of Tb and varies Fermi level of the samples.

reflection, absorbance, and transmission as well.

Feng et al. studied the drug adsorption capacity of HAp using absorption spectra [44, 101]. The respective drug adsorption capacity of the undoped HAp nanorod and the Eu3+/Gd3+ HAp was 653.5 and 841.4 mg/g. The interaction of the dopants with drug molecules might be strong due to the presence of oxygen vacancies and atomic defects which pave a path for sustained or rapid release of drug molecules. This revealed that the doped nanorods displayed a high drug adsorption compared to undoped sample with a reduced absorbance. The size of the dopant and its doping concentration play a predominant role in modifying the crystal structure thereby the Fermi level position gets altered leading to the enhancement or decrease of the band gap. When photon interacts with the material, either high absorbance or transmission of photons occurs due to the variation in energy levels of defects or vacancies. The optical response of the doped HAp is not only dependent on material property and type of dopant (drug or organic molecules) but also on the type of wavelength used. Moreover, different wavelengths have distinct capability of light

The carbon-based HAp showed enhanced optical absorption. N-doped carbon

dots (N-CDs) revealed absorption at about 240 and 340 nm, due to the Π▬Π\* transition of C〓C and Π▬Π\* transition of C〓O and C▬N/C〓N, respectively [56].

*DOI: http://dx.doi.org/10.5772/intechopen.93092*

electronic polarization and dielectric constant.

#### *Impact of Dopants on the Electrical and Optical Properties of Hydroxyapatite DOI: http://dx.doi.org/10.5772/intechopen.93092*

interactions of Er3+ ions [96]. As the erbium doping concentration increases, the intensity of the peaks increased due to the replacement of Ca2+ by Er3+ ions. Further, there was a blue shift observed on the doped HAp due to the reduced particle size [97]. The Er-HAp samples possess direct band gap (n = 1/2) so band to band transitions was allowed. As the erbium doping concentration increased, the band gap was enhanced to 4.46 eV from 4.02 eV. Hence the structure, phase, defects and particle size of HAp are the key parameters to alter either the absorption or reflection. Further, the symmetry of the HAp also plays a predominant role in modifying the electronic polarization and dielectric constant.

Apurba et al. studied the optical response of HAp films of various thicknesses on amorphous SiO2 substrates [98]. All the films showed 75–96% transparency in the visible region. There are two regions noticed, one is absorption and other a transparent oscillating region. Below 300 nm, the transmittance of the HAp films displayed robust absorption. When the film thickness enhances, the absorption edge was shifted to higher wavelength in turn altering electronic polarization Thus, these films revealed a strong absorption in UV region and transparency in the visible regions as well. The band gap of the varying thickness of HAp thin films was in the range 5.25–4.67 eV due to an increase in crystallinity and grain size. The band gap of thin films is similar to the band gap of bulk HAp (in the range of 5.4 to 4.51) [99]. For annealed films, the refractive indices were augmented with an increase in the film thickness [98]. Flores et al. reported that using the Kubelka-Munk function through the maximum of the first derivative, the absorption edge values were 5.62, 4.74 and 4.6 eV which correspond respectively to TbW0 [calcium-deficient (CD) HAp], TbW10 (terbium 10%-doped CDHA), and TbW12 (terbium 20%-doped CDHA) [100]. The optical band gap values of the TbW0, TbW10 and TbW12 samples were 5.41, 4.49 and 4.38 eV respectively. This demonstrates that as the Tb increases, the band gap was reduced. Here the host matrix having calcium vacancies was considered as n-type, whereas the Tb acts as a p-type dopant. When the Tb ions substitute at that calcium site then the electrons were delocalized to conduction band. Further, in the case of the TbW10 and TbW12 samples, the band gap was reduced. From the UV photoelectron spectra, the valence band energies of Ev0 (TbW0), Ev1 (TbW10) and Ev2 (TbW12) were 3.33, 2.55 and 2.40 eV, respectively [100]. It exhibits a decrease in the valence band at the higher concentration of Tb and varies Fermi level of the samples.

Feng et al. studied the drug adsorption capacity of HAp using absorption spectra [44, 101]. The respective drug adsorption capacity of the undoped HAp nanorod and the Eu3+/Gd3+ HAp was 653.5 and 841.4 mg/g. The interaction of the dopants with drug molecules might be strong due to the presence of oxygen vacancies and atomic defects which pave a path for sustained or rapid release of drug molecules. This revealed that the doped nanorods displayed a high drug adsorption compared to undoped sample with a reduced absorbance. The size of the dopant and its doping concentration play a predominant role in modifying the crystal structure thereby the Fermi level position gets altered leading to the enhancement or decrease of the band gap. When photon interacts with the material, either high absorbance or transmission of photons occurs due to the variation in energy levels of defects or vacancies. The optical response of the doped HAp is not only dependent on material property and type of dopant (drug or organic molecules) but also on the type of wavelength used. Moreover, different wavelengths have distinct capability of light reflection, absorbance, and transmission as well.

The carbon-based HAp showed enhanced optical absorption. N-doped carbon dots (N-CDs) revealed absorption at about 240 and 340 nm, due to the Π▬Π\* transition of C〓C and Π▬Π\* transition of C〓O and C▬N/C〓N, respectively [56].

*Biomaterials*

**4. Optical properties**

determined from the slope of logσ vs. logω and its value equal to 1 for the samples [80]. Eventually, it displays a lack of direct current conductivity [87]. Similar, s value was realized for the Te-doped HAp samples however, their ac conductivity enhanced at low concentration due to s ≥ 1 leading to localized hopping. At higher concentration, it was reduced due to s ≤ 1 leading to translation motion with a rapid hopping motion [88]. Ion implantation could also be employed to alter the ac conductivity, without using expensive and toxic chemicals. It can precisely modulate the ac conductivity up to a particular depth. Nitrogen ions implanted samples demonstrated higher ac conductivity at higher frequency owing to the strong arrangement of complex ions compared to pristine (**Figure 2B**). However, it was

The interaction of light on materials leads to transmission, absorptions and reflection. These parameters depend on refractive index, wavelength, dielectric constant, and dopants. Dopants play a vital role in creating the abundant defects/vacancies in the lattice of materials. When light interacts on the bound charges of the materials, they either transmit or reflect back. The structure and phase of HAp varied depending on the type of dopant [80] which also alters the optical properties of HAp. The optical properties of HAp were enhanced by developing smaller grains and low porosity in the HAp matrix. It is an optically anisotropic material and possesses a refractive index in the range 1.644 and 1.651 depending on light polarization and direction of propagation [89]. A highly transparent HAp nanosized grains were used to observe *in vivo* interactions with proteins/cells [90, 91]. For a proper insight of the optical properties, intrinsic correlated parameters/quantities say crystalline structure, dispersion of phonon, band structure, dielectric response, band gap, etc. are required. Density functional theory displays a local or semi local exchange-correlation interaction of HAp providing a band gap between 4.5 and 5.4 eV depending on the type of valence states [92, 93]. However, the (semi) local exchange-correlation underestimates the band gap of HAp, and thus its value is above 5.5 eV [94]. So, the defect free HAp is transparent to visible light under middle or far ultraviolet illumination having a band gap >6 eV. Interesting point to note is that the dopants produce defects level in between conduction band and valence band.

For instance, the thenoyltrifluoroacetonate (TTA)- and europium (Eu)-doped HAp were used to enhance transmission spectra by 20% compared to PMMA. The TTA shows a red shift and the transmission edge from 275 to 375 nm. Both the TTA and HAp mixture restrict the formation of PMMA clusters improving the transparency. In this case, the absorption and scattering losses were introduced by the TTA and the HAp nanoparticles [95]. The other example, Cd-doped HAp showed enhanced reflectivity at the Cd (40 at.%) due to the structural transformation to the monoclinic phase. Further, it alters the refractive index of the samples as well [80]. At a particular atomic percentage, the structural transformation occurred affecting the electronic polarization thereby, altering the reflectivity. In the case of erbium doping in HAp, seven absorption bands were noticed [43]. The bands observed at 1520, 980, 803, 657, 524, 490, and 448 nm corresponding to the electronic

I15/2 → 4

I15/2 → 4

F7/2, and 4

I13/2, 4

transition [92]. Further, the peaks were broadened inhomogeneously due to the f-f

F9/2, 4

I15/2 → 4

I11/2, 4

H11/2 transition had the highest intensity due to the hypersensitive

I15/2 → 4

F3/2, respectively [92]. Among all the transi-

I9/2, 4

I15/2 → 4

less at low frequency due to the weak turnover of ions [84].

**106**

I15/2 → 2

tions, the 4

transitions from ground state 4

I15/2 → 2

I15/2 → 4

H11/2, 4

In the case of HAp:Eu,Gd, the absorption at 200–280 nm due to the wide band gap of HAp with a peak (395 nm), was assigned to the intrinsic 4f-4f transition absorption of Eu [56] confirming the incorporation of Eu into HAp. The absorption of N-CDs/HAp:Eu,Gd was superior than single N-CDs and HAp:Eu,Gd and its band edge shifted to 550 nm due to the porous structure and multiple reflection [47]. The CDs based system could be used as a stable bioimaging candidate. Ni2+-doped calcium-lithium hydroxyapatite (CLHA) nanopowders were synthesized by mechanochemical synthesis by Reddy et al. [50]. The spectra of these powder possess four absorption band at 420, 718, 794, and 1189 nm attributed to the transitions of <sup>3</sup> A2g(F) → 3 T1g(P), 3 T1g(F), 1 E1g(D) (spin-forbidden transition), and 3 T2g(F), respectively [50]. The chemical potential of the CLHA is drastically affected leading to the modification of conductivity and dielectric properties. Iron, manganese, and cobalt ions doped HAp possess direct band gap (index number n = 1/2). The band gap of 5% Co-doped HAp was higher than Fe, Mn ions doped samples [7]. The band gaps were modified due to the presence of defect energy levels which shift Fermi level toward the valence or conduction band.

### **5. Photoluminescence**

The PL of HAp depends on the defect energy level formation, structure, and size of the nanoparticle. Moreover, its band gap is above 5.5 eV, when dopants added to HAp matrix, the band gap perhaps vary due to the formation of many defects energy levels in between valence and conduction band. When the HAp nanostructure was excited by different wavelengths, electrons were accordingly excited to the higher states sometimes, these electrons recombine with holes promptly to produce radiative emission. However, in some cases, the emission might be non-radiative. Proper understanding of the effect of dopant is required to create either light active or inactive centers which enable emission at different wavelengths. However, it also depends on the excitation wavelength as well. The defects energy level formation is a complex phenomenon containing the associated band position and the band bending process. It as well depends on the size, phase and atomic arrangements. In order to reduce non-radiative emission, the doping concentration must be properly tuned to avoid the occurrence of concentration quenching; otherwise, abundant non-radiative centers are facilitated [34].

Addition of sodium hydroxide (NaOH) varies PL of Sr-HAp samples [102]. By varying NaOH content, the shape of the spectra did not change drastically; however, the intensities vary significantly. The Sr-HAp sample (0.15 g NaOH) displays the strongest emission and the weakest intensity was at 0.40 g NaOH due to the variation in luminescent centers. Here, neither Sr2+ nor PO4 3− is responsible for the luminescence but it was due to the carbon and oxygen related active electronic defect centers on the host matrix [102]. Some reports are available for the self-activated luminescence due to the addition of trisodium citrate which enables the creation of carbon monoxide impurities [103]. In the case of the Tb-doped samples, the excitation peaks of TbW10 (terbium 10% CDHA) and TbW12 (terbium 20% CDHA) were matched with that of CDHA. Further, the vacancies and carbonate radicals act as acceptor chromophores and terbium ions as donor chromophores. The interaction of chromophores leads to a non-radiative relaxation through defects and impurities in each sample [100]. Two different wavelengths (255 and 264 nm) were used for excitation of the samples. The emission peaks were associated with <sup>5</sup> D4 → 7 F6 (489 nm), 5 D4 → 7 F6 (544 nm), <sup>5</sup> D4 → 7 F4 (585 nm), and 5 D4 → 7 F3 (622 nm) due to intraconfigurational 4f-5d transitions of the Tb3+ ion. At the 544 nm, a high intense green light emission was noticed in the doped samples

**109**

*Impact of Dopants on the Electrical and Optical Properties of Hydroxyapatite*

D4 → 7

D0 → 7

(TbW10) [104, 105]. There was a weak contribution of the PL emission from the host matrix of both samples and displayed a deformation of the peak shape with

TbW10 was high due to chemical composition, self-activated CDHA, whereas at higher Tb doping (TbW12), the intensity was reduced due to concentration quench-

Yang et al. reported the broad excitation spectrum at 250 nm due to the charge transfer between europium and oxygen ions and further, small peaks were noticed at longer wavelength due to f-f transitions of Eu3+ [28]. It was monitored

D4 → 7

F2 transition at 612 nm. The two peaks were prominently noticed at

HAp samples display a clear PL spectrum and it can be tracked by the luminescence intensity. The fascinating part is how the PL intensity varies with release rate of drug molecules. As the cumulative release rate increases, the PL intensity enhances and reached maximum when all drug molecules were released completely. The organic groups of the IBU highly quenched the emission due to europium in the IBU-Eu:HAp. The IBU creates high non-radiative centers in the matrix. However, the release of IBU augments with a weakening of quenching effect which was confirmed by enhanced PL intensity. Thus, it was used as a potential probe for

In the case of dual ions doped samples, the europium/gadolinium dual-doped HAp nanorods showed a maximum excitation peak at 394 nm [44]. There was no significant emission on the co-doping of europium (Eu3+)/gadolinium (Gd3+) compared to the lone europium doping. However, the PL intensity was altered and four emission peaks were noticed at 590, 615, 650, and 699 nm [44]. The prominent

D0 → 7

europium increases, the PL emission intensity was enhanced. The highest PL intensity occurred in 5 mol% Eu3+ but with an enhanced intensity compared to the dual doped samples. The PL intensity was higher at the doping ratio of Eu3+ to Gd3+ (1:2) than the ratio 1:1. On UV irradiation, the dual doped samples displayed a robust red emission from powder as well as from powder dispersed solution [44]. So, the PL intensity depends on the phase, particle size and defect based active centers which

In the case of Mg ions doping on HAp, the PL intensity was enhanced accompanied by a modification of the spectra. At higher Mg concentration, the peak was reduced due to weak radiative recombination [30]. Further, the defects create many active centers for non-radiative recombination and the centers might be very close to each other for a rapid decrease in the PL intensity. As the Mg and Ag ions in HAp augments, the PL intensity was enhanced, however, as the concentration raises, the intensity was reduced (**Figure 3A**) [34]. With further increase in co-doping, the PL intensity was enhanced but lesser than the lowest concentration of co-doping [34]. A similar emission was also seen in the iron and zinc co-incorporated HAp samples, but as the co-doping increases, the PL intensity was reduced (**Figure 3B**) [35]. So, the PL intensities were tuned by varying co-doping concentration which might alter the radiative recombination active center in the HAp matrix. Ion implantation is one of the effective routes to dope a particular element at the desired depth and varying defects energy level to tune the PL intensity. Nitrogen ion implantation enhanced the PL emission of HAp [84]. As the ion fluence increases, the PL intensity was enhanced due to the radiative recombination of electron-hole pair without the influence of concentration quenching effect (**Figure 3C**) [84]. There are reports on different synthesis routes for doping of various elements on HAp viz., microwave-assisted

other PL peaks were observed at 590, 650, and 699 nm owing to their respective

F4 transitions. The emission from

F2 (612 nm). Even the IBU-loaded europium-doped

F2 transition within Eu3+ ions. The

F4 transitions. As the doping concentration of

F6 and 5

*DOI: http://dx.doi.org/10.5772/intechopen.93092*

ing leading to non-radiative relaxation [100].

a small shift in the peaks 5

F1 (590 nm) and 5

monitoring the drug release [28].

peak was identified at 615 nm due to <sup>5</sup>

F3, and 5

D0 → 7

absolutely alter either the inter- or intra band transition and emission.

D0 → 7

by 5

5 D0 → 7

5 D0 → 7

F1, 5

D0 → 7

#### *Impact of Dopants on the Electrical and Optical Properties of Hydroxyapatite DOI: http://dx.doi.org/10.5772/intechopen.93092*

(TbW10) [104, 105]. There was a weak contribution of the PL emission from the host matrix of both samples and displayed a deformation of the peak shape with a small shift in the peaks <sup>5</sup> D4 → 7 F6 and 5 D4 → 7 F4 transitions. The emission from TbW10 was high due to chemical composition, self-activated CDHA, whereas at higher Tb doping (TbW12), the intensity was reduced due to concentration quenching leading to non-radiative relaxation [100].

Yang et al. reported the broad excitation spectrum at 250 nm due to the charge transfer between europium and oxygen ions and further, small peaks were noticed at longer wavelength due to f-f transitions of Eu3+ [28]. It was monitored by 5 D0 → 7 F2 transition at 612 nm. The two peaks were prominently noticed at 5 D0 → 7 F1 (590 nm) and 5 D0 → 7 F2 (612 nm). Even the IBU-loaded europium-doped HAp samples display a clear PL spectrum and it can be tracked by the luminescence intensity. The fascinating part is how the PL intensity varies with release rate of drug molecules. As the cumulative release rate increases, the PL intensity enhances and reached maximum when all drug molecules were released completely. The organic groups of the IBU highly quenched the emission due to europium in the IBU-Eu:HAp. The IBU creates high non-radiative centers in the matrix. However, the release of IBU augments with a weakening of quenching effect which was confirmed by enhanced PL intensity. Thus, it was used as a potential probe for monitoring the drug release [28].

In the case of dual ions doped samples, the europium/gadolinium dual-doped HAp nanorods showed a maximum excitation peak at 394 nm [44]. There was no significant emission on the co-doping of europium (Eu3+)/gadolinium (Gd3+) compared to the lone europium doping. However, the PL intensity was altered and four emission peaks were noticed at 590, 615, 650, and 699 nm [44]. The prominent peak was identified at 615 nm due to <sup>5</sup> D0 → 7 F2 transition within Eu3+ ions. The other PL peaks were observed at 590, 650, and 699 nm owing to their respective 5 D0 → 7 F1, 5 D0 → 7 F3, and 5 D0 → 7 F4 transitions. As the doping concentration of europium increases, the PL emission intensity was enhanced. The highest PL intensity occurred in 5 mol% Eu3+ but with an enhanced intensity compared to the dual doped samples. The PL intensity was higher at the doping ratio of Eu3+ to Gd3+ (1:2) than the ratio 1:1. On UV irradiation, the dual doped samples displayed a robust red emission from powder as well as from powder dispersed solution [44]. So, the PL intensity depends on the phase, particle size and defect based active centers which absolutely alter either the inter- or intra band transition and emission.

In the case of Mg ions doping on HAp, the PL intensity was enhanced accompanied by a modification of the spectra. At higher Mg concentration, the peak was reduced due to weak radiative recombination [30]. Further, the defects create many active centers for non-radiative recombination and the centers might be very close to each other for a rapid decrease in the PL intensity. As the Mg and Ag ions in HAp augments, the PL intensity was enhanced, however, as the concentration raises, the intensity was reduced (**Figure 3A**) [34]. With further increase in co-doping, the PL intensity was enhanced but lesser than the lowest concentration of co-doping [34]. A similar emission was also seen in the iron and zinc co-incorporated HAp samples, but as the co-doping increases, the PL intensity was reduced (**Figure 3B**) [35]. So, the PL intensities were tuned by varying co-doping concentration which might alter the radiative recombination active center in the HAp matrix. Ion implantation is one of the effective routes to dope a particular element at the desired depth and varying defects energy level to tune the PL intensity. Nitrogen ion implantation enhanced the PL emission of HAp [84]. As the ion fluence increases, the PL intensity was enhanced due to the radiative recombination of electron-hole pair without the influence of concentration quenching effect (**Figure 3C**) [84]. There are reports on different synthesis routes for doping of various elements on HAp viz., microwave-assisted

*Biomaterials*

of <sup>3</sup>

A2g(F) → 3

**5. Photoluminescence**

T1g(P), 3

T1g(F), 1

level toward the valence or conduction band.

non-radiative centers are facilitated [34].

D4 → 7

In the case of HAp:Eu,Gd, the absorption at 200–280 nm due to the wide band gap of HAp with a peak (395 nm), was assigned to the intrinsic 4f-4f transition absorption of Eu [56] confirming the incorporation of Eu into HAp. The absorption of N-CDs/HAp:Eu,Gd was superior than single N-CDs and HAp:Eu,Gd and its band edge shifted to 550 nm due to the porous structure and multiple reflection [47]. The CDs based system could be used as a stable bioimaging candidate. Ni2+-doped calcium-lithium hydroxyapatite (CLHA) nanopowders were synthesized by mechanochemical synthesis by Reddy et al. [50]. The spectra of these powder possess four absorption band at 420, 718, 794, and 1189 nm attributed to the transitions

respectively [50]. The chemical potential of the CLHA is drastically affected leading to the modification of conductivity and dielectric properties. Iron, manganese, and cobalt ions doped HAp possess direct band gap (index number n = 1/2). The band gap of 5% Co-doped HAp was higher than Fe, Mn ions doped samples [7]. The band gaps were modified due to the presence of defect energy levels which shift Fermi

The PL of HAp depends on the defect energy level formation, structure, and size of the nanoparticle. Moreover, its band gap is above 5.5 eV, when dopants added to HAp matrix, the band gap perhaps vary due to the formation of many defects energy levels in between valence and conduction band. When the HAp nanostructure was excited by different wavelengths, electrons were accordingly excited to the higher states sometimes, these electrons recombine with holes promptly to produce radiative emission. However, in some cases, the emission might be non-radiative. Proper understanding of the effect of dopant is required to create either light active or inactive centers which enable emission at different wavelengths. However, it also depends on the excitation wavelength as well. The defects energy level formation is a complex phenomenon containing the associated band position and the band bending process. It as well depends on the size, phase and atomic arrangements. In order to reduce non-radiative emission, the doping concentration must be properly tuned to avoid the occurrence of concentration quenching; otherwise, abundant

Addition of sodium hydroxide (NaOH) varies PL of Sr-HAp samples [102]. By varying NaOH content, the shape of the spectra did not change drastically; however, the intensities vary significantly. The Sr-HAp sample (0.15 g NaOH) displays the strongest emission and the weakest intensity was at 0.40 g NaOH due

for the luminescence but it was due to the carbon and oxygen related active electronic defect centers on the host matrix [102]. Some reports are available for the self-activated luminescence due to the addition of trisodium citrate which enables the creation of carbon monoxide impurities [103]. In the case of the Tb-doped samples, the excitation peaks of TbW10 (terbium 10% CDHA) and TbW12 (terbium 20% CDHA) were matched with that of CDHA. Further, the vacancies and carbonate radicals act as acceptor chromophores and terbium ions as donor chromophores. The interaction of chromophores leads to a non-radiative relaxation through defects and impurities in each sample [100]. Two different wavelengths (255 and 264 nm) were used for excitation of the samples. The emission peaks were

D4 → 7

the 544 nm, a high intense green light emission was noticed in the doped samples

F3 (622 nm) due to intraconfigurational 4f-5d transitions of the Tb3+ ion. At

F6 (544 nm), 5

D4 → 7

to the variation in luminescent centers. Here, neither Sr2+ nor PO4

F6 (489 nm), 5

E1g(D) (spin-forbidden transition), and <sup>3</sup>

T2g(F),

3− is responsible

F4 (585 nm), and

**108**

5 D4 → 7

associated with <sup>5</sup>

#### **Figure 3.**

*(A)Photoluminescence spectra of HAp and magnesium and silver ions co-incorporated HAp samples (reproduced from [34] with permission from Elsevier), (B) Photoluminescence spectra of (a) FZH0, (b) FZH1, (c) FZH2 and (d) FZH3 (reproduced from [35] with permission from Elsevier) and (C) Photoluminescence of (a) pristine, (b) 1HAp, (c) 2HAp and (d) 3HAp (reproduced from [84] with permission from Elsevier).*

route followed by ion implantation which is similar to dual ions doping [106]. However, the implantation was used to create/dope ions on the surface and subsurface with associated defects/vacancies without disturbing bulk properties of HAp. For instance, nitrogen ion implantation on magnesium ion incorporated HAp samples demonstrated that at low fluence of nitrogen, the PL intensity was enhanced [106]. However, at high fluence, the intensity was reduced due to concentration quenching and high non-radiative centers created on the surface. The PL intensity of unmodified Mg-doped HAp decreased drastically compared to the surface modified samples [106]. In the case of polymer (polyvinyl alcohol) doping in HAp, the PL intensity was raised [107] and the highest PL intensity was noticed at higher concentration due to the formation of abundant active sites created by the carbon and oxygen radicals in the polymer chains. So, the PL signal could be enhanced by optimizing the doping concentration, varying the band gap and creating active recombination defect sites and vacancies.

Machado et al. studied the photoluminescence of HAp nanorods at different excitation wavelengths (380 to 680 nm) [56]. They demonstrated a shift of emission from blue to red owing to the absence of the short wavelength emission with an increase in the excitation. Near-UV excitation, the intensity was higher with blue/green emissions. Eventually, the best emission intensity was in the range 330 to 430 nm [56]. Erbium-ytterbium (Yb)-molybdenum (Mo) tri-doped HA/β-TCP phosphor prepared by solid-state reaction was examined for intense green upconversion (UC) emission [51]. M5 ((0.5% mol) Er-(10% mol) Yb-(8% mol) Mo tri-doped HA/β-TCP) showed green emission intensity 650 times than that of the M1 (0.5% mol Er-doped HA/β-TCP), M2 ((0.5% mol) Er-(10% mol) Yb co-doped HA/β-TCP) due to energy transfer from 2 F7/2, 3 T2 > state of Yb3+ − MoO4 2− dimer to the 4 F7/2 of Er3+ ions. Further, red emission was weak because of feeble absorption cross-section of 4 I13/2 and the green emission band can be controlled by the Mo6+ doping concentrations. It aids to use the material for bioimaging application [51]. N-doped carbon dots/HAp:Eu,Gd (N-CDs/HAp:Eu,Gd) composite were studied [47] and the PL spectra of N-CDs/HAp:Eu,Gd simultaneously revealed the emission from N-CDs and HAp:Eu,Gd on single wavelength excitation. The PL emission depicted no shift under different wavelengths revealing excitation independent emission. It clearly shows that the electronic structure of N-CDs and Eu ions were not affected during the synthesis. So, both the N-CDs and HAp:Eu,Gd were preserved. The PL lifetime of both N-CDs/HAp:Eu,Gd and N-CDs illustrated a single exponential decay but the lifetime decreased from 11.7 ns (N-CDs) to 5.6 ns (N-CDs/HAp:Eu,Gd) owing to the photogenerated charges [47]. Further, it could be used for bioimaging with distinct differentiation of colors due to different emission from carbon dots and doped HAp samples. Polymer/inorganic based structures can also affect the PL. Karthikeyan et al.

**111**

**Acknowledgements**

*Impact of Dopants on the Electrical and Optical Properties of Hydroxyapatite*

The PL intensity at the lower fluence samples (1 × 1014 ions/cm2

described the formation of core-shell structures on argon ion implanted polymerbased zinc ions incorporated HAp and reported that the zinc ions and the polymer lead to formation of many defects which in turn produces additional energy states.

ment in the PL intensity due to the presence of higher radiative active centers. On

was decreased owing to the formation of more non-radiative centers [108]. The incorporation of metal ions such as iron not only play a major role in tailoring the optical properties but also when exposed to magnetic field, they tend to change their morphologies depending on the magnetic field strength and have potential biological application such as protein absorption [109]. It could also lead to the development of magnetic based prolonged PL emission. Hence, the structure and morphology play a prominent role in altering the radiative/non-radiative recombination sites which can

The effect of electric field, electromagnetic dispersion, dielectric relaxation and correlated structural variation of the doped HAp samples was discussed. The role of hydroxyl ions, frequency exponent and the mechanism of ac conductivity by translation and hopping motion in the doped HAp samples were elaborated. Further, the electrical studies showed that the polarized surface charge either positive or negative, could alter the growth of bone cells on HAp. The surface charge, microstructure, densification, porosity, phase, electrical dipole reversal and its strong orientation were responsible for the enhancement in the dielectric constant of the doped HAp samples. The optical response depends on the dopant size and its concentration and associated phase transformation. The formation of different energy levels and electronic level transitions on doping enabled the tuning of the PL intensity and multi-color emissions which are independent of the excitation wavelength. The drug-loaded doped HAp samples and their PL intensity were correlated to the rate of drug release. Thus, the europium ion-doped HAp behaves as a one-dimensional nanoprobe for tracking the drug release. Further, different synthesis routes, doping, and co-doping are capable of generating new energy levels between conduction and valence band. Moreover, it was understood that the respective radiative and non-radiative emission were due to the activated lumines-

cent centers and the concentration quenching of the doped samples.

lab facilities to carry out the characterization of the samples.

Electro-opto biomaterials could be developed for the rapid healing of bone defects and fracture owing to the enhanced bone cell proliferation and growth. Current, *in vitro* and *in vivo* analysis routes must be improved further to control infection and to enhance the cell growth after implantation. To have an exact biomimicking of the bone, multielemental incorporation of the HAp is essential. Still, there is a lack of advanced analysis technique to have an insight of the surface charge interaction with the bone cells and how these charges assist the precipitation, mineralization, and growth of bone tissues. Solar/photon and magneto/lightbased advanced biomaterials need to be developed for rapid healing and recovery.

The authors thank AMSS, IGCAR, Kalpakkam, Tamil Nadu, for providing the

and 1 × 1016 ions/cm2

) showed enhance-

, the PL intensity

*DOI: http://dx.doi.org/10.5772/intechopen.93092*

increasing the fluence viz., 1 × 1015 ions/cm2

be correlated to the PL intensity decay.

**6. Summary and future outlook**

*Impact of Dopants on the Electrical and Optical Properties of Hydroxyapatite DOI: http://dx.doi.org/10.5772/intechopen.93092*

described the formation of core-shell structures on argon ion implanted polymerbased zinc ions incorporated HAp and reported that the zinc ions and the polymer lead to formation of many defects which in turn produces additional energy states. The PL intensity at the lower fluence samples (1 × 1014 ions/cm2 ) showed enhancement in the PL intensity due to the presence of higher radiative active centers. On increasing the fluence viz., 1 × 1015 ions/cm2 and 1 × 1016 ions/cm2 , the PL intensity was decreased owing to the formation of more non-radiative centers [108]. The incorporation of metal ions such as iron not only play a major role in tailoring the optical properties but also when exposed to magnetic field, they tend to change their morphologies depending on the magnetic field strength and have potential biological application such as protein absorption [109]. It could also lead to the development of magnetic based prolonged PL emission. Hence, the structure and morphology play a prominent role in altering the radiative/non-radiative recombination sites which can be correlated to the PL intensity decay.

### **6. Summary and future outlook**

*Biomaterials*

**Figure 3.**

and vacancies.

to energy transfer from 2

F7/2, 3

route followed by ion implantation which is similar to dual ions doping [106].

*(b) 1HAp, (c) 2HAp and (d) 3HAp (reproduced from [84] with permission from Elsevier).*

However, the implantation was used to create/dope ions on the surface and subsurface with associated defects/vacancies without disturbing bulk properties of HAp. For instance, nitrogen ion implantation on magnesium ion incorporated HAp samples demonstrated that at low fluence of nitrogen, the PL intensity was enhanced [106]. However, at high fluence, the intensity was reduced due to concentration quenching and high non-radiative centers created on the surface. The PL intensity of unmodified Mg-doped HAp decreased drastically compared to the surface modified samples [106]. In the case of polymer (polyvinyl alcohol) doping in HAp, the PL intensity was raised [107] and the highest PL intensity was noticed at higher concentration due to the formation of abundant active sites created by the carbon and oxygen radicals in the polymer chains. So, the PL signal could be enhanced by optimizing the doping concentration, varying the band gap and creating active recombination defect sites

*(A)Photoluminescence spectra of HAp and magnesium and silver ions co-incorporated HAp samples (reproduced from [34] with permission from Elsevier), (B) Photoluminescence spectra of (a) FZH0, (b) FZH1, (c) FZH2 and (d) FZH3 (reproduced from [35] with permission from Elsevier) and (C) Photoluminescence of (a) pristine,* 

Machado et al. studied the photoluminescence of HAp nanorods at different excitation wavelengths (380 to 680 nm) [56]. They demonstrated a shift of emission from blue to red owing to the absence of the short wavelength emission with an increase in the excitation. Near-UV excitation, the intensity was higher with blue/green emissions. Eventually, the best emission intensity was in the range 330 to 430 nm [56]. Erbium-ytterbium (Yb)-molybdenum (Mo) tri-doped HA/β-TCP phosphor prepared by solid-state reaction was examined for intense green upconversion (UC) emission [51]. M5 ((0.5% mol) Er-(10% mol) Yb-(8% mol) Mo tri-doped HA/β-TCP) showed green emission intensity 650 times than that of the M1 (0.5% mol Er-doped HA/β-TCP), M2 ((0.5% mol) Er-(10% mol) Yb co-doped HA/β-TCP) due

T2 > state of Yb3+ − MoO4

ions. Further, red emission was weak because of feeble absorption cross-section of

I13/2 and the green emission band can be controlled by the Mo6+ doping concentrations. It aids to use the material for bioimaging application [51]. N-doped carbon dots/HAp:Eu,Gd (N-CDs/HAp:Eu,Gd) composite were studied [47] and the PL spectra of N-CDs/HAp:Eu,Gd simultaneously revealed the emission from N-CDs and HAp:Eu,Gd on single wavelength excitation. The PL emission depicted no shift under different wavelengths revealing excitation independent emission. It clearly shows that the electronic structure of N-CDs and Eu ions were not affected during the synthesis. So, both the N-CDs and HAp:Eu,Gd were preserved. The PL lifetime of both N-CDs/HAp:Eu,Gd and N-CDs illustrated a single exponential decay but the lifetime decreased from 11.7 ns (N-CDs) to 5.6 ns (N-CDs/HAp:Eu,Gd) owing to the photogenerated charges [47]. Further, it could be used for bioimaging with distinct differentiation of colors due to different emission from carbon dots and doped HAp samples. Polymer/inorganic based structures can also affect the PL. Karthikeyan et al.

2− dimer to the 4

F7/2 of Er3+

**110**

4

The effect of electric field, electromagnetic dispersion, dielectric relaxation and correlated structural variation of the doped HAp samples was discussed. The role of hydroxyl ions, frequency exponent and the mechanism of ac conductivity by translation and hopping motion in the doped HAp samples were elaborated. Further, the electrical studies showed that the polarized surface charge either positive or negative, could alter the growth of bone cells on HAp. The surface charge, microstructure, densification, porosity, phase, electrical dipole reversal and its strong orientation were responsible for the enhancement in the dielectric constant of the doped HAp samples. The optical response depends on the dopant size and its concentration and associated phase transformation. The formation of different energy levels and electronic level transitions on doping enabled the tuning of the PL intensity and multi-color emissions which are independent of the excitation wavelength. The drug-loaded doped HAp samples and their PL intensity were correlated to the rate of drug release. Thus, the europium ion-doped HAp behaves as a one-dimensional nanoprobe for tracking the drug release. Further, different synthesis routes, doping, and co-doping are capable of generating new energy levels between conduction and valence band. Moreover, it was understood that the respective radiative and non-radiative emission were due to the activated luminescent centers and the concentration quenching of the doped samples.

Electro-opto biomaterials could be developed for the rapid healing of bone defects and fracture owing to the enhanced bone cell proliferation and growth. Current, *in vitro* and *in vivo* analysis routes must be improved further to control infection and to enhance the cell growth after implantation. To have an exact biomimicking of the bone, multielemental incorporation of the HAp is essential. Still, there is a lack of advanced analysis technique to have an insight of the surface charge interaction with the bone cells and how these charges assist the precipitation, mineralization, and growth of bone tissues. Solar/photon and magneto/lightbased advanced biomaterials need to be developed for rapid healing and recovery.

### **Acknowledgements**

The authors thank AMSS, IGCAR, Kalpakkam, Tamil Nadu, for providing the lab facilities to carry out the characterization of the samples.

*Biomaterials*

### **Author details**

Kumaravelu Thanigai Arul1 \*, Jayapalan Ramana Ramya2 and Subbaraya Narayana Kalkura3 \*

1 Energy and Biophotonics Laboratory, Department of Physics, AMET (Deemed to be University), Chennai, Tamil Nadu, India

2 National Centre for Nanoscience and Nanotechnology, University of Madras, Guindy Campus, Chennai, Tamil Nadu, India

3 Crystal Growth Centre, Anna University, Chennai, Tamil Nadu, India

\*Address all correspondence to: thanigaiarul.k@gmail.com and kalkura@yahoo.com

© 2020 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

**113**

*Impact of Dopants on the Electrical and Optical Properties of Hydroxyapatite*

[9] Sundarabharathi L, Ponnamma D, Parangusan H, Chinnaswamy M, Maadeed MAAA. Effect of anions on the structural, morphological and dielectric properties of hydrothermally

nanoparticles. SN Applied Sciences.

[10] Nayak B, Misra PK. Recognition of the surface characteristics and electrical properties of a nanocrystalline hydroxyapatite synthesized from *Pila globosa* shells for versatile applications. Materials Chemistry and Physics.

[11] Tavangar M, Heidari F, Hayati R, Tabatabaei F, Vashaee D, Tayebi L. Manufacturing and characterization of mechanical, biological and dielectric

properties of hydroxyapatitebarium titanate nanocomposite scaffolds. Ceramics International.

[12] Khouri AE, Zegzouti A, Elaatmani M, Capitelli F. Bismuthsubstituted hydroxyapatite ceramics synthesis: Morphological, structural, vibrational and dielectric properties. Inorganic Chemistry Communications.

[13] Verma AS, Kumar D, Dubey AK. Dielectric and electrical response of hydroxyapatite – Na0.5K0.5NbO3 bioceramic composite. Ceramics International. 2019;**45**:3297-3305

[14] Helen S, Kumar AR. Study of structural, mechanical and dielectrical properties of ions doped apatite for antibacterial activity. Materials Chemistry and Physics. 2019;**237**:121867

[15] Obaid A, Omer K, Mai HSA, Zahran HY, Kilany M, Yahia IS, et al. Antimicrobial activity of Ga-doped hydroxyapatite nanostructures: Synthesis, morphological,

synthesized hydroxyapatite

2020;**2**:94

2019;**230**:187-196

2020;**46**:9086-9095

2019;**110**:107568

*DOI: http://dx.doi.org/10.5772/intechopen.93092*

[1] Soballe K, Larsen ST, Gelineck J, Fruensgaard S, Hansen ES, Ryd L, et al. Migration of hydroxyapatite coated femoral prostheses. Journal of Bone and

Joint Surgery. 1993;**75-B**:681-687

1999;**14**:239-247

**References**

[2] Chang YL, Stanford CM, Wefel JS, Keller JC. Osteoblastic cell attachment to hydroxyapatite-coated implant surfaces in vitro. The International Journal of Oral & Maxillofacial Implants.

[3] Schilephake H. Bone growth factors in maxillofacial skeletal reconstruction. International Journal of Oral and Maxillofacial Surgery. 2002;**31**:469-484

[4] Das A, Pamu D. A comprehensive review on electrical properties of hydroxyapatite based ceramic composites. Materials Science and Engineering: C. 2019;**101**:539-563

[5] Hendi AA, Yakuphanoglu F. Dielectric and ferroelectric properties of the graphene doped hydroxyapatite

ceramics. Journal of Molecular Structure. 2020;**1207**:127734

[6] Baskar S, Elayaraja K, Nivethaa EAK, Ramya JR, Sankar VD, Catalani LH, et al. Thermally modified iron-inserted calcium phosphate for magnetic hyperthermia in an acceptable alternating magnetic field. Physical Chemistry B. 2019;**123**:5506-5513

[7] Panneerselvam R, Anandhan N, Gopu G, Roselin AA, Ganesan KP, Marimuthu T. Impact of different transition metal ions in the structural,

[8] Ibrahim M, Dawood A. Influence of doping chromium ions on the electrical properties of hydroxyapatite. Egyptian Journal of Basic and Applied Sciences

mechanical, optical, chemicophysical and biological properties of nanohydroxyapatite. Applied Surface

Science. 2020;**506**:144802

(EJBAS). 2020;**7**:35-46

*Impact of Dopants on the Electrical and Optical Properties of Hydroxyapatite DOI: http://dx.doi.org/10.5772/intechopen.93092*

### **References**

*Biomaterials*

**112**

**Author details**

Kumaravelu Thanigai Arul1

Subbaraya Narayana Kalkura3

be University), Chennai, Tamil Nadu, India

Guindy Campus, Chennai, Tamil Nadu, India

provided the original work is properly cited.

\*, Jayapalan Ramana Ramya2

1 Energy and Biophotonics Laboratory, Department of Physics, AMET (Deemed to

\*Address all correspondence to: thanigaiarul.k@gmail.com and kalkura@yahoo.com

© 2020 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium,

2 National Centre for Nanoscience and Nanotechnology, University of Madras,

3 Crystal Growth Centre, Anna University, Chennai, Tamil Nadu, India

\*

and

[1] Soballe K, Larsen ST, Gelineck J, Fruensgaard S, Hansen ES, Ryd L, et al. Migration of hydroxyapatite coated femoral prostheses. Journal of Bone and Joint Surgery. 1993;**75-B**:681-687

[2] Chang YL, Stanford CM, Wefel JS, Keller JC. Osteoblastic cell attachment to hydroxyapatite-coated implant surfaces in vitro. The International Journal of Oral & Maxillofacial Implants. 1999;**14**:239-247

[3] Schilephake H. Bone growth factors in maxillofacial skeletal reconstruction. International Journal of Oral and Maxillofacial Surgery. 2002;**31**:469-484

[4] Das A, Pamu D. A comprehensive review on electrical properties of hydroxyapatite based ceramic composites. Materials Science and Engineering: C. 2019;**101**:539-563

[5] Hendi AA, Yakuphanoglu F. Dielectric and ferroelectric properties of the graphene doped hydroxyapatite ceramics. Journal of Molecular Structure. 2020;**1207**:127734

[6] Baskar S, Elayaraja K, Nivethaa EAK, Ramya JR, Sankar VD, Catalani LH, et al. Thermally modified iron-inserted calcium phosphate for magnetic hyperthermia in an acceptable alternating magnetic field. Physical Chemistry B. 2019;**123**:5506-5513

[7] Panneerselvam R, Anandhan N, Gopu G, Roselin AA, Ganesan KP, Marimuthu T. Impact of different transition metal ions in the structural, mechanical, optical, chemicophysical and biological properties of nanohydroxyapatite. Applied Surface Science. 2020;**506**:144802

[8] Ibrahim M, Dawood A. Influence of doping chromium ions on the electrical properties of hydroxyapatite. Egyptian Journal of Basic and Applied Sciences (EJBAS). 2020;**7**:35-46

[9] Sundarabharathi L, Ponnamma D, Parangusan H, Chinnaswamy M, Maadeed MAAA. Effect of anions on the structural, morphological and dielectric properties of hydrothermally synthesized hydroxyapatite nanoparticles. SN Applied Sciences. 2020;**2**:94

[10] Nayak B, Misra PK. Recognition of the surface characteristics and electrical properties of a nanocrystalline hydroxyapatite synthesized from *Pila globosa* shells for versatile applications. Materials Chemistry and Physics. 2019;**230**:187-196

[11] Tavangar M, Heidari F, Hayati R, Tabatabaei F, Vashaee D, Tayebi L. Manufacturing and characterization of mechanical, biological and dielectric properties of hydroxyapatitebarium titanate nanocomposite scaffolds. Ceramics International. 2020;**46**:9086-9095

[12] Khouri AE, Zegzouti A, Elaatmani M, Capitelli F. Bismuthsubstituted hydroxyapatite ceramics synthesis: Morphological, structural, vibrational and dielectric properties. Inorganic Chemistry Communications. 2019;**110**:107568

[13] Verma AS, Kumar D, Dubey AK. Dielectric and electrical response of hydroxyapatite – Na0.5K0.5NbO3 bioceramic composite. Ceramics International. 2019;**45**:3297-3305

[14] Helen S, Kumar AR. Study of structural, mechanical and dielectrical properties of ions doped apatite for antibacterial activity. Materials Chemistry and Physics. 2019;**237**:121867

[15] Obaid A, Omer K, Mai HSA, Zahran HY, Kilany M, Yahia IS, et al. Antimicrobial activity of Ga-doped hydroxyapatite nanostructures: Synthesis, morphological,

spectroscopic, and dielectric properties. Journal of Biomaterials and Tissue Engineering. 2019;**9**:881-889

[16] Nihmath A, Ramesan MT. Preparation, characterization, thermal, and electrical properties of chlorinated ethylene propylene diene monomer/ hydroxyapatite nanocomposites. Polymer Composites. 2016;**39**:1-8

[17] Horiuchi N, Madokoro K, Nozaki K, Nakamura M, Katayama K, Nagai A, et al. Electrical conductivity of polycrystalline hydroxyapatite and its application to electret formation. Solid State Ionics. 2018;**315**:19-25

[18] Sanchez AG, Prokhorov E, Barcenas GL, García AGM, Y, Muñoz EMR, Raucci MG, Buonocore G. Chitosan-hydroxyapatite nanocomposites: Effect of interfacial layer on mechanical and dielectric properties. Materials Chemistry and Physics. 2018;**217**:151-159

[19] Rivas M, Valle LJD, Armelin E, Bertran O, Turon P, Puiggalí J, et al. Hydroxyapatite with permanent electrical polarization: Preparation, characterization, and response against inorganic adsorbates. ChemPhysChem. 2018;**19**:1746-1755

[20] Yun J, Qin W, Benthem KV, Thron AM, Kim S, Han YH. Nanovoids in dense hydroxyapatite ceramics after electric field assisted sintering. Advances in Applied Ceramics. 2018;**117**:376-382

[21] Lakshmanaperumal S, Mahendran C. Structural, dielectric, cytocompatibility, and in vitro bioactivity studies of yttrium and strontium co-substituted nanohydroxyapatite by sol–gel method. Journal of Sol-Gel Science and Technology. 2018;**88**:296-308

[22] Maiti M, Sarkar D, Liu S, Xu S, Maiti B, Paul K, et al. Tungsten doped hydroxyapatite processed at different temperatures: Dielectric behaviour and anti-microbial properties. New Journal of Chemistry. 2018;**42**:16948-16959

[23] Yamashita K, Oikawa N, Kitagaki K, Umegaki T. Acceleration and deceleration of bone-like crystal growth on ceramic hydroxyapatite by electrical poling. Chemistry of Materials. 1996;**8**:12697-12700

[24] Ueshima M, Tanaka S, Nakamura S, Yamashita K. Manipulation of bacterial adhesion and proliferation by surface charges of electrically polarized hydroxyapatite. Journal of Biomedical Materials Research. 2002;**60**:578-584

[25] Ohgaki M, Kizuki T, Katsura M, Yamashita K. Manipulation of selective cell adhesion and growth by surface charges of electrically polarized hydroxyapatite. Journal of Biomedical Materials Research. 2001;**57**:366-373

[26] Mercado DF, Magnacca G, Malandrino M, Rubert A, Montoneri E, Celi L, et al. Paramagnetic iron-doped hydroxyapatite nanoparticles with improved metal sorption properties. A bioorganic substrates-mediated synthesis. ACS Applied Materials & Interfaces. 2014;**6**:3937-3946

[27] Silvia P, Monica M, Monica S, Michele I, Alessio A, Martina G, et al. Magnetic labelling of mesenchymal stem cells with iron-doped hydroxyapatite nanoparticles as tool for cell therapy. Journal of Biomedical Nanotechnology. 2016;**12**:909-921

[28] Stanica V, Janackovic D, Dimitrijević S, Tanasković SB, Mitrić M, Pavlović MS, et al. Synthesis of antimicrobial monophase silver-doped hydroxyapatite nanopowders for bone tissue engineering. Applied Surface Science. 2011;**257**:4510-4518

[29] Trujillo NA, Oldinski RA, Ma H, Bryers JD, Williams JD, Popat KC.

**115**

*Impact of Dopants on the Electrical and Optical Properties of Hydroxyapatite*

of Biomedical Materials Research.

[37] Bowen CR, Gittings J, Turner IG, Baxter F, Chaudhuri JB. Dielectric and piezoelectric properties of hydroxyapatite-BaTiO3

composites. Applied Physics Letters.

[39] Kobayashi T, Nakamura S,

by negative surface charges of

Yamashita K. Enhanced osteobonding

electrically polarized hydroxyapatite. Journal of Biomedical Materials Research. 2001;**57**:477-484

[40] Feng JQ, Yuan HP, Zhang XD. Promotion of osteogenesis by a piezoelectric biological ceramic. Biomaterials. 1997;**18**:1531-1534

[41] Nakamura S, Kobayashi T, Yamashita K. Extended bioactivity in the proximity of hydroxyapatite ceramic

surfaces induced by polarization charges. ournal of Biomedical Materials

[42] Popa CL, Ciobanu CS. Synthesis and characterization of fluorescent hydroxyapatite. Romanian Reports in

[43] Ammar ZA, Muhammed A, Goh YF, Rafiq M, Kadir A, Abdolahi A, et al. Structural characterization, optical properties and in vitro bioactivity of mesoporous erbium-doped

hydroxyapatite. Journal of Alloys and Compounds. 2015;**645**:478-486

[44] Feng C, Peng H, Zhu YJ, Wu J,

photoluminescence, drug delivery

multifunctional Eu3+/Gd3+ dual-doped

Zhang CL, Cui DX. The

and imaging properties of

Research. 2002;**61**:593-599

Physics. 2016;**68**:1170-1177

[38] Nakamura S, Takeda H, Kimihiro U. Proton transport polarization and depolarization of hydroxyapatite ceramics. Journal of Applied Physics.

2002;**60**:643-650

2006;**89**:132906

2001;**89**:5386

*DOI: http://dx.doi.org/10.5772/intechopen.93092*

deposited on titanium. Materials Science and Engineering: C. 2012;**32**:2135-2144

[30] Arul KT, Elayaraja K, Manikandan E, Bhalerao GM, Chandra VS, Ramya JR, et al. Green synthesis of magnesium ion incorporated nanocrystalline hydroxyapatite and their mechanical, dielectric and photoluminescence properties. Materials Research Bulletin.

[31] Ziani S, Meski S, Khireddine H. Characterization of magnesium-doped

hydroxyapatite prepared by solgel process. International Journal of Applied Ceramic Technology.

[32] Nan K, Wu T, Chen J, Jiang S, Huang Y, Pei G. Strontium doped hydroxyapatite film formed by microarc oxidation. Materials Science and Engineering: C. 2009;**29**:1554-1558

[33] Avci M, Yilmaz B, Tezcaner A, Evis Z. Strontium doped hydroxyapatite

biomimetic coatings on Ti6Al4V plates. Ceramics International.

Physicochemical characterization of the superhydrophilic, magnesium and silver ions co-incorporated nanocrystalline hydroxyapatite, synthesized by microwave processing. Ceramics International. 2014;**40**:13771-13779

[35] Ramya JR, Arul KT, Elayaraja K, Kalkura SN. Physicochemical and biological properties of iron and zinc ions co-doped nanocrystalline hydroxyapatite, synthesized by

ultrasonication. Ceramics International.

[36] Hoepfner TP, Case ED. The porosity dependence of the dielectric constant for sintered hydroxyapatite. Journal

2017;**43**:9431-9436

[34] Arul KT, Ramya JR, Bhalerao GM, Kalkura SN.

2014;**40**:16707-16717

2015;**67**:55-62

2014;**11**:83-91

Antibacterial effects of silver-doped hydroxyapatite thin films sputter

*Impact of Dopants on the Electrical and Optical Properties of Hydroxyapatite DOI: http://dx.doi.org/10.5772/intechopen.93092*

Antibacterial effects of silver-doped hydroxyapatite thin films sputter deposited on titanium. Materials Science and Engineering: C. 2012;**32**:2135-2144

*Biomaterials*

spectroscopic, and dielectric properties. Journal of Biomaterials and Tissue Engineering. 2019;**9**:881-889

hydroxyapatite processed at different temperatures: Dielectric behaviour and anti-microbial properties. New Journal of Chemistry. 2018;**42**:16948-16959

[23] Yamashita K, Oikawa N, Kitagaki K,

deceleration of bone-like crystal growth on ceramic hydroxyapatite by electrical

[24] Ueshima M, Tanaka S, Nakamura S, Yamashita K. Manipulation of bacterial adhesion and proliferation by surface charges of electrically polarized hydroxyapatite. Journal of Biomedical Materials Research. 2002;**60**:578-584

[25] Ohgaki M, Kizuki T, Katsura M, Yamashita K. Manipulation of selective cell adhesion and growth by surface charges of electrically polarized hydroxyapatite. Journal of Biomedical Materials Research. 2001;**57**:366-373

[26] Mercado DF, Magnacca G,

Interfaces. 2014;**6**:3937-3946

stem cells with iron-doped

[28] Stanica V, Janackovic D,

Science. 2011;**257**:4510-4518

Pavlović MS, et al. Synthesis of

[27] Silvia P, Monica M, Monica S, Michele I, Alessio A, Martina G, et al. Magnetic labelling of mesenchymal

hydroxyapatite nanoparticles as tool for cell therapy. Journal of Biomedical Nanotechnology. 2016;**12**:909-921

Dimitrijević S, Tanasković SB, Mitrić M,

antimicrobial monophase silver-doped hydroxyapatite nanopowders for bone tissue engineering. Applied Surface

[29] Trujillo NA, Oldinski RA, Ma H, Bryers JD, Williams JD, Popat KC.

Malandrino M, Rubert A, Montoneri E, Celi L, et al. Paramagnetic iron-doped hydroxyapatite nanoparticles with improved metal sorption properties. A bioorganic substrates-mediated synthesis. ACS Applied Materials &

Umegaki T. Acceleration and

poling. Chemistry of Materials.

1996;**8**:12697-12700

Preparation, characterization, thermal, and electrical properties of chlorinated ethylene propylene diene monomer/ hydroxyapatite nanocomposites. Polymer Composites. 2016;**39**:1-8

[16] Nihmath A, Ramesan MT.

[17] Horiuchi N, Madokoro K,

State Ionics. 2018;**315**:19-25

Physics. 2018;**217**:151-159

2018;**19**:1746-1755

2018;**117**:376-382

[21] Lakshmanaperumal S,

[18] Sanchez AG, Prokhorov E, Barcenas GL, García AGM, Y, Muñoz EMR, Raucci MG,

Nozaki K, Nakamura M, Katayama K, Nagai A, et al. Electrical conductivity of polycrystalline hydroxyapatite and its application to electret formation. Solid

Buonocore G. Chitosan-hydroxyapatite nanocomposites: Effect of interfacial layer on mechanical and dielectric properties. Materials Chemistry and

[19] Rivas M, Valle LJD, Armelin E, Bertran O, Turon P, Puiggalí J,

[20] Yun J, Qin W, Benthem KV,

Thron AM, Kim S, Han YH. Nanovoids in dense hydroxyapatite ceramics after electric field assisted sintering. Advances in Applied Ceramics.

Mahendran C. Structural, dielectric, cytocompatibility, and in vitro bioactivity studies of yttrium and strontium co-substituted nanohydroxyapatite by sol–gel method. Journal of Sol-Gel Science and Technology. 2018;**88**:296-308

[22] Maiti M, Sarkar D, Liu S, Xu S, Maiti B, Paul K, et al. Tungsten doped

et al. Hydroxyapatite with permanent electrical polarization: Preparation, characterization, and response against inorganic adsorbates. ChemPhysChem.

**114**

[30] Arul KT, Elayaraja K, Manikandan E, Bhalerao GM, Chandra VS, Ramya JR, et al. Green synthesis of magnesium ion incorporated nanocrystalline hydroxyapatite and their mechanical, dielectric and photoluminescence properties. Materials Research Bulletin. 2015;**67**:55-62

[31] Ziani S, Meski S, Khireddine H. Characterization of magnesium-doped hydroxyapatite prepared by solgel process. International Journal of Applied Ceramic Technology. 2014;**11**:83-91

[32] Nan K, Wu T, Chen J, Jiang S, Huang Y, Pei G. Strontium doped hydroxyapatite film formed by microarc oxidation. Materials Science and Engineering: C. 2009;**29**:1554-1558

[33] Avci M, Yilmaz B, Tezcaner A, Evis Z. Strontium doped hydroxyapatite biomimetic coatings on Ti6Al4V plates. Ceramics International. 2017;**43**:9431-9436

[34] Arul KT, Ramya JR, Bhalerao GM, Kalkura SN. Physicochemical characterization of the superhydrophilic, magnesium and silver ions co-incorporated nanocrystalline hydroxyapatite, synthesized by microwave processing. Ceramics International. 2014;**40**:13771-13779

[35] Ramya JR, Arul KT, Elayaraja K, Kalkura SN. Physicochemical and biological properties of iron and zinc ions co-doped nanocrystalline hydroxyapatite, synthesized by ultrasonication. Ceramics International. 2014;**40**:16707-16717

[36] Hoepfner TP, Case ED. The porosity dependence of the dielectric constant for sintered hydroxyapatite. Journal

of Biomedical Materials Research. 2002;**60**:643-650

[37] Bowen CR, Gittings J, Turner IG, Baxter F, Chaudhuri JB. Dielectric and piezoelectric properties of hydroxyapatite-BaTiO3 composites. Applied Physics Letters. 2006;**89**:132906

[38] Nakamura S, Takeda H, Kimihiro U. Proton transport polarization and depolarization of hydroxyapatite ceramics. Journal of Applied Physics. 2001;**89**:5386

[39] Kobayashi T, Nakamura S, Yamashita K. Enhanced osteobonding by negative surface charges of electrically polarized hydroxyapatite. Journal of Biomedical Materials Research. 2001;**57**:477-484

[40] Feng JQ, Yuan HP, Zhang XD. Promotion of osteogenesis by a piezoelectric biological ceramic. Biomaterials. 1997;**18**:1531-1534

[41] Nakamura S, Kobayashi T, Yamashita K. Extended bioactivity in the proximity of hydroxyapatite ceramic surfaces induced by polarization charges. ournal of Biomedical Materials Research. 2002;**61**:593-599

[42] Popa CL, Ciobanu CS. Synthesis and characterization of fluorescent hydroxyapatite. Romanian Reports in Physics. 2016;**68**:1170-1177

[43] Ammar ZA, Muhammed A, Goh YF, Rafiq M, Kadir A, Abdolahi A, et al. Structural characterization, optical properties and in vitro bioactivity of mesoporous erbium-doped hydroxyapatite. Journal of Alloys and Compounds. 2015;**645**:478-486

[44] Feng C, Peng H, Zhu YJ, Wu J, Zhang CL, Cui DX. The photoluminescence, drug delivery and imaging properties of multifunctional Eu3+/Gd3+ dual-doped hydroxyapatite nanorods. Biomaterials. 2011;**32**:9031-9039

[45] Baldassarre F, Altomare A, Corriero N, Mesto E, Lacalamita M, Bruno G, et al. Crystal chemistry and luminescence properties of Eu-doped polycrystalline hydroxyapatite synthesized by chemical precipitation at room temperature. Crystals. 2020;**10**:250

[46] Li L, Li D, Zhao W, Cai Q, Li G, Yu Y, et al. Composite resin reinforced with fluorescent europium-doped hydroxyapatite nanowires for in-situ characterization. Dental Materials. 2020;**36**:e15-e26

[47] Chang Q, Xu W, Chen Q, Xue C, Li N, Yang J, et al. One step synthesis of N-doped carbon dots/ hydroxyapatite:EuGd composite with dual-emissive and solid-state photoluminescence. Applied Surface Science. 2020;**508**:144862

[48] Kolesnikov IE, Nikolaev AM, Lähderanta E, Frank-Kamenetskaya OV, Kuz'mina MA. Structural and luminescence properties of Ce3+-doped hydroxyapatite nanocrystalline powders. Optical Materials. 2020;**99**:109550

[49] Zhang X, Xing Q, Liao L, Han Y. Effect of the fluorine substitution for – OH group on the luminescence property of Eu3+ doped hydroxyapatite. Crystals. 2020;**10**:191

[50] Reddy CV, Ravindranadh K, Ravikumar RVSSN, Shim J. A novel green-emitting Ni2+-doped Ca-Li hydroxyapatite nanopowders: Structural, optical, and photoluminescence properties. Journal of Materials Science: Materials in Electronics. 2020;**31**:5097-5106

[51] Van HN, Vu NH, Pham VH, Huan PV, Hoan BH, Nguyen DH, et al. A novel upconversion emission material based on Er3+−Yb3+−Mo6+ tridoped

hydroxyapatite/tricalcium phosphate (HA/β-TCP). Journal of Alloys and Compounds. 2020;**827**:154288

[52] Milojkov DV, Silvestre OF, Vojislav Dj S, Janjić GV, Mutavdžić DR, Milanović M, et al. Fabrication and characterization of luminescent Pr3+ doped fluorapatite nanocrystals as bioimaging contrast agents. Journal of Luminescence. 2020;**217**:116757

[53] Zhou R, Li Y, Xiao D, Li T, Zhang T, Fub W, et al. Hyaluronan-directed fabrication of co-doped hydroxyapatite as a dual-modal probe for tumorspecific bioimaging. Journal of Materials Chemistry B. 2020;**8**:2107-2114

[54] Wang Y, Xue Y, Wang J, Zhu Y, Wang X, Zhang X, et al. Biocompatible and photoluminescent carbon dots/ hydroxyapatite/PVA dual-network composite hydrogel scaffold and their properties. Journal of Polymer Research. 2019;**26**:248

[55] Xing Q, Zhang X, Wu D, Han Y, Wickramaratne MN, Dai H, et al. Ultrasound-assisted synthesis and characterization of heparin-coated Eu3+ doped hydroxyapatite luminescent nanoparticles. Journal of Colloid and Interface Science. 2019;**29**:17-25

[56] Machado TR, Leite IS, Inada NM, Li MS, da Silva JS, Andres J, et al. Designing biocompatible and multicolor fluorescent hydroxyapatite nanoparticles for cell-imaging applications. Materials Today Chemistry. 2020;**14**:100211

[57] Daneshvar H, Shafaei M, Manouchehri F, Kakaei S, Ziaie F. The role of La, Eu, Gd, and Dy lanthanides on thermoluminescence characteristics of nano-hydroxyapatite induced by gamma radiation. SN Applied Sciences. 2019;**1**:1146

[58] Barrera-Villatoro A, Boronat C, Rivera-Montalvo T, Correcher V,

**117**

*Impact of Dopants on the Electrical and Optical Properties of Hydroxyapatite*

[66] Constantz BR, Ison IC, Fulmer MT, Poser RD, Smith ST, Wagoner MV, et al. Skeletal repair by in situ formation of the mineral phase of bone. Science.

[67] Benard J. Combinaisons avec le phosphore. In: Benard E, Bouissieres G, Brusset H, et al., editors. Nouveau trait6 de chimieminerale. Vol. 4. Paris:

[68] Costantino PD, Friedman CD, Jones K, Chow LC, Pelzer HJ, Sisson GA Sr. Hydroxyapatite cement. I. Basic chemistry and histologic properties. Archives of Otolaryngology – Head & Neck Surgery. 1991;**117**:379-384

[69] Friedman CD, Costantino PD, Jones K, Chow LC, Pelzer HJ, Sisson GA. Hydroxyapatite cement. II. Obliteration and reconstruction of the cat frontal sinus. Archives of Otolaryngology – Head & Neck Surgery. 1991;**117**:385-389

[70] Khairoun I, Driessens FC, Boltong MG, Planell JA, Wenz R. Addition of cohesion promotors to calcium phosphate cements. Biomaterials. 1999;**20**:393-398

[71] Ciesla K, Rudnicki R. Synthesis and transformation of tetracalcium phosphate in solid state. Part I.

Synthesis of roentgenographically pure tetracalcium phosphate from calcium dibasic phosphate and calcite. Polish Journal of Chemistry. 1987;**61**:719-727

[72] Kaygili O, Dorozhkin SV, Ates T, Ghamdi AA, Yakuphanoglu F. Dielectric properties of Fe doped hydroxyapatite prepared by sol–gel method. Ceramics International. 2014;**40**:9395-9402

[73] Ryaby JT. Clinical effects of electromagnetic and electric fields on fracture healing. Clinical Orthopaedics.

[74] Scott G, King JB. A prospective, double-blind trial of electrical

1998;**355**:S205-S215

1995;**267**:1796-1799

Masson; 1958. pp. 455-488

*DOI: http://dx.doi.org/10.5772/intechopen.93092*

[59] Huang P, Zhou B, Zheng Q, Tian Y, Wang M, Wang L, et al. Nano wave plates structuring and index matching in transparent hydroxyapatite-YAG:Ce composite ceramics for high luminous efficiency white light-emitting diodes. Advanced Materials. 2020;**32**:1905951

[60] Luo H, Xie J, Xiong L, Yang Z, Zuo G, Wang H, et al. Engineering photoluminescent and magnetic lamellar hydroxyapatite by facile one-step Se/Gd dual-doping. Journal of Materials Chemistry B. 2018;**6**:3515

[61] Pogosova MA, González LV. Influence of anion substitution on the crystal structure and color

2018;**44**:20140-20147

2010;**114**:2918-2924

2010;**31**:3374-3383

[65] Bohner M. Calcium

Injury. 2000;**31**:37-47

[62] Al-Kattan A, Pascal D,

properties of copper-doped strontium hydroxyapatite. Ceramics International.

Jeannette DG, Christophe D. Preparation and physicochemical characteristics of luminescent apatite-based colloids. Journal of Physical Chemistry C.

[63] Zhang C, Li C, Huang S, Hou Z, Cheng Z, Yang P, et al. Self-activated luminescent and mesoporous strontium hydroxyapatite nanorods for drug delivery. Biomaterials.

[64] Yang P, Quan Z, Li C, Kang X, Lian H, Lin J. Bioactive, luminescent and mesoporous europium-doped hydroxyapatite as a drug carrier. Biomaterials. 2008;**28**:4341-4347

orthophosphates in medicine: From ceramics to calcium phosphate cements.

Garcia-Guinea J, Zarate-Medina J. Cathodo- and thermally stimulated luminescence characterization of synthetic calcium phosphates. Spectroscopy Letters. 2018;**51**:22-26

*Impact of Dopants on the Electrical and Optical Properties of Hydroxyapatite DOI: http://dx.doi.org/10.5772/intechopen.93092*

Garcia-Guinea J, Zarate-Medina J. Cathodo- and thermally stimulated luminescence characterization of synthetic calcium phosphates. Spectroscopy Letters. 2018;**51**:22-26

*Biomaterials*

2020;**10**:250

2020;**36**:e15-e26

2011;**32**:9031-9039

hydroxyapatite nanorods. Biomaterials.

hydroxyapatite/tricalcium phosphate (HA/β-TCP). Journal of Alloys and Compounds. 2020;**827**:154288

Vojislav Dj S, Janjić GV, Mutavdžić DR, Milanović M, et al. Fabrication and characterization of luminescent Pr3+ doped fluorapatite nanocrystals as bioimaging contrast agents. Journal of Luminescence. 2020;**217**:116757

[53] Zhou R, Li Y, Xiao D, Li T, Zhang T, Fub W, et al. Hyaluronan-directed fabrication of co-doped hydroxyapatite as a dual-modal probe for tumor-

specific bioimaging. Journal of Materials

Chemistry B. 2020;**8**:2107-2114

2019;**26**:248

[54] Wang Y, Xue Y, Wang J, Zhu Y, Wang X, Zhang X, et al. Biocompatible and photoluminescent carbon dots/ hydroxyapatite/PVA dual-network composite hydrogel scaffold and their properties. Journal of Polymer Research.

[55] Xing Q, Zhang X, Wu D, Han Y, Wickramaratne MN, Dai H, et al. Ultrasound-assisted synthesis and characterization of heparin-coated Eu3+ doped hydroxyapatite luminescent nanoparticles. Journal of Colloid and Interface Science. 2019;**29**:17-25

Inada NM, Li MS, da Silva JS, Andres J, et al. Designing biocompatible and multicolor fluorescent hydroxyapatite

[56] Machado TR, Leite IS,

nanoparticles for cell-imaging applications. Materials Today Chemistry. 2020;**14**:100211

[57] Daneshvar H, Shafaei M,

2019;**1**:1146

Manouchehri F, Kakaei S, Ziaie F. The role of La, Eu, Gd, and Dy lanthanides on thermoluminescence characteristics of nano-hydroxyapatite induced by gamma radiation. SN Applied Sciences.

[58] Barrera-Villatoro A, Boronat C, Rivera-Montalvo T, Correcher V,

[52] Milojkov DV, Silvestre OF,

[45] Baldassarre F, Altomare A, Corriero N, Mesto E, Lacalamita M, Bruno G, et al. Crystal chemistry and luminescence properties of Eu-doped polycrystalline hydroxyapatite synthesized by chemical precipitation at room temperature. Crystals.

[46] Li L, Li D, Zhao W, Cai Q, Li G, Yu Y, et al. Composite resin reinforced with fluorescent europium-doped hydroxyapatite nanowires for in-situ characterization. Dental Materials.

[47] Chang Q, Xu W, Chen Q, Xue C, Li N, Yang J, et al. One step synthesis of N-doped carbon dots/ hydroxyapatite:EuGd composite with dual-emissive and solid-state photoluminescence. Applied Surface

Science. 2020;**508**:144862

[48] Kolesnikov IE, Nikolaev AM, Lähderanta E, Frank-Kamenetskaya OV,

luminescence properties of Ce3+-doped hydroxyapatite nanocrystalline powders. Optical Materials. 2020;**99**:109550

[49] Zhang X, Xing Q, Liao L, Han Y. Effect of the fluorine substitution for – OH group on the luminescence property of Eu3+ doped hydroxyapatite. Crystals.

[50] Reddy CV, Ravindranadh K, Ravikumar RVSSN, Shim J. A novel green-emitting Ni2+-doped Ca-Li hydroxyapatite nanopowders: Structural, optical, and

[51] Van HN, Vu NH, Pham VH, Huan PV, Hoan BH, Nguyen DH, et al. A novel upconversion emission material based on Er3+−Yb3+−Mo6+ tridoped

photoluminescence properties. Journal of Materials Science: Materials in Electronics. 2020;**31**:5097-5106

Kuz'mina MA. Structural and

**116**

2020;**10**:191

[59] Huang P, Zhou B, Zheng Q, Tian Y, Wang M, Wang L, et al. Nano wave plates structuring and index matching in transparent hydroxyapatite-YAG:Ce composite ceramics for high luminous efficiency white light-emitting diodes. Advanced Materials. 2020;**32**:1905951

[60] Luo H, Xie J, Xiong L, Yang Z, Zuo G, Wang H, et al. Engineering photoluminescent and magnetic lamellar hydroxyapatite by facile one-step Se/Gd dual-doping. Journal of Materials Chemistry B. 2018;**6**:3515

[61] Pogosova MA, González LV. Influence of anion substitution on the crystal structure and color properties of copper-doped strontium hydroxyapatite. Ceramics International. 2018;**44**:20140-20147

[62] Al-Kattan A, Pascal D, Jeannette DG, Christophe D. Preparation and physicochemical characteristics of luminescent apatite-based colloids. Journal of Physical Chemistry C. 2010;**114**:2918-2924

[63] Zhang C, Li C, Huang S, Hou Z, Cheng Z, Yang P, et al. Self-activated luminescent and mesoporous strontium hydroxyapatite nanorods for drug delivery. Biomaterials. 2010;**31**:3374-3383

[64] Yang P, Quan Z, Li C, Kang X, Lian H, Lin J. Bioactive, luminescent and mesoporous europium-doped hydroxyapatite as a drug carrier. Biomaterials. 2008;**28**:4341-4347

[65] Bohner M. Calcium orthophosphates in medicine: From ceramics to calcium phosphate cements. Injury. 2000;**31**:37-47

[66] Constantz BR, Ison IC, Fulmer MT, Poser RD, Smith ST, Wagoner MV, et al. Skeletal repair by in situ formation of the mineral phase of bone. Science. 1995;**267**:1796-1799

[67] Benard J. Combinaisons avec le phosphore. In: Benard E, Bouissieres G, Brusset H, et al., editors. Nouveau trait6 de chimieminerale. Vol. 4. Paris: Masson; 1958. pp. 455-488

[68] Costantino PD, Friedman CD, Jones K, Chow LC, Pelzer HJ, Sisson GA Sr. Hydroxyapatite cement. I. Basic chemistry and histologic properties. Archives of Otolaryngology – Head & Neck Surgery. 1991;**117**:379-384

[69] Friedman CD, Costantino PD, Jones K, Chow LC, Pelzer HJ, Sisson GA. Hydroxyapatite cement. II. Obliteration and reconstruction of the cat frontal sinus. Archives of Otolaryngology – Head & Neck Surgery. 1991;**117**:385-389

[70] Khairoun I, Driessens FC, Boltong MG, Planell JA, Wenz R. Addition of cohesion promotors to calcium phosphate cements. Biomaterials. 1999;**20**:393-398

[71] Ciesla K, Rudnicki R. Synthesis and transformation of tetracalcium phosphate in solid state. Part I. Synthesis of roentgenographically pure tetracalcium phosphate from calcium dibasic phosphate and calcite. Polish Journal of Chemistry. 1987;**61**:719-727

[72] Kaygili O, Dorozhkin SV, Ates T, Ghamdi AA, Yakuphanoglu F. Dielectric properties of Fe doped hydroxyapatite prepared by sol–gel method. Ceramics International. 2014;**40**:9395-9402

[73] Ryaby JT. Clinical effects of electromagnetic and electric fields on fracture healing. Clinical Orthopaedics. 1998;**355**:S205-S215

[74] Scott G, King JB. A prospective, double-blind trial of electrical

capacitive coupling in the treatment of nonunion of long bones. The Journal of Bone and Joint Surgery. American Volume. 1994;**76A**:820-826

[75] Oishi M, Onesti ST. Electrical bone graft stimulation for spinal fusion: A review. Neurosurgery. 2000;**47**:1041-1055

[76] Goodwin CB, Brighton CT, Guyer RD, Johnson JR, Light KI, Yuan HA. A double-blind study of capacitively coupled electrical stimulation as an adjunct to lumbar spinal fusions. Spine (Phila Pa 1976). 1999;**24**:1349-1357

[77] Otter MW, McLeod KJ, Rubin CT. Effects of electromagnetic fields in experimental fracture repair. Clinical Orthopaedics and Related Research. 1998;**355**:S90-S104

[78] Marino AA, Becker RO, Bachman CH. Dielectric determination of bound water of bone. Physics in Medicine and Biology. 1967;**12**:367-378

[79] Lakes RS, Harper RA, Katz JL. Dielectric relaxation in cortical bone. Journal of Applied Physics. 1977;**48**:808-811

[80] Kaygili O, Keser S, Ates T, Ghamdi AA, Yakuphanoglu F. Controlling of dielectrical and optical properties of hydroxyapatite based bioceramics by Cd content. Powder Technology. 2013;**245**:1-6

[81] Iqbal N, Kadir MRA, Malek NANN, Mahmood NHB, Murali MR, Kamarul T. Characterization and antibacterial properties of stable silver substituted hydroxyapatite nanoparticles synthesized through surfactant assisted microwave process. Materials Research. 2013;**48**:3172-3177

[82] Horiuchi N, Endo J, Nozaki K, Nakamura M, Nagai A, Katayama K, et al. Dielectric evaluation of fluorine substituted hydroxyapatite. Journal of the Ceramic Society of Japan. 2013;**121**:770-774

[83] Shkir M, Kilany M, YahiaI S. Facile microwave-assisted synthesis of tungsten-doped hydroxyapatite nanorods: A systematic structural, morphological, dielectric, radiation and microbial activity studies. Ceramics International. 2017;**43**:14923-14931

[84] Thanigairul K, Elayaraja K, Magudapathy P, Mudali UK, Nair KGM, Sudarshan M, et al. Surface modification of nanocrystalline calcium phosphate bioceramic by low energy nitrogen ion implantation. Ceramics International. 2013;**39**:3027-3034

[85] ShKalil M, Beheri HH, Fattah WIA. Structural and electrical properties of zirconia/hydroxyapatite porous composites. Ceramics International. 2002;**28**:451-458

[86] Jonscher AK. The 'universal' dielectric response. Nature. 1977;**267**:673-679

[87] Dyre JC, Schrøder TB. Universality of ac conduction in disordered solids. Reviews of Modern Physics. 2000;**72**:873-892

[88] Yahia IS, Shkir M, AlFaify S, Ganesh V, Zahran HY, Kilany M. Facile microwave-assisted synthesis of Te-doped hydroxyapatite nanorods and nanosheets and their characterizations for bone cement applications. Materials Science and Engineering C. 2017;**72**:472-480

[89] Holzmann D, Holzinger D, Hesser G, Schmidt T, Knor G. Hydroxyapatite nanoparticles as novel low-refractive index additives for the long-term UV-photoprotection of transparent composite materials. Journal of Materials Chemistry. 2009;**19**:8102-8106

**119**

*Impact of Dopants on the Electrical and Optical Properties of Hydroxyapatite*

[98] Apurba D, Kumar AC, Bharti GP, Behera RR, Mamilla RS, Khare A, et al. Effect of thickness on optical and microwave dielectric properties of hydroxyapatite films deposited by RF magnetron sputtering. Journal of Alloys and Compounds. 2018;**739**:729-736

[99] Matsunaga K, Kuwabara A. Firstprinciples study of vacancy formation in hydroxyapatite. Physical Review B.

[100] Flores YJ, Quezada MS, Trigos JBR, Rojas LL, Suarez V, Mantilla A. Characterization of Tb-doped hydroxyapatite for biomedical applications: Optical properties and energy band gap determination. Journal of Materials Science. 2017;**52**:9990-10000

[101] Ma MY, Zhu YJ, Li L, Cao SW. Nanostructured porous hollow ellipsoidal capsules of hydroxyapatite and calcium silicate: Preparation and application in drug delivery. Journal of Materials Chemistry.

2007;**75**:014102

2008;**18**:2722-2727

[102] Zhanglei N, Zhidong C, Wenjun LI, Changyan S, Jinghua Z, Yang L. Solvothermal synthesis and optical performance of one-dimensional strontium hydroxyapatite nanorod. Chinese Journal of Chemical Engineering. 2012;**20**:89-94

[103] Zhang CM, Yang J, Quan ZW, Yang PP, Li CX, Hou ZY, et al.

with multiform morphologies: Controllable synthesis and luminescence properties. Crystal Growth & Design. 2009;**9**:2725-2733

[104] Ling L, Yukan L, Jinhui T, Ming Z, Haihua P, Xu X, et al. Surface modification of hydroxyapatite nanocrystallite by a small amount of terbium provides a biocompatible fluorescent probe. Journal of Physical Chemistry C. 2008;**112**:1229-12224

Hydroxyapatite nano- and microcrystals

*DOI: http://dx.doi.org/10.5772/intechopen.93092*

Observation of osteogenic differentiation cascade of living mesenchymal stem cells on transparent hydroxyapatite ceramics.

[90] Kotobuki N, Ioku K, Kawagoe D, Fujimori H, Goto S, Ohgushi H.

Biomaterials. 2005;**26**:779-785

2011;**31**:1533-1540

2003;**67**:134106

2011;**84**:134108

[91] Eriksson M, Liu Y, Hu J, Gao L, Nygren M, Shen Z. Transparent

[92] Calderin L, Stott M, Rubio A. Electronic and crystallographic

[93] Rulis P, Ouyang L, Ching WY. Electronic structure and bonding in calcium apatite crystals: Hydroxyapatite, fluorapatite,

[94] Slepko A, Demkov AA. Firstprinciples study of the biomineral hydroxyapatite. Physical Review B.

[95] Wiglusz RJ, Bednarkiewicz A, Strek W. Synthesis and optical properties of Eu3+ ion doped nanocrystalline hydroxyapatites embedded in PMMA matrix. Journal of

Rare Earths. 2011;**29**:1111-1116

[96] Mahraz ZAS, Sahar MR,

2013;**144**:139-145

Ghoshal SK, Dousti MR. Concentration dependent luminescence quenching of Er3+-doped zinc boro-tellurite glass. Journal of Luminescence.

[97] Pan A, Yang Z, Zheng H, Liu F, Zhu Y, Su X, et al. Changeable position of SPR peak of Ag nanoparticles embedded in mesoporous SiO2 glass by annealing treatment. Applied Surface

Science. 2003;**205**:323-328

Review B. 2004;**70**:155104

structure of apatites. Physical Review B.

chlorapatite, and bromapatite. Physical

hydroxyapatite ceramics with nanograin structure prepared by high pressure spark plasma sintering at the minimized sintering temperature. Journal of the European Ceramic Society.

*Impact of Dopants on the Electrical and Optical Properties of Hydroxyapatite DOI: http://dx.doi.org/10.5772/intechopen.93092*

[90] Kotobuki N, Ioku K, Kawagoe D, Fujimori H, Goto S, Ohgushi H. Observation of osteogenic differentiation cascade of living mesenchymal stem cells on transparent hydroxyapatite ceramics. Biomaterials. 2005;**26**:779-785

*Biomaterials*

capacitive coupling in the treatment of nonunion of long bones. The Journal of Bone and Joint Surgery. American

et al. Dielectric evaluation of fluorine substituted hydroxyapatite. Journal of the Ceramic Society of Japan.

[83] Shkir M, Kilany M, YahiaI S. Facile microwave-assisted synthesis of tungsten-doped hydroxyapatite nanorods: A systematic structural, morphological, dielectric, radiation and microbial activity studies. Ceramics International. 2017;**43**:14923-14931

[84] Thanigairul K, Elayaraja K,

Magudapathy P, Mudali UK, Nair KGM, Sudarshan M, et al. Surface modification of nanocrystalline calcium phosphate bioceramic by low energy nitrogen ion implantation. Ceramics International.

[85] ShKalil M, Beheri HH, Fattah WIA. Structural and electrical properties of zirconia/hydroxyapatite porous composites. Ceramics International.

[86] Jonscher AK. The 'universal' dielectric response. Nature.

of ac conduction in disordered solids. Reviews of Modern Physics.

[88] Yahia IS, Shkir M, AlFaify S, Ganesh V, Zahran HY, Kilany M. Facile microwave-assisted synthesis of Te-doped hydroxyapatite nanorods and nanosheets and their characterizations

for bone cement applications. Materials Science and Engineering C.

[89] Holzmann D, Holzinger D, Hesser G, Schmidt T, Knor G.

Hydroxyapatite nanoparticles as novel low-refractive index additives for the long-term UV-photoprotection of transparent composite materials. Journal of Materials Chemistry.

[87] Dyre JC, Schrøder TB. Universality

2013;**121**:770-774

2013;**39**:3027-3034

2002;**28**:451-458

1977;**267**:673-679

2000;**72**:873-892

2017;**72**:472-480

2009;**19**:8102-8106

Volume. 1994;**76A**:820-826

2000;**47**:1041-1055

1999;**24**:1349-1357

1998;**355**:S90-S104

1967;**12**:367-378

1977;**48**:808-811

[78] Marino AA, Becker RO, Bachman CH. Dielectric

determination of bound water of bone. Physics in Medicine and Biology.

[79] Lakes RS, Harper RA, Katz JL. Dielectric relaxation in cortical bone.

Journal of Applied Physics.

[80] Kaygili O, Keser S, Ates T, Ghamdi AA, Yakuphanoglu F.

Technology. 2013;**245**:1-6

hydroxyapatite nanoparticles

2013;**48**:3172-3177

Controlling of dielectrical and optical properties of hydroxyapatite based bioceramics by Cd content. Powder

[81] Iqbal N, Kadir MRA, Malek NANN, Mahmood NHB, Murali MR, Kamarul T. Characterization and antibacterial properties of stable silver substituted

synthesized through surfactant assisted microwave process. Materials Research.

[82] Horiuchi N, Endo J, Nozaki K, Nakamura M, Nagai A, Katayama K,

[75] Oishi M, Onesti ST. Electrical bone graft stimulation for spinal fusion: A review. Neurosurgery.

[76] Goodwin CB, Brighton CT, Guyer RD, Johnson JR, Light KI, Yuan HA. A double-blind study of capacitively coupled electrical stimulation as an adjunct to lumbar spinal fusions. Spine (Phila Pa 1976).

[77] Otter MW, McLeod KJ, Rubin CT. Effects of electromagnetic fields in experimental fracture repair. Clinical Orthopaedics and Related Research.

**118**

[91] Eriksson M, Liu Y, Hu J, Gao L, Nygren M, Shen Z. Transparent hydroxyapatite ceramics with nanograin structure prepared by high pressure spark plasma sintering at the minimized sintering temperature. Journal of the European Ceramic Society. 2011;**31**:1533-1540

[92] Calderin L, Stott M, Rubio A. Electronic and crystallographic structure of apatites. Physical Review B. 2003;**67**:134106

[93] Rulis P, Ouyang L, Ching WY. Electronic structure and bonding in calcium apatite crystals: Hydroxyapatite, fluorapatite, chlorapatite, and bromapatite. Physical Review B. 2004;**70**:155104

[94] Slepko A, Demkov AA. Firstprinciples study of the biomineral hydroxyapatite. Physical Review B. 2011;**84**:134108

[95] Wiglusz RJ, Bednarkiewicz A, Strek W. Synthesis and optical properties of Eu3+ ion doped nanocrystalline hydroxyapatites embedded in PMMA matrix. Journal of Rare Earths. 2011;**29**:1111-1116

[96] Mahraz ZAS, Sahar MR, Ghoshal SK, Dousti MR. Concentration dependent luminescence quenching of Er3+-doped zinc boro-tellurite glass. Journal of Luminescence. 2013;**144**:139-145

[97] Pan A, Yang Z, Zheng H, Liu F, Zhu Y, Su X, et al. Changeable position of SPR peak of Ag nanoparticles embedded in mesoporous SiO2 glass by annealing treatment. Applied Surface Science. 2003;**205**:323-328

[98] Apurba D, Kumar AC, Bharti GP, Behera RR, Mamilla RS, Khare A, et al. Effect of thickness on optical and microwave dielectric properties of hydroxyapatite films deposited by RF magnetron sputtering. Journal of Alloys and Compounds. 2018;**739**:729-736

[99] Matsunaga K, Kuwabara A. Firstprinciples study of vacancy formation in hydroxyapatite. Physical Review B. 2007;**75**:014102

[100] Flores YJ, Quezada MS, Trigos JBR, Rojas LL, Suarez V, Mantilla A. Characterization of Tb-doped hydroxyapatite for biomedical applications: Optical properties and energy band gap determination. Journal of Materials Science. 2017;**52**:9990-10000

[101] Ma MY, Zhu YJ, Li L, Cao SW. Nanostructured porous hollow ellipsoidal capsules of hydroxyapatite and calcium silicate: Preparation and application in drug delivery. Journal of Materials Chemistry. 2008;**18**:2722-2727

[102] Zhanglei N, Zhidong C, Wenjun LI, Changyan S, Jinghua Z, Yang L. Solvothermal synthesis and optical performance of one-dimensional strontium hydroxyapatite nanorod. Chinese Journal of Chemical Engineering. 2012;**20**:89-94

[103] Zhang CM, Yang J, Quan ZW, Yang PP, Li CX, Hou ZY, et al. Hydroxyapatite nano- and microcrystals with multiform morphologies: Controllable synthesis and luminescence properties. Crystal Growth & Design. 2009;**9**:2725-2733

[104] Ling L, Yukan L, Jinhui T, Ming Z, Haihua P, Xu X, et al. Surface modification of hydroxyapatite nanocrystallite by a small amount of terbium provides a biocompatible fluorescent probe. Journal of Physical Chemistry C. 2008;**112**:1229-12224

[105] Han Y, Wang X, Li S, Ma X. Synthesis of terbium doped calcium phosphate nanocrystalline powders by citric acid sol–gel combustion method. Journal of Sol-Gel Science and Technology. 2009;**49**:125-129

**Chapter 6**

**Abstract**

**1. Introduction**

**121**

Biogenic Source

*and Elvis Kwason Tiburu*

Biomaterial for Bone and Dental

Carbonated Hydroxyapatite from

There are several sources from which hydroxyapatite (HAp) can be obtained and may be broadly categorized as synthetic or biogenic. Elevated interest in recent times has pushed for the development of several procedures for extracting HAp from biogenic wastes due to their excellent composition and morphology resemblance to the human calcified tissue (B-type carbonated HAp). Notable biogenic sources reported for HAp extraction span bovine bones, fish scales, corals, eggshells, and snails among other calcium-rich sources. However, most of the synthetic methods are laborious and therefore result in high production costs. In this chapter, we discuss the synthesis of B-type carbonate substituted HAp from an untapped biogenic source, *Achatina achatina* shells, using a simple precipitation method and a controlled heat-treatment method. This unique treatment method affected the substitution resulting in different crystallographic parameters and revealed a novel

Implants: Synthesis of B-Type

*Bernard Owusu Asimeng, David Walter Afeke*

material for bone implants and enamel applications.

nal system, which consists of unconnected, PO4

**Keywords:** biogenic source, biomaterial, carbonated hydroxyapatite

Hydroxyapatite (HAp) is a member of the calcium apatite (group of phosphate)

stitial space and a chain of OH ions along the c-axis to balance the unit cell charges [4]. This hexagonal crystal structure allows for replacement (substitution) of ions into the structure. The substitution makes the HAp more reactive and biocompatible [6, 7]. The human calcified tissue (e.g., bone and tooth enamel) consists of mineral components similar to carbonated HAp (CHAp) [8, 9]. As a result, extensive research has been conducted to substitute carbonate into commercial HAp to achieve a suitable material for hard tissue replacement and implants coatings [10, 11]. There are two main types of carbonate (CO3) substitution that occur in hydroxyapatite, namely A-type and B-type. A-type occurs when CO3 replaces OH

<sup>3</sup> tetrahedra with Ca2+ in the inter-

family with a high concentration of hydroxyl group [1–3]. Stoichiometric HAp, Ca10(PO4)6(OH)2 can exhibit either monoclinic or hexagonal crystal structures [4, 5]. The most frequently reported hydroxyapatite crystal structure is the hexago-

[106] Arul KT, Ramya JR, Vanithakumari SC, Magudapathy P, Mudali UK, Nair KGM, et al. Novel ultraviolet emitting low energy nitrogen ion implanted magnesium ion incorporated nanocrystalline calcium phosphate. Materials Letters. 2015;**153**:182-185

[107] Arul KT, Ramya JR, Karthikeyan KR, Kalkura SN. A novel and rapid route to synthesize polyvinylalcohol/calcium phosphate nanocomposite coatings by microwave assisted deposition. Materials Letters. 2014;**135**:191-194

[108] Karthikeyan KR, Arul KT, Ramya JR, Nabhiraj PY, Menon R, Krishna JBM, et al. Core/shell structures on argon ions implanted polymer based zinc ions incorporated HAp nanocomposite coatings. Materials Science in Semiconductor Processing. 2019;**104**:104687

[109] Baskar S, Ramya JR, Arul KT, Nivethaa EAK, Mahadevan Pillai VP, Kalkura SN. Impact of magnetic field on the mineralization of iron doped calcium phosphates. Materials Chemistry and Physics 2018;**218**:166-171

### **Chapter 6**

*Biomaterials*

[105] Han Y, Wang X, Li S, Ma X. Synthesis of terbium doped calcium phosphate nanocrystalline powders by citric acid sol–gel combustion method. Journal of Sol-Gel Science and

Technology. 2009;**49**:125-129

Vanithakumari SC, Magudapathy P, Mudali UK, Nair KGM, et al. Novel ultraviolet emitting low energy nitrogen ion implanted magnesium ion incorporated nanocrystalline calcium phosphate. Materials Letters.

[106] Arul KT, Ramya JR,

[107] Arul KT, Ramya JR, Karthikeyan KR, Kalkura SN. A novel and rapid route to synthesize polyvinylalcohol/calcium phosphate nanocomposite coatings by microwave assisted deposition. Materials Letters.

[108] Karthikeyan KR, Arul KT, Ramya JR, Nabhiraj PY, Menon R, Krishna JBM, et al. Core/shell structures on argon ions implanted polymer based zinc ions incorporated HAp nanocomposite coatings. Materials Science in Semiconductor Processing.

[109] Baskar S, Ramya JR, Arul KT, Nivethaa EAK, Mahadevan Pillai VP, Kalkura SN. Impact of magnetic field on the mineralization of iron doped calcium phosphates. Materials

Chemistry and Physics 2018;**218**:166-171

2015;**153**:182-185

2014;**135**:191-194

2019;**104**:104687

**120**
