**3. Osseointegration of the implants**

immobilized on orthopaedic devices have been reported to enhance osteoblastic activity and favor implant integration [25]. The most commonly used growth factors in orthopae‐ dics are members of the transforming growth factor beta (TGF-β) superfamily including the BMP family, especially BMP2 and BMP7. Growth factors may be physically adsorbed or covalently grafted onto the implant surface and various studies have shown that the loading of implant with these factors can enhance interactions at the bone-implant interface and aid the remodeling process ultimately improving implant integration [26-28]. Howev‐ er critical factors in the successful use of growth factors in orthopaedic devices are the optimum dosage, exposure period and release kinetics, all have to be considered careful‐ ly to avoid the detrimental effects associated with growth factor use such as high initial burst rate, ectopic bone formation and short half-life. More recently, peptide sequences with the ability to target specific osteogenic cellular functions of differentiation and mineraliza‐ tion have been developed [29, 30]. These short functional fragments derived from the original protein have increased shelf life, can be synthetically produced and are more resistant to denaturizing effects. Their usage would provide significant clinical benefits over the use of conventional proteins. They can be linked to the implant surface to provide biological cues for bone formation. Additionally other peptide sequences in use include the RGD, YIGSR, IKVAV and KRSR which have been used to improve cellular adhesion and

Research in the recent years have concentrated on the development of bioactive compo‐ site coatings which mimics the structure of the bone tissue. These composite coatings would combine calcium phosphate with growth factors, peptides, antibodies etc. to enhance interactions at the bone-implant interface. However due to the fact that often high temperature or non-physiological conditions are needed in the preparation of calcium phosphate coatings, only physical adsorption is employed in deposition of the biomole‐ cules on the implant surface [34, 35]. However with physical adsorption techniques, initial high burst rate is often observed, which is not desired [36]. Therefore coating techniques that create a gentle sustained release kinetics are preferred. A recently published paper have shown that calcium phosphate coating combining slow release of antibiotics, aids in early success at recruitment of bone cells [37]. Many other studies have shown that depositing BMP2 and TGF-β onto the implant surface would greatly enhance bonebonding at the bone-implant interface [25, 34]. The biological efficacy of orthopaedic implants can be improved greatly by both physical and chemical modifications. The use of a wide multitude of engineering techniques in the manipulation of surface topography, morphology and incorporating the use of various inorganic and organic components would directly influence the response in the local bone-implant interface and the apposition of new bone. With the development of new techniques and strategies on composite coatings to better mimic the human bone structure this would result in a new generation of

orthopaedic implants with improved implant integration and bone healing.

bone matrix formation [31-33].

6 Titanium Alloys - Advances in Properties Control

**2.3. Organic–inorganic composite coatings**

The clinical strategies to manage musculoskeletal defects would center around three compo‐ nents: cells, structure and growth factors. For the design of implant materials, cells and proteins at the implant interface plays a critical role [38]. The utilization of biosignal proteins such as growth factors for development of bioactive implant materials holds great potential. Especially due to the scarcity of stem cells in the body, materials which regulates cellular functions such as adhesion, growth and differentiation are desired.

One of the most important process in determining the success of an orthopaedic implant is osseointegration. Osseointegration is defined as the formation of a direct structural and functional connection between the living bone and the surface of a implant [39, 40]. An implant is considered osseointegrated if there is no progressive relative movement between the implant and the bone it has direct contact with [40]. Under ideal conditions, implants could perma‐ nently become incorporated within the bone and persist under all normal conditions of loading, that is the two could not be separated without fracture. Vascularization which is the provision of blood supply is a critical component for the process of osseointegration. The differentiation of osteogenic cells is highly dependent on tissue vascularity and ossification is closely linked to the vascularization of differentiating tissue [40]. Therefore the success of tissue healing, regeneration and integration lies in the key process of revascularization which is crucial in improving the successful integration of implants [41, 42].

Bone healing around implants involves a cascade of cellular and biological events that take place at the bone-implant interface until finally the entire surface of the implant is covered by newly formed bone. This cascade of biological events is regulated by differentiation of cells stimulated by growth factors secreted at the bone-implant interface [40]. There has been considerable interest in modifying implant surfaces with growth factors to improve their cell functions and tissue integration capacity at the bone-implant interface. Enhanced cell functions and cell substrate interactions have been demonstrated with growth factors immobilized onto implant materials [26-28]. One of the more important growth factors for stimulating neovas‐ cularization (i.e. formation of new blood vessels) in target areas [43] would be angiogenic growth factors, crucial in improving the successful integration of implants both *in vitro* and *in vivo* [41, 42]. Of these angiogenic factors, vascular endothelial growth factor (VEGF) is the most potent and widely used key regulator of neovascularization [43, 44]. VEGF is a crucial factor in not only angiogenesis regulation but also in osteoblast [45] and osteoclast function [46-48] during bone repair. VEGF acts directly on osteoblasts, promoting cell functions such as proliferation, migration and differentiation [49, 50]. In addition, VEGF also indirectly affect osteoblasts via its influences on endothelial cells [51, 52]. VEGF is known to induce endothelial cells in surrounding tissues to migrate, proliferate and form tubular structures [53] and is an essential survival factor for endothelial cells [51] and new vessel formation [54]. Endothelial cells are needed to provide complex interactive communication networks in bone for gap junction communication with osteoblasts crucial to their formation from osteoprogenitors [55]. Furthermore VEGF stimulates endothelial cells in the production of beneficial bone forming factors acting on osteoblasts [50]. In all, the effects of VEGF on osteoblasts, osteoclasts and endothelial cells may synergistically act to enhance bone formation.

surface cell receptors through several binding modes. This enhances the formation of ligandreceptor complexes which are critical for signal transduction and the multivalent ligands are able to stabilize and prevent lateral diffusion of the formed complexes leading to the prolonged effect. Figure 3 shows the interactions of cells with the different forms of growth factor and

Titanium Alloys in Orthopaedics http://dx.doi.org/10.5772/55353 9

In order to effectively derive the effect from immobilized growth factors, strategies have to be developed that can optimize the structure to elicit the desired biological response. One of the problems encountered with implant materials for surface functionalizaton is the lack of suitable chemical groups on the surface. For more versatility and applicability, the concentra‐ tions of the OH group and other reactive groups such as amino or carboxyl groups have to be increased. The initial organic layer immobilized on the implant materials can then be used as a tether for biomolecular components used to mediate cell attachment. Another issue which merits investigation is the control of the retention and/or release of the biomolecules from the implant surface. The easiest and most common method employed for delivery of biomolecules is physical adsorption, which unfortunately provides little control over the delivery and orientation of the biomolecules. Bonding of the biomolecules and use of coatings incorporating them would be alternative methods of delivery to the bone-implant interface. Regardless, the preferred and chosen immobilization technique would depend on the specific working mechanism of the biomolecules. Given the above scenario, surface functionalization of biomaterials in order to enhance biocompatibility and promote osseointegration has great potential in addressing the problems of prosthetic joint implant longevity and survival.

Immobilization techniques are broadly classified into four categories, namely a) physical adsorption (via van der Waals or electrostatic interactions), b) physical entrapment (use of barrier systems), c) cross-linking and d) covalent binding. The choice of the technique would depend on the nature of the bioactive factors, substrates and its application. It will not be possible to have a universal means of immobilization, however developing a viable method‐

**Figure 3.** Effects of soluble growth factors compared to immobilized growth factors.

the enhanced mitogenic effects.

#### **3.1. Fixation of titanium implants**

The fixation of prosthetic components to the bone can be done with or without bone cement. In the cemented technique polymethylmethacrylate (PMMA) is used to "glue" the metal to the bone. In direct biological fixation, precise bone cuts are required to achieve maximum contact between metal and bone. The advantage of cement fixation is that the prosthetic components are instantly fixed, allowing movement immediately after surgery. However in the instances where revision surgery is required, it is extremely difficult to chip out all the cement during implant replacement. Cement fixation is usually employed on elderly patients over sixty-five where their bone stock is more osteoporotic with less likelihood of growing into the prosthesis and chances of revision is lower due to less demands on the implant and shorter remaining life expectancy compared to younger patients. Direct biological fixation is generally used for young patients due to better bone stock and ingrowth potential. The disadvantage of biological fixation is that it can take weeks or months to be fully complete during which weight bearing activity is restricted. However the final fixation achieved is more natural with complete incorporation of implant within the bone in ideal situations. Furthermore in case of young patients the chances for future revision surgery is higher and it would be easier to revise a cementless prosthesis with‐ out the need for cement removal. Another problem perceived was that cementless titani‐ um stems have been reported to be more resistant to osteolysis and mechanical failure compared to similar cemented titanium stems [56]. The features of titanium that are detrimental to the cement environment seems to have no effects in the cementless environ‐ ment and may in fact be beneficial leading to differences in performance of the two techniques. Therefore the enhancement of the bone implant interface especially in direct biological fixation with titanium implants would be extremely useful. This would greatly reduce the lag period in which osseointegration occurs between the prosthesis and the patient's bone.

#### **3.2. Surface functionalization by growth factors immobilization**

One promising way to incorporate growth factors usage with implant materials would be by surface functionalization of growth factors. Soluble growth factors work by binding with cognate receptors on cells to form complexes which would result in autophosphorylation of the cytoplasmic domains of the receptors and this phosphorylation activates intracellular signal transduction. The formed complexes are then aggregated and internalized into the cells by both clathrin-dependent and clathrin-independent mechanisms which leads to the recy‐ cling of the receptors for degradatory down-regulation [57]. Similarly immobilized growth factors work by forming complexes with the cell surface receptors, however the signal transduction is expected to last longer than soluble growth factors due to the inhibition of the internalization process. Multivalency is another important phenomenon responsible for this prolonged enhanced mitogenic effect. Multivalent ligands interact and bind avidly to multiple surface cell receptors through several binding modes. This enhances the formation of ligandreceptor complexes which are critical for signal transduction and the multivalent ligands are able to stabilize and prevent lateral diffusion of the formed complexes leading to the prolonged effect. Figure 3 shows the interactions of cells with the different forms of growth factor and the enhanced mitogenic effects.

**Figure 3.** Effects of soluble growth factors compared to immobilized growth factors.

factors acting on osteoblasts [50]. In all, the effects of VEGF on osteoblasts, osteoclasts and

The fixation of prosthetic components to the bone can be done with or without bone cement. In the cemented technique polymethylmethacrylate (PMMA) is used to "glue" the metal to the bone. In direct biological fixation, precise bone cuts are required to achieve maximum contact between metal and bone. The advantage of cement fixation is that the prosthetic components are instantly fixed, allowing movement immediately after surgery. However in the instances where revision surgery is required, it is extremely difficult to chip out all the cement during implant replacement. Cement fixation is usually employed on elderly patients over sixty-five where their bone stock is more osteoporotic with less likelihood of growing into the prosthesis and chances of revision is lower due to less demands on the implant and shorter remaining life expectancy compared to younger patients. Direct biological fixation is generally used for young patients due to better bone stock and ingrowth potential. The disadvantage of biological fixation is that it can take weeks or months to be fully complete during which weight bearing activity is restricted. However the final fixation achieved is more natural with complete incorporation of implant within the bone in ideal situations. Furthermore in case of young patients the chances for future revision surgery is higher and it would be easier to revise a cementless prosthesis with‐ out the need for cement removal. Another problem perceived was that cementless titani‐ um stems have been reported to be more resistant to osteolysis and mechanical failure compared to similar cemented titanium stems [56]. The features of titanium that are detrimental to the cement environment seems to have no effects in the cementless environ‐ ment and may in fact be beneficial leading to differences in performance of the two techniques. Therefore the enhancement of the bone implant interface especially in direct biological fixation with titanium implants would be extremely useful. This would greatly reduce the lag period in which osseointegration occurs between the prosthesis and the

endothelial cells may synergistically act to enhance bone formation.

**3.2. Surface functionalization by growth factors immobilization**

One promising way to incorporate growth factors usage with implant materials would be by surface functionalization of growth factors. Soluble growth factors work by binding with cognate receptors on cells to form complexes which would result in autophosphorylation of the cytoplasmic domains of the receptors and this phosphorylation activates intracellular signal transduction. The formed complexes are then aggregated and internalized into the cells by both clathrin-dependent and clathrin-independent mechanisms which leads to the recy‐ cling of the receptors for degradatory down-regulation [57]. Similarly immobilized growth factors work by forming complexes with the cell surface receptors, however the signal transduction is expected to last longer than soluble growth factors due to the inhibition of the internalization process. Multivalency is another important phenomenon responsible for this prolonged enhanced mitogenic effect. Multivalent ligands interact and bind avidly to multiple

**3.1. Fixation of titanium implants**

8 Titanium Alloys - Advances in Properties Control

patient's bone.

In order to effectively derive the effect from immobilized growth factors, strategies have to be developed that can optimize the structure to elicit the desired biological response. One of the problems encountered with implant materials for surface functionalizaton is the lack of suitable chemical groups on the surface. For more versatility and applicability, the concentra‐ tions of the OH group and other reactive groups such as amino or carboxyl groups have to be increased. The initial organic layer immobilized on the implant materials can then be used as a tether for biomolecular components used to mediate cell attachment. Another issue which merits investigation is the control of the retention and/or release of the biomolecules from the implant surface. The easiest and most common method employed for delivery of biomolecules is physical adsorption, which unfortunately provides little control over the delivery and orientation of the biomolecules. Bonding of the biomolecules and use of coatings incorporating them would be alternative methods of delivery to the bone-implant interface. Regardless, the preferred and chosen immobilization technique would depend on the specific working mechanism of the biomolecules. Given the above scenario, surface functionalization of biomaterials in order to enhance biocompatibility and promote osseointegration has great potential in addressing the problems of prosthetic joint implant longevity and survival.

Immobilization techniques are broadly classified into four categories, namely a) physical adsorption (via van der Waals or electrostatic interactions), b) physical entrapment (use of barrier systems), c) cross-linking and d) covalent binding. The choice of the technique would depend on the nature of the bioactive factors, substrates and its application. It will not be possible to have a universal means of immobilization, however developing a viable method‐ ology which can provide for a facile, secure immobilization with good interactions for orthopaedic implants is vital.

*3.2.3. Cross-linking*

Bioactive factors can also be immobilized through chemical cross-linking via homo- as well as heterobifunctional cross-linking agents. Among these glutaraldehyde cross-linking are the most popular due to its low cost, high efficiency and stability [58-60]. Glutaraldehyde is often used as an amine reactive homobifunctional crosslinker for biochemistry applications.

Titanium Alloys in Orthopaedics http://dx.doi.org/10.5772/55353 11

**Figure 6.** Schematic diagram showing glutaraldehyde cross-linking with proteins.

Covalent binding is another technique used for the immobilization of bioactive molecules. The functional groups investigated are usually the carboxyl, amino and phenolic group of tyrosine. Bioactive factors are covalently linked through functional groups in the factors not essential for the bioactivity. The covalent binding should be optimized so as to protect the active site

*3.2.4. Covalent binding*

and not alter its conformational flexibility.

#### *3.2.1. Physical adsorption*

This is the simplest of all the techniques available and does not alter the activity of the bioactive factors. Physical adsorption techniques are mainly based on ionic and hydrophobic interac‐ tions. If the bioactive factors are immobilized via ionic interactions, adsorption and desorption of the factors will depend on the basicity of the ion exchanger. A reversible dynamic equili‐ brium is achieved between the adsorbed factors and substrates which is affected by the pH as well as ionic strength of the surrounding medium. Hydrophobic interactions offer slightly higher stability with less loss of the factors from the surface of the substrates. Although physical adsorption systems are simple to perform and do not require extensive treatment to the bioactive factors and substrates used however there are certain drawbacks. These systems suffer from low surface loading and biomolecules may desorbed from the surface in an uncontrolled manner.

**Figure 4.** Schematic diagram showing physical adsorption system with proteins.

#### *3.2.2. Physical entrapment*

This method is employed with barriers including natural polymers like gelatin, agar and alginate entrapment systems. Other synthetic polymers employed include resins, polyur‐ ethane prepolymers etc. Some of the major limitations of the entrapment system is the diffusional problem where there is possible slow leakage during continuous use due to the small molecular size of bioactive factors, and steric hindrance which may affect the reactivity of the factors. Recent development of hydrogels and water soluble polymers attempt to overcome these drawbacks and have attracted much attention from the biomedical field.

**Figure 5.** Schematic diagram showing barrier system with proteins.

#### *3.2.3. Cross-linking*

ology which can provide for a facile, secure immobilization with good interactions for

This is the simplest of all the techniques available and does not alter the activity of the bioactive factors. Physical adsorption techniques are mainly based on ionic and hydrophobic interac‐ tions. If the bioactive factors are immobilized via ionic interactions, adsorption and desorption of the factors will depend on the basicity of the ion exchanger. A reversible dynamic equili‐ brium is achieved between the adsorbed factors and substrates which is affected by the pH as well as ionic strength of the surrounding medium. Hydrophobic interactions offer slightly higher stability with less loss of the factors from the surface of the substrates. Although physical adsorption systems are simple to perform and do not require extensive treatment to the bioactive factors and substrates used however there are certain drawbacks. These systems suffer from low surface loading and biomolecules may desorbed from the surface in an

This method is employed with barriers including natural polymers like gelatin, agar and alginate entrapment systems. Other synthetic polymers employed include resins, polyur‐ ethane prepolymers etc. Some of the major limitations of the entrapment system is the diffusional problem where there is possible slow leakage during continuous use due to the small molecular size of bioactive factors, and steric hindrance which may affect the reactivity of the factors. Recent development of hydrogels and water soluble polymers attempt to overcome these drawbacks and have attracted much attention from the biomedical field.

orthopaedic implants is vital.

10 Titanium Alloys - Advances in Properties Control

*3.2.1. Physical adsorption*

uncontrolled manner.

*3.2.2. Physical entrapment*

**Figure 4.** Schematic diagram showing physical adsorption system with proteins.

**Figure 5.** Schematic diagram showing barrier system with proteins.

Bioactive factors can also be immobilized through chemical cross-linking via homo- as well as heterobifunctional cross-linking agents. Among these glutaraldehyde cross-linking are the most popular due to its low cost, high efficiency and stability [58-60]. Glutaraldehyde is often used as an amine reactive homobifunctional crosslinker for biochemistry applications.

**Figure 6.** Schematic diagram showing glutaraldehyde cross-linking with proteins.

#### *3.2.4. Covalent binding*

Covalent binding is another technique used for the immobilization of bioactive molecules. The functional groups investigated are usually the carboxyl, amino and phenolic group of tyrosine. Bioactive factors are covalently linked through functional groups in the factors not essential for the bioactivity. The covalent binding should be optimized so as to protect the active site and not alter its conformational flexibility.

**Figure 7.** Schematic diagram showing polymerization of dopamine under alkaline pH and the equilibrium shift to‐ wards the quinone functional groups for reactivity with proteins.

63]. Furthermore it has been demonstrated that immobilized factors is more effective in promoting proliferation of cells compared to soluble factors [65]. Both immobilized and soluble factors bind to receptors on cells, however they have differing effects due to the fact that soluble factors are internalized and subsequently degraded, while immobilization inhibits internali‐ zation and prevents down regulation [64, 75], thereby enabling the factors to stimulate proliferation for an extended period of time. A comparison of cross-linking and covalent binding shows that they come quite close in terms of binding efficiency and there is no release

**Factor release overtime**

"/> 30% after 1 month

56% 0.449 Nil Immobilized Three steps

52% 0.841 Nil Immobilized Two steps

**Active form**

**Number of steps required for fabrication**

13

Titanium Alloys in Orthopaedics http://dx.doi.org/10.5772/55353

Soluble Single step

From the cytotoxicity indications (Table 1) follows that there is a lower cell viability with glutaraldehyde cross-linking compared to the other groups. This may be due to the fact that glutaraldehyde is known to be toxic and is able to kill cells quickly by cross-linking with their proteins. There have also been reports of its toxicity implicated in poor cell growth, attachment and apoptosis [58-60] by other groups. Although glutaraldehyde cross-linking effectively anchors a high density of factors onto the titanium substrate surface and the molecules are also more firmly attached than those which are physically adsorbed however the associated toxicity has made it unsuitable for clinical applications. The use of covalent immobilization with polydopamine looks promising. Polydopamine has been found to be able to form thin adherent films onto a wide variety of metallic substrates via covalent bonds and various strong inter‐ molecular interactions including metal chelation, hydrogen bonding and π-π interactions [76] which cannot be disrupted by normal mechanical forces. The use of this bioreactive layer for covalent bioconjugation with bioactive factors for orthopaedic applications holds great potential. Although it will not be possible to have a universal means of immobilization, however it is vital to develop a viable methodology which can provide for secure immobili‐ zation with good interactions for orthopaedic implants. The choice of the technique would depend on the nature of the bioactive factors, substrates and their application. The develop‐ ment of surface modification procedures that do not affect the integrity of the substrate and bioactivity of the growth factors are crucial in producing the desired surface functionalization effect. This would provide us with a secure and efficient method of attaching bioactive

of growth factors into the solution which is the preferred methodology.

**Cytotoxicity**

86% 0.677

**Binding Efficiency (50ng loading)**

Physical adsorption (via simple coating)

Cross-linking (via glutaraldehyde cross-linking)

Covalent Binding (via polydopamine conjugation)

**Table 1.** please add caption

#### *3.2.5. Comparison of the various immobilization techniques*

Several methods of immobilizing angiogenic growth factors onto substrates have been studied and reported [61-66]. A summary of a short study investigating the efficacy of immobilization of VEGF via various modes of functionalization on Ti-6Al-4V including physical adsorption, cross-linking and covalent binding (adapted for orthopaedic applications) is presented here to evaluate the effectiveness of each technique. As physical entrapment is not suitable in this case of improving the bone-implant interface via the surface of the implant material, therefore this system is not investigated. Table 1 summarizes the parameters of the binding efficiency, cytotoxicity, release profile and number of steps required for the fabrication of the substrates.

Although physical adsorption had the highest rate of binding however there was also uncontrolled release of the factors from the substrate which may be undesirable [67-69]. A measurement of the percentage of factors released into the solution over a 30 day period showed that more than 30% of the factors were released. A number of studies have examined simple coating or loading of factors onto implants [67-73] in order to provide local and sustained delivery after implantation. However with this strategy some studies showed an uncontrolled initial burst in the release kinetics of factors from such implants [67-69]. High levels of factors in the local microenvironments of these implants may be detrimental to healing and may promote tumorogenesis [74]. To avoid the deleterious effects, secure immobilization strategy would be preferred [61, 64-66]. Immobilization of growth factors on implants have been shown to promote desirable cell substrate interactions and enhance cell functions [62,


**Table 1.** please add caption

**Figure 7.** Schematic diagram showing polymerization of dopamine under alkaline pH and the equilibrium shift to‐

Several methods of immobilizing angiogenic growth factors onto substrates have been studied and reported [61-66]. A summary of a short study investigating the efficacy of immobilization of VEGF via various modes of functionalization on Ti-6Al-4V including physical adsorption, cross-linking and covalent binding (adapted for orthopaedic applications) is presented here to evaluate the effectiveness of each technique. As physical entrapment is not suitable in this case of improving the bone-implant interface via the surface of the implant material, therefore this system is not investigated. Table 1 summarizes the parameters of the binding efficiency, cytotoxicity, release profile and number of steps required for the fabrication of the substrates.

Although physical adsorption had the highest rate of binding however there was also uncontrolled release of the factors from the substrate which may be undesirable [67-69]. A measurement of the percentage of factors released into the solution over a 30 day period showed that more than 30% of the factors were released. A number of studies have examined simple coating or loading of factors onto implants [67-73] in order to provide local and sustained delivery after implantation. However with this strategy some studies showed an uncontrolled initial burst in the release kinetics of factors from such implants [67-69]. High levels of factors in the local microenvironments of these implants may be detrimental to healing and may promote tumorogenesis [74]. To avoid the deleterious effects, secure immobilization strategy would be preferred [61, 64-66]. Immobilization of growth factors on implants have been shown to promote desirable cell substrate interactions and enhance cell functions [62,

wards the quinone functional groups for reactivity with proteins.

12 Titanium Alloys - Advances in Properties Control

*3.2.5. Comparison of the various immobilization techniques*

63]. Furthermore it has been demonstrated that immobilized factors is more effective in promoting proliferation of cells compared to soluble factors [65]. Both immobilized and soluble factors bind to receptors on cells, however they have differing effects due to the fact that soluble factors are internalized and subsequently degraded, while immobilization inhibits internali‐ zation and prevents down regulation [64, 75], thereby enabling the factors to stimulate proliferation for an extended period of time. A comparison of cross-linking and covalent binding shows that they come quite close in terms of binding efficiency and there is no release of growth factors into the solution which is the preferred methodology.

From the cytotoxicity indications (Table 1) follows that there is a lower cell viability with glutaraldehyde cross-linking compared to the other groups. This may be due to the fact that glutaraldehyde is known to be toxic and is able to kill cells quickly by cross-linking with their proteins. There have also been reports of its toxicity implicated in poor cell growth, attachment and apoptosis [58-60] by other groups. Although glutaraldehyde cross-linking effectively anchors a high density of factors onto the titanium substrate surface and the molecules are also more firmly attached than those which are physically adsorbed however the associated toxicity has made it unsuitable for clinical applications. The use of covalent immobilization with polydopamine looks promising. Polydopamine has been found to be able to form thin adherent films onto a wide variety of metallic substrates via covalent bonds and various strong inter‐ molecular interactions including metal chelation, hydrogen bonding and π-π interactions [76] which cannot be disrupted by normal mechanical forces. The use of this bioreactive layer for covalent bioconjugation with bioactive factors for orthopaedic applications holds great potential. Although it will not be possible to have a universal means of immobilization, however it is vital to develop a viable methodology which can provide for secure immobili‐ zation with good interactions for orthopaedic implants. The choice of the technique would depend on the nature of the bioactive factors, substrates and their application. The develop‐ ment of surface modification procedures that do not affect the integrity of the substrate and bioactivity of the growth factors are crucial in producing the desired surface functionalization effect. This would provide us with a secure and efficient method of attaching bioactive molecules to titanium implant material surface conferring enhancement of cell-implant interactions beneficial for orthopaedic applications.

[2] Niinomi M. Mechanical biocompatibilities of titanium alloys for biomedical applica‐

Titanium Alloys in Orthopaedics http://dx.doi.org/10.5772/55353 15

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### **4. Conclusions**

There is an ever growing need for orthopaedic advancement with the high prevalence and impact of musculoskeletal diseases. 50% of the world's population over 65 suffer from joint diseases and more than 25% of population over 65 require health care for joint related diseases. The instances for failed joint replacements associated with osteolysis and bone defects is increasing. There is an urgency to increase the success of bone implant fixation and the longevity of implant. Fixation of orthopaedic implants has been one of the most challenging and difficult problem faced by orthopaedic surgeons and patients. Fixation can often be achieved via direct biological fixation by allowing tissues to grow into the surfaces of the implants or with the use of bone cement acting as a grouting material. Whether cemented or cementless fixation are employed, the problems of micromotion and the generation of wear particles may eventually necessitate further surgery. Revision surgery poses increased risks like deep venous thrombosis, infection and dislocation, in addition to being an economic burden to the patient. Therefore the enhancement of implant integration would bring enor‐ mous benefits. Titanium alloy is one of the most frequently used material in orthopaedic implants. However despite the good inherent bioactivity and biocompatibility exhibited by titanium alloys, osseointegration with host tissue is still not definite, the lack of bioactivity may cause implant failure at times. Fixation of orthopaedic implants has been one of the most challenging and difficult problem faced by orthopaedic surgeons and patients. With the ever growing number of patients requiring orthopaedic reconstructions the development and evolvement of titanium alloys with structural and biological potential to manage bone healing impairment and defects would be desirable.

### **Author details**

Wilson Wang\* and Chye Khoon Poh

Department of Orthopaedic Surgery, National University of Singapore, Kent Ridge, Singa‐ pore

## **References**

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[2] Niinomi M. Mechanical biocompatibilities of titanium alloys for biomedical applica‐ tions. J Mech Behav Biomed Mater. 2008;1:30-42.

molecules to titanium implant material surface conferring enhancement of cell-implant

There is an ever growing need for orthopaedic advancement with the high prevalence and impact of musculoskeletal diseases. 50% of the world's population over 65 suffer from joint diseases and more than 25% of population over 65 require health care for joint related diseases. The instances for failed joint replacements associated with osteolysis and bone defects is increasing. There is an urgency to increase the success of bone implant fixation and the longevity of implant. Fixation of orthopaedic implants has been one of the most challenging and difficult problem faced by orthopaedic surgeons and patients. Fixation can often be achieved via direct biological fixation by allowing tissues to grow into the surfaces of the implants or with the use of bone cement acting as a grouting material. Whether cemented or cementless fixation are employed, the problems of micromotion and the generation of wear particles may eventually necessitate further surgery. Revision surgery poses increased risks like deep venous thrombosis, infection and dislocation, in addition to being an economic burden to the patient. Therefore the enhancement of implant integration would bring enor‐ mous benefits. Titanium alloy is one of the most frequently used material in orthopaedic implants. However despite the good inherent bioactivity and biocompatibility exhibited by titanium alloys, osseointegration with host tissue is still not definite, the lack of bioactivity may cause implant failure at times. Fixation of orthopaedic implants has been one of the most challenging and difficult problem faced by orthopaedic surgeons and patients. With the ever growing number of patients requiring orthopaedic reconstructions the development and evolvement of titanium alloys with structural and biological potential to manage bone healing

Department of Orthopaedic Surgery, National University of Singapore, Kent Ridge, Singa‐

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interactions beneficial for orthopaedic applications.

14 Titanium Alloys - Advances in Properties Control

impairment and defects would be desirable.

and Chye Khoon Poh

**Author details**

Wilson Wang\*

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pore

**4. Conclusions**


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**Chapter 2**

**Sputtered Hydroxyapatite Nanocoatings on Novel**

Titanium and titanium alloys have been extensively studied for many applications in the area of bone tissue engineering. It was believed that the excellent properties of titanium al‐ loys, e.g. lightweight, excellent corrosion resistance, high mechanical strength and low elas‐ tic modulus compared to other metallic biomaterials such as stainless steels and Cr-Co alloys, would provide enhanced stability for load-bearing implants. However, they usually lack sufficient osseointegration for implant longevity, and their biocompatibility is also an important concern in these applications due to the potential adverse reactions of metallic ions with the surrounding tissues once these metallic ions are released from the implant sur‐ faces. One approach for consideration to improve the healing process is the application of a hydroxyapatite nanocoating onto the surface of biomedical devices and implants. Hydrox‐ yapatite, with its excellent biocompatibility, and similar chemistry and structure to the min‐ eral component of bone, provides a bioactive surface for direct bone formation and apposition with adjacent hard tissues. The deposition of a SiO2 interlayer between the im‐ plant surface and the hydroxyapatite nanocoating is necessary to further improve the bio‐ compatibility of metal implants, as SiO2 has its own excellent compatibility with living tissues, and high chemical inertness, which lead to enhanced osteointegrative and functional

Therefore, SiO2 and hydroxyapatite nanocoatings were deposited onto titanium alloys using electron beam evaporation and magnetron sputtering techniques, respectively, with differ‐ ent process parameters to optimize the deposition conditions and so achieve desired proper‐ ties. Surface characteristics are essential due to their role in enhancing osseointegration. Surface morphology and microstructure were observed using a scanning electron micro‐

> © 2013 Mediaswanti et al.; licensee InTech. This is an open access article 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.

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.

© 2013 Mediaswanti et al.; licensee InTech. This is a paper distributed under the terms of the Creative Commons

**Titanium Alloys for Biomedical Applications**

Kun Mediaswanti, Cuie Wen, Elena P. Ivanova,

Additional information is available at the end of the chapter

Christopher C. Berndt and James Wang

http://dx.doi.org/10.5772/54263

properties of the system as a whole.

**1. Introduction**

