**2. Plasma-sprayed hydroxyapatite coating**

One of the ongoing research fields in the scientific community is the design of novel materials which can stimulate the bone regenerative process because the bone regeneration is a constant and continuous process in our lives, although the resultant regeneration speed shows a decreased tendency as a function of the age [11]. In addition, several studies indicate that every year over 2 million people worldwide require bone grafting surgery in order to repair large bone defects which are a very common problem in orthopedic surgery, being the main alternatives to repair these defects the use of autologous bone grafts, allografts, or biocompatible synthetic materials [12, 13].

Bones as well as other calcified tissues are considered as natural anisotropic composites consisting of biominerals embedded in a protein matrix, other organic materials, and water [14]. More specifically, the biomineral phase can be one or more types of calcium phosphates (CP) salts which vary in the resultant chemical formula and solubility value, respectively. Among all the known CP salts, hydroxyapatite (HA) stands out because it is the main calcium phosphate phase constituent of bones, comprising around 70% in comparison with water (10%) and organic phase (collagen) which constitutes the remaining part (around 20%), providing elastic resistance [15]. Due to this, multiple works can be found in the bibliography related to a wide variety of chemical methods (dry, wet, or high-temperature processes) for the fabrication of synthetic HA for biomedical applications (bone scaffold, bone filler, implant coating, or drug delivery systems) [14, 16, 17]. It has been demonstrated that synthetic HA shows a wide number of advantages such as an excellent biocompatibility, bioactivity, noninflammatory, affinity to biopolymers, and high-osteoconductive as well as osteointegrative properties without causing any systemic toxicity, rejection, or foreign body response [18–20]. A representative example can be observed in **Figure 1** where the in vivo bone repair experiments demonstrate that a new type of porous scaffold such as poly (γ-benzyl-L-glutamate)-modified hydroxyapatite/(poly (L-lactic acid) (PBLG-g-HA/PLLA) induced higher levels of new bone formation (rat femur defect) in comparison with blank (control), poly (L-lactic acid) (PLLA), and hydroxyapatite/poly (L-lactic acid) (HA/PLLA), respectively. These results indicate the potential applications for bone tissue engineering by demonstrating favorable osteogenic properties [21].

in different areas of the body ranging from scaffolds, coronary stents, and heart valves to hip and knee prostheses [2–5]. Although there are characteristics common to all the different prostheses such as resistance to corrosion and biocompatibility (they should not "harm" the patient), the requirements of each of them are in many cases very different [6]. A replacement plate of a cranial bone or a hip prosthesis inserted into the femur should have a good capacity for osseointegration. However, a coronary stent should prevent cell growth in its internal part [7]. This differentiation occurs not only between different prostheses but also between different zones in the same prosthesis. This determines that different materials are used not only for different prostheses but also in different areas of the same prosthesis; for instance, hip prostheses have a basal titanium alloy, with a femoral head of CoCr or ceramic that rotates on

Although the development of materials and treatments has allowed the durability and performance of these devices to be very high, there are still several problems associated with the prosthesis [8]. The most known are premature wear and dislocation in hip and knee prostheses, infection and rejection in dental prostheses, restenosis and heavy metal release in coronary stents, among others. In this way, infections are frequent complications in hospitals with dramatic consequences. It has been estimated that 80% of the infections in hospitals involve bacterial biofilms that have up to 1000 times higher resistance to antimicrobials than bacteria

In this context, advanced surface treatments are playing an essential role in improving performance and increasing the life of prostheses. The treatments of thermal projection of hydroxyapatite (HA) are an extended solution to increase the capacity of growth of the cells of the bone on alloys of titanium and stainless steels. Physical vapor deposition (PVD) treatments, both ceramic and diamond like carbon (DLC), are an effective tool for increasing wear resistance. Ionic implantation is a method used to decrease the migration of heavy ions to the body, and laser texturing is being effectively used to obtain antimicrobial surfaces in an effective strategy to interrupt infections. This work includes a review of the state of the art and industrial implementation of some of the most used surface treatment techniques for the improvement of the different types of prostheses, paying special attention to the most used

One of the ongoing research fields in the scientific community is the design of novel materials which can stimulate the bone regenerative process because the bone regeneration is a constant and continuous process in our lives, although the resultant regeneration speed shows a decreased tendency as a function of the age [11]. In addition, several studies indicate that every year over 2 million people worldwide require bone grafting surgery in order to repair large bone defects which are a very common problem in orthopedic surgery, being the main alternatives to repair these defects the use of autologous bone grafts, allografts, or biocompat-

solutions and the future possibilities of advanced surface treatment techniques.

**2. Plasma-sprayed hydroxyapatite coating**

a high-density polyethylene cup.

200 Advanced Surface Engineering Research

in the planktonic form [9, 10].

ible synthetic materials [12, 13].

Other aspect of great relevance is that the presence of hydroxyapatite particles can be also employed for the inhibition growth of different types of cancer cells [22–24]. In this sense, it is well documented that the use of nanosized hydroxyapatite particles can significantly increase the biocompatibility and bioactivity of man-made materials [25, 26]. A clear example can be found in [26] where a highly biocompatible hydroxyapatite nanopowder (known as GoHAP) has been successfully synthesized in a very short of period of time (range of 90 s). These GoHAP nanoparticles showed excellent biocompatibility properties (confirmed by in vitro tests) because no vacuolization or cell membrane lysis was found on the surface and the resultant cells presented a correctly flattened phenotype, maintaining morphology typical for bone cells. The experimental results clearly indicate that GoHAP could be a promising material for resorbable bone implant fabrication.

One of the most important applications is that as coatings deposited onto bioinert metallic implants can promote early bonding of bones with an increase of biological fixation. In this sense, it has to be mentioned that as coatings, they are not intended to substitute existing materials, although these HA coatings are used for an enhancement of a fully functional implant. Due to this, different deposition techniques such as sol-gel process [27–29], pulsed laser deposition [30–32], electrospinning [33, 34], sputtering [35–37], or plasma spray [38] can be found in the bibliography related to design of optimal HA coatings onto the surface of metallic implants. Among all these fabrication methodologies, it has to be mentioned that plasma spray technique is the only process which has been approved by US Food and Drug Administration (FDA) for coating implants with biocompatible materials [39]. The plasma

scanning electron microscopy (SEM), whereas the determination of the roughness has been performed by optical interferometry (IFM), as it can be observed in **Figure 3**. The experimental results indicate that lamellar bone formation in close contact with implant surfaces has

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**Figure 2.** Schematic diagram (cross section) of plasma spray torch. Reprinted with permission of [40].

A consideration related to the material concerned (HA) is that it reacts strongly to rapid solidification following the plasma spray, yielding the formation of amorphous or metastable phases. According to this, the presence of an amorphous phase is undesirable because the natural bone is crystalline, being the integrity of the bone-implant compromised [42]. In this sense, an ideal HA coating for biomedical implants should have low porosity, high cohesive strength, a good adhesion to the substrate, high degree of crystallinity, high chemical purity, and phase stability [43]. According to this, the optimization of the coating properties by just varying the plasma spray parameters is a concern for obtaining a coating with the desired characteristics. An interesting work can be found in [44] where several steps are recom-

However, mechanical tests indicate that the resultant HA coatings suffer poor mechanical properties (tensile strength, wear resistance, hardness, toughness, or fatigue), limiting its long-term application due to the relative movement between the implant and human bone, respectively. In order to overcome these mechanical limitations of HA coatings, the addition of different bioinert ceramic materials into HA matrix for reinforcement such as aluminum oxide [45, 46], zirconia [45], mixture of titania and zirconia [47], yttria-stabilized zirconium [48], or nanodiamond particles [49] have been evaluated. A representative

example can be found in [45] where two reinforced HA coatings with alumina (Al<sup>2</sup>

phase formation, and mechanical properties (hardness and tensile bond strength) as a function of as-sprayed coating and after postthermal treatment at 700°C for 1 h. The results indicate that after postcoating heat treatment, a dual effect has been observed such as an increase in the crystallinity and a decrease in the resultant porosity. This heat treatment enables an enhancement in cross-sectional hardness, although a decrease in bond strength

), respectively, have been analyzed in order to investigate the microstructure,

O3 ) and

mended in order to produce stable and adherent HA coatings.

been observed.

zirconia (ZrO2

has been also observed.

**Figure 1.** In vivo bone formation assessed by microcomputerized tomography (μ- CT) of control (A–C), poly (L-lactic acid) (PLLA) (D–F), HA/PLLA (G–I), and poly (γ-benzyl-L-glutamate)-modified hydroxyapatite/(poly (L-lactic acid) (PBLG-g-HA/PLLA) (J–L) scaffolds at 2, 4, and 8 weeks postimplantation. Reprinted with permission of [21].

spray process demands a control of several parameters for the design of optimal coatings (particle size range, distance between gun and substrate, arc current, power setting, particle morphology, plasma gas mixture, postspray treatment, etc.) [40]. A schematic representation of plasma spray torch is shown in **Figure 2**.

A novel study about the physical and chemical characterization of bioactive ceramic-coated plateau root form implant surface by using plasma-sprayed hydroxyapatite (PSHA) is presented in [41]. The surface characterization of the PSHA coatings has been performed by Advanced Surface Treatments for Improving the Biocompatibility of Prosthesis and Medical… http://dx.doi.org/10.5772/intechopen.79532 203

**Figure 2.** Schematic diagram (cross section) of plasma spray torch. Reprinted with permission of [40].

scanning electron microscopy (SEM), whereas the determination of the roughness has been performed by optical interferometry (IFM), as it can be observed in **Figure 3**. The experimental results indicate that lamellar bone formation in close contact with implant surfaces has been observed.

A consideration related to the material concerned (HA) is that it reacts strongly to rapid solidification following the plasma spray, yielding the formation of amorphous or metastable phases. According to this, the presence of an amorphous phase is undesirable because the natural bone is crystalline, being the integrity of the bone-implant compromised [42]. In this sense, an ideal HA coating for biomedical implants should have low porosity, high cohesive strength, a good adhesion to the substrate, high degree of crystallinity, high chemical purity, and phase stability [43]. According to this, the optimization of the coating properties by just varying the plasma spray parameters is a concern for obtaining a coating with the desired characteristics. An interesting work can be found in [44] where several steps are recommended in order to produce stable and adherent HA coatings.

However, mechanical tests indicate that the resultant HA coatings suffer poor mechanical properties (tensile strength, wear resistance, hardness, toughness, or fatigue), limiting its long-term application due to the relative movement between the implant and human bone, respectively. In order to overcome these mechanical limitations of HA coatings, the addition of different bioinert ceramic materials into HA matrix for reinforcement such as aluminum oxide [45, 46], zirconia [45], mixture of titania and zirconia [47], yttria-stabilized zirconium [48], or nanodiamond particles [49] have been evaluated. A representative example can be found in [45] where two reinforced HA coatings with alumina (Al<sup>2</sup> O3 ) and zirconia (ZrO2 ), respectively, have been analyzed in order to investigate the microstructure, phase formation, and mechanical properties (hardness and tensile bond strength) as a function of as-sprayed coating and after postthermal treatment at 700°C for 1 h. The results indicate that after postcoating heat treatment, a dual effect has been observed such as an increase in the crystallinity and a decrease in the resultant porosity. This heat treatment enables an enhancement in cross-sectional hardness, although a decrease in bond strength has been also observed.

spray process demands a control of several parameters for the design of optimal coatings (particle size range, distance between gun and substrate, arc current, power setting, particle morphology, plasma gas mixture, postspray treatment, etc.) [40]. A schematic representation

**Figure 1.** In vivo bone formation assessed by microcomputerized tomography (μ- CT) of control (A–C), poly (L-lactic acid) (PLLA) (D–F), HA/PLLA (G–I), and poly (γ-benzyl-L-glutamate)-modified hydroxyapatite/(poly (L-lactic acid)

(PBLG-g-HA/PLLA) (J–L) scaffolds at 2, 4, and 8 weeks postimplantation. Reprinted with permission of [21].

A novel study about the physical and chemical characterization of bioactive ceramic-coated plateau root form implant surface by using plasma-sprayed hydroxyapatite (PSHA) is presented in [41]. The surface characterization of the PSHA coatings has been performed by

of plasma spray torch is shown in **Figure 2**.

202 Advanced Surface Engineering Research

Concerning to the PVD processes, there are three main methods that are more widespread such as the electron beam (EB), the cathodic arc (CA), and the magnetron sputtering (MS), respectively. On the other hand, new concept in magnetron sputtering systems has been developed using high-power pulses (HIPIMS), making possible a significant increase in plasma ionization, and as a result, a considerable enhancement in the resultant adhesion of the coatings has been obtained [56, 57]. In **Figure 5**, a comparison between high-power impulse magnetron sputtering (HIPIMS), direct current magnetron sputtering (DCMS), and modulated pulsed power magnetron sputtering (MPPMS) is presented as function of power

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One of the research lines where PVD coatings have shown a high degree of novelty is in the protection against joint wear. The first experiences date back to the decade of the 1980s of the last century where TiN in total join arthroplasty was used as well as clinical trials were started

**Figure 4.** Scheme of the Metaplas Ionon MZR 323 arc evaporation PVD system. Courtesy of AIN.

(HIPIMS), and modulated pulsed power magnetron sputtering (MPPMS), respectively.

**Figure 5.** Power versus time in direct current magnetron sputtering (DCMS), high-power impulse magnetron sputtering

and time, respectively.

in the 1990s in knee and hip arthroplasty [58, 59].

**Figure 3.** SEM intermediate micrograph (a) high magnification micrograph (b) and IFM three-dimensional reconstruction for PSHA coatings (c). Reprinted with permission of [41].
