Deposition Of Hydroxyapatite Coatings Research Articles

Advances in coatings on Mg alloys and their anti-microbial activity for implant applications

Magnesium matrix composites reinforced by calcium phosphate could not show the desired effect on the magnesium breakdown rate. Rapid disintegration rate limited the magnesium alloys used as biodegradable implant material. The rate of degradation can be minimized and biological activity can be improved in the magnesium alloy by Hydroxyapatite (HA) coating with the improvement of bone induction and conduction abilities. Various alkali post-treatment and conversion coating methods are applied to deposit HA coatings and biocompatible dicalcium phosphate dihydrate (DCPD) on magnesium alloy so that corrosion resistance and surface biocompatibility can be improved to be used in bone tissue engineering applications. Magnesium's corrosion resistance will weaken its antibacterial properties, which are linked to and proportional to the alkaline pH at the time of breakdown. The goal of this study is to bring together and compare contemporary research on different coatings on magnesium and related alloys in relation to antibacterial functionalized activities. A though review has been performed on in vivo and in vitro cytocompatibility, material property, corrosion resistance, and antibacterial properties of the coatings. Increased degradation behavior, biocompatibility, and bioactivity have been achieved following multiple procedures such as alkali treatment with HA electrochemical deposition on magnesium alloy. Multifunctional coatings can make safe and bioactive magnesium alloy surfaces for biodegradable implant applications.

Deposition of hydroxyapatite coatings by axial plasma spraying: Influence of feedstock characteristics on coating microstructure, phase content and mechanical properties

Axial plasma spray is one of the thermal spray techniques to deposit multifunctional advanced coatings. The present work explores the use of this process to deposit thin, continuous, and adherent Ca5 (PO4)3OH (hydroxyapatite, HAp) coatings and characterize its microstructure, phases, hardness and adhesion strength. Three different suspension-deposited HAp coatings were investigated and compared with powder-deposited HAp coating on a Ti6Al4V substrate. The effect of mean solute particle size and solid-loading in the suspension has been explored on the evolution of microstructure, phase content and mechanical properties of axial suspension plasma sprayed (ASPS) coatings. Phase-characterization has shown retention of hydroxyapatite phase and coating crystallinity in the deposited coatings, whereas the adhesion strength of the HAp coating decreased from ∼40 MPa to ∼13 MPa when bioglass was added to the feedstock material. The lower solid load content and lower mean solute particle size in the suspension were found to be beneficial in achieving porous, rougher, and well-adhering coatings. This work concludes that ASPS can potentially deposit thin HAp coatings (< 50 μm) with high adhesion strength.

Pulsed electrodeposition of compact, corrosion resistant, and bioactive HAp coatings by application of optimized magnetic field

NiTi shape memory alloy possess spectacular properties, so it has a great potential to use as bio-implants. However, releasing toxic Ni ions from its surface have restricted its successful use in human's body. The present research attempts to modify the surface conditions of NiTi through deposition of calcium phosphate (CaP) coatings to overcome the abovementioned challenge, besides improving the other critical characteristics required for successful use of NiTi in bio-related applications. CaP deposits were deposited on NiTi by means of pulsed electrodeposition under magnetic field. Since the hydroxyapatite (HAp) has the closest Ca/P ratio to that of human's bone among the other CaP types, and considering its superior bioactivity, the processing variables including pulse current density, deposition time, as well as magnetic field direction and magnitude were adjusted in such a way that the Ca/P ratio of HAp obtains. The aim behind application of magnetic field was to attain favorable morphological and structural properties. Then, the characteristics of coatings electrodeposited under optimized conditions were investigated via Fourier transform Infrared (FTIR) and X-ray diffraction (XRD). Electrochemical potentiodynamic polarization studies test in Ringer's solution was performed to assess the corrosion behavior of the coatings. A noticeable increment in polarization resistance of NiTi, about 10 times, is obtained with deposition of the optimized coatings. Ni ion release was found to decrease by 89% after deposition of HAp coatings on NiTi. In vitro bioactivity of the coatings were studied through immersion of HAp coatings in simulated body fluid (SBF). Nucleation of apatite particles all over the coating microstructure demonstrate the desirable bioactivity of the deposits. In general, compact, corrosion resistive, and bioactive HAp coating with minimal Ni ion release were deposited on NiTi by controlling the operating conditions.

Atmospheric Plasma-Sprayed Hydroxyapatite Coatings with (002) Texture

Hydroxyapatite (HA) coatings are being widely used in biomedical applications owing to their excellent biocompatibility and osteoconductivity. Recent studies have demonstrated that the crystallographic texture plays an important role in improving the chemical stability and mechanical properties of HA coatings. In this study, optimized APS parameter was selected to deposit HA coatings with strong (002) crystallographic texture, high phase purity and enhanced melting state. Cross-sectional SEM images show uniformly distributed columnar grains perpendicular to the coating surface. To study the formation conditions of columnar grains, coatings with distinct microstructure were deposited with different spray parameters. Moreover, HA coatings were deposited on substrates with varying temperatures such as 25, 300 and 600 °C at a long stand-off distance to evaluate the role of the substrate temperature in the formation of columnar grains. The results indicate that completely molten in-flight particles and slow cooling rate are necessary conditions to form a strong crystallographic texture. The present study suggests that the crystalline structure of HA coatings deposited and formed by APS could be well controlled by modifying spray parameters and substrate temperature.

Bone marrow derived mesenchymal stem cell response to the RF magnetron sputter deposited hydroxyapatite coating on AZ91 magnesium alloy

The aim of this study was to investigate adhesion of bone marrow stromal cells (BMSCs) and in vitro degradation of hydroxyapatite (HA) coatings deposited on the surface of magnesium alloy (AZ91). The uniform and pore-free HA coating was successfully obtained via RF magnetron sputter deposition at the substrate bias of −25 and substrate bias of −100 V followed by post-deposition annealing. The structural features of the HA coating have been characterized and biological performance of the HA coating has been examined via in vitro test. The X-ray diffraction and Energy dispersive X-ray analysis data revealed that the phase and elemental composition of the deposited HA coatings were similar, but the surface roughness and morphology were influenced by the substrate bias voltage. The coating deposited at −25 V consists of multiple grain-boundaries. In contrast, no grain boundary but rather flat surface was observed for the sample deposited at −100 V. Atomic force microscopy (AFM) was used to obtain topographic images of the obtained coatings and quantitative assessment of the surface roughness. The roughness parameters of the deposited coatings were found to display nanometer scale surface roughness (Sa and Sq) from 92 to 130 nm over areas of 5 × 5 μm2. The in vitro direct culture experiment showed that the HA coating deposited at the substrate bias of −100 V significantly reduced the release of Mg ions in post-culture media and revealed higher BMSCs adhesion density than that on the HA coating deposited at the bias of −25 V and the uncoated AZ91 alloy. Thus, this study demonstrated that the negative pulse substrate bias applied for HA coating deposition allows to improve the surface features of the AZ91 magnesium alloy and enhance BMSCs adhesion in vitro.

Self‐Assembly of L‐Arginine on Electrophoretically Deposited Hydroxyapatite Coatings

AbstractIn this study, L‐Arginine (L‐Arg) molecules are self‐assembled on HA coatings for potential orthopedic applications. By immersing HA coatings in L‐Arg solution in phosphate buffer saline (PBS) with a pH of 7.4 at room temperature for 24 hours, L‐Arg assembled on HA. The samples were characterized by using Scanning Electron Microscopy (SEM), X‐ray Photoelectron Spectroscopy (XPS), Attenuated Total Reflection‐Fourier Transform Infrared Spectroscopy (ATR‐FTIR), X‐ray Diffraction (XRD), Atomic Force Microscopy (AFM) and ninhydrin assay. The response of human bone marrow derived mesenchymal stem cells (hBMSCs) to HA and L‐Arg/HA coatings both in growth and osteogenic differentiation medium are reported. In the growth medium, cells are found to be more spread out on L‐Arg/HA coatings than HA coatings. Moreover, cell aggregates which are indicators of differentiation are observed on HA coatings spontaneously in the growth medium and cell aggregates are only observed on L‐Arg/HA coatings in the osteogenic differentiation medium. Overall, this study showed that L‐Arg self‐assembles on HA and it hinders osteogenic differentiation of hBMSCs in the growth medium.

Hydroxyapatite coatings on Mg-Ca alloy prepared by Pulsed Laser Deposition: Properties and corrosion resistance in Simulated Body Fluid

Magnesium (Mg) alloys are very promising biocompatible materials for biodegradable biomedical implants, however, the main problem in using them is their fast degradation in the conditions of human body. In this work, we coated Mg-Ca (calcium) alloy substrate with hydroxyapatite (HA) to improve its resistance to corrosion and to control the in vitro degradation. Pulsed Laser Deposition technique was applied to deposit HA coatings. Their properties were investigated by X-ray diffraction, atomic force microscopy, scanning electron microscopy, high resolution transmission electron microscopy, Vickers microhardness, and Tafel plot electrochemical technique. The substrate temperature was ranged in the interval from room up to 400 °C. According to the obtained results, the recommended substrate temperatures are 200–300 °C, since higher ones lead to the HA decomposition. Furthermore, the HA films deposited at 200 and 300 °C show good corrosion resistance in Simulated Body Fluid. The obtained coatings are promising for a perspective use in biomedical implant applications.

Mejoramiento de la adherencia en recubrimientos de Hidroxiapatita elaborados mediante proyección térmica oxiacetilénica, a partir de resultados de simulación numérica

Los resultados de la simulación numérica realizada con el software Jets et Poudres fueron utilizados para depositar recubrimientos de hidroxiapatita mediante proyección térmica oxiacetilénica, con el fin de aumentar su cohesión y adhesión sobre sustratos de Ti6Al4V. Se determinó el efecto de una llama neutra, una oxidante y una súper-oxidante, así como de la distancia de proyección (7, 9,5 y 12 cm) entre el sustrato y la antorcha, sobre el número adimensional de K-Sommerfeld (K). Dicho número adimensional es influenciado por la distancia de proyección y no por las llamas evaluadas. Para recubrimientos fabricados a una distancia de 12 cm el número de Sommerfeld varía entre 34 y 37, lo que les confiere una estructura más homogénea que la de aquellos depositados a 7 y 9,5 cm en los que existe un alto contenido de partículas parcialmente fundidas (K=0), o con salpicaduras (K&gt;75), que reducen la adhesión de los recubrimientos.

Surface morphology and in vitro bioactivity of biocompatible hydroxyapatite coatings on medical grade S31254 steel by RF magnetron sputtering deposition

Hydroxyapatite (HA) is popularly used as a biocompatible coating material for metallic implants, in view of its improved bone fixation property, leading to an increased life of the implant. However, the deposition of HA on medical grade UNS S31254 stainless steel (SS254) for orthopaedic implant applications by the radio-frequency magnetron sputtering technique is unreported in the literature so far. The surface morphology of the deposited HA coatings was characterised using scanning electron microscopy and atomic force microscopy, while their phase composition was determined using X-ray diffraction. The thickness and adhesive strength of the HA coatings were determined using an ellipsometer and a tensometer, respectively. Finally, the antibacterial efficacy and bioactivity of the deposited coatings were confirmed using fluorescence activated cell sorting and immersion testing in simulated body fluid environment. The results obtained showed that the HA coatings grown on SS254 using magnetron sputtering possess desirable surface properties as well as good adhesion and biocompatibility properties, ideally suited for potential applications in orthopaedic implants.

RF magnetron-sputtered coatings deposited from biphasic calcium phosphate targets for biomedical implant applications

Bioactive calcium phosphate coatings were deposited by radio-frequency magnetron sputtering from biphasic targets of hydroxyapatite and tricalcium phosphate, sintered at different mass % ratios. According to Raman scattering and X-ray diffraction data, the deposited hydroxyapatite coatings have a disordered structure. High-temperature treatment of the coatings in air leads to a transformation of the quasi-amorphous structure into a crystalline one. A correlation has been observed between the increase in the Ca content in the coatings and a subsequent decrease in Ca in the biphasic targets after a series of deposition processes. It was proposed that the addition of tricalcium phosphate to the targets would led to a finer coating's surface topography with the average size of 78 nm for the structural elements.

Electrophoretic deposition (EPD) of nanohydroxyapatite - nanosilver coatings on Ti13Zr13Nb alloy

Abstract Titanium and its alloys are the biomaterials most frequently used in medical engineering, especially as parts of orthopedic and dental implants. The surfaces of titanium and its alloys are usually modified to improve their biocompatibility and bioactivity, for example, in connection with the deposition of hydroxyapatite coatings. The objective of the present research was to elaborate the technology of electrophoretic deposition (EPD) of nanohydroxyapatite (nanoHAp) coatings decorated with silver nanoparticles (nanoAg) and to investigate the mechanical and chemical properties of these coatings as determined by EPD voltage and the presence of nanoAg. The deposition of nanoHAp was carried out at two voltage values, 15 and 30 V. The decoration of nanoHAp coatings with nanoAg was carried out using the EPD process at a voltage value of 60 V and a deposition time of 5 min. The thickness of the undecorated coatings was found to be 2.16 and 5.14 µm for applied EPD voltages of 15- and 30-V, respectively. The release rate of silver nanoparticles into an artificial saliva solution increased with exposure time and EPD voltage. The corrosion current, between 1 and 10 nA/cm 2 , was significantly higher for undecorated nanoHAp coatings and close to that of the substrate for decorated nanoHAp coatings. The hardness of the undecorated nanoHAp coatings obtained at 15 and 30 V of EPD voltage attained 0.2245±0.036 and 0.0661±0.008 GPa, respectively. Resistance to nanoscratching was higher for thicker coatings. The wettability angle was lower for coatings decorated with nanoAg.

Tribological behaviour of RF-magnetron sputter deposited hydroxyapatite coatings in physiological solution

The aim of the paper is to explore the tribological performance of hydroxyapatite (HA) coatings deposited by radio frequency (RF) magnetron sputtering on AZ31 magnesium alloy (96% Mg, 3% Al, 0.7% Zn, 0.3% Mn) for biomedical applications. In this study, the position of the samples on a substrate holder, relative to a target erosion zone was taken into consideration in order to elucidate its impact on the coating characteristics, such as composition, morphology, surface topography and tribology. Substrate rotation and arc-movement were foreseen in the experimental set-up to increase the uniformity of thin film properties. The deposited HA thin films were revealed to exhibit an increase of the Ca/P ratio from 1.83 to 1.97, a decrease of (002) texture and thickness, as the samples were shifted towards the target erosion zone. By coatings, the roughness of Mg alloy was decreased (Ra Mg alloy=31.3nm; Ra coating=29nm and 21nm). The coating placed in the centre of the substrate holder showed high hardness and Young's modulus (H =8.3±0.9GPa; E=89±10GPa) than the coating prepared under the target erosion zone (H =6.9±1.1GPa; E=75±6GPa). The coating deposited under target erosion zone exhibits superior friction behaviour in simulated body fluid environment, with the friction coefficient (μ) of 0.184, while the sample located in the centre of the substrate holder possesses the friction coefficient (0.306) comparable to the AZ31 substrate (0.307). The low wear rate was determined in the case of coating deposited under target erosion zone (4.83×10−5mm3 N−1 m−1) than uncoated AZ31 substrate (0.00518mm3 N−1 m−1) or than coating placed in the centre of the substrate holder (0.00294mm3 N−1 m−1).

The deposition of strontium and zinc Co-substituted hydroxyapatite coatings.

The in vitro and in vivo performance of hydroxyapatite (HAp) coatings can be modified by the addition of different trace ions, such as silicon (Si), lithium (Li), magnesium (Mg), zinc (Zn) or strontium (Sr) into the HAp lattice, to more closely mirror the complex chemistry of human bone. To date, most of the work in the literature has considered single ion-substituted materials and coatings, with limited reports on co-substituted calcium phosphate systems. The aim of this study was to investigate the potential of radio frequency magnetron sputtering to deposit Sr and Zn co-substituted HAp coatings using Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The FTIR and XPS results highlight that all of the Sr, Zn and Sr-Zn co-substituted surfaces produced are all dehydroxylated and are calcium deficient. All of the coatings contained HPO42- groups, however; only the pure HAp coating and the Sr substituted HAp coating contained additional CO32- groups. The XRD results highlight that none of the coatings produced in this study contain any other impurity CaP phases, showing peaks corresponding to that of ICDD file #01-072-1243 for HAp, albeit shifted to lower 2θ values due to the incorporation of Sr into the HAp lattice for Ca (in the Sr and Sr-Zn co-substituted surfaces only). Therefore, the results here clearly show that RF magnetron sputtering offers a simple means to deliver Sr and Zn co-substituted HAp coatings with enhanced surface properties. (a) XRD patterns for RF magnetron sputter deposited hydroxyapatite coatings and (b)-(d) for Sr, Zn and Sr-Zn co-substituted coatings, respectively. The XPS spectra in (b) confirms the presence of a HA sputter deposited coating as opposed to

Hydroxyapatite thin films grown by pulsed laser deposition and matrix assisted pulsed laser evaporation: Comparative study

Pulsed Laser Deposition (PLD) and Matrix Assisted Pulsed Laser Evaporation (MAPLE) techniques were applied for growing hydroxyapatite (HA) thin films on titanium substrates. All experiments were conducted in a reaction chamber using a KrF* excimer laser source (λ=248nm, τFWHM≈25ns). Half of the samples were post-deposition thermally treated at 500°C in a flux of water vapours in order to restore crystallinity and improve adherence.Coating surface morphologies and topographies specific to the deposition method were evidenced by scanning electron, atomic force microscopy investigations and profilometry. They were shown to depend on deposition technique and also on the post-deposition treatment. Crystalline structure of the coatings evaluated by X-ray diffraction was improved after thermal treatment. Biocompatibility of coatings, cellular adhesion, proliferation and differentiation tests were conducted using human mesenchymal stem cells (MSCs). Results showed that annealed MAPLE deposited HA coatings were supporting MSCs proliferation, while annealed PLD obtained films were stimulating osteogenic differentiation.

Surface morphology, bioactivity, and antibacterial studies of pulsed laser deposited hydroxyapatite coatings on Stainless Steel 254 for orthopedic implant applications

Coating of hydroxyapatite using the pulsed laser deposition technique, on medical grade UNS S31254 stainless steel (254SS), to yield a biomaterial for potential orthopedic implant applications, is unreported so far in the literature. In this paper, the pulsed laser deposition process was used to improve the physiological response of 254SS. The surface morphology of the deposited hydroxyapatite coatings was characterized using scanning electron microscopy and atomic force microscopy, while the phase composition of the deposited hydroxyapatite coatings was determined using the X-ray diffraction method. The thickness and adhesive strength of the hydroxyapatite coatings were determined using an ellipsometer and a tensometer, respectively. The antibacterial efficacy of the deposited hydroxyapatite coatings was confirmed using the modern technique of fluorescence-activated cell sorting. Finally, the bioactivity of hydroxyapatite coatings was investigated by conducting immersion test in simulated body fluid environment. The scanning electron microscopy and atomic force microscopy results revealed higher (∼8 nm) average surface roughness, which is likely to facilitate better osseointegration. X-ray diffraction analysis confirmed that postdeposition annealing is essential to achieve the desired crystallinity and uniformity of coatings. Tensile pull-out tests confirmed adhesive strength of hydroxyapatite coatings beyond the standard expected values. Immersion tests inferred high bioactivity of pulsed laser deposition hydroxyapatite coatings. The promising results obtained in this research signify the potential application of hydroxyapatite coatings in orthopedic implants.

The Effect of Pores in Dual Nano Hydroxyapatite Coating on Thermally Oxidized Commercial Pure Titanium : Mechanical and Histological Evaluation

Background:In this study,TiO2 layer was thermally grown as a diffusion barrier on CP Ti substrate prior to electrophoretic deposition of HA coatings, to improve the coating’s compatibility also macro and micro pores in nano Hydroxyapatite dual coatings were created and their effect on the bond strength between the bone and implant was evaluated. Materials and methods: Electrophoretic Deposition technique (EPD) was used to obtain coatings for each one of four types of Hydroxyapatite(HA)on CP Ti screws (micro HA, nano HA, dual nano HA with micro pores, dual nano HA with macro pores) where carbon particles used as fugitive material to be removed by thermal treatment to create porosity.For examination of the changes occurred on the substrate, SEM, SPM and XRD used, coatings characterized by XRD, SEM and interfacial shear strength measurements. Results:The results mentioned the formation of rutilenano TiO2 with, SEM showed that the size of pores in HA coatings corresponded to the size of carbon particles. Statistical analysis of the removal torque tests showed highest means of the single nano HA coating at 2 and 4 weeks implantation intervals. Histological analysisrevealed a faster reaction of bone and higher osteoblasts activity towards thermally oxidized CP Ti implants coated with single nano HA coating. Conclusion:Carbon particles as a fugitive material within nano HA coat produced porosity.Presence of pores ˃ 1µ in nano HA coats did not achieve highest removal torque values nor highest osteoblasts activity in 2 and 4 weeks implantation intervals. Keywords:Titanium, Thermal oxidization, Nano Hydroxyapatite, Coatings, porosity.

Real-time measurement of protein adsorption on electrophoretically deposited hydroxyapatite coatings and magnetron sputtered metallic films using the surface acoustic wave technique

Surface acoustic wave (SAW) biosensors are highly sensitive for mass binding and are therefore used to detect protein–protein and protein–antibody interactions. Whilst the standard surface of the chips is a thin gold film, measurements on implant- or bone-like surfaces could significantly enhance the range of possible applications for this technique. The aim of this study was to establish methods to coat biosensor chips with Ti, TiN, and silver-doped TiN using physical vapor deposition as well as with hydroxyapatite by electrophoresis. To demonstrate that protein adsorption can be detected on these surfaces, binding experiments with fibronectin and fibronectin-specific antibodies have been performed with the coatings, which successfully proved the applicability of PVD and EPD for SAW biosensor functionalization.

Electrophoretic Deposition of Hydroxyapatite Coatings on AZ31: The Effect of Nanoparticle Multiple Coating Approach

Magnesium and its alloys are potential biodegradable implant materials. However, they are characterized by rapid degradation in the electrolytic environment of the body. This phenomenon might result a sudden implant failure before bone restoration was complete, or inflammation subsided. This research will explore ways to improve the corrosion resistance of AZ31 magnesium alloy by improving the coating layer of hydroxyapatite (HA) through multiple coating layers by an electrophoretic deposition (EPD) process. In this study, the quality of the coating layer was improved by multiple coating processes without using any binders. X-ray diffraction spectrometer (XRD) showed that an amorphous structure of HA was successfully deposited on the AZ31 alloy. Scanning electron microscopy (SEM) has been used to observe that the morphology of the AZ31 alloy coated with multiple layers of HA has a denser coating structure with improved adhesion at the interface as compared to the single coating layer. A denser coating structure with greater bonding between the coating layer and the substrate is expected to protect the substrate from a high corrosion rate, thus resulting in a longer period of biodegradation as in implant in the electrolytic environment.

Titanium Substrate Surfaces Coated with Hydroxyapatite by Magnetron Sputtering

The deposition of hydroxyapatite coatings on titanium via sputtering techniques has been quite studied on commercial dense substrates, for use as a biomaterial. In this work, porous titanium samples produced by powder metallurgy and commercially dense titanium sheet, used as control, were used as substrates. The coatings were deposited by radio frequency magnetron sputtering using a hydroxyapatite target in argon atmosphere with different deposition times. Samples characterization was performed by Optical Microscopy, Scanning Electron Microscopy/Energy Dispersive Spectroscopy and low-angle X-ray Diffraction. Hydroxyapatite coating depositions were obtained on both titanium substrates. The results indicated the potential of this methodology for titanium substrates with homogeneous hydroxyapatite coatings.

Development of electrophoretically deposited hydroxyapatite coatings on anodized nanotubular TiO2 structures: Corrosion and sintering temperature

Hydroxyapatite (HA) coatings in and onto anodized TiO2 nanotube arrays were presented and prepared by electrophoretic deposition technique (EPD). Coatings were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). EPD proved to be an innovative and versatile technique to coat HA on and into nanotubular structures of TiO2 with enhanced adhesion between nanotubes and HA particles provided by mechanical interlocking. After EPD of HA on TiO2 layer, samples were sintered at 400°C, 600°C and 800°C for 2h in an Ar atmosphere. Effect of EPD processing parameters on thickness of the deposits and rate of deposition was elucidated for HA coatings on the nanotubular TiO2 structures. It was shown that higher applied voltages increase deposition rate and thickness of the coatings. Potentiodynamic polarization measurements proved corrosion protection caused by both HA coating and nanotubular TiO2 structure in simulated body fluid (SBF). Effect of sintering temperature on adhesion strength of HA coatings on TiO2 nanotubes and their composition were also studied.