ISSN 20751133, Inorganic Materials: Applied Research, 2011, Vol. 2, No. 5, pp. 474–481. © Pleiades Publishing, Ltd., 2011. Original Russian Text © S.V. Gnedenkov, Yu.P. Scharkeev, S.L. Sinebryukhov, O.A. Khrisanfova, E.V. Legostaeva, A.G. Zavidnaya, A.V. Puz’, I.A. Khlusov, 2011, published in Perspektivnye Materialy, 2011, No. 2, pp. 49–59.
MATERIALS FOR INSURING HUMAN LIFE ACTIVITY AND ENVIRONMENT PROTECTION
Formation and Properties of Bioactive Surface Layers on Titanium S. V. Gnedenkova, Yu. P. Scharkeevb, S. L. Sinebryukhova, O. A. Khrisanfovaa, E. V. Legostaevab, A. G. Zavidnayaa, A. V. Puz’a, and I. A. Khlusovc a
Institute of Chemistry, Far East Branch, Russian Academy of Sciences, Vladivostok, 690022 Russia email:
[email protected],
[email protected],
[email protected],
[email protected],
[email protected] b Institute of Strength Physics and Materials Science, Siberian Branch, Russian Academy of Sciences, Tomsk, 634021 Russia email:
[email protected],
[email protected] c Siberian State Medical University, Tomsk, 634050 Russia email:
[email protected] Received July 29, 2010
Abstract—The plasma electrolytic oxidation technique was used to form a calcium phosphate coating con taining hydroxyapatite on a titanium implant surface. The composition, morphology, and anticorrosion and mechanical properties of the obtained layers were studied. The optimal conditions of the polarization mode and electrolyte composition were found. Experiments performed in vivo and in vitro showed that the bioac tivity of deposited films depends on the chemical composition (concentrations and Ca/P ratio) and the sur face roughness of calcium phosphate layers. Keywords: titanium, hydroxyapatite, implant, plasma electrolytic oxidation, anticorrosion properties, elasto plastic properties, osteogenesis. DOI: 10.1134/S2075113311050133
INTRODUCTION Artificial materials are widely used across different fields of modern medicine as prostheses for damaged tissues and organs. Depending on conditions, these implants are intended to gradually dissipate, being replaced by living tissues, or remain unchanged, func tioning for a long time. At present, titanium and its alloys are often the compounds of choice due to their high though not perfect anticorrosion and mechanical properties. The ideal implant material for bone replace ment should be noncorrosive with the mechanical properties similar to natural bones. It has to also stimu late osteogenesis and induce no immune response while integrating well with the surrounding bone tissue. Bioactive ceramics based on hydroxyapatite and cal cium phosphate are considered as prospective implant compounds owing to their biocompatibility and chem ical composition that is similar to the mineral compo nent of bone tissue [1]. Thus, the physicochemical properties of hydroxyapatite Ca10(PO4)6(OH)2 provide nearly ideal biocompatibility by stimulating processes of osteogenesis and regeneration of bone tissue, and it is no wonder that hydroxyapatitemade implants have been used for some time already in dental and trauma tological medicine and orthopedic and plastic surgery. Apart from it, nonstoichiometric hydroxyapatite (Ca10 – x(HPO4)x(PO4)6 – x(OH)2 – x, where 0 < x < 1) [2], and calcium phosphatebased materials are also widely applied in the reconstructive surgery of injured
bones. At the same time, it was shown [3] that, besides the chemical composition, the implant surface mor phology plays an important role largely determining the immune response of the body. In addition, a certain roughness and porosity of the titanium surface are required to increase the adhesive strength between a calcium phosphate bioactive coating and substrate. Because of this, the surface of titanium implants is often subjected to some sort of mechanical treatment or chemical etching [4]. Presently, hydroxyapatite in the form of fine parti cles of different sizes and shapes can be synthesized by several solutionbased techniques [5]. Also, a few types of bioceramic materials able to form coatings on the implant surface were developed: for instance, inert alumina (Al2O3), surfaceactive (bioglasses), and resorbable (tricalcium phosphate and hydroxyapatite) ones [6]. Generally, solgel, electrochemical oxida tion, and plasma spraying are commonly used tech niques for depositing a calcium phosphate film on tita nium. The presence of such a bioactive layer signifi cantly reduces the side effects accompanying the application of an implant without one. Currently, one of the most popular technologies of metal surface modification is plasma electrolytic oxi dation (PEO), also known as microarc oxidation (MAO) [7]. Recently, we described the application of the PEO technique for producing a calcium phosphate layer containing hydroxyapatite on titanium (trade
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mark VT10) [8]. The results on the morphology, phase and elemental composition, and anticorrosion, mechanical, and elastoplastic properties of the obtained film were found to be promising for applica tion in implant surgery. Up to now, there have not been enough data on what is the optimal combination of physiochemical properties (porosity, surface roughness, etc.) that are required for successful application of produced bioac tive layers as implants. The aim of the present work is to study the bioac tivity of PEOobtained calcium phosphate coatings as a function of their composition, morphology, and other physical and chemical parameters. EXPERIMENTAL The tested samples of bioactive calcium phosphate films were formed on the surface of squareshaped tita nium plates with round corners (10 mm × 10 mm × 1 mm) by the plasma electrolytic oxidation technique as described in [9]. All plates were mechanically pretreated to standardize the titanium surface. The electrolytes of various composition were composed of aqueous solu tions of calcium citrate (Ca3(C6H5O7) ⋅ 4H2O or cal cium acetate Ca(CH3COO)2 ⋅ 2H2O and sodium monohydrophosphate Na2HPO4 ⋅ 2H2O as the sources of calcium and phosphate ions, respectively. The plasma electrolytic oxidation was performed under the unipolar (anode) and bipolar current (anode–cathode) modes generated by a processing system consisting of a TER4100.460N22UKhL4 reversible thyristor power supply and an automated command and control system. These allowed real time monitoring and changing of the parameters of the technological process. The bioactive coatings were formed by immersing the titanium samples into the electrolyte solution fol lowed by the PEO process under the bipolar or the uni polar mode. The potential of 350–380 V and current density of 0.67 A/cm2 were applied for 10 min under galvanostatic (unipolar) conditions. In the bipolar mode for 5 min the anodic component was ramped from 0 up to 280 V at the rate of 0.25 V/s, while the cathodic component was kept in galvanostatic mode at a current density of 1.0 A/cm2. The electrochemical properties of the formed lay ers were studied by potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) using a 12558 WB electrochemical system (Solartron Ana lytical, UK) and CorrWare/ZPlot (Scribner Associ ates Inc., USA) software. All measurements were con ducted in a 0.5 M aqueous solution of NaCl at room temperature using a niobium mesh coated with plati num as the counter electrode and a saturated KCl Ag/AgCl electrode as the reference electrode (E = +0.201 V, all potentials in present work are given ver sus Ag/AgCl electrode). The surface area of working electrode (titanium specimen) was 0.72 cm2. The INORGANIC MATERIALS: APPLIED RESEARCH
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impedance spectra were acquired using a sinusoidal ac perturbation with an amplitude of 10 mV. The mechanical and elastoplastic properties (microhardness, elastic modulus) of the studied coat ings were examined with a DUHW201 dynamic ultramicrohardness tester (Shimadzu, Japan) using a Berkovich indenter tip (a diamond threesided pyra mid). The film thickness was measured by observing a vertical crosssectional profile using the optical system of the microhardness tester. The phase composition of coatings was studied using a D8 Advance Xray diffractometer (XRD) (Bruker, Germany) with a CuKα source. The elemen tal composition and the surface morphology were characterized with a scanning electron microscope (SEM) (S5500, Hitachi, Japan). The in vivo studies of coated titanium plates were performed using 15 healthy male BALB/c mice at the Institute of Pharmacology of the Tomsk Science Center (Siberian Branch, Russian Academy of Medical Sci ences) by inserting two implants under the skin of each anesthetized animal. The plates were pretreated with a column of syngeneic bone marrow (average surface area of 7.5 mm2) taken from the femurs under aseptic condi tions to provide a source of multipotent mesenchymal stromal cells (MMSCs) and growth factors. The adhe sion of bone marrow on implant specimens was per formed by cultivating of a cell culture for 45 min in RPMI 1640 medium (ICN) with 5% fetal calf ferum (ICN). No formation of bone tissue (tissue aggregates) was observed when marrow samples and implants were introduced under the skin separately. The coated tita nium plates were harvested 45 days after implantation and the images were captured by a digital camera under fixed parameters. For histological imaging, a standard brightfield microscopy technique was used. The tissue aggregates grown on implants were decalcified, embed ded in paraffin, thinsectioned (10 μm) perpendicular to the surface of titanium plate, and then treated with hematoxylin and eosin stains. The surface roughness was estimated with a Taly surf 5120 profilometer (resolution up to 1 nm). Pro file roughness parameter Ra was found according to the Russian standard GOST 278973 as the arithmetic average of vertical deviations of the roughness profile over several sampling lengths. Computerassisted morphometry was conducted for quantitative assessment of the obtained sections using ImageJ 1.34p image analysis software (gray level differ ence statistics algorithm). Statistical data analyses were performed using the Excel 2007 and Statistica 6.0 software packages. Appli cation of the Kholmogorov–Smirnov test to the obtained data sets showed some deviation from the assumed normal distribution. Therefore, the Mann– Whitney Utest was used to evaluate the statistical sig nificance of observed difference. The regression analy sis and Spearman’s rank correlation coefficient (r) were No. 5
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Ca10(PO4)6(OH)2 Ca3(PO4)2
30 20 10 0 10
20
30
40 50 Angle, deg
60
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Fig. 1. XRD diffractogram of calcium phosphate coating formed from an acetatebased electrolyte.
used to determine the statistical dependence between studied variables. These were considered to be indepen dent at a value of ρ < 0.05. RESULTS AND DISCUSSION As a result of our investigation, various calcium phosphate coatings including one that contained hydroxyapatite were formed on the surface of titanium VT10 using the plasma electrolytic oxidation tech nology (Fig. 1). With the aim to promote the formation of bioactive layers with a Ca/P ratio value similar to the one in human bone tissues (1.67), different salts of calcium were used as a source of Ca in the working electrolyte. The table presents the physicochemical properties of formed calcium phosphate films as a function of elec trolyte composition and conditions of the experiment. As one can see, the PEO process performed in electro
lytes made of calcium acetate resulted in coatings con taining hydroxyapatite. In turn, the use of calcium cit ratebased electrolytes led to formation of layers com posed of crystalline titanium dioxide and amorphous calcium phosphate compounds according to the data of Xray diffraction spectroscopy and elemental anal ysis. The mode of polarization is another important fac tor determining the formation of hydroxyapatite under the studied conditions. Thus, deposition of hydroxya patite film from the calcium acetate electrolyte was observed only under the bipolar mode of plasma elec trolytic oxidation, while in the citratebased electro lyte application of the bipolar mode resulted in higher concentration levels of Ca and P in the obtained layers compare to unipolar PEO experiments. Presumably, the pulsed polarization leads to saturation of the near electrode space with both Ca2+ and (HPO4)2– ions, which interact further with formation of calcium phosphate compounds, such as Ca10(PO4)6(OH)2, Ca3(PO4)2, and CaHPO4 ⋅ 2H2O. In addition, gener ally, bipolar plasma electrolytic oxidation provides more intense electrochemical synthesis than the uni polar one. Under pulsed polarizing current, the ion transport of oxidative reagents to a working electrode is slowed down owing to reconfiguration of the electric double layer (Helmholtz or Gouy–Chapman mod els). The latter process is accompanied by the increase in charge transfer resistance (space charge region) in the material of the electrode. This leads to more pow erful plasma discharges on the anode under a subse quent positive bias and, therefore, to a situation where an extra amount of electrolyte material is involved in formation of the plasma. Also, compared to lighter anions, the thermal energy produced during the destruction of citrate or acetate ionic complexes pro vides an extended period of heat impact on the form
Physicochemical properties of calcium phosphate coatings studied in experiments in vivo Sample no.
Electrolyte composition, g/l; polarization mode
1
Ca3(C6H5O7)2 ⋅ 4H2O, 30; Na2HPO4 ⋅ 2H2O, 30; unipolar Ca3(C6H5O7)2 ⋅ 4H2O, 20; Na2HPO4 ⋅ 2H2O, 20; unipolar Ca3(C6H5O7)2 ⋅ 4H2O, 40; Na2HPO4 ⋅ 2H2O, 40; unipolar Ca3(C6H5O7)2 ⋅ 4H2O, 20; Na2HPO4 ⋅ 2H2O, 20; bipolar Ca(CH3COO)2 ⋅ 2H2O, 50; Na2HPO4, 25; bipolar
2
3
4
5
Number Ra, of sections μm
Coating phase composition
Elemental S bone S composition, at % Ca/P bone, marrow, ratio mm2 mm2 Ti Ca P
17
2.15 TiO2 + Xray 0.4483 0.4318 11.32 3.18 3.69 0.86 amorphous phase
17
2.86 TiO2 + Xray 0.4749 0.8140 11.49 3.11 3.28 0.95 amorphous phase
11
9.13 TiO2 + Xray 0.3346 0.2950 amorphous phase
9
17
6.29 9.28 5.24 1.77
0.7635 0.3670 19.07 11.73 7.21 1.63 1.97 TiO2 + Xray amorphous phase 1.79 Ca5(PO4)3OH + Ca3(PO4)2
0.7453 0.5450
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FORMATION AND PROPERTIES |Z|, Ω cm2
477 (a)
105 104 2 3
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3 (а)
2
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1
101 10–2 10–1 100 θ, deg
101
102
103
104 f, Hz
102
103
104 f, Hz
(b) –80 –60
(b)
–40
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–20
Fig. 2. SEM images of calcium phosphate coatings formed by the PEO technique using (a) citratebased and (b) ace tatebased electrolyte.
0 10–2 10–1 100
ing oxide coating, which facilitates the transformation of metastable intermediates into stable compounds. Subsequently, this changes the morphology of the sur face layer and its anticorrosion properties. According to the elemental analysis data, all obtained coatings contain calcium and phosphorous (table), while the XRD phase analysis showed no signs of calcium phos phate compounds in some of the samples. It is reason able to assume that these compounds are present in coatings in the amorphous state or in quantities less than 10% (limit of detection for the Xray diffraction phase analysis). The method of energy dispersive spectroscopy using an SEM (EDS/SEM) was used for the chemical char acterization of studied specimens making it possible to determine the value of the Ca/P ratio and evaluate, therefore, the compositional resemblance of these coat ings to the mineral component of natural bone. The scanning electron microscope images revealed the welldeveloped, porous structure of calcium phos phate films obtained both from citrate and acetate based electrolytes (Figs. 2a, 2b) which could be important for producing composite heterostructures. For instance, the pores could be filled with antibiotics to reduce chances of an inflammatory response to implants. Subsequent partial sealing of these pores with superdispersed polytetrafluoroethylene powder in an oven at 100°C would provide gradual delivery of drugs, thereby prolonging their therapeutic effect [10]. INORGANIC MATERIALS: APPLIED RESEARCH
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Fig. 3. Bode plot representation of (a) impedance modulus and (b) phase angle as a function of frequency for studied coatings: (1) natural titanium dioxide film; (2) unipolar mode/citratebased electrolyte (calcium phosphate coat ing); (3) bipolar mode/citratebased electrolyte (calcium phosphate coating).
On the other hand, the porosity of the implant sur face itself along with its chemical composition similar ity would assist the ingrowth of bone tissue to the implant and speed up the healing process (Fig. 2). The EIS technique was used to study the electro chemical properties of formed calcium phosphate layers (Fig. 3). It is clearly seen from the impedance spectrum of the surface natural titanium dioxide film (Fig. 3, curve 1) that the corresponding graph has a maximum charac teristic of nonporous homogeneous films. Meanwhile, the spectra of calcium phosphate layers (Fig. 3, curves 2 and 3) show the presence of an extra inflection point and changes in the shape of graphs due to an additional highfrequency time constant that appeared as a result of changes in surface morphology (roughness, porosity etc.). The images from a scanning electron microscope (Fig. 2) reconfirm this observation. It also follows from Fig. 3 that film resistance in the implants with and with out a coating is about the same because of the porosity PEOformed layers and their high permeability to the electrolyte solution. No. 5
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0.5 2 3 0 1 –0.5 10–10
10–9
10–8
20 μm
(а) 10–7
10–6 j, A/cm2
Fig. 4. Polarization curves of studied coatings: (1) natural titanium dioxide film; (2) unipolar mode/citratebased elec trolyte (calcium phosphate coating); (3) bipolar mode/cit ratebased electrolyte (calcium phosphate coating).
Figure 4 presents polarization curves recorded in the physiological solution of coatings formed in the citratebased electrolyte. As one can see, while the corrosion currents remained in the range of 20– 60 nA/cm2 for both coated and uncoated samples, some improvement of the free corrosion potential was observed in specimens with a calcium phosphate layer. It is important for bone implant materials to have mechanical properties that closely match the proper ties of natural bone. The modulus of elasticity of tita niummade prostheses is E = 80 GPa (microhardness H = 2.7–3.3 GPa), which is significantly higher than corresponding values of bone tissue (E = 20 GPa and H = 0.6–0.8 GPa) [11]. This leads to boundary ten sion at the implant–bone interface that often ends up in fracturing and necrosis of bone tissue. For these rea sons, the mechanical and elastoplastic properties of the calcium phosphate coating on the titanium surface were studied in the course of our investigation. While the microhardness of obtained films was slightly less than the microhardness of titanium substrate (2.2 GPa and 2.7 GPa, respectively), the value of Young’s mod ulus of calcium phosphate layers (30 GPa) is 2.5 times lower than titanium (80 GPa); i.e., the mechanical properties of the coated implant resemble one of nat ural bone. Owing to this similarity, the chances of interfacial splitting between bone and implant are greatly reduced. The in vivo and in vitro studies are the crucial part of any medical research. The preliminary investigations on bioactivity and biocompatibility of coated titanium specimens were performed in vitro using simulated body fluid (SBF) that imitates thuman blood plasma. The samples were immersed in an SBF solution renewed every two days for a month while maintaining a constant temperature of 37 ± 0.5°C. The composition of SBF includes NaCl, NaHCO3, KCl, K2HPO4 ⋅ 3H2O, MgCl2 ⋅ 6H2O, CaCl2, Na2SO4, and deionized
10 μm
(b)
Fig. 5. SEM images of calcium phosphate coating formed by the PEO technique using acetatebased electrolyte (a) before and (b) after soaking in the SBF solution for 30 days.
water [12]. The solution acidity was adjusted to pH = 7.4 by addition of tris(hydroxymethyl)aminomethane ((CH2OH)3CNH2) and 1M HCl. Figure 5 presents the SEM images of coated tita nium samples formed by the PEO method from the calcium acetatebased electrolyte before and after soaking in the SBF solution. The exposition of implants to the SBF solution saturated with Ca2+ and 3– PO 4 ions led to surface formation of flakelike crystals of hydroxyapatite according to data of the XRD phase analysis. Similar results were obtained for the films prepared from citratebased electrolyte. These observations are in good agreement with published data [13–17] showing that a calcium phos phate coating of certain roughness may induce forma tion of crystalline hydroxyapatite. Thus, according to [17], the growth of hydroxyapatite crystals on a film exposed to the SBF solution may be considered as a criterion of the film’s bioactivity. It is well known that titanium dioxide in the forms of rutile and anatase is often formed along with calcium phosphate com pounds during the described plasma electrolytic oxi dation procedure. Owing to the isoelectric points of rutile and anatase (4.6 ± 0.4 and 5.9 ± 0.2, respectively [18]), the coated surface becomes negatively charged in the SBF solution and attracts Ca2+ cations to the interface between the coating and solution, and conse 2– quently OH– and HPO 4 anions to the extent that hydroxyapatite precipitates on the coated surface.
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FORMATION AND PROPERTIES I, arb. units
1200
479 2
(a) Ca Kα
O Kα P SiESC
3
900 600
P Kα Ti Kα
300
1
CKα Ti Lα1
0 1 I, arb. units
2
3
20 μm
Fig. 7. Historical crosssections of the tissue around cal cium phosphate coating in the test of ectopic bone forma tion in mice: (1) fragments of calcium phosphate coating; (2) bone plate; (3) spaces filled with red bone marrow.
4 E, keV (b) Ca Kα
1500 P Kα 1000
O Kα CKα P SiESC
500
Ca Kβ1 Mg Kα 0
1
2
3
4 E, keV
Fig. 6. EDS spectrum of calcium phosphate coating (a) before and (b) after soaking in the SBF solution for 30 days.
The method of energydispersive spectroscopy (EDS) was used to study the calcium phosphate films before and after immersion for 30 days in the SBF solution (Fig. 6). No signals of titanium in the spectra of soaked samples (Fig. 6b) were observed owing to formation of a dense hydroxyapatite layer. The EDS data also show the increase in Ca and P concentrations and the Ca/P ratio in the specimens soaked in the stimulated body fluid. At the same time, it was shown earlier [19] that bone implant osteointegration depends on many fac tors that can be properly evaluating only during in vivo experiments. Therefore, the biocompatibility of PEOproduced calcium phosphate coatings on titanium was exam ined by implanting samples directly under the skin of mice, and no implantrelated adverse reactions were observed. We discovered as well that the coating com position has to have a certain Ca/P ratio value in order to induce the osteogenesis. In addition, the experi ments showed that the surface roughness and absolute values of the calcium and phosphorous concentrations of the coating are equally important for the in vivo induction of osteogenesis. INORGANIC MATERIALS: APPLIED RESEARCH
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The histological sectioning of tissues grown on the surface of implants revealed coarsefibered bone tissue containing cavities filled with bone marrow (Fig. 7). The quantitative evaluation of the histological compo sition (bone, bone marrow), the profile roughness parameter Ra, phase composition, and Ca/P ratio of the studied coatings are given in the table. These data confirm the bioactivity of all obtained calcium phos phate coatings. While some of them seemingly consist of only titanium dioxide according to the data of XRD phase analysis, the presence of calcium phosphate compounds in the amorphous state was established by elemental analysis that revealed high values of the Ca/P ratio. The highest Ca/P ratio (1.92) among stud ied specimens belongs to sample no. 5 (table), which is the only one consisting of crystalline hydroxyapatite and calcium phosphate phases. At the same time, the bioactivity of the films was evaluated by the quantity of bone tissue grown on the surface of implants. This study showed no linear dependence between the surface roughness parameter Ra and the osteogen esis efficiency determined as the surface area of bone (S bone, mm2) and bone marrow (S bone marrow, mm2) (Fig. 8). At the same time, there is a local max imum of bioactivity corresponding to the surface roughness of 2–3 μm. Besides the roughness of the surface, the elemental composition of formed coatings (concentrations of calcium and phosphorous and the values of the Ca/P ratio) is an important factor in the process of bone tis sue ingrowth. According to our data (table), the most intense osteogenesis was observed for samples 4 and 5, which despite the difference in phase composition (amorphous calcium phosphate compounds (no. 4) and crystalline hydroxyapatite (no. 5)) both showed the highest (among other studied samples) values of the Ca/P ratio. On the other hand, while sample 3 has a fairly high value of the Ca/P ratio as well, the con centration of calcium and phosphorous in this coating is relatively low. In addition, the roughness parameter No. 5
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ACKNOWLEDGMENTS
S, mm2 0.8 0.7 0.6 0.5 0.4 0.3 1 2
0.2 0
2
4
6
8 Ra, μm
Fig. 8. Graphical representation of the surface area of (1) bone and (2) bone marrow as a function of the surface roughness parameter.
value Ra for this sample (9.13 μm) is quite far from the seemingly optimal value of 2–3 μm. Samples 1 and 2, on the contrary, have Ra values just about right (2.15 and 2.86 μm, respectively (Fig. 8, table)), while the concentrations of Ca and P and the Ca/P ratio are sig nificantly lower than in samples 4 and 5. Presumably, because of these reasons, the growth of bone tissue is not as significant for the coating of samples 1, 2, and 3 as for samples 4 and 5. This confirms our earlier obser vation about the surface roughness being important but not the crucial factor of osteogenesis induction. This is supported by the results of previous in vitro investigations that showed the existence of an optimal value of surface roughness which favors the differentia tion of human multipotent mesenchymal stromal cells. Thereby, it is reasonable to assume that this value of the Ra parameter is rather close to the value obtained through the in vitro studies of mouse MMSCs. CONCLUSIONS 1. Bioactive calcium phosphate coatings were formed on the surface of titanium via the plasma elec trolytic oxidation technique. The phase and elemental composition, surface morphology, and anticorrosion and mechanical properties of the obtained layers were studied and the resemblance between the mechanical properties of these coatings and natural bone were established. 2. The bioactivity of studied samples was evaluated in experiments in vitro using simulated body fluid. 3. The performed in vivo studies showed that the bioactivity of prepared calcium phosphate films depends on a number of factors, such as the concen tration of Ca and P, the Ca/P ratio, and the profile sur face roughness.
This work was supported by the Interdisciplinary Integration Project of the Siberian and Far East Branches of the Russian Academy of Sciences (grants no. 126 and no. 09IISO04001), by the Federal Target Program “Research and Development on Pri ority Directions of the Scientific and Technological Complex of Russia for 2007–2012” (project no. 02.512.11.2285 from March 10, 2009), and by the Russian Foundation for Basic Research (grant no. 09 0400287a). REFERENCES 1. Han, Y., Hong, S.H., and Xu, K., Structure and in Vitro Bioactivity of TitaniaBased Films by MicroArc Oxidation, Surf. Coat. Techn., 2003, vol. 168, pp. 249– 258. 2. Putlyaev, V.I., Contemporary Bioceramical Materials, Soros. Obrazov. Zh., 2004, vol. 8, no. 1, pp. 44–50. 3. Suchanek, W., Yashima, M., Kakihana, M.., and Yoshimura, M., Hydroxyapatite Ceramics with Selected Sintering Additives, Biomaterials, 1997, vol. 18, pp. 925–933. 4. Legostaeva, T.V., Sharkeev, Yu.P., Tolkacheva, T.V., Tolmachev, A.I., and Uvarkin, P.V., RF Patent 2385740, Bull. Izobret., 2010, no.10. 5. Orlovskii, V.P., Ezhova, Zh.A., Rodicheva, G.V., et al., Study of Conditions of Formation of Gydroxuapaptite in CaCl2–(NH4)2HPO4–NH4OH–H2O (25°C), Zh. Neorg. Khim., 1992, vol. 37, no. 4, pp. 881–883. 6. Mamaev, A.I. and Mamaeva, V.A., StrongCurrent Pro cesses in Electrolyte Solutions, Novosibirsk: Sib. Otd. Ross. Akad. Nauk, 2005. 7. Gnedenkov, S.V., Khrisanfova, O.A., and Zavid naya, A.G., Plasma Electrolytic Oxidation of Metals and Alloys in TartrateContained Solutions, Vladivostok: Dal’nauka, 2008. 8. Gnedenkov, S.V., Khrisanfova, O.A., Sineb ryukhov, S.L., Puz’, A.V., and Nistratova, M.V., RF Patent 2348744, Bull. Izobret., 2009, no. 7. 9. Gnedenkov, S.V., Khrisanfova, O.A., Sineb ryukhov, S.L., Puz’, A.V., and Nistratova, M.V., Forma tion of Surface Layers Contained Hydroxiapatite on Titanium, Korroziya: Materialy, Zashchita, 2008, no. 8, pp. 24–30. 10. Gnedenkov, S.V., Khrisanfova, O.A., Sineb ryukhov, S.L., Puz’, A.V., and Gnedenkov, A.S., Com position Protective Coatings on the Surface of Tita nium Nickelide, Korroziya: Materialy, Zashchita, 2007, no. 2, pp. 20–25. 11. Zysset, P.K., Guo, X.E., Hofler, C.E., Moore, K.E., and Goldstein, S.A., Elastic Modulus and Hardness of Cortical and Trabecular Bone Lamellae Measured by Nanoindentation in the Human Femur, J. Biomechan ics, 1999, vol. 32, pp. 1005–1012. 12. Kokubo, T. and Takadama, H., How Useful Is SBF in Predicting in vivo Bone Bioactivity? Biomaterials, 2006, vol. 27, pp. 2907–2915.
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FORMATION AND PROPERTIES 13. Wei, D., Zhow, Y., Jia, D., and Wang, Y., Biomimetic Apatite Deposited on Microarc AnataseBased Ceramic Coatings, Ceramic Int., 2008, vol. 34, pp. 1139–1144. 14. Wei, D., Zhow, Y., Yia, D., and Wang, Y., Characteris tics and in vitro Bioactivity of a MicroarcOxidized TiO2Based Coating after Chemical Treatment, Acta Biomater., 2007, vol. 3, pp. 817–827. 15. Wei, D., Zhow, Y., Jia, D., and Wang, Y., Characteris tics of Microarc Oxidized Coatings in Titanium Alloy Formed in Electrolytes Containing Chelate Complex and NanoHA, Appl. Surf. Sci., 2007, vol. 253, pp. 5045–5050. 16. Ryu, H.S., Song, W.H., and Hong, S.H., Biomimetic Apatite Induction of PContaining Titania Formed by Microarc Oxidation before and after Hydrothermal
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Treatment, Surf. Coat. Technol., 2008, vol. 202, pp. 1853–1858. 17. Huang, P., Xu, K.W., and Han, Y., Preparation and Apatite Layer Formation of Plasma Electrolyte Oxida tion Film on Titanium for Biomedical Application, Mater. Lett., 2005, vol. 59, pp. 185–189. 18. Hanawa, T., Kon, M., Doi, H., et al., Amount of Hydroxyl Radical in CalciumIon Implanted Titanium and Point of Zero Charge of Constituent Oxide of the Surface Modified Layers, J. Mater. Sci.: Mater. Med., 1998, vol. 9, pp. 89–92. 19. Kim, M. and Kawashita, M., Novel Bioactive Materials with Different Mechanical Properties, Biomaterials, 2003, vol. 24, pp. 2161–2175.
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