Materials Science and Engineering C 33 (2013) 15–20

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Surface characteristics and electrochemical corrosion behavior of NiTi alloy coated with IrO2 M. Li a, Y.B. Wang a, X. Zhang a, Q.H. Li a, Q. Liu a, Y. Cheng a, b,⁎, Y.F. Zheng a, b, c,⁎, T.F. Xi a, b, S.C. Wei a, b, d a

Center for Biomedical Materials and Tissue Engineering, Academy for Advanced Interdisciplinary Studies, Peking University, Beijing 100871, China Biomedical engineering research center, Shenzhen institution of Peking University, Shenzhen 518057, China Department of Advanced Materials and Nanotechnology, College of Engineering, Peking University, Beijing 100871, China d Department of Oral and Maxillofacial Surgery, School and Hospital of Stomatology, Peking University, Beijing 100871, China b c

a r t i c l e

i n f o

Article history: Received 27 January 2012 Received in revised form 21 June 2012 Accepted 17 July 2012 Available online 22 July 2012 Keywords: Iridium oxide NiTi shape memory alloy Thermal decomposition

a b s t r a c t The aim of this work is to investigate the surface characteristics and corrosion behavior of NiTi (50.6 at.% Ni) shape memory alloy coated by a ceramic-like and highly biocompatible material, iridium oxide (IrO2). IrO2 coatings were prepared by thermal decomposition of H2IrCl6 · 6H2O precursor solution at the temperature of 300 °C, 400 °C and 500 °C, respectively. The surface morphology and microstructure of the coatings were investigated by scanning electron microscope (SEM) and glancing angle X-ray diffraction (GAXRD). X-ray photoelectron spectroscopy (XPS) was employed to determine the surface elemental composition. Corrosion resistance property of the coated samples was studied in a simulated body fluid at 37± 1 °C by electrochemical method. It was found that the morphology and microstructure of the coatings were closely related to the oxidizing temperatures. A relatively smooth, intact and amorphous coating was obtained when the H2IrCl6·6H2O precursor solution (0.03 mol/L) was thermally decomposed at 300 °C for 0.5 h. Compared with the bare NiTi alloy, IrO2 coated samples exhibited better corrosion resistance behavior to some extent. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Nickel-titanium alloy, a near-equiatomic intermetallic compound, has been found widespread applications in the area of stents, dentistry, guide wires and other biomedical applications [1–4] due to its unique shape memory effect, superelasticity, as well as biocompatibility [5]. Although this marvelous shape memory alloy has acceptably biocompatibility, a limitation of the NiTi alloy is the release of Ni ions into the host body through corrosion and wear process, and Ni is believed to be toxic, allergic, and carcinogenic [6]. What's more, another problem of using NiTi alloy as biomedical materials is its poor resistance to localized corrosion in chloride containing environment. As a result, great deals of researches have been done to improve its biocompatibility and corrosion resistance such as mechanical and electrochemical treatments [7], chemical etching [8], heat treatments [9], plasma ion immersion implantation and deposition [10,11], etc. Iridium oxide exhibits well corrosion resistance as a ceramic like material [12] and high biocompatibility for releasing no toxic or allergic

⁎ Corresponding authors at: Center for Biomedical Materials and Tissue Engineering, Academy for Advanced Interdisciplinary Studies, Peking University No.5 Yi-He-Yuan Road, Hai-Dian District, Beijing 100871, China. Tel./fax: +86 10 62753404. E-mail address: [email protected] (Y. Cheng). 0928-4931/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.msec.2012.07.026

compounds into the body and being a natural catalyst for the decomposition of hydrogen peroxide [13]. Hydrogen peroxide (H2O2) is produced at a metal (cobalt, zinc, nickel, copper, silver, chromium, and some of their alloys) surface when it is corroded [14], and it can be harmful to the artery and cause inflammatory reactions, because of its strong oxidation effect. IrO2 can promote the conversion of H2O2 into water and oxygen, which may lead to a less pronounced inflammatory reaction and an acceleration of the endothelial coverage of the stent [13]. Consequently, it is an ideal coating materials for stents to potentially reduce the late in stent restenosis rates [15]. Iridium oxide coatings can be prepared by many different methods, for instance, by sol-gel process [16], pulsed laser deposition [17], sputtering [18] and so on. To our knowledge, no researches about the corrosion resistance of the NiTi alloy coated with IrO2 were reported. In our study, IrO2 coating was prepared by thermal decomposition of H2IrCl6·6H2O precursor solution on NiTi shape memory alloy. The chemical reaction processes may involved in the precursor solution are as follows [19]. H 2 IrCl6
6H 2 O→6H 2 O þ H 2 IrCl6

ð1Þ

H 2 IrCl6 →IrCl4 þ 2HCl

ð2Þ

IrCl4 þ O2 →IrO2 þ 2Cl2

ð3Þ

M. Li et al. / Materials Science and Engineering C 33 (2013) 15–20

2.1. Preparation of IrO2 coatings The experimental alloy had a composition of Ti-50.6 at.% Ni, and all samples were cut into 10 mm× 10 mm× 1 mm from the as-received cold rolled NiTi alloy sheet. For all samples, both sides with the area of 10 mm× 10 mm were mechanically polished down to #2000 to get a mirror-like surface and then cleaned ultrasonically in acetone, alcohol and distilled water for 15 min, respectively. The coating process mainly consists of three steps: (1) preparing the deposition solution; (2) dipping the NiTi samples into the solution prepared in step (1) for a while and then drawing them out slowly; (3) thermal oxidation of the samples prepared in step (2). Concretely speaking, for step (1), H2IrCl6·6H2O was dissolved in ethanol/isopropanol (1:1) solvent, and the concentration of iridium was about 0.03 mol/L; for step (2), the samples were dipped into the H2IrCl6·6H2O solution for 0.5 h at 40 °C and then drew out from the deposition solution automatically by a vertical drawing machine with the drawing rate at 40 mm/min; for step (3), the wet samples were dried at a temperature of 100 °C for 10 min,and then oxidized at temperature ranging from 300 °C to 500 °C for 1 h under air flow. The fabrication procedure described above is shown in Fig. 1.

Electrochemical experiment was performed by a computer-controlled Autolab CHI 650 C in simulated body fluid (SBF) [20] at 37±0.1 °C (pH= 7.4) in order to study the corrosion resistance property. Anodic polarization experiments started after the specimen immersed in the experimental solution for 1 h under open-circuit conditions and performed at a rate of 60 mV/min. 2.4. In vitro cytotoxicity evaluation In vitro cytotoxicity of the IrO2 coated NiTi alloys to murine firoblast L-929 cell line was evaluated by using an indirect method according to ISO 10993-5:2009 [21]. All the samples were sterilized by ultravioletradiation for at least 2 h, and then they were incubated in the DMEM serum free medium for 72 h in a humidified atmosphere with 5% CO2 at 37 °C. After that, the supernatant culture solution was withdrawn and centrifuged to prepare the extraction medium, then refrigerated at 4 °C before the cytotoxicity test. Cells were seeded at a density of 5×103 cells/100 μL per well in 96-well cell culture plates and incubated in the Dulbecco's modified Eagle's medium (DMEM) supplemented with 10% fetal bovine serum, 100 U mL-1 penicillin and 100 μg mL-1 streptomycin in a humidified atmosphere with 5% CO2 at 37 °C. After 24 h incubation, the culture medium in each well was replaced with the prepared extracts supplemented with 10% fetal bovine serum. In the control groups, DMEM medium was used as negative control and 10% DMSO DMEM as positive control. After 3 days incubation, 10 μL CCK-8 solution was added to each well for 3 h incubation, respectively. The measurement of the supernatant optical density (OD) was performed by a spectrophotometer (Elx-800, bio-Tek instruments) at 450 nm wavelength, with a reference wavelength of 630 nm. The cell viability was expressed in the percentage as following: Cell viabiltyð% Þ ¼

ODðtest Þ−ODðblankÞ  100% ODðnegative controlÞ−ODðblankÞ

2.2. Characterization of IrO2 coatings

3. Results and discussions

The microstructural characteristics of the samples were examined by glancing angle X-ray diffraction (XRD; D8-Discover, Bruker Co. Ltd.) with an incident angle of 1o, using CuKα radiation (λ=1.540598 Å) at 40 kV, and the scanning rate was 4 o/min. The surface morphology of the coated samples was observed by scanning electron microscope (SEM; 1910FE, Amray Co. Ltd.). The chemical composition of iridium oxide was identified by X-ray photoelectron spectrum (XPS; AXIS Ultra, Kratos Analytical Ltd.).

3.1. Microstructure of the IrO2 coatings

NiTi(020)

Intensity/a.u.

1400

IrO 2(200)

1600

1200 1000

NiTi(110)

IrO 2(110)

1800

IrO 2(101)

Fig. 2 shows GAXRD patterns of the bare and IrO2 coated NiTi alloys oxidized at 300 °C, 400 °C and 500 °C, respectively. The (020), (110), (200) and (211) diffraction peaks of the bare NiTi substrate are located at 39.2°, 42.8°, 62.0° and 78.2°, respectively [22]. IrO2 peaks appear for the sample oxidizied at 500 °C, and the XRD peaks can be indexed to

500 oC

NiTi(211)

2. Experimental

2.3. Electrochemical tests

NiTi(200)

It may be speculated that, during the heat treatment, most of the H2O, HCl, and Cl2, as decomposition products of H2IrCl6·6H2O, evaporated into the air. This preliminary and exploratory research mainly discusses the preparation of IrO2 coatings by thermal decomposition of H2IrCl6·6H2O precursor solution on NiTi shape memory alloy and investigates the influence of oxidizing temperatures on the surface characteristics and anti-corrosion properties of IrO2 coating, with the purpose to widen the application of IrO2 in biomedical area and enhance the NiTi alloy's biocompatibility and corrosion resistance.

IrO 2(211)

16

800

400 oC

600

o

400

300 C

200

uncoated

0 10

20

30

40

50

60

70

2Theta/degree Fig. 1. Schematic diagram of the process for preparation of iridium oxide coatings.

Fig. 2. GAXRD patterns of the bare and IrO2 coated NiTi alloys.

80

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the (110), (101), (200) and (211) diffraction peaks for tetragonal IrO2 at 28.1°, 34.7°, 40.1° and 54.0°, respectively. The fairly narrow peaks suggest higher crystallinity of the coatings. However the IrO2 peaks are not detected for the samples oxidizied at 300 °C and 400 °C, suggesting that IrO2 coatings presents amorphous microstructure when the oxidizing temperature below 500 °C. Similar results were also obtained from Roginskaya [23]. As for the IrO2 coating produced by thermal decomposition of H2IrCl6, only amorphous hydrated iridium oxides at temperature below 300 °C exist. IrO2·xH2O was crystallized with a rutile structure when the temperature was above 300 °C. At temperature above 400 °C, crystallites of anhydrous IrO2 emerged [23,24]. 3.2. Surface morphology of the IrO2 coatings Surface morphology is one of the most important factors in determining the biocompatibility of biomedical materials. Fig. 3(a–c) shows the SEM images of the surface and cross-section of the IrO2 coatings oxidized at 300 °C, 400 °C and 500 °C, respectively. The 1.5 μm thick and relatively smooth coating layer can be clearly seen in Fig. 3(a, a-1) without obviously cracks. As the oxidizing temperatures increase to 400 °C, much more cracks can be observed from Fig. 3(b, b-1) with the coating thickness of 1.6 μm. Distributed exposed substrate area resulted from the detachment of coatings and cracks can be seen on the surface of the samples oxidized at 500 °C from Fig. 3(c, c-1) and the coating thickness increased to 1.9 μm. A relatively intact and smooth IrO2 coating can be obtained by thermal decomposition of H2IrCl6·6H2O precursor solution at low oxidizing temperature (300 °C). The detachment and cracks shown by the SEM images in Fig. 3 indicate that iridium oxide coatings have relatively weak adhesion strength between the NiTi alloy substrate, especially at high oxidizing temperatures. The cracks and detachment of the coatings maybe originate from the difference of thermal expansion coefficient between NiTi alloy (6–11× 10−6 K−1 [25] ) and iridium oxide (3.6–5.5 × 10−6 K −1[26] ), as well as the inherent low tensile strength and fracture toughness of IrO2 [12]. And the increase

17

of coating thickness from 1.5 μm at 300 °C to 1.9 μm at 500 °Ccould deteriorate the coating integrity during the heat treatment as well. The binding strength between the coating and substrate is of vital importance in coating applications, especially in biomedical implant areas. Good adhesions of the coating and substrate can prolong the lifetime service in the implanting physiological environment and enhance the corrosion resistance property of the implant. Therefore the bonding strength of the IrO2 coatings was qualitatively measured by pressing a piece of 3 M tape down firmly on the sample and peel off quickly, and then the surfaces were observed by SEM as shown in Fig. 3(a-2, b-2 and c-2). The peel off test also indicated that the binding strength of the prepared coatings on NiTi alloy decreased with the oxidizing temperature of the precursor solution. The mechanical compatibility of the designed coatings with the superelasticity and shape memory effect of NiTi alloy is still a complicated issue for surface modification/coating of NiTi alloy [27], and a desirable coating on NiTi alloy should perform its function in accordance with its capacity of relatively large amounts of deformation. Therefore further studies should be conducted to increase the adhesive strength of the prepared IrO2 coatings by introducing an intermediate transitional layer between the coating and substrate or exploiting alternative coating methods with the advantage of high binding strength, such as plasma immersion ion implantation and deposition [28]. 3.3. XPS analysis of the IrO2 coatings Fig. 4 shows survey XPS spectra of IrO2 coatings at different oxidizing temperatures. All major features of Ir, Cl and O are visible in the spectra. The spectra indicate that iridium oxide is fabricated successfully on the NiTi shape memory alloy. No evident change in the spectra shape is observed under different oxidizing temperatures, except for the peak height of O1s and Cl 2p. It is clearly shown that, higher oxidizing temperature could result in lower Cl content in the IrO2 coatings. The quantitative analysis of Cl atomic concentrations, as

Fig. 3. SEM images of the surface, cross section and after peel test surface of the IrO2 coatings oxidized at 300 °C (a, a-1, a-2), 400 °C (b, b-1, b-2) and 500 °C(c, c-1, c-2), respectively.

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M. Li et al. / Materials Science and Engineering C 33 (2013) 15–20

1.2x106 Ir4f 5/2

1.0x106

o

500 C

o

500 C o 400 C o 300 C 55

536

534

532

530

528

526

Ir4f

60

Cl2p

o

400 C

3.4. Electrochemical property of the IrO2 coatings

o

300 C

0.0 1000

800

600

400

200

0

Binding Energy/(eV) Fig. 4. Survey XPS spectra for IrO2 coatings and the inset shows the high resolution XPS spectra of Ir4f and O1s at different oxidizing temperatures.

well as O and Ir, is shown in Table 1, it is found that Cl atomic concentration in IrO2 coatings is significantly reduced with the increase of oxidizing temperature from 32.36% at 300 °C to 7.86% at 400 °C and 2.63% at 500 °C. The residual chloride shown in the XPS spectra in Fig. 4 may be attributed to the incomplete thermal decomposition of H2IrCl6·6H2O. It has been reported that the relative concentrations of the residual chloride in the surface of the IrO2 coatings depend strongly on the calcination temperature [29]. Therefore, the IrO2 coatings should be fabricated at high oxidizing temperature to reduce the content of Cl (IrCl4) to enhance its biocompatibility and anti-corrosion property. However, according to the SEM results in Fig. 3, raising the oxidizing temperature could not obtain an intact and smooth coating surface, further study to get both Cl- free and intact IrO2 coatings is needed. The binding energy of Ir4f7/2 in iridium chlorides (IrClx) is 62.90 eV [30], and that in pure IrO2 is 62.0 eV [31]. The photoelectron lines in the inset of Fig. 4 show that the binding energies of Ir4f7/2 were 62.85 eV, 62.55 eV and 62.12 eV respectively, as the oxidizing temperature increased from 300 °C to 500 °C. This demonstrates that the higher oxidizing temperature results in lower binding energy of Ir4f7/2, and much more iridium oxide is obtained. Another inset of Fig. 4 shows the XPS spectra for O1s of the IrO2 coatings and it is found that the O1s spectra shift to low binding energy direction with the rise of oxidizing temperature. The O1s spectra may consist of three parts, spectra of O1s in H2O, X-OH and X-O (X=Ir, with low content of Ni and Ti), with the binding energy of 533±1 eV,531±0.5 eV and 529±0.5 eV [29,32]. The formation of iridium oxide results in the shifting of O1s spectra to low binding energy direction. The stoichiometry of IrO2 (O/Ir ratio) is estimated based on the O1s and Ir4f spectra data. The O/Ir ratio in the IrO2 coatings oxidized at 300 °C is 1.23:1, lower than the stoichiometric value of 2:1. The oxygen deficiency under low oxidizing temperatures can be partially attributed to the incomplete thermal decomposition of H2IrCl6·6H2O. And when the oxidizing temperatures increased to 400 °C and 500 °C the ratio are 2.56:1 and 2.95:1, higher than the reasonably stoichiometric value. The oxygen surplus in the O/Ir ratio under high oxidizing temperatures Table 1 Binding energy of O, Ir and Cl in IrO2 coatings and their atomic concentrations at different oxidizing temperatures. Oxidizing temp./°C

Ir4f at.%

300 400 500

B.E./eV

13.27 62.85 21.33 62.55 19.17 62.12

O1s at.%

B.E./eV

16.33 531.85 54.52 530.95 56.58 530.22

Cl2p at.%

Ti2p Ni2p

B.E./eV

at.%

at.%

32.26 199.55 7.86 199.35 2.63 198.92

0.21 0.37 1.59

0.71 1.59 2.28

The open-circuit potential (OCP) represents thermodynamic stability of materials, corrosion resistance and stability of the IrO2 coating on NiTi alloy can be evaluated under the open circuit conditions. It can be seen from Fig. 5 that during the whole immersion time in SBF, the OCP values for the coated NiTi alloys are less negative than the uncoated one. The OCP curves for the samples produced at 300 °C and 400 °C, having a slight variation with the immersion time, ultimately remains at −0.01 V and −0.09 V respectively. The results indicate that NiTi alloy coated with IrO2 have better corrosion resistance stability than NiTi. Besides, we can also find that the OCP-t curve for the coated samples oxidized at 500 °C declines from − 0.02 V down to − 0.16 V during the primary stages (0–1250 s), and finally stabilized at − 0.15 V. However, markedly decrease of the OCP values for the samples prepared at 300 °C and 400 °C are not detected, compared with the sample prepared at 500 °C. This difference suggests that NiTi substrate coated with IrO2 at high temperature is unstable. The anodic polarization plots in Fig. 6 show that, the IrO2 coated NiTi alloys have higher Ecorr values than the uncoated one. And the Ecorr values of the samples oxidized at 300 °C, 400 °C and 500 °C are − 0.021 V, − 0.195 V, and − 0.151 V, respectively, higher than − 0.309 V of the uncoated one. This indicates that, compared to the uncoated NiTi alloys, the IrO2 coated NiTi alloys exhibit better corrosion resistance property in simulated body fluid (SBF) at 37 ± 0.1 °C (pH = 7.4). The corrosion current density (Icorr) of the NiTi alloy is 2.312 × 10 − 6 A/cm 2, higher than that of the NiTi alloys coated with IrO2. Comparing the Icorr values from samples produced at different oxidizing temperatures (0.919× 10-6 A/cm 2 at 300 °C, 1.614 × 10-6 A/cm 2 at 400 °C, and 2.068 × 10−6 A/cm2 at 500 °C), it can be found that the higher the oxidizing temperature is, the higher the value of Icorr is. 0.5

uncoated

0.4

Potential vs. SCE(V)

2.0x105

65

Ir4d3/2 Ir4d5/2

o

500 C

o

300 C

Ir4p1/2

6.0x10

70

400 C

O1s Ir4p3/2

75 5

o

OKLL

CPS

8.0x105

4.0x105

may be attributed to the detachment of the coatings and the oxidation of the exposed NiTi alloy substrate. It is reported that the surface chemical compound of the NiTi alloy after heat treatment is NixTiy with the binary and ternary metal oxides [33,34]. According to the SEM results, it is shown in Fig. 3 that cracks and exposed substrate areas are observed under high oxidizing temperatures. The XPS analysis area in our study is approximately 300 μm×700 μm, much larger than the exposed substrate area. It can be seen from Table 1 that the atomic concentrations of Ni and Ti in the surface increase with the rise of the oxidizing temperatures, which maybe mainly come from the exposed NiTi substrate due to the detachment of the IrO2 coatings.

O1s

Ir4f 7/2

o

300 C

0.3

400 C

0.2

500 C

o o

0.1 0.0 -0.1 -0.2 -0.3 -0.4 -0.5 0

500

1000

1500

2000

2500

3000

3500

Time/sec Fig. 5. Open-circuit potential vs time curves for the uncoated and IrO2 coated NiTi alloys.

M. Li et al. / Materials Science and Engineering C 33 (2013) 15–20

Potential vs. SCE(V)

1.5

1.0

300-IrO2, and almost the same with 500-IrO2. A possible explanation for this is that 300-IrO2 coatings contained a certain degree of IrCl4, which exhibited cytotoxicity to L929 growth to a certain extent [35]. Besides, from the results of XPS, it can be seen that, although the detachment and cracks existed in the coating for 400-IrO2 and 500-IrO2, the higher temperature may be also beneficial for the oxidation of the exposed surface of NiTi substrate, which could contribute to the improvement of the biocompatibility. The inset in Fig. 7 shows the optical micrographs of L929 cells cultured in the uncoated and IrO2 coated NiTi alloys, as well as the negative control and positive control after 3 days incubation. The cells cultured in the negative control and all the IrO2 coated sample extracts exhibited healthy morphology of cells with a flattened spindle shape. Both the CCK-8 test and microscopic observation indicated that the IrO2 could increase the biocompatibility of the NiTi substrate, especially when they were oxidized at higher temperatures.

uncoated 300 oC 400 oC 500 oC

0.5

0.0

-0.5

1E-9

1E-8

19

1E-7

1E-6

1E-5

1E-4

1E-3

0.01

Current density (A/cm2) Fig. 6. Anodic polarization plots for the uncoated and IrO2 coated NiTi alloys.

One possible explanation for this phenomenon is that much more cracks and detachment present on the surface of IrO2 coatings with the increase of the oxidizing temperatures, which make it easier for the electrolyte to get into the NiTi substrate and result in the aggravated corrosion. Another possible reason for this is that, the bonding strength of the prepared coatings on NiTi alloy decreased with the oxidizing temperature. Open circuit potential measurements, combined with the anodic polarization tests, indicate that the NiTi alloy coated with IrO2 exhibits better corrosion resistance than the NiTi substrate to some extent, and the oxidizing temperature has great influence on its anti-corrosion property.

4. Conclusions IrO2 coating has been successfully fabricated by thermal decomposition of H2IrCl6·6H2O precursor solution on the surface of NiTi alloy in our study. The oxidizing temperatures have a significant influence on the morphology and microstructure of the coatings. A relatively smooth, intact and amorphous coating has been obtained when the H2IrCl6·6H2O precursor solution (0.03 mol/L) is thermally decomposed at 300 °C for 0.5 h. Compared with the bare NiTi alloy, IrO2 coated NiTi alloy exhibits a better corrosion resistance property to some extent and improvement of biocompatibility. As a preliminary and exploratory research, our current experimental results may provide valuable and useful information on the surface modification of NiTi alloys by IrO2 coatings, widen the application of IrO2 in biomedical area and make NiTi alloys more suitable for biomedical application, especially in the cardiovascular area.

3.5. The in vitro cytotoxicity of the IrO2 coated NiTi alloys Acknowledgement Fig. 7 shows the in vitro L929 cell viability of the uncoated and IrO2 coated NiTi alloys extraction medium after 3 days incubation. The samples prepared at 300 °C, 400 °C and 500 °C are coded as 300-IrO2, 400-IrO2 and 500-IrO2, respectively. The cells cultured in the extraction of 400-IrO2 shows higher viability than those of the NiTi substrate and

This work is supported by National High Technology Research and Development Program of China (2011AA030103), National Basic Research Program (973) of China (NO. 2009CB930004), and National Natural Science Foundation of China (NO. 30870623, NO. 31070846).

Fig. 7. The in vitro L929 cell viability of the uncoated and IrO2 coated NiTi alloys extraction medium after 3 days incubation and the inset shows the corresponding optical photos of the cells incubating in the extractions of each sample. * represents p b 0.05.

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References [1] L. Torrisi, Biomed. Mater. Eng. 9 (1999) 39–47. [2] M. Thier, G. Kubla, D. Drescher, C. Bourauel, J. Phys. IV 1 (1991) 181–186. [3] S.B. Vearick, M.D.O. Michelon, L. Schaeffer, R.G. Xavier, G. Kuhl, P.R.S. Sanches, M.E.S. Duarte, J. Biomed. Mater. Res. B 83B (2007) 216–221. [4] B. O'Brien, W. Carroll, Acta Biomater. 5 (2009) 945–958. [5] M. Es-Souni, M. Es-Souni, H. Fischer-Brandies, Anal. Bioanal. Chem. 381 (2005) 557–567. [6] J.K. Dunnick, M.R. Elwell, A.E. Radovsky, J.M. Benson, F.F. Hahn, K.J. Nikula, E.B. Barr, C.H. Hobbs, Cancer Res. 55 (1995) 5251–5256. [7] W. Simka, M. Kaczmarek, A. Baron-Wiechec, G. Nawrat, J. Marciniak, J. Zak, Electrochim. Acta 55 (2010) 2437–2441. [8] S. Shabalovskaya, G. Rondelli, J. Anderegg, B. Simpson, S. Budko, J. Biomed. Mater. Res. B 66B (2003) 331–340. [9] D. Vojtech, M. Voderova, J. Fojt, P. Novak, T. Kubasek, Appl. Surf. Sci. 257 (2010) 1573–1582. [10] J.H. Sui, W. Cai, L.H. Liu, L.C. Zhao, Mater. Sci. Eng. A Struct. 438 (2006) 639–642. [11] D.G. Castner, B.D. Ratner, Surf. Sci. 500 (2002) 28–60. [12] U.K. Mudali, T.M. Sridhar, B. Raj, Sadhana Acad. Proc. Eng. Sci. 28 (2003) 601–637. [13] C. Di Mario, E. Grube, Y. Nisanci, N. Reifart, A. Colombo, J. Rodermann, R. Muller, S. Umman, F. Liistro, M. Montorfano, E. Alt, Int. J. Cardiol. 95 (2004) 329–331. [14] Z.H. Zhao, Y. Sakagami, T. Osaka, Can. J. Microbiol. 44 (1998) 441–447. [15] B. O'Brien, C. Chandrasekaran, in: Medical Device Materials II: Proceedings from the Materials & Processes for Medical Devices Conference 2004, 2005, pp. 301–306. [16] K. Nishio, Y. Watanabe, T. Tsuchiya, Thin Solid Films 350 (1999) 96–100.

[17] L.M. Zhang, Y.S. Gong, C.B. Wang, Q. Shen, High Perform. Ceram. IV (Pts 1-3) (2007) 336–338 (2215-2217). [18] J.D. Klein, S.L. Clauson, S.F. Cogan, J. Vac. Sci. Technol. A Vac. Surf. Films 7 (1989) 3043–3047. [19] J.M. Hu, H.Q. Zhang, C.N. Cao, Thermochim. Acta 403 (2003) 257–266. [20] T. Kokubo, H. Takadama, Biomaterials 27 (2006) 2907–2915. [21] ANSI/AAMI, Arlington, VA, 2009. [22] M. Iijima, W. Brantley, W. Guo, W. Clark, T. Yuasa, I. Mizoguchi, Dent. Mater. 24 (2008) 1454–1460. [23] Y.E. Roginskaya, M.D. Gol'dshtein, O.V. Morozova, L.D. Glazunova, Russ. J. Electrochem. 37 (2001) 1065–1071. [24] Y.E. Roginskaya, I.D. Belova, B.S. Galyamov, Prot. Met. 27 (1991) 518–523. [25] A. Vitiello, G. Giorleo, R.E. Morace, Smart Mater. Struct. 14 (2005) 215–221. [26] G. Bayer, H.G. Wiedemann, Thermochim. Acta 11 (1975) 79–88. [27] S. Shabalovskaya, J. Anderegg, J. Van Humbeeck, Acta Biomater. 4 (2008) 447–467. [28] Y. Cheng, Y. Zheng, Thin Solid Films 515 (2006) 1358–1363. [29] C.C. Chang, T.C. Wen, C.H. Yang, Y.D. Juang, Mater. Chem. Phys. 115 (2009) 93–97. [30] B.D. Elissa, A. Katrib, R. Ghodsian, B.A. Salsa, S.H. Addassi, Int. J. Quantum Chem. 33 (1988) 195–216. [31] M. Peuckert, Electrochim. Acta 29 (1984) 1315–1320. [32] K. Endo, Y. Katayama, T. Miura, T. Kishi, J. Appl. Electrochem. 32 (2002) 173–178. [33] Q. Wei, Z.D. Cui, X.J. Yang, J. Shi, Appl. Surf. Sci. 255 (2008) 462–465. [34] G.S. Firstov, R.G. Vitchev, H. Kumar, B. Blanpain, J. Van Humbeeck, Biomaterials 23 (2002) 4863–4871. [35] A. Yamamoto, R. Honma, M. Sumita, J. Biomed. Mater. Res. 39 (1998) 331–340.