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Corrosion Science 103 (2016) 173–180

Contents lists available at ScienceDirect

Corrosion Science

j ourna l h omepage: www.elsev ier .com/ locate /corsc i

ize-dependent corrosion behavior and cytocompatibility of Ni–Ti–Oanotubes prepared by anodization of biomedical NiTi alloy

uiqiang Hanga,b,∗, Yanlian Liua, Si Liua, Long Baia, Ang Gaob, Xiangyu Zhanga,iaobo Huanga, Bin Tanga, Paul K. Chub,∗∗

Research Institute of Surface Engineering, Taiyuan University of Technology No. 79 Yingze West Road, Taiyuan, ChinaDepartment of Physics and Materials Science, City University of Hong Kong Tat Chee Avenue, Kowloon, Hong Kong, China

r t i c l e i n f o

rticle history:eceived 4 August 2015eceived in revised form3 November 2015ccepted 14 November 2015vailable online 18 November 2015

a b s t r a c t

We fabricate Ni–Ti–O NTs with different size on NiTi alloy through varying anodization voltages andevaluate their corrosion behavior, Ni release, and cytocompatibility. Our results show the NTs influencethe corrosion behavior and cytocompatibility of NiTi alloy in a size-dependent manner. Worse corrosionresistance and more Ni release are observed from large NTs because of their high specific surface area.However, cytocompatibility is improved after anodization especially for the sample anodized at 25 V.These results thus indicate the release level of Ni ions from NiTi alloy is well tolerated by osteoblasts and

eywords:ickel–titanium alloynodizationanotubesorrosion behavior

surface nanotubular structure contribute to its cytocompatibility.© 2015 Elsevier Ltd. All rights reserved.

ytocompatibility

. Introduction

Since Zwilling et al. successfully fabricated TiO2 nanotubes (NTs)y anodizing titanium (Ti) and its alloy in a fluoride (F)-containinglectrolyte in 1999 [1], there have been extensive researches onhe related synthesis, properties, and potential applications of the

aterials [2–7]. The initial electrolytes used to synthesize the NTsre aqueous solutions containing hydrofluoric acid (HF) or F salts,hich leads to limited NT length (normally less than 5 �m) andoorly self-organized structure [1,8]. On the contrary, in subse-uently developed viscous organic electrolytes (such as ethylenelycol) that supplemented with small amount of H2O and F salts,ong TiO2 NTs (approach to 1000 �m) with highly ordered struc-ure can be produced [9,10]. TiO2 NTs possess unique electrical,ptical, biological properties [11]. In addition, these properties

an be further enhanced by elemental doping [12,13]. Pure andoped TiO2 NTs have shown great potential in energy, environ-ental, biomedical and other fields [11,14–18]. Specially, TiO2 NTs

∗ Corresponding author at: Research Institute of Surface Engineering, Taiyuanniversity of Technology No. 79 Yingze West Road, Taiyuan, China. Fax.: +86526010540.∗∗ Corresponding author.

E-mail addresses: hangruiqiang@tyut.edu.cn (R. Hang), paul.chu@cityu.edu.hkP.K. Chu).

ttp://dx.doi.org/10.1016/j.corsci.2015.11.016010-938X/© 2015 Elsevier Ltd. All rights reserved.

have attracted much attention in biomedical field due to severaladvantages. Firstly, anodization can produce TiO2 NTs on Ti-basedbiomaterials with a complex shape economically and reproducibly[11]. Secondly, the NTs with proper dimensions grown on pure Tican enhance its corrosion resistance [19]. Thirdly, the NTs withadjustable dimensions can modulate the behavior and functionsof various types of cells including mesenchymal stem cells (MSCs)[20], endothelial cells [21], smooth muscle cells [21], macrophages[22], osteoblasts [23], and others. Fourthly, NTs with open endsmay serve as carriers to deliver drugs [24,25]. Although the bond-ing strength of NTs to the substrates is relatively poor because ofthe presence of F-rich layer between them, many approaches havebeen proposed to overcome the issue and significant improvementhas been achieved [26–29].

Nearly equiatomic nickel–titanium (NiTi) alloy is widely used inbiomedical field on account of its unique shape memory effect andsuperelasticity. The main concern of NiTi alloy is its poor corrosionresistance that leads to Ni release, because spontaneously formedoxide film on its surface is very thin with poor self-healing abil-ity [30]. Although a trace amount of Ni is essential for human bodybecause it participates in some critical physiological processes [31],

excessive Ni may produce high cytotoxicity, allergic reactions, andeven genotoxicity [32,33] thus is generally considered to adverselyaffect the biocompatibility of NiTi alloy. It is well known that cor-rosion resistance of metallic biomaterials can be improved through

dx.doi.org/10.1016/j.corsci.2015.11.016http://www.sciencedirect.com/science/journal/0010938Xhttp://www.elsevier.com/locate/corscihttp://crossmark.crossref.org/dialog/?doi=10.1016/j.corsci.2015.11.016&domain=pdfmailto:hangruiqiang@tyut.edu.cnmailto:paul.chu@cityu.edu.hkdx.doi.org/10.1016/j.corsci.2015.11.016

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74 R. Hang et al. / Corrosion

roper surface treatments [34–36]. As previously mentioned, con-tructing highly ordered nanotubular structure on pure Ti and itslloys through anodization is an emerging and promising methodo improve their corrosion resistance and other desired properties,o producing NTs on NiTi alloy with the same technique may ben efficient approach to solve its aforementioned concern. How-ver, anodizing NiTi alloy is relatively difficult compared to purei because of the presence of a large amount of Ni. Until 2010,im et al. successfully fabricated Ni–Ti–O NTs through anodizingiTi alloy [37]. Our recent work systematically investigated the

nfluence of anodization parameters on the formation ability andharacterizations of the NTs on NiTi alloy, and found under opti-ized experimental conditions highly ordered ones with different

izes can be produced [38].Previous studies have shown the size of TiO2 NTs is a key fac-

or to determine their corrosion behavior. Yu et al. found TiO2 NTsith small diameter grown on pure Ti could lower its corrosion

ate, but large NTs have an opposite effect [39]. Improved corrosionesistance of pure Ti with small NTs may be ascribed to relativelyhick oxide layer formed during anodization at the substrate/NTnterface, whereas large NTs possess a high specific surface area toontact with electrolyte thus beneficial to the diffusion of corrosive

ons and corrosion products. Another work reported by Liu et al.lso corroborate the phenomenon [40]. In addition to corrosionehavior, cellular functions can also be significantly influenced byT size. For instance, TiO2 NTs with small diameter promote adhe-

ig. 1. Surface SEM images of polished (NiTi-MP) and anodized (NiTi-5 V, NiTi-15 V, and Niorphologies of the Ni–Ti–O NTs and the diameter (D) and length (L) of NTs are shown i

ifficult to determine).

ce 103 (2016) 173–180

sion and proliferation of human mesenchymal stem cells (hMSCs),while large NTs induce hMSCs elongation that leads to their selec-tive differentiation [20]. The results described above imply Ni–Ti–ONTs may influence the corrosion behavior and cytocompatibility ofNiTi alloy in a size-dependent manner. In this work, Ni–Ti–O NTswith different size are fabricated on NiTi alloy at different anodiza-tion voltages and the size-dependent corrosion behavior, Ni release,as well as cytocompatibility are studied.

2. Experimental

2.1. Sample preparation

Rolled and annealed NiTi alloy (50.8 at% Ni) sheets with highpurity (>99.5 wt.%) were cut into small pieces with dimensions of15 × 15 × 2 mm. Each piece was successively ground with a seriesof SiC papers and finally polished with 1 �m diamond paste, fol-lowed by ultrasonically cleaning in acetone, alcohol, and distilledwater for 5 min sequentially and air dried. Anodization was con-ducted on a 250 ml plastic two-electrode cell with a Pt foil as thecathode and NiTi sheet as the anode. The electrolyte with a vol-ume of 100 ml was composed of ethylene glycol containing 0.2 wt%

NH4F and 0.5 vol% H2O. The anodization temperature was kept at30 ◦C using a thermostatic bath. To prepare the NTs with differ-ent size, the anodization voltages supplied by a DC power supplywere varied from 5 V to 35 V at steps of 10 V. To dissolve the surface

Ti-25 V) NiTi sheets after annealing at 450 ◦C for 2 h. The insets show cross-sectionaln the high-magnification images (L of the NTs fabricated at 5 V is so small that it is

R. Hang et al. / Corrosion Science 103 (2016) 173–180 175

Fig. 2. (a) XPS survey spectra, (b) high-resolution Ti 2p spectra, and (c) high-resolution Ni 2p spectra acquired from polished (NiTi-MP) and anodized (NiTi-5 V, NiTi-15 V,and NiTi-25 V) NiTi sheets after annealing at 450 ◦C for 2 h.

Table 1Elemental concentrations (at.%) on the surface of polished (NiTi-MP) and anodized(NiTi-5 V, NiTi-15 V, and NiTi-25 V) NiTi sheets determined by XPS. All samples wereannealed at 450 ◦C for 2 h.

Sample Atomic concentrations (at.%) Ni/Ti ratio

Ti Ni O C N

NiTi-MP 2.8 1.7 60.6 33.8 1.1 0.61

ida53twmc

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7m

NiTi-5V 8.7 2.3 66.7 21.6 0.7 0.26NiTi-15V 11.8 3.2 69.1 15.4 0.5 0.27NiTi-25V 12.7 2.9 70.1 14.0 0.3 0.23

rregular layer and expose the underlying NTs, a long anodizationuration is required for samples anodized at a low voltage. Thenodization time for the samples anodized at 5 V (denoted as NiTi-

V), 15 V (denoted as NiTi-15 V), 25 V (denoted as NiTi-25 V), and5 V (denoted as NiTi-35 V) were 12 h, 6 h, 1.5 h, and 1.5 h respec-ively. After anodization, the samples were rinsed with distilledater, air dried, and annealed at 450 ◦C for 2 h. For comparison,irror-polished and annealed NiTi alloy (NiTi-MP) was used as the

ontrol.

.2. Characterization

Field-emission scanning electron microscopy (FE-SEM, JSM-001F, JEOL) was performed at 15 kV to observe the surfaceorphology of the anodized samples and the elemental composi-

Fig. 3. XRD patterns of polished (NiTi-MP) and anodized (NiTi-5 V, NiTi-15 V, andNiTi-25 V) NiTi sheets after annealing at 450 ◦C for 2 h.

tion and chemical states were determined by X-ray photoelectronspectrometer (XPS, PHI5802). The survey spectra were acquiredat a constant pass energy of 187.85 eV at binding energies of200–1400 eV at 0.8 eV/step. The high-resolution spectra were

obtained at a constant pass energy of 11.75 eV at 0.1 eV/step. All thebinding energies were referenced to C1s (284.8 eV). The crystallinestructure of the Ni–Ti–O NTs was determined by X-ray diffraction(XRD, DX-2700, Haoyuan) using Cu K� radiation at an incident

176 R. Hang et al. / Corrosion Science 103 (2016) 173–180

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(50 �l for each sample) for 1 h at 37 ◦C, followed by examination ofthe labeled cells by confocal laser scanning microscopy (CLSM, C2Plus, Nikon).

Fig. 4. (a) Low and (b) high magnification TEM and SAED (Inset in (b)) ima

ngle of 2◦. The microstructure of the NTs anodized at 25 V wasxamined by transmission electron microscopy (TEM, Tecnai G2-TWIN) at 200 kV.

.3. Corrosion tests

The corrosion behavior of the anodized samples was evalu-ted on an electrochemical workstation (CHI6144D, Chenhua) byotentiodynamic polarization in phosphate buffered solution (PBS,H 7.4), which is composed of 137 mM NaCl, 2.7 mM KCl, 1.5 mMH2PO4, and 8 mM Na2HPO4. The hardware consisted of a 50 ml

hree-electrode cell with a saturated calomel electrode (SCE) as reference electrode, platinum (Pt) foil as the counterpart, andnodized sample with an exposed area of 1 cm2 as the workinglectrode. The experiments were performed at a constant temper-ture of 37 ± 0.1 ◦C. After immersion in PBS for 2 h, the polarizationurves were obtained over the potential range between −0.8 V and.5 V at a constant scanning rate of 1 mV/s. Corrosion potentialsEcorr), current densities (Icorr), and cathodic Tafel slopes (ˇc) of theamples were acquired from Tafel extrapolation method.

.4. Ni release

To evaluate Ni release from the anodized samples, five non-orking surfaces were sealed with 704 silicone rubber. Each sample

ig. 5. Potentiodynamic polarization curves of polished (NiTi-MP) and anodizedNiTi-5 V, NiTi-15 V, and NiTi-25 V) NiTi sheets after annealing at 450 ◦C for 2 h.

the Ni–Ti–O NTs fabricated at 25 V followed by annealing at 450 ◦C for 2 h.

was immersed in 3 ml of PBS on 12-well plates at 37 ± 0.3 ◦C. Tomimic the in vivo dynamic environment, the PBS was refreshedevery day during immersion for 30 days. The solutions collectedat days 1, 5, 10, 20, and 30 were analyzed by inductively-coupledplasma mass spectrometry (ICP-MS, 7500, Agilent).

2.5. Cytocompatibility

The cytocompatibility of the anodized samples was assayedusing the Live/Dead® viability/cytotoxicity kit for mammalian cells(Invetrogen) and the experimental procedures were similar tothose in our previous work [4]. Briefly, MC3T3-E1 subclone 14 pre-osteoblasts were planted on the alcohol-sterilized samples at adensity of 2.0 × 104 cells/cm2 and cultured in the complete culturemedium for 1 and 3 days. At each prescribed time point, the sampleswere rinsed thrice with PBS and incubated with the assay reagents

Fig. 6. Ni release profiles of polished (NiTi-MP) and anodized (NiTi-5 V, NiTi-15 V,and NiTi-25 V) NiTi sheets after annealing at 450 ◦C for 2 h. The solution wasrefreshed every day for each sample and the release amount of Ni ions over 1 daywas collected, measured, and shown. The exact values of the data that approach tozero have been added to the graph.

R. Hang et al. / Corrosion Science 103 (2016) 173–180 177

Table 2Corrosion potentials (Ecorr), current densities (Icorr), and cathodic Tafel slopes (ˇc)of polished (NiTi-MP) and anodized (NiTi-5 V, NiTi-15 V, and NiTi-25 V) NiTi sheetsderived from the polarization curves. All samples were annealed at 450 ◦C for 2 h.

Sample Ecorr/V vs. SCE Icorr/A cm−2 ˇc/V decade−1

NiTi-MP −0.27 1.08 × 10−8 −0.141NiTi-5V −0.49 3.20 × 10−8 −0.139

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Table 3Elemental concentrations (at.%) on the surface of polished (NiTi-MP) and anodized(NiTi-5 V, NiTi-15 V, and NiTi-25 V) NiTi sheets after immersion in PBS for 30 daysdetermined by XPS. All samples were annealed at 450 ◦C for 2 h before immersion.

Sample Atomic concentrations (at.%) Ni/Ti ratio

Ti Ni O C N

NiTi-MP 0.5 0.1 29.0 64.9 5.5 0.20NiTi-5V 1.6 0.1 36.9 53.3 8.1 0.06

NiTi-15V −0.48 5.90 × 10−8 −0.138NiTi-25V −0.53 7.48 × 10−8 −0.104

. Results and discussion

.1. Materials characterization

Fig. 1 shows the surface SEM images of mirror-polished andnodized samples after annealing. There is no definite structuren NiTi-MP except slight marks left from grinding. In comparison,

nanotubular structure is clearly observed from all of the anodizedamples. The diameter (D) and length (L) of the NTs are deter-ined from the high-magnification SEM images except L of the

ample anodized at 5 V because of the difficulty encountered duringbservation. D and L of the NTs increase with anodization voltage.owever, an excessively high anodization voltage (for example,5 V) does not alter the size of NTs and macroscopic pits can bebserved on the surface.

The XPS survey spectra in Fig. 2(a) show that the surfaces of theolished and anodized samples are relatively clean. Besides Ti, Ni,nd O, only C and N surface contamination can be detected. C ishe predominant contaminant and the N concentration is gener-lly less than 1 at.%. The high-resolution Ti 2p and Ni 2p spectra areresented in Fig. 2(b) and (c), respectively. There are two peaks at59.2 and 465.0 eV for Ti 2p, indicating that Ti is oxidized to TiO241]. The Ni 2p 3/2 and Ni 2p1/2 peaks are located at 856.4 and74.0 eV, respectively, indicating the presence of NiO [42]. Table 1ummarizes the elemental concentrations. The Ni/Ti ratios on allhe anodized samples are smaller than that of the NiTi substrate.

ith regard to NiTi-MP, the deficiency of Ni on the surface can bescribed to preferential oxidation of Ti during annealing resulting ini accumulation underneath the TiO2 layer [43]. The Ni-rich sublay-rs may serve as reservoirs responsible for long-lasting Ni releasefter the outer TiO2 layer is damaged. During anodization, Ti and

i are oxidized to TiO2 and NiO [37], respectively, and NiO prefer-ntially dissolves in the electrolyte under field-assistant attack of−, thus leading to reduced Ni concentration in the NTs and betteriosafety.

ig. 7. XPS survey spectra of polished (NiTi-MP) and anodized (NiTi-5 V, NiTi-5 V, and NiTi-25 V) samples after immersion in PBS for 30 days. All samples werennealed at 450 ◦C for 2 h before immersion.

NiTi-15V 5.5 0.1 40.9 49.9 2.6 0.02NiTi-25V 7.1 1.4 40.2 49.3 2.0 0.20

Although the fabrication of well-defined nanotubular structureson NiTi alloy by anodization is generally more difficult than that onpure Ti, our previous study has shown that it is possible by control-ling the anodization parameters [38]. The diameter of the Ni–Ti–ONTs is smaller than that of TiO2 NTs prepared under similar condi-tions [11]. D of the NTs is related to growth factor (f) of the metalto be anodized and the relationship can be expressed as f = D/2U[11], where, U is the effective voltage of the electrode. Typically, fis in the range of 2–4 nm/V for valve metals and about 2.5 nm/V forTi [11,44]. Using the nominal voltage instead of effective one, thevalue of f of NiTi alloy is estimated to be 1 nm/V, which is far lessthan that of pure Ti thus indicating that the field-assisted chem-ical dissolution rate of Ni–Ti–O mixed oxide is more rapid thanpure TiO2. XPS provides evidence of this phenomenon stemmingfrom preferred dissolution of NiO, because the Ni to Ti ratio of theNTs is less than that of the NiTi substrate. Rapid dissolution of NiOalso makes it difficult to produce long Ni–Ti–O NTs. Another fac-tor that influences D of the NTs is the temperature of electrolyte.Although the nominal temperature of the electrolyte is controlledto be 30 ◦C, resistive heating in the system cannot be ignored if theanodization voltage is high enough (for example, 35 V). An elevatedtemperature accelerates dissolution of Ni–Ti–O mixed oxides thusreducing D and L of NTs, but on the other hand, it may result in theoccurrence of active sites, breakdown of the oxide film, and finallyformation of macroscopic corrosion pits [45]. The pits may becomeshort-circuit channels leading to current self-amplification (run-away anodization) and further heating the electrochemical system[46].

Fig. 3 shows the XRD patterns of polished and anodized sam-ples after annealing. Only diffraction peaks from the NiTi substratecan be detected indicating the Ni–Ti–O NTs annealed at 450 ◦C areamorphous. The amorphous characteristic is corroborated by TEM.As shown in Fig. 4, the selected-area electron diffraction (SAED)pattern only shows a dim halo characteristic of an amorphous struc-ture. Generally, the as-formed TiO2 NTs are amorphous and can beconverted into anatase at about 300–500 ◦C. Recent studies haveshown that alloying elements such as niobium (Nb), zirconium (Zr),and tungsten (W) in TiO2 NTs elevate the transition temperatureat which the amorphous structure is converted into the anatasephase [47–49]. The as-formed Ni–Ti–O NTs are amorphous too [50],and XRD and TEM do not indicate the formation of the anatasephase after annealing at 450 ◦C. It is consistent with previous results[51] implying that Ni also retards the transformation into anatase.Anatase has been observed to be absent from the surface of NiTialloy after oxidation at 300–500 ◦C, and only rutile phase can bedetected after oxidation at 600–800 ◦C [52]. These results suggestthat it is difficult to achieve in situ growth of the anatase phase onNiTi alloy during high-temperature oxidation.

3.2. Corrosion behavior and Ni release

Fig. 5 displays the potentiodynamic polarization curves of pol-ished and anodized samples and the electrochemical parametersderived from Tafel extrapolation method are listed in Table 2. Since

178 R. Hang et al. / Corrosion Science 103 (2016) 173–180

Fig. 8. Live/dead fluorescent images of osteoblasts after culturing for 1 day on polished (NiTi-MP) and anodized (NiTi-5 V, NiTi-15 V, and NiTi-25 V) NiTi sheets (Live cells aregreen and dead cells are red). All samples were annealed at 450 ◦C for 2 h. Scale bars: 200 �m.

Fig. 9. Live/dead fluorescent images of osteoblasts after culturing for 3 days on polished (NiTi-MP) and anodized (NiTi-5 V, NiTi-15 V, and NiTi-25 V) NiTi sheets (Live cellsare green and dead cells are red). All samples were annealed at 450 ◦C for 2 h. Scale bars: 200 �m.

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R. Hang et al. / Corrosion

he extrapolation was started at least 100 mV away from Ecorr,nd the cathodic branches of the curves showed good linearity onemilogarithmic scale over one decade of Icorr, good accuracy ofhe results can be expected [53]. NiTi-MP possesses higher Ecorr−0.27 V) and smaller Icorr (1.08 × 10−8 A/cm2) than the anodizedamples, implying low corrosion tendency and rate. The corro-ion resistance of the anodized samples decreases with increasingT size as shown by the small Ecorr and large Icorr. No obviousreakdown is observed from the oxide film on the samples exceptiTi-25 V for which the current shows a sudden increase at about.92 V but repassivation occurs quickly at an elevated potential. Theradual increase in the polarization currents at about 1.0 V may bescribed to water electrolysis. The theoretically initial voltage forater electrolysis is 1.23 V [54] and the standard electrode poten-

ial of SCE versus standard hydrogen electrode (SHE) is 0.2415 V55], and so an over-potential of 1.0 V versus SCE is sufficient toecompose water. Some small bubbles appear on the anode at aigh potential also suggesting electrolysis of water. The corrosionesistance deteriorates with increasing NT size and it is believedo arise from the larger surface area in contact with the electrolytend resulting mass transport at the electrolyte/electrode interface.

Fig. 6 shows Ni release profiles for as long as 30 days. Themounts of Ni leached form the samples over 1 day decrease withime and the anodized samples release more Ni than the polishedne. Generally, larger NTs release more Ni. It is believed that theurface oxide contributes to the good biocompatibility of NiTi alloyy inhibiting Ni release from the bulk materials [43]. However, ashown by XPS, the surface oxide layer on the NiTi alloy containsome NiO which can dissolve in the electrolyte during spontaneouslectrochemical corrosion. Anodization increases the surface areaf the oxide layer due to the nanotubular structure which may behe major reason for the difference in the release profiles of Ni. Nin the NTs may be depleted over time (Fig. 7 and Table 3) and sohe amounts of released Ni from different samples are expected athe same after immersion for a long time. Nonetheless, one shouldeep in mind that although the anodized samples release more Nihan the polished one, the quantities of leached Ni are much lesshan the tolerable limit of Ni in vivo [56] and so the anodized NiTilloy should be biologically safe.

.3. Cytocompatibility

Fig. 8 shows the live/dead fluorescent images of osteoblasts onolished and anodized sample after culturing for 1 day. No deadells can be observed and the number of cells on the anodized sam-les is larger than that on the polished one. The anodization voltageas little influence as well. The cells on NiTi-MP generally have aon-spread morphology. In contrast, the cells on anodized samplesre well spread showing a lot of cross-linked pseudopodia. Afterulturing for 3 days, the difference in the cell quantities betweenhe polished and anodized groups is more obvious and the numberf cells on NiTi-25 V is larger than that on NiTi-5 V and NiTi-15 VFig. 9).

Although a lot of works aim at depressing Ni release to improvehe cytocompatibility of NiTi alloy through various surface mod-fication techniques [57], our results indicate Ni release has noppreciable side effects on its cytocompatibility and even mayas positive effects. Gursoy et al. have found promoted epithelialell proliferation after exposure to a low concentration of Ni butncreased cytotoxicity at a high dose [58]. Apart from Ni release,urface morphologies of biomaterials also influence their cyto-

ompatibility. The proper surface morphology, for instance, highlyrdered and vertically oriented NTs, have been shown to enhancehe cell viability and proliferation [59–61] as consistent with ouresults.

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ce 103 (2016) 173–180 179

4. Conclusion

Ni–Ti–O NTs with different size are fabricated on biomedicalNiTi alloy by varying the anodization voltages. The NTs are amor-phous after annealing at 450 ◦C for 2 h and the Ni concentration inthe NTs is less than that in the NiTi substrate because of prefer-ential dissolution during field-assisted chemical etching. Becauseof the larger surface area, the NTs exhibit worse corrosion resis-tance and more Ni release compared to the polished sample. Onthe other hand, the cytocompatibility is improved after anodiza-tion especially the sample anodized at 25 V. This provides evidencethat the amount of leached Ni is well tolerated.

Acknowledgments

The work was jointly supported by the National Natural Sci-ence Foundation of China (31400815), the Specialized ResearchFund for the Doctoral Program of Higher Education of China(20131402120006), and Hong Kong Research Grants Council (RGC)General Research Funds (GRF) Nos. CityU 112212 and 11301215,as well as City University of Hong Kong Strategic Research Grant(SRG) No. 7004188.

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