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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: [email protected] (R. Hang),
[email protected]. 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:[email protected]:[email protected]/10.1016/j.corsci.2015.11.016
-
1 Scien
psatsaeTKNicms
twrrticiabN
Fmd
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
2
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
ges of
aeU
2
appKtaaeac1(s
2
w
F(
(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
3
3
aoasmsoH3o
patap4[8stWaNeaNeFb
F1a
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.
[
[
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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