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Colloids and Surfaces B: Biointerfaces 84 (2011) 400–405

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Colloids and Surfaces B: Biointerfaces

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / c o l s u r f b

In vitro corrosion study of different TiO2 nanotube layers on titanium in solutionwith serum proteins

Wei-qiang Yu, Jing Qiu, Fu-qiang Zhang ∗

Departmentof Prosthodontics, School of Stomatology, ShanghaiNinth People’s Hospital,ShanghaiJiao TongUniversity School of Medicine,ShanghaiResearch Instituteof Stomatology,

639 Zhizaoju Road, Huangpu District, Shanghai 200011, China

a r t i c l e i n f o

Article history:

Received 10 October 2010Received in revised form 18 January 2011

Accepted 22 January 2011

Available online 5 March 2011

Keywords:

Titanium

TiO2 nanotube layer

Corrosion

Serum proteins

a b s t r a c t

Titanium oxide nanotubes prepared by anodization have received considerable attention in the bioma-

terials domain. The objective of this study was to demonstrate the electrochemical behavior of differentdiameter TiO2 nanotube layers on titanium in phosphate buffered saline (PBS) and Dulbecco’s minimumessential medium + 10% fetalcalf serum (D-FCS) usingopen circuit potentials (OCP), electrical impedance

spectroscopy (EIS), and a potentiodynamic polarization test. The results showed that the nanotubes hadhigher OCP, higher resistance of the inter barrier layer ( Rb), and lower I pass in the two test solutions

compared to the smooth Ti, especially the 30 nm diameter nanotubes. The corrosion resistance of thenanotubes in D-FCS was higher than in PBS because of protein adsorption from the D-FCS solution as

suggestedby scanning electron microscope (SEM) images. In addition,protein aggregates of 30nm diam-eter nanotubescaused themodelof EISspectra to transformfrom two-layerto three-layer.The corrosion

behavior of the nanotubes for use as a dental implant material is discussed.Crown Copyright © 2011 Published by Elsevier B.V. All rights reserved.

1. Introduction

Titanium is currently one of the most important materials inbiomedical and dental implants [1]. Chemical composition andtopography of Ti-implant were modified by various methods to

improve osseointegration of endosseus implants, which couldeventuallyplayan importantrole in clinical outcomes [2]. Recently,the researchers began to modify the implant surface in nanometerregime because of the in vivo nanometer physiological environ-

ment that bone cells are accustomed to [3].Many studies have shown that the nano-Ti implant canenhance

osteoblast adhesion and function more significantly than whatthe conventional Ti implant dose [4–13]. Corrosion studies are

also essential because higher corrosion rate means more ionrelease, which mayinterfere with cell metabolismin tissues aroundimplants, even induces implant failure [14,15]. Furthermore, the

greater real surface area in nano-Ti implants may exhibit a lowercorrosion resistance than the conventional Ti implant [5,6,16].However, up to now only a few corrosion studies have been carriedout on nano-implants [17–19]. Zheng et al. studied the corro-sion behavior of Ti–TiC–TiC/diamond-like carbon (DLC) gradient

nano-composite films on NiTi alloy in hank’s solution. They foundthat nano composite films coated NiTi showed excellent corrosionresistance properties than uncoated NiTi [17]. Karpagavalli et al.

∗ Corresponding author. Tel.: +86 021 23271699 5694; fax: +86 021 63136856.

E-mail addresses: [email protected], [email protected] (F.-q. Zhang).

reported that Ti6Al4 V with nanostructured TiO2 films deposited

eletrolytically could enhance the corrosion resistance compared tothe bare Ti6Al4V [18]. But some reports exhibited contrary cor-rosion performance. For example, Garbacz et al. found that thenano-Ti by hydrostatic extrusion demonstrated lower corrosion

resistance than the micro-Ti in a NaCl solution did [19]. So thecorrosion behaviors of nano-Ti in physiological environment needfurther investigation.

Recently, the biological characteristics of TiO2 nanotube lay-

ers by anodization have been extensively studied and shownpromising results in biomaterial application [20–26]. The tita-nium with nanotube layers can not only significantly increaseosteoblast or bone-forming cell adhesion and function in vitro

[22–24], but also promote in vivo bone formation around implantscompared to their unanodized counterparts [25,26]. However, upto now, few articles about corrosion behavior of nanotubes have

been published [27,28]. Saji et al. studied the corrosion behaviorof Ti–35Nb–5Ta–7Zr alloy with amorphous nanotubular oxide inRinger’s solution at 37 ◦C. Their results showed that this nanotubelayer had good corrosion resistance due to forming an immedi-ate and effective passivation [27]. Yu et al. tested the corrosion

characteristics of titanium with amorphous and anatase nanotubesin Hank’s solution and found that this nanotube layer, especiallythe anatase nanotubes, exhibited better corrosion resistance thansmooth-Ti [28]. The Ringer’s and Hank’s solution only mimic inor-

ganic components of body fluids [29]. Many studies have shownthat the serum proteins of body fluids can play an important rolein corrosion of the implant materials [30–34]. So it is more rep-

0927-7765/$ – see front matter. Crown Copyright © 2011 Published by Elsevier B.V. All rights reserved.

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W.-q. Yu et al. / Colloids and Surfaces B: Biointerfaces 84 (2011) 400–405 401

resentative of physiological environment to test the corrosion of TiO2 nanotube layers in solution with serumproteins.Furthermore,

the diameter of nanotubes has been proved to affect the adsorbof serum proteins, which can influence cell behavior [35,36]. It isnot known whether the adsorb of serum proteins can change thecorrosion of nanotubes on titanium.

The present work was to test corrosion behavior of titaniumwith different diameter nanotube layers in PBS solution and fluidswith serum proteins and detected by scanning electron micro-scope (SEM), electrochemical impedance spectroscopy (EIS) and

potentiodynamic polarization. This study aims to enhance theunderstanding of corrosion characteristics of nanotubes on tita-nium in physiological environment.

2. Materials and methods

2.1. TiO 2 nanotube layer preparation

Ti thin foils (0.25 mm thick, 99.5%, Alfa Aesar) were used to pre-

pare the nanotubes. Firstly, titanium foils were immersed in anmixture (2 mL 48% HF, 3mL 70% HNO3 (both reagent grade chemi-cals) and 100 mL deionized water) for 3 min to remove the naturaloxide layer, then ultrasonically washed in deionized (DI) water for

5 min and dried by nitrogen stream. The nanotube layers were pre-pared by anodization in 1 M H3PO4 and 0.5 wt% HF solution for 3 hat different voltages (5 V and 15 V) to obtain different diameternanotubes [35]. The counter electrode was platinum plate. After

anodization, the samples were ultrasonically washed in DI waterand dried by nitrogen stream. The Ti foil was polished with SiCemery papers No. 240, 400, 600, 800, 1000 and 1200 grit sizes inseries for use as a control group sample.

2.2. Surface characterization

Some samples were immersed in30 mL test solutionsat 37 ◦Cfor

half an hour. Then the samples were picked up and gently washedwith DI water. Their microstructures were observed by Scan-

ning Electron Microscopy (Philips, The Netherlands, Sirion200). Allspecimens were sputter-coated with Au before imaging using a

HUMMER I Sputtercoater for 3 min.

2.3. Preparation of corrosion test

The three kinds of test specimens used in this research were (a)smooth Ti (S), (b) the nanotube layers at 5V (N-5), and (c) the nan-otube layers at 15 V (N-15). The mechanical and electrical contact

between the specimen and thecupper wire were obtained by elec-tric conduction paste with copper. Thecold epoxy resin wasused tocarefullymount the samples. The exposed test surface (area:1 cm2)was ultrasonically washed in deionized water, ethanol, and then

deionized water for about 5 min, respectively. Three specimens

were prepared for each condition to corrosion test.Two synthetic solutions simulating physiological environment

were used in this study. That is phosphate buffered saline (PBS)

and Dulbecco minimum essential medium + 10% fetal calf serum(D-FCS). The solution was prepared from deionized water, analyticgrade agents, Dulbecco minimum essential medium, and fetal calf serum. The main components of solution areshown in Table 1 [32].

2.4. Corrosion test

The apparatus for electrochemical measurement consisted of

a computer-controlled potentiostat (PARSTAT 2273 AdvancedElectrochemical System) with research corrosion software (Pow-erSuite), an Ag/AgCl with a saturated potassium chloride as a

reference electrode, a platinum plate as a counter electrode, and a

Table 1

The component (mol/l) of solutions.

Component PBS D-FCS

NaCl 1.37×10−1 1.16×10−1

KCl 2.68×10−3 5.36×10−3

CaCl2 1.80×10−3

Na2HPO4 8.10×10−3 8.98×10−4

KH2PO4 1.47×10−3

MgSO4 8.11×10−4

NaHCO3 2.38–2.62×10−2

Amino acid 5.5×10−3

Proteins (g/l) <10

standard 3-electrode microcell. After the specimen was immersedin PBS and D-FCS solution, the open circuit potential (E ocp) was

measured for half an hour at 4 s intervals. The electrochemicalimpedance spectra (EIS) measurements were taken at the E open

achieved in the E vs. t test using a frequency response analyzercoupled withthe potentiostat. The frequencyspan analysed ranged

from 10−2 to105 Hz with a perturbing signal of 10mV. The EIS datafitting was done using the suitable equivalent circuit by the ZSim-pWin 3.21 software. The potentiodynamic polarization behavior of specimens wasrecorded after immersed for90 min in testsolution.

The scan range of the potentiodynamic polarization was −400mVto +3000 mV (vs.opencircuit potential)at a scanning rate of 1 mV/s.Alltests were maintained at 37 ◦C. Allthe tests were repeated threetimes with each sampleand solution.The reproducibilityof thedata

by repeatingmeasurements on a seriesof electrodes waswithin5%.All experiment data were statistically analysed using ANOVA andScheffeˇ ıs test (˛= 0.05) of SPSS 11.0.

3. Results

3.1. The morphology of surfaces

The morphology of nanotube layers in different test solutionswas shown by Fig. 1. The N-5 and N-15 is the nanotube layers with

approximately 30 nm and 70 nm diameter by anodized at 5 V and15V (Fig. 1A). The morphologies of all the surfaces after immerged

in PBS were the same as before (Fig. 1B). However, SEM imagesrevealed that protein aggregates deposited from D-FCS settle onthe surfaces after immerged in D-FCS for half an hour. Few proteinaggregates were found in S and N-15 surfaces, but lots of pro-

tein aggregates were deposited on N-5 surface so that the poresof nanotubes were sealed.

3.2. E ocp in different solutions

Open circuit potential (E ocp) data after half an hour immersionin PBS and D-FCS solutions were reported in Fig. 2. The E ocp of N-5wassignificantlyhigherthanthatofSandN-15intwotestsolutions

(P < 0.05). The E ocp of S and N-15 inD-FCS became lower thanin PBStest solutions (P > 0.05). But the E ocp of N-5 in D-FCS became higherthan in PBS test solutions (P >0.05).

3.3. EIS analysis

Fig. 3 shows the electrochemical impedance data for S, N-5, and

N-15 in PBS and D-FCS solutions for half an hour. The Bode-phaseplots showed clear two time constants in the N-15 surfaces. Butonly one time constant was seen in the S and N-5 surfaces.

The spectra obtained for S was interpreted by using one time

constant due to passive oxidelayer. The N-5 in PBS and N-15 in PBSand D-FCS solutions were interpreted by using a model with twoconstants because of the nanotubes with an outer tube layer and

an inner barrier layer [27] (even though only one time constant



402 W.-q. Yu et al. / Colloids and Surfaces B: Biointerfaces 84 (2011) 400–405

Fig. 1. High magnification SEM images of the samples in different test solutions ((A) unimmersed, (B) immersed in PBS, and (C) immerged in Dulbecco minimum essential

medium + 10% fetal calf serum and (a) smooth Ti, (b) the nanotube layers at 5 V, and (c) the nanotube layers at 15V, ×150,000).

in N-5 shown by Bode-phase plots). Furthermore, the N-5 in D-FCS solution was interpreted by using three time constant because

protein aggregates were deposited on nanotubes surface. In ourstudies, a constant-phase element(CPE) wasuse to replace thepurecapacitance in the fitting of the EIS because of the rough surfaceacquired by 1200-SiC-abrase paper on S and the porous surface on

N-5 and N-15. The impedance of CPE is defined as [31],

Z CPE = [Y 0( jw)n]−1

(1)

where n is the empirical exponent of the CPE, −1 < n <1, if n =−1,

CPE is the pure inductance; if n = 1, CPE is the pure capacitance; if

n = 0, CPE is the pure resistance.

The components of this equivalent circuit are electrolyte resis-tance (Rs), resistance of outer tube layer (Rt), resistance of inter

barrier layer (Rb), and resistance of the proteinaggregates (Rp). Therepresentative graphic examples of the excellent fitting accuracyobtained with simulation of model in Fig. 4 were shown in Fig. 5.The 2-values 10−3 for S and 10−4 for N-5 and N-15 have shown

the good fit quality between experimental points and fitted points(Table 2).

The resistance, constant-phase element, and n values obtainedby choosing suitable model of equivalent circuit for all samples

were given in Table 2. The higher barrier layer resistance (Rb)suggested better corrosion resistance of titanium with 30 nm and70 nm diameter nanotubes compared to the smooth Ti in two test

Fig. 2. (A) Representative E vs. T -curves and (B) Open circuit potential ( E ocp) on surfaces in the test solutions. (PBS: phosphate buffered saline, D-FCS: Dulbecco minimum

essential medium+ 10% fetal calf serum, S: smooth Ti, N-5: the nanotube layers at 5 V, and N-15: the nanotube layers at 15 V; mean (standard deviation), n =3, *P <0.05).




Fig. 3. The Bode-phase plots for smooth Ti and nanotubes in PBS (phosphate

buffered saline) and D-FCS (Dulbecco minimum essential medium+ 10% fetal calf

serum) solutions (S: smooth Ti, N-5: the nanotube layers at 5 V, and N-15: the

nanotube layers at 15 V).

Fig. 4. Model of equivalent circuit obtained from the EIS-results: (A) Smooth Ti

in phosphate buffered saline and Dulbecco minimum essential medium + 10% fetal

calfserum solutions. (B)The nanotubelayersat 5 V in phosphate bufferedsalineand

the nanotube layers at 15 V in phosphate buffered saline and Dulbecco minimum

essential medium+ 10% fetal calf serum solution. (C) The nanotube layers at 5 V in

Dulbecco minimum essential medium + 10% fetal calf serum solution.

solutions. The highest Rb is shown in N-5 surface (33×105) inPBS solutions, but in N-15 surface (56 ×105) in D-FCS solutions

(P < 0.05). Furthermore, the Rb values of all the surfaces in PBS arelower than those in D-FCS (P < 0.05). The n values of barrier layersin all samples are about 0.9. The higher Rt of N-15 is shown com-

pared to N-5, but the Y 0-CPEt tendency is opposite (P < 0.05). TheRp, Y 0-CPE, and n results of N-5 have also shown that proteins of D-FCS could aggregate on the 30 nm nanotubes after immerged inD-FCS for half an hour.

Fig. 5. Bode plot for representative samples ((A) smooth Ti in phosphate buffered

saline andDulbecco minimum essentialmedium+ 10%fetalcalf serum solutions, (B)

the nanotube layers at 5V in phosphate buffered saline and the nanotube layers at

15 V in phosphate buffered saline and Dulbecco minimum essential medium + 10%

fetal calf serum solution, and (C) the nanotube layers at 5 V in Dulbecco minimumessential medium+ 10% fetal calf serum solution). EIS-result with the experimentalpoints of phase ( ) and module | Z | (Msd.) and the fitted points (Calc.) obtained with

the simulation of the model shown in Fig. 4.

Table 2

Values for the elements of the equivalent circuit (R, CPE, n) shown in figure and 2.

Group Rb Y 0-CPE n Rt Y 0-CPE n Rp Y 0-CPE n 2

PBS: S 7 (1.3) 4.4 (0.21) 0.91 (0.002) 10−3

PBS: N-5 33.2 (2.3) 2.3 (0.41) 0.91 (0.002) 42.8 (2.2) 59.2 (14.3) 0.53 (0.013) 10−4

PBS: N-15 25.1 (6.4) 1.9 (0.61) 0.93 (0.015) 264.0 (108.0) 19.5 (6.2) 0.57 (0.97) 10−4

D-FCS: S 26.7 (2.9) 3.4 (0.23) 0.90 (0.004) 10−3

D-FCS: N-5 42.1 (3.3) 4.4 (0.43) 0.9 9 (0.004 ) 18.4 (2.4) 194.5 (5 2.1) 0.5 3 (0.042) 2.7 (1.3) 7.5 (1.53) 0.746 (0.03 2) 10−4

D-FCS: N-15 56.5 (5.0) 8.8 (1.67) 0.93 (0.032) 231.6 (90.1) 21.9 (5.3) 0.59 (0.029) 10−4

Values: mean (standard deviation); Rt (); Y 0-CPE (Scm−2 s−n ×10−5); Rb and Rp (×105 ), n = 3.



404 W.-q. Yu et al. / Colloids and Surfaces B: Biointerfaces 84 (2011) 400–405

Fig. 6. Typical diagrams of the potentiodynamic polarization for the samples (PBS:

phosphate bufferedsaline, D-FCS: Dulbecco minimum essential medium + 10%fetal

calf serum, S: smooth Ti, N-5: the nanotube layers at 5 V, and N-15: the nanotube

layers at 15 V).

3.4. Potentiodynamic polarization

Fig. 6 shows the typical potentiodynamic polarization curves

for all surfaces in the two test solutions. The corrosion parame-ters about I pass, E break , and I corr were acquired by potentiodynamicpolarization curve analysis [29,37]. The I pass which is passive cur-rent density was obtained from the potentiodynamic polarization

curve at 1 V. The E break is the potential where the current densityremarkably increases with increasing potential. I corr means thecor-rosion rate. The corrosion parameters are showed in Table 3. Thevalue trends of I pass are N-5 < N-15< S acquired by one-way ANOVA

(˛= 0.05) in the two test solutions. Moreover, the I pass values of allthe surfaces in PBS are higher than those in D-FCS, but not statis-

tically significant. The E break values for N-15 were highest amongthe surfaces in the two test solutions. The N-15 I corr values for PBS

and D-FCS were 145 and 142 nA/cm2, respectively, and were sig-nificantly higher than that of S and N-5 (P <0.05). The I corr valuesof S and N-5 in PBS are significantly higher than those in D-FCS(P <0.05).

4. Discussion

In this study, the OCP of all the surfaces in the two test solu-tions rose rapidly and then gradually reached a stable value. Thisis because a titanium oxide film began to grow and thicken afterthe samples were immerged in the solutions, which resulted in

increased corrosion resistance and subsequent a relatively stable

state [30,38]. Our results have shown that the OCP of S and N-15in PBS is higher than that in D-FCS at 37 ◦C. Contu et al. [39] sug-gested that the slope of the oxygen cathodic reduction in PBS was

lower than that in the solution containing proteins. The anodic and

Table 3

Corrosion parameters tested in this research. The parameters are mean (standard

deviation), n = 3.

Group I pass (nA/cm2) E break (mV) I corr (nA/cm2)

PBS: S 6380 (81) 1276 (80) 76 (9)

PBS: N-5 1473 (419) 1580 (79) 45 (7)PBS: N-15 4390 (331) 1643 (15) 145 (6)

D-FCS: S 5840 (65) 1293 (28) 23 (3)

D-FCS: N-5 578 (99) 1386 (51) 20 (5)

D-FCS: N-15 3920 (127) 1650 (60) 142 (21)

cathodic reactions of electrode determine the OCP. In this respect,the corrosion resistance of S and N-15 decreased due to proteins

contained in D-FCS. However, the OCP value of N-5 in PBS is lowerthan in D-FCS, which may because a protective smear layer wasformed on N-5 surface by the protein aggregates deposited from D-FCS(Fig.1). Furthermore,the OCPvaluesfor N-5and N-15stabilized

in the two test solutions suggested the better corrosion resistanceproperties than those for S, especially, there was statistically sig-nificant difference in OCP values between N-5 and S. This tendencylies in the fact that the N-5 and N-15 have not only TiO2 tube layer

but also TiO2 barrier layer, which is thicker than the native oxidefilm of S [29].

In our studies, a two-layer (barrier oxide layer and nanotubelayer) model was established to interpret the EIS spectra of N-5 in

PBS and N-15 in the two test solutions. This agrees with reports of Saji and Yu et al. [28,29]. After the N-5 immerged in D-FCS for houran half, a three-layer (barrier oxide layer, nanotube layer, and pro-tein layer) was used to interpret the EIS spectra. The protein layer

on N-5 was confirmed by SEM image in Fig. 1C-b. Oh et al. [36]found that serum proteins proned to aggregate smaller nanotubelayer. We verified this tendency. At thesame time, our results indi-cate that thetwo-layermodelof EISspectra canbe transformed to a

three-layerone when the corrosion behaviors of smaller nanotubes

are investigated on solution with serum proteins.Theresults of bode-phase spectra (phaseof Z vs. frequency) sug-

gested that a compact passive film was present on the S and N-15

surfaces in the two test solutions. The cause may lie in the factthat the phase angles of the S and N-15 surfaces are close to −90◦

over a wide frequency range, a characteristic of the compact pas-sive film reported by Delgado-Alvarado and Sundaram [40]. At the

same time, the surface of the N-15 may form the compact passivefilm in the two test solutions due to the phase angle about −90◦ athigh frequency range. The EIS data were analysed using the modelof Fig. 4 to determine all the R and CPE. The barrier layer resis-

tance (Rb) is an important parameter whose values are positivelyproportional to corrosion resistance [18,29]. The N-5 and N-15 lay-ers have higher Rb values compared to the S surfaces in the two

test solutions. On one hand, the barrier oxide layers of the N-5and N-15 by anodization are thicker than that of the S surfaces. Onthe other hand, the nanopore and nanotube structure could adsorbmore proteins compared with the S surfaces in the D-FCS solution[4–6], which is highlighted by the physical evidence provided by

the SEM images in Fig. 1. Our results have shown that all the sam-ples in D-FCS have higher Rb values than in PBS. It is possible thatthe adsorbed proteins from the D-FCS increase the corrosion resis-tance of all the surfaces by covering the electrode surface [41]. The

protein adsorption is confirmed by SEM in Fig. 1. Furthermore, theRt values of the N-15 are higher than that of the N-5, and the rea-son may be that the longer nanotubes of the N-15 increases the

Rt value compared to the N-5. The length of nanotubes becomes

longer when the voltages of anodization increase from 5 V to 15 V

[42]. (Our results about nanotube length have been used in anotherarticle.) The n values of barrier layers for all the samples are 0.9,which indicates that the barrier layers of all the samples show a

similar pure capacitive behavior [43]. The n value of tube layers forthe N-5 and N-15 are about 0.55, which means that the two sur-faces have a considerable deviation from ideal capacitor behaviorbecause of their porous surface [29,43].

As for corrosion data acquired from the potentiodynamic polar-ization curves, the N-5 and N-15 exhibited lower current densityof I pass than the S surfaces. This may be due to an immediate andeffective passivation on the N-5 and N-15 surfaces [28]. But current

densityof I corr fornanotubes (N-15)is greater than forthe smoothTi(S),which meanslower corrosionresistancefor the nanotubes com-pared to the smooth Ti. Saji et al. [27] also found this phenomenon

when studying corrosion characteristic of nanotubular oxide on




Ti–35Nb–5Ta–7Zr alloy in Ringer’s solution. They thought it wasthe tube bottom interface shaped by distinctly separated barrier

oxide/concave that contributed to the lower corrosion resistanceof the naotubes. Our nanotubes also formed the similar interface,which seem to be a reason for lower corrosion on N-15. The I pass

and I corr in D-FCS all became lower than in PBS, especially the I pass

of the N-5 surfaces. This is because the proteins can deposit on thesurfaces to alleviate corrosionprocess.Therefore,the protein aggre-gates on the N-5 surface performed effective passivation, whichsignificantly decreased the current dentisy of I pass [30,41].

5. Conclusions

The nanotube layers of titanium with different diameter, espe-cial at 30 nm, show better corrosion resistance than smooth Ti due

to higher OCP, Rb, and lower I pass in PBS and D-FCS solution. Thecorrosion resistance of nanotubes in a solution containing serumproteins increasedin comparison with that inPBS by analysingOCP,EIS, and potentiodynamic polarization. Moreover, the model of EIS

spectra cantransform from a two-layerto a three-layer onebecauseof proteins deposited from D-FCS solution, which indicates that itshould be careful to choose the EIS spectra model when investigat-ing the corrosion behavior of nano-implant materials in solution

with serum protein. Our results exhibit that the titanium withnanotubes can improve the corrosion resistance in physiologicalenvironment.

Acknowledgements

The authors would like to thank Bang-shang Zhu (Instrumen-tal Analysis Center, Shanghai Jiao Tong University) for helpfulassistance in experiments. This work was supported by Shang-

hai Leading Academic Discipline Project (Project No.: S30206) andScience and Technology committee of Shanghai (08DZ2271100,1052nm04300, and 10JC1408600) and Shanghai Leadind AcademicDiscipline Project (T0202), and National Natural Science Founda-

tion of China (81070866).

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