Surface treatment of NiTi shape memory alloy by modified advanced oxidation process

Available online at www.sciencedirect.comTransactions ofScienceDirectNonferrous Metals恶感业ScienceSociety of ChinaELSEVIER PressTrans. Nonferrous Met. Soc. China 19(2009) 575-580www.unmsc.cnSurface treatment of NiTi shape memory alloy bymodified advanced oxidation processCHU Cheng-lin(储成林)', WANG Ru-meng(F.如萌)', YIN Li-hong(尹立红),PU Yue-pu(浦跃朴)尸, DONG Yin-sheng(董寅生)' , GUO Chao(郭超)',SHENG Xiao-bo(盛晓波), LIN Ping -hua(林萍华), CHU Paul-K(朱剑豪)1. Jiangsu Key Laboratory for Advanced Metallic Materials, School of Materials Science and Engineering,Southeast University, Nanjing 211189, China;2. School of Public Health, Southeast University, Nanjing 210096, China;3. Department of Physics and Materials Science, City University of Hong Kong, Hong Kong, ChinaReceived 7 November 2008; accepted 4 January 2009Abstract: A modified advanced oxidation process(AOP) utilizing a UVlelectrochenmically generated peroxide system was used tofabricate titania films on chemically polished NitTi shape memory aloy(SMA). The microtructure and biomedical properties of thefilm were characterized by scanning electron microscopy(SEM), X-ray photoeletron spectroscopy(XPS),inductively coupledplasma mass spectrometry(ICPMS), hemolysis analysis, and blood platelet adhesion test. It is found that the modified AOP has ahigh processing effectiveness and can result in the formation of a dense titania film with a Ni-fee zone near its top surface. Incomparison, Ni can still be detected on the outer NiTi surface by the conventional AOP using the UV/H2O2 system. The depthprofiles of O, Ni, Ti show that the flm possesses a smooth graded interface structure next to the NiTi substrate and this structureenhances the mechanical stability of titania film. The titania film can dramatically reduce toxic Ni ion release and also improve thehemolysis resistance and thromboresistance of biomedical NiTi SMA.Key words: NiTi shape memory alloy; titania fim; advanced oxidation process(AOP); biomedical propertiesultraviolet (UV)/H2O2 photocatalytic system[12], where1 Introductionhydroxyl radicals (OH) are generated by directdecomposition of H2O2 with Fe2* ions or UV iradiation.Deposition of a titania film improves the biologicalSuch a nonselective and highly reactive oxidant has beensafety of NiTi shape memory alloys(SMA) used inutilized to remove organic pollutants from water[13-14].cardiovascular products[1 -4] because the titania film asSince :OH (with oxidation potential of 2.8 V) is aa barrier layer can commendably block leaching of toxicstronger oxidant than H2O2 (oxidation potentials equalnickel ions from the NiTi substrate in patients[5] and can1.80 and 0.87 V at pH=0 and 14, respectively), it can bealso improve the blood compatibility of NiTi SMA byused to oxide NiTi SMA to form a titanium oxide layerpreventing thrombosis formation[6]. There have hithertoon NiTi substrate.been a pumber of reports on the fabrication of titaniaHowever, in our previous studies, it is also foundfilms on NiTi SMA by different methods, includingthat during the AOP of NiTi SMA, the advancedsol-gel technique[7], heat oxidation[8], laser oxidationoxidation reaction in the UV/H2O2 system is not stable[9], H2O2 oxidation[10], etc.because the quantity of OH produced by catalyticRecently, we have reported that the surface structuredecomposition of H2O2 under UV inradiation decreasesof NiTi SMA can be modified by advanced oxidationdue to continuous consumption of H2O2. This intrinsicprocesses(AOP) in a Fe'/H2O2 system[11] or anrame hv a mndified AOP utilizing a中国煤化工Foundation item: Project(NCET-06-0464) supported by the Program for New Centuryv of Education of China;Projcct(BK2007515) supported by the Natural Science Foundation ofCNMHG3Z445) supported by theNational lightech Rescarch and Development Program of China; Project supported by Nppon Sheet Glass Foundation for MaterialsScience and Engineering (NSG Foundation): Projec(7001999) suppred by SRG Grant from the Research Comitee of the CityU of HKCorresponding author: CHU Cheng-lin; Tel: +86 25-52090683; E-mail: clchu@gseu.edu.cnDOl: 10.1016/S1003-6326(08)60315-5576CHU Cheng-lin, et al/Trans. Nonferrous Met. Soc. China 19(2009) 575- -580UVlelectrochemically-generated peroxide system, inmonochromatic Al Ka (1486.6 eV) X-ray radiation. Thewhich H2O2 can be produced continuously at the cathodebase pressure in the analysis chamber was better thanunder rather mild conditions by the reduction of oxygen.10- Pa. High-resolution Ti 2p, 0 ls and Ni 2p spectraThe electrochemically generated H2O2 subsequentlywere acquired at a 20 eV pass energy to determine thereacts with UV to supply *.OH continuously[13]. In thischemical states and concentrations. The XPS depthcase, there is a stable :.OH source for the formation of theprofiles were obtained by using a rastered 3 keV Ar^ iontitania flm on NiTi substrate by AOP.beam. The argon pressure during depth profiling wasThe aim of this work is to fabricate a titania flm onabout 10~ Pa and the sputtering rate was estimated to bechemically polished NiTi SMA by this modified AOP inabout 20 nm/min.a UV/electrochemically-generated peroxide system. Themicrostructure and biomedical properties of the film are2.3 Ni releaseTwo samples of each type were immersed in 25 mLdetermined by scanning electron microscopy(SEM),X-ray photoeletron spectroscopy(XPS), inductively-simulated blood fluid(SBF) in a polypropylene bottle.coupled plasma mass spectrometry (ICPMS), hemolysisThe bottles were closed tightly and incubated in athermostatic chamber at (37+0.1)C for 2 weeks and 5analysis, and blood platelet adhesion test.weeks, respectively. At each time point for each group,2 Experimentalthe SBF was taken out and analyzed by inductively-coupled plasma mass spectrometry(ICPMS) to determine2.1 Fabrication of titania flm on NiTi SMA bythe average amount of Ni leached from the 4 specimensin two bottles.modified AOPA commercially available NiTi (50.8% Ni, molar2.4 Hemolysisfraction) SMA plate for medical applications with a8 mL of fresh blood was collected from a rabbit andmartensite initiation temperature(M) of -12.8 C andthen diluted with 10 mL 0.9% saline. Each sample wasaustenite finish temperature (Ad) of33.4 C was cut intoput into a test tube with 10 mL saline and incubated at 37small rectangular blocks with dimensions of 10 mmX 10C for 30 min. Afterwards, 0.2 mL of the diluted bloodmmX 1 mm. The samples were chemically polished withwas added to each test tube and incubation continued fora solution containing H2O, HF, and HNO3 in a ratio ofanother 60 min. After incubation, the suspension was5:1:4 for 5 min. The samples were then ultrasonicallycentrifuged at 2 500 r/min for 5 min. The absorbance ofwashed in acetone for 10 min and deionized water for 10he supernatant fluid was measured by amin. They were divided into two groups. The first groupspectrophotometer (UV240, China). The positive controlwas used as the control (denoted as the CP). The secondvas a mixture of blood and deionized water, and thegroup was treated by the modifed AOP to form a surfacenegative control was a mixture of blood and saline. Thetitania flm at a constant current of0.3 A for 60 min in anhemolysis results were averages of three measurements.electrolytic cell consisting of a graphite cathode and theNiTi sample as the anode. The electrolyte consisted of a2.5 Blood platelet adhesion0.02 mol/L Na2SO4 aqueous solution at pH= -3.0. DuringThe samples were put into a 24-well tssue culturethe modified AOP, fresh air was continuously blown intoplate. A 3.8% (mass fraction) citrate acid solution wasthe Na2SO4 electrolyte near the graphite cathode. Theadded to the blood with a blood to citrate acid volumeelectrolyte was iradiated by UV (254 nm) vertically,ratio of 9:1. The solution was centrifuged to form aleading to photocatalytical decomposition of theplatelet-rich plasma(PRP) and erythrocyte. 0.1 mL of theelectrochermically-generated H2O2. Afterwards, thePRP was added to each well. After incubation at 37 Csamples were ultrasonically washed in acetone for 1for 3 h, the PRP was taken out from the wells. Amin and deionized water for 10 min (denoted as thephosphate buffer solution(PBS) was added to the wellsmodified AOP).nd gently rinsed 2- -3 times to get rid of plateletsadsorbed loosely on the sample surface. The samples2.2 Microstructureswere. then. soaked in 2 5% glutaraldehyde at roomThe surface morphology was asssed by atempe中国煤化工ered platelets. Thefield-emission type SEM (Sirion 2000, FEI Co.) at 20 kVsampl:CHCNMHGted in 50%, 75%,accelerating voltage after the surface was coated with0%，allu1uu7o CllaIlUI Iul Iu 1111 sequentially. Aftergold. The samples were analyzed by XPS on a VGdehydration, the residual alcohol on the samples wasScientificESCALAB 5 spectrometer withcleaned off in 50%, 75%, 90% and 100% isoamyl acetateCHU Cheng-lin, et al/Trans. Nonferrous Met. Soc. China 19(2009) 575- -58077water solutions for 10 min. After critical point drying,the samples were coated with thin gold films. And the(a)distribution and morphology of the platelets wereobserved by SEM.3 ResultsSEM photographs (Fig.1(a)) show that the surfaceL3of the CP NiTi SMA is relatively smooth and a whitephase that appears in the parent phase is confrmed to beTi2Ni according to energy-dispersive X-ray analysis号(EDS) performed in conjunction with SEM. ln contrast, adense oxide film is formed on the modified AOP sample,10008006004002000as indicated in Fig.1(b). The typical XPS survey spectraBinding energy/eVin Fig.2 show that the dominant elements on the CP(b)surface are Ni, Ti, O and C whereas Ti, 0 and C on themodified AOP one. The Ni concentration on the CPsample surface can reach as high as 11 .4% (molarfraction) but there is no detectable Ni on the surface ofthe modified AOP sample. The results suggest that thelatter is covered by a dense titanium oxide film that hasno detectable Ni near its top surface. The presence of C .results from physical adsorption of carbon-containingmolecules onto the surface.1 000800Fig.2 XPS survey spectra of surface of NiTi SMAs: (a) CPsample; (b) Modified AOP sampleexhibits two dominant peaks that can be identified to beTi+* (TiO2) 2p3z at 458.8 eV and Ti+ (TiO2) 2piz at464.6 eV, as displayed in Fig.3. In contrast, no Ni in anychemical state can be seen from the Ni2p XPS spectrumb(not shown here), further confirming the absence of Ni inthe outermost surface of the modified AOP sample. Thehigh resolution 0 ls XPS spectrum acquired from thesurface of the modified AOP one is shown in Fig.4together with the fted curves. The dominant peak is alsoat 530.5 eV that can be assigned to oxygen in metaloxides. Other oxygen states such as adsorbed water andTi- OH can also be detected.The XPS depth profiles of Ni, Ti and 0 acquiredfrom the suface of the modified AOP sample are shownin Fig.5. The oxygen profile shows a peak close to theFig.1 Surface morphologies of NiTi SMAs: (a) CP sample;op surface whereas the Ti profile shows a minimum(b) Modified AOP sampleconcerl中国煤化工on the outermostsurfacait. Afterwards, theHigh resolution XPS spectra were taken tofYHCN MHGro and the Niinvestigate the Ti and Ni binding energies on the surfaceconcentration increases to a steady-state value of aboutof the modified AOP sample. The Ti 2p XPS spectrum 50% after 14 min sputtering. If the oxide thickness is578CHU Cheng-lin, et al/Trans. Nonferrous Met. Soc. China 19(2009) 575-580estimated by taking the depth where the 0 signal dropsTable 1 summarizes the ICPMS results of the Ni ionto 50% of the maximum value, the thickness of theconcentrations in SBF for different immersion time andtitania film on the modified AOP NiTi SMA is about 200hemolysis ratios of NiTi SMAs. The surface titania filmnm.on the modified AOP sample can significantly reduce Nirelease from the NiTi substrate. For the two immersiontimes (2 weeks and 5 weeks), the amounts of Ni leached| Ti2pfrom the modified AOP NiTi are only about 2% of those2p32(i4+)leached from the CP NiTi. Obviously, the titania filmformed by modified AOP can mitigate out-diffusion ofNi from the substrate.Table 1 Concentrations of release Ni ions in SBF for different2p/)(i44)immersion times and hemolysis ratios ofNiTi SMAsNiTi SMANi ion concentration in SBF/109 Hemolysis2 weeks5 weeksratio/%CP sample715.64.26470465460455Modifed.915.70.52Binding energy/eVAOP sampleFig3 Ti 2p high resolution XPS spectrum of surface onmodifed AOP sampleThe hemolytic activity is assessed by determininghemoglobin release under static conditions. Thehemolysis ratios determined from the CP and modified|O1sAOP sample are 4.26% and 0.52%, respectively. A lowerMetal oxideshemolysis ratio means that less hemolysis occurs on theOHsurface of the modified AOP sample. The results showthat the hemolysis resistance of NiTi can be improved bythe formation of a titania film using the modified AOP.The morphologies of the adhered blood platelets onH2Qthe NiTi SMAs after 3 h of incubation are shown in Fig.6.The number of adherent platelets on the modified AOPNiTi is much less than that on the CP one. Accumulationand pseudopodium of platelets are also serious on the CP538 536 534532一530528526sample as indicated by the three-dimensional structuresconnected by pseudopodium in Fig.6(a). In contrast, theFig.4 0 1s high resolution XPS spectrum of surface onplatelets on the modified AOP sample are isolatedmodified AOP sample(Fig.6(b)). There is no sign of accumulation and onlyslight pseudopodium can be observed. Hence, platelet80adhesion is reduced remarkably on the NiTi SMA after●一O1sthe modified AOP and the thromboresistance of NiTi4- Ni2pSMA can also be improved by fabrication of a titania60-film.4 Discussion旨40The conventional AOP using a UV/H2O2 system is209unstable because the quantity of produced OHdiminishes continuously on account of continuousconsu中国煤化工-odified AOP, H2O210o 25can b; cathode under theSputtering time/minacidicfMHCN M H Gf oxygen from airFig.5 XPS depth profiles of composition changes of surface onblown into the electrolyte[13]:02+2H20+2e一H2O2+20H~(1)CHU Cheng-lin, et al/Trans. Nonferrous Met. Soc. China 19(2009) 575- -580579and Ni last for the same sputtering time of about 8 min,during which the molar ratio of O to Ti is stable near 2:1.This suggests that the titanium oxide film is mainlycomposed of TiO2. The XPS depth profiles also showthat the titania film has a graded structure at the interfacewith the NiTi substrate. It is likely that this gradedinterface structure improves the bonding strength of thetitania flm. SEM and XPS results obviously reveal theformation of a dense titania flm of about 200 nm inthickness after the modified AOP for 1 h, which”reow3oHT 里indicates that the modified AOP has a higher processingb)effectiveness than the conventional AOP in a givenUV/H2O2 system[12].Our results indicate the titania film produced by themodified AOP method is more efective in impeding theout-difusion of Ni from NiTi SMA during the entirefive-week immersion period. Platelet adhesion is alsoreduced remarkably on the NiTi SMA after the modifiedAOP method and the thromboresistance of NiTi SMAcan also be improved by fabrication of titania film. Theimprovement may be related to the intrinsic electricalcharacteristics of the titania film[7].Fig.6 SEM morphologies of adherent platelets on surface ofNiTi SMAs after 180 min incubation in PRP: (a) CP sample;5 Conclusions(b) Modifed AOP sample1) A modified AOP conducted in a UV/Under UV iradiation, the electrochemically-electrochemically generated peroxide system has agenerated H2O2 reacts to continuously replenish OHhigher processing effectiveness and can result in theaccording to the following reactions[13], which canformation of a dense titania film with a Ni-free zone nearprovide a stable OH source for the formation of theits top surface.titania film on NiTi substrate by oxidation:2) A trace amount of Ni can still be detected fromH2O2- UV→2.0H(2)the NiTi surface after undergoing the conventional AOPin a given UV/H2O2 system.As discussed in Ref.[11], Ti has a stronger affinity3) The titania film can dramatically reduce Nito 0 chemisorptions than Ni because the formationrelease and also improve the hemolysis resistance andenthalpy of TiO2 (- -956 kJ/mol) is four times that of NiOthromboresistance of biomedical NiTi SMA.(- 241 kJ/mol)[15]. Therefore, Ti on the NiTi surface can4) There is a smooth graded interface structurebe oxidized by .OH to form TiO2, whereas Ni mayenbancing the mechanical stability of the titania film.remain unchanged and can be removed from the Ni-Timatrix due to acidic etching in the aqueous solution.ReferencesMoreover, the formation of a titania flm can be furtheraccelerated by anodic oxidation of the NiTi substrate[1] CHENG Y, CAI w, LI H T, ZHENG Y F, ZHAO L C. Surfacecharacteristics and corosion resistance properties of TINi shapeserving as the anode. The exact in-situ formationmemory aly coated with Ta [] Surface and Coatings Technology,mechanism of the titania film on NiTi SMA during the2004, 186(3) 346 -352.modified AOP is studied in more details and new[2] OTSUKA K, WAYMAN C M. Shape mermory materials [M].findings will be reported in due course.Cambridge: Cambridge University Press, 1998.As indicated by these results, the modified AOP can[3DUERIG T, PELTON A. STOCKEL D. An overview of nitinolmedical apolications [J]L. Mater Sci Ene A, 1999, A273/275: 149-result in the formation of a dense titania flm with a中国煤化工_Ni-free zone near its top surface, whereas a trace of Ni[4D. MAN H C. Bioactivecan still be detected on the upper NiTi surface after theTYHC N M H Gding iplaos [I Matarconventional AOP in a UV/H2O2 system, as reportedSci Eng C. 2004, 24: 497-502.previously[12]. The titanium oxide plaforms of both 0[5RONDELLI G VICENTINI B. Localized corrosion behaviour in580CHU Cheng lin, et al/Trans. Nonferrous Met. Soc. China 19(2009) 575- -580simulated human body fuids of commercial Ni-Ti orthodontic wires[1] CHUCL, HU T. wusL, WANG R M, DONG Y s, LINP H,0 Biomaterials, 199 20: 785- -792.CHUNG C Y, CHU P K In situ synthesis of nanostructured titania[6] BAIER RE, DUTTON RC. Iitial events in itencton of bod withfilm on NitTi shape memory alloy by Fenton's oxidation method小]foreign surfaces [n. J Biomed Mater Res, 1969, 3: 191-206.Trans Nonferous Met Soc China, 2007, 17(): 902- 906.[7] LIUJX, YANG D z, SHI F, CAI Y J. Sol-gel deposited TiO, flm on[12]WANGRM,CHUcL,HUT,DONGYs,GUOC,SHENGXB,NiTi surgical lly for biocompatibility improvement [1]. Thin SolidLIN P H CHUNG C Y, CHU P K. Surface XPS caneterization ofFilms, 2003, 429: 225- 230.NiTi shape memory aly afer advanced oxidation processes in[8] FIRSTOV G s, VITCHEV R G KUMAR H, BLANPAIN B,UVH2O2 photocatalytic system [小] Applied Surface Science, 2007,HUMBEECK J V. Surface oxidation of NiTi shape memory aly U253: 8507-8512.Biomaterials, 2002, 23: 4863- -4871.[13] NEYENS E, BAEYENS J. A review of cassic Fenton's peroxidation[9] CHENGF τ, SHIP, PANGGK H, WONG M H, MAN HC.as an advanced oxidation technique [I]. Jourmal of HazardousMicrostructural characterization of oxide flm formed on NiTi byMaterials, 2003, B98: 33-50.anodization in acetic acid J]. Jourmal of Alloys and Compounds,[14] BERGENDAHL J A, THIES T P. Fenton's oxidation of MTTBE with2007, 438(12): 238-242.zro-valen iron []. Water Research, 2004. 38: 327- 334.[10] HUT, CHU CL, YINLH, PUYP, DONG Y s, GUO C, SHENG x[15] WEAST R C, ASTLE MJ. CRC Handbook of chenistry and physicsB, CHUNG C Y。CHU P K. In vitro biocompatibility of[M]. Boca Raton, Flonda: CRC Press, 1982: 36.titanium-nickel aloy with titanium oxide flm by H2O2 oxidation [D].(Edited by YANG Bing)Trans Nonferrous Met Soc China, 2007, 17(3): 553-557.中国煤化工MYHCNMHG