Influence of nitinol wire surface treatment on oxide thickness and composition and its subsequent effect on corrosion resistance and nickel ion release B. Clarke, 1 W. Carroll, 1 Y. Rochev, 1 M. Hynes, 2 D. Bradley, 3 D. Plumley 3 1 National Centre for Biomedical Engineering Science, National University of Ireland, Galway, Ireland 2 Department of Chemistry, National University of Ireland, Galway, Ireland 3 Fort Wayne Metals Research Products Corporation, Fort Wayne, Indiana 46809 Received 7 October 2005; revised 9 December 2005; accepted 30 December 2005 Published online 6 June 2006 in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/jbm.a.30720 Abstract: Medical implants and devices are now used suc- cessfully in surgical procedures on a daily basis. Alloys of nickel and titanium, and in particular Nitinol are of special interest in the medical device industry, because of their shape memory and superelastic properties. The corrosion behavior of nitinol in the body is also of critical importance because of the known toxicological effects of nickel. The stability of a NiTi alloy in the physiological environment is dependant primarily on the properties of the mostly TiO 2 oxide layer that is present on the surface. For the present study, a range of nitinol wires have been prepared using different drawing processes and a range of surface prepara- tion procedures. It is clear from the results obtained that the wire samples with very thick oxides also contain a high nickel content in the oxide layer. The untreated samples with the thicker oxides show the lowest pitting potential values and greater nickel release in both long and short-term experiments. It was also found that after long-term immer- sion tests breakdown potentials increased for samples that exhibited lower values initially. From these results it would appear that surface treatment is essential for the optimum bioperformance of nitinol. © 2006 Wiley Periodicals, Inc. J Biomed Mater Res 79A: 61–70, 2006 Key words: nitinol; medical devices; surface treatments; cor- rosion resistance; nickel release INTRODUCTION Alloys of nickel and titanium, and in particular Nitinol (50% Ni, 50% Ti) are of special interest in the medical device industry, because of their shape mem- ory and superelastic properties. The use of nitinol for medical purposes was first reported in the late 1960s 1 and since then it has been used in a variety of appli- cations. These applications include orthodontics, 2 or- thopedics, 3 and clinical instruments. 4 In more recent years, the widespread commercial use of nitinol for stent applications has made a breakthrough in medi- cine. Nitinol stents are primarily used in peripheral and coronary bypass graft interventions. 5 Whether nitinol is fabricated for stents (vascular and nonvas- cular) or other devices or instruments its stability in the complex body fluid environment relies almost ex- clusively on the stability of its surface oxide, which is mainly composed of TiO 2. The surfaces of NiTi alloys show a tendency towards preferential oxidation of titanium which results in the TiO 2 surface oxide layer. This behavior is in agreement with the fact that the free enthalpy of formation of TiO 2 is negative and exceeds in absolute value the enthalpy of formation of nickel oxides by at least two or three times. 6 As a result of this, any nickel present on nitinol surfaces is mainly present in its elemental state. Considering the known toxicological effects of nickel, primary con- cerns regarding nitinol as an implant material include the high nickel content of the alloy, its elemental sur- face state and the possible influence of this on biocom- patibility. The main functionality of the TiO 2 oxide layer is (1) to increase the stability of surface layers by protecting the bulk material form corrosion and (2) to create a physical and chemical barrier against nickel oxidation and release. 7 The natural oxides formed on nitinol surfaces have also been found to contain nickel, which can alter the stability of the material, and to a large extent, effect the corrosion resistance. 8 The corrosion resistance of nitinol in vitro has been evaluated in a number of studies, indicating that it Correspondence to: B. Clarke; e-mail: brenda.clarke@ nuigalway.ie Contract grant sponsor: Irish Research Council for Sci- ence, Engineering and Technology (IRSCET): Funded by the National Development Plan © 2006 Wiley Periodicals, Inc.