Nickel-titanium (NiTi) alloy derives its biocompatibility and good corrosion resistance from a homogeneous oxide layer mainly composed of TiO(2), with a very low concentration of nickel. In this article, we described the corrosion behavior of NiTi alloys after mechanical polishing, electropolishing, and sterilization processes using cyclic polarization and atomic absorption. As a preparative surface treatment, electropolishing decreased the amount of nickel on the surface and remarkably improved the corrosion behavior of the alloy by increasing the mean breakdown potential value and the reproducibility of the results (0.99 +/- 0.05 V/SCE vs. 0.53 +/- 0. 42). Ethylene oxide and Sterrad(R) sterilization techniques did not modify the corrosion resistance of electropolished NiTi, whereas a steam autoclave and, to a lesser extent, peracetic acid sterilization produced scattered breakdown potential. In comparing the corrosion resistance of common biomaterials, NiTi ranked between 316L stainless steel and Ti6A14V even after sterilization. Electropolished NiTi and 316L stainless-steel alloys released similar amounts of nickel after a few days of immersion in Hank's solution. Measurements by atomic absorption have shown that the amount of released nickel from passive dissolution was below the expected toxic level in the human body. Auger electron spectroscopy analyses indicated surface contamination by Ca and P on NiTi during immersion, but no significant modification in oxide thickness was observed.

Because orits good radiopacity, superelasticity, and shape memory properties, nickel-titanium (NiTi) is II potential material for fabrication of stents because these properties Clln facilitate their implantation and precise positioning. However, ill vitro studies of NiTi alloys report the dependence of alloy biocompatibility and corrosiun behavior on surface conditions. Surface oxidation seems to be very promising for improving the corrosion resistancc and biocompatibility of NiTi. In this work, we studied the effect on corrosion resistancc and surface chanlcteristics of electropolishing, heat treatment, and nitric acid passivation of NiTi stents. Characteri zation techniques such as potentiodyna mic polarization tests, scanning electron microscopy, Auger electron spectroscopy, and X-ray photoelectron spectroscopy were used to relute corrosion behavior to surface ch.mlcteristics and surface treatments. Results show that all of these surface treatments improve the corrosion resistance of the alloy. This imprOl'ement is attributed to the plastically deformed nath'e oxide layer removul und replacement by a newly grown, more uniform one. The uniformity of the oxide layer, rather than its thickness and composition, seems to he the predominant factor to explain the corrosion resistance improvement. 0 t998 John Wiley & Son~. Inc.

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SummaryNitinol is a very attractive material for manufacturing minimally-invasive therapy devices and tools because of its unique superelasticity and shape-memory properties. While several studies have shown it to possess good biocompatibility, its high nickel content and possible dissolution during corrosion still remain a concern . However, passivation and electropolishing can significantly decrease nickel dissolution from Nitinol by forming a corrosion-resistant titanium oxide surface layer. In general, passivated and electropolished Nitinol exhibits equivalent, if not better, static corrosion behaviour and ability to resist and repassivate (repair) surface damage when compared with 316L stainless steel (SS). Combining Nitinol with SS, titanium and tantalum does not significantly affect its corrosion behaviour. However, combining Nitinol with gold, platinum and platinum-iridium alloy can result in an order of magnitude increase in corrosion rate. Nickel release from Niti nol decreases from well below dietary levels to nearly non-detectable levels in the first few days following immersion in a physiological medium. Finally, in vivo studies indicate minimal corrosion of Nitinol during im plantation, with released nickel concentration in surround ing tissues or organs being equivalent to that released by 316L SS.

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