Assessing the corrosion behaviour of Nitinol for minimally-invasive device design

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Abstract
Summary Nitinol 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 duri ng corrosion sti ll 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 electropoli shed 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 wit h SS, titanium and tantalum does not significantly affect its corrosion behaviour. However, combining Nitinol with gold, plat inum and platinum- iridium alloy can result in an order of magnitude increase in corrosion rate. Nickel release from Ni ti 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 surrounding tissues or organs being equivalent to that released by 316L SS.