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

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 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 Nitinol 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 implantation, with released nickel concentration in surrounding tissues or organs being equivalent to that released by 316L SS.