Effects of Oxide Layer Composition and Radial Compression on Nickel Release in Nitinol Stents

There is a public health need to understand the effects of surface layer thickness and composition on corrosion in nickel-containing medical devices. To address this knowledge gap, five groups of Nitinol stents were manufactured by various processing methods that altered the titanium oxide layer. The following surfaces were created: >3500 nm thick mixed thermal oxide (OT), ~420 nm thick mixed thermal oxide (SP), ~130 nm thick mixed thermal oxide (AF), ~4 nm thick native oxide (MP), and an ~4 nm thick passivated oxide (EP). Radially compressed and not compressed devices were evaluated for nickel (Ni) ion release in a 60-day immersion test. The results indicated that OT stents released the most Ni, followed by stents in the SP and AF groups. For OT and SP stents, which exhibited the thickest oxide layers, radial compression significantly increased Ni release when compared to non-compressed stents. This result was not observed in AF, MP, SP stents indicating that the increased Ni release may be explained by cracking of the thicker oxide layers during crimping. Strong correlations were observed between oxide layer thickness and cumulative Ni release. These findings elucidate the importance of oxide layer thickness and composition on uniform corrosion of laser-cut Nitinol stents.