Understanding the Biocompatibility of Nitinol Biomedical Devices

This presentation explores the effects of oxidation and passivation on the corrosion resistance, ion release, and biocompatibility of medical grade Nitinol. High spatial and angular analytical techniques such as SEM, FIB, AES, as well as synchrotron techniques (microdiffraction, SAXS, and XPS) were used to characterize the composition, phase distribution, and thickness of Nitinol surfaces after various treatments. We will discuss these findings in terms of anodic polarization behavior and Ni ion release data of implant devices. There is a strong inverse relationship between the thickness and structure of surface oxide and the localized and uniform corrosion resistance of the material. Properly passivated Nitinol surfaces consist of Ni-free amorphous titanium oxide, whereas thermal oxide surfaces are composed of crystalline TiO₂ (rutile) with Ni-rich phases (Ni and Ni₃Ti). Long-term extraction tests show minimal Ni release rate for passivated Nitinol, whereas thermally oxidized Nitinol shows an increased ion release. In this presentation, we also review corrosion-related failure analyses of medical explants. These observations, in combination with scientific investigations, provide comprehensive research that probes the structure of Nitinol biomedical devices surfaces and their interactions with biological fluids.  We believe, therefore, that these results provide a first rational explanation for the different biological response of nominally very similar passivation barriers.  In addition, this presentation will also present scientifically sound strategies for increasing the inertness of Nitinol devices in vivo.