The role of surface oxide thickness and structure on the corrosion and nickel elution behavior of nitinol biomedical implants

Biocompatibility is an important factor for metallic medical device implants, and corrosion resistance of implantable alloys is one aspect of biocompatibility. Corrosion behavior of nitinol is strongly dependent upon the nature of the surface oxide, which forms during processing. The surface oxide is comprised of a mixture of titanium and nickel oxides, and subsequent thermal exposure (e.g., during shape setting) and surface removal (e.g., electropolishing, mechanical polishing, etching, etc.) influence its structure. Corrosion behavior is often assessed through testing methods such as cyclic potentiodynamic polarization (e.g., ASTM F2129) and nickel ion release. Studies have suggested that a correlation exists between oxide thickness and nickel ion release, with thicker oxides eluting more nickel. It is hypothesized that the composition of the surface oxide, and not only its thickness, influences the corrosion performance of nitinol. To investigate this, nitinol wire samples were processed to produce surface oxides with different structures both in terms of thickness and composition. These samples were tested per ASTM F2129 and nickel ion release testing.