(V) Accelerated Method to Predict Long-Term Nickel Release from Nitinol

Nitinol is commonly used in medical devices because of its unique shape memory and superelasticity properties. However, nitinol is also known to release nickel ions from its surface, and this can lead to local and systemic toxicity concerns in patients. Therefore, nickel release from nitinol-based devices is typically characterized through time consuming in vitro immersion tests. In this study we explore elevated temperature as a potential method to expedite this testing using four different surface finishes of nitinol (electropolished, chemically etched, amber oxide and black oxide). Nickel ion release immersion testing was characterized for four different temperatures (37 ⁰C, 52 ⁰C, 67 ⁰C and 87 ⁰C) using inductively-coupled plasma mass spectrometer. Pitting corrosion susceptibility was assessed using ASTM F2129 and auger electron spectroscopy was used to characterize elemental compositional depth profiles and oxide thickness. We found that for the three materials with relatively thin oxide layers (electropolished, chemical-etch and amber-oxide), nickel release exhibited Arrhenius behavior over the entire temperature range. However, nickel release from black oxide nitinol, which had a much thicker oxide layer, did not follow Arrhenius behavior. Our expedited protocol illustrated that a 50-day nickel release profile at 37 ⁰C can be accurately captured by testing for less than one week at 67 ⁰C. Additional testing is needed to validate temperature scaling that can be routinely used to evaluate nickel release from nitinol devices of different surface finish.