Corrosion behavior of nitinol in blood serum and PBS containing amino acids

The susceptibility to localized corrosion of metallic, implant medical devices is typically evaluated in a simulated physiological solution, such as phosphate buffered saline (PBS). For implant devices in contact with blood, the underlying premise is that proteins and other components in blood do not significantly change the corrosion susceptibility. This study examined the corrosion behavior of nitinol in bovine serum and PBS containing amino acids (cysteine, glutamine, and tryptophan). Tests were performed on mechanically polished (MP) and black oxide (BO) nitinol wire using cyclic potentiodynamic polarization and electrochemical impedance spectroscopy. They showed that the susceptibility to pitting corrosion of MP nitinol in PBS can be influenced by the presence of amino acids, depending on the type of amino acid. However, the pitting susceptibility of MP and BO nitinol did not differ significantly (based on a t-test, with p < 0.05 being statistically significant) between serum and PBS, suggesting that the combination of proteins and amino acids in serum had little effect. The impedance spectra showed near-capacitive behavior in serum and in PBS alone and with amino acids, and the data could be fitted by a parallel resistance-capacitance (as a constant phase element) circuit associated with the passive oxide film. The capacitance indicated that serum proteins were adsorbed on nitinol but that little, if any, adsorption of the three amino acids under study occurred at the corrosion potential. There did not appear to be a correlation between breakdown and the adsorption of organic compounds in serum.