The electrochemical characteristics of native Nitinol surfaces

The present study explored the avenues for the improvement of native Nitinol surfaces for implantation obtained using traditional procedures such as mechanical polishing, chemical etching, electropolishing and heat treatments for a better understanding of their electrochemical behavior and associated surface stability, conductivity, reactivity and biological responses. The corrosion resistance (cyclic potential polarization, open circuit potential and polarization resistance) of Nitinol disc and wire samples were evaluated for various surface states in strain-free and strained wire conditions. The surface response to tension strain was studied in situ. Surface chemistry and structure were explored using XPS and Auger spectroscopy and photoelectrochemical methods, respectively. It was found that the polarization resistance of the Nitinol surfaces varied in a range from 100 kΩ to 10 MΩ cm2 and the open circuit potentials from −440 mV to −55 mV. The surfaces prepared in chemical solutions showed consistent corrosion resistance in strain-free and strained states, but mechanically polished and heat treated samples were prone to pitting. Nitinol surface oxides are semiconductors with the band gaps of either 3.0 eV (rutile) or 3.4 eV (amorphous). The conductivity of semiconducting Nitinol surfaces relevant to their biological performances is discussed in terms of oxide stoichiometry and variable Ni content. Such biological characteristics of Nitinol surfaces as Ni release, fibrinogen adsorption and platelets behavior are re-examined based on the analysis of the results of the present study.