Physicochemical and biological aspects of Nitinol as a biomaterial

The rapid increase in the development of medical devices using nearly equiatomic Ni-Ti alloys (Nitinol) provides evidence of their advantages as biomaterials. Two generic properties of Nitinol, thermally induced shape recovery and superelasticity, are successfully employed for medical uses. However, several aspects of the alloys' short and long term efficacy have yet to be comprehensively determined, especially parameters relating to surface and corrosion performance, The present review surveys studies carried out internationally over about the past 10 years aiming at an improved understanding of Nitinol's surface, corrosion, and biocompatibility features. A critical survey is given of the surface condition of alloys modified using various advanced techniques, oxidised in air-water-steam mixtures, and passivated using chemical-electrochemical procedures, Surface parameters and their importance to the improvement of Nitinol's corrosion resistance and biological performance are discussed on the basis of XPS, Auger, SEM, and BSEM studies. In vivo and in vitro corrosion studies using potentiodynamic, potentiostatic, and scratch tests on as cast alloys, wire, and actual devices are surveyed. The effects of stress, strain, heat treatment, microstructure, precipitates, and alloy composition on corrosion are evaluated and compared with data for stainless steels, cobalt base alloys, and titanium. Release of nickel, observed in vitro and in vivo, is also surveyed. Biocompatibility aspects are briefly discussed with emphasis on their possible correlation with surface condition.