Corrosion properties of the superelastic shape memory Ni-Ti alloy for medical implants

Abstract

The Ni-Ti alloy (Nitinol, approx. 50 at. % Ni) is a biomaterial showing the shape memory and superelasticity effects. These characteristics make this material of interest for biomedical applications, especially for manufacture of stents, i.e., tubular implants for restoring damaged blood vessels, oesophageus etc. The corrosion resistance in the human body environment is a very important factor determining the life time of implants. An insufficient corrosion resistence may lead to fractures of implants, formation of dangerous fragments and to serious health problems for patients. The corrosion behavior is influenced mainly by the surface structure and chemistry of a material. But the surface state is strongly modified by chemical and heat treatment processes used in the implant manufacture. Therefore, in this study the relationships between chemical treatment and heat treatment regimes, surface state of Nitinol and its corrosion resistance in simulated physiological solution are demonstrated. It is shown that, unlike chemical pre-treatment, heat treatment used in the manufacture of stents generally negatively influences the corrosion resistance. The findings are discussed in relation to the surface state and chemistry of the material.