ABSTRACT
In the paper, a review of modification methods which have been applied to Nitinol intermetallic compound used as biomaterial for medical applications, is carried out. A variety of methods used for Nitinol improvement, beginning from its manufacture, covers mechanical treatment, heat treatment, chemical processing including water boiling, electropolishing, plasma ion implantation, coating to improve the corrosion resistance, minimize nickel leaching, improve osseointegration, and/or vascular compatibility, sterilization and disinfection. Nitinol alloying by addition of a third element to replace Ti or Ni has an enormous effect on phase transformation, corrosion resistance and biocompatibility of the newly created ternary Nitinol alloy. Unfortunately, the ternary nitinol alloys – apart from NiTiCu in actuators and NiTiCr in wires used as a pulling-pushing tool in minimally invasive spine operation – have not found a widespread industrial application yet. One of the most effective surface finishing operations used for metallic biomaterials, with a special attention directed to Nitinol, appears to be magnetoelectropolishing (MEP). A uniform magnetic field used in MEP process can be generated by permanent magnets or by electromagnets. MEP leads to refinement of surface chemical composition impossible to achieve by standard electropolishing. During MEP of alloys and intermetallic compounds, ferromagnetic elements, such as Fe (from stainless steels) or Ni, are primarily removed from the surface so that the passive film on Nitinol is totally composed of titanium oxide. One more unique feature of metal samples after MEP is their de-hydrogenation noticed both in stainless steels, niobium, titanium and Nitinol. In fact, the fatigue resistance of Nitinol after MEP referred to as-received, dependent on the refinement and displacement of inclusions, increases from three to seven times.
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