Nitinol (nickel-titanium) alloys exhibit a combination of properties which make these alloys particularly suited for self-expanding stents. Some of these properties cannot be found in engineering materials used for stents presently. This article explains the fundamental mechanism of shape memory and superelasticity, and how they relate to the characteristic performance of self-expanding stents. Nitinol stents are manufactured to a size slightly larger than the target vessel size and delivered constrained in a delivery system. After deployment, they position themselves against the vessel wall with a low, "chronic" outward force. They resist outside forces with a significantly higher radial resistive force. Despite the high nickel content of Nitinol, its corrosion resistance and biocompatibility is equal to that of other implant materials. The most common Nitinol stents are listed and described.

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In this study, the oxidation of the surface of a 9 / 5 0 (at.%) NiTi alloy was studied by XPS, AFS and SEM. Samples of the NiTi alloy were oxidized at 23 "C and high temperatures at an oxygen pressure of lo-' Torr and in atmosphere. At lo-' Torr of 0, and 23"C, titanium was oxidized preferentially while nickel remained in the metallic form even after 40 min of exposure. At the same oxygen partial pressure and 400 "C, a mixture of TiO, , Ti,O, and TiO, was formed, and nickel remained metallic. Eventually, after 40 min of 0, exposure, the whole surface was covered with a layer of TiO, . In atmosphere and at 23 "C, titanium again was oxidized prefereotially, but NiO was detected on the surface after the first minute of air exposure. At a higher temperature of 450°C, areas comprising a mixture of NiO and TiO, , and areas consisting of TiO, only were found. Below the top surface layer, a complete layer of TiO, was detected. Underneath the TiO, layer, a nickel-rich layer was present.