1.3 Shape Memory Alloys for Use in Medicine

This chapter initially describes some fundamental aspects of shape memory alloy systems, including essential crystallographic features and how shape memory and superelastic characteristics are derived from these crystal structures and their temperature or stress dependence. Practical shape memory materials such as nitinol are introduced, and the importance of composition, melting methods, and component fabrication and heat treatment is reviewed. Microstructural changes during heat treatment and how these influence material transformation temperatures and mechanical properties are described. Methods for measuring transformation temperatures are briefly reviewed. Minimally invasive device applications for nitinol are reviewed in some detail, with an emphasis on cardiovascular stents, where the materials have been widely utilized. The advantages of the unique features of nitinol in stenting applications are presented and these are substantiated with a review of some clinical applications in coronary, peripheral, and aortic vessel applications. Stenting for other nonvascular indications and other devices such as vena cava filters and septal occluders are also presented. The use of the material in orthodontic and orthopedic applications is summarized, with porous nitinol structures offering interesting opportunities in relation to improved tissue integration. Important performance characteristics such as fatigue, corrosion, and imaging compatibility are also addressed. In conclusion, it is predicted that new applications for nitinol will continue to evolve.