A shape memory alloy (SMA) (also known as memory metal or smart wire) is a metal that remembers its geometry. After it is deformed, heated to a specific temperature it regains its original geometry by itself.

The two main types of SMA are the copper-zinc-aluminium alloys, and the generally more expensive nickel-titanium alloys. The temperature at which the SMA changes its structure is characteristic of the alloy, and can be tuned by varying the elemental ratios.

The nickel-titanium alloys were first developed in 1965 by the Naval Ordnance Laboratories and commercialised under the trade name Nitinol (an acronym for NIckel TItanium Naval Ordnance Laboratories).

The range of applications for SMAs has been increasing in recent years, with one major area of expansion being medicine, for example the development of dental braces that exert a constant pressure on the teeth. However, these materials are not currently appropriate for applications such as robotics or artifical muscles, due to limited energy inefficiency, slow response times, and large hysteresis.

Metal alloys are not the only thermally responsive materials, as shape memory polymers have also been developed, becoming commercially available in the late 1990's.

There is another type of SMA called ferromagnetic shape memory alloys (FSMA), that change shape under strong magnetic fields.

External links

• Nitinol technology http://www.nitinol.com/3tech.htm (from Nitinol Devices & Components)
• Nitinol Technical Data/Application Notes http://www.jmmedical.com/html/nitinol_technical_information.html#NitinolTechnica
  lInfo (from Johnson Matthey, Inc.)
• introductions and comparisons of active materials http://www.mide.com/active_materials.html (from Midé Technology Corporation)
• BBC report on medical applications of Nitinol http://news.bbc.co.uk/1/hi/health/755819.stm