MATERIAL METAMORPHOSIS: When heated, biodegradable shape-memory polymer transforms from a temporary shape (top) to its parent shape (bottom) within 20 seconds. Image: © 2001 National Academy of Sciences, U.S.A.
Special plastic materials able to change shape in response to temperature may soon find applications in a variety of extreme climes¿from the warm, moist environs of human blood vessels to cold, wet and windy mountaintops. These plastics have a "memory" that allows them to be deformed into a temporary configuration and then be restored to the original parent geometry by applying heat. Shrink-wrap is perhaps the most familiar example of a shape-memory polymer (SMP). But since the mid-1980s chemists, materials scientists and engineers have been working to develop SMPs as a kind of "smart" material¿a substance that can respond to environmental changes as desired.
Shape-memory substances are not new: certain metallic compounds exhibited the effect in the 1930s, and alloys such as nickel-titanium (Nitinol) have since found use in actuators and medical devices such as dental braces and endovascular implants. These metals switch from a temporary to a parent shape above a certain transition temperature. Below that temperature, the shape-memory alloy (SMA) can be bent into various configurations.