Shape shifter shifts twice.

Certain plastics known as shape-memory polymers switch to predetermined shapes when triggered by heat or light. Now, researchers have developed more-versatile versions of such polymers. When heated, each of the new triple-shape polymers switches to a second shape. Then, at a higher temperature, the plastic changes to a third form.

"For some applications, [these] more-complex deformations are required," says chemist Andreas Lendlein of the GKSS Research Center in Teltow, Germany. He, chemical engineer Robert Langer of the Massachusetts Institute of Technology, and their colleagues have already made prototype devices from the new substances.

One such device is a tube that could force open partially blocked blood vessels. The tube, a removable stent, would start out with a squashed shape. Body heat would then expand the stent, which would push outward on the vessel walls. Later, further heating of the stent would shrink it for easy removal.

Each of the two new triple-shape plastics is composed of two different polymer components whose chain like molecules don't mix easily. Certain chemical bonds, however, join the polymers in specific places.

The composites' shape-shifting capabilities arise because below specific temperatures, the molecules of the two polymer components become attached to other molecules of the same kind, creating networks of linked chains within the material, Lendlein explains. Because those networks form at different temperatures for the different components, each composite plastic has three possible states--with no networks, with one network, and with two networks. A step during fabrication of an object associates a shape with each state.

The researchers describe the new materials and their potential applications in the Nov. 28 Proceedings of the National Academy of Sciences.--P.W.