The researchers who developed self-tying sutures that change shape when exposed to light have now made morphing structures that can take on three consecutive shapes in response to changes in temperature. The shape-changing polymers could eventually be employed as removable stents and self-closing fasteners used in assembling complex parts.

The structures are made of shape-memory polymers, a class of materials that change from one preset shape to another in response to a new condition, such as increased heat. In the past few years, various research groups have created polymers that respond to light or a magnetic field. But now researchers at MIT and the GKSS Research Center, in Teltow, Germany, have added the ability to morph polymers into a third shape. "This is the first time you can make a material go from shape A to B to C," says Robert Langer, MIT chemical-engineering professor and one of the researchers.

In the Proceedings of the National Academy of Science (PNAS) this week, the researchers describe two prototype structures that use the new materials. In the first prototype, a flattened plastic tube expands after being heated, forming an open tube that maintains this shape even after cooling. When heated to a still higher temperature, the tube shrinks in diameter. A potential application: a stent that can be opened once placed within a patient's body, such as inside an artery, and then, after the stent has served its purpose, be heated again to make it small enough to remove.

The other prototype unfolds and then extends two arms to fasten itself in place (see a video of the process here). This design could be useful on assembly lines for fastening together difficult-to-reach parts, says Andreas Lendlein, head of the Institute for Polymer Research at the GKSS Research Center and one of the authors of the PNAS paper.