Morphing Materials Take On New Shapes

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.

Lendlein says the key to the new structures was developing two types of polymers that have distinct melting points. At room temperature, the material holds its first shape. But when heated above a certain temperature, areas throughout the material soften, allowing it to change to an intermediate shape. At a yet higher transition temperature, the rest of the material softens, allowing the structure to take its final shape.

Although the researchers designed polymers specifically for this project, Lendlein says that the results demonstrate a general principle that could work with a variety of polymers. Indeed, he says that for specific applications researchers may need to change materials to make them compatible for use in the body and to decrease costs for use in manufacturing. Lendlein is working with mNemoscience, based in Aachen, Germany, a spin-off of MIT and RWTH Aachen University, to commercialize the new technology.

Richard Vaia, a researcher at the Air Force Research Labs, says the work is part of a trend in shape-memory research toward more-complex, higher-performance materials. While early applications, such as shrink-wrap, were simple, researchers are using nanoparticles to increase the amount of force the materials can apply as they change shape, and they’re developing new ways of triggering the changes, Vaia says. Another major goal would go beyond Lendlein’s multishape materials to those that can change shape reversibly, returning to the original shape for applications that require repeated movements.