Smart drugs, smart bombs, smart mice--there seems to be nothing upon which science can't bestow a higher IQ. But smart plastics? That's what a team of scientists has come up with: so-called shape-memory polymers that remember forms they once held. The plastics might one day be used to develop "smart" medical devices or self-repairing materials.
Shape-memory was first achieved with other materials. In the 1950s scientists discovered that certain metal alloys can switch between two shapes when they are heated above their so-called transition temperature, from a temporary form to a chemically predetermined one. Doctors use wires made of a nickel-titanium alloy that change their shape, for instance, by expanding to a mesh when the body warms them. The alloys are also used to make self-repairing eyeglass frames. But the materials are expensive and only allow for a deformation of about 10%, says polymer chemist Andreas Lendlein of the Technical University Aachen, Germany.
So Lendlein and his colleagues decided to look for a better alternative. The team focused on plastics, because it's easy (and cheap) to change their properties, such as elasticity, by varying the recipe. After tinkering with several ingredients, they struck gold with a mixture of two chemicals, oligo(e-caprolactone) dimethacrylate and n-butyl acrylate. The resulting plastic can be programmed with ultraviolet radiation to assume any permanent shape, and it can switch back to that shape even after being deformed by more than 100%, the team reports in the 30 January issue of the Proceedings of the National Academy of Sciences. If the plastic is deformed past the point of recovery, heating it to its transition temperature will bring back the programmed shape.
"It's a really clever story," says polymer chemist Ulrich Suter of the Swiss Federal Institute of Technology in Zürich. The new materials, he adds, could be used to construct almost anything, from medical implants to car bodies. Lendlein agrees: "Think about it. You get into an accident, your car has a dent, you take out your hairdryer and heat up the dent--and the damage is fixed." Now that would be a smart car.