Teaching an Old Polymer Memory Tricks

A polymer takes on four shapes depending on the temperature.

Researchers have uncovered shape-memory properties in a commercially available polymer that's widely used to make fuel-cell membranes. The polymer, Nafion, can take on four different shapes in response to temperature changes--researchers have made triple-shape polymers before. "It's arguably the most versatile smart polymer ever discovered," says Tao Xie, a polymer scientist at the GM Research and Development Center in Warren, MI, who published his findings in this week's Nature.

Morphing magic: Researchers have trained the polymer Nafion to take on different shapes at different temperatures. Clockwise from top left: the polymer’s original shape; at 140 ºC; at 107 ºC; and at 68 ºC. Credit: Tao Xie, GM Research and Development Center

Shape-memory polymers go from a predefined shape to another in response to triggers such as temperature, light, or magnetic field. Most of these materials have one temporary shape and a permanent state they revert to when triggered. The polymers are being developed for self-adjusting orthodontic braces, self-tying sutures for laparoscopic surgery, medical devices such as blood vessel stents and bone implants, and aircraft with wings that morph during flight.

The versatility of Nafion as a multishape polymer is surprising, as it was not initially developed for this purpose, says Andreas Lendlein, a shape-memory polymer pioneer at GKSS Research Center in Teltow, Germany. Xie says his findings suggest that a broad range of polymers with similar properties might be able to change shape multiple times.

Nafion's shape-shifting properties also indicate that it could take on more than four shapes. The polymer can be programmed to morph at arbitrary temperatures within a broad range as long as these temperatures are well separated. Xie was able to program three transitions, giving a total of four shapes. "You wonder how many they can remember," he says.

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"The more shapes you can memorize, the more complex you can make the structure and function," says Christoph Weder, a polymer chemist at the University of Fribourg in Switzerland. It could, for one, expand current applications: aircraft wings that take on four different forms; spacecraft antenna that expand to different widths; and adjustable-width stents that could be moved from a wider blood vessel to a narrower one or that could be placed at the branch of an artery.

In 2006, Lendlein and his colleagues developed a material that can take on three shapes in response to heat changes. The key was to develop two types of polymers with distinct melting points. The material kept one shape at room temperature, shifted to a second shape when heated above one melting point, and a final shape when heated to an even higher temperature.