Curl . . . and release. A strip of polymer spirals when exposed to UV light, then uncurls under a shorter wavelength.

Teaching An Old Polymer New Tricks

Researchers have cooked up a new plastic that switches shape in response to various types of light. The technology may someday be used for self-stitching sutures, artificial muscles, or staples that pop open on command.

Plastics are made of polymers, long repetitive chains of molecules that wiggle and loop like a tangled ball of string. Whether a polymer is flexible or stiff depends on how many molecular bonds link its strands. The more cross-links it has, the stiffer it becomes. These links determine the shape of the polymer and typically are set during the initial processing of the material.

In the 14 April Nature, Andreas Lendlein at the GKSS Research Center in Teltow, Germany, and colleagues demonstrate a polymer with cross-links that can be broken and rearranged via exposure to ultraviolet light. The researchers took a common flexible polymer and added cinnamic acid, a molecule containing chemical bonds that break when exposed to ultraviolet light. When the researchers molded the plastic into a specific shape and exposed it to UV light of a wavelength longer than 260 nanometers, the cinnamic acid bonds formed cross-links, freezing the plastic in its new shape. When the team shined UV light shorter than 260 nanometers onto the plastic, the cross-links broke and reverted back to their original configuration, returning the polymer to its old form.

The shape-shifting polymer is the first of its kind to be sensitive only to light, say the researchers. Previous shape-memory polymers were primarily temperature-sensitive, limiting how well they could be controlled. Temperature-sensitive sutures with preformed knots, for example, often flipped into shape from body heat before they could be tugged into place.

"These are fairly normal polymers that no one has thought to use in this way before," says Dave Putnam, a biomedical engineer at Cornell University in Ithaca, New York. To make them truly useful, he adds, the researchers need to find a way to get the light-sensitive polymers to snap into shape as quickly as thermosensitive plastics do, in seconds instead of minutes.

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