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INSPIRING CREATURE: A new polymer made to act similarly to a sea cucumber can soften when its contacted by water.
COURTESY OF FRED CARPENTER

Stealing a trick from a tiny, pickle-shaped creature that dwells in the depths of the ocean, scientists have designed a new polymer that, when exposed to water, can instantly change its rigidity and strength. The inventors say the innovation could be useful in biomedical applications, such as brain implants for patients suffering from Parkinson's disease, spinal cord injuries or stroke. Those inserts, say some researchers, may fail because they need to be very stiff when they are “installed.”

When contacted by water, the material transforms from a hard plastic, like that of a CD case, to a soft rubber, explains Stuart Rowan, an associate professor of macromolecular science and engineering in the chemistry department at Case Western Reserve University in Cleveland and a co-author on the new study, which appears in Science.

Sea cucumbers, of course, are always in contact with water. The animals perform their rigid-to-soft "trick" when they sense a threat, hardening up their skin as a sort of body armor.

Christoph Weder, an associate professor in the same department at Case, says he and Rowan thought of copying the sea cucumber's adaptation more than five years ago. Working with marine biologists, they determined that the deep-sea animal accomplished its transformation thanks to fibers made of a protein known as collagen. The tightness of the connections between those fibers determines how stiff the cucumber's skin is, and is controlled by the animal's nervous system.

To get their polymer to do the same thing, the Case scientists used fibers found in another deep sea dweller, sea squirts, and also in cotton. When they mixed those fibers—known as cellulose nanofibers—with the rubbery polymer ethylene oxide–epichlorohydrin, they formed a stiff network, "almost glued to each other," says Weder.

Due to the nature of the bonds between the polymer and the fibers, however, water gets between the two substances, weakening the fibers' adhesion. The material then becomes soft.

The most immediate application for the new technology, which Weder says the team is exploring, is brain implants that use electrical impulses to control movement. The effectiveness of such devices—which are largely experimental—tends to drop sharply within a few months of being implanted. Some researchers say that is because the devices are too stiff—a requirement for proper insertion—thereby causing eventual brain damage. Weder and Rowan imagine that the new polymer would be rigid during implantation and then relax once it interacts with brain fluids.

Wim L. C. Rutten, a professor of neurotechnology at the University of Twente in the Netherlands, says there is a theoretical benefit to the polymer. "The cucumber paper touches upon an interesting idea, and it would certainly help if a device of the future has a polymer matrix which may be softened after implantation, on 'command,'" says Rutten, who wasn't involved in the research. The "stiff" status, however, may not be rigid enough to penetrate the tough membranes that surround the brain, he says.

Still, Weder is optimistic. He adds that he and Rowan would like to develop a version of the polymer matrix whose strength can be altered using electricity. He envisions applications such as a bulletproof vest that can harden when a soldier goes into combat (but be more comfortable when the wearer is out of harm's way). He also sees other medical applications, such as a knee brace that can become more pliable when the user needs to move and still provide the necessary support.