© Urban, Science/AAAS
OPEN AND CLOSED CASEA scratch in a polyurethane film, which contains a substance found in lobster and shrimp shells, self-repairs upon exposure to UV light. From left to right: the initial scratch, the crack 15 minutes later, and the mend nearly complete 30 minutes later.
Sunshine may not cure all ills, but it could offer a quick fix for a scratched car. Scientists have used a substance from the shells of shrimp to create a new material that repairs itself when exposed to ultraviolet light. The properties of the polymer, described in the March 13 Science, are still being investigated, but it could in a matter of years make its way into all kinds of coatings, such as paints, and surfaces on everything from surgical instruments to countertops.

"It's some interesting chemistry," comments Nancy Sottos, a materials scientist at the University of Illinois at Urbana-Champaign. The self-repair market is big with a lot of potential applications, she says.

Self-repairing materials come in two basic kinds. One approach involves filling microcapsules or hollow fibers with a liquid healing agent and then mixing the capsules into a polymer-based substance that could later become part of a dashboard or countertop. When the substance is damaged, the capsules break open and "bleed," healing the wound. The other approach is to make a material with molecules that break and remake bonds through an interaction with an external stimulus such as heat, water or, in this case, UV light.

The key ingredient of the new material created by polymer scientist Marek Urban and his doctoral student Biswajit Ghosh is a dash of a precursor to chitosan, a structural element in the shells of crabs, shrimp, insects and fungi. To the chitosan precursor, the scientists added oxetane, a ringed molecule that can be broken without much energy. Then they incorporated this new OXE-CHI molecule into polyurethane, the material that might need repair.

Scratching the new material breaks the oxetane rings, exposing a reactive end hungry to bind to something, explains Urban, of the University of Southern Mississippi in Hattiesburg. UV light then cleaves the chitosan component, which in turn binds to the reactive ends made by the scratch.

Test scratches made with a razor blade healed after less than an hour of exposure to a 120 watt florescent UV lamp, a little more power than you'd get from sitting in the sun, the researchers report.

Although Urban and Ghosh were rigorous in their analysis, comments Michael Kessler of the Iowa State University in Ames, it is very difficult to tell what's happening at the level of molecules. "Clearly, covalent bonds are being cleaved. And we know new bonds are forming," he adds.

The scientists used three tools for seeing molecular structures up-close: Raman spectroscopy, nuclear magnetic resonance imaging and internal reflection infrared imaging, which uses a special magnifying lens to detect compounds on a material's surface.

The new work presents a lot of opportunities for further exploration, says Kessler, including investigating the mechanical properties of the material, the precise mechanism by which the crack repairs and the effects of long-term exposure to UV light, like that a car might get while parked in the sun. While commercial use is probably a ways off, the material "has promise," Kessler says.

Urban would like to develop imaging techniques that allow more precise spying on the healing. He says the work is exciting. "It's not easy, but it is a lot of fun."