Polymers give new life to the nervous system

Nerve damage is difficult to repair because once a nerve is severed its cells don't regenerate themselves easily. Nerve cells cannot reconnect beyond a gap wider than a centimeter. Eventually, that part of the neuron "downstream" from the wound dies. "Peripheral nervous system axons-the part of the nerve cell that carries the impulses-normally have a connective tissue sheath of myelin to guide their growth. And without mat guidance, they aren't able to grow productively," explains Surya Mallapragada, a professor of chemical engineering at Iowa State University and an associate in materials chemistry at the U.S. Department of Energy's Ames Laboratory. She has developed a method of using superthin, biocompatible, and biodegradable polymer films to help nerve cells re-grow properly. Mallapragada and her team use films only a few hundred microns thick, much thinner than a human hair. They men etch patterns onto the film, and these patterns guide the regenerating cells. They've devised two methods to do this. A "casting" technique first etches me patterns onto a silicon wafer, which is then imprinted onto the film. "We also use a direct etching method using lasers and atomic-force microscopy," she says. The latter method is slower but gives the researchers more flexibility.

In recent trials, tiny bits of rats' sciatic nerves were sliced away. The damaged nerves were then spliced together using the film. The rats began showing movement in their legs after three weeks and were functioning normally after six weeks. "We're essentially mimicking what nature does," Mallapragada says. Now that they've proved it works on the peripheral nervous system, she and her colleagues are turning their attention to the central nervous system, specifically trying the procedure on rats' optic nerves. In these tests they are trying to further promote nerve-cell growth by the addition of adult stem cells. The commercial polymers needed for this treatment is relatively costly, she admits, but not prohibitively so.