Douglas Smith mechanically stretches living nerves to grow resilient transplants

In a lab at the University of Pennsylvania, a plastic dish holds two rows of tiny black dots, pairs of them connected by dozens of thin, hairlike filaments. Each dot is a cluster of thousands of neurons, explains Douglas Smith, who is a professor of neurosurgery and the director of Penn's Center for Brain Injury and Repair. The fibers that stretch between them actually comprise thousands of axons, long, slender projections that conduct electrical impulses away from each neuron's central body. These bundles--each one a lab-engineered nerve--represent physical bridges that Smith hopes will help researchers like him mend previously irreparable injuries.

To make the long nerve transplants, Smith and his team first collect sensory neurons--­cells that transmit information to the brain--from the spinal cords of fetal rats. Research technician Kevin Browne then pipettes a gelatinous pink protein called collagen onto two adjacent films in a specially built chamber. About the size of a shoebox, it houses a stretching apparatus made up of a vertical block attached to metal rods. One of the small, clear films, called the towing membrane, is suspended at one end by the block and curves down almost to the base of the chamber, where it overlaps the second membrane. Browne places one set of neurons in the collagen on the towing membrane and another on the bottom membrane. At this point, the two groups are less than 100 micrometers--two hairs' width--apart. He puts the whole setup into a humming incubator that runs at 37 °C, mimicking the internal temperature of a rat.