The next day, Browne drips a solution of enzymes and other proteins onto the m­embranes using a pipette; the solution encourages the neurons to sprout axons. Slowly, an axon from a single neuron reaches out and forms a synaptic connection with a neuron across the way. After about five days, the axons have securely connected to their neighboring neurons, and Browne attaches the chamber’s rods to a computer-­controlled motor. The motor pulls the towing membrane away from the bottom membrane at a varying rate that has been determined by trial and error.

After about three to five days of gradually increasing the tension, the team can begin stretching the axons as fast as one centi­meter per day (roughly 100 times the speed at which regenerating nerves grow in the body), though shorter transplants can be stretched more slowly.

Repair Work
After about a week of slow stretching, Browne takes the elongation box out of the incubator. He uses a pipette to add more collagen, which acts like a soft glue, on top of the cells. Then he rolls the nerve fibers and the attached neurons off the films. With microscopic forceps, Browne drops the new nerve, now about a centimeter long, into a strawlike tube that has been split lengthwise. The tube, made of a biodegradable material that dissolves inside the body, serves as a synthetic nerve sheath. Browne sutures or glues it securely shut with the nerve inside.

In initial experiments designed to test the transplant’s ability to repair nerve injuries, Smith removes about a centimeter of a rat’s sciatic nerve, which runs through the b­uttocks and down the back of each leg to the ankle and foot, carrying messages from the spinal cord to the various leg muscles. He then places the tube into the space where the nerve was. Using forceps, he gently pushes a stump of the rat’s sciatic nerve sheath into each end of the tube and seals it with fibrin glue. Without the implant in place, the part of the nerve sheath below the cut would degenerate, and the rat would lose movement in that leg. The lab-grown nerves provide a living pathway for regeneration, encouraging the rat’s own motor neurons to grow in the right direction and keeping the sheath alive.

Smith says that in tests performed on more than 40 rats, his group has had almost 100 percent success at restoring the animals’ ability to walk. When the researchers dissected those rats, they found that new axons had grown from their spinal cords and intertwined with the transplanted nerves. The neurons inside the tubes had also given rise to new axons that extended out of the tube in both directions and further mingled with the rats’ own regenerating axons.

Smith and his team think that longer nerve implants could help repair more extensive injuries; so far, the longest nerve they have grown is approximately 10 centi­meters. They have also shown that the stretching process works on human neurons from organ donors. Smith hopes to start testing the human-derived implants in patients with nerve injuries in the next two years.