Now researchers at the University of Washington are using the same principle for biomedical applications. Mixing chitosan, found in the shells of crabs and shrimp, with an industrial polyester creates a promising new material for the tiny tubes that support repair of a severed nerve, and could serve other medical uses. The hybrid fiber combines the biologically favorable qualities of the natural material with the mechanical strength of the synthetic polymer.
After an injury that severs a peripheral nerve, such as one in a finger, nerve endings continue to grow. But to regain control of the nerve surgeons must join the two fragments. For large gaps surgeons used to attempt a more difficult nerve graft. Current surgical practice is to attach tiny tubes, called nerve guides, that channel the two fragments toward each other.
Left panel shows a closeup of chitosan and polyester fibers woven at the nanometer scale. The middle panel shows a nerve cell growing on the resulting mesh, which has a texture similar to the body's fibrous connective tissue. The right panel shows a cross-section of the synthetic nerve guide. Arrows point to nerve cells that have attached to the inner and outer surfaces of the tube. Photo Credit: University of Washington
"A nerve guide requires very strict conditions. It needs to be biocompatible, stable in solution, resistant to collapse and also pliable, so that surgeons can suture it to the nerve," said Miqin Zhang, a UW professor of material science and engineering and lead author of a paper now available online in the journal Advanced Materials. "This turns out to be very difficult."
Today's commercial nerve guides are made from collagen, a structural protein derived from animal cells. But collagen is expensive, the protein tends to trigger an immune response and the material is weak in wet environments, such as those inside the body. The strength of the nerve guide is important for budding nerve cells.
"This conduit serves as a guide to protect the neuron from injury," Zhang said. "If the tube is made of collagen, it's difficult to keep the conduit open because any stress and it's going to collapse."