Crab shell ‘fabric’ patches damaged nerves

U. WASHINGTON (US)—Mixing natural and synthetic fibers is nothing new, but researchers at the University of Washington are applying that principle in a novel way. They have created a new biomedical material by combining a substance derived from crab shells with an industrial polyester.

The team mixes chitosan, found in the shells of crustaceans, with polyester to form 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.

“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,” says Miqin Zhang, a University of Washington professor of material science and engineering and lead author of a paper available online in the journal Advanced Materials. “This turns out to be very difficult.”

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.

Today’s commercial nerve guides are made from collagen, a structural protein derived from animal cells. But collagen is expensive and the protein tends to trigger an immune response. 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 explains. “If the tube is made of collagen, it’s difficult to keep the conduit open because any stress and it’s going to collapse.”

Zhang and colleagues developed an alternative. The first component of their material, polycaprolactone, is a strong, flexible, biodegradable polyester commonly used in sutures. It is not suitable on its own because water-based cells don’t like to grow on the polyester’s water-repelling surface.

The second component, chitosan, is found in the shells of crustaceans. It’s cheap, readily available, biodegradable, and biocompatible, which means it won’t trigger an immune response. Chitosan has a rough surface similar to the surfaces found inside the body to which cells can attach. But chitosan swells in water, making it weak in wet environments.

Researchers combined the fibers at the nanometer scale by first using a technique called electrospinning and then wove the fibers together. The resulting material has a texture similar to that of the nanosized fibers of the connective tissue that surrounds human cells.

Zhang and colleagues built prototype nerve guides and tested the chitosan-polyester blend against another biomaterial under study, polylacticcoglycolic acid, and a commercially available collagen guide.

Of the three materials, the chitosan-polyester weave showed the most consistent performance for strength, flexibility, and resistance to compression under both dry and wet conditions. Under wet conditions, which the researchers say best mimics those in the body, the chitosan-polyester blend required twice as much force to push the tube halfway shut as the other biomaterial, and eight times as much force as the collagen tube.

The new material showed promise for nerve guides but Zhang suspects it will also work well for wound dressings, heart grafts, tendons, ligament, cartilage, muscle repair, and other biomedical applications.

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