Nanocomposite Material of Squid Beaks Model for Better Medical Implants

The special properties of squid beak, consisting of a nanocomposite material, could help to develop better medical implants in the future. While various kinds of implants often require hard materials, they also have to connect to or pass through soft body tissue, a mechanical mismatch that can lead to problems including a breakdown of the skin from abdominal feeding tubes or where wires pass through the chest to power heart pumps.

A squid’s beak seamlessly connects these two extremes, being harder than human teeth at the tip, but the base is as soft as the animal’s Jell-O-like body. In order to connect the two, a major part of the beak has a mechanical gradient that acts as a shock absorber so the animal can bite a fish with bone-crushing force, yet suffer no wear and tear on its fleshy mouth.

Researchers believe they could use nature’s technology to make a range of medical devices more comfortable and safer for patients, including glucose sensors for diabetics and prosthetic arms and legs that attach to amputees’ bones.

“We’re mimicking the architecture and the water-enhanced properties of the squid to generate these materials,” says Stuart J. Rowan, professor of engineering at Case Western Reserve University and senior author of a new study published in the Journal of the American Chemical Society.

The structure of a squid’s beak is a nanocomposite made of a network of chitin fibers embedded within cross-linked structural proteins. The gradient is present when the beak material is dry, but is dramatically enhanced when in water, the squid’s natural environment.

Rowan and Jeffrey R. Capadona, assistant professor of biomedical engineering, previously reported a material that mimics the sea cucumber’s skin, which is soft and pliable when wet and stiff and hard when dry.

They thought that material, in the form of a film, could be cross-linked with nanofibers to maintain stiffness when wet. They filled the film with functionalized cellulose nanocrystals that, when exposed to light, form cross-links. Then, to increase stiffness across the film, one end was exposed to no light and subsequent sections to increasingly more light. The longer the film was exposed, the more cross-links formed.

Just like the beak, the grade from soft to hard was steeper when wet. Water switches off the weaker non-covalent bonds that form when the material is dry. The wet environment inside the body will enhance the gradient just as well, which makes this technology especially attractive for implants, the researchers say.

“There are all sorts of places in medicine where we’re using hard materials, but we’re mostly soft,” says Paul D. Marasco, a principal investigator at the Advanced Platform Technology Center at the Louis Stokes Cleveland Department of Veterans Affairs Medical Center.

The contrast is a recipe for sores and infection, poor performance, and implant failure.

Needles in diabetics’ insulin pumps, metal stents inserted in blood vessels, and electrodes inserted in muscles or brains could be safer and more effective if materials would remain hard where they need to be but buffer surrounding soft tissues.

“Prosthetic limbs are connected to the arm or leg with a socket of hard plastic that fits over the residual limb,” Marasco says. “But bone moves around under the socket and can damage the soft tissue inside, while the socket can be hard on the skin where it makes contact.”

A better solution would be to run a metal insert into the bone inside the body and attach a prosthesis directly outside the body using this kind of mechanical buffer where the hard metal passes through the soft skin.

The researchers already are working on the next generation of materials and cross-linking strategies to make the buffer gradient steeper. The tip of a squid’s beak is 100 times harder than its softest portion, while this first mimic’s hard tip is five times harder than its soft end.

“This is a proof of concept,” Rowan says. “Now that we have shown the concept works, we’re now getting a wee bit more complicated and targeting materials that will allow us to move closer to applications.”