Researchers led by scientists at Case Western Reserve University have turned to an unlikely model to make medical devices safer and more comfortable-a squid's beak.
Many medical implants require hard materials that have to connect to or pass through soft body tissue. This mechanical mismatch leads to problems such as skin breakdown at abdominal feeding tubes in stroke patients and where wires pass through the chest to power assistive heart pumps. Enter the squid.
The tip of a squid's beak is harder than human teeth, but the base is as soft as the animal's Jello-like body. In order to connect these two mechanically dissimilar parts of the squid 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.
Nature's technology could make a range of medical devices more comfortable and safer for patients, from glucose sensors for diabetics to prosthetic arms and legs that attach to amputees' bones, the researchers say. Their work is published today in the Journal of the American Chemical Society.
"We're mimicking the architecture and the water-enhanced properties of the squid to generate these materials," said Stuart J. Rowan, the Kent H. Smith professor of macromolecular science and engineering at Case Western Reserve, and senior author.
Rowan worked with PhD student Justin D. Fox and assistant professor of biomedical engineering Jeffrey R. Capadona at CWRU, and Paul D. Marasco, who, like Capadona, is a principal investigator at the Advanced Platform Technology Center at the Louis Stokes Cleveland Department of Veterans Affairs Medical Center.
Other researchers have shown the structure of the beak is a nanocomposite comprised of a network of chitin fibers embedded within increasingly cross-linked structural proteins from mouth to tip. The gradient is present when the beak material is dry, but is dramatically enhanced when in water, the squid's natural environment.
Rowan and Capadona were among a team of researchers who had 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.
To ramp up stiffness across the film, one end was exposed to no light and subsequent sections to increasingly more light. The longer the exposure, the more cross-links that 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.