Scientists at Case Western University have made a biopolymer that switches rapidly between rigid and flexible states, using material inspired by sea cucumbers. The new material softens in the presence of a water-based solvent, and it stiffens back up as the solvent evaporates. Christoph Weder, lead researcher and professor of macromolecular science and engineering, says that such a material may be useful in the design of implantable electrodes able to record brain activity over long stretches of time, with minimal scarring compared with conventional electrodes.
One of the challenges facing researchers developing neural implants to help paralyzed patients is that the electrodes are typically made of metal. Such brittle and stiff material can cause tissue damage over time. (See “Stretchable Electronic Skin.”) Indeed, over a couple of months, the electrode’s hard exterior rubs against soft brain matter, causing scar tissue to form and significantly decreasing the electrode’s recording ability. “We need a new generation of electrodes that are different than the usual metal electrodes that produce all sorts of damage after a while and don’t work anymore,” says MIT Institute Professor Emilio Bizzi, who was not involved in the study.
To overcome this problem, Weder and his colleagues looked for biocompatible materials that could transform from rigid to flexible states, and they found an ideal model in the sea cucumber. As a sea cucumber maneuvers its way across the ocean floor, its pliable structure makes it easy to worm through cracks and crevices. At the first sign of danger, its skin stiffens, forming a rigid armor against likely predators. Researchers have found that the sea cucumber’s skin is composed of an ultrafine network of cellulose fibers, or “whiskers.” In defensive mode, surrounding cells release molecules that cause the whiskers to bind together, forming a rigid shield. In a relaxed state, other cells release plasticizing proteins, loosening fibers and making the skin pliable.
Weder’s team isolated stiff cellulose fibers from the mantles of tunicates, sea creatures with skin similar to that of sea cucumbers. The researchers then combined the fibers with a rubbery polymer mixture. The fibers formed a uniform matrix throughout, reinforcing the softer polymer material. These intersecting points hold the network together, creating an inflexible material. “It’s like a three-dimensional web in which these nanofibers overlap at certain points, and wherever they overlap, they stick to each other,” says Weder.
He says that cellulose fibers are particularly good at binding with each other because they contain many hydroxyl groups on their surface. In the absence of any other hydrogen-containing molecule, these hydroxyl groups stick together, forming a fibrous web. In order to break the fiber bonds and loosen the web, Weder’s team injected a water-based solvent into the material that contained competitive hydrogen groups. In response, cellulose fibers decoupled as their hydrogen groups combined with the water solution. Alternately, as water evaporated from the mixture, fibers reconnected, becoming stiff again.
“In the stiff state, the material is like a hard, rigid plastic, much like your CD case,” says Weder. “When the material becomes soft, it’s more like a rubber.” He says that if such a material were used to design neural electrodes, it could be engineered to respond to fluid in the brain, softening as it comes in contact with nerve tissue.
MIT’s Bizzi says that such a pliable electrode would lengthen the recording time within the brain that’s possible with neural implants, and provide valuable data for treating conditions such as Parkinson’s disease, Tourette’s syndrome, and spinal-cord injuries. “The field needs new technology to make it possible to record for longer periods of time from the brain,” says Bizzi. “If it works, it would be a godsend.”
In electrode applications, the material would only have to transform once, from rigid to soft, once inside the brain. Weder says that the cellulose-based material may be used for other applications that require shifting back and forth from stiff to softer states. “You could think about a smart cast, where you would want to stiffen your cast, but every now and then, you want to soften it up so you can move your arm,” says Weder. “So in that application, you would like a reversible material.”
Weder adds that cellulose fibers can be obtained from sources other than sea cucumbers, such as wood and cotton–an avenue that his team plans to explore.