Joined: 16 Mar 2004
|Posted: Thu Jan 08, 2009 2:54 pm Post subject: Adaptable Polymer Inspired by Sea Cucumbers
|Adaptable Polymer Inspired by Sea Cucumbers
New material promises safer brain implants
A sudden stiffening of the skin can help the humble sea cucumber defend itself from predators. Now, a new composite material has been designed that mimics this feat and has demonstrated potential for biomedical applications such as neural implants.
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. 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.
To overcome this problem, Christoph Weder, a materials scientist, and his colleagues at Case Western Reserve University looked for biocompatible materials that could transform from rigid to flexible states, and 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.
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 current application that we're interested in is to make materials for biomedical applications that are originally rigid, but when you implant them in a biological material, they soften in a very controlled way," Weder said
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."
However, Weder added that it was important to come up with a hard-to-soft technology that didn't depend merely on soaking up a liquid. To make sure that their material worked instead by responding a chemical switch, they dipped it into a different liquid, isopropyl alcohol, and found that the plastic retained its stiffness. That demonstrated that the stiff-to-floppy transition could be chemically controlled.
Weder said the hard-to-soft plastic is currently being tested in animal studies - but he acknowledged that "it's very difficult to say how long this is going to take," due to the protracted period required to certify such materials for medical purposes.
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.
Dr. Rodolfo Llinas, a neuroscientist at New York University working on a different kind of microelectrode technology commented that the soft-polymer technology looked promising:
"The use of polymer-based electrical stimulating/recording probes is clearly the next step in direct interface with the central nervous system. I suspect that as we learn more about such materials, their optimal utilization in neuroscience will afford stunning new experimental designs. Indeed, the 'Holy Grail' in central nervous system research, to record/stimulate in freely moving animals with hundreds of probes, over a protracted time, might become a reality."