From hard to soft in seconds

A sea cucumber can activate its body armor in a matter of seconds, by secreting chemicals that stiffen its soft skin. Now researchers are adapting that trick to create plastics beefed up with nanomaterials that can switch from hard to soft, or vice versa, with the flick of a signal.

Writing in Friday's issue of the journal Science, the researchers say such plastics could eventually be used for future biomedical implants, such as brain electrodes or … well … whatever.

"Where would you want to have materials that can electrically switch between floppy and rigid? I'd just encourage you to use your imagination," said Christoph Weder, a materials scientist at Case Western Reserve University and one of the senior authors behind the research paper.

Building on previous research published last year in Nature Nanotechnology, Weder and his colleagues took a page from the sea cucumber's cookbook - and changed the ingredients to suit their own purposes. The sea cucumber's skin has rigid collagen nanofibers embedded in soft connective tissue. When the creature senses a threat, it secretes chemicals that cause the nanofibers to bind together, hardening the skin into more of a shell.

"We've taken that architecture - not the chemistry and the materials that nature is using - but we've taken the architecture to produce an artificial material that does a similar trick," Weder told me.

To mimic the sea cucumber's skin, the researchers substituted cellulose nanofibers for the collagen, and embedded them in a rubbery, porous polymer. The nanofibers were specially treated so that they would stick together in a matrix when dry, but would separate when exposed to a hydrogen-bond-forming solvent - like water, for example.

The resulting material is as hard as the plastic in a CD case when it's dry - but goes limp and floppy like soft rubber when it's immersed in water. When the material dries out, it gets hard again.

"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.

Going soft in the brain

For now, the technology is being developed primarily for microelectrodes that can be implanted in the brain. Such devices are being used experimentally in "artificial nervous systems" to treat medical conditions ranging from strokes and spinal cord injuries to Parkinson's disease.

Some experiments have shown that the brain signals degrade within a few months after implantation, and researchers suspect that this occurs because the stiff microelectrodes do damage to the surrounding brain tissue over time. Electrodes that go soft "could alleviate this problem," Dustin Tyler, another member of the research team, said in a Case Western news release.

"That's why we designed our first materials to respond to water," Weder said. "This allows the rigid electrodes to become soft when implanted into the water-rich brain."

You might wonder whether there are similar ways to achieve the same result. For example, even a dry sponge can get all floppy when it's thrown into a bucket of water. But Weder said it was important to come up with a hard-to-soft technology that didn't depend merely on soaking up a liquid. If their material swelled up like a sponge in the brain ... well, you can imagine how bad that could be.

To make sure that their material worked instead by responding a chemical switch, they dipped the stuff in 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.

Dr. Rodolfo Llinas, a neuroscientist at New York University, is working on a different kind of microelectrode technology and was not involved in Weder's work. Nevertheless, after reviewing the Science paper, he told me in an e-mail 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."

Beyond the brain

Weder told me he doesn't intend to stop with microelectrodes. On the biomedical side of things, you could make implantable stents from plastic instead of wire. You could also create "smart casts" for injured limbs, Weder said.

"There are conditions where, every now and then, it is desirable for the system to soften up so you can stretch your arm, and then the cast can harden back up," he explained.

All sorts of other gee-whiz devices become marketable if researchers can figure out a way to control the stiff-vs.-floppy switch with an electrical signal rather than a chemical signal. (I can imagine the erectile-dysfunction ads already!) Weder took the high road in his own example, saying the technology could lead to "smart bulletproof vests that you can switch from soft to rigid."

That would bring the concept around full circle: What started out as body armor for the sea cucumber could someday lead to next-generation body armor for humans.

In addition to Weder and Tyler, the authors of the Science paper include Jeffrey Capadona, Kadhiravan Shanmuganathan and Stuart Rowan of Case Western. Capadona, Tyler, Rowan and Weder also are affiliated with Cleveland DVA Medical Center.