Changeable plastic mimics sea cucumber's trick

By Julie Steenhuysen

CHICAGO (Reuters) - A new material inspired by a defense mechanism in sea cucumbers can change easily from hard and rigid to soft and floppy, a feature that may make it suited for medical implants, U.S. researchers said on Thursday.

When wet, the material changes from a stiff plastic to a rubber-like state in seconds, and it can change back just as quickly, they said.

The material mimics a trick done by sea cucumbers. The invertebrate sea creatures can quickly change the stiffness of their skin, forming a kind of armor in response to a threat.

"We used the skin of these sea cucumbers as the basis of a new class of artificial material that can change their mechanical properties on command," said Chris Weder, a researchers at Case Western Reserve University in Cleveland, whose study appears in the journal Science.

Weder and colleagues plan to use the material in medical applications, such as pliable brain electrodes used in treatments for patients with Parkinson's disease, stroke or spinal cord injuries.

The material could be stiff to make implanting it easier, then become flexible in the water-rich brain to more closely resemble surrounding tissue.

"If you look at the tissue of the brain, it is much, much softer than the typical electrode you would implant," Weder said in a telephone interview.

Experimental studies have shown electrodes that remain stiff can degrade surrounding tissue over time. The new material is designed to overcome this mechanical mismatch, Weder said.

It is made from two compounds: a rubber-like polymer and tiny cellulose fibers that add stiffness. Where the cellulose fibers cross, they form hydrogen bonds that make the whole material hard.

"These nanofibers are glued to each other wherever they intersect. If you add water, the water will unglue those intersections," said Weder. He said the water acts as a hydrogen de-bonding agent.

Weder and colleagues were able to glue and unglue the fibers in several experiments. The researchers are testing the material in animals to see how it affects brain tissue.

He said the material has potential for other uses as well.

"I think there is a range of applications in the biomedical implant area, such as stents that one could envision to be realized with these materials," he said.

Weder said the same principles could be used to develop electrically switchable materials, such as a type of "smart" ankle or body cast that could be stiff or flexible as needed.

And it could even be used in law enforcement.

"Think of an electrically switchable bulletproof vest that would be comfortable to wear, but that you could switch on to become bulletproof," he said. "It could be really broadly important."

(Editing by Will Dunham and Vicki Allen)