News in Science

Sea cucumber makes hard plastic go soft

A new material inspired by sea cucumbers can change easily from hard and rigid to soft and floppy, US researchers say.

When wet, the material changes from a stiff plastic to a rubber-like state in seconds, and it can change back just as quickly, the researchers report today in the journal Science.

It's a feature they say may make it suited for medical implants.

The material mimics a trick that sea cucumbers perform. The invertebrate sea creatures can quickly change the stiffness of their skin, forming a kind of armour 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," says Professor Chris Weder, a researcher at Case Western Reserve University.

Weder and colleagues plan to use the material in medical applications, such as pliable brain electrodes used in treatments for people 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 says.

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 says.

What's it made from?

The material is made from two compounds: a rubber-like polymer and tiny cellulose fibres that add stiffness.

Where the cellulose fibres cross, they form hydrogen bonds that make the whole material hard.

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

Weder and colleagues glued and unglued the fibres in several experiments.

The researchers are testing the material in animals to see how it affects brain tissue.

Weder says the material also has potential for other biomedical implants, like stents.

He says the same principles could be used to develop electrically switchable materials, such as a type of 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 says.