Humidity Powers Artificial Muscles Formed from Spider Silk

Nanowerk Spotlight has reported another medical advance involving spider silk, a biopolymer that is stronger than steel and more elastic than rubber. Most of the world’s 40,000 species of spiders produce a silken thread with a unique combination of mechanical properties: strength (its tensile strength is about five times as strong a steel of the same density), extensibility (up to 30%) and toughness (its ability to absorb a large amount of energy without breaking). Researchers are experimenting with spider silk to design better adhesives; advanced materials that are both stretchy and strong; and to get clues for protein engineering.

Yet the impressive performance of spider silk is not limited solely to tensile mechanics. Researchers have now shown that silk also exhibits powerful cyclic contractions that are precisely controlled by changes in humidity, allowing it to act as a high performance mimic of biological muscles.

“My colleague from Biology, Todd Blackledge, was discussing his recent results on spider silk,” Ali Dhinojwala says. “He was restricting a very thin thread of spider silk (5 micron diameter) in a force machine and was cycling humidity and was observing a cyclic force exerted by the silk. This cyclic behavior was not discussed in the literature.

Dhinojwala, the H.A. Morton Professor in the Department of Polymer Science at the University of Akron was intrigued by this observation because materials scientists have been looking for synthetic materials that can work similar to biological muscles.

“I felt that this cyclic humidity response could be an interesting approach to design biomimetic muscles using spider silk” he says. “We made a slightly modification in our experiments. Instead of restraining the spider silk on a force machine, we decided to hang weights. Interestingly, as we cycled the humidity, the weight was lifted up and down in response to changes in humidity.”

The calculations by Dhinojwala’s and Blackledge’s team showed that silk generates work 50 times greater than the equivalent mass of human muscle. The researchers point out that these numbers are also much better than most of the synthetic materials developed so far.

More information is available at Nanowork Spotlight and from Todd Blackledge’s website.

Tags: Ali Dhinojwala, biopolymer, spider silk, Todd Blackledge