Spider silk is already known for its exceptional strength-to-weight ratio, flexibility, and toughness. But another newly discovered property might lead to new kinds of robotic control devices or even artificial muscles.

Other researchers had recently found that spider silk fibers can suddenly shrink in response to changes in moisture. A team led by Professor Markus Buehler, head of the Department of Civil and Environmental Engineering, discovered that when the relative humidity is above 70%, they also twist, providing a strong torsional force.

This twisting “is a new phenomenon,” says Buehler, whose collaborators included former postdoc Anna Tarakanova and undergraduate Claire Hsu. Through detailed molecular modeling, Tarakanova and Hsu found that when water molecules interact with a particular kind of protein building block in the spider silk, they disrupt its hydrogen bonds in an asymmetrical way. That causes the rotation, which goes in only one direction. “This could be very interesting for the robotics community,” Buehler says. In particular, it could be a novel way of controlling certain kinds of sensors or control devices known as actuators.

Spider silk’s ability to generate torsional force makes possible “a whole new class of materials,” he adds. “Maybe we can make a new polymer material that would replicate this behavior.”