Spider silk, known as one of the strongest materials for its weight, turns out to have another unusual property. It might lead to new kinds of artificial muscles or robotic actuators, researchers have found.
The fibres, according to study experts, respond very strongly to changes in humidity.
Above a certain level of relative humidity in the air, they suddenly contract and twist, exerting enough force to potentially be competitive with other materials being explored as actuators -- devices that move to perform some activity such as controlling a valve.
The study, led by Markus Buehler that was reported in the journal Science Advances, saw former postdoc Anna Tarakanova and undergraduate student Claire Hsu at MIT; Dabiao Liu, an associate professor at Huazhong University of Science and Technology in Wuhan, China; and six others accompany him.
Researchers recently discovered a property of spider silk called supercontraction, in which the slender fibres can suddenly shrink in response to changes in moisture. The new finding is that not only do the threads contract, they also twist at the same time, providing a strong torsional force. "It's a new phenomenon," Buehler says.
"We found this by accident initially," Liu says adding, "My colleagues and I wanted to study the influence of humidity on spider dragline silk."
To do so, they suspended a weight from the silk to make a kind of pendulum, and enclosed it in a chamber where they could control the relative humidity inside. "When we increased the humidity, the pendulum started to rotate. It was out of our expectation. It really shocked me."
The team tested a number of other materials, including human hair, but found no such twisting motions in the others they tried. But Liu said he started thinking right away that this phenomenon "might be used for artificial muscles."
"This could be very interesting for the robotics community," Buehler says, as a novel way of controlling certain kinds of sensors or control devices. "It's very precise in how you can control these motions by controlling the humidity."
Spider silk is already known for its exceptional strength-to-weight ratio, its flexibility, and its toughness, or resilience. A number of teams around the world are working to replicate these properties in a synthetic version of the protein-based fibre.