Unusual property in spider silk could be used in robotic muscles

Spider silk is already thought to be a pretty amazing material. A spider’s web has exceptional strength but is also incredibly stretchy – a characteristic that allows it to soften and stiffen when pulled. These properties make spider silk incredibly versatile.

Now, another unusual property of the ultra-strong material could be harnessed for twisting or pulling motions, which could lead to new kinds of artificial muscles or robotic actuators, according to MIT researchers.

The findings were reported in a paper in Science Advances by Prof Markus Buehler, head of the department of Civil and Environmental Engineering at MIT, along with former postdoctoral scholar Anna Tarakanova and undergraduate student Claire Hsu of MIT; as well as Dabiao Liu, an associate professor at Huazhong University of Science and Technology in Wuhan, China; and six others.

An accidental discovery

The team discovered that the resilient fibres in spider silk 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 or artificial muscles.

Researchers had already recently discovered that spider silk had a property called supercontraction, in which the fibres can suddenly shrink in response to changes in moisture. However, these new findings show that not only do the fibres contract, but they also twist, providing a strong torsional force.

Liu said the team discovered this by accident as they initially 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 these.

Wide range of applications

While the researchers have not yet found a biological purpose for the twisting motion, they have been able to determine how the twisting mechanism works. It is based on the folding of a particular kind of protein building block called proline. The protein has a rotational symmetry built in and, through its torsional force, it could make a whole new class of materials possible, according to the team.

Buehler said this could be very interesting for the robotics community 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.”

Tarakanova, who is now an assistant professor at the University of Connecticut, said this property in spider silk can be exploited to design responsive, silk-based materials that can be precisely tuned at the nanoscale. “Potential applications are diverse, from humidity-driven soft robots and sensors, to smart textiles and green energy generators.”

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