Spider silk could be used as a robotic muscle

The team discovered that resilient fibers respond very strongly to changes in humidity. Above a certain level of relative humidity in the air, they contract and twist suddenly, exerting enough strength to be potentially competitive compared to other materials crawled as an airframe. actuators, devices that move to perform an activity such as controlling a valve.

The results are reported today in the diary Progress of sciencein an article by MIT professor Markus Buehler, head of the civil and environmental engineering department, along with Anna Tarakanova, a former postdoctoral fellow, and Claire Hsu, an undergraduate student at MIT; Dabiao Liu, associate professor at the Huazhong University of Science and Technology in Wuhan, China; and six others.

Researchers have recently discovered a property of spider silk called supercontraction, in which thin fibers can contract suddenly in response to changes in humidity. The new conclusion is that not only do the wires contract, but they twist at the same time, providing significant torsional force. "It's a new phenomenon," says Buehler.

"We found this by accident early," Liu said. "My colleagues and I wanted to study the influence of moisture on the silk of the draglines." To do this, they hung a silk weight to make it a kind of pendulum and locked it in a room where they could control the relative humidity inside. "When we increased the humidity rate, the pendulum started spinning.It was unexpected.This really shocked me."

The team tested a number of other materials, including hair, but did not find such twisting movements in others. But Liu said that he immediately began to think that this phenomenon "could be used for artificial muscles".

"This could be very interesting for the robotics community," says Buehler, as a new way to control certain types of sensors or control devices. "The way you can control these movements by controlling the humidity is very accurate."

Spider silk is already recognized for its exceptional strength / weight ratio, flexibility and strength. A number of teams around the world are striving to replicate these properties in a synthetic version of the protein-based fiber.

From the spider's point of view, the purpose of this twisting force is unknown, but researchers believe that supercontraction in response to moisture can be a way to ensure that a web is tight in response to the morning dew, perhaps protecting it from damage and maximizing its reactivity to vibration so that the spider can detect its prey.

"We did not find any biological significance" for the twisting motion, says Buehler. But thanks to a combination of laboratory experiments and computer molecular modeling, they were able to determine how the torsion mechanism works. It turns out to be based on the folding of a particular type of protein building block, called proline.

The study of this underlying mechanism required detailed molecular modeling, performed by Tarakanova and Hsu. "We tried to find a molecular mechanism for what our collaborators found in the lab," says Hsu. "And we actually found a potential mechanism," based on proline. They showed that with this particular proline structure in place, torsion always occurred in simulations, but without it, there was no twisting.

"Spider floss is a fiber of protein," says Liu. "It is composed of two main proteins, called MaSp1 and MaSp2." The proline, crucial for the torsion reaction, is found in MaSp2 and, when the water molecules interact with it, they break its hydrogen bonds asymmetrically, which causes the rotation. The rotation only goes in one direction and is at a relative humidity threshold of about 70%.

"The protein has an integrated rotational symmetry," says Buehler. And thanks to its torsional force, it allows "a brand new class of materials". Now that this property has been found, he suggests, perhaps it could be reproduced in a synthetic material. "Maybe we could make a new polymer material that would replicate this behavior," says Buehler.

"Silk's unique propensity for supercontrol and torsional behavior in response to external triggers such as moisture can be exploited to design responsive silk-based materials that can be precisely tuned to the surface. 39, nanoscale, "says Tarakanova, currently assistant professor at the University of Connecticut. "Potential applications are varied: from flexible robots and moisture-based sensors to smart textiles and green energy generators."

It can also be found that other natural materials present this property, but if so, it has not been noticed. "This kind of twisting movement could end up in other documents that we have not looked at yet," says Buehler. In addition to the possible artificial muscles, the results could also lead to accurate moisture sensors.

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More information:
"Spider flirty silk as a torsion actuator driven by moisture" Progress of science (2019). advance.sciencemag.org/content/5/3/eaau9183