Spider silk could be used as a robotic muscle

Researchers have discovered that spider silk, already known as one of the most resistant materials for its weight, has another unusual property that could lead to new types of artificial muscle or muscle. robotic actuators.

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, the pendulum began to turn. It was out of our expectations. It really shocked me. "

The researchers were able to decode the molecular structure of the two main proteins, represented here, that make up the silk of the draglines. One of them, MaSp2, contains proline, which interacts with water molecules to produce the newly discovered twisting motion.

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. "It's very precise in how to control these movements by controlling the humidity."

"It's a fantastic discovery, because the twist in the silk of the strap is huge, a full circle every millimeter or so," says Pupa Gilbert, professor of physics, chemistry and materials science at the University of Toronto. Wisconsin to Madison, who has not been involved in this work. Gilbert adds: "It's like a rope that twists and loosens depending on the humidity of the air. The molecular mechanism leading to this outstanding performance can be exploited to build soft robots or intelligent tissues driven by moisture. "

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 discovered 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, essential for the torsion reaction, is 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 whole new class of materials". Now that this property has been found, he suggests, it may be reproduced in a synthetic material. "Maybe we could make a new polymer material that would replicate that behavior," says Buehler.

"Silk's unique propensity for supercontrol and torsional behavior in response to external triggers such as moisture can be harnessed to design responsive silk-based materials that can be precisely adjusted to the point of use." 39, nanoscale, "says Tarakanova, currently assistant professor at the University of Connecticut. "Potential applications are varied: robots and software sensors based on moisture, 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 be found in other materials that we have not yet examined," says Buehler. In addition to the possible artificial muscles, the results could also lead to accurate moisture sensors.

These researchers "have used the known high sensitivity of silk to moisture and have demonstrated that it can also be used in interesting ways to create very precise torsion actuators," said Yonggang Huang, engineering professor. Civil and Environmental and Mechanical Engineering at Northwestern University. has not been involved in this work. "The use of silk as a torsion actuator is an innovative concept that could find applications in various fields ranging from electronics to biomedicine, for example, the hygroscopic artificial muscles and the moisture sensors, "he explains.

Huang adds, "What is particularly remarkable in this work is that it combines molecular modeling, experimental validation and in-depth understanding through which elemental changes in the chemical bond are transformed into macroscopic phenomena. This is very significant from the point of view of basic science and also interesting for applications. "

The work included collaborators from the Huazhong University of Science and Technology and Hubei University, both located in Wuhan, China, and Queen Mary University in London. It was supported by the National Natural Science Foundation of China, the National Science Foundation of Hubei Province, the High Performance Young Scientists Sponsorship Program by CAST, the National Institutes of Undergraduate research opportunities from MIT and the Office of Naval Research.