Spider silk could be used as robotic muscle

March 1, 2019, Massachusetts Institute of Technology
Forced silking from a Nephila pilipes spider. Credit: Liu et al., Sci. Adv. 2019; 5 : eaau9183

Spider silk, already known as one of the strongest materials for its weight, turns out to have another unusual property that might lead to new kinds of artificial muscles or robotic actuators, researchers have found.

The resilient fibers, the team discovered, 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 being explored as actuators—devices that move to perform some activity such as controlling a valve.

The findings are being reported today in the journal Science Advances, in a paper by MIT Professor Markus Buehler, head of the Department of Civil and Environmental Engineering, along with 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.

Researchers recently discovered a property of called supercontraction, in which the slender fibers 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.

Forced silking from a Nephila pilipes spider. Credit: Liu et al., Sci. Adv. 2019; 5 : eaau9183
"We found this by accident initially," Liu says. "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 fiber.

Nephila pilipes spider in Lab. Credit: Liu et al., Sci. Adv. 2019; 5 : eaau9183

While the purpose of this twisting force, from the spider's point of view, is unknown, researchers think the supercontraction in response to moisture may be a way to make sure a web is pulled tight in response to morning dew, perhaps protecting it from damage and maximizing its responsiveness to vibration for the spider to sense its prey.

"We haven't found any biological significance" for the twisting motion, Buehler says. But through a combination of lab experiments and molecular modeling by computer, they have been able to determine how the twisting mechanism works. It turns out to be based on the folding of a particular kind of protein building block, called proline.

Investigating that underlying mechanism required detailed molecular modeling, which was carried out by Tarakanova and Hsu. "We tried to find a molecular mechanism for what our collaborators were finding in the lab," Hsu explains. "And we actually found a potential mechanism," based on the proline. They showed that with this particular proline structure in place, the twisting always occurred in the simulations, but without it there was no twisting.

"Spider dragline silk is a protein fiber," Liu explains. "It's made of two main proteins, called MaSp1 and MaSp2." The proline, crucial to the twisting reaction, is found within MaSp2, and when water molecules interact with it they disrupt its hydrogen bonds in an asymmetrical way that causes the rotation. The rotation only goes in one direction, and it takes place at a threshold of about 70 percent relative humidity.

Torsion pendulum made of spider dragline silk. Credit: Liu et al., Sci. Adv. 2019; 5 : eaau9183
"The protein has a rotational symmetry built in," Buehler says. And through its torsional force, it makes possible "a whole new class of materials." Now that this property has been found, he suggests, maybe it can be replicated in a synthetic material. "Maybe we can make a new polymer material that would replicate this behavior," Buehler says.

"Silk's unique propensity to undergo supercontraction and exhibit a torsional behavior in response to external triggers such as humidity can be exploited to design responsive silk-based materials that can be precisely tuned at the nanoscale," says Tarakanova, who is now an assistant professor at the University of Connecticut. "Potential applications are diverse: from humidity-driven soft robots and sensors, to smart textiles and green energy generators."

It may also turn out that other natural materials exhibit this property, but if so this hasn't been noticed. "This kind of twisting motion might be found in other materials that we haven't looked at yet," Buehler says. In addition to possible artificial muscles, the finding could also lead to precise sensors for .

Explore further: Strange silk: Why rappelling spiders don't spin out of control

More information: "Spider dragline silk as torsional actuator driven by humidity" Science Advances (2019). advances.sciencemag.org/content/5/3/eaau9183

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Display comments: newest first

not rated yet Mar 01, 2019
Interesting, but I suspect that the response time is very slow for both contracting and relaxing. Probably way to slow to be useful.
Whydening Gyre
5 / 5 (1) Mar 01, 2019
Interesting, but I suspect that the response time is very slow for both contracting and relaxing. Probably way to slow to be useful.

I suppose that would depend on the application requirements...
Might take longer to dry out than humidify (unless you're in a heated environment), tho...
5 / 5 (4) Mar 01, 2019
Well, this spider has a use for its twisty shrinking thread
as in the air, a damp threads dries quickly
that is its probable use
when it rains
it web gets soaked
heavy with water droplets
its thread twists, maintaining tension on a soggy water laden web
it does not sag
to using it for muscles
Just think about the neat trick spiders pull of
hanging 20ft of the end of their thread
they do not spin
they stay stabile unspinning
Exactly the opposite of a wind up spinning thread on a damp day
not rated yet Mar 01, 2019
Heard several years ago of similar effects achieved with twisted nylon response to temperature changes.
Mar 02, 2019
This comment has been removed by a moderator.
Thorium Boy
not rated yet Mar 02, 2019
More testing, more TESTING!! They've been at that for 30 years and nothing has come of it.
not rated yet Mar 02, 2019
Maybe they should treat their spiders somewhat more carefully. After all the critters turn out to be useful. You don't stick sheep to boards and put them under intense light.
2.3 / 5 (3) Mar 02, 2019
oh ponyboy, you really do get off on embarrassing yourself,

This is a biological product being developed for industrial products.
"Nature" doesn't cooperate worth a shit!
Now the hard part.
Using psychology to convince investor angels to risk their money.
2 / 5 (4) Mar 04, 2019
. A number of teams around the world are working to replicate these properties in a synthetic version of the protein-based fiber.

It's OK to copy the design, but it would be very courteous to acknowledge the designer and pay tribute to the great genius that created the silk in the first place. The Creator, otherwise known as God. The God of Abraham, Isaac and Jacob. There is only ONE God.
Da Schneib
1 / 5 (2) Mar 04, 2019
@FreddyJoe insists jebus made it.

Back to your sister, the book about the super magic sky daddy by the drunken stone-age sheep herders, and the trashbarrel fire in the trailer park, @FreddyJoe.
Captain Stumpy
1 / 5 (1) Mar 04, 2019
The God of Abraham, Isaac and Jacob
actually, no: it would be Iktomi, Arachne, Anansi, Arianrhod, Unilisi Kananeski, Gede Zariyen, or some other deification of the spider which predated your abrahamic tome

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