Nav: Home

Spider silk could be used as robotic muscle

March 01, 2019

CAMBRIDGE, Mass. -- 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 materials 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 spider silk 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.

"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.

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.

"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 humidity.
-end-
The work included collaborators at Huazhong University of Science and Technology and Hubei University, both in Wuhan, China, and Queen Mary University of London. It was supported by the National Natural Science Foundation of China, the National Science Foundation of Hubei Province, the Young Elite Scientist Sponsorship Program by CAST, the National Institutes of Health, the MIT Undergraduate Research Opportunities Program, and the Office of Naval Research.

ADDITIONAL BACKGROUND:

ARCHIVE:

How to build better silk

ARCHIVE:

Spinning a new version of silk

ARCHIVE:

How spider webs achieve their strength

Massachusetts Institute of Technology

Related Spider Silk Articles:

Preventing infection, facilitating healing: New biomaterials from spider silk
New biomaterials developed at the University of Bayreuth eliminate risk of infection and facilitate healing processes.
Mixing silk with polymers could lead to better biomedical implants
Spun by spiders and silkworms, silk has mystified human engineers who have yet to figure out how to artificially recreate it.
Spider silk inspires new class of functional synthetic polymers
Synthetic polymers have changed the world around us. However, It is hard to finely tune some of their properties, such as the ability to transport ions.
The earliest cat on the Northern Silk Road
Dr. Irina Arzhantseva and Professor Heinrich Haerke from the Centre for Classical and Oriental Archaeology (IKVIA, Faculty of Humanities, HSE University) have been involved in the discovery of the earliest domestic cat yet found in northern Eurasia.
Spider silk made by photosynthetic bacteria
A research team in Japan reported that they succeeded in producing the spider silk -- ultra-lightweight, though, biodegradable and biocompatible material -- using photosynthetic bacteria.
Spider silk can create lenses useful for biological imaging
Spider silk is useful for a variety of biomedical applications: It exhibits mechanical properties superior to synthetic fibers for tissue engineering, and it is not toxic or harmful to living cells.
Dynamics of silk proteins are key to outstanding stability of spider silk as biomaterial
Scientists from the universities of Mainz and Würzburg in Germany discovered that methionine is highly abundant in some spider silk proteins.
New silk materials can wrinkle into detailed patterns, then unwrinkle to be 'reprinted'
Tufts engineers have developed silk materials that can wrinkle into highly detailed patterns -- including words, textures and images as intricate as a QR code or a fingerprint.
Spider silk: A malleable protein provides reinforcement
Scientists from the University of Würzburg have discovered that spider silk contains an exceptional protein.
Combination of wood fibers and spider silk could rival plastic
Combination of wood fibres and spider silk could rival plastic The unique material outperforms most of today's synthetic and natural materials by providing high strength and stiffness, combined with increased toughness
More Spider Silk News and Spider Silk Current Events

Trending Science News

Current Coronavirus (COVID-19) News

Top Science Podcasts

We have hand picked the top science podcasts of 2020.
Now Playing: TED Radio Hour

Listen Again: IRL Online
Original broadcast date: March 20, 2020. Our online lives are now entirely interwoven with our real lives. But the laws that govern real life don't apply online. This hour, TED speakers explore rules to navigate this vast virtual space.
Now Playing: Science for the People

#574 State of the Heart
This week we focus on heart disease, heart failure, what blood pressure is and why it's bad when it's high. Host Rachelle Saunders talks with physician, clinical researcher, and writer Haider Warraich about his book "State of the Heart: Exploring the History, Science, and Future of Cardiac Disease" and the ails of our hearts.
Now Playing: Radiolab

Falling
There are so many ways to fall–in love, asleep, even flat on your face. This hour, Radiolab dives into stories of great falls.  We jump into a black hole, take a trip over Niagara Falls, upend some myths about falling cats, and plunge into our favorite songs about falling. Support Radiolab by becoming a member today at Radiolab.org/donate.