Spider silk, already known as one of the strongest materials for its weight, can be used to create artificial muscles or robotic actuators, scientists say. According to researchers from Massachusetts Institute of Technology (MIT) in the US, the resilient fibres 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.
Researchers recently discovered a property of spider silk called supercontraction, in which the slender fibres can suddenly shrink in response to changes in moisture.
The findings, published in the journal Science Advances, is that not only do the threads contract, they also twist at the same time, providing a strong torsional force.
"We found this by accident initially. My colleagues and I wanted to study the influence of humidity on spider dragline silk," said Dabiao Liu, an associate professor at Huazhong University of Science and Technology in China.
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," said Liu. The team tested a number of other materials, including human hair, but found no such twisting motions in the others they tried.
"This could be very interesting for the robotics community, as a novel way of controlling certain kinds of sensors or control devices," said Markus Buehler, a professor at MIT.
"It's very precise in how you can control these motions by controlling the humidity," said Buehler.
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 fibre.
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 maximising its responsiveness to vibration for the spider to sense its prey.
"We haven't found any biological significance for the twisting motion," said Buehler.
However, 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.
"Silk's unique propensity to undergo supercontraction and exhibit a torsional behaviour 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," said Anna Tarakanova, a former postdoctoral fellow at MIT.
"Potential applications are diverse: from humidity-driven soft robots and sensors, to smart textiles and green energy generators," said Tarakanova, who is now an assistant professor at the University of Connecticut. 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 said. In addition to possible artificial muscles, the finding could also lead to precise sensors for humidity.
