A research team led by scientists from the Massachusetts Institute of Technology (MIT) is looking at one of nature’s strongest materials to develop new types of artificial muscles, robotic actuators, smart textiles and green energy generators.
The researchers found that spider silk—one of the strongest materials in the world—will respond to a certain level of relative humidity by suddenly contracting and twisting with enough force to where it is similar to other materials currently being explored as actuators.
Researchers previously knew that spider silk contains a property called supercontraction, where the fibers suddenly shrink in response to changes in moisture, but it was not until recently that they found that not only do the threads contract, they also twist at the same time to provide a strong torsional force.
In the lab, the researchers suspended a weight from the spider silk to make a pendulum and then enclosed it in a chamber that enabled them to control the relative humidity inside. The results were surprising.
“We found this by accident initially,” Dabiao Liu, an associate professor at Huazhong University of Science and Technology, said in a statement. “My colleagues and I wanted to study the influence of humidity on spider dragline silk. When we increased the humidity, the pendulum started to rotate. It was out of our expectation. It really shocked me.”
The researchers tried a number of other materials to try to replicate these properties, including human hair, but were unable to duplicate the twisting motions found in the spider silk.
After conducting various lab experiments, coupled with computer molecular modeling, the researchers determined that the twisting mechanism is based on the folding of a protein building block called proline.
“We tried to find a molecular mechanism for what our collaborators were finding in the lab,” MIT undergraduate student Claire Hsu said in a statement. “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 silk is made of two proteins called MaSp1 and MaSp2, with the proline needed for the twisting reaction is found within MaSp2. When water molecules interact with MaSp2, they disrupt its hydrogen bonds in an asymmetrical way that causes the rotation in one direction at a threshold of about 70 percent relative humidity.
“The protein has a rotational symmetry built in,” MIT professor Markus Buehler, head of the Department of Civil and Environmental Engineering, said in a statement. “Maybe we can make a new polymer material that would replicate this behavior.”
Along with its strength-to-weight ratio, spider silk is known for both its flexibility and resilience.
The researchers believe that the supercontraction properties are a way the spider makes sure their web is pulled tight and protect it from damage in response to morning dew.
