To most onlookers, a small rotating pendulum might have seemed uninspiring. To Dabiao Liu and his fellow researchers, however, the pendulum’s motion signified the opening of a door—one that may lead to synthetic muscles, soft robotics, and numerous other applications. Liu, a materials scientist at Huazhong University of Science and Technology, China, co-authored an article published earlier this month in Science Advances where he reported that a single thread of spider dragline silk (independently tested from three different spider species) could generate enough rotational force to move a pendulum weighing more than 36,000 times its own weight. In other words, Liu and his research team had shown that spider dragline silk could act as a rotational actuator.

The project started nearly four years ago, shortly after Liu earned his PhD degree. At the time, spider silk was gaining attention due to its ability to contract to about half its original length while swelling to twice its diameter under high humidity conditions—a process known as supercontraction. This action raised the possibility that silk could act as a potential actuator. However, no one had reported on the torsional force that silk might exert under these conditions. Having recently built a chamber capable of measuring such a force, Liu was in a good position to test it.

Liu and his colleagues showed that increasing the relative humidity around spider dragline silk (specifically tested with silk from Nephila pilipes, Nephila edulis, and Argiope versicolor) causes the silk fibers to rapidly, and forcefully, twist. “That really shocked me,” says Liu, “We checked the experiment again, and again. We turned the humidity from 50% to 100% repeatedly, and still the torsional deformity occurred.”

This result was both surprising and exciting because dragline silk may one day be a viable material for engineering and design. Proteins in spider dragline silk are comprised of natural block copolymers that alternate between a rigid protein secondary structure—technically called antiparallel β-sheets—and an amorphous matrix. Together, these domains form a semi-elastic protein fiber with tensile strength comparable to that of Kevlar.

A single spider can produce up to seven different types of silk, each differing in mechanical properties. Dragline silk is used to form the spokes and outer rim of a spider’s web and is of interest to materials researchers due to its unique combination of high tensile strength and high elasticity. This type of silk is formed using two different proteins, MaSp1 and MaSp2, which differ very slightly in their amino acid composition—most notably, the inclusion of an inflexible amino acid, proline, in MaSp2. Liu and his team believed that silk’s torsional force is a result of this amino acid’s presence.

To test this hypothesis, they partnered with a team of researchers at the Massachusetts Institute of Technology, led by Markus Buehler, who modeled the potential effects of water on the silk proteins’ structure. The modeling suggested that high levels of humidity were likely to force water molecules into the protein structure where they could disrupt key hydrogen bonds. MaSp2, rich in prolines, has fewer hydrogen bonds when compared to MaSp1 and is more prone to twisting (a natural consequence of proline’s unique structure). Buehler’s model suggested that these two factors likely explain the torsional force observed under high humidity.

In supporting this model, Liu’s team tested the effect of humidity on torsional rotation using materials that lack proline: silk from Bombyx mori (silkworm), human hair, and Kevlar. As predicted, negligible amounts of rotation occurred with these materials. 

Findings like these become all the more important as researchers draw closer to scalably producing spider silk for human use. Despite its impressive features, spider silk has not been adopted for materials applications outside of the laboratory—there are some important processes that go into making spider silk that are not fully understood, and spiders’ cannot be domesticated in the same way as silkworms. Based on the several advances that have been made, however, Liu’s findings could have some exciting applications.

Shengjie Ling, a materials researcher from ShanghaiTech University, China, says that these findings are “fascinating because we didn’t know that spider silk could provide such a strong and torsional force under these conditions. These results are sure to trigger further research on developing animal silks in new applications, such as actuators, artificial muscles, and sensors.”

Following torsional deformation from a rise in humidity, Liu and his team noted that silk did not recover when humidity levels were returned to baseline. Future studies will need to explore the properties governing silk torsion and whether it can be returned to its starting point.

Read the article in Science Advances.