Spider silk is already known for its exceptional strength-to-weight ratio, flexibility and resilience. Researchers have added another property to this list of material descriptors -- supercontraction.

On exposure to a certain level of humidity, spider silk quickly shrinks and twists, demonstrating a trait which could be of use in engineering artificial muscles and actuators. Such supercompaction was discovered by suspending a weight from spider silk in a chamber in which relative humidity could be controlled. The weight Schematic diagram of the apparatus for measuring torsional actuation of silks or other fibers driven by relative humidity. Source: MITstarted to rotate, demonstrating torsional forces, as the humidity level increased to about 70%.

Molecular modeling conducted to unravel the mechanism responsible for this action identified proline, a protein building block, as crucial to the twisting reaction. Interaction with water molecules disrupts its hydrogen bonds in an asymmetrical way that causes one-way rotation. The molecular mechanism leading to this action can be exploited to design humidity-driven soft robots or smart fabrics.

Spider silk is also a material of interest for a large number of biomedical applications. Silk-based nanowires under development could be used in cancer therapy or fabricated as sheets for use as cell growth scaffolds. Gold-coated spider silk has also been demonstrated to improve the performance of hearing aids and microphone components.

Researchers from Huazhong University of Science and Technology (China), Hubei Key Laboratory of Engineering Structural Analysis and Safety Assessment (China), MIT, National University of Singapore, Hubei University (China) and Queen Mary University of London contributed to this study, which is published in Science Advances.