A strand of spider silk roughly 10 centimetres long can twist itself through nearly 100 rotations when exposed to moisture, an attribute that might be useful for making artificial muscles.

During﻿ the past decade or so, scientists have discovered that spider silk — known for its extraordinary strength — can twist and contract in humid conditions. To explore this property further, Dabiao Liu at Huazhong University of Science and Technology in Wuhan, China, Markus Buehler at the Massachusetts Institute of Technology in Cambridge and their colleagues dangled a pendulum from a strand of silk and placed the apparatus in a chamber in which humidity could be controlled.

When the relative humidity reached about 70%, the silk (and pendulum) began to twist. The team tested silk from three species of spider and found that a one-millimetre-long section of silk from Nephila pilipes could twist by almost 360 degrees — a greater range of rotation than can be achieved with devices made of carbon nanotubes and powered by electricity. The twisting could be halted, and then resumed, by lowering and raising the humidity across the 70% threshold.

This capability could one day be used to create soft robots or smart fabrics woven from twisting threads, the authors write.