It's no secret that spider silk is one of nature's most incredible materials. It's light and stretchy, as strong as steel and tougher than Kevlar – and now scientists have discovered a weird new ability. A team led by MIT has found that when exposed to a certain level of humidity, spider silk suddenly shrinks and twists, which could make it useful in artificial muscles.

Although the two motions happen at the same time, only the shrinking was previously known. This "supercontraction" was found to occur in response to moisture, which is believed to help keep the fibers taut in the morning dew. After all, tight fibers conduct vibrations better, so the spider can better hear the dinner bell.

But while investigating this reaction to humidity, the team working on the current study discovered it wasn't just shrinking but twisting too. The researchers suspended a weight from silk as a makeshift pendulum, then cranked up the humidity in a chamber. To their surprise, the pendulum started spinning once the humidity reached a certain point.

To figure out just how this twisting occurs, the team examined spider silk in the lab and created computer models of its molecular makeup. The culprit turned out to be an amino acid called proline, which is found in a key silk protein called MaSp2. Water molecules were found to break some of its hydrogen bonds in an uneven way, kicking off the spinning motion.

The team also found that the rotation only ever goes in one direction, and starts when relative humidity hits about 70 percent. While the scientists aren't yet sure why the spider needs fibers that twist when wet, there are plenty of useful applications in robotics or other devices.

"Silk's unique propensity to undergo supercontraction and exhibit a torsional behavior 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," says Anna Tarakanova, an author of the study. "Potential applications are diverse: from humidity-driven soft robots and sensors, to smart textiles and green energy generators."

The research was published in the journal Science Advances.

Source: MIT

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