Science

DNA-inspired artificial muscles contract by "supercoiling"

DNA-inspired artificial muscle...
An example of supercoiling in the artificial muscles (right), compared to the looser twist (left)
An example of supercoiling in the artificial muscles (right), compared to the looser twist (left)
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An example of supercoiling in the artificial muscles (right), compared to the looser twist (left)
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An example of supercoiling in the artificial muscles (right), compared to the looser twist (left)

Tiny robots could serve all kinds of useful functions, but shrinking their actuators has proven challenging. Now researchers at the University of Wollongong (UOW) in Australia have made artificial muscles that surpass your puny natural ones, inspired by the “supercoiling” of DNA strands.

DNA performs one of nature’s most impressive feats of contraction, cramming strands about 2 m (6.6 ft) long into a single human cell. To do so, DNA makes use of a process called supercoiling – essentially, if you twist two fibers together and keep going after they’ve wound completely around each other, they will deal with the extra force by buckling out to the side. Besides DNA, you can see this supercoiling effect in everything from a tangled garden hose to the wires of your earbuds.

For the new study, the UOW scientists set out to replicate this phenomenon in their artificial muscles. They made them out of composite polyester fibers, coated in a hydrogel that swells up when it gets wet. These were twisted together into the familiar helix shape of DNA, then immersed in water to get them to swell up.

Normally this swelling would cause the fibers to unravel, but the team found that if they clamped the ends the fibers undergo supercoiling instead. As such, they contract upwards, generating a relatively large amount of mechanical force.

The supercoiled fibers shrank down to just 10 percent of their original length, generating the equivalent of 1 joule of energy per gram. The team says the mechanical work this muscle can perform is up to 36 times higher than a comparable skeletal muscle.

To put the new artificial muscles to the test, the team used them to make tiny tools, like micro-scissors and tweezers, with 7-mm long arms. And sure enough, the design worked well for these kinds of applications.

That said, the muscles currently move quite slowly, thanks to the hydrogel mode of action. But the team says that this could be sped up by tweaking the materials or methods.

“We do believe that the speed can be increased by making smaller diameter fibres, but right now the applications are limited to those that need a slower response,” says Professor Geoffrey Spinks, lead author of the study. “Developing faster supercoiling muscles would open up further applications. We hope that others will explore different means for generating a volume change – such as by electrical heating – that can lead to a faster response.”

Other artificial muscles are also in development, some of which already use electrical currents to induce the contraction through heating.

The new study was published in the journal Science Robotics. The team demonstrates the new artificial muscles in the video below.

Supercoiling artificial muscles

Sources: University of Wollongong via The Conversation

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