Soft autonomous earthworm robot at MIT

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Earthworms creep along the ground by alternately squeezing and stretching muscles along the length of their bodies, inching forward with each wave of contrac...

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The team set out to design a similar soft, peristalsis-driven system. The researchers first made a long, tubular body by rolling up and heat-sealing a sheet of polymer mesh. The mesh, made from interlacing polymer fibers, allows the tube to stretch and contract, similar to a spring.

They then looked for ways to create artificial muscle, ultimately settling on a nickel-titanium alloy. 'It's a very bizarre material,' Kim says. 'Depending on the [nickel-titanium] ratio, its behavior changes dramatically.'

Depending on the ratio of nickel to titanium, the alloy changes phase with heat. Above a certain temperature, the alloy remains in a phase called austenite - a regularly aligned structure that springs back to its original shape, even after significant bending, much like flexible eyeglass frames. Below a certain temperature, the alloy shifts to a martensite phase - a more pliable structure that, like a paperclip, stays in the shape in which it's bent.

The researchers fabricated a tightly coiled nickel-titanium wire and wound it around the mesh tube, mimicking the circular muscle fibers of the earthworm. They then fitted a small battery and circuit board within the tube, generating a current to heat the wire at certain segments along the body: As a segment reaches a certain temperature, the wire contracts around the body, squeezing the tube and propelling the robot forward. Kim and his colleagues developed algorithms to carefully control the wire's heating and cooling, directing the worm to move in various patterns.

The group also outfitted the robot with wires running along its length, similar to an earthworm's longitudinal muscle fibers. When heated, an individual wire will contract, pulling the worm left or right.

As an ultimate test of soft robotics, the group subjected the robot to multiple blows with a hammer, even stepping on the robot to check its durability. Despite the violent impacts, the robot survived, crawling away intact.

'You can throw it, and it won't collapse,' Kim says. 'Most mechanical parts are rigid and fragile at small scale, but the parts in Meshworms are all fibrous and flexible. The muscles are soft, and the body is soft ... we're starting to show some body-morphing capability.'

Kellar Autumn, a professor of biology at Lewis and Clark College, studies the biomechanics of animal motion in designing soft robotics. Autumn says robots like the Meshworm may have many useful applications, such as next-generation endoscopes, implants and prosthetics.

'Even though the robot's body is much simpler than a real worm - it has only a few segments - it appears to have quite impressive performance,' Autumn says. 'I predict that in the next decade we will see shape-changing artificial muscles in many products, such as mobile phones, portable computers and automobiles.'

This research was supported by the U.S. Defense Advanced Research Projects Agency.

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