Soft, autonomous robot mimics movements of earthworm

MIT researchers have engineered a soft, autonomous robot that mimics the way earthworms creep along the ground by alternately squeezing and stretching the muscles along the length of their bodies.

Developed in collaboration with Harvard University and Seoul National University, the Meshworm robot is made up almost entirely of soft materials and in testing was able to inch away unscathed after being stepped on and even bludgeoned with a hammer. The researchers believe it could be of use in many applications, such as next generation endoscopes, implants and prosthetics. The Meshworm robot takes its name because of the flexible, mesh-like tube that makes up its body. This was created by rolling up and heat-sealing a sheet of polymer mesh, which the researchers created from interlacing polymer fibres; allowing the tube to stretch and contract, similar to a spring. A nickel-titanium alloy was chosen to mimic artificial muscle, which depending on the ratio of nickel to titanium, 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. Algorithms were specially developed 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 fibres. When heated, an individual wire will contract, pulling the worm left or right. "You can throw it, and it won't collapse," said Sangbae Kim, assistant professor of mechanical engineering at MIT. "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."