Engineers at the Massachusetts Institute of Technology in the US have developed a soft, autonomous robot that moves via peristalsis, crawling across surfaces by contracting segments of its body, much like an earthworm.
The robot, named Meshworm, is made almost entirely of soft materials and has the potential to be developed to navigate rough terrain or squeeze through tight spaces.
Meshworm is based on wire made of nickel and titanium – a shape-memory alloy that stretches and contracts with heat. The researchers wound the wire around a tube, creating segments along its length, similar to the segments of an earthworm. They then applied a small current to the segments of wire, squeezing the mesh tube and propelling the robot forward.
In the past few decades, many engineers have looked for ways to engineer soft robotic systems. Without bulky, breakable hardware, soft robots might be able to explore hard-to-reach spaces and traverse bumpy terrain.
A significant challenge in soft robotics has been in designing soft actuators, or motors, to power such robots. One solution has been to use compressed air, carefully pumped through a robot to move it. But Sangbae Kim, assistant professor of mechanical engineering at MIT, said that air-powered, or pneumatic, robots require bulky pumps. “Integrating micro air compressors into a small autonomous robot is a challenge.”
Instead, Kim and his colleagues looked to the earthworm for design guidance. They noted that the creepy-crawly has two main muscle groups for motion: circular fibres that wrap around the worm’s tubelike body, and longitudinal fibres that run along its length. Both muscle groups work together to inch the worm along.
The team set out to design a similar soft, peristalsis-driven system. They first made a long, tubular body by rolling up and heat-sealing a sheet of polymer mesh. The mesh, made from interlacing polymer fibres, allows the tube to stretch and contract, similarly to a spring.
They then looked for ways to create artificial muscle, ultimately settling on a nickel-titanium alloy. “It’s a bizarre material,” said Kim. “Depending on the nickel-titanium ratio, its behaviour changes dramatically.”
Depending on the ratio of nickel to titanium, the alloy changes phase with heat. Above a certain temperature, it remains in a phase called austenite – a regularly aligned structure that springs back to its original shape, even after significant bending, in the same way as flexible eyeglass frames. Below a certain temperature, the alloy shifts to a martensite phase – a more pliable structure that, as with 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 fibres 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, in a similar way to an earthworm’s longitudinal muscle fibres. 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 it. Despite the violent impacts, it survived, crawling away intact.
“You can throw it and it won’t collapse,” said Kim. “Most mechanical parts are rigid and fragile at small scale, but the parts in Meshworms are fibrous and flexible. The muscles are soft, and the body is soft – we’re starting to show body-morphing capability.”
The researchers hope to continue working on the Meshworm, establishing devices to navigate rough terrain or squeeze through tight spaces, which could have application in the military or humanitarian response sectors.