Soft autonomous robot mimics the actions of an earthworm

Researchers at the Massachusetts Institute of Technology (MIT), Harvard University and Seoul National University have engineered a soft autonomous robot that moves through peristalsis, crawling across surfaces by contracting segments of its body.

Sangbae Kim, the Esther and Harold E Edgerton assistant professor of mechanical engineering at MIT, said in a statement that such a soft robot may be useful for navigating rough terrain or squeezing through tight spaces.

The robot is named Meshworm for the flexible, mesh-like tube that makes up its body.

To make the robot, researchers created artificial muscle from wire made of nickel and titanium — a shape-memory alloy that stretches and contracts with heat. They wound the wire around the tube, creating segments along its length. They then applied a small current to the segments of wire, squeezing the mesh tube and propelling the robot forward.

A significant challenge in soft robotics has been in designing soft actuators to power robots such as Meshworm. One solution has been to use compressed air pumped through a robot to move it, but Kim said integrating micro air compressors into a small autonomous robot is a challenge.

Instead, Kim and his colleagues noted that the earthworm is made up of two main muscle groups: circular muscle fibres that wrap around the worm’s tube-like body and longitudinal muscle 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. 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 fibres, 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 said. ‘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, the alloy remains in a phase called austenite — a regularly aligned structure that springs back to its original shape, even after significant bending. Below a certain temperature, the alloy shifts to a martensite phase, a more pliable structure that 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 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.

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 said. ‘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.’

The team — which included graduate student Sangok Seok and postdoctoral student Cagdas Denizel Onal at MIT, associate professor Robert J Wood at Harvard, assistant professor Kyu-Jin Cho of Seoul National University and Daniela Rus, professor of computer science and engineering and director of MIT’s Computer Science and Artificial Intelligence Laboratory — recently published details of the design in the journal IEEE/ASME Transactions on Mechatronics.