Roboticists build a better earthworm

Examples abound of animals that get around simply by repeatedly contracting and lengthening their bodies. For instance, the muscle layers on an earthworm generate peristaltic waves to propel it underground or atop a surface. Several research groups over the past decade have developed robots to mimic such motion. All of them, however, demonstrated the limitations of peristaltic design—motion by elongation and compression—because each robot had to switch between producing the longitudinal and radial forces.

Riddhi Das , a postdoctoral researcher, and his colleagues at the Italian Institute of Technology in Genoa have now designed and built a robotic earthworm that better mimics the muscular behavior of the real animal by combining the bidirectional forces in a single actuator system. Much like bellows whose motions work antagonistically, each of the five peristaltic soft actuators (PSAs), shown in the prototype here, does both jobs—longitudinal and radial movements—at once.

The PSAs, which are round and connected in series, are each filled with fluid that controls the internal pressure needed to exert the forces that enable localized and independent movement. Each PSA elongates when air is pumped into it and compresses when the air is drawn out. That is, each segment first becomes longer and thinner and then—mimicking a muscular contraction—shorter and fatter.

A single actuator elongates to a maximum length of 11 mm at 1 bar of positive pressure and squeezes by about the same amount at half a bar of negative pressure. The prototype is 45 cm long and weighs 600 g. That’s small and light enough to carry aboard a spaceship for remote planetary exploration—one of the potential applications Das and colleagues have in mind. Fortunately, the robots are just as able to explore, excavate, and search confined underground regions much closer to home as well. (R. Das et al., Sci. Rep. 13, 1571, 2023 .)