Earthworm-inspired multimodal soft actuators

Earthworms can crawl on the ground and burrow through the soil through the sequential actuation of two types of muscles. The elongated soft body of an earthworm extends by contraction of circularly arranged external muscles and swells by contraction of longitudinally oriented muscles. Despite their slow movement, earthworms offer a rich model for developing next-generation limbless soft robots for many applications, including automated sensing of soil properties and microbiomes, gastrointestinal tract endoscopy, and sewer pipe inspection. Here, we take inspiration from the interwoven morphology of earthworms' musculature to create a multimodal soft actuator. We devised a prototyping technique for fabricating composite pneumatic actuators by coiling prestretched inflatable tubes around a cylindrical soft actuator at varying tension. We conducted comprehensive experiments and characterized the evolution of pressure and elongation of these multimodal actuators while inflating the inner and outer actuators in various sequential orders. Finally, we harnessed one of the identified actuation sequences to achieve in-pipe locomotion.