Kirigami pop-up spikes improve soft robot anchoring and locomotion under soil

Earthworms can move beneath soil by expanding parts of their bodies radially; bristles called chetae can work as anchors during surface locomotion but their efficacy during subsurface movement is unknown. We designed a soft, worm-like robot which models the putative earthworm anchoring mechanisms by combining Kirigami skin with radially-expanding pneumatic actuators. The robot consists of three pneumatic actuator segments: head and tail segments that expand radially as anchors, and a middle segment that elongates body. The Kirigami structure pops up when an actuator is radially expanded, forming bristle-like spikes, which penetrate the soil and improve the anchoring ability of segment. Improvements in anchoring are studied by measuring the differences in segment drag forces beneath soil. The performance of robot locomotion in soil terrain with or without Kirigami skin was measured in several terrain conditions (within a wet garden soil channel, buried in wet garden soil, and buried in sand) as the robot dragged payloads behind it. The Kirigami skin-covered robot exhibits a greater maximum drag force (improved from 0.43N to 1.35N), greater forward displacement per gait cycle, and higher traction (e.g., with an 80g payload, the 1.11cm/gait cycle improved to a 2.18cm/gait cycle).