Kirigami Skin Improves Soft Earthworm Robot Anchoring and Locomotion Under Cohesive Soil

Earthworms can move beneath soil by expanding parts of their bodies radially; bristles called setae work as anchors during surface locomotion but their efficacy during subsurface movement is unknown. We designed a soft, wormlike 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 the body. The Kirigami structure pops up when an actuator is radially expanded, forming bristle-like spikes which are perpendicular to surface, and folds back down when deflated, forming a smoother skin structure. The robustness of Kirigami skin is improved by introducing a novel silicone-plastic layer fabrication method. The pop-up features penetrate the soil without significantly deforming the soil surface, improving the anchoring ability of the segment. Improvements in anchoring are studied systematically 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 cohesive garden soil channel, buried in cohesive soil) as the robot dragged payloads behind it. The Kirigami skin-covered robot exhibits a greater maximum drag force (improved from 2.1 +/- 0.3N to 5.5 +/- 0.5N in 25mm hole diameter condition), greater forward displacement, and higher traction (e.g., with an 40g payload, the 3.7 +/- 2.8cm improved to a 12.5 +/- 0.1cm in 6 gait cycles).