Biohybrid Crawling Robot With Spiral-Spring Backbone and Inclined Legs for Enhanced Locomotion

Biohybrid robots powered by muscle tissues offer flexibility and silent actuation, and ring-shaped muscle tissues (muscle rings) are promising modular actuators. However, locomotion in muscle-ring-driven robots remains limited by bending resistance and leg geometry of their skeletons. At reliably printable dimensions, straight-beam backbones resist bending and restrict muscle-driven deformation. Thinning the beam increases bending compliance but reduces torsional stiffness and conflicts with fabrication limits. Predominantly vertical legs, oriented perpendicular to the substrate, limit conversion of contraction-relaxation cycles into directional displacement. Here, we present a biohybrid crawling robot combining two complementary structural elements. Compared with a straight beam producing the same bending displacement, the spiral-spring backbone maintains a larger, printable cross-section and exhibits greater resistance to unintended torsional deformation, supporting axial postural stability. This geometry facilitates transmission of muscle-generated force into large-amplitude body deformation. Additionally, outward-inclined legs with unequal front-rear lengths introduce phase-dependent frictional anchoring that converts cyclic deformation into directional strides. Integrating these elements, the robot achieved a forward speed of 2.0 mm s^{- 1} (14 body lengths per minute), exceeding those of previously reported muscle-powered crawling and walking robots. These results establish a structural design principle that balances bending compliance, torsional resistance, printability, and directional anchoring to improve biohybrid locomotion.