Puzzle Piece Robots: Inverse-Designed Shape-Morphing Docking for Spherically Reconfigurable Soft Robots

Abstract

Modularity in robots enhances versatility, enabling shape morphing and reconfiguration. In modular soft robots, the use of soft materials allows dimensional transformations across different architectures - from chains (1D) to lattices (2D) and spheres (3D). All this enables a swarm of robots to exhibit multi-modal locomotion - such as millipede-like, starfish-like, and soccer-ball-like movement patterns. However, achieving such reconfiguration remains challenging, especially in soft robots, where docking is difficult to realize without compromising compliance. Conventional approaches - such as rigid inserts, magnetic actuators, and adhesives - face challenges due to rigid–soft fabrication mismatch, interference with body compliance and limited holding strength. To address these challenges, this work proposes a geometric, active shape-morphing docking mechanism for spherically reconfigurable soft robots, that combines concepts of topology design and mechanical metamaterials. The robot module edges are designed to create geometric interlocks between adjacent edges (similar to jigsaw puzzle pieces) with an internal structure that deforms under actuation by inlaid shape memory alloy (SMA) wires. The metamaterial internal structure is obtained through inverse design optimization of a computational deformation model created in Abaqus CAE. The constraint-aware optimization strategy blends random search and genetic algorithm features to handle a large number of bounded variables and nonlinear objective function, driving convergence toward a global minimum via geometric decay of the search space. The resulting optimal geometry is designed to buckle under high localized forces, enabling docking and undocking, while remaining minimally deformed under distributed forces, thereby passively maintaining coupling during operation. The docking mechanism is experimentally validated by confirming that the deformation achieved under actuation can facilitate the docking operation and th