Design and Characterization of a Dual‐Mode Soft Robotic Flapping Propeller With Pneumatic Stiffness Modulation

Soft robotic fish typically suffer from a tradeoff between propulsive efficiency and maneuvering flexibility due to fixed body stiffness. To address this limitation, this article proposes a novel soft robotic flapping propeller capable of active stiffness modulation and dual‐mode locomotion. The system features a hybrid actuation strategy: a pneumatic origami‐inspired accordion structure for stiffness regulation and axial deformation, coupled with a cable‐driven mechanism for lateral flapping. This unique architecture enables two distinct swimming modes: (1) a body and/or caudal Fin (BCF) mode with tunable stiffness to optimize propulsive transmission, and (2) an earthworm‐inspired wriggling mode for navigation in confined spaces, facilitated by passive directional flapping foils. Extensive underwater experiments characterize the kinematic relationship between actuation and end‐effector response. Crucially, thrust tests reveal a nonlinear coupling between stiffness and flapping frequency. An optimal operating domain (15 kPa stiffness at 1.2 Hz) was identified, yielding a maximum net thrust of 741.2 mN while maintaining a low coefficient of variation (CV = 0.8), thereby balancing high propulsion with output stability.