From Anguilliform to Ostraciiform: Soft Robotic Caudal Fins for Adaptive Propulsion

Fish caudal fins exhibit diverse morphologies and mechanical properties adapted to distinct swimming modes, from the highly flexible anguilliform motion of eels to the rigid, oscillatory propulsion of boxfish. Drawing inspiration from five representative species—eel (anguilliform), trout (subcarangiform), mackerel (carangiform), tuna (thunniform), and boxfish (ostraciiform)—we designed and characterized soft robotic caudal fins to study how stiffness and kinematics affect thrust generation. Each fin integrates stiff fin rays within a compliant membrane, fabricated via multi-material additive manufacturing to replicate natural bending, cambering, and twisting observed in biological fins. Actuation through waterproof RC servos enables oscillations across biologically derived tail-beat frequencies (0.9–7.0 TB s⁻¹) and amplitudes (10°–30°), spanning the fast, low-amplitude undulation of tuna to the slower, large-amplitude oscillation of boxfish. Robophysical experiments quantify thrust and wake structures using particle image velocimetry (PIV), revealing how geometry and compliance shape vortex shedding and propulsive efficiency—informing future robotic fin designs for adaptive, efficient underwater locomotion.