FROM OCEAN TO PROSTHETICS: SADDLE-SPRING ANKLE–FOOT DESIGN

This research introduces a quasi-passive, variable-stiffness ankle–foot prosthesis inspired by the mantis shrimp’s doubly curved saddle spring. Unlike passive commercial devices that cannot adjust stiffness, the presented design utilizes a hyperbolic-paraboloid composite spring to provide fast response, user-specific stiffness modulation while remaining lightweight and energy efficient. Analytical modeling, geometric parameterization, and multi-parameter optimization were conducted to match and span the range of biological ankle stiffness from slow to fast walking. Mechanical bench tests validated the theoretical model, demonstrating dorsiflexion stiffness adjustability from 8.63 to 18.43 Nm/° for a 100 kg user, with the device capable of adjusting from minimum to maximum stiffness within 0.3 s. Comparative finite element analysis showed that, relative to conventional leaf springs, the saddle spring stores more strain energy, provides more uniform stress distribution, and achieves up to threefold higher strain-energy density within biological dorsiflexion angles. These advantages allow reductions in carbon/glass fiber usage, lowering material demands and manufacturing costs. Overall, the findings highlight doubly curved composite springs as strong candidates for next-generation prosthetic feet that enhance energy return, reduce material consumption, and improve gait performance.