Mantis Shrimp-Inspired Quasi Passive Ankle-Foot Prosthesis

The biological ankle joint adjusts stiffness to adapt to changing walking speed and environments. However, commercial passive ankle-foot prostheses lack the ability to adjust stiffness, which causes gait issues. While powered prostheses can address these problems, they require large actuators, making them heavy, bulky, and energy insufficient. In this study, we introduce a quasi-passive, variable-stiffness anklefoot prosthesis that employs unique characteristics of mantis shrimp's saddle spring. Mantis shrimps employ the saddle spring's energy storing and return mechanism to effectively punch their prey with their appendages. To integrate this saddle spring, we developed governing equations for parameterizing the geometry of the saddle spring. From the parameterization, the curvature and thickness of the saddle spring were optimized to match the biological ankle stiffness in normal walking speed. The moment arm of the saddle spring is then optimized to provide a range of biological ankle stiffness for slow to fast walking speeds. A theoretical model of the saddle spring is validated with experimental bench tests. The designed saddle spring can adjust dorsiflexion stiffness of the prosthesis from 8.63 to $18.43 \text{Nm} /{ }^{\circ}$ for a 100 kg user. The stiffness adjustment time from the lowest to highest stiffness is 0.3sec. Our prosthesis shows a promising ability to provide prompt user-specific stiffness in daily living while maintaining compact, lightweight, and energy efficiency.