An Unpowered Exoskeleton With a Bionic Multi-Segment Foot Structure for Walking Assistance.

Lower limb exoskeletons have been designed to assist human walking. However, current unpowered exoskeletons primarily targeted the ankle joint, but neglected the energy storage and recoil within the foot. In this study, we proposed an unpowered foot-ankle exoskeleton with a bionic multi-segment foot structure to mimic energy conservation strategies of natural barefoot walking. An arch-like structure was built in the exoskeleton with springs aligning with the medial longitudinal arch and the plantar fascia to store elastic energy in the early and mid-stance phases and recoil energy in the late stance phase of walking. We found that compared with mass-matched experimental shoes, the exoskeleton reduced the total net metabolic cost of walking by $8.5~\pm ~3.1$ % for healthy individuals, and caused a 10.6% reduction in the average activity and a 25.9% reduction in the tissue oxygen saturation index of the soleus. As expected, the exoskeleton did not interfere with the natural motion of the midfoot and ankle joints during walking. The exoskeleton may help reduce energetic penalties by assisting the soleus concentric contraction for propulsion and permitting sufficient intra-foot motion. Our results highlight the importance of utilizing elastic energy storage of intra-foot structures for walking assistance, and imply that implementing a multi-foot structure have a potential to further improve the performance of walking assistance devices.