Background: Biomechanical energy harvesting–generating electricity from people during daily activities–is a promising alternative to batteries for powering increasingly sophisticated portable devices. We recently developed a wearable knee-mounted energy harves-ting device that generated electricity during human walking. In this methods-focused paper, we explain the physiological principles that guided our design process and present a detailed description of our device design with an emphasis on new analyses. Methods: Effectively harvesting energy from walking requires a small lightweight device that efficiently converts intermittent, bi- irectio-nal, low speed and high torque mechanical power to electricity, and selectively engages power generation to assist muscles in per-forming negative mechanical work. To achieve this, our device used a one-way clutch to transmit only knee extension motions, a spur gear transmission to amplify the angular speed, a brushless DC rotary magnetic generator to convert the mechanical power into elec-trical power, a control system to determine when to open and close the power generation circuit based on measurements of knee angle, and a customized orthopaedic knee brace to distribute the device reaction torque over a large leg surface area. Results: The device selectively engaged power generation towards the end of swing extension, assisting knee flexor muscles by producing subs-tantial flexion torque (6.4 Nm), and efficiently converted the input mechanical power into electricity (54.6%). Consequently, six subjects walking at 1.5 m/s generated 4.8 ± 0.8 W of electrical power with only a 5.0 ± 21 W increase in metabolic cost. Conclusion: Biomechanical energy harvesting is capable of generating substantial amounts of electrical power from walking with lit-tle additional user effort making future versions of this technology particularly promising for charging portable medical devices.

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