Development of a Biomechanical Energy Harvester

Background: Biomechanical energy harvesting–generating electricity from people during dailyactivities–is a promising alternative to batteries for powering increasingly sophisticated portabledevices. We recently developed a wearable knee-mounted energy harvesting device that generatedelectricity during human walking. In this methods-focused paper, we explain the physiologicalprinciples that guided our design process and present a detailed description of our device designwith an emphasis on new analyses.Methods: Effectively harvesting energy from walking requires a small lightweight device thatefficiently converts intermittent, bi-directional, low speed and high torque mechanical power toelectricity, and selectively engages power generation to assist muscles in performing negativemechanical work. To achieve this, our device used a one-way clutch to transmit only kneeextension motions, a spur gear transmission to amplify the angular speed, a brushless DC rotarymagnetic generator to convert the mechanical power into electrical power, a control system todetermine when to open and close the power generation circuit based on measurements of kneeangle, and a customized orthopaedic knee brace to distribute the device reaction torque over alarge leg surface area.Results: The device selectively engaged power generation towards the end of swing extension,assisting knee flexor muscles by producing substantial flexion torque (6.4 Nm), and efficientlyconverted the input mechanical power into electricity (54.6%). Consequently, six subjects walkingat 1.5 m/s generated 4.8 ± 0.8 W of electrical power with only a 5.0 ± 21 W increase in metaboliccost.Conclusion: Biomechanical energy harvesting is capable of generating substantial amounts ofelectrical power from walking with little additional user effort making future versions of thistechnology particularly promising for charging portable medical devices.