Using liquid metal coils to harvest the body’s kinetic energy

Abstract Harvesting energy from human motion is a promising technique for powering wearable devices on the body. Achieving both significant amounts of power and comfort in the same system is however challenging; electromagnetic harvesters based on moving magnets are capable of generating sizeable amounts of power from the body’s motion but only at a significant cost in weight and discomfort in limiting practical use. In this paper we present a stretchable electromagnetic energy harvester based on a sliding magnet through coils made of the room temperature liquid metal eutectic gallium indium (EGaIn). The harvester is fabricated by injecting EGaIn into commercial silicone tubing wrapped around 3D printed preforms, with molded silicone used to hold the coils in place followed by removal from the preforms. The behavior of the liquid metal coils with a moving magnet was first modelled using an analytical model and compared with experimental testing using a sliding drop tower test to verify the electromagnetic characteristics for different velocities. A motorized shaker in the laboratory was then used to characterize the harvested energy during vibration, followed by testing on a person running on a treadmill. Strain testing also demonstrated that the harvester could operate up to 20% mechanical strain, sufficient for use in close contact with the body. The device generates a peak power of 160 μW and average power of 17 μW during the laboratory test, and 100 μW peak and 14 μW average for the wearable version on the treadmill, nearly an order of magnitude higher power than past on-body stretchable electromagnetic harvesters.