Wearable Energy Harvesting: From body to battery

Energy Harvesting (EH) technologies provide promising solutions to overcome the short lifetime of wearable devices. In the last decade, EH has matured as a technology and found use in many application scenarios, such as smart grid and wireless sensor networks. Recently, advances have been made in miniaturizing EH devices to supply wearable devices by exploiting ambient energy in the form of motion, thermal gradients, light and electromagnetic radiation. However, harvesting energy from the body for powering wearable devices is more challenging due to strict constraints in terms of size, weight and cost. In this paper, we present a taxonomy of technologies, architectures and design trade-offs for efficient EH systems suitable for wearable devices. Additionally, we provide implementation details, including the conversion stages for kinetic and thermal EH, optimized for the human body. We quantify the energy that it is possible to harvest in real application scenarios, which is in the range of 200-700 mJ per day, depending the source, and result in up to 1.5 J per day if coupled. The design guidelines and experimental evaluations we present, with in-field measurement, will be of benefit to designers of future EH wearable systems.