Powering smart wearable systems with flexible solar energy harvesting

Recent advances in electrical sensing and flexible devices have demonstrated great potential in a wide range of applications including wearable devices for fitness and health care. Flexible plastic substrates, such as polyimide, or transparent conductive polyester are used to fabricate flexible devices that increase the comfort when they are worn by users or patients. However, one challenge that still limits the success of wearable devices is the limited lifetime especially in biomedical applications. Energy harvesting technology is one of the most promising approaches to address the short lifetime of wearable devices. However, harvesting energy for powering wearable devices is more challenging due to strict constraints in terms of size, weight and cost. In this work, we present the design of a wearable smart bracelet that uses thin-film small form factor flexible photovoltaic panels as energy source. The solar energy harvesting subsystem has been designed to maximize the energy conversion efficiency (up to 90%) to achieve a self-sustainable wireless wearable system. The full-system integration has been developed and assembled using polyamide film to realize a fully flexible smart bracelet for long term monitoring of patients or elderly people in healthcare applications. Preliminary in-field experiments show that a single flexible solar panel can harvest up to 16mW of power in outdoor and 0.21mW in indoor scenarios. We demonstrate that, the developed device, combining low power design and flexible energy harvesting, achieves perpetual work, acquiring one blood oxygenation measurement per minute and sending data via Bluetooth.