Solar Energy Harvesting Node for Battery-Free Physiological Monitoring Wearable Wristband*

Capitalising on the potential benefits of digital healthcare is heavily dependent on engineering progress on wearable devices capable of continuous sensing and extraction of diverse physiological parameters, such as heart rate, blood pressure, or body temperature. Consequently, it is important for wearable devices to maintain seamless operation without the necessity for frequent battery replacement or recharging. This paper presents a battery-free flexible solar energy harvesting node designed to power low-power medical wearables, with a focus on wearability and flexibility during the design. The system includes a solar panel, a power management unit (PMU), a supercapacitor, and a wireless sensor node, and is designed to be flexible, compact, and lightweight to optimise wearability, consequently improving the potential user experience under long-term monitoring. The proposed system can harvest 0.45 mW of power under typical office lighting (500 lx) and 0.14 m W under low lux levels (200 lx). Experimental results demonstrate an 80 % efficiency for solar energy harvesting. With sufficient solar illumination at 500 lux for 3 hours per day, the proposed system can power a PPG sensor node without using a battery. By eliminating the requirement for battery replacement or recharging, this design holds the potential to substantially diminish maintenance costs associated with wearable devices, making them a more practical and cost-effective solution for long-term monitoring applications.