Solar powered wrist worn acquisition system for continuous photoplethysmogram monitoring

We present a solar-powered, wireless, wrist-worn platform for continuous monitoring of physiological and environmental parameters during the activities of daily life. In this study, we demonstrate the capability to produce photoplethysmogram (PPG) signals using this platform. To adhere to a low power budget for solar-powering, a 574 nm green light source is used where the PPG from the radial artery would be obtained with minimal signal conditioning. The system incorporates two monocrystalline solar cells to charge the onboard 20 mAh lithium polymer battery. Bluetooth Low Energy (BLE) is used to tether the device to a smartphone that makes the phone an access point to a dedicated server for long term continuous storage of data. Two power management schemes have been proposed depending on the availability of solar energy. In low light situations, if the battery is low, the device obtains a 5-second PPG waveform every minute to consume an average power of 0.57 mW. In scenarios where the battery is at a sustainable voltage, the device is set to enter its normal 30 Hz acquisition mode, consuming around 13.7 mW. We also present our efforts towards improving the charge storage capacity of our on-board super-capacitor.