Integrated Wearable and Mobile Ultrasound/Photoplethysmography Device via Transparent Ultrasound Transducer

Fusion sensors, including photoplethysmograms, cameras, microphones, ultrasound sensors, and accelerometers, are commonly used in mobile and wearable healthcare electronics to measure bio-signals. However, small size is in high demand, but integrating multiple sensors into small mobile or wearable devices is challenging. This study presents two new opto-ultrasound sensors: (1) a wearable device with both photoplethysmography (PPG) and ultrasound (US) capabilities, and (2) a PPG sensor built-in mobile smartphone with an integrated US sensor using a transparent ultrasound transducer (TUT). The TUT has a center frequency of 6 MHz, a 50% bandwidth, and is 82% transparent in the visible and near-infrared ranges. To demonstrate its potential, we developed a wearable device combining photoplethysmography and ultrasound capabilities and fused the TUT to the smartphone. We used this setup to measure heart rates optically and acoustically in human subjects and to calculate oxygen saturation optically through the TUT. This proof-of-concept represents a unique fusion of sensors for small mobile and wearable devices that aim to improve digital healthcare. The results of this research can serve as a basis for innovative development of sensor-based high-tech industrial applications such as healthcare, automobiles, robots, and drones.