MEMS ultrasonic fingertip heart rate sensor

The heartbeat is one of the main human physiological activities and is closely related to psychological and physiological states. Since limited solutions exist for convenient heart rate measurements at the fingertip, such as photoplethysmography, this paper provides an alternative solution enabled by a piezoelectric micromachined ultrasonic transducer (PMUT). With fingertip vascular expansion and retraction, the change in ultrasound time-of-flight (TOF) between the PMUT and phalanx is used to extract heart rate data. A PMUT with a resonant frequency of 4.3 MHz is designed and fabricated as a 25 by 25 array of elements, and Parylene and polydimethylsiloxane (PDMS) are used for packaging and better coupling with fingertips, respectively. The emitting ultrasound has a spatial-peak temporal average power density of 36 nW/mm2, far below the FDA safety limit. The proposed solution paves the way for ultrasound-based heart rate monitoring at the fingertip, and it features convenient use, easy integration and biosafety.