Fingertip plethysmography using a sensitive wafer-level-packaged capacitive mems strain sensor

We introduce a highly sensitive out-of-plane capacitive silicon strain sensor designed for wearable health monitoring applications, specifically targeting finger plethysmography. Featuring a compact wafer-level-packaged die size measuring 2.5 mm x 2.2 mm x 0.85 mm, the sensor exhibits a strain sensitivity of 1.1 fF/mu epsilon in a linear range of 700 mu epsilon. We demonstrate that fingertip strain plethysmography (SPG), facilitated by this novel device, effectively tracks photo-plethysmography (PPG) recordings that are conventionally used for pulse oximetry. SPG delivers accurate monitoring of heart rate (HR), heart rate variability (HRV), and respiratory rate (RR). It also holds promise for non-invasive continuous measurement of blood pressure (BP) through the application of the pulse transit time (PTT) principle. The high sensitivity of the device enables detection of small physiological strains even after being mounted onto a small PCB board, showcasing great potential for use in challenging wearable health-monitoring platforms.