Windows to the Sole: Prototyping Soft Sensors for Wearable Ballistocardiography

Continuous measurement of cardiovascular parameters is important for monitoring cardiovascular health. Ballistocardiography is a noninvasive method of recording cardiovascular events. Here, we present a sensor system prototype for recording of the full-body ballistocardiogram in a wearable. An array of soft bladders in each sole are filled with water and connected to barometric pressure sensors. We demonstrate the use of the prototype to estimate the pulse transit time against continuous blood pressure in a validation experiment (n=14). Participants wore the sensor shoes while standing on a reference weight-scale. Simultaneous recordings were taken of the sole pressure arrays, finger-clip photoplethysmography, and continuous blood pressure via the volume-clamp method. Measurements were taken at rest, during cold-pressor intervention for 60 seconds, and 3 minutes following end of intervention. The waveform of the ballistocardiograms captured by the proposed sensor system corresponded well to the simultaneously collected waveforms from the reference weigh-scale. Pulse-transit time estimated from shoe BCG and PPG show inverse correlation to vasoconstriction-induced blood pressure increase. By demonstrating the use of the system to compute a vascular transit time, we show the potential of ballistocardiographic insoles as a wearable sensor interface for cardiovascular monitoring.