Insole Ballistocardiography for Unobtrusive Respiratory and Heart Rate Monitoring Using 3D-Printed Piezoelectric Sensors

There is a growing interest in monitoring vital signs outside of clinical settings for early detection of gradual health deterioration. Ballistocardiography (BCG) is a viable approach for unobtrusive everyday use without requiring direct skin contact. In this work we propose insole-based BCG using highly sensitive, 3D-printed ferroelectret sensors as a wearable system. We analyze the measurement data obtained from three participants in a standing, upright posture. A continuous wavelet transform is performed to filter the main frequency bands (6 to 8 Hz) in the time-frequency domain. Peak detection reveals the beat-to-beat heart rate, which agrees with the reference of a chest strap. Respiration is extracted from the heartbeat data through amplitude and frequency demodulation. Both characteristics can be fused for a breath-to-breath measurement, minimizing influence of body movements and non-respiratory arrhythmia. The heart rate variability of ± 4 beats per minute is consistent with values from the literature on respiratory sinus arrhythmia. In conclusion, the presented highly sensitive ferroelectret insole combined with appropriate signal processing is suitable for health monitoring, paving the way for unobtrusive sensor solutions.