Heart Rate Measurement Using an Earphone-Type Wearable Device Equipped with Biodegradable Piezoelectric Sensors

In recent years, there has been increased interest in using wearable devices for continuous physiological monitor- ing, which has promising applications in personal health man- agement and clinical practice. This study proposes a method for estimating heart rate using an earphone-type device equipped with a low-power, structurally simple piezoelectric sensor that captures pressure pulse wave signals in the external auditory canal. The proposed method applies two-stage noise removal filtering based on physiologically plausible R–R intervals (RRI) and signal amplitude components, enabling stable heart rate detection even under conditions with external disturbances. When applied to both resting and stimulating periods, the proposed method achieved an agreement rate exceeding 95% after noise removal and demonstrated a strong correlation with heart rate values obtained from ECG. Notably, motion and pain stimuli introduced noise into the signal during the stimulus periods; nevertheless, the proposed method effectively suppressed spurious peak detections arising from transient disturbances.