A New, Simple Wrist-Mounted PVDF Sensor for Continuous Heart Rate Monitoring

Wrist-worn smartwatches are widely used to monitor heart rate unobtrusively, generally, by means of photoplethysmography sensors. Practically, heart rate is only intermittently measured because of the high-power consumption of the LEDs. Alternatively, this study proposes a sensor based on an off-the-shelf polyvinylidene fluoride (PVDF) piezoelectric transducer inserted into a rubber 3D-printed wristwatch strap that can capture the weak mechanical vibrations generated by the radial artery pulse. The sensor requires much less power consumption and allows continuous and prolonged heart rate monitoring. Sensor output signal and an electrocardiography (ECG) lead were simultaneously acquired from 10 healthy subjects, to assess sensor performance and heart rate accuracy. Sphygmic pulses were automatically identified by a template matching technique, independently of the ECG signal. The template matching approach detected heartbeats with sensitivity and positive predictive value of 98.0 % and 99.0 %, respectively. Inter-beat intervals were then estimated and compared with those obtained from the ECG by means of Passing-Bablok linear regression and Bland-Altman analyses. The results show high agreement between the two measurements: linear regression coefficient of determination R2 of 0.994, and Bland-Altman limits of agreement of ± 17.0 ms. These preliminary findings suggest that the proposed sensor can offer a continuous and accurate heart rate monitoring.