Smart wristband based on a magnetic soft sensor for pulse wave measurement

Optical sensors, commonly used in wearable devices for pulse wave monitoring, face significant limitations such as dependency on sensor positioning and variations related to skin tone. To overcome these challenges, this paper presents a novel smart wristband using a magnetic soft sensor for pulse wave and contact force monitoring. The system combines a Hall effect sensor with a deformable silicone structure that encapsulates a permanent magnet. It detects magnetic field variations caused by radial artery pulsations, enabling the measurement of distance changes between the magnet and the sensor due to the pulse wave, while monitoring contact force during placement. Metrological characterization showed an average sensitivity of 0.36 V/mm over the entire measurement range. Moreover, within a 1 mm displacement interval starting at a 4 mm sensor-to-magnet distance, the sensor exhibits a nearly linear response with a higher sensitivity of 0.68 V/mm. Dynamic testing under simulated heart rate conditions (60 bpm - 120 bpm) was performed under cyclic loading, showing hysteresis errors from 5.2% to 7.3%. The discrimination threshold was established at 0.05 mm. Feasibility assessment on eight volunteers was carried out to test the system’s capability to detect systolic, diastolic peaks and dicrotic notch, compared to a reference PPG sensor. Analysis revealed promising performance, with systolic peaks showing 0.1% false positives and 0.2% false negatives. Using these peaks, beat-to-beat heart rate was estimated, with a mean absolute error of 0.6 bpm. The smart wristband integrates pulse wave detection and contact force measurement, offering a potential contribution toward more standardized monitoring.