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 article presents a novel smart wristband using a magnetic soft sensing element for pulse wave monitoring. The system combines the Hall effect sensor with a deformable silicone structure that encapsulates a permanent magnet. It detects magnetic field variations induced by the pulsatile activity of the radial artery, enabling the measurement of distance changes between the magnet and the sensor generated by the pulse wave, thereby providing additional information on wearable system positioning. 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 (HR) conditions [60 beats per minute (bpm)–120 bpm] was performed under cyclic loading, showing hysteresis errors from 5.2% to 7.4%. The discrimination threshold (DT) was established in the range of 0.01 mm–0.05 mm. A feasibility assessment on eight volunteers was carried out to test the system’s capability to detect systolic peaks, diastolic peaks, and dicrotic notch, compared to a reference photoplethysmography (PPG) sensor. Analysis revealed promising performance, with systolic peaks showing 0.1% false positives (FPs) and 0.2% false negatives (FNs). Using these peaks, beat-to-beat pulse rate was estimated, with a mean absolute error (MAE) of 0.6 bpm. The smart wristband integrates pulse wave detection and sensor-to-magnet distance monitoring, offering a potential contribution toward more standardized monitoring.