Soft Force Sensor for Breathing Monitoring: Design, Development and Feasibility Assessment

This study presents the design, development, and metrological characterization of two Hall effect-based soft force sensors for monitoring respiratory activity. The sensors consist of a rigid component embedding a Hall sensor and a soft element housing a neodymium magnet, made up of two silicon matrices with different stiffnesses: the more flexible Ecoflex 00-30 (called Eco 30 sensor) and the more rigid Ecoflex 00-50 (called Eco50 sensor). Calibration curves (sensor's output vs. force) were obtained through quasi-static compression tests, revealing a decrease in sensitivity with increasing stiffness of the material (sensitivity of 1.88 V/N for Eco30 sensor and 0.60 V/N for Eco50 sensor). The measuring range is also different, reaching 2.22 N for the stiffer sensor, compared to 0.76 N for the softer sensor. Dynamic tests simulating different breathing frequencies demonstrated low hysteresis errors (from 2.92% to 12.59%). The higher sensitivity of the Eco30 sensor compared to the Eco50 sensor is also reflected in a higher amplitude of the respiratory signal collected during the preliminary tests (max amplitude of 0.47 V and 0.15 V for Eco30 and Eco50, respectively), even if the force values are comparable during the trials. Overall, both sensors allow capturing robust respiratory signal, distinguishing between shallow and deep breathing and identifying both end-inhalation and end-exhalation apnea phases.