Highly Flexible Strain Sensors Based on CNT-Reinforced Ecoflex Silicone Rubber for Wireless Facemask Breathing Monitoring via Bluetooth

Abstract

Highly stretchable strain sensors based on carbon nanotube (CNT)-reinforced Ecoflex silicone rubber are developed for breathing monitoring purposes. The addition of CNTs promotes an improvement in electrical conductivity and mechanical properties (Young’s modulus and tensile strength) due to its good dispersion in Ecoflex. The evaluation of strain monitoring response, in both tensile and compression conditions, indicates a wide strain detection range and an ultrasensitive response at high strain levels, reaching a gauge factor of around 104 at 70% or 105 at 300% for 0.3 and 0.7 wt % CNT-reinforced sensors, respectively. They show a quite stable electrical response under 2000 cycling loads and different levels of frequencies. Moreover, the response and recovery times are in the range of milliseconds (∼600 and ∼800 ms, respectively). Finally, a proof-of-concept of wireless facemask breathing monitoring was carried out with Bluetooth Low Energy technology and a platform that has been developed to acquire, filter, visualize, and store the breathing signal. With this, the respiration rate can be unequivocally monitored as well as the difference between inspiration and expiration. Thus, this type of trial is proposed for breath monitoring in medical analysis, emergency teams, or first aid.