Humidity sensor based on cobalt Chloride/Cellulose Filter-Paper for respiration monitoring

With the spread of COVID-19, more and more attention has been paid to the self-monitoring of healthy breathing. In this paper, we propose a humidity sensor which is simple to manufacture, low cost, good flexibility and visualization. The humidity sensor based on cellulose filter paper (FP) exhibited a good moisture-dependent voltage response over a wide relative humidity (RH) range due to the capillary structure of porous cellulose films and highly hydrophilic material of cobalt chloride (CoCl2), which facilitated water molecule adsorption and diffusion. The cellulose film colour shifted from blue to red when RH changed from 11% to 98%, and this colour shift was reversible. A moisture-driven sensor, based on the diffusive flow of water in cellulose networks, can provide an output voltage of 200 mV in a highly moist environment. Due to the high flexibility of cellulose fibres, the sensor maintained a constant output voltage even after 3000 folds. Moreover, the device was successfully applied for human respiratory monitoring and movement frequency tracking. The uniqueness of this new wearable humidity sensor technology can establish a new application field for the development of intelligent textiles and personal health products in the future Internet-of-things.