fig4

Figure 4. (A) Photos of humidity sensors based on interdigital electrodes and the humidity sensing mechanisms of their various parts; (B) The influence of sweat electrolytes on the sensor response current when RH changes; (C) Response and recovery characteristics of the sensor at different RH levels, with RH switching intervals of 25 s; (D) Nose-breathing response curve with different breathing frequencies; (E) Contactless sensing: effect of finger distance from sensor height on response current; (F) Selectivity evaluation of the humidity sensor against common breath volatiles. Some image elements designed by Freepik (https://www.freepik.com/); (G) Photographs of the contactless switch sensing system; (H) Programmable 3 × 3 humidity sensors and LED arrays to be used to display the shapes of the letters “C”, “H”, “L”, and “U”. RH: Relative humidity; LED: light-emitting diode; AG: agarose.