Laser Fabrication\nof Humidity Sensors on Ethanol-Soaked\nPolyimide for Fully Contactless Respiratory Monitoring

Humidity-sensor-based fully contactless respiratory monitoring\ncan eliminate the discomfort and infection risks associated with any\nwearable device. However, challenges in the facile fabrication of\nhighly sensitive humidity sensors continue to hinder their widespread\napplication for fully contactless respiratory monitoring. In this\nstudy, we introduce a simple method to fabricate highly sensitive\nhumidity sensors. Our method employs laser-induced graphene (LIG)\non an ethanol-soaked polyimide (PI) film as the electrode of the humidity\nsensor. The ethanol-soaked PI between adjacent LIG electrodes functions\nas the sensing material, enabling ion-conductive humidity sensing.\nCompared to the LIG humidity sensors fabricated on untreated PI films,\nLIG humidity sensors fabricated on ethanol-soaked PI films exhibit\nsuperior performance with higher linearity (<i>R</i><sup>2</sup> = 0.9936), reduced hysteresis (Δ<i>H</i> =\n5.1% RH), and increased sensitivity (0.65%/RH). Notably, the LIG humidity\nsensor fabricated on the ethanol-soaked PI film can detect a person’s\nbreathing from a distance of 30 cm, a capability not achieved by sensors\nfabricated on untreated PI films. Moreover, incorporating these LIG\nhumidity sensors into an array further enhances both the detection\ndistance and the sensitivity for respiratory monitoring. Experimental\nresults demonstrate that the LIG humidity sensor array can be employed\nfor fully contactless on-bed respiration monitoring and for continuous,\nfully contactless monitoring of the respiratory rate during treadmill\nexercise. These results highlight the great potential of our LIG humidity\nsensors for various practical applications in medicine and sports.