Fig. 1. Design and fabrication of flexible capacitive humidity sensors. (a) Fabrication processes of flexible Ga₂O₃/LM humidity sensors, including ultrasonication, spraying coating and laser sintering. (b) Schematic of the mechanism to form GWLM films by laser sintering and the sensing mechanism of Ga₂O₃/LM-based humidity sensors.

Fig. 2. Characterizations of flexible humidity sensors. SEM images of GWLM (a–d) with and (a, e) without laser sintering. (f) EDX images of the Ga, In, and O distributions. (g) Histogram of diameter size distribution for the unsintered GWLM particles on the PI film. (h) Resistivity of the laser induced conductive GWLM paths at different laser fluences. (i) The minimum resolution of sintered LM path at a laser fluence of 9.4 J/cm². (j–l) Schematics of Ga₂O₃/LM-based humidity sensors with various fabrication parameters (i.e. widths and lengths of electrodes, UV laser fluence) (top). Cycle measurements of Ga₂O₃/LM-based humidity sensors by periodically varying the humidity from 30% RH to 95% RH (bottom).

Fig. 3. (a) Photo of a flexible Ga₂O₃/LM-based humidity sensor. (b) Capacitance change of Ga₂O₃/LM-based humidity sensor at different RHs. The inset shows the image of this sensor. (c) A temperature dependent test of Ga₂O₃/LM-based humidity sensor via varying temperatures from 25 °C to 45 °C. The temperature in the oven was recorded by a commercial thermal sensor. (d,e) Long-term stability measurement (50 cycles) under 95% RH. (f) Cycle measurements of four different batches of humidity sensors by periodically varying the humidity from 30% RH to 95% RH.

Fig. 4. (a) Photos of a humidity sensor on a commercial mask worn on the subject’s face. (b) Human respiration test of a subject by mouth at a rest state. (c) Response and recovery time of the sensor. (d–f) Real-time monitoring of respiratory rate by nose of a subject at a rest state. Real-time monitoring of palm moisture while (g) drinking hot water and (h) exercising.