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Aerosol-jet printing of flexible green graphene humidity sensors for IoT applications

This research focuses on designing and developing highly sensitive flexible graphene sensors for humidity detection. First, green graphene inks were prepared with triton X-100 (Ge-GTr) as a dispersant and loaded gelatin as a binder. Then, flexible graphene sensors were fabricated by printing the graphene inks with aerosol-jet on top of screen-printed carbon electrodes. High gelatin-modified sensors (0.5Ge-GTr and 1Ge-GTr) exhibited a good linear response in relative humidity (RH) range of 30%RH–90%RH with a good sensitivity of 0.55/%RH at 25°C and with fast response in a second range. Moreover, they showed good stability to temperature fluctuation ranged from 22°C to 70°C. The humidity sensing mechanism depends on the surface coverage of graphene by the hydrophilic coating. The electrons transfer can explain the sensing mechanism for sensors of GTr, and 0.1Ge-GTr. In contrast, sensors' response could be better explained by gelatin swelling for 0.25Ge-GTr, 0.5Ge-GTr, and 1Ge-GTr sensors upon humidity detection. The proposed humidity sensors (0.5Ge-GTr and 1Ge-GTr) are green, highly sensitive, and with a fast response to human breathing, making them good candidates for healthcare applications such as respiration sensors for facial masks.