Rapid, highly sensitive, and highly repeatable printed porous paper humidity sensor

Paper based humidity sensors are low-cost, flexible, and biodegradable. However, the previously reported paper based humidity sensors exhibit poor repeatability, low sensitivity, high drift, and slow response. Here, we reported a new strategy to monolithically fabricate humidity sensor based on porous paper (PPHS) using robust and precise screen and gravure printings. PPHS is highly sensitive, with the maximum 1480% change at 1 kHz and the maximum 411.4% change at 10 kHz in capacitance, respectively. PPHS is highly repeatable (a 1.05% standard derivation in sensitivity for 3 continuous runs) and is stable and insensitive to a mild temperature change (15-30 °C). PPHS rapidly responds (0.8 s) and recovers (0.78 s). PPHS is also highly foldable and twistable. The high repeatability, high sensitivity, quick response/recovery, high foldability/twistability, and good stability of PPHS could be attributable to the combined effects of the high specific surface area of porous paper and the robust and precise printings. By comparison, PPHS is more sensitive than the previously reported paper-based humidity sensors and several humidity sensors fabricated using different substrates, materials, and printings. Due to these unique features, PPHS shows great potential applications in the humidity sensing, the breath detection, the proximity detection, and non-touch switch.