Facile assembly of flexible humidity sensors based on nanostructured graphite/zinc oxide-coated cellulose fibrous frameworks for human healthcare.

The development of flexible, cost-effective, highly efficient, and reliable humidity monitoring sensors is in high demand owing to their wide-range of applications in industrial domains. In this study, a humidity sensor was fabricated based on graphite/zinc oxide nanoparticle (G/ZnO-NP)-coated cellulose paper. A bar device was designed using computer software, and its sketch was printed on cellulose paper, with graphite bars then added using the pencil-drawing method, and then ZnO-NP paste was coated on the graphite patterns. Scanning electron microscopy and X-ray diffraction analysis were used to respectively inspect the morphological and structural features of the samples. For sensor fabrication, copper wires were attached to the electrodes using copper tape. The fabricated device was placed into a chamber with varying relative humidity (RH) levels of 11%, 24%, 43%, 62%, 84%, and 97%, controlled using the salt solutions inside the chamber. The response of the sensor was recorded in terms of the change in resistance of the device upon exposure to different humidity environments. The sensor delivered a response time as short as 4.31 s for the 24% RH condition, and a recovery time as short as 10.05 s for 43% RH. Moreover, the sensor exhibited a sensitivity of 717% for the 97% RH condition. The sensor was also evaluated for human breath monitoring, showing distinctive responses for inhalation and exhalation.