Printed, all-carbon-based flexible humidity sensor using a cellulose nanofiber/graphene nanoplatelet composite

•Flexible, high-performance, eco-friendly humidity sensor using simple and highly productive printing method was developed.•The humidity sensor is prepared from CNF/GNP; abundant, biodegradable, and biocompatible materials; not only for sensing layer but also electrodes.•This sensor showed a high resistive response of 240% for relative humidity (RH) in the range of 30%−90% with response and recovery times of 17 and 22 s, respectively. Eco-friendly flexible humidity sensors with high sensing performance are desired for the next-generation wearable electronics. In this work, we developed a high-performance fully printed flexible humidity sensor using all-carbon functional materials. The electrodes and sensing layer of the sensor were printed using all-carbon-based cellulose nanofiber/graphene nanoplatelet (CNF/GNP) composites. The newly developed CNF/GNP ink can be prepared by simple mixing without any complex processes. These sensors showed a high resistive response of 240% over the relative humidity (RH) range of 30% to 90%, with response and recovery times of 17 s and 22 s, respectively, and good mechanical flexibility. Since CNF and GNPs are abundant in nature, degradable, and biocompatible, this work suggests the possibility of manufacturing inexpensive, eco-friendly sensors with high performances. As a proof-of-concept, we demonstrate the application of our sensor in human respiration detection, non-contact proximity sensing, and baby-diaper wetness monitoring.