Drawn a flexible, low-cost, eco-friendly, and multifunctional humidity sensor on paper using carbon ink

Paper is known to have good flexible and hydrophilic characteristics, making it a potential candidate material to fabricate flexible humidity sensor. The key technology for fabricating paper-based (PB) humidity sensor is to prepare electrodes on the surface of paper. However, the preparation of electrodes on the flexible paper usually involves complex processes and expensive raw materials. In this work, inspired by the traditional writing art with carbon ink, a facile PB humidity sensor is fabricated by drawing the daily available carbon ink on paper as the electrodes. The results show that the PB humidity sensor exhibits an excellent humidity sensing response of more than 1000 as well good linearity (R2 = 0.9981) within the humidity range from 18.7% to 91.5% relative humidity (RH). Owing to the good flexibility and fast response rate (~7.5 nA/s, current change rate of the humidity sensor from18.7% to 91.5% RH), endowing the PB humidity sensor has multifunctional applications for breath rate, baby diaper wetting, and vertical space humidity distribution monitoring. Moreover, the PB humidity sensor can be directly disposed by a simple and cost-saving combustion method. This work provides a helpful guidance for the preparation of flexible, low-cost, eco-friendly and multifunctional humidity sensor, and expands the application of daily available carbon ink in the field of electrodes.