Paper and carbon ink enabled low-cost, eco-friendly, flexible, multifunctional pressure and humidity sensors

Abstract For a long time, the daily paper and carbon ink have been used for writing and painting. With the development of the electronic technology, they are expected to play new roles in electronic devices. Herein, combining the unique characteristics of the paper (rough surface, hydrophilicity) and carbon ink (conductivity), this work rationally proposed two low-cost, eco-friendly, flexible, multifunctional pressure and humidity sensors. The results show that as-fabricated paper-based (PB) pressure sensor has a good sensitivity of 0.614 kPa −1 in the pressure range of 0–6 kPa. The PB humidity sensor has a large response of ∼2120 (current ratio at 91.5% relative humidity (RH) and 0% RH). The PB pressure sensor is proven to be useful for multiple pressure related contact detections, and the PB humidity sensor can be used for many humidity related non-contact detections. Interestingly, combining the different detecting modes of pressure and humidity sensors, some same detecting functions (e.g. switch, respiratory frequency and speech recognition) are realized from contact to non-contact using PB pressure and humidity sensors, which greatly enhance the wearable comfort. Compared with previous reports, this work demonstrates a much simpler approach without expensive raw materials, toxic reagents and high temperature treatment to achieve outstanding sensing performances of the multifunctional pressure and humidity sensors.