Paper-based printed impedance sensors for water sorption and humidity analysis

The hygroscopic nature of cellulose-based materials poses a challenge in utilising paper for printed electronics. Adsorbed moisture strongly modulates the conductive and dielectric properties of paper. While affecting the function of printed electronic elements, this behaviour is beneficial for low-cost humidity sensing. Here we describe a simple, sensitive paper-based sensor with a pair of inkjet-printed interdigitated electrodes for analysing water sorption of cellulose over a wide range of relative humidity from 10% to 90%. Impedance spectroscopy between 100 Hz and 1 MHzreveals the humidity-dependent electrical properties of paper. Weshow in particular that measurements at 1 MHzprovide reliable information about the water adsorption and desorption of paper. In contrast, testing at frequencies below 200 kHz is dominated by ionic electrode polarisation, masking the water sorption kinetics of the sensor. Transient adsorption follows nearly an exponential function, while desorption is described by a stretched exponential law. At equilibrated conditions, the effective paper permittivity measured during adsorption and desorption is characterised by a GuggenheimAnderson- de Boer sorption isotherm which allows for a quantitative evaluation of the sorption hysteresis. Our means of sensing provides a simple and practical method of recording and analysing simultaneously conductivity and permittivity as well as wetting and drying of moist paper.