Sustainable Fully Inkjet-Printed Humidity Sensor Based on Ionic Liquid and Hydroxypropyl Cellulose

The current digitalization entails the implementation of sensors to collect information from the surrounding environment, in every facet of society. Among the environmental factors, the monitoring of humidity levels is essential in agricultural productivity, food and drug storage, comfort levels, ecosystems, or meteorology, among others. Due to the ubiquity demand of those sensors, certain requirements need to be met, such as being lightweight, flexible, stretchable, efficient, low cost, and of simple fabrication. However, sustainability concerns may arise from the use of non-degradable and synthetic materials in temporary or disposable electronics. This work focuses on the development of advanced humidity sensors using water-soluble cellulose derivatives and ionic liquids to meet performance requirements while promoting sustainability.<br/>We fully inkjet printed a humidity sensor composed of hydroxypropyl cellulose (HPC) as a matrix comprising a 50 wt.% of the ionic liquid: 1-butyl-3-methylimidazolium tetrachloronickelate(II) ([Bmim]₂[NiCl₄]). Further, the ionic liquid used presents a color change from transparent when hydrated to blue when relatively dehydrated. The developed ink based on these materials was dissolved using only water and ethylene glycol. The sensor consists of two layers: i) a silver interdigitated electrode (IDE) printed on a cellulose diacetate substrate and ii) the formulated ink printed on the IDE as an active layer.<br/>The relative humidity (RH) was measured by recording the impedance moduli at different frequencies (10^-1–10⁶ Hz). The sensor exhibits a linear response in the range between 30% RH and 90% RH with low hysteresis. Moreover, the color change has been monitored by UV-Vis spectroscopy where a linear increase of the transmittance was detected for decreasing RH at ≤ 30%, making it an electrical and a visual sensor. Therefore, this sensor allows to electrically measure the humidity levels in normal conditions and visually indicates exceptionally dry conditions. Thus, it can find applications to conserve water in agriculture or prevent the burning of stubble that can cause forest fires.