Flexible CNF/CB-based humidity sensors with optimized sensitivity and performance

Abstract

Humidity monitoring is ubiquitously used in industries like robotics, human–machine interfaces, and electronic skins, where humidity sensors ensure device protection and data-gathering accuracy. Therefore, this work focuses on developing sustainable, flexible, and highly sensitive cellulose-based humidity sensors. To achieve this goal, a Taguchi design of experiments was suggested to optimize four key ink formulation parameters: carbon black (CB) fillers, cellulose nanofibers (CNF), polyvinyl pyrrolidone (PVP) binders, and glycerol plasticizers. The resulting formulations were subjected to material, electrical resistance, adhesion, water contact angle, and hygroresistive characterization. The results detail how the factors of interest influence the electrical response of the sensors, their sensitivity, and ink adhesion to the substrate. Moreover, increasing the CNF, glycerol, and PVP while reducing CB enhances sensitivity and ink performance. Optimized sensors demonstrated high responsiveness between 20% and 90% relative humidity, with an exponential growth rate of 9.6–10.0% RH⁻¹. The optimized sensors were also assessed regarding their repeatability across 10 cycles, stability to bending, and insensitivity to temperature variation. These findings highlight the role of formulation interactions in sensor performance and demonstrate the potential of eco-friendly and highly sensitive humidity sensors for next-generation flexible electronics.