Packaging-Grade Paper Humidity Sensors Made by Flexography Only: From Sustainable Manufacturing to Transient Applications

Printed electronics offer a scalable and sustainable route for integrating sensing systems into everyday environments; however, the use of flexography remains highly limited, and fully printed sensors fabricated exclusively with industrial flexographic technology have not been previously reported. This study evaluates the feasibility and practical limits of fabricating resistive humidity sensors for relative humidity (RH) measurements using flexography only, relying on commercial infrastructure, packaging-grade substrates, and low-temperature processing. Silver interdigitated electrodes and a carbon-based sensing layer were printed using solvent-based electronic inks, industrial aniloxes (12 and 20 cm3/m2), and standard flexographic conditions (10 m/min, ≤120 °C drying), without any post-processing. The sensing layer was optionally modified with adsorptive additives (≤5 wt% MgO; additionally, Al2O3 and Al) to enhance moisture interaction while maintaining rheological compatibility. Sensors were fabricated on recyclable paper substrates and PET for comparison. Under controlled conditions (10%–90% RH at 23 °C), devices exhibited a maximum relative resistance change of ~75% at 90% RH (referenced to 40% RH), low hysteresis (≤~5%), rapid visible response (<1 min), and stabilization within ~30 min. MgO increased relative response by 20%–233%, depending on humidity. Paper-based sensors showed higher responses but single-use behavior under flooding, while PET enabled repeatable cycling. Rather than targeting state-of-the-art performance, this work defines the functionality reliably achievable using flexography only, clarifying trade-offs among substrate choice, layer thickness, and additives for sustainable, humidity and disposable flood monitoring.