A Flexible Humidity Sensor Based on Vanillin‐Modified Polyvinyl Alcohol Ionogel for Wearable Health Monitoring

Flexible hydrogel‐based humidity sensors play a pivotal role in wearable health monitoring devices. However, their practical application is significantly hindered by the inherent instability of hydrogels, which remains a largely unsolved problem. Herein, we overcome these limitations by fabricating an intrinsically transparent and robust ionogel sensor. This is achieved through the chemical modification of polyvinyl alcohol (PVA) with vanillin (VPVA), which critically strengthens hydrogen‐bonding with the ionic liquid (IL), thereby resolving compatibility issues and bolstering mechanical integrity. This sensor demonstrates a suite of superior performance metrics, including exceptional linearity (R 2 = 0.9969), a wide detection range of 8%–90% relative humidity (RH), minimal hysteresis (<3.86%), and maintains highly stable humidity sensing performance even after exposure to extreme temperatures (−20°C to 80°C). Elucidation of the sensing mechanism reveals a response driven by ion migration, which is promoted by the plasticizing effect of water molecules. The sensor's practical utility is validated in diverse applications, including real‐time respiratory monitoring, non‐contact sensing, and quantitative skin moisturization assessment. This chemical modification strategy provides a scalable and robust platform for developing ionogel‐based wearable sensors.