Localized‐Casting Enabled Humidity Sensors with Ultrahigh Sensitivity for Long‐Range Non‐Contact Respiratory Monitoring

ABSTRACT

Non‐contact respiratory monitoring based on capacitive humidity sensors is highly desirable due to its superior comfort, but it faces challenges including attenuated signals and limited working distance. Herein, we present a high‐performance humidity sensor fabricated via a novel localized‐casting method, based on a well‐defined porous moisture‐sensitive material consisting of hygroscopic lithium chloride (LiCl) as the moisture‐capturing component, hydrophobic poly(vinylidene fluoride‐trifluoroethylene) [P(VDF‐TrFE)] as the matrix, and polyvinylpyrrolidone (PVP) as the compatibilizer. Humidity‐induced reversible phase separation of LiCl in the P(VDF‐TrFE) skeleton improves the utilization efficiency of water molecules. Localized‐casting precisely deposits the material into the electrode gaps, allowing direct diffusion of water molecules to the effective sensing area. The device demonstrates an ultrahigh sensitivity of 2 µF·%RH‐1 and a fast response time of 0.9 s. Furthermore, the device exhibits robust stability under cyclic humidity variations, temperature fluctuations, long‐term storage, and bending deformations. Benefiting from excellent performance, the device realizes reliable non‐contact respiratory monitoring at a maximum distance of 50 cm, and proximity sensing up to 5 cm for LED switching control. For practical applications, a fully contactless, bed‐mounted sleeping pod for apnea monitoring is developed, enabling interference‐free and high‐comfort monitoring and presenting significant potential for home‐based healthcare applications.