From Moisture to Signal: A Comprehensive Review of Flexible Capacitive Humidity Sensors

ABSTRACT

Water covers more than 75% of the Earth's surface, and consequently, moisture is ubiquitous in nearly every environment. Humidity directly influences the performance, reliability, and lifespan of electronic devices, as well as the quality and safety of pharmaceutical, food, and industrial products. This underscores the need for highly sensitive, reliable, and reproducible humidity sensors capable of accurately detecting moisture levels across diverse conditions. Conventionally, humidity sensors have been fabricated on rigid substrates such as silicon, glass, or ceramic. However, emerging applications—including wearable health monitoring, skin hydration tracking, respiratory diagnostics, and human–machine interfaces—demand sensors that are mechanically flexible, lightweight, and conformable to curved or dynamic surfaces. Among the various transduction mechanisms, capacitive humidity sensors have attracted particular attention due to their fast response, low power consumption, good linearity, and relative insensitivity to lower temperature dependence under specified operating conditions. This review provides a comprehensive overview of recent advances in flexible capacitive humidity sensors, with emphasis on sensing mechanisms, dielectric material selection (including metal oxides, two‐dimensional materials, and polymers), electrode design, and substrate engineering strategies that enable mechanical flexibility without compromising sensing performance. Key performance metrics—sensitivity, response/recovery time, humidity range, and frequency range—are critically compared across reported devices. This review aims to serve as a valuable reference for researchers working toward the development of next‐generation flexible humidity‐sensing technologies.