Recent advances in mass-sensing humidity sensors: mechanisms, materials, and applications

Humidity detection is of great significance in industry, agriculture, healthcare, and daily life. A humidity sensor is a core electronic component of humidity detection. Among the various humidity sensors, mass-sensing humidity (MSH) sensors have received widespread attention due to their excellent humidity sensing performances. This review systematically explores recent advancements in MSH sensors, including quartz crystal microbalance (QCM) humidity sensors and surface acoustic wave (SAW) humidity sensors. Firstly, the unique mass-frequency transduction mechanisms and performance test methods of QCM and SAW humidity sensors are elucidated. Next, we comprehensively categorized and analyzed critical advancements in humidity sensing materials for MSH sensors, including oxides, organic materials, two-dimensional materials, other single materials, and composite materials, highlighting their roles in enhancing humidity sensing performances. Furthermore, the potential of MSH sensors is demonstrated through cutting-edge applications in human body-related humidity monitoring, including respiratory behavior analysis and non-contact human-machine interaction. Finally, we critically noted existing challenges and outlined future directions. We expect that this review will provide readers with a comprehensive understanding of MSH sensors and serve as a guide for researchers in the field of MSH sensors.