Recent advances in flexible paper-based humidity sensors: fabrication, mechanisms, performances, and applications

Humidity sensors, as key electronic devices for monitoring humidity, have a wide range of applications, including environmental and human-related humidity detections. Paper has become a promising material for flexible humidity sensors due to its inherent advantages such as low cost, hydrophilicity, environmental friendliness and flexibility. After years of development, paper-based humidity (PBH) sensors have made significant achievements, but many challenges remain in this field. This review aims to systematically discuss the recent developments in PBH sensors (mainly focusing on the past 5 years). First, we overview the main performance parameters and test methods of humidity sensors. Then, the fabrication techniques, as well as the humidity sensing mechanisms and performances of different types of PBH sensors (resistance, capacitance, impedance, triboelectricity, electrochemical, and ion-gradient), are systematically reviewed and discussed. In addition, the emerging applications of PBH sensors for human-related humidity detection are discussed. Finally, the challenges and future trends of PBH sensors are analyzed from different aspects. We hope that this review will provide important insights and guidance for the development of PBH sensors.