Rapid-response, reversible and flexible humidity sensing platform using a hydrophobic and porous substrate

Flexible humidity sensors enabling the real-time monitoring of the humidity level in the environment or around the human body have increasing application prospects in electronic skin and personal healthcare. Tremendous efforts have been devoted to improve the performance of humidity sensors, but little effort has been made to investigate the effect of the substrate. This work reveals the vital role of the sensor substrate in determining the humidity sensing performance by performing a comparative study. A porous, hydrophobic and flexible liquid crystal polymer (LCP) substrate is employed to speed up the response and recovery processes and boost the sensitivity simultaneously. Graphene oxide-based sensors fabricated on the LCP substrate exhibit 8.1 and 3.2 times higher response and recovery speeds, respectively, and 1.37 times higher sensitivity than corresponding sensors generated on a smooth SiO₂ substrate. Similar performance improvement is observed when reduced graphene oxide and carbon nanocoils are deployed as the humidity sensing materials. These distinct structure-related properties enable the fabrication of humidity sensors to monitor various human activities, including fast and normal respiration, skin hydration, finger movement, etc. Furthermore, a positive correlation between the sensitivity and the oxygen content in carbon materials is revealed.