Paper-based humidity sensor for wearable based on near-field electrohydrodynamic direct writing

Flexible humidity sensors have been widely used in wearable electronics, owing to their flexibility and compatibility with electronic device substrates. This study fabricated a flexible paper-based humidity sensor with a grid-like sensing film using the near-field electrohydrodynamic direct writing (NFEDW) method. A grid-like sensing layer with a dense network of carbon nanotubes/cellulose nanofiber (CNTs/CNF) was prepared, and induced by a voltage field applied during printing. The effects of the substrates and pattern width on the performance of the sensor were systematically investigated. The prepared sensor exhibited high sensitivity of 101 % at 95 % relative humidity (RH). An interlocking structure formed between the CNTs/CNF composite and the paper substrate, endowing the sensor with outstanding bending (minimum diameter of 5 mm) and folding (up to 100 times) durability. Furthermore, the prepared sensor exhibited rapid response performance (17 % within 10 s). This addresses the issue of long response time, which is typically associated with CNTs-based sensors, making these sensors more suitable to practical applications. The above-mentioned advantages enable the sensor to be used for respiration monitoring and even for vocal recognition. Finally, a real-time respiration monitoring platform was developed to demonstrate the sensor's potential in wearable devices.