Flexible Humidity Sensor with High Sensitivity and Durability for Respiratory Monitoring Using Near-Field Electrohydrodynamic Direct-Writing Method.

The humidity of breath can serve as an important health indicator, providing crucial clinical information about human physiology. Significant progress had been made in the development of flexible humidity sensors. However, improving its humidity sensing performance (sensitivity and durability) is still facing many challenges. In this work, near-field electrohydrodynamic direct writing (NFEDW) was proposed to fabricate humidity sensors with high sensitivity and durability for respiration monitoring. Due to the applied electric field, dense carbon nanotube/cellulose nanofiber (CNT/CNF) networks formed during the printing process that enhance the sensitivity of the sensor. The prepared sensor showed excellent humidity responses, with a maximum response value of 61.5% (ΔR/R0) at 95% relative humidity (RH). Additionally, the sensitivity film prepared by the NFEDW method closely fits the poly(ethylene terephthalate) (PET) substrate, endowing the sensor with outstanding bending (with a maximum curvature of 4.7 cm-1) and folding durability (up to 50 times). The sensitivity of the prepared sensor under different simulated conditions, namely, nose breathing, mouth breathing, coughing, yawning, breath holding, and speaking, was excellent, demonstrating the potential of the sensor for the real-time monitoring of human breath humidity. Thus, the high-performance flexible humidity sensor is suitable for human respiration and health monitoring.