Hydrophobic Multifunctional Flexible Sensors with a Rapid Humidity Response for Long-Term Respiratory Monitoring

Slow humidity response and poor tolerance to high humidity environments limit the application of humidity sensors for respiratory monitoring. Herein, a flexible humidity sensor with a structure composed of multiwalled carbon nanotubes (MWCNTs) firmly attached to the surface wrinkles of a natural latex film was fabricated by a simple swelling-self-assembly method. The weak interaction bond between MWCNTs and water molecules affords the sensor a humidity response time that is as low as 0.7 s and a wide humidity detection range (0–100%), enabling it to be used to accurately detect human respiration at a frequency of 1 Hz. The wrinkled structure of the sensor surface increases the contact area between the MWCNTs and the airflow, thus improving the sensitivity to humidity in the airflow. The near-superhydrophobic surface (contact angle = 143.5°; roll-off angle = 5°) enables the sensor to be used to accurately detect human respiration for extended periods of time in high humidity environments. The firmly attached MWCNTs ensure the stability and robustness of the sensor for humidity detection. In addition, the sensor can be used to detect trace organic solvents in water in a noncontact mode. The above advantages enable the sensor to be used to monitor a user's respiration in real time and even to identify the vocal characteristics of the user, indicating its potential application in wearable devices.