Nanocellulose-Based Composites for Flexible and Biodegradable Humidity Sensors for Breath Monitoring

In this letter, capacitive humidity sensors were made from flexible and biodegradable films of nanocellulose-containing composites. Nanocellulose (NC) was extracted from wheat straw by acid hydrolysis. The composites were made by mixing NC suspension and polyvinyl alcohol (PVA) solution in different ratios of 25%:75%, 50%:50%, and 75%:25% by weight. The self-standing films were obtained by drying the solution in a Petri dishes. Measuring electrodes were deposited on the surface of NC-PVA composite films by magnetron deposition technique through a special stencil in the form of an unfolded capacitor. Such device parameters as sensitivity, response, reversibility, repeatability, response time, recovery time, and stability were measured. It was determined that sensors based on pure PVA have the highest value of sensitivity and response: 6,36 pF/%RH and 85.6 pF, respectively. However, the sensors made from pure nanocellulose show the highest speed (response time is 710 s and recovery time is 60 s) and the best short-term stability (signal deviation does not exceed 5%). At the same time, as the PVA content in the composite increased, the strength is decreased, and the plasticity of the material is increased. This makes it possible to develop sensors with specified characteristics. Such sensors have also been successfully used to monitor human respiratory activity.