Fabrication of a flexible wearable humidity sensor and its respiratory detection experiment

Respiratory monitoring is essential for determining the vital characteristics of the human body and diagnosis of sleep-disordered breathing, critical diseases, and neonates. Moreover, micro/nanosensors play an important role in healthcare. In particular, they can detect changes in humidity around the nose and mouth for analysis of respiratory behavior. Herein, a high-performance flexible wearable humidity sensor was developed using a chitosan/conductive carbon black (CS/CB) hybrid film as the sensitive layer for respiratory detection.

The humidity sensor was fabricated by screen-printing a CS/CB film on a flexible poly (ethylene terephthalate) substrate with interdigitated electrodes. Scanning electron microscopy (SEM) was conducted to demonstrate the successful synthesis and combination of the CS/CB hybrid film. The SEM results showed that the CB particles are uniformly embedded in the CS/CB film with porous and rough nanostructures, which can help water molecules diffuse into the sensitive film. The humidity performance of the humidity sensor was tested by switching the device between different RH levels established by several saturated salt solutions (11%–97% RH) at room temperature. The resistance of the CS/CB sensor was measured with a high-precision data logger. The CS/CB humidity sensor had good electrical conductivity, and its resistance was increased with the increase in RH with good linearity. The sensor was experimentally evaluated for its performance parameters, such as sensitivity, response recovery characteristics, and repeatability. The humidity switching test demonstrated that the sensor has a fast response rate and good discrimination to different humidity environments. The sensor under the humidity rise reduction test showed good symmetry and coincidence, indicating that the hysteresis effect can be ignored. The CS/CB humidity sensor demonstrated good repeatability, rapid response/recovery time, and negligible hysteresis. A flexible wearable film-type breath detection platform was developed using a single-chip microcomputer and LabVIEW technologies, which achieved real-time data reading and processing of humidity, communication between the microcontroller and computer terminal, and display of breath waveforms. The detection platform has low production costs and is easy to operate, providing new insights into the practical application of high-performance humidity sensor detection technology. The sensor is placed in the mask for respiratory behavior monitoring, including normal breathing, slow breathing, rapid breathing, stop breathing, Biot’s breathing, wave breathing, tidal breathing, febrile patient breathing, etc. The breathing frequency and intensity can be observed clearly.

The flexible wearable humidity sensor could monitor and analyze human respiratory conditions with high accuracy and reliability. It demonstrated good application in real- time human breath monitoring. The humidity-sensitive mechanism of the sensor was ascribed to the good hydrophilicity and many active sites (hydrophilic groups and vacancies) on the CS/CB nanomaterials to absorb water molecules. In addition, the CS/CB nanomaterial with high surface area had numerous surface-active sites to adsorb water molecules.

The developed CS/CB humidity sensor has great practical potential in wearable applications, especially in humidity sensing detection. It also promotes students’ in-depth understanding of the experiments and applications of flexible wearable sensors, effectively enhancing their motivation and effectiveness of teaching.