Existing flexible humidity sensors often suffer from limitations such as poor adhesion at crease areas, electrode damage during bending, and slow response times. While many studies focus on enhancing sensitivity and flexibility, the response time, crucial for applications like respiration monitoring requiring high-frequency signal capture (e.g., coughing, asthma), remains often overlooked. Slow response times can lead to inaccurate respiration strength measurement, signal drift, and inability to capture the full breathing waveform. This research addresses these limitations by introducing a flexible RH sensor designed for high-speed sensing applications.

Previous attempts to create fast-response humidity sensors have utilized materials like silicon-nanocrystal films and tunable graphene polymer heterogeneous nanosensing junctions, achieving response/recovery times in the tens of milliseconds. However, these sensors may still not capture the nuances of rapid breathing patterns and often only provide information on respiration strength, not detailed waveforms. The current work aims to improve upon these existing limitations by creating a sensor with enhanced response time while maintaining other key performance indicators.

The sensor's fabrication involves several key steps. First, VACNT electrodes are created using a process including a 2-nm Fe seed layer deposition on a silicon wafer, patterning into an interdigital finger shape via lift-off, and VACNT forest synthesis using microwave plasma-enhanced chemical vapor deposition (PECVD). A double-layer flexible substrate is then prepared using PDMS and Parylene C. Trichloro(1H,1H,2H,2H-perfluorooctyl) silane (PFOCTS) treatment is applied to the silicon wafer to reduce PDMS adhesion and ensure substrate integrity during peeling. PDMS is spin-coated, followed by Parylene C deposition. The VACNT electrodes are then peeled off and cut into individual sensor pieces. Graphene oxide (GO) solution, with varying concentrations (1, 0.5, and 0.25 mg/ml), is dripped onto the VACNT electrodes, and the sensor is baked at 70 °C for 10 min. The VACNTs, substrate, and GO film are characterized using SEM, Raman spectroscopy, and XPS to confirm morphology, structure, and chemical composition. The hydrophobic and hydrophilic properties are evaluated using a contact angle meter. Sensor performance is tested using a custom setup involving mass flow meters, a reference RH sensor (Sensirion, RH-C-SHT20), an Arduino Yun board, and an RS LCR meter. A separate setup using a mechanical chopper is utilized for accurate response time measurement.

The sensor exhibits high sensitivity (16.7 pF/%RH at 1 kHz for 1 mg/ml GO solution), a low hysteresis (<0.44%), good repeatability (maximum Cr values of 2.7% and 2.1% for 20%-80% and 20%-60% RH step signals, respectively), and reasonable long-term stability (though sensitivity attenuation was observed over two months). The ultrafast response time was demonstrated in a custom-built setup. The performance of the sensor is significantly influenced by the concentration of the GO solution used; higher concentrations leading to higher sensitivity. The sensor shows a good performance at lower excitation frequencies, particularly 1 kHz. The sensor without GO showed minimal sensitivity change, indicating the crucial role of GO in humidity sensing. SEM analysis confirmed the successful integration of the VACNTs and GO layers. Raman and XPS analyses characterized the structure and chemical composition of the GO.

The findings demonstrate the successful fabrication of a flexible, highly sensitive, and ultrafast-responding RH sensor. The use of VACNTs as electrodes provides robustness against deformation, and GO as the sensing material contributes to high sensitivity and water adsorption due to its hydrophilic nature. The ultrafast response time is critical for applications such as real-time respiration monitoring. The device's performance characteristics meet the necessary requirements for high-frequency monitoring scenarios. The observed long-term stability attenuation warrants further investigation and potential improvements. The results highlight the potential of this sensor in various applications.

This work successfully demonstrates a novel flexible RH sensor with superior performance in terms of response time, sensitivity, and stability. The sensor's applicability in diverse areas such as respiration monitoring, pipe leakage detection, and non-contact electric safety warning has been shown. Future work should focus on improving long-term stability and exploring further miniaturization for enhanced wearable applications.

The study notes a sensitivity attenuation over long-term use, suggesting further optimization is needed for extended periods. The response time measurement setup was custom-built, and cross-validation with other established methods could strengthen the findings. The long-term stability might be affected by the residual water molecules in the graphene oxide film. Further optimization is required to reduce the hysteresis observed at high humidity levels.