Microwave humidity sensor based on shorted split ring resonator with interdigital capacitance and α-Al2O3 nanoflakes/chitosan sensitive film for respiration monitoring

Abstract

A microwave relative humidity (RH) sensor based on shorted split ring resonator with interdigital capacitance (SSRR-IDC) as a test platform and α-Al2O3 nanoflakes/chitosan (CS) composite as sensitive film is proposed for the detection of human respiration. The SSRR-IDC was designed by optimizing the finger spacing and width of the IDC. Electromagnetic simulation of the designed SSRR-IDC confirms a strong electric field distribution and high dielectric sensitivity in the IDC region. The porous α-Al2O3 nanoflakes/CS composite was coated onto the IDC region of the SSRR to obtain a microwave humidity sensor. Abundant hydrophilic groups and large specific surface area of the α-Al2O3 nanoflakes/CS contribute to the adsorption of water molecules. The proposed microwave humidity sensor shows excellent sensitivity in wide RH range, especially at high humidity condition. The working principle and the sensitivity enhancement mechanism of the proposed microwave sensor were explained by combining a circuit model analysis and the dielectric property and surface microstructure changes of the sensitive materials. Furthermore, the microwave sensor also demonstrates short response/recover time and outstanding repeatability, long-term stability, temperature stability, and selectivity. Additionally, the prepared microwave humidity sensor successfully records respiratory data from different volunteers. Changes in respiratory depth before and after drinking water of the volunteers can also be distinguished by this sensor.