Biodegradable RF Metamaterial Perfect Absorber for Wireless Soil pH Monitoring

Abstract A wireless soil pH sensor is proposed using a sheet‐type perfect absorber metamaterial. The sensor utilizes a high‐impedance surface metamaterial, which functions as a perfect absorber at the resonant frequency. This sensor has a uniform water‐soluble Mg film at the bottom coated with hydroxyapatite that degrades depending on the pH, and a periodic square split ring resonator metamaterial structure made on top of a lossy dielectric plate. The sensor detects the dissolution of the metamaterial due to soil pH conditions through the electromagnetic wave reflection response at GHz frequency. Because of the perfect absorbing characteristics, the influence of soil on the metamaterial's electromagnetic response is blocked, allowing for robust measurement regardless of soil type. By coating the Mg with hydroxyapatite, the difference between acidic (pH = 3.0) and neutral soils can be detected by the time difference by a factor of three in the reflectance change from 40% to 75% reflectance change, indicating a transition from absorptive to reflective of the metamaterial's response, at the resonant frequency of 4.12 GHz.