Wireless Soil PH Sensing Using Fully-Degradable Sprit-Ring-Resonator Array with Isotropic Electromagnetic Response

This paper reports a two-dimensional isotropic and biodegradable sensor for soil pH monitoring. The sensor is operated wirelessly through split-ring resonators (SRRs) that consist of patterned hydroxyapatite (HA)-coated magnesium (Mg) on a poly-lactic acid (PLA) film. In acidic conditions, the degradation of each SRR results in a change in the electromagnetic response of the sensor. We demonstrate the operation of the sensor in two different acidic soils (pH>5 and <5) to distinguish the acidic soil wirelessly. Furthermore, devising the design and arrangement of SRRs, we have developed the SRR array with a highly isotropic transmission response that is invariant under two-dimensional rotation. We believe that our biodegradable sensor can be applied to low-cost, easy-to-use, and environmentally friendly monitoring of soil pH.