Entirely Biodegradable Wireless pH Sensor with Split-Ring Resonators for Soil pH Monitoring

This paper describes a wireless and battery-free soil pH sensor made entirely of degradable materials. Soil pH monitoring is significant for precision agriculture and environmental conservation because soil health can be estimated by measuring soil pH. Soil pH can be measured by collecting soil samples and analyzing them in a laboratory environment or by inserting an electrode-type sensor into the soil and measuring soil pH on site. However, neither method is suitable for wide-area soil pH sensing for precision agriculture or environmental conservation. Herein, a fully biodegradable wireless pH sensor is presented that can be dispersed to outdoor environment to detect soil acidity. The sensor is operated wirelessly through split-ring resonators (SRRs) that consist of patterned octacalcium phosphate (OCP)-coated magnesium (Mg) on a poly-lactic acid (PLA) sheet. At acidic soil pH, the degradation of each SRR results in a change in the electromagnetic response of the sensor. The operation of the sensor in six different types of acidic soils is demonstrated to distinguish the acidic soil wirelessly. It is expected that the biodegradable sensor can be applied to low-cost, easy-to-use, and environmentally friendly monitoring of soil pH.