A Tuned Microwave Resonator on Flexible Substrate for Nondestructive Water Content Sensing in Fruits

This work aims to develop a planar microwave sensor fabricated on a flexible polyimide substrate to monitor the water content of fruits nondestructively. The sensor is based on a planar loop resonator tuned with a concentric metal pad that features improved resonance, compact size, and flexibility to conform to the curved surface of the fruit. The sensing mechanism is to detect electromagnetic resonance that is susceptible to dielectric property changes by water content variations. The robust resonance provides electric fields that penetrate deeper into the fruit tissues, compared with an untuned one, with a sufficient spectral resolution to reach high sensitivity. Experiments were conducted, including long-term continuous water content monitoring and total water content measurements. The sensors demonstrated clear frequency shifting trends when fresh apples became dehydrated, and their initial resonant frequencies indicated total water contents. Simulations were conducted to examine measurement discrepancies induced by inhomogeneous water evaporation and surface curvatures. The feasibility of sensing the watercore defects inside apples was demonstrated with simulations. In addition, the sensor was used to demonstrate the feasibility of measuring water content in potatoes. The promising results show the great potential of the noninvasive and continuous water-content sensor applications in agriculture to study the growth, maturity, anomaly, and storage of fruits and in food processing applications to achieve optimal quality.