Electronic-free and low-cost wireless sensor tag for monitoring fish freshness

Real-time monitoring of fish products throughout the supply chain is critically important for proper tracking of fish quality and spoilage to eliminate food waste. However, most of the existing food freshness monitoring technologies are based on complex and costly sensors that lack specificity to direct biomarkers of product quality rendering them impractical for scalable low-cost production and integration into standard food packaging products. To address these issues, we present an electronic-free wireless sensor tag that can selectively monitor hypoxanthine (HX) as a specific biomarker to indicate the spoilage of fish products. The sensor tag is composed of a working and reference coil in a self-calibrating, dual-resonator configuration. The working coil is covered with a stimuli-responsive bilayer polymer structure that degrades only when high levels of HX are produced in spoiled fish products, resulting in a discernible change in the resonant characteristics of the coil. Detecting this change with respect to the reference antenna using a contactless external reader provides a differential mode of sensing to avoid environmental noise and enhance accuracy. The sensor fabrication process takes advantage of scalable and cost-effective manufacturing technologies including laser processing of metalized films and solution casting approaches. Systematic design of resonant coils and optimization of functional HX responsive coatings were performed to identify structures that provided the required spoilage detection threshold of (>2.25 mM HX) which is an indicator of fish spoilage in package products. As a proof of concept, the sensor tag's performance was validated in a commercially packaged fish product, highlighting its ability to effectively differentiate spoiled products from fresh products in refrigerated conditions. The high selectivity exhibited by this technology puts forth a new platform that merges stimuli-responsive sensing with electronic-free wireless telemetry to develop biomarker-specific sensors for a wide range of intelligent food packaging applications.