Time–Temperature Excursion Monitoring Using Chipless RFID Tags and Organic Oils

A food-safe cost-effective time–temperature indicator (TTI) sensor for cold chain disruption detection at the item level is proposed. The sensor is based on the radar cross Section (RCS) readout from a chipless square split ring resonator (SSRR) exposed to organic oils with customizable melting temperatures and defined flow paths. The inclusion of several oil mixtures into the same sensor allows for the determination of a range of configurable temperatures/times. The same sensor has two modes of operation: one for threshold detection and another for gradual change detection. These modes depend on the orientation of the sensor on the packaging and the influence of gravity. The provided design, along with a convenient signal conditioning strategy, accurately detects four time exposure thresholds in the 7–30 min range when placed in upright position at ambient temperature, while it exhibits linear response between 10 and 30 min just by turning it by 90°. Prospective future directions are also discussed.