Cold and ultra-cold chain integrity monitoring via embedded resonant sensor indicators

Abstract

An increasing number of products require transport at cold and ultra-cold temperatures, < 5 and −60°C respectively. Time-temperature indicators (TTIs) are used to monitor the integrity of the cold chain, however, they are frequently based on colorimetric changes that can be inaccurate for quantification and not possible to read through closed shipment materials. Here, TTIs were prototyped using microfluidic channels containing fluids (oils) that are tuned to melt above target temperatures. By coupling these TTIs to resonant sensors, the time-temperature information can be transmitted wirelessly and quantitatively by observing shifts in resonant frequency. Multiplexed TTIs containing fluids that melt at 9, 13.5, and 19.5°C are demonstrated. When coupled to the resonant sensor, the fluid flows as the temperature increases incrementally and generates a frequency shift of 3.5, 4.5, and 6 MHz at each threshold. TTI for ultra-cold environments were also designed using microfluidic channels with melt responsive fluids (ethanol). The resonant sensor coupled TTI was placed in a Styrofoam container containing dry ice, and over 1 MHz frequency shift is observed when read external to the container as the dry ice evaporates and the temperature exceeds −35°C. This augurs potential uses in inexpensive monitoring of food and pharmaceutical shipments that must remain cold for adequate quality.