This color will self-destruct

Messenger RNA vaccines for COVID-19 must be stored at subzero temperatures to remain effective over their full lifetimes. But how can a pharmacist ensure that a particular vial of vaccine hasn’t warmed up and then been re-cooled? New labels that irreversibly change color from green or red to colorless could be used to signal when vaccines, as well as medicines and organs, have been exposed to degrading temperatures.

Scientists led by Yadong Yin at the University of California, Riverside, and Xuemin Du at the Shenzhen Institute of Advanced Technology developed the labels, which use self-destructing liquid microcrystal arrays to produce the color-changing phenomenon. The labels have a layer of liquid colloidal photonic crystals to produce structural color—hues that arise from reflecting certain wavelengths of light rather than dye molecules. That layer is topped with a layer of triggering agent, like polyethylene glycol, which can be tuned to melt at a particular temperature. When the triggering agent melts, it seeps into the liquid microcrystal array, breaking the crystals up and destroying the color (ACS Nano 2023, DOI: 10.1021/acsnano.3c00467).

There are other methods for tracking temperature variations, but these rely on electronics, which can be tampered with, or materials that don’t work at subzero temperatures. The scientists say that these structural color liquids could also find use in wearable sensors and photonic displays.