Abstract
In the precarious situation caused by the COVID-19 pandemic, the use of messenger ribonucleic acid (mRNA) vaccines is promising for prevention against the infection. However, this type of vaccine has not been effectively commercialized because it needs to be stored and transported at ultracold conditions. mRNA vaccines exposed to undesired temperatures may not show any visible changes but can deteriorate and cause negative effects. Consumers' demand for vaccine authenticity requires logistics to develop a robust monitoring tool to ensure the integrity of ultracold supply chain from manufacturing until vaccination. Here, we report a time-temperature indicator (TTI) that can detect a relatively small change in temperature within subzero ranges, for example, from -70 to -60 °C, which cannot be achieved by current TTIs operating at room temperature. A dyed noneutectic ethylene glycol/water mixture that melts near the mRNA conservation temperature (-69 °C) diffuses into a white absorbent and leaves a colored trace. In addition, the heterogeneous ice particles in the noneutectic mobile phase can prevent absorption during short-term exposure to room temperature. Therefore, the proposed TTI will not record inevitable "meaningless" short-term exposure to room temperature during the cold supply chain but monitor the "meaningful" relatively long-term exposure above -60 °C. These findings help facilitate the safe distribution of the COVID-19 mRNA vaccines.