Know by the color of its packaging if a medicine has been well preserved

Some medicines, as is the case with many vaccines against COVID-19, must be kept cold. In the search for a robust and stable technique for when drugs of this class, as well as foods that also need refrigeration, exceed the conditions in which they are safe, researchers have discovered a type of brightly colored microcrystals in materials which become colorless when undergoing certain temperature changes.

Cold rooms and refrigerated trucks usually maintain set temperatures, but accidents can happen. It is possible to control the temperature of each product with wireless sensors, but these devices generate a lot of electronic waste.

Recently, initiatives have emerged to promote the use of materials that act as visual indicators so that this information can be obtained without generating so much waste. However, some of the current options that use color reactions or dyes produce shades that can fade. In other cases, they can only detect temperatures above freezing, which is not useful for some COVID-19 vaccines that can start to break down below freezing.

For this reason, the international team led by Chao Huang, from the Shenzhen Institute of Advanced Technology (SIAT), under the Chinese Academy of Sciences, set out to develop a material with better color change and with a modifiable melting point that would allow recording a wide range of temperatures.

The researchers used structural colors instead of dyes for their indicator system. The team made glycerol-coated silicon dioxide nanoparticles, which looked bright green or red when they clumped together to form microcrystals in water. Next, they created liquids with different melting points by mixing different ratios of polyethylene glycol or ethylene glycol and water. When these two parts come into contact, the color would irreversibly disappear when the temperature-sensitive solution dissolved and the microcrystals fell apart.

The green structurally colored material in the cap of a tube (left) becomes colorless (right) irreversibly when heated high enough. (Photos: adapted from ACS Nano 2023, DOI: 10.1021/acsnano.3c00467)

The materials could be modified to record exposures to temperatures between 37 degrees Celsius above zero and 70 degrees below zero, lasting from a few minutes to several days. In other experiments, two-part indicator systems were included on flexible round tube labels with a QR code. These systems are very sensitive, and they effectively detected when materials were at too high a temperature. The researchers say that materials of this type can be useful for the various situations that arise in cold chains for medical supplies.

Huang and his colleagues discuss the technical details of their system in the academic journal ACS Nano, under the title “Self-Destructive Structural Color Liquids for Time-Temperature Indicating.” (Source: American Chemical Society)