Shenzen, China—It is one of the pharmacists’s worst nightmares: Because of a problem with a refrigerator or freezer, drugs required to be kept cold become too warm. One of the key concerns is how to detect when safe limits have been exceeded.

A report from an American Chemical Society (ACS) publication, ACS Nano, explained how research is coming up with a solution.

“Some foods and medicines, such as many COVID-19 vaccines, must be kept cold,” noted a press release from the ACS. “As a step toward a robust, stable technique that could indicate when these products exceed safe limits, researchers in ACS Nano report a class of brilliantly colored microcrystals in materials that become colorless over a wide range of temperatures and response times. As a proof of concept, the team packaged the color-changing materials into a vial lid and QR code.”

The problem is that walk-in freezers and refrigerated trucks may sometimes fail. “Wireless sensors can monitor the temperature of individual products, but these devices produce a lot of electronic waste,” according to the press release.

The ACS pointed out that researchers have suggested using materials that act as visual indicators to provide this information with less waste but have found that some current options using colorful reactions or dyes produce hues that can fade. Another issue is that some of the proposed products can track only above-freezing temperatures, which is not useful for some COVID-19 vaccines that can start breaking down below freezing—above –4 degrees F or below –94 degrees F.

Because of this, according to the ACS, a team of Chinese researchers set out to “develop a better color-changing material with tunable melting to track a wide range of temperatures.”

The journal article noted that vaccines that must be kept cold, such as those used against COVID-19, “often face significant challenges in reliable cold chain transport. Despite extensive efforts to monitor their time–temperature history, current time–temperature indicators (TTIs) suffer from limited reliability and stability, such as difficulty in avoiding human intervention, inapplicable to subzero temperatures, narrow tracking temperature ranges, or susceptibility to photobleaching.”

The solution, according to the report, is a class of structural-color materials that “harnesses dual merits of fluidic nature and structural color, enabling thermal-triggered visible color destruction based on triggering agent-diffusion-induced irreversible disassembly of liquid colloidal photonic crystals for indicating the time–temperature history of the cold chain transport.”

The indicator system uses structural colors, instead of dyes, for its indicator system. The researchers created glycerol-coated silicon dioxide nanoparticles, which appeared bright green or red when they clustered together into microcrystals in water. Then, by mixing different proportions of polyethylene glycol or ethylene glycol and water, the researchers were able to develop liquids with variable melting points.

“When these two parts were put together, they could produce an irreversible color loss when the temperature-triggered solution melted and the microcrystals broke apart,” stated the ACS press release. “The materials could be customized to track temperature exposures from –94 to +99 degrees Fahrenheit that lasted from a few minutes to multiple days.”

When the two-part indicator systems were packaged into flexible round vial labels and a QR code was added, they “were very sensitive and successfully indicated when the materials got too warm,” the ACS stated.

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