“This sticker, once exposed to room temperature, cannot be restored to its original state even if one attempts to refrigerate or freeze it again. Also, room-temperature exposure time cannot be manually adjusted. This means that there is virtually no room for any manipulation.”Dongyeop Oh
The cold chain is the supply chain that ensures uninterrupted distribution and storage in a temperature controlled environment, and is used to prolong the life of fresh products such as fruit and vegetables, and frozen food, as well as the distribution of medical supplies. It is crucial that it is subject to a quality management system to analyse, control, document, measure and validate such products.
However, if this system isn’t working properly it is difficult to tell if cold-chain food products have gone bad, particularly if examining by sight or smell don’t work, as the bacteria that forms when refrigerated or frozen foods become exposed to room temperature are not easy to identify. To that end, a new study from the Korea Research Institute of Chemical Technology, reported in Advanced Materials [Choi et al. Adv. Mater. (2020) DOI: 10.1002/adma.201907064], has developed a cold-chain safety sticker that creates an image when exposed to room temperature, indicating if food products inside have gone off.
The sticker, which is thin, flexible, easy to read, comparatively cheap to produce and can tolerate damage, shows the room temperature exposure history and time throughout the delivery process, and cannot be manually edited. The technology is based around on a nanofiber film with a stable structure at low temperatures, where thin threads intersect each other and appear opaque since the light is scattered. However, on exposure to room temperature for a certain period, the nanofiber structure collapses with the threads beginning to melt and becoming entangled. This allows light to transmit through the film, making it appear transparent.
From this mechanism, when the film on the front surface of the sticker becomes transparent on exposure to room temperature, the image produced on the back is visible from the front, which shows if the food products have spoiled. The team also demonstrated how to control the time required for the nanofiber film to become transparent, making it suitable for a range of foods that take different times to go bad – by controlling the composition and thickness of the nanofiber, it can operate similarly to a timer.
Competing technology involves the use of chemical reactions of inks to indicate exposure to room temperature, but are made of thick plastic, difficult to attach and much more expensive, with there also being a risk of the ink leaking if damaged. As researcher Dongyeop Oh points out, “This sticker, once exposed to room temperature, cannot be restored to its original state even if one attempts to refrigerate or freeze it again. Also, room-temperature exposure time cannot be manually adjusted. This means that there is virtually no room for any manipulation.”