Researchers have created a thin-film polymer metamaterial that can be incorporated inside packaging that could change colour if internal conditions within the packaging change.
Designed by materials scientists at Rice University and the Massachusetts Institute of Technology (MIT), the product mixes polymers into metamaterial that changes colour – once they’re exposed to ions in the environment – depending on the ions’ ability to infiltrate the hydrophilic (water-loving) layers.
Ned Thomas, the William and Stephanie Sick Dean of Rice’s George R. Brown School of Engineering, clarified that the film is easy to make and fairly inexpensive so it will be able to cope with the industry demand.
He said the technology is so that the shift can be to longer to shorter wavelengths, so for example, from green to blue or from green to red.
He added: “If it’s inside a sealed package and the environment in that package changes because of contamination or aging or exposure to temperature, an inspector would see that sensor change from blue to red and know immediately the food is spoiled.
“And you can read these sensors with low [level technology], either with your own eyes or a spectrophotometer to scan things”.
The films are made of nanoscale layers of hydrophobic polystyrene and hydrophilic poly(2-vinyl pyridine) (P2VP). The micron-thick material is called a photonic gel.
Nano-sized layers of hydrophilic and hydrophobic molecules self-assemble into a black copolymer called a phototonic gel which changes colour depending on the amount of water absorbed by the hydrophilic layers.
The products work when the polystyrene molecules clump together to keep water molecules out while the P2VP forms its own layers between the polystyrene. On a substrate, the layers form into a transparent stack of what the researchers called alternating “nano-pancakes”.
The research team exposed their films to various solutions and found different colours depending on how much solvent was taken up by the P2VP layers. For example, with a chlorine/oxide/iron solution that is not readily absorbed by the P2VP, the film is transparent.
They turned a clear film to blue (with thiocyanate), to green (iodine), to yellow (nitrate), to orange (bromine) and finally to red (chlorine) with the changes being reversible.
Thomas added that the next step was to either licence the technology, which could also be used for security and multiband optical elements in laser-driven systems, or create a start-up company.
By Aneka Chohan