Polymer opal films shed light on spoiled foods

New polymer opal films use nanotechnology to change colour, a quality that can be used to indicate when foods have become spoiled, say scientists. The food packaging application is one of the promising commercial possibilities for a new type of flexible plastic film developed by scientists at the Britain's University of Southampton and the Deutsches Kunststoff-Institut (DKI) in Darmstadt, Germany.
The new films have the potential to help food manufacturers prevent recalls and food contamination incidents from affecting their brands.
The "polymer opal films" belong to a class of materials known as photonic crystals. The crystals are built of many tiny repeating units, and are usually associated with large contrasts in the components' optical properties.
A sheet of the opal film may change to a different colour depending on the angle it is viewed.
Photonic crystals are also present in opals, butterfly wings, certain species of beetle, and peacock feathers, which all feature arrays of tiny holes, neatly arranged into patterns.
Researcher Jeremy Baumberg and his team developed polymer opals to combine the precise structure of man-made photonic crystals with the colour of natural structures.
The polymer opal films are made of arrays of spheres stacked in three dimensions, rather than layers.
They also contain carbon nanoparticles wedged between the spheres. The light also scatters off the nanoparticles, and not just from the interfaces between the plastic spheres and the surrounding materials.
The technique makes the film look intensely coloured, even though they are made from only transparent and black components that are environmentally benign.
Additionally, the material can be "tuned" to only scatter certain frequencies of light simply by making the spheres larger or smaller, the scientists say.
Researchers in both Britain and Germany then developed a means to mass-produce the photonic crystals. They have developed a manufacturing process that can be successfully applied to photonic crystals to produce long rolls of polymer opal films.
The films are "quite stretchy" and when they stretch, they change colour, according to researcher Jeremy Baumberg.
"This, too, makes them ideal for a wide range of applications, including potential ones in food packaging, counterfeit identification and even   defence."