Designer colours

Military clothing that changes colour to warn of chemical attack, or food packaging that shifts in tone to indicate just how long it has been on the shelf are just two of the applications for a technology currently under development in the UK.

Researchers are producing materials with bright, vibrant colours that are created through the manipulation of nanoparticles rather than using harmful toxic dyes, which are of increasing concern for their impact on the environment.

This new flexible plastic material could be designed to degrade safely over time and could have numerous applications in mobile electronics, fashion and automotive industries.

The new technique manipulates tiny polymer spheres which refract light in certain ways depending on how they are stacked together. Each of the spheres is only about around 200nm wide, or roughly the same size as the wavelength of light.

When light strikes them it is reflected around inside the structure, in much the same way that opals reflect light, according to lead researcher, Prof Jeremy Baumberg, head of the quantum, light and matter group in the Department of Physics at

.

Industrial scale

But whereas opals have taken hundreds of thousands of years to grow — and even laboratory-based experiments to grow artificial opals take months — this research hopes to be able to create these structures on a huge, industrial scale. It is this development that has piqued the interest of big industrial partners such as

,

and

.

At present, Baumberg's researchers are collaborating with colleagues at the

— the German centre for colour technology.

The technique uses these plastic spheres which then flow over each other in a plastic melting machine. As they line up they create large sheets of plastic material that shimmers with irridescent colours.

'The colours are set primarily by the size of the spheres,' explained Baumberg. 'If you look at different angles you get different colours. What we have also discovered is that if you add tiny amounts of nanoparticles of carbon, for example, you get a very strong, metallic green colour. This is the effect we are trying to understand.'

By adding these nanoparticles, which are hundreds of times smaller in size than the spheres, it has proved possible to make an enormous variety of new sorts of materials which have 'smart' colour. For instance, the films are elastic and they can drastically change colour when they are either stretched or bent.

The defence research laboratory, DSTL, has been keeping a close eye on the technology and has also added funds to the project's EPSRC grant. According to Baumberg, the potential military applications for the material are huge.

Further tinkering

'DSTL is interested in smart sensors on the battlefield. So if you make fibres out of this material and chemically sensitise them the clothing could, for example, change colour at low concentration of nerve gas,' he said.

With further tinkering the materials could be made to harvest power to run the array of electronic equipment that future soldiers will have to carry. 'DSTL has a watching brief on smart materials and this is one of the most interesting ones,' he said.

De La Rue, the company that adds the security features to the UK's banknotes, is interested in looking at using the films for added security applications.

Unilever, too, is a partner on the project and Baumberg said that the materials could be used to create packaging that could indicate shelf life at a glance as the colours changed over time.

There are, however, a number of challenges that Baumberg's team need to overcome before the technology can be fully commercialised, not least of which is predicting how the spheres are going to interact.

'We need to prove it can be made in the specification you want, on demand, completely predictably,' said Baumberg. 'The scattering inside this periodic set of spheres is very different to normal light scattering, so we are trying to understand that, as sometimes it works better than others — which is no good for industrial applications.'

Niall Firth