Sensitive reaction: Photonic gel crystals demonstrate the 'tunability' of materials made from alternating layers of hard and soft polymers. The soft polymers are easily swollen with liquid or vapor causing the materials to reflect different colors of light based on the way their molecules are chemically 'tuned', said the researchers.

Credit: Donna Coveney

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ADELAIDE: Temperature, pressure and humidity can all be used to make a new 'photonic gel' change colour through the entire visible spectrum – the material could be used to make new kinds of sensor.

Both physical and chemical forces cause significant changes to pigmentation of the gel, said lead scientist Edwin Thomas, a professor of materials science and engineering at Massachusetts Institute of Technology (MIT) in Boston, USA.

Colour-changing swimmers

One possible application is in food processing plants where the colour of a sensor could change if food was overly exposed to humidity or heat. A more eye-catching possibility is using it to create swimwear that shifts to a different shade when wet, said Thomas. "Not only that, but if the wearer was swimming in the ocean, a river, or a swimming pool the colour would differ due to the different salt concentrations and pH."

The researchers say that the new technology differs to other colour changing substances in that it can respond to a wide range of everyday stimuli. The reaction is fully reversible and can occur in fractions of a second, they report in a study in the British journal Nature Materials.

The gel is made up of thin alternating layers of polymers, and it is the thickness of these layers that determines what frequencies of light are reflected. We then perceive the different frequencies of light as different colours.

Positively charged

Some of the layers contain chains of a long molecule polymer (poly-2-vinyl-pyridine), which has positive charges along its backbone. If the charges along the chain are shielded from each other – such as by adding large numbers of charged salt ions to the gel – then the backbone of the chain collapses. The expansion and collapse of the chains changes the thickness of the gel, and therefore the colour that it reflects.

"The simple fabrication process, wide tunability, and the fast response make this system potentially very useful for many photonic applications," commented chemist Yadong Yin of the University of California in Riverside. "The sub-second response time is truly surprising."

The MIT team are now developing a gel that will change colour in response to electrical currents.