Elastic gels change colour when exposed to UV light

Researchers have created elastic gels that change colour when exposed to ultraviolet (UV) light and change back when the UV light is removed or the material is heated up.

The gels, developed at North Carolina State University, are impregnated with a type of photochromic compound called spiropyran. Spiropyrans change colour when exposed to UV light and the colour they change into depends on the chemical environment surrounding the material.

The researchers made the gels out of an elastic silicone substance, which can be chemically modified to contain various other chemical compounds — changing the chemical environment inside the material. Changing this interior chemistry allows researchers to fine-tune how the colour of the material changes when exposed to UV light.

’For example, if you want the material to turn yellow when exposed to UV light, you would attach carboxylic acid,’ said Dr Jan Genzer, Celanese professor of chemical and biomolecular engineering at NC State and co-author of a paper describing the research. ’If you want magenta, you’d attach hydroxyl. Mix them together and you get a shade of orange.’

Photochromic compounds are not new, but this is the first time they’ve been incorporated into an elastic material without impairing the material’s elasticity.

According to a statement, the researchers were also able to create patterns by using a shaped mould to change the chemical make-up of specific regions in the material.

Applying hydroxyl around a star-shaped mould on the material would result in a yellow star-shaped pattern appearing on a dark magenta elastic when it is exposed to UV light.

’There are surely applications for this material — it’s flexible, changes colour in UV light, reverts to its original colour in visible light and can be patterned,’ said Genzer. ’At this stage, we have not yet identified the best application.’

The paper, ’Photochromic materials with tunable colour and mechanical flexibility’, was published and featured on the cover of Soft Matter this month.

The research was supported in part by the US Defense Threat Reduction Agency.

The image to the left shows one of the photochromic gels before exposure to UV light. The image on the right shows the elastic material after exposure to UV light
The image to the left shows one of the photochromic gels before exposure to UV light. The image on the right shows the elastic material after exposure to UV light