Elastic fibres appear to change colour as they are stretched

Colour-changing clothes and more flexible optical fibres could be on the cards following the creation of a material that appears to change colour as it stretches.

Researchers at Exeter, Cambridge and Harvard universities developed the elastic fibres after being inspired by a type of South American berry, which reflects light in way that generates a rainbow of colours similar to those seen on soap bubbles or oil patches.

The special microstructure of the polymer fibres means that they reflect light at different wavelengths depending on how thin they are, which causes their colour to change as they are stretched.

Prof Peter Vukusic, one of the Exeter researchers, said the fibres could one day be used in place of a kind of optical fibre known as a photonic crystal fibre (PCF) that traps light inside it due to its microstructure.

‘Whereas the traditional PCFs are made of glass, silica or other materials and you can’t generally stretch them, the bio-inspired fibres fabricated in this work can be stretched,’ he told The Engineer. ‘And this leads to the potential for changing the light that is confined within it.’

Another potential application could be in sports clothes that change colour in areas of muscle tensions or that sense when an object is placed under strain as a result of heat.

Multiple scales of repeating architecture can be seen in the skin of the hogberry

The fibres are made from two polymer materials featuring the structure inspired by the fruit of the margaritaria nobilis (commonly called ‘bastard hogberry’), layered on top of each other and then rolled into cylinders tens of microns thick.

The top and bottom layers reflect light separately and the resulting beams interfere in a way that produces bright colour, which changes when the fibre is stretched and the angles at which the light waves are reflected are altered.

The researchers, led by Harvard’s Dr Mathias Kolle, created the fibres by dissolving each of the materials in a solution, spinning them into thin films and layering them on top of each other on a glass substrate.

Two polymers were spun into thin layers then rolled into a thin fibre.

‘Then we had to lift it off the glass substrate and that actually took a bit of doing,’ said Vukusic. ‘So we floated them in water so the bottom layer detached itself from the glass. Then we used a really thin glass fibre as kind of a rolling pin, grabbed one side of the bilayer and rolled it up, which was a huge undertaking.

‘The more you roll it, the more these layers end up becoming part of a concentric cylinder. We were then able to remove the rolling pin, leaving a completely flexible, cylindrical, multi-layered structure.’

The team now believes the fabrication method can be scaled up for industrial production.

The researchers were funded by the US Air Force Office of Scientific Research Multidisciplinary University Research Initiative, EPSRC, and the Bonn-based Alexander von Humboldt Foundation.

A paper on their work is published in the journal Advanced Materials.