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Bio-inspired fibres change colour when stretched

29 January 2013

A team of materials scientists at Harvard University in the US and the University of Exeter in the UK, have invented a new fibre that changes colour when stretched.

The so-called bastard hogberry, shown here floating in water (which changes its apparent colour), has inspired a new type of photonic fibre (image courtesy of Peter Vukusic)
The so-called bastard hogberry, shown here floating in water (which changes its apparent colour), has inspired a new type of photonic fibre (image courtesy of Peter Vukusic)

Inspired by nature, the researchers identified and replicated the unique structural elements that create the bright iridescent blue colour of a tropical plant’s fruit. The multilayered fibre could lend itself to the creation of smart fabrics that visibly react to heat or pressure.

“Our new fibre is based on a structure we found in nature, and through clever engineering we’ve taken its capabilities a step further,” says Mathias Kolle of the Harvard School of Engineering and Applied Sciences (SEAS). “The plant, of course, cannot change colour. By combining its structure with an elastic material, however, we’ve created an artificial version that passes through a full rainbow of colours as it’s stretched.”

The photonic fibres are made by wrapping multiple layers of polymer around a glass core, which is later etched away. The thickness of the layers determines the apparent colour of the fibre, which can range across the entire visible spectrum of light. 

Since the evolution of the first eye on Earth more than 500 million years ago, the success of many organisms has relied upon the way they interact with light and colour, making them useful models for the creation of new materials. For seeds and fruit in particular, bright colour is thought to have evolved to attract the agents of seed dispersal, especially birds.

The fruit of the South American tropical plant, Margaritaria nobilis, commonly called 'bastard hogberry' is an intriguing example of this adaptation. The ultra-bright blue fruit, which is low in nutritious content, mimics a more fleshy and nutritious competitor. Deceived birds eat the fruit and ultimately release its seeds over a wide geographic area.

“The fruit of this bastard hogberry plant was scientifically delightful to pick,” says principal investigator Peter Vukusic, Associate Professor in Natural Photonics at the University of Exeter. “The light-manipulating architecture its surface layer presents, which has evolved to serve a specific biological function, has inspired an extremely useful and interesting technological design."

Vukusic and his collaborators at Harvard studied the structural origin of the seed’s vibrant colour. They discovered that the upper cells in the seed’s skin contain a curved, repeating pattern, which creates colour through the interference of light waves. (A similar mechanism is responsible for the bright colours of soap bubbles.) The team’s analysis revealed that multiple layers of cells in the seed coat are each made up of a cylindrically layered architecture with high regularity on the nano-scale.

The team replicated the key structural elements of the fruit to create flexible, stretchable and colour-changing photonic fibres using an innovative roll-up mechanism perfected in the Harvard laboratories.

“For our artificial structure, we cut down the complexity of the fruit to just its key elements,” explains Kolle. “We use very thin fibres and wrap a polymer bilayer around them. That gives us the refractive index contrast, the right number of layers, and the curved, cylindrical cross-section that we need to produce these vivid colours.”

The researchers say that the process could be scaled up and developed to suit industrial production.

“Our fibre-rolling technique allows the use of a wide range of materials, especially elastic ones, with the colour-tuning range exceeding by an order of magnitude anything that has been reported for thermally drawn fibres,” says Joanna Aizenberg, Professor of Materials Science at Harvard SEAS.

The fibres’ superior mechanical properties, combined with their demonstrated colour brilliance and tunability, make them very versatile. For instance, they can be wound to coat complex shapes. Because the fibres change colour under strain, the technology could lend itself to smart sports textiles that change colour in areas of muscle tension, or that sense when an object is placed under strain as a result of heat.

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