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Replicating the structure of this iridescent blue seed is the key to a new colour-tunable material

Bio-inspired fibres change colour when stretched

A team of materials scientists at Harvard University and the University of Exeter has invented a new fibre which changes colour when stretched. Inspired by nature, the researchers identified and replicated the unique structural elements, which create the bright iridescent blue colour of a tropical plant’s fruit.

The multilayered fibre, described in the journal Advanced Materials, 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 lead author Dr Mathias Kolle, a postdoctoral fellow at 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.”

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 in which 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.”

Professor Vukusic and Exeter PhD student Alfie Lethbridge worked with collaborators at Harvard to study 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 nanoscale.

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 coauthor Joanna Aizenberg, Amy Smith Berylson Professor of Materials Science at Harvard SEAS, and Kolle’s adviser. Aizenberg is also a Director of the Kavli Institute for Bionano Science and Technology and a Core Faculty Member at the Wyss Institute for Biologically Inspired Engineering at Harvard.

The fibres’ superior mechanical properties, combined with their demonstrated colour brilliance and tuneability, make them very versatile. For instance, the fibres can be wound to coat complex shapes. As 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.

Additional coauthors included Moritz Kreysing at Ludwig Maximilians University in Germany, and Jeremy B Baumberg, Professor of Nanophotonics at the University of Cambridge.

This research was supported by the US Air Force Office of Scientific Research Multidisciplinary University Research Initiative, by the UK Engineering and Physical Sciences Research Council, and through a postdoctoral research fellowship from the Alexander von Humboldt Foundation. The researchers also benefited from facilities at the Harvard Center for Nanoscale Systems, which is part of the National Nanotechnology Infrastructure Network supported by the US National Science Foundation.

Date: 29 January 2013

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