Could banknotes made like butterfly wings deter forgery?

Posted by Bridget in Science in Society

Scientists say they have created artificial surfaces that mimic the stunning colours found on the wings of tropical butterflies. The findings could be used to help make banknotes and credit cards harder to forge.

The striking iridescent colours displayed on beetles, butterflies and other insects have long fascinated physicists and biologists.

However, imitating nature’s most colourful, eye-catching surfaces has proved elusive. This is partly because these colours are produced by light bouncing off microscopic structures on the insects’ wings. This differs from the better-understood process of using pigments, or molecules that absorb specific colours out of white light and thereby produce other colours.

Researchers at the University of Cambridge studied the Indonesian Peacock or Swallowtail butterfly, or Papilio blumei, whose wing scales consist of intricate, microscopic structures that resemble the inside of an egg carton. These structures produce intense colours because of their shape and the fact that they consist of alternate layers of air and cuticle, a hard material.

Unlocking one of nature’s secrets

Using a combination of procedures for fabrication at minute scales, or nanofabrication, the researchers reported that they made structurally identical copies of the butterfly scales that produced the same vivid colours as the wings. The nanofabrication methods including techniques known as self-assembly and atomic layer deposition.

‘We have unlocked one of nature’s secrets and combined this knowledge with state-of-the-art nanofabrication to mimic the intricate optical designs found in nature,’ said researcher Mathias Kolle of Cambridge. ‘Although nature is better at self-assembly than we are, we have the advantage that we can use a wider variety of artificial, custom-made materials to optimise our optical structures.

Clever colouring keeps it safe

‘These artificial structures could be used to encrypt information in optical signatures on banknotes or other valuable items to protect them against forgery. We still need to refine our system but in future we could see structures based on butterflies wings shining from a £10 note or even our passports.’

Intriguingly, the butterfly may also be using its colours to encrypt itself – appearing one colour to potential mates but another colour to predators, he added. ‘The shiny green patches on this tropical butterfly’s wing scales are a stunning example of nature’s ingenuity in optical design. Seen with the right optical equipment these patches appear bright blue, but with the naked eye they appear green.

‘This could explain why the butterfly has evolved this way of producing colour. If its eyes see fellow butterflies as bright blue, while predators only see green patches in a green tropical environment, then it can hide from predators at the same time as remaining visible to members of its own species.’

The findings are published in the May 30 issue of the research journal Nature Nanotechnology.

Source: World Science Net (http://www.world-science.net/), courtesy of the University of Cambridge and World Science staff

Thumbnail caption: The bright green wings of the P. blumei butterfly result from the mixing of the different colours of light that are reflected from different regions of the scales found on the wings of these butterflies. (Image: Mathias Kolle, University of Cambridge)