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Could banknotes made like butterfly wings deter forgery?

May 31, 2010
Courtesy of the University of Cambridge
and World Science staff

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

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

The bright green wings of the P. blumei but­ter­fly re­sult from the mix­ing of the dif­fer­ent col­ors of light that are re­flected from dif­fer­ent re­gions of the scales found on the wings of these but­ter­flies. (Cred­it: Ma­thi­as Kolle, U. of Cam­bridge)

But im­i­tat­ing na­ture’s most col­or­ful, eye-catch­ing sur­faces has proved elu­sive. This is partly be­cause these col­ors 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 col­ors out of white light and there­by pro­duce oth­er col­ors.

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 col­ors be­cause of their shape and the fact that they con­sist of al­ter­nate lay­ers of air and cu­ti­cle, a hard ma­te­ri­al.

This scan­ning elec­tron micro­graph shows that the sur­face of a wing scale is cov­ered with con­cav­i­ties. (Cred­it: Ma­thi­as Kolle, U. of Cam­bridge)

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 col­ors 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 op­ti­mize our op­ti­cal struc­tures.

Artificial sur­face mi­mick­ing a but­ter­fly wing, and con­sis­ting of a stack of 11 al­ter­nat­ing lay­ers of ti­tan­ia and al­u­mi­na. (Cred­it: Ma­thi­as Kolle, U. of Cam­bridge)

“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 col­ors to en­crypt it­self – ap­pear­ing one col­or to po­ten­tial mates but anoth­er col­or 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 col­or. 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.

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Scientists say they have created artificial surfaces that mimic the stunning colors found on the wings of tropical butterflies. The findings could be used to help make banknotes and credit cards harder to forge, they say. The striking iridescent colors displayed on beetles, butterflies and other insects have long fascinated physicists and biologists. But imitating nature’s most colorful, eye-catching surfaces has proved elusive. This is partly because these colors 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 colors out of white light and thereby produce other colors. 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 colors because of their shape and the fact that they consist of alternate layers of air and cuticle, a hard material. Using a combination of procedures for fabrication at minute scales, or nanofabrication, the researchers made structurally identical copies of the butterfly scales that produced the same vivid colors as the butterflies’ 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 optimize our optical structures. “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 colors to encrypt itself – appearing one color to potential mates but another color 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 color. 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.