Creating colour using nature's pallete

Challenge:  Iridescent colour

Natural Solution:  Butterfly wings, feathers and beetles using light-interfering structures

 

Brilliant iridescent colour, pigment-free – so no toxic chemicals, non fading, energy efficient and visible in direct sunlight – all these valuable properties have become possible in many new technologies thanks to butterfly and beetle wings, peacock feathers and some mollusc shells.  The new technologies inspired by these natural properties include electronic display Indian beetlewing tea-cosy (1885-1900) © Hampshire County Museums Servicepanels, mobile phone and MP3 player displays, e-readers, paint, fabrics and more!

 

Iridescence in nature has long been appreciated aesthetically. Indeed, natural specimens of iridescent material have been incorporated in many cultural artefacts that are centuries old. Historically bird feathers have been used to make royal cloaks in several Polynesian cultures.  Mother of pearl has been used in jewellery around the globe; artisans from the Japanese to the Ottoman and Aztec cultures have carved and used mother of pearl as decoration and to display wealth and status1.

 

Scarf with iridescent beetle wing-case embroidery © Hampshire County Museums Service

 

Iridescent beetles, such as Sternocera aequisignata, a species of jewel beetle, have been  highly prized from the 17th century onwards with the elytra, or wing cases, stitched into ceremonial costumes, dresses, headdresses, among other decorative fabrics, jewellery2 and art work3.  In decorating the Belgium Royal Palace artists used over one and a half million beetle wing cases to cover a ceiling, associated panels and chandelier4.  These beetles, have the ability to be farmed, are eaten through much of their range5, (Thailand, India and Burma) with the residual wing cases then sold on for jewellery still today.

 
 
 

Iridescent Blue morpho (Morpho Rhetenor) © Amanda Gregory

 

Many butterflies too have evolved to produce an array of brilliant, iridescent colours on their wings.  Browns and blacks are produced by pigments of melanin on the scales, but the brilliant greens, blues and reds are produced quite differently. The Blue Morpho (Morpho rhetenor) found in the Amazon region of South America is one species of butterfly that beautifully demonstrates this structural colour6.  While its wings display a brilliant iridescent cobalt-blue colour, they actually contain no blue pigment.  So how do butterflies produce their vivid colours?   

 
 

Butterfly wings are covered with highly evolved set of scale structures.  These thin, nano-sized, transparent, chitin-and-air layered scales overlap, selectively cancelling out certain colours through wavelength interference while reflecting others, depending on the exact structure and interspatial distance between diffracting layers.  It is this reflected light that we see as colour.  This system of producing colour allows for the dynamic control of light flow and wavelength interaction, which butterflies rely upon for camouflage, thermoregulation, and signalling7, 8.

 

Iridescent coloration is broadly distributed in the animal kingdom and appears to have evolved independently in a number of different taxonomic groups9 with similar metallic iridescent colour is seen in a number of species of coleoptera beetles as noted above.  These light-interacting structures have provided the model for a number of new technologies that can produce colour without toxic heavy-metals, pigments and dyes10.

 

  

New technological developments

Several new technologies have been inspired by the physical light manipulating properties of these beautiful creatures from the natural world. Among the most interesting and economically promising is the new electronic display technology used on the personal electronic equipment so ubiquitous in our daily lives.  Butterfly wings have also inspired other products with colour shifting capability such as new, pigment-free paints and fabrics, among other uses.

 

Hisense C108 with mirasol display © Qualcomm

 
Electronic displays

The mirasolTM electronic display has been developed to use ambient light to reflect colour from screen, thus using the same basic principles as butterfly wings to project colour.  The display consists of two conductive plates.  One is a thin film stack on a glass substrate and the other, a reflective membrane suspended over the substrate.  The two are separated by a gap filled with air.  When ambient light strikes the structure, it is reflected both off the top of the thin-film and off the reflective membrane.  Depending upon the height of the optical cavity different wavelengths of light are reflected off each surface.  This allows a predetermined wavelength of light to emerge or be suppressed11, 12.  When a voltage is applied, the image will be seen and different spatially ordered elements reflect in the red, green and blue wavelengths providing a full-colour display.

 

mirasol e-reader featuring structural colour display © Qualcomm

 

Using ambient light rather than backlight to illuminate the screen has many benefits.  The display screen is clearly visible in outdoor conditions, including strong daylight, as well as being extremely energy efficient only using energy as the image changes13.  This display technology is now present in the Inventec V112 Smartphone, Hisense C108 phone, MP3 player, a mini GPS system designed for golfers, a waterproof digital audio player, Acoustic Research Bluetooth® headsets and an e-reader that also displays video14.

 
 
 

Pigment-free iridescent paint and fabrics

These include pigment-free paint, eg ChromaFlairTM and SpectraFlairTM developed by JDSU, and others, that use ultra-thin film flakes to produce fade-free iridescent colour by varying the thickness of each individual flake so the colour changes with the angle it is viewed from.Chrysler PT Cruiser with ChromFlair iridescent paint © JDSU

 

Car manufacturers such as General Motors in their Cadillac brand have used the paint to great effect.

 

A number of companies have developed colour shifting fabric, e.g. MorphotexTM by Teijin Fibers Limited.  Morphotex fibres are laminated using nanotechnology: thin films of 70 nm thickness consisting either of polyester or nylon are laminated alternatively in 61 layers. Four basic colours, such as red, green, blue and violet, are allowed to be developed by precisely controlling the thickness of the layers according to visible wavelength15.  The process reveals a rainbow of colours according to the intensity and angle of light due to the unique structure of the fibre.

 

Another advantage of structural colour is that it cannot be mimicked by chemical pigments or dyes and that it is unaffected by fading. What is more, multiple colours can be displayed using a single material simply by varying the dimension of the tiny film structures.  These artificial structures could be used to encrypt information in optical signatures on banknotes or other valuable items to protect them against forgery 16, 17, 18.

 

Environmental implications of the new technology

Broadly exhibited across the animal kingdom, natural iridescence has served as a model for the development of a number of new technologies as we have seen above. The innovation of artificial means to duplicate the natural process means that human beings can now continue to appreciate this aspect of nature without the need to destroy the source.

 

While some of the iridescent insects are not endangered across their range some, others are in grave danger. For instance, the Queen Alexandra’s Birdwing (Ornithoptera alexandrae), the largest butterfly in the world, is threatened by illegal collection and habitat loss and is classified as endangered on Appendix I of the Convention on International Trade in Endangered Species (CITES), and as an Annex A species in the EU Council Regulation on trade of wild species. Both of these international agreements prohibit international trade in this species.  Although not collected for use in the production of secondary products, exploitation for whatever reason can threaten species with extinction that will result in the loss of any possibility of understanding such physical features forever.

 

A secondary benefit offered by this new technology is its ability to avert serious environmental pollution.  The World Bank estimates that 17 to 20 percent of industrial water pollution comes from textile dyeing and treatment19. The pollution is mainly due to the non-biodegradable nature of the dyes along with the strong presence of toxic trace metals, acids, alkalis and carcinogenic aromatic amines in the effluents20.  Principal air pollutants from these processes include volatile organic compounds (VOCs), nitrogen oxides (NOx), hydrogen chloride (HCI), and sulphur oxides (SOx).  Liquid effluents including suspended solids, oil and grease and solid waste includes spent acids, and process residue from the pigments21 affect biochemical oxygen demand and chemical oxygen demand in recipient water bodies.  Of the 700,000 tons of dyes produced annually worldwide, approximately 10 to 15 percent is disposed of as effluent from dyeing operations22.  In many areas of the world the dyeing industry has left many cities with severe pollution of both surface and ground water23, 24.  As no actual dyeing is involved in producing colour shifting fabrics and paints, it reduces the consumption of the prodigious amount of water and energy common in conventional dyeing25 thus saving energy and minimizing industrial waste when compared with conventional methods26.

 

Economic potential

The potential for not only improved product quality but better energy efficiency and environmental benefits would suggest that these new technologies would have a promising future.  The economic advantage is clear both at the consumer as well as the corporate level. 

 

Compared to a mobile device using a conventional LCD display, a mobile device using a mirasolTM display could consume 33.7% less energy, which could in turn extend the battery life by 51%.  Based on a simple lifecycle analysis, this would result in 94% less carbon dioxide emitted in the use phase of the display. In addition, this efficiency advantage would result in about 58 fewer recharge cycles over the course of a year and would extend the life of the battery for an additional 1.25 years27.  This represents approximately a 10% recharge saving and 50% additional battery life (based on with 10-24 month life or 400-500 charges 1-2 years).

 

To the e-reader market the new technology offers colour display with video capability as well as offering efficiency benefits.  With 67% of the global population having mobile subscriptions, millions of mobile electronic devises being sold globally each year28 and the e-reader market projected to reach 35 million by 201429 the efficiency savings potential for this new electronic equipment could be huge.

 

Pigment-free iridescent paint and fabrics might not have the immediate consumer desirability as the electronic display technology but nevertheless have significant potential. Within the automotive industry the hottest growth area is in the sport compact/import performance niche30.  After-market products (non-factory parts, accessories and upgrades), including speciality paints, are worth around US$1 billion and this colour-shifting paint is expected to further expand this market31.

 

The global market for colour pigments and dyes is estimated at US$20 billion today32.  The global market for nano-fibre products is estimated to reach $101.5 million by the end of 2010 with the market forecast to grow to nearly $2.2 billion in total revenues by 202033.

 
 

Image Sources

Qualcomm for Mirasol display technology

JDSU Colour shifting paints - ChromaFlair and SpectraFlair

Hampshire County Museums Service

School of Physics, Exeter University

 
 

References

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  2. Anon (2009)  Beetles  Causes of colour.   Web exhibits.  Accessed October 2010
  3. Rivers, V  (2010)  Beetles in textiles   Insects.org  Accessed October 2010
  4. Lennon, S  (2009)  Royal Belgian ceiling glows with Flemish sculptor's beetles arrangement  projo.com online journal   Accessed October 2010
  5.  De Foliart, GR  (2002)  The Human Use of Insects as a Food Resource:  A Bibliographic Account in Progress.  Chapter 24 South-eastern Asia: Thailand  University of Wisconsin-Madison  On-line book Accessed October 2010
  6. Lee, RT and Glenn S  (2009)  Detailed electromagnetic simulation for the structural colour of butterfly wings.  Smith School of Electrical and Computer Engineering, Georgia Institute of Technology, Atlanta, Georgia
  7. Prum, RO, Quinn, T & Torres, RH. (2006)  Anatomically diverse butterfly scales all produce structural colours by coherent scattering. Journal of Experimental Biology. 209(4): 748-765.  Full article online  Accessed September 2010
  8. Anon (2010)  Wing scales diffract and scatter light: Morpho butterflies.   Ask Nature  Accessed October 2010
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  10. Anon 2010  Wing scales diffract and scatter light: Morpho butterflies.   Ask Nature  Accessed October 2010
  11. McKaeg, T  (2009b)  Green Biz  Online Journal  Accessed September 2010
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  13. Qualcomm  (2009)  Organisation website  Accessed September 2010
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