On replicating the Morpho Butterfly\’s Structural Color

Abstract:

The objective of this article is to explore the existing efforts in the study of replicating the Morpho butterfly’s structural color in order to create artificial colorings that may be longer lasting and more environmental-friendly which has the potential to offer a better way to color various products. The article gives an overview of different attempts at creating structural color for different industries. This literature review addresses the hypothesis of how replicating butterfly structural color can contribute to artificial coloration that does not fade and is better for the environment. In the end, efforts of replicating this technology were found both in a laboratory setting and in the commercial market but most still need a lot of development in order to be cost effective and mass produced.

Introduction:

Morpho butterflies are a type of butterfly that are found mostly in Mexico and South America. Blue Morphos have iridescent metallic blues in the upper wing surface, and brown, black, gray or red in the lower wing surface^[1].
Many of the most vivid colors in the animal kingdom are not obtained from pigments – organic molecules that give most things their color. Rather, animals use structural color. This is color created by very small structures that interact with light^[2]. This is especially common with the color blue. Examples of structural color can be found in peacock\’s feathers, blue eyes and beetles. Animals normally get their color from pigments in their diet, with the exception of blue which cannot be processed from dietary pigments. In response, the vast majority of animals with blue coloration evolved structural color^[3]. Based on this information, the research questions addressed in this report is: what are the existing efforts of replicating the morpho-structural color and why do researchers want to replicate the morpho butterfly’s structural color in the first place?
If the methods for creating structural color are studied and replicated, the structure could potentially help bring an improved type of man-made color without fading or waste. Replicating an organism that is already adapted to their environment may assist in creating a more vibrant, environmentally favorable, and durable artificial color.
A review of the primary literature was conducted to find out replication methods and their application to artificial coloring. The research was performed in order to show the numerous examples of experiments that replicate structural color, as well as examples of early products coming into the market.

Figure 1: Picture of the Morpho Butterfly. Credit: William Warby Flikr.

Relevance:

Applying color to alter human appearance and the environment has been recorded since the distant past. New ideas in the pigment industry are necessary for the advancement and continued application in line with current norms that avoid pollution and enhance durability as well as brightness. For example, structural colors can lessen the environmental impact of the pigment industry^[4]. Possible applications of structural color could potentially help the automobile, cosmetic, textile, paint or any coloration-requiring industry^[5] . As the techniques to produce structural color reach higher volumes, they can help decrease production costs, making them more accessible for the common buyer.
One of the benefits of replicating structural color is that structural color does not fade easily. A great example that shows the long durability of structural color can be found in a 50-million-year-old beetle carcass found in Germany in 1998. While the rest of the fossils around had dulled, this particular beetle was still a metallic blue^[3]. Pigments produce color by absorbing and reflecting certain wavelengths. Dyes tend to fade over time especially when exposed to sunlight since it slowly breaks the bonds in the molecules. Structural colors are more resistant to fading because the color is produced by interactions with microscopic structures. Structural color can also produce purer colors^[6].
There are certain environmental and durability issues related to current use of blue pigments. Some types of blues may cause cancer or release cyanide. Other blue pigments are not entirely safe when exposed to heat or acidic conditions^[7]. Also, the waste created as a result of producing some dyes, pigments, food safe dyes, medicine, and cosmetics is in the list of hazardous wastes of the EPA K181^[8].
The toxic nature of current commercial dyes has become a grave concern for the environment and has an adverse effect on all forms of life^[4]. Over 10,000 different dyes and pigments are used industrially and over 735 tons of synthetic dyes are annually produced worldwide^[7]. A lot of dyes contain sulfur, nitrates, acetic acid, soaps, enzymes, chromium compounds and heavy metals, not to mention the other harsh chemicals based on dye fixing agents. The wastewater from the dyeing process is very challenging to manage, so it leaches into rivers and makes the water undrinkable. This affects local populations that rely on this water for crop irrigation and sanitation. Exposure to these chemicals can deteriorate health and damage local ecosystems. Dyeing requires large quantities of water. Thirty to fifty liters of water is used in the dyeing process of every kilogram of fabric^[4]. Using structural color would possibly mean little water use and reduction of hard chemicals leaching into public waterways.

Organisms with Blue Coloration

Living organisms via 3.8 billion years of evolution have been adapting through natural selection and have adapted to their environment in order to survive and pass on their genes ^[9]. In the case of Morpho butterflies, their bright wings may be used to communicate and identify other members of their species given that it can be noticed from very far away. Communication between males is also thought to be the major selective agent responsible for the brilliant coloration in male butterflies.^[10] These organisms have had time to evolve while ensuring the survival of future generations.
Organisms with blue coloration can be found in many different species like fish, birds, and insects.^[3] Most colors are generally created by pigments. However, as stated before, many living organisms especially those with blue coloration obtain their color via structural color.^[11]

Structural Color

Most of the structural colors in nature originate from four optical processes and their combinations:

1. A thin-film interference (see glossary) with only one layer where light bending causes some wavelengths to cancel out. This is what can be observed in bubbles.
2. A multilayer interference which is when various layers pile up and some wavelengths pass through reflecting in each layer
3. A diffraction grating effect, where light scatters in many directions. Photonic crystals in which birds like peacocks create their color where little melanin balls are arranged and piled in a structure that lets the desired wavelengths pass through.
4. Irregular spongy structure that bends the light in the right angles which appears in birds like the kingfisher. ^[12]

Structural Color in Morpho Butterflies
Butterflies wings are covered with chitin scales of 0.104 millimeters^[2]. Each scale has a series of ridges in which there are nano structural layers that form a grid pattern. If that structure is cut and seen laterally, a structure resembling pine trees can be seen. This is what interacts with light to create structural color^[1]. The variation in distance between the layers and their measurement causes the different shades of blue in each species. There are normally 6 to 8 horizontal structures in ridge and the brightest color is achieved when there are more layers closer together.^[2]

Figure 2: Image captured by electron microscope of nanostructure in a morpho butterfly wing. Credit: Wikimedia Commons

The nanostructures in morpho butterflies make the wings appear blue interacting with light in the following way:

1. The morpho butterfly uses multilayer interference to create color. This happens in the top most layers of the structure. Multilayer interference is when there are several layers with space in between them. Light gets reflected through each of the layers. This will cause the rest of the wavelengths which are not blue to reflect out of face and therefore cancel out ^[12]. However, the blue wavelengths get reflected in sync. As a result, blue is the color perceived by the human eye. When the light enters from a slightly different angle, there is a minor change in the wavelengths reflected. This is why the wings slightly shift their color.^[2]
2. There is diffraction involved in the upper layers of the nanostructure which is when light is dispersed like a prism.
3. The brown pigment underneath absorbs stray rays of light.^[12] In order to produce the color we see, the structure cancels, disperses, and absorbs other wavelengths and only leaves blue. The structures are also arranged at different heights to prevent the interference of light which would cause areas where the coloration doesn’t show. ^[2]
4. There is a cover scale that makes the wing appear less glossy.^[10]

Figure 3: Diagram showing multilayer interference, diffraction and absorption.

Research and innovation in replicating the structural color in the Morpho Butterfly

Researchers and companies of various fields have been replicating the Morpho butterfly’s structural color. This section shows the efforts being made in replication research in development. Examples shown are of commercial products with structural color in the textile, security, computational and automobile industries. The following are some examples of the efforts being made in order to replicate structural color.
A group of researchers published at World Scientific made a structure of windows of the same size with two sides offsets with silicate. This was inspired by the wings of a Morpho butterfly. Then they conducted numerical and light scattering simulations to test its optical properties. They found out that their structure could scatter light in the same color tone (monochromatically). The structure is easy to fabricate, and the researchers want to use it in designing future computational materials^[5].
Harvard University researchers used self-assembling techniques to make nanostructures that produce different structural colors including yellow, red and blue. Their goal was to make an angle-independent structural color. Most structural color is angle dependent which causes iridescence. Iridescence is achieved by making ordered structures called photonic glasses which get close to the Morpho butterflies structural color, but with more stable coloration. Therefore, to create an angle-independent structural color the structures need to be disordered. The structure was made with spherical colloidal particles. Microcapsules were used to make a “microfluidic device.” This device makes a droplet that is surrounded by a thin shell of monomer (what forms a polymer). Inside the droplets were micro (colloidal) particles with a core-shell made of polystyrene that has a high refractive index. The shell is made of hydrogel which has a low refractive index. The core-shell allows them to separate the particles and choose the distance between them which is what makes the structural color and determines how opaque it is. They concentrated the particles and arranged them in a disordered manner. They changed the sizes of the cores and shells and made different colors including blue.^[11]
A team of three scientists in France, Magali Thomé, Lionel Nicole and Serge Berthier attempted replicating the iridescent properties which was hard using current nanostructure technology. They used natural structures as a mold. However these are very fragile and this could not be adequate for a production process. They used the butterfly’s scales as a template and deposited silica on top of them. Then a chemical solution was placed, they used the sol-gel method (method of producing solid things out of tiny molecules, solution evaporation process and dip coating then dipping something in a solution to create a thin film).^[13]
Blue Morpho wings can be viewed from half mile away without the need for a highly lit environment so based on that principal angle independent reflectors were also created. The material was flexible, could be viewed the same from different angles and did not lose the structural color if bent or folded.^[14]
For the textile industry Kuraray Corp took inspiration from the structural color of the Morpho butterfly to create a polyester fabric with low reflectivity, but vivid coloration. This fabric (now named Diphorl) was made using bicomponent polyester fibers with a rectangular cross-section so the fibers were intertwined perpendicularly. The fibers were spun from two polyester components with different thermal properties (approx. 80–120 twists per inch). The structure’s alternating horizontal/vertical structure causes repeated reflection and absorption of the incident light, so it can produce vibrant colors. Teijin Fibres Ltd in Japan began producing the fabric ‘Morphotex’ that mimics the microstructure of Morpho butterfly wings and produces structural color. This is made out of nylon or polyester and has more than 60 laminated layers. An exact replica of the Morpho wing structure was produced using atomic layer deposition (a chemical technique in which vapor is used to produce thin films materials) of Al₂O₃ on real butterfly wing template.^[15]
Nanostructures like Morpho colours are very difficult to replicate. It is for this reason that they may be used as an authentication device. Nanotech security corp, has developed a luminescent material to stamp into security items, such as bank notes. The result was an angle-dependent optical filter with at least one structural color that can be seen from at least one viewing angle. In this case, the optical device is applied to a surface like paper or plastic. The structural color is created by placing nanoholes in the material in a grid pattern. Therefore, when the light is reflected via thin film interference it creates the color. Variation to the size of the holes determines the different tones obtained.^[16]
Nanotech is also working on making this type of recreation of structural color more convenient to produce by replicating nanostructures using a master stamp which will allow a larger volume of production and will be more cost-effective. With the stamp, they can create circular structures in any arrangement and size. The stamp consists of a 1 cm area with a pattern of 200 nm diameter nano-holes spaced 520 nm apart. The stamp can work on polymers which are then coated with a Cr/Au (5/100 nm) film. The goal will be to use this technique to color materials like textiles, or human hair.^[17]
Lastly, Lexus has developed “structural blue”: an angle-independent structurally colored car paint. In order to achieve this, a structural blue pigment was developed and dispersed in a binder. The structurally colored flakes have multiple layers and a thickness range between 0.5 to 5 microns, and a diameter of between 5 and 50 microns. The structure in the flakes can have a reflective band of fewer than 200 nanometers when it is viewed from angles between 0 and 45 degrees.^[18]

Conclusion

Research papers have found the exact structure of Morpho butterfly nanostructures, the function of these structures, postulated reasons for the evolution of them, and have now begun replicating them. Companies like Nanotech Security Corp. and Kuray corp. like shown above have begun to utilize these structures for the development of car paint, textiles, authentication devices, reflectors and others.
This will be of great benefit in order to adapt to current demand of man-made color. Studying and replicating this structure will help create less harmful, longer-lasting, more secure, and vibrant colors. Assuming that there will always be a demand for artificial color, the applications of structural colors in this realm have yet to be tested. Structural color is a very promising lead, especially as its analysis and replication process improves.
Color will continue being a crucial part of how we see the world, express ourselves and communicate. It is an integrated part of our culture and can be found anywhere in our lives. Which is why we must strive to color our products in the most ethical and environmentally friendly way possible. Problems with our current dye and pigment industries should be addressed in order to develop better alternatives. Since colors are very present in our lives, this technology can be applied from luxury cars to clothing and security, making the possibilities endless. There will always be a demand for artificial coloration and the development of structural color can be an environmentally conscious tool of satisfying this demand, that is present in almost any industry.
Although developments in this area will continue, the majority of these innovations are still in a laboratory setting, and still need to be cost effective and easily replicable. Mass production is still very difficult because current technologies use molds that need a fragile butterfly wing, which makes it difficult to replicate quickly.

Glossary:

Interference: The effect formed when waves meet. This can either cancel out the waves or amplify them. ^[23]
Diffraction: When waves spread around obstacles. ^[24]
Photonic crystals: Structures that can allow or stop the propagation of electromagnetic waves in some frequencies. ^[25]
Melanin: A biological dark pigmented material found in skin, hair, feathers, scales, eyes, and membranes. ^[26]
Chitin: A biological material found in insect exoskeletons, fungi, and invertebrates.^[27]
Nanostructures: Any structure that is measured in the nanometer scale, which is 10^-9m.^[28]
Silicate: A mineral found throughout much of the solar system made of silicon-oxygen compounds.^[29]
Self-assembling: When the components of a system like molecules, polymers or microscopic particles organize themselves into structures without external direction.^[30]
Angle-independent structural color: When the color remains the same even if you are viewing it from a different angle or rotating the material.^[11]
Angle-dependent structural color: When the color changes depending on the viewing angle or the rotation of the material. This creates iridescence.^[11]
Photonic glasses: Structural colors that are not iridescent because their structure is disorganized.^[11]
Colloidal particles: Solid microscopic particles floating in a fluid.^[31]
Microcapsules: Very small capsules that have a thin polymer coating. On the inside of the polymer coating there can be liquid droplets, small solid particles or both.^[32]
Microfluidic device: Device used to create microcapsules and manage fluids precisely. ^[11]
Refractive index: The measurement of how much a ray of light bends when it passes from one medium to another.^[33]
Hydrogel: Polymer networks that trap water in the space between the molecules.^[34]
Silica: Silicone dioxide usually takes the form of crystals. Silica is used in ceramics, concrete and many other products.^[35]
Reflectivity: How much light is reflected when light meets an absorbing material.^[36]
Binder: The main ingredient in paint, it coats the pigments and is responsible for a good adhesion.^[37]

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Figure References:

1. William Warby, “Blue Morpho Butterfly”, flikr, September 5, 2009, Accessed October 18, 2019. https://www.flickr.com/photos/wwarby/3895947534
2. “Morpho sulkowskyi wings.jpg”, Wikimedia Commons, November 16, 2015, Accessed October 18, 2019. https://en.wikipedia.org/wiki/File:Morpho_sulkowskyi_wings.jpg

About the Author

Tania Santiago is 16 years old and lives in Mexico City. She is currently in 10th grade at Technologico de Monterrey High School. She is fascinated by science, material engineering and biomimicry and would love to one day work as as a nature inspired materials engineer. She also enjoys taking care of her home aquaponics system, dancing, drawing the natural world, and spending time with her cat.