Metamaterials are synthetic materials created by humans that use periodic structures called “meta-atoms” to interact with light for a desired effect. For a material to be considered a metamaterial, it must operate on a microscopic-scale. For many metamaterials, individual meta-atom structures cannot be detected by the unaided human eye. Generally, these types of materials do not appear naturally as they are incredibly complex, however, some organisms have naturally developed their own form of metamaterial tissue that changes their colour without pigment.
Butterflies receive their vibrant colours from two different sources: ordinary (or pigmented) colour and structural colour. The ordinary colour comes from traditional chemical pigments that absorb certain wavelengths of light and reflect others. For example, the pigment, chlorophyll, makes plants appear green. The chlorophyll soaks up the blue and red colors of the spectrum, but not the green which you see when it bounces back to your eye. Most butterflies get their different shades of brown and yellow from melanin, the same pigment that makes your skin darken in summer and gives some people freckles.
Butterflies are cold blooded organisms and rely on external heat sources such as the sun. These insects are able to absorb heat rays through their wings. Because darker pigments can absorb more heat than light colours, butterflies exist in a variety of patterns based on where they live in the world.
The morpho butterfly has wings that are made of very thin layers of hardened protein called chitin. Chitin is derived from glucose and therefore can arrange itself into periodic patterns in crystalline nanofibrils. While chitin is common among many organisms such as crustaceans, fungi, and anthropoids, the blue morpho butterfly uses it in a completely different way. It arranges the chitin into thousands of layers of tiny scales that give the butterfly a stunning iridescent blue colour.
However, the blue morpho doesn’t use pigment in their wings. Instead, they rely on a unique type of natural metamaterial that gives their wings an iridescent blue colour. With this incredible trait, the blue morpho selectively absorbs most wavelengths of light but reflects blue to fend off predators as a camouflaging agent.
The structural colour of butterflies is where things get interesting. This type of colour stems from the specific structure of the butterflies’ wings and explains why some of a butterfly’s colours seem to shift and appear so intense. This quality of changing colours as the observer moves is known as iridescence and occurs very frequently in nature. Mother of pearl seashells, fish and peacocks are just a few examples of animals with this quality, but it is most pronounced in the butterfly family. The effect happens because light passes through a mostly transparent, multilayered surface and is partially reflected from each layer. Natural selection evolved the layer thicknesses such that the multiple reflections compound one another and intensify specific colours, while absorbing others.
Although the definition of a metamaterial is that of a human-made material, biological tissues can also exhibit properties of metamaterials. This is apparent not only in butterflies, but in the human eyeball as well. If you’ve ever wondered why some people have brown eyes and others have green, blue, or hazel eyes, that is because of another strange biological anomaly. People with blue eyes are actually lacking the pigment called melanin that people with brown eyes have. Without this pigment, the human iris appears blue, but not because of a different type of pigment, it’s because of the structure of eyeball tissue itself. Just like the blue morpho, the human eye can exhibit a natural form of metamaterial that reflects blue but selectively absorbs other colours. Likewise, green or hazel eyes are a result of having less pigment than those with brown eyes. The tissue structure reflects blue while the pigment reflects brown, creating a variety of brown, green, grey, or blue colours.
Because of their vast complexity, most practical applications of metamaterials are still in development stages. However, with the help of computational electromagnetics and advances in optical physics, we are beginning to see the beginnings of a brighter future infused with metamaterial breakthroughs. Despite its complexity, metamaterial science promises a technological revolution that will disrupt how we view the world. With their precision and unprecedented capabilities, perhaps their use will become as commonplace as red dye number 40. Maybe someday we will wear fabrics that are fortified with metamaterials to strike the iridescent blue that only the blue morpho butterfly can boast at this time.