Sepia latimanus (Reef cuttlefish) dark coloration

Bio-inspired photonics or bio-inspired optical materials are the application of biomimicry (the use of natural models, systems, and elements for human innovations) to the field of

photonics Photonics is a branch of optics that involves the application of generation, detection, and manipulation of light in form of photons through emission, transmission, modulation, signal processing, switching, amplification, and sensing. Though ...

(the science and application of light generation, detection, and manipulation). This differs slightly from

biophotonics The term biophotonics denotes a combination of biology and photonics, with photonics being the science and technology of generation, manipulation, and detection of photons, quantum units of light. Photonics is related to electronics and photons. P ...

which is the study and manipulation of light to observe its interactions with

biology Biology is the scientific study of life. It is a natural science with a broad scope but has several unifying themes that tie it together as a single, coherent field. For instance, all organisms are made up of cells that process hereditary i ...

. One area that inspiration may be drawn from is

structural color Structural coloration in animals, and a few plants, is the production of colour by microscopically structured surfaces fine enough to interfere with visible light instead of pigments, although some structural coloration occurs in combination wit ...

, which allows color to appear as a result of the detailed material structure. Other inspiration can be drawn from both static and dynamic

camouflage Camouflage is the use of any combination of materials, coloration, or illumination for concealment, either by making animals or objects hard to see, or by disguising them as something else. Examples include the leopard's spotted coat, the b ...

in animals like the chameleon or some cephalopods. Scientists have also been looking to recreate the ability to absorb light using molecules from various

plant Plants are predominantly photosynthetic eukaryotes of the kingdom Plantae. Historically, the plant kingdom encompassed all living things that were not animals, and included algae and fungi; however, all current definitions of Plantae exclu ...

s and

microorganism A microorganism, or microbe,, ''mikros'', "small") and ''organism'' from the el, ὀργανισμός, ''organismós'', "organism"). It is usually written as a single word but is sometimes hyphenated (''micro-organism''), especially in olde ...

s. Pulling from these heavily

evolved Evolution is change in the heritable characteristics of biological populations over successive generations. These characteristics are the expressions of genes, which are passed on from parent to offspring during reproduction. Variation ...

constructs allows engineers to improve and optimize existing photonic technologies, whilst also solving existing problems within this field.

History

One of the earliest encounters with biological photonics was as early as the 6th century B.C.E (before common era). The Greek philosopher Anaximander, widely regarded as the first scientist, had a student named Anaximenes, who had the first documented mention of bioluminescence. He described seeing a glow in the water when striking it with an oar. Similarly,

Aristotle Aristotle (; grc-gre, Ἀριστοτέλης ''Aristotélēs'', ; 384–322 BC) was a Greek philosopher and polymath during the Classical period in Ancient Greece. Taught by Plato, he was the founder of the Peripatetic school of ph ...

also experienced the same phenomena, which he documented in works like ''Meteorologica'' and ''De Coloribus''. He mentions seeing "things which are neither fire nor forms of fire seem to produce light by nature." Although it was experienced that early, there was still no explanation to why it was occurring. It was not until the early

microscope A microscope () is a laboratory instrument used to examine objects that are too small to be seen by the naked eye. Microscopy is the science of investigating small objects and structures using a microscope. Microscopic means being invisi ...

s, utilized by Robert Hooke in the mid-1600s, that allowed humans to observe nature in greater detail. Hooke himself published what he had seen in the text ''

Micrographia ''Micrographia: or Some Physiological Descriptions of Minute Bodies Made by Magnifying Glasses. With Observations and Inquiries Thereupon.'' is a historically significant book by Robert Hooke about his observations through various lenses. It ...

'' in 1665. Here he describes various biological structures such as the feathers of colorful

bird Birds are a group of warm-blooded vertebrates constituting the class Aves (), characterised by feathers, toothless beaked jaws, the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart, and a strong yet lightweig ...

s, wing and eyes of

flies Flies are insects of the order Diptera, the name being derived from the Greek δι- ''di-'' "two", and πτερόν ''pteron'' "wing". Insects of this order use only a single pair of wings to fly, the hindwings having evolved into advanced m ...

, and pearlescent scales of silverfish. This ability to look at the microstructures of nature, gave scientists information on the mechanisms behind the interactions between biology and light. The Theorie of Imperfection, published by the Russian biophysicist Zhuralev and American biochemist Seliger, is the first working hypothesis about the ultra-weak emission of photons by biological systems. Further developments in microscopy, like scanning electron microscopy (SEM), only increased this and would allow scientists to mimic these observed structures. In addition, the concept of biomimicry was spurred by many scientist including

Leonardo da Vinci Leonardo di ser Piero da Vinci (15 April 14522 May 1519) was an Italian polymath of the High Renaissance who was active as a painter, Drawing, draughtsman, engineer, scientist, theorist, sculptor, and architect. While his fame initially res ...

. He spent a great deal of time studying the

anatomy Anatomy () is the branch of biology concerned with the study of the structure of organisms and their parts. Anatomy is a branch of natural science that deals with the structural organization of living things. It is an old science, having it ...

of birds and their flight capabilities. As demonstrated by various sketches and notes he left behind, he even attempted to create a "flying machine". Although unsuccessful, it was one of the earliest examples of biomimicry. Dinoflagellate luminescence

Molecular biomimetics

Molecular biomimetics involves the design of optical materials based on specific molecules and/or macromolecules to induce coloration. Molecularly Imprinted Polymers (MIPs) are specifically aimed at sensing macromolecules. They can also form them into specific structures that change color.

Pigment A pigment is a colored material that is completely or nearly insoluble in water. In contrast, dyes are typically soluble, at least at some stage in their use. Generally dyes are often organic compounds whereas pigments are often inorganic compou ...

-inspired materials aiming for specific molecular

light absorption In physics, absorption of electromagnetic radiation is how matter (typically electrons bound in atoms) takes up a photon's energy — and so transforms electromagnetic energy into internal energy of the absorber (for example, thermal energy). A ...

have been developed as for example

melanin Melanin (; from el, μέλας, melas, black, dark) is a broad term for a group of natural pigments found in most organisms. Eumelanin is produced through a multistage chemical process known as melanogenesis, where the oxidation of the amino ...

-inspired films prepared by

polymerization In polymer chemistry, polymerization (American English), or polymerisation (British English), is a process of reacting monomer molecules together in a chemical reaction to form polymer chains or three-dimensional networks. There are many fo ...

of melanin precursors such as dopamine and 5,6-dihydroxyindole to provoke

color saturation Colorfulness, chroma and saturation are attributes of perceived color relating to chromatic intensity. As defined formally by the International Commission on Illumination (CIE) they respectively describe three different aspects of chromatic ...

Polydopamine Dopamine (DA, a contraction of 3,4-dihydroxyphenethylamine) is a neuromodulatory molecule that plays several important roles in cells. It is an organic chemical of the catecholamine and phenethylamine families. Dopamine constitutes about 80% ...

is a synthetic polymer with color properties similar to melanin. It can also act to enhance the vibrancy and stability of structural colors. Materials based on the multi-layer stacking of

guanine Guanine () ( symbol G or Gua) is one of the four main nucleobases found in the nucleic acids DNA and RNA, the others being adenine, cytosine, and thymine (uracil in RNA). In DNA, guanine is paired with cytosine. The guanine nucleoside is c ...

molecular crystals found in living organisms (e.g.

fish Fish are aquatic, craniate, gill-bearing animals that lack limbs with digits. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish as well as various extinct related groups. Approximately 95% of ...

and chameleons) have been proposed as potential reflective coatings and solar reflectors.

Protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, res ...

-based optical materials, for instance self-assembling

reflectin Reflectins are a family of intrinsically disordered proteins evolved by a certain number of cephalopods including ''Euprymna scolopes'' and ''Doryteuthis opalescens'' to produce Iridescence, iridescent camouflage and signaling. The recently identi ...

proteins found in cephalopods and

silk Silk is a natural protein fiber, some forms of which can be woven into textiles. The protein fiber of silk is composed mainly of fibroin and is produced by certain insect larvae to form cocoons. The best-known silk is obtained from the ...

, have incited interest in artificial materials for

systems,

electronic paper Electronic paper, also sometimes electronic ink, e-ink or electrophoretic display, are display devices that mimic the appearance of ordinary ink on paper. Unlike conventional flat panel displays that emit light, an electronic paper display ...

(e-paper) and biomedical applications. Non-protein biological macromolecules such as DNA have also been utilized for bio-inspired optics. The most abundant biopolymer on earth,

cellulose Cellulose is an organic compound with the formula , a polysaccharide consisting of a linear chain of several hundred to many thousands of β(1→4) linked D-glucose units. Cellulose is an important structural component of the primary cell w ...

, has been also utilized as a principal component for bio-optics. Modification of wood or other cellulose sources can mitigate scattering and absorption of light leading to optically interesting materials such as transparent wood and paper. Pressure and solvent polarity affect the color of a manufactured cellulose membrane, to the point of detection by the naked eye. Cellulose can also be used as nanofibrils or nanocrystals after treatments. One such treatment involves a nitrating agent to form

nitrocellulose Nitrocellulose (also known as cellulose nitrate, flash paper, flash cotton, guncotton, pyroxylin and flash string, depending on form) is a highly flammable compound formed by nitrating cellulose through exposure to a mixture of nitric acid and ...

. Cellulose nanocrystals can polarize light.

Bioinspired periodic/aperiodic structures

Structural color Structural coloration in animals, and a few plants, is the production of colour by microscopically structured surfaces fine enough to interfere with visible light instead of pigments, although some structural coloration occurs in combination wit ...

is a type of coloration that arises from the interaction of light with nano-sized structures. This interaction is possible because these photonic structures are of the same size as the wavelength of light. Through a mechanism of constructive and destructive

interference Interference is the act of interfering, invading, or poaching. Interference may also refer to: Communications * Interference (communication), anything which alters, modifies, or disrupts a message * Adjacent-channel interference, caused by extr ...

, certain colors get amplified, while others diminish. Photonic structures are abundant in nature, existing in a wide range of organisms. Different organisms use different structures, each with a different morphology designed to obtain the desired effect. Examples of this are the photonic crystal underlying the bright colors in

peacock Peafowl is a common name for three bird species in the genera '' Pavo'' and '' Afropavo'' within the tribe Pavonini of the family Phasianidae, the pheasants and their allies. Male peafowl are referred to as peacocks, and female peafowl are r ...

feathers or the tree-like structures responsible for the bright blue in some ''

Morpho The morpho butterflies comprise many species of Neotropical butterfly under the genus ''Morpho''. This genus includes more than 29 accepted species and 147 accepted subspecies, found mostly in South America, Mexico, and Central America. ''Morph ...

'' butterflies. An example of bio-inspired photonics using structures is the so-called

moth Moths are a paraphyletic group of insects that includes all members of the order Lepidoptera that are not butterflies, with moths making up the vast majority of the order. There are thought to be approximately 160,000 species of moth, many of w ...

eye. Moths have a structure of ordered cylinders in their eyes that do not produce color, but instead reduce reflectivity. This concept has led to creation of antireflective coatings. A combination of chemical structure and how it interacts with visible light creates color within organisms' nature. The creation of specific biological photonics requires identifying the chemical components of the structure, the optical response created by the physics and the structure's function. The complex structures created by nature can range from simple, quasi-ordered structures to hierarchical complex formations.

2-D Structures

Simple Array Structure (Peacock Feathers)

Nature sometimes manipulates the

nanostructure A nanostructure is a structure of intermediate size between microscopic and molecular structures. Nanostructural detail is microstructure at nanoscale. In describing nanostructures, it is necessary to differentiate between the number of dimens ...

, such as its crystal lattice parameters in order to create its patterns and colors. The

Barbule Feathers are epidermal growths that form a distinctive outer covering, or plumage, on both avian (bird) and some non-avian dinosaurs and other archosaurs. They are the most complex integumentary structures found in vertebrates and a premier ...

(the individual strands of a feather that hold its color) of the

is made of an outer layer of keratin and an inner layer containing an array of

rods connected by

keratin Keratin () is one of a family of structural fibrous proteins also known as ''scleroproteins''. Alpha-keratin (α-keratin) is a type of keratin found in vertebrates. It is the key structural material making up scales, hair, nails, feathers, ho ...

with holes separating them. When the melanin rods are parallel to the lattice arrangement of the structure of the keratin outer layer it creates the brown color. The rest of the colors of the feather are created by changing the spacing of the melanin layers. Types of Structures

Aperiodic Photonic Structures

Aperiodic A periodic function is a function that repeats its values at regular intervals. For example, the trigonometric functions, which repeat at intervals of 2\pi radians, are periodic functions. Periodic functions are used throughout science to desc ...

Photonic Structures do not have a unit cell and are capable of creating band gaps without the requirement of a high index of refraction difference. Also known as quasi-ordered crystal structure creates blue and green coloring.

3-D Structures

Helicoidal Multilayers

These are twisted multilayers where fibers are aligned in the same direction and each layer they are slightly rotated. This structure allows nature to reflect polarized light and creates an intense value due to Bragg reflection.

Application Examples

Bioinspired antibacterial structural color hydrogel

As a form of application, biophotonics are used in order to indicate antibacterial and self-healing properties. Since the existence of silver nanoparticles prevent bacterial adhesion (there is already bacteria existing in the hydrogel) it causes hydrogel degradation and color fading. This allows for the engineered hydrogel to display with color its integrity after self-healing.

Photonic nanoarchitectures in butterflies and beetles

Nanoarchitectures contribute to the iridescence of butterflies and beetles. Multilayers are common, typically in a 1-D or 3-D structure, 2-D structures are more rare. Disorder and irregularity in the structure are “intentional” and adapted to the habitat. The structure has been successfully recreated and can be used as a coating. It is also used in some applications where stable, vibrant color is required. It is flexible enough that it can be designed to have a pattern.

Mimicking fireflies to improve LED efficiency

When observing

fireflies The Lampyridae are a family of elateroid beetles with more than 2,000 described species, many of which are light-emitting. They are soft-bodied beetles commonly called fireflies, lightning bugs, or glowworms for their conspicuous production ...

(''Photuris sp.'') using SEM, it was observed that their light emitting cuticle had a specific 2D periodic structure. It is structured following a “factory roof” like pattern with scales oriented at a tilted slope and a sharp edge on the protruding side of the scales. When modeling a similar structure using a

photoresist A photoresist (also known simply as a resist) is a light-sensitive material used in several processes, such as photolithography and photoengraving, to form a patterned coating on a surface. This process is crucial in the electronic industry. ...

layer on

light emitting diodes A light-emitting diode (LED) is a semiconductor device that emits light when current flows through it. Electrons in the semiconductor recombine with electron holes, releasing energy in the form of photons. The color of the light (cor ...

(LEDs), it resulted in a 68% power increase and 55% increase in light extraction efficiency (LEE). This technology reduces the amount of energy consumed to produce the same amount of light.

Responsive materials

Responsive materials are materials or devices that can respond to external stimuli as they occur. A little bit of time is taken to adjust to the new surroundings, but the idea remains consistent with what is seen in nature. The most commonly used examples are the chameleon or octopus, as their responsive skin allows them to change the color or even the texture of their skin. The mechanisms behind these tactics are called

chromatophore Chromatophores are cells that produce color, of which many types are pigment-containing cells, or groups of cells, found in a wide range of animals including amphibians, fish, reptiles, crustaceans and cephalopods. Mammals and birds, in contrast, ...

s, which are pigment-filled sacs that uses muscles and nerves to change the animal's external appearance. These chromatophores are activated by neuronal activity, so an animal can change its color just by thinking about it. The animal uses another mechanism to be able to know what color or shape to take; a photo-sensitive cell within their skin called

opsin Animal opsins are G-protein-coupled receptors and a group of proteins made light-sensitive via a chromophore, typically retinal. When bound to retinal, opsins become Retinylidene proteins, but are usually still called opsins regardless. Most ...

is able to detect light (and possibly color). The animal can use these opsins to their advantage to quickly assess their surroundings, before turning on their chromatophores to accurately camouflage to their circumstances. A lot of creatures have camouflage incorporated into their bodies — take the fish in the figure on the right for example. In this hypothetical, the animal can appear in two different ways depending on their surroundings: in the middle of the ocean away from all solid objects, it can appear near-translucent; near the sea floor where potential predators will only sense it from above, it can turn darker to naturally blend in with the rocky bottom. Many fish, such as the marine hatchetfish, use a combination of camouflage techniques to achieve these appearances.

Silvering Silvering is the chemical process of coating a non-conductive substrate such as glass with a reflective substance, to produce a mirror. While the metal is often silver, the term is used for the application of any reflective metal. Process Mos ...

, a common tactic, utilizes highly reflective scales to reflect the surrounding light effectively enough to make the scales appear invisible from the side.

Counterillumination Counter-illumination is a method of active camouflage seen in marine animals such as firefly squid and midshipman fish, and in military prototypes, producing light to match their backgrounds in both brightness and wavelength. Marine animals of ...

, a tactic used more by deep-sea dwellers, uses a luminous organ located in the bottom of the body to emit light in order to appear brighter from underneath. At this angle, the light emitted is at an intensity meant to replicate the sunlight as it appears on the surface of the water. Thus, from below the creature is essentially invisible to many predators. Within the luminous organ is a laminar structure of photocytes and nerve branches, with relatively small gap junctions between them. It is thought that the vast interconnectivity and the layered structure of these neuro-photocyte units is what allows a deep-sea fish to rapidly respond to a situation with spontaneous

luminescence Luminescence is spontaneous emission of light by a substance not resulting from heat; or "cold light". It is thus a form of cold-body radiation. It can be caused by chemical reactions, electrical energy, subatomic motions or stress on a crys ...

. Because all of the nerves are directly connected to the spinal cord (and by extension, the brain), researchers believe that electronic signals can trigger these photocytes to react. With this line of thinking, scientists are working to develop technology using this type of neuro-photocyte unit. These biologically inspired materials can be applied in many different circumstances. This technology can be used to camouflage objects, create a device that can mold its shape yet still retain its desired properties, or even help people in relation to biomedical applications. A coating of this technology can help incorporate a foreign body into a living ecosystem, i.e. a human body. The technology of this device allows a person's antibodies to detect the new object as a non-threat, thus permitting easier acceptance of manmade tools into the body, such as a cardiac pacing device to the chest.

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Series A, Mathematical, Physical, and Engineering Sciences , volume = 376 , issue = 2112 , pages = 20170182 , date = February 2018 , pmid = 29277747 , pmc = 5746562 , doi = 10.1098/rsta.2017.0182 Bioinspiration Photonics