Scientists are always trying to find more effective ways of making high performance materials with minimum consumption of energy and resources, minimum waste production and, of course, maximum functionality. In other words, they are trying to make materials that are economically viable, environmentally friendly and versatile. Living organisms are examples of design that consume the least amount of energy and materials. They are designed strictly for function, yet they excel in engineering. For an increasing number of scientists, biological materials in nature represent future innovations for material synthesis in terms of complexity and functionality. What captures the imagination is the way relatively simple building blocks can be constructed into highly precise functional hierarchical structures. In fact, there are numerous design examples in nature that engineers have only been able to dream about until now. As scientists more closely examine the cellular and molecular workings of nature, they are starting to find information which they can apply to everything from advanced optics to robotics. The result is a new field called biomimicry, biomimetics, or biologically- inspired design. Biomimetics is the application of methods and systems found in nature to the study and design of engineering systems and modern technology. The conscious copying of examples and mechanisms from natural organisms and ecologies is a form of applied case-based reasoning, treating nature itself as a database of solutions that already work.
The innovations implemented in nature have the potential to improve the way we do everything, from desalinating water, gluing things together, to streamlining cars. Where there is a design problem, there is a solution for it in nature created by nature’s Designer. We can distinguish the levels in biology that technology can be modeled after as i) mimicking the natural methods in the manufacture of chemical compounds to create new ones, and ii) imitating mechanisms found in nature. There are a few examples of biomimetic materials that are already part of our daily lives. Velcro, for instance, is a brand name of a fabric that consists of hook and loop fasteners used to connect objects. It was invented by Georges de Mestral, a Swiss engineer/inventor. The idea came to him after he took a close look at the Burdock seeds which stuck to his clothes and his dog’s fur on their daily walk in the Alps. He closely examined the hook-and-loop system that the seeds used under a microscope, and realized that the same approach could be used to join other things together. Velcro is commonly used in many different areas, such as in the automotive industry, clothing, shoe making, and for bringing rigid or soft surfaces together. The lotus, which possesses tiny wax crystals on the surface of its leaves, remains pristine and white, even in the midst of swampy, contaminant-rich conditions. For some, the lotus plant is even a symbol of cleanliness.
The lotus effect in material science is defined as the observable self-cleaning property found in the lotus plant. The characteristics of the lotus brought about a new application of biomimetics to the self-purification of surfaces, such as paints and roof tiles that maintain a clean surface like the lotus, by creating a surface that is similar to that of the lotus plants.1 The figure shows that dirt particles are unable to adhere to the paint and simply flow away with the rain. Everybody knows about the vivid colors of butterflies. But where does this color come from? One would naturally think that butterflies must use pigments, as in the paint industry. Actually, there are two fundamental mechanisms by which color is produced on butterfly wings. One leads to what we call ordinary color, and the second leads to the spectacular iridescent color. The ordinary color is due to the presence of chemical pigments, which absorb certain wavelengths and transmit or reflect others. The iridescent color is produced not by pigmentation, but by the interference of light due to multiple reflections within the physical structure of the material. The parts of a butterfly wing are shown in the Figure 3 in the following order, from left to right: Wing > Scales > Veins > Ridges. The size and periodicity of arrangement of the features on the wings causes interference with the visible light, creating color. Using this concept, structures and physical mechanisms that produce a shining color, like that found on the wings of butterflies, have been reproduced in carbon by an international team based at Allied Signal in Morristown, N.J. These highly periodically patterned novel carbon materials possess unique and potentially useful properties. 2 Another striking example that inspires design principles is the box-fish. These are rigid-bodied marine fish that live predominantly in shallow-watered, highly energetic, tropical reef environments. They are remarkably stable and agile swimmers.
They are able to maintain smooth swimming trajectories with minimal pitching, rolling, or yawing, even in highly turbulent waters. Moreover, they are capable of swimming rapidly (> 6 body lengths s-1), can spin around with a minimal turning radius, and can maintain precise control of their position and orientation.3 What applications could these types of properties be used for? In fact, one of the leading car manufacturers produced a bionic concept car that is based on the contours of the boxfish carapace and takes advantage of its drag reduction benefits. Not only the shape, but also the organizational composition of living organisms is highly advanced.
Therefore, great efforts are made to study and understand the formation of the hierarchical structures of these creatures. The shell of the abalone, for instance, is known for being exceptionally strong. It is made of microscopic calcium carbonate tiles that are stacked like bricks. Between the layers of tiles is a sticky protein substance. Even though calcium carbonate is one of the softest materials in nature, when the abalone shell is struck, the tiles slide, instead of shattering and the protein stretches to absorb the energy of the blow. Material scientists at the University of California, San Diego are studying the tiled structure for insight into stronger ceramic products, such as body armor. Researchers at Princeton, working on a grant from NASA, are analyzing the remarkable strength of abalone shells to help make impact-resistant coatings for thermal tiles. There are numerous groups that are working towards a better understanding of the structure and the governing mechanisms involved in the assembly of natural composite systems that have amazing mechanical properties. In synthetic composite structures, the hardness of the material is proportional to the inorganic/mineral content. However, there are striking examples of design in nature in which almost negligible amounts of minerals are used in a specially tailored environment, and very high levels of hardness, comparable to human dentine, can be achieved. An interesting example is sea-worms. Although mainly consisting of soft tissue, these worms have very hard jaws that have an exceptionally low amount of inorganic consistency. The jaw material is of particular interest because of its hard, lightweight and abrasive-resistant properties due to some gradient elements. The chemical surrounds and forms of these elements are not clear enough to be able to identify or mimic the arrangement/structure. These jaws, in addition to their extraordinary mechanical properties, are very good examples of natural gradient materials that have a perfect interface between the hard and soft tissues. Although many high-tech analysis techniques have been devised to understand how such a composite could be formed, particularly in highly unfavorable salty sea or ocean water, and how they have such great mechanical strength, the findings are still incomplete.
The information gathered is like the scattered pieces of a puzzle; to finish the puzzle, the missing pieces must be found with new advancements in analytical tools. What about mussels then? “If we have Batman and Spider-Man, why don’t we have any mussel super heroes?” asks Professor Herbert Waite of the University of California, Santa Barbara. Mussels may not be the biggest or the flashiest creatures in the sea, but they do one thing exceedingly well. They make a glue that lets them anchor themselves firmly to a rock and remain there-drenched by water, buffeted by the ocean’s waves. “I don’t know any other adhesive that can do that,” says Waite.6 Not only the glue, but the threads they make to attach themselves to the rocks are very significant in terms of both their composition and complexity, according to Niels Holten, who is conducting research on these systems at the University of California, Santa Barbara. These threads can elongate and relax with extraordinary mechanical flexibility under great impacts from ocean waves. The Waite group research on these thread cuticles reveals a very important aspect of material science, the significance of which has only very recently been understood: interface engineering. These threads have a very low amount, ca. 1-2 wt%, of metal ions in a polymeric matrix holding together large polymeric chains, which is possibly what gives the structure its flexibility and extensibility. Man-made structures cannot compete with the mechanical performance of these threads, especially at such a low volume of metal ion ingredients.
The ultimate goal of ongoing research is to understand the formation principles of these features so that similar structures can be made, using the same set of principles in laboratory conditions. In fact, the perfections of designs that are implemented in nature turn out to be an enormous fountain of ideas. Jewel beetles, which lay their eggs in freshly charred trees, can detect fires from miles away; the defense industry is studying these beetles for clues to design new low-cost, military-grade infrared detectors. Meanwhile, one of the leading car manufacturers is tapping the locusts’ famed ability to fly in dense swarms without colliding for a possible key to anti-collision devices in cars. And the Defense Advanced Research Projects Agency is funding development of a robot that can climb vertical surfaces, using the same principle that geckos use to walk up walls and saunter upside down across ceilings. There are several examples that could be given on this matter, but, due to limited space, we can only briefly summarize some of them. However, our understanding of the mechanism in nature is very limited, and it is expected that better insight will be gained with the advancement of available analytical tools. The great diversity of product designs in nature is produced from only a few common components, whereas we use a great number of materials and components to achieve new designs. Such high control and hierarchy in design in nature can only be attributed to an artist or designer who hides the perfection of his creation in the details. It is up to us to find out, see, and appreciate these perfections. Material scientists, of course, have the duty of transferring the findings from nature for the service of humankind by turning them into applicable forms in our daily lives.