SEB Bulletin January 2007
Biomimetics - In Pursuit of Natural Sources of Inspiration
Bullet-proof pheasants (ref 1), geckos feet (ref 2), wood, worms and potatoes (ref 3,4) have all provided inspiration for modern biomimetic research (ref 5,6,7). “The field of biomimetics (or bionics) began to gather pace about 15 years ago, when researchers in material science and medical research realised that much of what they wanted was already available in nature” recalls Professor George Jeronimidis, director of the Centre for Biomimetics at the University of Reading (ref 8). Coined by Otto Schmitt in the 1950's, the term 'biomimetics' did not become consensus until 1991 when the US Air Force Office of Scientific Research began to formalise the field and ran a workshop to investigate what biology could offer in terms of the design and processing of materials. Since then the remit has broadened and a better definition was devised by Professor Julian Vincent who heads the Centre for Biomimetic and Natural Technologies at the University of Bath (ref 9): “The abstraction of good design from nature”. Over millions of years of evolution nature has solved many engineering problems, undoubtedly making many mistakes along the way, by turning to nature engineers can tap into this wealth of knowledge and avoid making the same mistakes.
Although biomimetics, in its current form, is a relatively new field of research, the concept is ancient, with many examples. The Chinese tried to develop a synthetic silk over 3000 years ago, Joseph Paxton is believed to have based the structure of the Crystal Palace on a lilypad and the Eiffel Tower takes inspiration from the human thigh bone. But perhaps the most famous example is Velcro (ref 10): George Mestral invented Velcro after studying the burrs that stuck to his dog's coat, he observed that the burrs consisted of hundreds of tiny hooks that locked around the soft fur. With the help of French weaving experts, he mimicked the burrs and fur to produce Velcro, which was patented in 1952.
Biomimetics however, is not simply a process of copying nature as Dr. Thomas Speck of the Biokon Centre at the University of Freiburg (ref 11,12) is keen to emphasise, “Biomimetics means an independent and creative development initiated by natural models”. Nor is biomimetics a one-way street from biologist to engineer, knowledge of the mechanics of living organisms can also help biologists to gain a better understanding of them. Current biomimetic research is helping to develop a great many new products from fabrics that respond to its wearers needs, through new building materials to potential solutions for global warming and its true potential is only now being realised.
Pines cones, sharks and hi-tech fabrics
The humble pine cone has provided inspiration for the development of a smart fabric, capable of reacting to the activity of the wearer (ref 13,14). Pine cones open and close in response to changes in humidity, opening as they dry to release their seeds. Professor Vincent demonstrated that this movement is due to the large ovuliferous scales, which comprise two easily distinguishable tissues, one on the outer surface and one on the inner surface of the cone. Just as a bimetallic strip bends with heat, the ovuliferous scales bend with increased humidity due to the outer tissue expanding more readily than the inner tissue, which results in the cone closing (ref 15).
A fabric with the ability to react to humidity could revolutionise clothing and this is precisely what Professors Jeronimidis and Vincent and their teams developed as part of a Ministry of Defence contract. The design brief was a material that could be used in the desert that would be cool during the day and provide warmth at night when temperatures plummet, alleviating the need to carry extra clothing. Taking inspiration from the pine cone, their smart material is made of two bonded fabric layers interspersed with tiny flaps that open with increased humidity. Thus as the wearer sweats the flaps open, increasing air-flow and cooling the wearer. There is an additional waterproof layer under the opening flaps to provide protection from rain. The material is currently being developed for fashion and sports wear and commercial prototypes should be available soon.
Another fabric whose origin was inspired by nature is the sharkskin swimsuit (ref 16). A shark is covered in V-shaped structures that look like scales. These scales reduce drag by channelling the water generating microscopic vortices, which allow the shark to glide through it. The sharkskin swimsuit channels water along grooves in the fabric reducing drag in much in the same way. Interestingly, in the swimming events at the Sydney Olympics, 28 of the 33 gold medals were won by swimmers wearing the sharkskin suit.
Plant stems and structures
Whilst visiting The University of Montpellier Dr. Thomas Speck and his wife Dr. Olga Speck, noticed the fantastic mechanical properties of the giant reeds that grew there. The reeds combined both stiffness and elasticity and appeared never to break, properties that would be invaluable in a building material and so with the help of their colleagues began investigating the structure of the reed's stems (ref 17). They studied both the Dutch rush (aka horsetail, Equisetum hyemale) and the giant reed (Arundo donax). “Very quickly we realised the technical potential of the structure” recalls Dr. Speck. From these observations and in collaboration with Dr. Markus Milwich of ITV Denkendorf (ref 18) Dr. Speck developed the 'Technical Plant Stem', an innovative fibrous compound material (ref 19). The hollow structure of the horsetail stem consisting of two rings of strengthening tissue, connected by T-shaped pillars, which sandwich a third tissue type with canals (vallecular canals) running through it, provided the design for an ultra-lightweight structure with excellent mechanical properties. The giant reed uses densely packed fibres associated with vascular bundles in regions of highest stress to prevent breaking. Learning from this several structural gradients were included in the new material. Finally, inspired by the observation of bundles of cellulose fibrils arranged in helices in wood fibres and the arrangement of fibre bundles in the giant reed's stems a helical arrangement of fibres was also incorporated in the final product to provide stability and vibration dampening. The resulting 'Technical Plant Stem' offers a material that is stiff, strong and lightweight with high energy dampening and benign fracturing behaviour - features that improve on currently available materials. It is also extremely versatile and is currently being developed for use in the automotive and aircraft industries and may soon be used in the production of the latest skis, snowboards and surfboards.
Feathers and insulation
Down feathers have long been used as an effective insulating material due to their ability to trap large volumes of air. However, when wet they clump together reducing their insulating properties. Despite being poorer insulators, synthetic materials are often used in place of feathers, especially in extreme weather clothing, because their efficiency is not reduced when wet. The challenge now is to develop more efficient synthetic materials. Dr. Richard Bonser of the Centre for Biomimetics at Reading is currently studying the mechanics and geometry of the domestic goose (Anser anser), domestic duck (Anas platyrhyncos) and Gentoo penguin (Pygoscelis papua) down feathers in the hope they will pave the way to new synthetic insulators (ref 20).
All feathers share a common structure with a central stem called a rachis that has side branches (barbs), which in turn have smaller branches (barbules). Dr. Bonser believes we can learn a lot from nature's insulators “at their most advanced current synthetics are little more than a tube with four holes running through it, in contrast to the incredibly intricate structures of down”. It is however, not just the shape of the feathers that is important, but also how they interact. Penguin feathers, for example, compress in water but on land spring back to shape, restoring vital insulating air spaces. By understanding the mechanics of this shape restoration and the geometry of feathers Dr. Bonser feels “we perhaps are only a step away from a synthetic down insulator”.
Bacteria and solar cells
Not only has inspiration been taken from nature to provide engineering solutions, but as the work of Dr. Silviu Balaban of the Institute for Nanotechnology within the Research Center Karlsruhe and the newly formed Karlsruhe Insitute of Technology (ref 21) demonstrates, biomimetics can be used to give back to the natural world. With global warning and concerns over the safety of nuclear power, attention is turning to renewable energy sources. The sun is possibly the most readily available source of renewable energy; our entire energy needs for one year could be met, if it was possible to harness all of the solar energy that reaches the Earth's surface, in just one hour (ref 22,23). As plants get all of their energy requirements from the sun through photosynthesis, they would seem a logical place to look for inspiration. However, Dr. Balaban points out that the photosynthetic machinery of plants is “very complicated with complex protein structures required to assemble the antennae (structures which contain the pigment molecules that harvest light), which is not possible to replicate in the laboratory”. His research therefore focuses instead on the much simpler systems found in the light harvesting organelles, called chlorosomes, of the photosynthetic bacteria, Chlorobium tepidum (Green Sulphur bateria) and Chloroflexus aurantiacus. Like plants, these bacteria capture light energy using pigment molecules (chromophores) and convert it into biochemical energy, but unlike plants the building blocks of bacterial antennae are self-assembled into functional structures. Self-assembly is very economical requiring no external input as it works by exploiting the predictable manner with which different molecules and small reactive groups within the chromophores interact, using “soft” or “supramolecular” bonds formed between them to glue the structure together. By understanding this process Dr. Balaban has been able to mimic nature in the laboratory and using only synthetic chemicals has produced functioning antennae, which could be used to harvest light in solar cells. The advantage of bacterial antennae solar cells is they could be up to two and a half times more efficient at harvesting the available light energy than current solar cells, making them a more viable alternative to fossil fuels and nuclear power. Whilst it will take many years Dr Balaban believes “that scientists, will come up sooner or later with better solutions than the presently, costly silicon solar cells” an ambition he hopes witness being fulfilled in the next 20 years.
Nature is full of surprises and it is impossible to predict where the next practical application will come from but as Prof. Jernomidis' “first lesson in Biomimetics” teaches, much can be achieved “with a great deal of ingenuity, design skills and creative thinking”.
Rebecca Poole
University of Bristol
References
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15. Dawson, C., Vincent, J., Rocca, A.M. (1997)
How pine cones open. Nature 390, 668
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23. DOE report Basic Research Needs for
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