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Physiology and Biochemistry |
2Institute of Textile Technology and Process Engineering (ITV) Denkendorf, Koerschtalstrasse 26, D-73770 Denkendorf, Germany; 3Plant Biomechanics Group, Botanischer Garten, Universitaet Freiburg, Schaenzlestrasse 1, D-79104 Freiburg, Germany; 4‘Competence Networks Biomimetics' Baden-Württemberg, Germany and ‘BIOKON' Germany
Received for publication March 16, 2006. Accepted for publication June 30, 2006.
ABSTRACT
The significance of inspiration from nature for technical textiles and for fibrous composite materials is demonstrated by examples of already existing technical solutions that either parallel biology or are indeed inspired by biological models. The two different basic types of biomimetic approaches are briefly presented and discussed for the "technical plant stem." The technical plant stem is a biomimetic product inspired by a variety of structural and functional properties found in different plants. The most important botanical templates are the stems of the giant reed (Arundo donax, Poaceae) and of the Dutch rush (Equisetum hyemale, Equisetaceae). After analysis of the structural and mechanical properties of these plants, the physical principles have been deduced and abstracted and finally transferred to technical applications. Modern computer-controlled fabrication methods for producing technical textiles and for structuring the embedding matrix of compound materials render unique possibilities for transferring the complex structures found in plants, which often are optimized on several hierarchical levels, into technical applications. This process is detailed for the technical plant stem, a biomimetic, lightweight, fibrous composite material based on technical textiles with optimized mechanical properties and a gradient structure.
Key Words: Arundo donax • biomimetics • bionics • Equisetum hyemale • gradient structures • technical plant stem • technical textiles
INTRODUCING BIOMIMETICS
Biomimetics or bionics is a relative new term for a process as old as humankind: borrowing ideas from nature for shaping and creating our surroundings. "Ancient" biomimetics led to the development of tools, clothing, and housing. With the dawn of the technical age and later the industrial age, technical developments deviated from natural prototypes, because looking closely, those principles were often too complex to be transferred to engineering techniques. Instead of using nature's mostly flexible, soft, and force-adaptive structures, technical constructions mostly went into rigid rectangular shapes, which could be more easily calculated and mechanically tooled. Moreover, the invention of the wheel and the processing of metal had no natural example, and they marked milestones of the alienation of mankind from nature (Vogel, 1998
; Speck and Neinhuis, 2004
).
Modern biomimetics is a systematic approach for researchers who know that new developments and insights can only be achieved in transdisciplinary collaborations. If this is true for examples of co-working between physics or chemists and engineers, why not networking with biologists? Most helpful and catalytic for this kind of collaboration was the development of new measuring methods and instruments, which are able to unearth natural principles or regularities hitherto unknown. The systematic approach toward comprehending biological structures, processes, and functionality resulted in new and astonishing technical solutions such as self-cleaning surfaces (modeled after the lotus plant) (Barthlott and Neinhuis, 1997
; Scherrieble et al., 2005
), lightweight shape-optimized technical structures (modeled after trees and bones) (Mattheck, 1990
, 1996
, 1998
), new propeller designs (modeled after bird wings) (Bannasch, 2005
), or widely improved sonar transmission techniques (modeled after dolphin communications) (Bannasch, 2006
).
We also have to mention that biomimetics is only, in very rare cases, a mere copy or mimicry of natural structures or processes and their transfer into technical constructions. In many cases, after understanding a biological model, a creative human mind must abstract and modify those biological concepts to reach a technical solution that is better than already existing, often already highly sophisticated solutions. The new biomimetic solution is hopefully, but not intrinsically, the more sustainable, ecologically superior solution. Better often means faster or cheaper, however, to industry and consumers who are not always very concerned about questions of ecology.
The biomimetic process is very transdisciplinary, encompassing approximately seven different fields of bionic research (the suggested structure is according to the subdivisions of the German Federal Bionics Competence Network BIOKON): (1) architecture and design; (2) lightweight construction and materials; (3) surfaces and interfaces; (4) fluid dynamics, swimming, and flying; (5) biomechatronics and robotics; (6) communication and sensorics; and (7) optimizaton.
The boundaries between individual fields, however, are often somewhat blurry, and the transition between subdivisions is gradual (Speck and Harder, 2006
; Speck et al., 2006a
). In addition, the history of biomimetics is a field attracting increasing interest.
INTRODUCING TEXTILES WITH HIGH FUNCTIONALITY: "SMART" TECHNICAL TEXTILES AND "SMART" CLOTHING
Textiles are no longer just simple means of clothing, seat covering, or carpets. So-called smart textiles with high functionality conquer the markets. New fiber developments are stronger, chemical- and fire-resistant, electrically conductive, or adjustably biodegradable. Using high-tech fibers, textiles nowadays are used for environmental pollution control or lifesaving implants (Milwich and Mueller, 2005
). Snowboarding jackets can play MP3s and have incorporated GPS transmitters (Stollbrock, 2005
). Incorporated sensors in carpets, called "thinking" or "smart" carpets, will give emergency light, can detect the heat of a fire, call a nurse if a patient falls and remains on the ground, and detect footsteps of an intruder (Lauterbach, 2005
).
A common research topic for all these examples is the integration of "smart" functionalities that preferably should be integrated into the fiber or the textile as deep as possible. That means that any "smart" functionality should not be a rigid, sharp-angled form, which stands out from the textile or fibers as a foreign body, but should be as smooth as the textile surrounding it. Thus, the smart function itself should be made of the special fibers or textiles. Doing this, we come full circle back to nature's soft, flexible, and adaptive structures. A good example for the deep integration of a smart function is the Baby Body Clothing, developed by ITV Denkendorf (Linti and Horter, 2005
). Without any hard edges or planes, it records body functions of the baby and calls for help if breathing or heart beats are unsteady (Fig. 1).
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; Harder, 2006
). With the use of strong, specially developed fibers such as glass, carbon, or aramid and their embedment in polymer or ceramic matrices, very strong and lightweight composite materials and structures can be processed. An advantage of using flexible fiber material is that they can be laid exactly in the direction of the strain lines of a designed structure. Fiber composites can be found in airplanes, space shuttles, or in racing cars. American architect Peter Testa has proposed building a skyscraper of thick strands of helically and circumferentially wound carbon fiber composites arranged similarly to a mesh, with the spaces in between the strands filled with glass plates.
FIBER-REINFORCED PLASTICS (COMPOSITES) ARE FUNCTIONAL TECHNICAL TEXTILES
Fiber composites are a classic example for biomimetic translation of nature's wisdom into technology. Many principles of composites have their counterpart in nature. Bones, plant stems, and wood have highly optimized the use of fibers in the exact directions of effective loads (cf. e.g., Mattheck, 1990
, 1996
, 1998
; Vogel, 1998
) and are emulated by various textile methods. The optimized microscopic fiber arrangement in biological materials finds its extension in an optimal macroscopic arrangement of struts for load-carrying structures. Figure 2 shows the macroscopic fiber arrangement of the plant stem of a cactus, which found its biomimetic counterpart in the Rotex robotic arm, made manually by DLR Germany, with 0°, 90°-fiber bundles (tensile/compression/bending forces) and 45°-fiber bundles (torsional loads). Figure 3 shows the thorax of a dragonfly, in which the ribs themselves are weight-optimized structures and are macroscopically arranged as weight-saving spacer structures connected by a thin layer of outer skin.
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) based on (ultra-)structural features in wood (Gordon and Jeronimidis, 1980
; Jeronimidis, 1980
, 1991
). The orientation of the fibers in this biomimetic composite is based on angles found in microfibrils of wood tracheids.
Composites now are used widely and are undergoing development. Research goes into reducing production costs, developing more sophisticated methods for constructing ultimate lightweight structures, or creating higher functionality, the functionality preferably deeply integrated into the textile or the fiber. Examples for such functionalities are (1) passive, form-optimizing adaptive wings for maximum energy yield of airplane wings or wind turbine blades (Breitbach and Sinapius, 2004
), (2) integrated (fibrous) glass sensors for damage control in bridges and airplanes, and (3) active damping of disturbing or harmful vibration with piezo-ceramic fibers (Monner, 2005
).
Ultimate lightweight composite structures are manufactured with so-called gradient textile techniques. As in nature, every single fiber strand is exactly laid within the structure in the direction necessary to neutralize outer forces so that no unnecessary fibers or weight are incorporated. As an example for a gradient textile process, Fig. 4 shows an extreme lightweight carbon fiber reinforced plastic Robot-arm of the German company Kuka Roboter GmbH, produced by a process called tailored fiber placement. This is a stitching process, where, on a slightly altered textile stitching machine, up to 10 stitching heads place every single carbon fiber strand next to the preceding fiber strand (Fig. 5).
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, 1996
, 1998
). His propagation of using tension force loaded machine elements instead of using pressure loaded elements under the motto "Thinking in Ropes" for lightweight construction will further advance the use of custom-made composites (Mattheck et al., 2004
). In case of the development of the "technical plant stem," we will show how biomimetics can further enhance the performance of composites. STARTING THE BIOMIMETIC DIALOGUE
In the future, traditional engineering will still be the basis of most new technical developments. Biomimetics cannot and shall not replace this established and well-tested approach. But new developments, whenever possible and ingenious, should be stimulated by solutions from nature and equalized with nature's wisdom, thus generating a pool of ideas and knowledge for further use. The mostly superficial, functional knowledge gained from past research can now be supplemented with new findings about the fine structure of materials or the functions of boundary layers using new measuring methods.
Usually, there are two approaches of doing biomimetics (Speck et al., 2006a
, b
). (1) Top-down approach: Engineers are searching for ways to optimize existing products or processes with the aid of biologists and their pool of biological knowledge. Because of the differing technical languages of those involved, it is very helpful if biologists acquire knowledge of various physical contexts, whereas engineers should be equally open-minded and willing to think in unusual directions. After identifying the most promising solutions with additional, and sometimes very extensive, structural and functional analyses, biologists and engineers will abstract nature, deduce the functional principles, and create a modified and appropriate technical solution. Then engineers have to look for an optimal translation into techniques with appropriate production methods and materials. This strategy is also usable for optimizing already existing biomimetic technical solutions. (2) Bottom-up approach: Biologists do fundamental research into nature's structures, processes, and functional modes of operations. New principles are discovered and analyzed, then communicated to engineers, and the two groups work together to abstract and transfer insights into new technical solutions.
EXAMPLE FOR BIOMIMETIC ENGINEERING: THE TECHNICAL PLANT STEM
Conscious that the knowledge of biological structures, processes, and functionality in plants and animals will deeply enrich engineering work, biologists from the universities of Freiburg and Tuebingen and engineers from the Institute of Textile Technology and Process Engineering in Denkendorf (ITV) founded the Competence Network Biomimetics (Speck and Speck, 2003
). The interdisciplinary approach of the network members and current R & D projects ensure that results from basic research are transferred into industrial products throughout the whole value chain. Because of superior mechanical and lightweight properties of their stems, Dutch rush (Equisetum hyemale, Equisetaceae) (Fig. 6) and giant reed (Arundo donax, Poaceae) (Fig. 7) were identified by biologists from the Plant Biomechanics Group Freiburg and engineers from ITV Denkendorf as particularly promising biomimetic templates for the construction of ultra-lightweight technical structures with an interesting combination of mechanical properties (Speck and Spatz, 2001
; Speck et al., 2006a
). The hollow aerial stem of horsetail (Equisetum hyemale) represents an extremely lightweight construction. Functional analyses identified a double ring structure in strengthening tissues, consisting of an outer ring of fibrous collenchymatous tissue that are connected to the inner, double-layered endodermis by "pillar-like structures" having the appearance of T-struts in cross-section in the stem periphery (Fig. 6). Between the collenchyma and the endodermis, which resemble a technical sandwich structure, is a thicker layer of parenchymatous tissue with remarkably large so-called vallecular canals, significantly reducing the weight of the hollow stem (Speck et al., 1998
; Spatz and Emanns, 2004
). The hollow stems of the giant reed Arundo donax grow up to a height of 6 m with a basal outer diameter of approximately 2 cm. They also have excellent mechanical properties under both static and dynamic loading conditions (Spatz et al., 1997
; Speck, 2003
; Speck and Spatz, 2003
). If the dense stands are subjected to dynamic wind loads, the slender culms respond with bending vibrations and pronounced damping (Speck, 2003
; Speck and Spatz, 2004
).
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, 1992
, 1997
; Mattheck, 1996
, 1998
, 2006
; Spatz and Speck, 1994
; Spatz et al., 1997
; Speck et al., 1998
; Mattheck et al., 2006
). So far, fibrous cores in tubular composites are very seldomly applied. However, Figure 9 shows a newly developed, relatively expensive but very lightweight tubular composite material, where helically fiber strands are stitched onto the inner side of a thin tubular skin.
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; Speck et al., 2006a
).
|
; Cave and Walker, 1994
; Reiterer et al., 1999
; Burgert et al., 2004
, 2005
).
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The advantages of the new composite material were self evident in such a way that specialists from prominent composite companies—seeing the first prototypes of the technical plant stem—encouraged the inventors to patent it. The now-patented material, called the "technical plant stem," has generated interest by several composite companies from the fields of aerospace, automotive, building, and sporting goods, which are willing to finance future research work.
PRODUCTION METHODS AND MACHINERY EQUIPMENT FOR THE TECHNICAL PLANT STEM
Nevertheless, the production costs for combining these functionalities into a new, unique product, must always be considered. In case of composite beams, the pultrusion process is a cost-efficient production method for endless-fiber-reinforced plastics. Compared to metals, the profiles are corrosion resistant and to a large degree maintenance free. They are very safe in having good electrical and thermal isolation, installation costs are lower and lighter foundations can be realized. In thermo-set pultrusion, impregnated high-performance fibers are pulled through a form-shaping die and are consolidated by heat and pressure during the transit through the die (Fig. 12). To incorporate diagonal fiber bundles into the technical plant stem, a braiding machine was installed into the pultrusion process (Figs. 13, 14).
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Many other, astonishing, plant functionalities are waiting to be discovered. The transdisciplinary collaboration of researchers with biological, chemical, physical, and engineering backgrounds and a systematic biomimetic approach to comprehend biological structures, processes, and functionality will bring new insights for the development of new technical solutions. In our opinion, technical textiles and fibrous composite materials offer a brilliant opportunity for transferring ideas inspired by biological models via biomimetic approaches into innovative technical structures, because composite materials based on technical textiles allow production processes comparable to those used by nature (Fig. 21).
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FOOTNOTES
1 The authors gratefully acknowledge the support of the research project by the Competence Network ‘Plants as Concept Generators for Biomimetic Materials and Technologies' of the State of Baden-Wuerttemberg, Germany. 
5 Author for correspondence (Markus.Milwich{at}itv-denkendorf.de
)
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