Biomimetics and Biotemplating of Natural Materials
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ight of large columns is typically limited by the buckling of the column under compressive load from the top (Euler buckling). Wood is ideally suited for this purpose because it has an E/ρ2 value larger than most engineering materials (see Figure 1). A first lesson from this observation is that materials harvested from nature are ideally used in functions close to their original ones for which they have evolved. Wood, for example, is supposed to work in hydrated conditions and is optimized for its typical loading pattern in axial compression and under bending by an adaptation of shape, fiber orientations, and distribution of internal strains. Cutting shelves out of a trunk means losing some of its good properties; wood workers know how to divide a trunk in the best way in order to keep as much of its natural strength as possible. A well-known difficulty in the use of renewable materials is the natural variability of their properties. What appears to be a weakness at first glance could be linked to a major strength of natural materials in adapting to environmental conditions. Wood, for example, changes its inherent properties during growth, which allows it to have a flexible trunk at a young age and a stiffer trunk later in life to
MRS BULLETIN • VOLUME 35 • MARCH 2010 • www.mrs.org/bulletin
103 ceramics t.
Introduction
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Natural materials display a wealth of structures and fulfill a variety of functions. Hierarchical structuring is one of the keys to providing multifunctionality and to adapting to varying needs of an organism. As a consequence, the natural environment represents not only a direct and renewable source of useful materials, such as wood, plant fibers, or even proteins of pharmaceutical importance, but also an enormous “database” of structures with exceptional mechanical, optical, or magnetic properties. Rather than focusing on the direct use of natural materials, this article discusses the use of structures that appeared in evolution and have been implemented in artificial materials of an entirely different type and chemical composition. This may be done either by directly copying the structure (biotemplating) or by extracting the design principles encoded in them for the fabrication of novel bioinspired materials.
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Abstract
alloys
/ρ 2 =
Oskar Paris, Ingo Burgert, and Peter Fratzl
E
and Biotemplating of Natural Materials
Young’s Modulus E (GPa)
Biomimetics
prevent buckling.2 Moreover, this adaptive growth also allows the tree to cope with side winds and other environmental challenges.3 Following such unusual mechanical stimuli, the tree develops a special kind of tissue called reaction wood, thereby creating beneficial mechanical property gradients throughout the tissue, as well as internal stresses to compensate for the increasing load of branches and leaves.4 This teaches another lesson that the variability of properties in natural materials is not random but corresponds to structural adaptation. Hence, a better understanding of the structure-function relations in natural tissu
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