Biomimetic Ceramics and Composites
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Biomimetic Ceramics and Composites Paul Calvert Introduction An obvious parallel of structure and function exists between a rhinoceros and a tank, and between a beetle shell and the skin of an aircraft. We can also draw comparisons at the microstructural level between these biological and synthetic materials. Significant differences also exist, however, and the rigid biological materials such as bone and shell have much to teach us. In particular, they are distinctly composite structures. Although they bear loads in much the same way as synthetic composites or ceramics, they have far more complex architectures. The goal in considering the group of mineralized biological materials as described, for example, in the article by Fink et al. in this issue, and in devising modifications of them, which is the focus of this article and of Mann's, is to learn to devise arrangements of synthetic materials that work more efficiently than the homogeneous substances of simple composites that we use now. In addition to designing better microstructural arrangements we may also learn, again by analogy to the biological materials, how best to process these structures and how to recycle them after use. Biological structural materials are optimized for their high strength- or stiffnessto-weight ratio. Achieving this in synthetic materials for nonbiological application, for example in cars and airplanes, would be of obvious value. Our own interest here has focused on cuticle and bone as models for our synthetic work. Another property of biomineralized materials, for example biological ceramics, is their increased toughness. In this case we will discuss tooth enamel mimicking. Bone, Cuticle, and Carbon FiberReinforced Polymers Insect cuticle has a simple composite structure of chitin fibers (made of N-acetyl
MRS BULLETIN/OCTOBER 1992
glucosamine polymers) embedded in a protein matrix. The cuticle shows a vast range of properties1 depending on the fiber content, and whether the protein is soft, or tanned (heavily cross-linked) and rigid. The cuticle consists of three layers. The outer epicuticle is a waxy protective layer, very similar to the fiber-free gel coat that is applied to glass-fiber yachts to protect the composite from corrosion. Next is a rigid layer of fibers in a tanned matrix, the exocuticle. The inner endocuticle has a matrix that is untanned and soft. The tanning chemistry is not well understood but is related to cross-linking of amine side groups on the protein by phenolic compounds. The tanning is coupled with a loss of water from the matrix and deplasticization. The chemistry is closely related to that used to make phenolic resins. Overall, this structure is similar to that of fiber-reinforced composites. The cuticle fulfills a similar requirement for a beetle that the composite does for an aircraft: to provide a cylinder of maximum stiffness for minimum weight. Improved properties, in both naturally occurring and synthetic composites, depend both on the matrix and the fibers. Comparison to a biological system is ins
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