Hierarchical Structure of a Natural Composite: Insect Cuticle
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HIERARCHICAL STRUCTURE OF A NATURAL COMPOSITE: INSECT CUTICLE
STEPHEN L. GUNDERSON*, KATIE E. GUNNISON*, and JOHN W. SAWVEL** *University of Dayton Research Institute, 300 College Park, Dayton, OH 45469-0168 **Ohio Northern University, Department of Biology, Ada, OH 45810
ABSTRACT The insect cuticle is an excellent example of a natural, fiber-reinforced, polymeric composite consisting of chitin fibers embedded in a protein matrix. Optical and electron microscopy have been used to examine the structure and interaction of the constituents of the bessbeetle (Odontotaeniusdisjunctus) cuticle from the molecular to the macroscopic levels. Molecular chains of the polysaccharide chitin (N-acetylglucosamine) are grouped together to form "fibrils" which are either dispersed throughout the matrix or combined to form larger "fibers". The fibers are unidirectionally oriented within individual sheets or laminae which are stacked on top of one another at various angles forming a laminated structure. The protein matrix is ductile upon initial deposition but then undergoes a crosslinking process which increases its shear stiffness, thereby improving load transfer between fibers. The matrix is bound to the chitin via beta linkages holding it together at both the fibril and fiber levels. The matrix has a fibrous morphology which provides adequate toughness in spite of the high degree of crosslinking. Reference is made to designs observed in the bessbeetle cuticle which could be applied to man-made composites for improved performance primarily in the areas of damage tolerance and strength and stiffness coupled with low weight. For these designs to be implemented using synthetic materials, new or modified processing and fabrication methods are needed. INTRODUCTION A composite can be considered as a multiphase material or structure consisting of two or more distinctly different constituents which, when combined properly, provide improved performance over the individual components acting alone. This philosophy has been epitomized by nature as evidenced by many complex biological composite structures including wood, bone, mollusc shell, and insect cuticle. The constituents of natural composites are relatively weak and simple; consisting primarily of sugars, proteins, minerals, and water; but through unique material combinations and organization, extremely complex and intricate systems are created that possess excellent performance characteristics [1]. Natural materials and structures grow under ambient conditions with their formation finely controlled from the beginning molecules to the final macrostructure, creating a complex hierarchy with excellent interaction between structural levels. In fact, it is often very difficult to determine where one level ends and another begins for the purpose of distinguishing between material, structure, and system. Some natural composites are subjected to external loads during formation allowing the structure to be designed/developed under stress and thus improving its in-situ characteristics. Through
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