Image analyses of two crustacean exoskeletons and implications of the exoskeletal microstructure on the mechanical behav
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The microstructures of exoskeletons from Homarus americanus (American lobster) and Callinectes sapidus (Atlantic blue crab) were investigated to elucidate the mechanical behavior of such biological composites. Image analyses of the cross-sectioned exoskeletons showed that the two species each have three well-defined regions across the cuticle thickness where the two innermost regions (exocuticle and endocuticle) are load bearing. These regions consist of mineralized chitin fibers aligned in layers, where a gradual rotation of the fiber orientation of the layers results in repeating stacks. The exocuticle and endocuticle of the two species have similar morphology, but different thicknesses, number of layers, and number of stacks. Mechanics-based analyses showed that the morphology of the layered structure corresponds to a nearly isotropic structure. The cuticles are inter-stitched with pore canal fibers, running transversely to the layered structure. Mechanics-based analyses suggested that the pore canal fibers increase the interlaminar strength of the exoskeleton.
I. INTRODUCTION
Exoskeletons are the hard, external covering of arthropods such as crustaceans (e.g., lobsters, crabs, and shrimp) and insects (e.g., beetles). Many exoskeletons have outstanding material properties1 with multiple functions such as carrying the body weight, serving as a filter for chemicals and environmental exposure, and providing armor against enemies.2 To achieve this, nature primarily uses one material—the biopolymer chitin—to develop a complex hierarchical structure, together with connective tissue consisting of proteins and minerals. Variations in the morphology of the chitin network and mineralization within the structure lead to a range of properties.1 Arthropod exoskeletons are commonly composed of three major regions including epicuticle, exocuticle, and endocuticle (Fig. 1). The outmost region, the epicuticle, is thin (2–4 m) compared to the other regions. It is a waxy layer lacking chitin, acting as a diffusion barrier. The exocuticle and endocuticle are mainly made of the chitin-protein marcofibrils, with minerals deposited in between for species with calcified exoskeletons.3,4 Chitin is a natural polysaccharide that is commonly found in most arthropods. These chitin polymers with their asso-
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2008.0375 2854
J. Mater. Res., Vol. 23, No. 11, Nov 2008
ciated proteins form microfibrils.3,5 Bundles of microfibrils form the so-called “macrofibrils,” which can be observed under electron microscopy. The deposited minerals in calcified samples of crustaceans include calcite, vaterite, and hydroxyapatite.3 The exocuticle and endocuticle are the load-carrying structure in the cuticle3 and vary in thickness, depending on species. Although the shell of nacre has received significant attention in the last decade (e.g., Refs. 6–9), the architecture and mechanical behavior of most arthropods exoskeletons are not well-understood. By investigating an
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