Influence of Structural Principles on the Mechanics of a Biological Fiber‐Based Composite Material with Hierarchical Organization: The Exoskeleton of the Lobster Homarus americanus

Abstract

The cuticle of the lobster Homarus americanus is a nanocomposite, such as most structural biological materials. It consists of a matrix of chitin‐protein fibers associated with various amounts of crystalline and amorphous calcium carbonate in the rigid parts of the body, and is organized hierarchically at all length scales. One prominent design principle found in the hierarchical structure of such biological fibrous composite materials is the twisted plywood structure. In the lobster cuticle, it is formed by superimposing and gradually rotating planes of parallel aligned chitin‐protein fibers. To adjust the mechanical properties to the requirements on the macroscopic level, the spatial arrangement and the grade of mineralization of the fibers can be modified. A second design principle of lobster cuticle is its honeycomb‐like structure, generated by the well‐developed pore canal system, whose twisted ribbon‐shaped canals penetrate the cuticle perpendicular to its surface. Due to the hierarchical structure, the mechanical properties of the lobster cuticle have to be investigated at different length scales, which is essential for the understanding of the structure–mechanical function relations of mineralized tissues (e.g., potentially also bone and teeth). In order to investigate the influence of the structural principles on the mechanical properties on the macroscopic scale miniaturized tensile, compression, and shear tests were carried out to obtain integral mechanical data. Characterization of the microstructure included scanning electron microscopy (SEM) combined with energy dispersive X‐ray (EDX) measurements.

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“…16b) [135]. A series of pore canals traverse the lamellae of both the exocuticle and endocuticle, creating a honeycomb-like structure that contributes to the strong, lightweight design [136]. Generally, the endocuticle is the thickest and most variable layer.…”

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“…[32,33] Finally, the cuticle shows a well-organized, honeycomb-like structure of pore canals with the scale of 10-1000 µm, facilitating water-mediated transport of mineral ions and reduced density (IV). [11] Compared with the harder exocuticle, the endocuticle demonstrates lower mineral content, larger canal pores, and a longer stacking height. [11] Water acting as a plastifier contributes to the remarkable mechanical properties of wet lobster cuticle.…”

“…[11] Compared with the harder exocuticle, the endocuticle demonstrates lower mineral content, larger canal pores, and a longer stacking height. [11] Water acting as a plastifier contributes to the remarkable mechanical properties of wet lobster cuticle. [28,34] Under wet conditions, the twisted plywood structure can be strongly distorted and split up when loaded.…”

“…[11,27,28] The cuticle consists of three layers: a thin, waxy epicuticle, an exocuticle, and an endocuticle; the exocuticle is the hardest and stiffest. [27,29] The highly ordered hierarchical architecture of the exocuticle has been well investigated.…”

“…Discovery: Lobster-cuticle image: reproduced with permission. [11] Copyright 2009, Wiley-VCH; Bone image: reproduced with permission. [8] Copyright 2016, Nature Publishing Group; Brick-and-mortar, planar-twisted-plywood, and concentric-twisted-plywood schematics: reproduced with permission.…”