Mechanical Properties of the Lobster Cuticle
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Mechanical Properties of the Lobster Cuticle D. Raabe, C. Sachs Max-Planck-Institut für Eisenforschung, Max-Planck-Str. 1, 40237 Düsseldorf, Germany ABSTRACT We present experiments on the mechanical and structural gradients through the cuticle of homarus americanus (lobster). The exocuticle (outer layer) is characterized by a very fine woven structure of the chitin-protein matrix (Bouligand structure) and by a high stiffness (8.5–9.5 GPa). The hardness increases within the exocuticle between the surface region (130 MPa) and the region close to the interface to the endocuticle (270 MPa). In the endocuticle which is characterized by a much coarser twisted plywood (Bouligand) structure both, the stiffness (3–4.5 GPa) and hardness (30–55 MPa) are much smaller than in the exocuticle. The transition in mechanical properties and structure between the exo- and endocuticle is abrupt.
INTRODUCTION The exoskeleton material of arthropods consists of mineralized fibrous chitin-based tissue . The most characteristic feature of this biological nano-composite material is its strictly hierarchical organization which reveals various structural levels [1-3]. The most characteristic level in the hierarchy, visible already at optical magnification, is referred to as twisted plywood or Bouligand pattern [4-6]. This structure is formed by the helicoidal stacking sequence of fibrous crystalline chitin-protein planes. The thickness of such a plywood layer corresponds to a certain stacking density of planes which are gradually rotated about their normal axis, thereby creating complex structures which appear as mesoscale arches when cut in cross sections. This study presents observations which substantiate that this structural picture of the exoskeleton material of arthropods must be completed by an additional feature, namely, by the occurrence of a pronounced mesoscopical structural gradient of the Bouligand pattern through the thickness of the exoskeleton material. Corresponding microindentation tests which were carried out through the cuticle thickness suggest further that the structure gradients observed play an important role for the micromechanical design strategy inherent to such materials. The experiments are conducted on the cuticle of the lobster homarus americanus which is a large arthropod (joint–limb animal) in the class of the crustaceans. Its cuticle consists, like that of most arthropods, of the three main layers epicuticle, exocuticle, and endocuticle which are secreted by a single layer of epidermis cells. The epicuticle (outer skin) is a very thin and waxy functional layer which acts as a diffusion barrier to the surrounding. The exocuticle and endocuticle layers, which carry the mechanical loads, consist of a hard mineralized fibrous chitin–protein tissue. The data shown in this study stem from these two hard layers. The exoskeleton was analyzed by using microscopy and mechanical testing. The sample material consisted of dry specimens cut from the right cheliped (pincher claw). Mechanical testing was conducted