Microstructure and Internal Stresses in the Exoskeleton of a Homarus americanus Lobster

Artificial tissues are being increasingly used in the replacement of damaged structural parts of the body. These artificial structures do not only require appropriated mechanical properties, but also biocompatibility and hemostatic properties. The emulation of natural scaffolds represents, therefore, an interesting possibility to combine good mechanical properties with biocompatibility. Natural extracellular skeletons often consist of ordered fibrous composites, which are hierarchically arranged in twisted plywoods structures. Composite materials contain, in general, significant residual stresses due to differences in the physical properties of matrix and reinforcement. Also biological composites are, therefore, expected to have residual stresses, which are supposedly correlated to their formation process and function. The aim of these investigations was, thus, a first time characterization of the residual stress state within a biocomposite, particularly in the cuticle of a lobster Homarus americanus, which is a hierarchically organized natural material. The microstructure of the exoskeleton and its residual stress state are revealed by electron microscopy and synchrotron X-ray diffraction studies. The microstructure of the exoskeleton revealed by optical (OM) and scanning electron microscopy (SEM) studies is subdivided into three distinct sub-layers (Fig.2). The outmost part of the exoskeleton is called epicuticle. It is a thin and waxy layer, which acts as a diffusion barrier to the surrounding environment. Beneath the epicuticle, two different fibrous layers can be distinguished. These two inner layers, so-called epicuticle and endocuticle, consist of a twisted plywood pattern of mutually misoriented stacking sequences of chitin-protein microfibrils filled with microscopic biominerals. Each microfibril is formed by a bundle of protein-wrapped crystalline α-chitin nanofibrils. Such patterns are typical for arthropod cuticles according the pioneering works of Bouligand [1] and Giraud-Guille [2]. Furthermore, the epicuticle differs from the endocuticle only in their stacking density of planes. The exocuticle (about 200μm thickness) appears immediately below the epicuticle and exhibits clearly a higher stacking density than the inmost layer – the endocuticle (up to 1.5mm thickness).