Microstructure and mechanical properties of the exoskeleton of the lobster Homarus americanus as an example of a biological composite material

The main focus of this study was the characterization of a mineralized biological tissue with respect to its microstructure and mechanical properties using the cuticle or shell of the American lobster as a novel model material. The cuticle of the lobster Homarus americanus is a nano-composite material like 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 on all length scales. One prominent design principle found in the hierarchical structure of 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 changed. 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 mechanical properties on the macroscopic scale, miniaturized tensile, compression and bending tests combined with the digital image correlation method (strain mapping) were carried out to obtain global mechanical data and to examine underlying deformation mechanisms. On the microscopic scale the local mechanical properties were investigated by micro- and nano-indentation tests. To examine the underlying mechanical properties of the fibers depending on their orientation and their grade of mineralization, micro- and nanoindentation is an excellent tool which makes it possible to probe small volumes with high spatial resolution. Characterization of the microstructure included scanning electron microscopy (SEM) combined with energy dispersive x-ray (EDX) measurements and thermo-gravimetric analysis (TGA) for evaluating the grade of mineralization. The outstanding mechanical properties and the multi-functionality of biological materials make their investigation particularly interesting from a biomimetic viewpoint. The identification of essential structural features responsible for their efficiency may help designing artificial systems with similar properties