Contribution of Collagen, Mineral and Water Phases to the Nanomechanical Properties of Bone
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Contribution of Collagen, Mineral and Water Phases to the Nanomechanical Properties of Bone Amanpreet K. Bembey1, Vanessa Koonjul1, Andrew J. Bushby1, Virginia L. Ferguson1, 3 and Alan Boyde2 1 Department of Materials, Queen Mary, University of London, London, UK. 2 Dental Biophysics, Queen Mary, University of London, London, UK. 3 BioServe Space Technologies, Department of Aerospace Engineering Sciences, University of Colorado, Boulder, CO, USA. ABSTRACT Cortical bone is an anisotropic material, and its mechanical properties are determined by its composition as well as its microstructure. Mechanical properties of bone are a consequence of the proportions of, and the interactions between, mineral, collagen and water. Mid-shaft palmar cortical tissue from the equine third metacarpal bone is relatively dense and uniform with low porosity. The mainly primary osteons are aligned to within a few degrees of the long axis of the bone. Beams of compact cortical bone were prepared to examine effects of dehydration and embedding and to study contribution of collagen and mineral to nano-scale material properties. Five beams were tested: untreated (hydrated); 100% ethanol (dehydrated); or embedded in polymethylmethacrylate (PMMA) for one normal, one decalcified, and one deproteinated bone sample. Elastic modulus was obtained by nanoindentation using spherical indenters, with the loading direction transverse [1] and longitudinal to the bone axis. By selectively removing water, mineral and organic components from the composite, insights into the ultrastructure of the tissue can be gained from the corresponding changes in the experimentally determined elastic moduli. INTRODUCTION In recent years, nanoindentation has been applied to the study of calcified tissues to determine mechanical properties at the scale of individual microstructural elements such as lamellae in bone [2], individual tidemarks in articular calcified cartilage [3], and enamel prisms [4] and peritubular dentine [5] in dental tissues. Nanoindentation may also be able to elucidate further information at the ultrastructural level and can be conducted in different tissue orientations to investigate anisotropy of individual microstructural components [6]. Storage and preparation methods influence measurements of bone mechanical properties. Surface effects complicate nanoindentation of physiologically relevant bone samples [1]. Thus, most studies have examined dried, embedded, or included bone. At the ultrastructural level, bone can be thought of as a 3 phase co-continuous composite consisting of collagen and other proteins, bone mineral and water. The relationships between collagen orientation and mineral content, and how these influence mechanical properties of bone at this scale, are poorly understood. The role of water in the structure may also be complex. NMR studies have shown that water can act as a plasticizer in collagen fibrils [7,8]. Water can also act as a particle separator, as in the galleries of clay minerals and as a binder as in Portlan
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