Nanoindentation of bone: Comparison of specimens tested in liquid and embedded in polymethylmethacrylate
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V.L. Ferguson Department of Materials, Queen Mary, University of London, London E1 4NS, and Department of Anatomy and Developmental Biology, University College London, London WC1E 6BT, United Kingdom
A. Boyde Department of Anatomy and Developmental Biology, University College London, London WC1E 6BT, and Dental Biophysics, Centre for Oral Growth and Development, Queen Mary, University of London, Institute of Dentistry, London E1 2AD, United Kingdom (Received 16 June 2003; accepted 8 October 2003)
Elastic modulus of bone was investigated by nanoindentation using common methods of sample preparation, data collection, and analysis, and compared to dynamic mechanical analysis (DMA: three-point bending) for the same samples. Nanoindentation (Berkovich, 5 m and 21 m radii spherical indenters) and DMA were performed on eight wet and dehydrated (100% ethanol), machined equine cortical bone beams. Samples were embedded in polymethylmethacrylate (PMMA) and mechanical tests repeated. Indentation direction was transverse to the bone long axis while DMA tested longitudinally, giving approximately 12% greater modulus in DMA. For wet samples, nanoindentation with spherical indenters revealed a low modulus surface layer. Estimates of the volume of material contributing to elastic modulus measurement showed that the surface layer influences the measured modulus at low loads. Consistent results were obtained for embedded tissue regardless of indenter geometry, provided appropriate methods and analysis were used. Modulus increased for nanoindentation (21 m radius indenter) from 11.7 GPa ± 1.7 to 15.0 GPa ± 2.2 to 19.4 GPa ± 2.1, for wet, dehydrated in ethanol, and embedded conditions, respectively. The large increases in elastic modulus caused by replacing water with ethanol and ethanol with PMMA demonstrate that the role of water in fine pore space and its interaction with collagen strongly influence the mechanical behavior of the tissue.
I. INTRODUCTION
Nanoindentation has proven to be a powerful technique for characterizing the mechanical properties of small volumes of material, such as thin films on dissimilar substrates. The technique has only recently been applied to the examination of biological materials and complements existing larger scale assays by allowing the study of individual micrometer- and submicrometersized structures. Nanoindentation provides a means to study many of the principal microstructural components in bone and other mineralized tissues, which often have dimensions of only a few micrometers and usually cannot be separated for independent study. Bone is a heterogeneous material containing three main phases: mineral, collagen, and water, arranged in a series of hierarchical structures. Collagen contributes to J. Mater. Res., Vol. 19, No. 1, Jan 2004
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bone’s elastic and viscoelastic behavior while mineral stiffens the overall material. The degree of mineralization and collagen orientation highly influences mechanical properties.1,2 Mineralization
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