Nanoindentation Measurements of Bone Viscoelasticity as a Function of Hydration State
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0898-L07-04.1
Nanoindentation Measurements of Bone Viscoelasticity as a Function of Hydration State 3
Amanpreet K. Bembey 1, Michelle L. Oyen 2, Andrew J. Bushby 1, Alan Boyde 1 Department of Materials, Queen Mary, University of London, London, E1 4NS, UK 2 Center for Applied Biomechanics, Department of Mechanical and Aerospace Engineering, University of Virginia, Charlottesville, VA 22902 3 Biophysics Section, Centre for Oral Growth and Development, Dental Institute, Queen Mary, University of London, London, E1 1BB, UK
ABSTRACT: Bone is an anisotropic material, and its mechanical properties are determined by its microstructure as well as its composition. Mechanical properties of bone are a consequence of the proportions of, and the interactions between, mineral, collagen and water. Water plays an important role in maintaining the mechanical integrity of the composite, but the manner in which water interacts within the ultrastructure is unclear. Dentine being an isotropic two-dimensional structure presents a homogenous composite to examine the dehydration effects. Nanoindentation methods for determining the viscoelastic properties have recently been developed and are a subject of great interest. Here, one method based on elastic-viscoelastic correspondence for ‘ramp and hold’ creep testing (Oyen, J. Mater. Res., 2005) has been used to analyze viscoelastic behavior of polymeric and biological materials. The method of ‘ramp and hold’ allows the shear modulus at time zero to be determined from fitting of the displacement during the maximum load hold. Changes in the viscoelastic properties of bone and dentine were examined as the material was systematically dehydrated in a series of water:solvent mixes. Samples of equine dentine were sectioned and cryo-polished. Shear modulus was obtained by nanoindentation using spherical indenters with a maximum load hold of 120s. Samples were tested in different solvent concentrations sequentially, 70% ethanol to 50% ethanol, 70 % ethanol to 100% ethanol, 70% ethanol to 70% methanol to 100% methanol, and 70% ethanol to 100% acetone, after storage in each condition for 24h. By selectively removing and then replacing water from the composite, insights in to the ultrastructure of the tissue can be gained from the corresponding changes in the experimentally determined moduli, as well as an understanding of the complete reversibility of the dehydration process. INTRODUCTION: Nanoindentation has become a leading technique for measuring mechanical behavior of a large variety of materials, including polymers and biological tissues. However, standard elastic (Hertzian contact [1]) or elastic-plastic (Oliver-Pharr [2]) analyses of indentation data obtained with either spherical or sharp (conical or pyramidal) indentation tips are inappropriate for materials exhibiting time-dependent mechanical behavior [3,4]. In many cases, direct measurement of the viscoelastic behavior of a polymer or tissue is desired, as opposed to techniques that seek to eliminate viscoelastic effects and isolate the e
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