Hydration effects on the micro-mechanical properties of bone
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A. Boyde Biophysics Section, Centre for Oral Growth and Development, Dental Institute, Queen Mary, University of London, London E1 1BB, United Kingdom
V.L. Ferguson Department of Mechanical Engineering, University of Colorado, Boulder, Colorado 80309
M.L. Oyenb) Center for Applied Biomechanics, Department of Mechanical and Aerospace Engineering, University of Virginia, Charlottesville, Virginia 22902 (Received 11 January 2006; accepted 15 February 2006)
Bone is a composite material with hydroxyapatite mineral, collagen, and water as primary components. Water plays an important role in maintaining the mechanical integrity of the composite, but the manner in which water interacts within the collagen and mineral at ultrastructural length-scales is poorly understood. The current study examined changes in the mechanical properties of bone as a function of hydration state. Tissues were soaked in different solvents and solutions, with different polarities, to manipulate tissue hydration. Mineralized bone was characterized using nanoindentation creep tests for quantification of both the elastic and viscoelastic mechanical responses, which varied dramatically with tissue bathing solution. The results were considered within the context of solution physical chemistry. Selectively removing and then replacing water provided insights into the ultrastructure of the tissues via the corresponding changes in the experimentally determined mechanical responses.
I. INTRODUCTION
Biological tissues have mechanical responses that are inherently time-dependent by virtue of the intrinsic hydration associated with physiological conditions. While the time-dependent response is dominant in soft tissues, it is also present in mineralized tissues such as bone. In mineralized tissues, the time dependence is often attributed to the hydrated collagen phase that comprises approximately half the tissue by volume.1 The mechanical response of bone can be altered by manipulating the tissue hydration state. Ethanol, a polar solvent, is commonly used to dehydrate biological tissues for histological preparation; water is more polar than most organic solvents. Polar molecules possess large dipole moments and large dielectric constants and are a)
Address all correspondence to this author. e-mail: [email protected] b) These authors were editors of this focus issue during the review and decision stage. For the JMR policy on review and publication of manuscripts authored by editors, please refer to http:// www.mrs.org/publications/jmr/policy.html. DOI: 10.1557/JMR.2006.0237 1962 J. Mater. Res., Vol. 21, No. 8, Aug 2006 http://journals.cambridge.org Downloaded: 07 Feb 2015
miscible with water. In previous work, samples of cortical bone beams tested using dynamic mechanical analysis (DMA) in three-point bending exhibited a 20% increase in the storage modulus with increasing dehydration in a graded series of ethanol baths.2 Dehydration and altered hydrogen bonding in the bone beams was the most likely cause of the increased stiffness. In related studies
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