Nanoindentation of Bone in a Physiological Environment
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Nanoindentation of Bone in a Physiological Environment R. Akhtar, S. Morse, P.M. Mummery School of Materials, The University of Manchester, Manchester, M1 7HS, United Kingdom ABSTRACT Nanoindentation has been established as an effective method to measure the mechanical properties of bone tissue at the micron and sub-micron length scale. Although it is welldocumented that the mechanical properties of macroscopic bone specimens vary depending on whether the samples are tested dry or wet, nanoindentation is generally conducted on dehydrated bone tissue at room temperature, primarily because nanoindentation systems are extremely sensitive to changes in environmental conditions such as humidity and temperature. In this study, these problems were overcome by using a specially constructed liquid cell with an extension piece that allowed the indenter tip to be submerged under 5 mm of liquid. The custom setup was used to test cortical bovine bone and cancellous human bone specimens in three distinct conditions – dehydrated, rehydrated in simulated body fluid (SBF) at 20°C, and rehydrated in SBF at 37.5°C. A heating element with a temperature control unit was used to test at 37.5°C. The hardness and elastic modulus of the bone samples were found to decrease when dry specimens were rehydrated and tested in physiological conditions. It is suggested that nanoindentation in physiological conditions gives a better estimate of the mechanical properties of the microstructural components of bone in vivo rather than nanoindentation under conventional conditions. INTRODUCTION Nanoindentation has been used for almost a decade to determine the mechanical properties of small volumes of bone tissue. The technique has been found to be particularly useful because it can give a spatial resolution which is better than 1 µm [1], and thus can be used to measure the mechanical properties of individual bone structural units such as single osteons and individual trabeculae directly. However, the results obtained from nanoindentation by various researchers have been found to be consistently higher than those obtained from other test methods [2]. For example, the modulus of individual trabeculae obtained by microbending where specimens were kept moist has been measured to be 5.4 GPa [3] whereas the value determined by nanoindentation was 13.4 GPa [1]. Such large differences in measured values may partially be due to the effect of drying bone prior to testing [1]. It is well-documented that when bone is dehydrated it becomes much more brittle and less tough, and has a lower strain at fracture [4], and because water is one of the major components of bone [5], its role in the mechanical properties of bone cannot be neglected. The importance of indenting hydrated bone has been acknowledged by most researchers [1,2]; however, due to experimental complications, the majority of nanoindentation studies to date have been conducted on dehydrated specimens. Nanoindentation is an extremely sensitive technique and difficulties arise when attempting
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