Size effects in indentation of hydrated biological tissues
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Matteo Galli Department of Cambridge University Engineering, Cambridge CB2 1PZ, United Kingdom; and Laboratoire de Mécanique Appliqée et d’Analyse de Fiabilité, École Polytechnique Fédérale Lausanne, Lausanne 1015, Switzerland (Received 5 May 2011; accepted 31 August 2011)
Fluid flow in biological tissues is important in both mechanical and biological contexts. Given the hierarchical nature of tissues, there are varying length scales at which time-dependent mechanical behavior due to fluid flow may be exhibited. Here, spherical nanoindentation and microindentation testings are used for the characterization of length scale effects in the mechanical response of hydrated tissues. Although elastic properties were consistent across length scales, there was a substantial difference between the time-dependent mechanical responses for large and small contact radii in the same tissue specimens. This difference was far more obvious when poroelastic analysis was used instead of viscoelastic analysis. Overall, indentation testing is a fast and robust technique for characterizing the hierarchical structure of biological materials from nanometer to micrometer length scales and is capable of making quantitative material property measurements to do with fluid flow. I. INTRODUCTION
Biological tissues, including bone and articular cartilage, are multiphase materials with a porous “solid” skeleton and a hydrating fluid phase. Natural tissues are hierarchically structured materials,1 exhibiting different structural and mechanical characteristics over a range of length scales.2 As a consequence of the varied structure, tissues exhibit fluid flow across different length scales3 and distinct levels of porosity can often be identified.4 The study of fluid flow in tissues during deformation is fundamental both to understand how natural tissue functions and to develop biomimetic materials for tissue repair and replacement.5,6 Fluid flow in tissues gives rise to time-dependent mechanical behavior, which has been considered both within viscoelastic7 and poroelastic4 mechanical frameworks. A poroelastic framework is a continuum mechanical description of the fluid–solid composite material8; while analytically more complicated than a linear viscoelastic framework, this approach has the advantage that material property values can be related directly to tissue microstructure, including physical pore size. Indentation is a common technique for testing the mechanical behavior of almost any material at length scales ranging from nanometers, for nanoindentation, to millimeters in the case of traditional indentation. In considering a)
Address all correspondence to this author. e-mail: [email protected] This author was an editor of this journal 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/jmr-editor-manuscripts/ DOI: 10.1557/jmr.2011.322 J. Mater. Res., Vol. 27, No. 1, Jan 14, 2012
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