ABSTRACT
On the other hand, the structural complexity in biological so tissues has made the mechanical properties of so tissue heterogeneous and anisotropic. If the mechanical properties of so tissues can be measured microscopically, then the relationship between function, structure and material properties can be understood in a better way, which would advance the basic research in disease pathology, tissue repair and tissue engineering. In certain situations, the dimension of the tissue, such as the articular cartilage in a mouse model, which has only a thickness of several hundred micrometers, is also limited. erefore, the spatial resolution of measurement should be increased correspondingly. In order to measure the mechanical properties of so tissues on a microscopic scale, the nanoindentation technique in material testing has also been introduced in this eld. Based on the dierence of scale for measurable deformation, the general concept of nanoindentation includes the instrumented indentation and the atomic force microscopy-enabled indentation. ey have dierent applications in dierent elds of biological measurement and have their own advantages and disadvantages in applications (Van Vliet 2011). In biological applications, the nanoindentation was rst used in the elasticity measurement of two mineralised hard tissues, i.e. bone (Rho et al. 1997; Turner et al. 1999; Ferguson et al. 2003; Haque 2003) and dental tissue (Habelitz et al. 2001; Cuy et al. 2002; Kinney et al. 2003; Angker and Swain 2006). ese two materials received broad application of nanoindentation because they, compared to so tissues, are more similar to traditional engineering materials such as metals or ceramics. erefore, the measurement range in force and deformation is comparable to that of engineering materials, and it is relatively easier to adopt the nanoindentation instrument for these two hard tissues. Furthermore, these two tissues have an obvious layered
structure (osteons and lamellae in bone; dentine, enamel and cement in dental tissue), which makes the measurement of localised elasticity more meaningful. However, a nanoindentation test of biological tissues is easily aected by a large number of factors, such as the condition of hydration, the surface preparation method (polished or unpolished), the storage medium, duration of storage and the loading frequency (static, quasi-static or dynamic) (Lewis and Nyman 2008). erefore, the method of nanoindentation of industrial materials should be modied for testing biological tissues. Based on successful experience in nanoindentation of hard tissues, it is further adopted for the measurement of the interface between hard and so tissues. Gupte et al. (2005) utilised nanoindentation to measure the elasticity at the interface of bone and calcied cartilage and its relationship with the content of mineralisation. It was found the relationship between stiness and mineral content was dierent for bone and the calci-ed cartilage. Among the various so tissues, cartilage has been one of the most frequently reported tissues where nanoindentation was used for the measurement of its mechanical properties (Ebenstein et al. 2004; Simha et al. 2004, 2007; Li et al. 2006; Franke et al. 2007, 2011). Nanoindentation has also been reported in the study of elasticity in the stratum corneum (Yuan and Verma 2006) and vascular tissues (Ebenstein et al. 2009). e factors aecting the nanoindentation measurement of so tissues are different from those in the case of hard tissues; for example, it is aected by the shape of the indenter and the adhesion between the indenter and so tissues (Ebenstein and Pruitt 2006), which will be introduced in Section 4.3.
