NANOINDENTATION OF BONE - Nanoindentation with Biological Applications - page 212

Site-specific measurements of material properties can be collected
on a scale (<1 micron) that is relevant to the tissue-level organization
of bone. Structural features at the tissue-level are commonly at least
several microns in width and ideally suited for nanoindentation testing
(Section 4.3 below). However, the power of nanoindentation in studying
bone's heterogeneity comes from the ease at sampling many different
sites and combining mechanical measurements with adjunctive
techniques.
Nanoindentation within a heterogeneous material enables statistical
sampling of variability within the tissue's properties. Statistical sampling
can be used to predict composition-based mechanical performance. For
example, a large grid of indents on a well-characterized composite
material, i.e. titanium-titanium monoboride (Ti-TiB), allowed for
identification of the morphological arrangement and volumetric
proportions of multiple, discrete, mechanically dissimilar phases. 126 A
similar approach has been used to study bone and other mineralized
tissues, 50,127,128 however, the complexity of quantifying the contributions
of each component leaves more questions than answers. While some
basic models have been able to relate mineral content to mechanical
property variations, 42 more advanced paradigms are required to fully
incorporate all sources of heterogeneity within bone that include mineral
content, composition, and orientation, crystallinity, collagen content and
organization, porosity, and the level of hydration as dictated by the
chemical properties of bone's underlying extracellular matrix.
Depending on factors that include age, 129 type, 45,57,119,129-131 and
genetics or disease state, 50 the properties of bone can vary significantly.
Simple composites theory has been used to show that no simple
composites model can describe the relationship between the elastic
modulus and mineral content of bone. 132 Composite bounds, for
example, merely predict bounds on bone's elastic behavior. Although
many studies have shown a positive correlation between modulus and
mineral content, the relationship is not exact and is influenced by degree
of anisotropy, organization, and porosity. 133
Nanoindentation, to examine the tissue-level of bone, has helped to
better elucidate the relationship between modulus and mineral volume
fraction (mineral V f ). 127 Until recently, this relationship was only studied