NANOINDENTATION OF BONE - Nanoindentation with Biological Applications - page 188

the resistance to plastic deformation via Vickers hardness, residual
indentation impressions are large and often extend far beyond fifty
micrometers in width. 40 Nanoindentation provides an advantage that
none of these techniques possess: the ability to avoid the influence of
bone's complicated three-dimensional geometry on measured properties
and to probe bone at the level of the tissue itself.
In order to understand bone's mechanical properties at multiple
scales, the basic structural building blocks must also be investigated.
Nanoindentation is the ideal tool as it allows testing of small volumes of
bone and excludes the effect of large-scale porosity on mechanical
property measurements. Additionally, nanoindentation can be used
to determine the nanomechanical properties of bone's various
microstructures ( e.g. , individual lamellae), which are on the order of
micrometers and are thus ideally suited for indentation testing.
Nanomechanical data provide insight into tissue-level alterations in bone
that are caused by factors including bone diseases, disuse or
weightlessness, and even fossilization.
At any scale, the mechanical response of bone depends heavily on
extrinsic factors that include preparation and preservation techniques,
sample geometry ( e.g. , testing a whole bone versus a machined section),
hydration state, and mechanical test type and parameters ( e.g. , strain
rate). Additionally, a wide variety of testing parameters such as loading
rate and indenter tip shape must be considered in order to extract a
specific set of mechanical properties. The Oliver-Pharr formulation used
to commonly extract elastic modulus from an indentation unloading
curve assumes that the test material is homogenous, isotropic, and elastic-
plastic. 41 Instead, bone is heterogeneous, anisotropic, and viscoelastic.
Caution must therefore be taken when testing and interpreting
nanoindentation results to understand bone's unique structural and
mechanical response.

2. Testing Methodologies and Considerations

Nanomechanical testing of bone necessitates the careful selection of test
parameters, well-controlled sample collection and preparation, and
consideration of alterations within the tissue before, during, and after