NANOINDENTATION OF BONE - Nanoindentation with Biological Applications - page 189

indentation testing is performed. A key concern lies in maintaining strict
control during cutting, polishing, and storage to avoid unintentional
modifications of the bone material. Factors ranging from alterations
within the tissue itself to changes at the sample's surface are of critical
importance. Although alterations in bone with preparation and storage
have been addressed for testing bulk, macroscopic-scale specimens, little
literature is available regarding the tissue-level changes that would likely
affect bone's nanomechanical response.
2.1. Indentation test parameters
Every aspect of the nanoindentation test can have an effect on the
measured mechanical properties. Nanoindentation of bone has been used
by numerous investigators with a large range of testing procedures and
sample preparation methods (key published data are shown in Table 7-1 ) .
Tip selection, loading rate, maximum load and other variables can play a
significant role in the type of data that can be extracted from the test.
This section provides an overview of nanoindentation test parameters as
they relate to and influence the measured properties of bone tissue.
2.1.1. Tip geometry
The selection of the optimal tip geometry is critical for obtaining high
quality indentation data. The biggest limitation in tip selection lies in our
limited understanding of what occurs within the collagen, mineral and
pore spaces within the bone material, at nanometer length scales, during
an indentation event. For example, plastic deformation under an indenter
tip likely results in compaction of the mineral platelets, possible
generation of microcracks within the material of bone, pore collapse, and
permanent displacement of water (for bone in a hydrated state). 42 The
mechanics of how each tip contacts and indents into the bone material is
therefore of critical importance and should be considered when
determining the experimental approach to testing elastic, “plastic”, or
viscous behaviors. However, nanoindentation measurements in bone
sample a three-dimensional volume of material, where Berkovich and
spherical tips produce roughly comparable values of modulus for bone