NANOINDENTATION OF BONE - Nanoindentation with Biological Applications - page 196

multiple loading cycles to establish a repeatable loading history. 68 Over
successive loading cycles in soft tissues, the stress versus strain
responses shift to the right and eventually converge. 69 However,
preconditioning is seldom performed in large-scale mechanical testing of
bone as it would likely result in micro-fractures and other permanent
structural and material alterations prior to the onset of data collection.
Most nanoindentation studies of bone also avoid preconditioning.
However, preconditioning is considered here as several studies have
employed what amounts to a preconditioning protocol, through repeated
loading cycles at a single indentation site. Collection of nanomechanical
property measurements is then made on the final loading cycle. 61,66,70,71
In soft tissues, preconditioning is used to establish a reference state
that has a significant effect on the resulting calculations of stress and
strain. 68 In large or small-scale mechanical testing of bone, a reference
state is naturally set by the rigidity of the material through the
organization of collagen and mineral, three-dimensional tissue structure,
mineral content, and other factors. Specific to nanoindentation, the
reference state of a bone sample is set by the physical boundaries of the
half-space - that is, the prepared surface of the bone sample itself. It is
this generally stiff surface that provides a consistent definition of the
reference state for nanoindentation through a consistent reference point
for subsequent data calculations ( e.g. , indenter contact depth).
The effects of nanometer-scale preconditioning, or repeated loading,
on the material bone are unclear. Fan and Rho (2002) hypothesized that,
during a plastic indentation, the bone material underneath the indenter tip
undergoes radial expansion parallel to the surface and permanent
compression perpendicular to the surface. 56 It is not unreasonable to
surmise that preconditioning may compact the mineral crystals and, in a
hydrated specimen, force water out of the indented region prior to the
onset of data collection. While these phenomena have not been explicitly
considered in the literature, they likely occur and have a profound
influence on the measured nanomechanical properties. Given the
difficulties of standardizing preconditioning protocols in areas of
soft tissue research, 68 the implementation of such procedures for
nanoindentation of bone would also require standardization. Further, it is