NANOINDENTATION OF BONE - Nanoindentation with Biological Applications - page 214

bound) and compliant particles in a continuous stiff phase (upper
bound). 134 In contrast to bone, site-matched measurements of indentation
modulus and mineral volume fraction for dental enamel fall outside of
the H-S bounds but lie entirely within the Voigt-Reuss bounds
(unpublished data).
Elastic indentation modulus increases as a function of mineral volume
fraction (as calculated from calibrated quantitative backscattered electron
analysis at each indentation site). 50,127 The heterogeneity in factors that
include the material composition ( i.e. , the makeup of the mineral phase)
and crystallinity, collagen orientation, and micrometer or nanometer-
scale porosity result in the diffuse relationship that exists between
modulus and mineral V f throughout the range of bone types. Further,
variance in measurements of modulus and mineralization decreases in
tissues containing collagen fibrils that are, in general, uniformly well-
aligned such as articular calcified cartilage 50 and in regions of uniform
enamel prism orientation. 49
3.3. Viscoelasticity
Bone behaves in a time-dependent manner due in part to its organic
phase 135,136 and its water content with a corresponding poroelastic flow
through the bone material. 5 Viscoelasticity in bone has been shown to
correlate with hydration state 47,135,137 and mineral content. 136 Historically,
the general approach for nanoindentation of bone has been to employ a
range of techniques to eliminate creep effects, rather than to complicate
nanoindentation data analysis with time-dependent behavior. A long
creep hold at maximum load can eliminate time-dependent behavior on
unloading and thus permits measurement of elastic properties. 138,139
Rapid unloading following no hold at peak load enables determination of
the unrelaxed modulus. 140 A third method eliminates viscoelastic effects
by correcting the unloading stiffness 41 with the creep rate and contact
depth during creep (measured during a hold at max load). 141,142
Rather than eliminate viscous effects during indentation, more recent
works have embraced nanoindentation as a tool to explore the viscous
behavior of bone. The use of indentation to study the elastic and viscous
responses of biological materials is well established. Further, while