NANOINDENTATION OF BONE - Nanoindentation with Biological Applications - page 215

nanoindentation was initially developed to study stiff, elastic-plastic
engineering materials, its use for examining polymers is becoming
widespread. 46,138,143-146
Early work on the viscoelasticity of dry bone, included in a non-
infiltrating epoxy resin, examined the role of loading rates or holding
times (at max applied load). 66 The use of various loading rates (with a
constant 180 s creep hold) demonstrated that elastic modulus was
overestimated at 'fast' indentation rates and that an extended hold time
(at least 30 s) was required to eliminate the time-dependent plastic effect.
Additional testing on dry bone specimens used a viscoelastic correction
procedure 141 to produce a corrected elastic modulus. Both of these
studies utilized a pointed, Berkovich tip and employed a multiple loading
protocol at each indentation site prior to collecting data. The pointed tip
may complicate the interpretation of the bone material's response by
causing an immediate plastic deformation upon loading. 46 Single loading
indentations made into a bone-dental implant interface using a pointed
tip, analyzed using a viscous-elastic-plastic model, 147 produced modulus
and hardness results that were comparable to those obtained by using
traditional elastic-plastic analysis techniques. Further, the variations
in the local resistance to plastic deformation dominated the overall
response.
It can be argued that a spherical indenter tip provides significant
advantages in probing bone - a material possessing elastic, plastic, and
viscous behavior. Several studies utilizing a spherical tip have evaluated
the viscous behavior of bone via creep testing following a finite-rate
ramp loading. 47,48,148 Analytical expressions, based on curve-fits to the
load-time or displacement-time data during a creep hold at maximum
load, were used to calculate measurements of the creep or stress
relaxation functions. This approach was used to examine the role of
hydration in the viscous behavior of bone, where cortical bone soaked in
varying concentrations of ethanol demonstrated a loss of viscosity with
increasing dehydration. 48 Similarly, bone soaked in solvents possessing
varying degrees of polarity (and Hansen's solubility parameter for
Hydrogen bonding) revealed a similar dependence on hydration and a
distinct role of hydrogen bonding in their viscous response. 47 While
elastic stiffness, as measured by the values of time-zero shear modulus