NANOINDENTATION OF SOFT TISSUES AND OTHER BIOLOGICAL MATERIALS - Nanoindentation with Biological Applications - page 308

In recent years, more rigorous viscoelastic models have been
developed that measure the time-dependent behavior rather than allowing
it to dissipate. 82 Many of these methods are reviewed in Chapter 5 .
Applications of viscoelastic models to biological tissues include stress-
relaxation analysis of arteries 26 and demineralized dentin 28 , dynamic
indentation of the stratus corneum layer of skin, 27 and mapping of
dynamic mechanical properties in spider silk cross-sections. 9 More
complex viscous-elastic-plastic models have been applied to the study of
bone. 48,49,82 Biphasic poroelastic models that they take into account
both fluid flow and the deformation of the solid matrix seem like the
most promising future direction for characterization hydrated, compliant
biological materials, and have recently been used for analysis of
nanoindentation data. 98 Because biphasic poroelastic tests typically
involve long hold periods to allow fluid flow to reach equilibrium and
evaluate matrix behavior, the challenges of thermal drift correction will
need to be overcome to develop reliable poroelastic analysis methods.
However, since most biological materials exhibit time-dependent
properties, the continued development and application of explicit
viscoelastic or poroelastic analysis techniques is likely to be central to
future nanoindentation studies of biological materials.
4.3.2 . Adhesion
In addition to time-dependent properties, many biological materials
have a high surface energy, and adhesion between the tip and the sample
can interfere with measurements of modulus using the compliance
method. 63,83-85 Adhesion can sometimes be observed as the load falling
below zero during the unloading portion of the load-displacement
curve, as shown in the cartilage indents in Fig. 9-3 and the idealized soft
tissue load-displacement curve in Fig. 9-4 . (Note that negative loads can
also result from a combination of open-loop control and creep, as
illustrated in Fig. 9-4 and described in Section 2.4.3. So care must be
taken in interpreting negative loads observed in open-loop load-
displacement curves, as one or both factors may be playing a role.)
Recent studies of soft polymers (silicone elastomers) have demonstrated
that the compliance method overestimates the sample modulus when