NANOINDENTATION OF SOFT TISSUES AND OTHER BIOLOGICAL MATERIALS - Nanoindentation with Biological Applications - page 307

4.3 . Data acquisition and analysis
The challenges and concerns outlined in the previous sections focused on
issues relating to sample and tip preparation for indentation, and areas for
development of improved instrumentation. The unique properties of soft
tissues, namely their time-dependent properties, the high amounts of
adhesion sometimes observed between the tip and sample during
indentation of soft tissues, and their anisotropy, are also leading to the
development of new data acquisition and analysis methods for
nanoindentation. Since the models behind time-dependent properties and
adhesion are presented in other chapters, the following sections will
focus on specific methods proposed or used for indentation of biological
materials to address the issues of time-dependent properties, adhesion,
and anisotropy.
4.3.1 . Time-dependent properties
All soft tissues and most biological materials exhibit time-dependent
properties due to their water content and protein-based structures. Time-
dependent mechanical behavior of soft tissues can be analyzed using
viscoelastic or biphasic poroelastic models in bulk testing. However, in
the field of nanoindentation, the majority of tissue studies have not used
rigorous viscoelastic or biphasic poroelastic analysis to characterize
time-dependent behavior. 7 Instead, traditional quasi-static analysis
methods are used except that a trapezoidal load function (load-hold-
unload) replaces the typical triangular (load-unload) load function. The
hold period allows time for creep to dissipate prior to unloading, and
then the unloading curve can be analyzed using the traditional
compliance method. 73 This approach has been utilized in the majority of
indentation studies of biological materials, with hold periods ranging
from 3 to 120 seconds. 18-20,22,24,27,50,53,58,67,74 Because thermal drift and
other constraints may limit the practical hold period time that can be
imposed during indentation, other researchers have developed post-hoc
methods to correct trapezoidal indentation data to remove any remaining
influence of creep. 75-78 These numerical correction methods have been
applied to recent studies of bone 79,80 and teeth, 81 revealing substantial
error in modulus calculations when the corrections were not used.