NANOINDENTATION OF SOFT TISSUES AND OTHER BIOLOGICAL MATERIALS - Nanoindentation with Biological Applications - page 296

changes in load-displacement behavior. However, instrumentation issues,
such as open-loop indentation vs closed-loop load- or displacement-
control also influence the behavior of soft tissues. Hence, it is not
surprising that adaptations to traditional nanoindentation techniques have
focused on both data collection and instrumentation concerns.
This section will highlight some of the major concerns and identified
challenges in sample and tip preparation, instrumentation, data
acquisition and analysis, and data interpretation that have been identified
as traditional nanoindentation techniques have been adapted to the study
of soft tissues and other biological materials. Where relevant, examples
of some common approaches to addressing these concerns will also be
provided. Soft tissues will be the primary focus of this section, since soft
tissue samples show the most extreme differences from traditional
indentation substrates and hence represent the most challenging materials
to test. However, subsets of these concerns and challenges apply to each
type of biological material presented in this topic.
Many of the techniques developed to adapt nanoindentation
instrumentation and methodologies to the study of biological materials
were derived from studies on polymers and hydrogels. Hydrogels in
particular have similar properties to soft tissues, including high
compliance, hydration, and time-dependent mechanical properties.
However, they have the advantage of being more homogeneous than soft
tissues, and can be produced in large, relatively uniform quantities,
which makes them an optimal control material for development of
nanoindentation protocols for soft tissues.
4.1 . Sample and tip preparation
In preparing for a nanoindentation experiment, there are many challenges
associated with the use of soft tissues as sample materials. These include
both sample preparation issues, such as methods to maintain sample
hydration and prepare the sample surface for indentation, and tip
preparation issues, such as selecting an appropriate tip geometry and
calibrating the tip area function. The challenges and concerns related to
these aspects of sample and tip preparation, as well as approaches that