NANOINDENTATION OF SOFT TISSUES AND OTHER BIOLOGICAL MATERIALS - Nanoindentation with Biological Applications - page 288

2.4 . Conclusions
These cartilage studies show the variety of research questions that can be
investigated through cartilage nanoindentation studies. Nanoindentation
studies of cartilage repair tissue in rabbit and minature pig knee models
illustrated the importance of using mechanical characterization in
conjunction with histology to demonstrate the functional behavior of
repair tissue. In addition, nanoindentation was successfully used to
investigate structure-property relationships in rabbit finger joint cartilage,
and to measure fracture properties in bovine knee cartilage. Most of these
studies could not have been performed using bulk mechanical testing
techniques due to the small size of the regions of interest in the tissue
samples, and hence demonstrate the value of having a tool that can
measure mechanical properties in soft tissues with high spatial resolution.
In addition, these cartilage studies reveal some valuable insights into
the interpretation of past studies and the development of future studies of
soft tissues using nanoindentation techniques. Some basic issues were
raised for future experimental design, such as the importance of selection
of appropriate tip geometries, tip dimensions and loading conditions to
measure physiologically relevant properties. In addition, the importance
of sample hydration and the choice of hydrating fluid were addressed.
Limitations associated with inhomogeneous tissue properties and
indenter tip size effects were also raised. Methods for dealing with
hydration, tip selection, time-dependent properties, substrate effects, and
other challenges when indenting soft tissues will be discussed in more
detail in Section 4.
Despite these ongoing challenges, one important insight into data
interpretation that can be gained by comparing these studies is that even
if the traditional quantitative parameters (modulus, hardness, etc.) are
not physically meaningful for every experimental set-up, they or other
functional parameters can still provide a valuable qualitative measure
of relative mechanical properties. Often, this relative assessment is
sufficient to answer research questions such as “Does the repair cartilage
have the same mechanical behavior as healthy cartilage?” Material
parameters that can translate to the macroscale are not always necessary.
While these qualitative comparisons are effective within studies,