CONCLUSIONS AND OUTLOOK - Nanoindentation with Biological Applications - page 345

AFMs are far advanced compared with commercially available indenters.
However, the cantilever-based loading mechanism introduces challenges
for time-dependent characterization, due to a lack of direct closed-loop
control of the load or position of the the indenter tip.
Instrumentation challenges are only one factor preventing the
widespread adoption of nanoindentation techniques in the context of
soft tissues. Instrument manufacturers appear reluctant to introduce
options for testing and data analysis into the software driving current
commercial nanoindentation systems. Clearly the widespread adoption of
nanoindentation techniques for analysis of engineering materials was
driven by the incorporation of Oliver-Pharr elastic-plastic analysis into
indenter software. Mechanical testing underwent a change almost
overnight, as samples could be inserted into what was almost a “black
box” mechanical property meter, where mechanical tests could be
performed and data analyzed with just a few clicks of a mouse. This has
resulted in the widespread reporting of the “hardness-and-modulus” as if
these were the relevant mechanical properties for all materials. Such an
approach assumes that there is a one-size-fits-all testing method for
nanoindentation and that an elastic-plastic analysis is appropriate in
all cases.
We therefore encounter the first significant challenge that must be
addressed prior to the routine implementation of nanoindentation testing
for mechanical analysis of all biological materials: one or more
commercial instruments must be developed, including both hardware and
software modifications compared with current commercial systems,
meant to address the needs of the biological materials community. It is
useful to note at this point that there are compliant nonbiological
materials for which similar testing capabilities would be useful, such as
polymer foams, elastomers and hydrogels (Fig. 1-11), and the market for
such instruments would be larger than just the biological materials
community. As emerging areas of biomedical research develop, the need
will emerge for both researchers and medical device developers to
mechanically test materials with compliances comparable to biological
soft tissues, such as hydrogel coatings and tissue-engineered implants.