INTRODUCTION - Nanoindentation with Biological Applications - page 16

cross-linkers. In addition, there has been significant recent interest in
materials that are purified from natural components. Gel materials are
quite common in this arena, including agarose gel, fibrin gel, and gelatin
(denatured collagen) gels.
Finally, biomimetic composite materials are being engineered to
resemble natural materials. Examples include bone-like materials made
from gelatin and hydroxyapatite 14 and nacre-like materials made from
polyelectrolytes and clays, producing the organic and inorganic layers
similar to those in the nacre. 8 Issues associated with the nanoindentation
of biomimetic materials are similar to those encountered in studies of the
material being mimicked, and as such there is no separate treatment of
these classes of biomimetic materials in this volume.
3. Nanoindentation
In nanoindentation testing, a probe is pressed into a material surface,
typically under load-control, and the load and displacement are
monitored during the full loading-unloading contact cycle. Following the
publication of the landmark paper by Oliver and Pharr in 1992, 15 a
straightforward mechanism has existed for the deconvolution of elastic
modulus ( E ) and hardness ( H ) of an elastic-plastic half-space based on
the indentation load-displacement ( P - h ) data. The Oliver-Pharr algorithm
is incorporated directly into the software shipping with most commercial
nanoindentation instruments such that automated, high-throughput
mechanical testing is facilitated. This actually introduces a problem, in
that the Oliver-Pharr procedure is not appropriate for materials that are
not characterized as an elastic-plastic half-space, 16 including essentially
all polymers and biological materials. Further complications arise when
there is substantial adhesion between the sample and the indenter tip, as
is often the case with “softer” materials including polymers and soft
tissues. As such, the values for E and H that are ejected automatically
from a commercial nanoindenter can be meaningless 16 in the context of
the materials of interest in this volume.
An additional issue arises with the interpretation of “hardness” data
from Vickers or nanoindentation tests of biological materials. Clearly the
parameter H has meaning, but it is in this context quite unlike in