NANOINDENTATION OF SOFT TISSUES AND OTHER BIOLOGICAL MATERIALS - Nanoindentation with Biological Applications - page 286

indentation depth were not significantly different from the values
measured in post-hoc histological analysis. 21
This fracture-toughness study differs largely from the previous
studies in that it develops a new experimental approach to characterizing
a biological material, rather than simply trying to adapt nanoindentation
techniques developed for stiff, elastic engineering materials to compliant,
time-dependent materials. This is a path that should be pursued more in
the future if quantitative materials characterization, with parameters
comparable to macroscale properties, are desired. The results of this
paper also present some interesting implications for future soft tissue
studies. The ability to penetrate the cartilage surface using a blunted
indenter tip raises some concerns over the use of sharp ( i.e. small
included angle conical, cube-corner, or Berkovich) indenter tips to indent
soft tissues. The authors noted that they chose the truncated cone because
it was difficult to detect when penetration occurred using an unflattened
60° included angle conical tip. 21 Presumably, however, penetration
would occur more easily with a sharp tip than with the blunter tip used in
this study. Hence, in future studies, care should be taken when
considering the use of sharp tips to indent soft tissues, particularly if
large loads or displacements are used.
The final cartilage indentation study, from the same group, highlights
several challenges associated with nanoindentation testing of cartilage, 22
many of which will apply to other biological materials as well. The goal
of the study was to investigate the effect of indenter size on the elastic
modulus of cartilage measured using nanoindentation techniques. In this
study, cartilage samples were indented using a series of flat-ended 60°
cone and cylindrical indenter tips with end diameters ranging from 5 μm
to 4 mm. An elastic model was used to analyze each load-displacement
curve and calculate an equilibrium modulus. The study revealed that
there was a size effect when indenting cartilage, with the equilibrium
modulus decreasing as the tip diameter increased. 22 The authors
speculated that the inhomogeneous material properties of cartilage, such
as variations in tissue organization and composition with depth in the
tissue or surface characteristics, could have been the source of the size
effect. 22 However, no direct correlations were investigated in this study.