NANOINDENTATION OF SOFT TISSUES AND OTHER BIOLOGICAL MATERIALS - Nanoindentation with Biological Applications - page 285

in situ , with the cartilage surface supported by the underlying bone, and
the bone being mounted in the indentation apparatus. In addition, sample
hydration was maintained in all three cases, either through the use of
fluids-soaked gauze, 18 submerging the sample during the entire indent
process, 19 or keeping the sample fully submerged in a fluid cell until just
before indentation. 20 The choice of tip geometry varied among the
studies, with two groups using a 100 μm spherical tip 18,20 while the
cartilage repair study by Franke et al. used the Berkovich tip typically
used for stiff materials. 19 However, all three studies used fairly standard
nanoindentation techniques with only slight modifications to the
protocols used for traditional nanoindentation substrates. One of the key
modifications was the incorporation of the hold period at peak load to
allow time for creep to dissipate prior to unloading, a common method
for dealing with time-dependent behavior of polymers and soft tissues,
but a method which does not allow for rigorous biphasic poroelastic
or viscoelastic analysis. These and other methods for dealing with
hydration, tip selection, time-dependent properties, and other concerns
when indenting soft tissues will all be discussed in more detail in
Section 4.
While the three studies described above represent only slight
modifications to traditional quasi-static indentation analysis, the fourth
study developed an entire new model and methodology for indentation of
soft tissues. The goal of this study by Simha et al. 21 was to develop a
technique for measuring the fracture toughness of articular cartilage
using nanoindentation. A truncated diamond conical tip with a 67°
included angle and 10 micrometer end diameter was used in the study,
with the intent of penetrating the tissue surface rather than elastically
deforming it. A method was developed to detect penetration into the
surface using power as an indicator, and only indents that exhibited
penetration were analyzed. Penetration depths ranged from 100-250
micrometers, much deeper than indents typically performed with
nanoindenters (<1 m), so the authors called the technique

“micropenetration”. 21 The predicted fracture toughness from indentation
testing was not significantly different from the values measured in
macroscopic notch tests, and penetration depths predicted by the

μ