NANOINDENTATION OF SOFT TISSUES AND OTHER BIOLOGICAL MATERIALS - Nanoindentation with Biological Applications - page 299

4.1.3 . Tip selection
When testing biological samples, the selection of the geometry and size
of tip will depend on the length scale of interest and the mechanical
properties being measured. For characterization of the mechanical
properties in stiffer, small-diameter biological materials, such as sponge
spicules and spider silk, sharp Berkovich (3-sided pyramidal) tips are
commonly used to provide high spatial resolution. 9,45 For fracture testing
of cartilage, flattened conical tips with small included angles and end
diameters were used to ensure penetration into the samples. 21 However,
for most soft tissue characterization, flat punch tips or large diameter
(> 20 μm) spherical tips are typically preferred. 7,18,20,24-28,50 These blunter
tips are preferred because the contact areas between the tip and the
sample are large enough to sample tissue level properties (larger than
individual cells and protein fibers), 7,22,50 and there is less danger of
puncturing or fracturing the sample. 22 In addition, the larger contact
area between the tip and the sample facilitates surface detection in
compliant materials where initial contact stiffnesses are quite small. Only
a few soft tissue studies have used Berkovich tips 19,27 due to concerns
about contact sizes, puncturing the surface, and surface detection. For an
illustration of contact sizes vs. cell sizes for different tip geometries, see
Fig. 9-5 .
The flat punch tip has the advantage of a known, constant contact
area independent of depth. This is particularly helpful if load-relaxation
or creep tests are being performed to measure time-dependent properties.
However, for this contact area to be valid, full contact is needed with the
surface. Hence, the flat punch is optimal only for very flat surfaces or at
deep depths, so that full contact can be ensured. An additional concern
associated with the use of the flat punch is that stress concentrations may
arise at the perimeter of the punch. Spherical tips, on the other hand, do
not generate stress concentrations and allow for some inaccuracy in the
approach to the sample since they will contact the surface with an orb
shape even if there is a slight tilt to the sample. In addition, many
adhesion models and other indentation methods have been developed for
the spherical tip, which can make this geometry advantageous. However,
the solutions to some indentation problems are more difficult for