NANOINDENTATION OF SOFT TISSUES AND OTHER BIOLOGICAL MATERIALS - Nanoindentation with Biological Applications - page 301

the mechanical properties measured through indentation, leading to
the development of correction factors that compensate for this
influence. 12,14,17 Although rigorous studies of nanoindentation substrate
effects in thin layers of soft tissues have not been performed, it is
expected that the substrate will affect nanoindentation measurements
when indenting soft tissues with large tips or deep penetration depths.
Hence, this is an area that should be pursued in future studies.

4.1.4 . Tip calibration

For traditional nanoindentation testing of engineering materials, the
standard tip calibration procedure is to perform a series of indents at
different loads in a fused silica sample of known modulus. Since fused
silica is presumed to have a constant elastic modulus of 72 GPa and
Poisson's ratio of 0.16-0.18 as a function of depth, analysis of the load-
displacement curves can be coupled with knowledge of the mechanical
properties of the fused silica sample to extract a tip area function as
a function of penetration depth (based on elastic analysis via the
compliance method 6 ). At a peak load of 10 mN, the highest value for
some commercial nanoindentation systems, the penetration depth of a
sharp Berkovich tip is typically between 300 and 400 nm. When
indenting in fused silica with a blunt tip, such as a large diameter
spherical tip, the penetration depth will be much smaller. However,
indents in compliant materials like soft tissues typically involve
penetration depths on the order of micrometers, not nanometers. Hence,
the traditional method of tip calibration does not calibrate the tip to
sufficient penetration depths to be valid for soft tissue characterization, at
least not using a sample as stiff as fused silica.
For this reason, researchers in the areas of polymer and soft tissue
indentation have been seeking improved tip calibration materials and
methods. Since soft tissues have moduli on the order of kPa to MPa, it
has been difficult to identify a suitably compliant calibration material
that does not have time-dependent or depth-dependent properties, and
has repeatable behavior and minimal sample to sample variability.
Polycarbonate ( E ~ 3 GPa) has been used to calibrate 20 m radius of

curvature spherical tips for some stiff polymer studies, 63 while urethane

μ