INSTRUMENTATION AND EXPERIMENTATION - Nanoindentation with Biological Applications - page 67

3.4.4. Quasistatic load-depth hystereses
The deformation via AFM cantilever penetration is neither load- nor
indenter displacement-controlled, as the feedback loop only modulates
the displacement of the cantilever base over a specified time and extent.
Thus, load reversal at a specified cantilever base displacement will not
deform all materials to the same depth; and load reversal at a specified
cantilever free end deflection will deform all materials to the same
maximum load. Further, this immediately indicates that the material
is not deformed at a constant loading rate or a constant indenter
displacement rate. It is technically possible to calculate these quantities
on-the-fly and conduct signal feedback on either load or indenter probe
displacement in any commercial AFM via software modification.
However, owing to the historical application of AFM for imaging rather
than indentation, these feedback loops are not currently standard features
on commercial instrumentation. Thus, comparisons of biomaterial
response between AFM-enabled indentation and instrumented
indentation must bear in mind the differing load histories and control
variables, even if applied strains are comparable. This is particularly
important in consideration of nonlinear elastic and viscoelastoplastic
materials that comprise the majority of biological materials.
In typical experiments, the extent of piezoactuator displacement and
the time interval of that displacement are specified. The condition for
actuator reversal ( i.e ., unloading of the sample) may also specified,
which could be the default displacement of the cantilever base or
alternatively a limiting deflection (and thus force) attained on the sample
surface. The P-h responses appear qualitatively similar to those obtained
via instrumented indentation, but with important caveats. First, by default
data is acquired as soon as the piezoactuator displaces, resulting in
possible acquisition of ample data before and after the probe-sample
contact. From such traces, it is clear that drift in the deflection signal
[nm/s] can be comparable to displacement rates into the sample surface,
and are typically subtracted from the data post-test. Second, the high
force sensitivity of such cantilevered probes—sensitivity in terms of
deflection that may be due to surface charge and other interactions
besides mechanical contact—results in well-known difficulties in