INSTRUMENTATION AND EXPERIMENTATION - Nanoindentation with Biological Applications - page 68

objective identification of the contact point P = h = 0. Various methods
have been posed in the literature to unambiguously choose the point of
actual probe-sample contact. 10,12 Automation of this step is considered
important in terms of mechanical property calculation accuracy and in
light of the large number of replicate experiments conducted for
structurally heterogeneous samples. 13
3.4.5. Dynamic loading
As in instrumented indentation, the AFM cantilever can be used to
cyclically contact and load the biomaterial surface. This is commonly
executed in AFM imaging of compliant material surfaces, and is then
called tapping or intermittent contact or AC mode imaging. In such
imaging, the probe scans the surface while maintaining a feedback loop
between user-specified cantilever free-end amplitude and the cantilever
base piezoactuation. A lock-in amplifier maintains a driving amplitude
that is dissipated and shifted in phase via probe interaction with the
surface. The error signal in such imaging is the actual free-end
amplitude, which differs from the user-specified amplitude due to more
rapid variations in mechanical energy dissipation across the surface than
can be accommodated by the gains of the feedback loop. Additionally,
the phase lag indicates energy dissipation at every pixel in the sample
surface. This type of pixel-resolved, qualitative and yet visual
representation of energy dissipation was the basis of the early work that
preceded dynamic mechanical analysis via instrumented indentation. 14
Despite the widespread use of this dynamic contact imaging to visualize
regions of disparate energy dissipation in structurally heterogeneous
nanocomposites including tissues and cells, there is little quantitative
interpretation of amplitude reduction and phase lag in terms of
mechanical properties. This is due primarily to the multiple, complex
interactions at the probe-biomaterial surface that dissipate contact
energy, in addition to the mechanical dissipation of biopolymeric phases.
Imaging biomaterials in aqueous environments via dynamic contact is
notororiously difficult, due to both the high mechanical compliance of
fully hydrated biomaterials and the considerable mechanical coupling
and noise associated with rapid oscillation of the microscale cantilever in