INSTRUMENTATION AND EXPERIMENTATION - Nanoindentation with Biological Applications - page 59

A further advantage of these probes, as compared to diamond
conospherical probes of comparable radii, is that these probe materials
are easily modified with biochemical moieties to enhance or mitigate
specific interactions between the probe and the biomaterial. These AFM
cantilevered probes can even be used within commercial instrumented
indenters, simply by mounting the fractured cantilever to the indenter
shaft. In this case, the cantilever “legs” are essentially handles to position
a well defined, spherical probe and achieve loads and loading profiles
typical of instrumented indenters. 9 Figure 3-4 illustrates this application
for the particular case of an instrumented indenter mounted with a
borosilicate sphere of m-scale radius.

The second approach is to modify the AFM in order to maintain
nanoscale imaging resolution via cantilever scanning, while achieving
nanoindentation loading via a separate device more similar to
instrumented indenters. For example, some commercial AFMs include an
indentation module that actuates diamond indenters within the same
mount that holds the AFM cantilever, but is comparably rigid and
enables normal approach to the sample surface. The compromise in such
designs is that the exquisite load resolution of AFM cantilevers (pN) is
sacrificed.
Contact creep compliance and stress relaxation cannot be inferred
reliably from AFM contact experiments with biological samples, due to
the control variables inherent in AFM contact. For example, contact
creep compliance requires that contact load is maintained constant for an
appreciable duration while contact depth increases. This condition
requires that cantilever free-end deflection is maintained constant and, as
the cantilever base position is the signal in feedback, would require an
additional calculated feedback loop to attain. Additionally, the signal
drift in AFMs can be tens to hundreds of nanometers per second, owing
to the sensitivity of these cantilevers to thermal fluctuations; this can
exceed the expected range of depth changes during creep or stress
relaxation experiments. In short, for such experiments it is strongly
preferable to use instrumented indentation capable of either true load or
displacement control at the point of contact.

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