INSTRUMENTATION AND EXPERIMENTATION - Nanoindentation with Biological Applications - page 62

polymers to o-ring gasketed stages with injection inlets and outlets for
media exchange. In addition to the chemical stability of such fluid cells
that was noted above, it is also important to confirm the duration over
which the fluid cell is impervious to leaks (especially gradual wicking of
moisture toward electronic components) and to corrosion (given that
saline is often a component of biomaterial imaging fluids).

3.3. Selection of indenter probes

As distinct from the selection of instrumented indenter probes, the
mechanical stiffness of the AFM cantilevers strongly affects
experimental design. This stiffness directly controls the range of contact
forces and indentation depths attainable over the linear range of the
position sensitive photodiode that monitors cantilever free-end
deflection. Consider a typical contact-mode Si 3 N 4 V-shaped cantilever of
spring constant k = 0.1 N/m and an associated OLS (for a particular
AFM) of 100 nm/V. For a photodiode that is maximized at an output
voltage of 10 V, the maximum free-end cantilever deflection that can be
measured is 1 μm, and thus the maximum force that can be imposed on
the biomaterial surface is 0.1 μN or 100 nN. This, of course, assumes
that the entire range of the photodiode is linearized.
If one needs to impose more force than allowed by this cantilever,
one would choose a stiffer cantilever geometry. As will be discussed in
Section 3.5, the choice of cantilever stiffness is further complicated by
the fact that indentation with a cantilever that is much stiffer than the
biomaterial sample results in penetration without deflection, and with
one that is much more compliant results in deflection without
penetration.
There is much more limited choice of indenter probe geometry in
AFM-enabled indentation. Essentially, there exist the as-fabricated
pyramidal cones of Si and Si 3 N 4 cantilevers, terminating in radii of
~25 nm; colloidal spheres of m-scale radius, comprising glass or

polystyrene beads that are glued to tipless cantilevers, and diamond
probes glued or brazed to stiff, tipless cantilevers. These diamond probes
are typically from the same vendors who supply the instrumented

μ