INSTRUMENTATION AND EXPERIMENTATION - Nanoindentation with Biological Applications - page 45

also be important to regulate the gaseous environment. This can be
achieved readily via gas inlets and outlets in all commercial indenters.
However, care must be taken to either seal the sample chamber well or to
monitor the actual gas content of the chamber if gas content is an
important variable. As a rule, the smaller the environmental chamber
surrounding the sample and indenter, the easier it is to maintain known
environmental parameters. In the particular case of gaseous content, the
gas flow must be sufficiently slow and laminar that acoustic and
mechanical noise are not created within the chamber. As a result,
stabilization of a gaseous environment also typically requires several
hours of lead time prior to data acquisition.
2.2.3 . Fluid cells for hydrated biological materials
Many biomaterials require immersion in aqueous environments (or at
least high humidity environments) to maintain structure and concurrent
mechanical properties. Chambers that maintain sample submersion can
be readily constructed, but peculiarities of the instrumented indenter load
train must be considered carefully. For example, a typical instrumented
indenter displaces the tip vertically into the sample surface; if the
indenter shaft were to plunge through an air-water interface to contact a
submerged sample, a meniscus will form and move readily along the
shaft perimeter. The associated surface tension of the water will deflect
the indenter shaft, indicating a load that is due not to sample stiffness but
to this moving meniscus. If properly calibrated and minimized compared
to the indentation loads of interest, such artifacts can be accounted for
during data interpretation. Alternatively, a horizontal orientation of the
pendulum-based system eliminates the moving meniscus, as the extended
indenter can remain fully submerged during sample contact and
indentation loading ( Fig. 3-1 ) . Constantinides et al. have shown that such
a horizontal fluid cell design enables measurement of mechanical
properties for hydrogels and animal tissues over durations of observation
on the order of 24 hrs.
Note that immersion of the biomaterial also requires consideration of
fixation to the underlying substrate, such that the biomaterial should not
float (if it is oriented horizontally) or sag (if it is oriented vertically).