INSTRUMENTATION AND EXPERIMENTATION - Nanoindentation with Biological Applications - page 51

2.4.2.3. Roughness
As most models of contact loading assume normal contact of the probe
with respect to the sample surface, surface roughness of calibration-
standard materials would be minimized. One possible exception would
be in calibration of experiments for which the biomaterials are inherently
rough and this roughness confers the mechanical property of interest, as
in the case of adhesion of patterned elastomers. 6
2.5. Sample mounting and preparation
Mounting of nonbiological samples to indentation stages typically
involves coarse clamping via vacuum pressure, cyanoacrylate adhesives,
or high-temperature cements. In such cases, the goal is to minimize the
mechanical compliance due to the adhesive, while maintaining a
stationary sample position over the course of testing. This requirement
remains in analysis of biological samples, but the presence of hydration
and the organic nature of such materials constrains the methods of
sample preparation.
2.5.1. Extent of hydration
Most biological material samples exist at superambient states of
humidity and hydration. To the extent possible, the physiological degree
of hydration, salinity, and overall ion content of the biological solutions
surrounding the biomaterial in vivo should be recapitulated in
experiments. This is because the structure of these materials is
maintained by such hydration, and is responsible for the mechanical
properties of the biomaterial. For biological materials such as soft tissues
that are fully hydrated in vivo , it is optimal to conduct instrumented
indentation experiments within a fluid cell that is filled with solution of
appropriate ionic strength and pH. For biological materials that are in
humidified but not necessarily submerged environments, it is optimal to
maintain relative humidity feedback and avoid condensation onto
electronic components.