INSTRUMENTATION AND EXPERIMENTATION - Nanoindentation with Biological Applications - page 48

2.3.2 . Adhesion
Actual adhesion of the probe to the biomaterial surface is initiated by
attractive interactions, and may be due to specific or nonspecific
interactions between the probe and sample surfaces. Under conditions of
adhesion, the contact load can be increased and the contact stress
essentially reduced due to increased probe-material contact area. Elastic
moduli can be computed directly from the adhesion force, according to
Johnson-Kendall-Roberts (JKR) theory (see Chapter 4 ). In synthetic
polymers, it has been shown that various means of calculating elastic
moduli (including the JKR theory and Oliver-Pharr method) give a wide
range of results. This indicates that the extent of adhesion between a
probe and a biomaterial remains a valid concern in the analysis of
mechanical properties, though the most accurate way to infer such
properties for sticky, compliant materials is a matter of current research.
2.4 . Experimental approaches
2.4.1. Calibration over appropriate signal range
For quantitatively accurate measurements, load and depth signals must
be calibrated over a range appropriate to the conditions of interest.
Although in certain commercial indenters the internal signal calibrations
are not readily accessible to the user, such full calibration is
recommended on at least an intermittent basis to confirm the stability of
such critical conversion factors. If direct calibration is not possible, the
next best alternative is a standardized series of experiments on a
reference sample. Interestingly, there is no clear standard for mechanical
properties of polymers and biological materials; this is in contrast to
standards for stiffer and harder materials, which tend to be fused silica
(stiff ) and aluminum (ductile). Several potential materials are currently
under consideration, including relatively stiff, amorphous polymers such
as polycarbonate and polystyrene, and relatively compliant elastomers.
As noted above, load as [N/V] and displacement as [m/V] are signal
transducer conversion factors that can be calibrated by the user via
methods such as weighing of calibrated masses placed on the load train,