INSTRUMENTATION AND EXPERIMENTATION - Nanoindentation with Biological Applications - page 46

Standard adhesives such as cyanocrylate may not be stable in submerged,
saline environments typical of tissue analysis, but biomedically
specialized adhesives such as Cell-Tak™ can be employed. In the case of
vertically oriented, hydrated samples, both continuum simulations and
experimental observations indicate that the buoyancy of such samples
minimizes any surface deflections over immersion for at least 24 hrs
( Fig. 3-1 ) .
Figure 3-1. (A) Indentation of porcine skin within aqueous fluid cell. Probe is maintained
at a constant submersion depth during indentation. Meniscus shown for reference of fluid
level. (B) Quasistatic force-depth response of skin changes appreciably over several
hours of sustained hydration, due ostensibly to tissue degradation. Courtesy J. F. Smith,
G. Constantinides, Z. I. Kalcioglu, and K. J. Van Vliet.
2.3. Indenter probe materials
Although biological materials tend to be more mechanically compliant
than the metals and ceramics for which diamond indenters were
originally developed, the choice of indenter material is not as simple as
selecting any material which is mechanically stiffer and harder than the
biomaterial sample. If that were the case, polystyerene spheres could be
used to measure the mechanical response of soft tissues, and glass
spheres or cones could be considered in the analysis of mineralized
collagen. Rather, the chemical functionality of the probe surface is an
increasingly important consideration in the mechanical analysis of
biomaterial surfaces, which often exhibit appreciable surface charge and
nonspecific adhesion. These interactions can effect unintended probe
interactions during approach and retraction from the surface.