INSTRUMENTATION AND EXPERIMENTATION - Nanoindentation with Biological Applications - page 52

2.5.2. Limiting biodegradation rates

If biological materials are hydrated in full or in part, it is common for
these materials to denature or degrade over time. This is due to the
presence of enzymes that act on proteins or nucleic acids in natural
degradation processes. As these structural changes will modify the
mechanical properties of such samples, it is optimal to limit the rate of
degradation during mechanical analysis. This can be achieved by
lowering the sample temperature below room temperature (4°C), as is
often done in other analyses of biological materials and processes, or by
limiting the number or activation of proteolytic enzymes. Alternatively
and least optimally, one could attempt to minimize degradation simply
by analyzing the biological material for short intervals at room
temperature, followed by storage in saline conditions at lower
temperature (4°C). To minimize degradation of the biomaterial samples
over the course of experimental analysis, RNAase and proteinase
neutralizers can be added to the bathing solution.

2.5.3 . Compatible adhesives

Given that secure mounting of biological samples is a requirement of
accurate measurement, care must be taken to ensure that any adhesives
used are both minimally compliant and also stable under the physical
environment of test (e.g., immersion in saline). Cyanoacrylate adhesive
meets the first requirement, in that such thermoplastic adhesives of low
viscosity (<5 cP) wet and spread easily to contribute a maximum
thickness of ~50 m. Based on the mechanical properties of such

adhesives ( E = 4 GPa), we can estimate the maximum compliance due to
compression of such a linear elastic layer to be 2 nm/N. 5 Although this is
a sufficiently low compliance for most testing, this adhesive is not stable
over months in submerged aqueous environments. Other adhesives have
been developed especially for sustained adhesion of hydrated cells and
tissues, such as BD Cell-Tak™ (BD Biosciences, Franklin Lakes, NJ).
These adhesives are of similar compliance, but considerably more
expensive than cyanoacrylate. The chief advantage of such proprietary
formulations is that proteinaceous biomaterials adhere well to including

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