NANOINDENTATION OF BONE - Nanoindentation with Biological Applications - page 208

wet tissues may be limited by the physical set-up of the indentation
system, where some instruments lend themselves better to testing in
fluid than others, and the challenge of maintaining stable surface
characteristics over time. 43,64,85 The selection and validation of an
appropriate storage solution and associated conditions, for example, is
critical in preventing the appearance of a softer layer on the bulk sample,
as was seen by Bushby et al. (2004) 43 or altered near-surface properties
seen with storage in solutions that cause mineral dissolution or
deposition and consequential alterations in nanomechanical behavior. 89,90
Finally, the surface roughness is greater in wet tissues due to difficulties
in obtaining a fine polish. 43,74 Overall, the value of testing in wet
conditions must be evaluated since obtaining high quality data, especially
at shallow contact depths, is full of many challenges.
2.2.6. Summary
Physiological conditions are ideal for testing any biological material.
Bone is no exception. Many researchers have attempted to determine the
relevance of properties of bone in vivo and live bone ex vivo . However
the benefits of testing under the ideal conditions must be weighed with
the practicality and the benefits of physiologically relevant testing,
versus the collection of high-quality data from well-controlled tests that
lack degrees of physiological relevance. On macroscopic samples,
mechanical testing of bone is often performed in an environment that
mimics critical physiological conditions of hydration, ionic solution,
and temperature. The accessibility of some nanoindentation systems to
controlling such variables is limited, but adaptations to the systems
can be made to enable testing under wet or temperature-controlled
conditions.
3. Nanoindentation Behavior of Bone
Conventional analytical techniques for nanoindentation assume an
elastic, isotropic, and homogenous material; yet bone fits none of these
criteria. Historically, bone nanoindentation results have been analyzed
using the Oliver-Pharr method using a Berkovich tip 41 or Hertzian