INSTRUMENTATION AND EXPERIMENTATION - Nanoindentation with Biological Applications - page 44

heating both the sample and the indenter to eliminate any quenching at
the probe-sample interface.
In all cases, it is critical that the increased temperature does not result
in significantly increased rate of change of the output signals ( i.e .,
thermal drift), and that this thermal drift is known. As in the case of
contact creep compliance, these drift rates can be measured via sustained
contact loading of rigid surfaces. However, it is always preferable to
modify the instrument or the operating condition to minimize drift in
such signals by minimizing heat differentials and fluxes on the
electronics, rather than to subtract a supposedly constant drift function
h drift ( t ) or P drift ( t ) from the experimental data. Theoretically, this can be
achieved via heating of the entire apparatus to a uniform temperature, but
this can be difficult due to the size of these instruments. Alternatively,
the electronic components can be physically shielded from the heated
components ( e.g ., the sample stage). As long is care is taken to avoid
construction of effective chimneys that lead to thermal drafts around the
instrument, this solution has routinely demonstrated electronic drift
levels that are comparable to room temperature signal stability.
Expansion of all heated components would be expected due to finite
thermal expansion coefficients of these materials, so the time required to
achieve a stable testing temperature is considerably greater than for
ambient experiments. Typically, one allows 12-24 hrs to achieve thermal
stability of the system inclusive of the sample. This pause can be reduced
if load train components are machined from materials of low thermal
expansion coefficients. When the sample must also be maintained in a
hydrated state and may be prone to proteolytic degradation, this extended
pause requires special care to maintain biomaterial integrity. Finally, the
elevated temperatures of testing are necessarily transmitted through
the indenter probe, such that attachment of the probe to the indenter
holder within the load train requires either mechanical fixture or
high-temperature adhesives.
2.2.2 . Controlled gaseous atmosphere
As the structure and mechanical properties of many biomaterials can be
sensitive to %CO 2 , oxygen tension, and ambient air composition, it may