INSTRUMENTATION AND EXPERIMENTATION - Nanoindentation with Biological Applications - page 43

loading. Alternatively, direct measurement of C f can be achieved via
fully elastic contact between a flat punch version of the indenter and the
stage surface, provided that normal contact can be achieved to eliminate
air gaps and compression of surface asperities at the indenter-stage
interface. The frame compliance will depend in part of the details of
the instrumented indenter mount and other load train components.
As expected, the significance of the frame compliance to inferred
mechanical properties is nonlinear and depends on the stiffness of the
material and maximum loads applied. For a hydrogel case above, the
additional frame deflection is clearly negligible (10 -7 nm); for the bone
the additional deflection due to frame compliance can be more
substantial.
2.2 . Adapations required of environmental control
One of the major differences between instrumented indentation of
synthetic vs. biological materials is that the latter are typically stable only
in non-ambient environments. For example, hydrogels require immersion
in water to maintain the structure that confers the low elastic moduli of
these materials; tissues rapidly denature if not maintained at a near-
neutral pH in aqueous media that is devoid of degrading proteolytic
enzymes; and cells do not maintain the molecular adhesions to
underlying surfaces unless specific levels of CO 2 and metal ions in the
surrounding media are maintained at approximately body temperature
(37 ο C). In all of these cases, the physical environment and signal stability
of instrumented indenters require special consideration.
2.2.1. Temperature and associated thermal drift
For biomaterials that exist or will be used at body temperature, it is
relatively straightforward to elevate the testing or sample temperature.
This is achieved differently in various commercial indenters, either by
heating the entire indenter enclosure to achieve a stable operating
temperature at which P-h data is collected, by heating only the sample
via devices such as a thermoelectric or Peltier stage, or by resistively