Biomedical Engineering Reference
In-Depth Information
sample, which can also lead to a false triggering of the setpoint. To
avoid accidental triggering of the setpoint by environmental factors, it
may be necessary to use a setpoint value even higher than the default,
which will in turn lead to a more substantial sink-in and associated
modulus calculation errors. Recently Kaufman
et al.
97
have looked at the
problems and potential solutions surrounding nanoindentation of very
compliant materials.
Methods to overcome the difficulty in surface
detection under hydrated conditions include using manual methods of
surface detection such as intentionally starting indents out of contact with
the sample or using continuous stiffness measurements to more
sensitively detect the surface.
69,72
However, to fully resolve the issue of
surface detection and sink-in there is a clear need for a more sensitive
method of surface detection that is more suitable for soft tissue
applications.
4.2.2
. Large displacements
Because soft tissues are so compliant (
E
~ kPa or MPa), they exhibit
large displacement under very low loads. To accommodate these large
displacements, calibrations need to take into account any nonlinear
behavior of the transducers at large displacements. This can typically be
achieved by performing an indent in air (or the hydrating fluid) to the
maximum displacement anticipated during the indent process, and then
subtracting that control curve off the experimental curve.
25
Most
commercial indentation systems have this type of calibration built into
their software, though you need to be sure to do the calibration in the
appropriate environment (
i.e
. air vs. fluid). When performing dynamic
testing at large displacements, additional calibrations may also be needed
to account for dynamic calibration constants that vary with transducer
displacement.
72
The impact of many challenges associated with indentation of soft
tissues, such as surface roughness, sink-in effects and adhesion, can be
minimized by performing deep indents in the material. In addition, the
high compliance of soft tissues can mean that a high stroke length is
necessary to observe the material's behavior at physiologically relevant
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