Biomedical Engineering Reference
In-Depth Information
rubbers ( E ~ 4 MPa) have occasionally been used to calibrate larger
diameter tips for applications in more compliant polymers and soft
tissues. 21 However, there is currently no industry-accepted standard for
calibrating tips to depths on the order of micrometers. A common
alternative to tip calibration for blunt spherical tips is the use of an ideal
shape function (
A
(
h
)
= π
(
2
Rh
h
2
)
, where h is the penetration depth
and R is the radius of curvature of the tip) to infer contact areas. 18,20,50
This method has the advantage that it relies only on knowledge of the
radius of the tip, which can be determined under a microscope or using
the nominal value supplied by the manufacturer. However, it also
assumes a perfect tip of known constant radius with no surface defects.
For flat punch tips, which have a constant contact area with depth, no
calibration is needed.
4.2 . Instrumentation limitations
The challenges and concerns outlined in the previous section focused on
issues relating to sample and tip preparation for indentation. The high
compliance and time-dependent behavior of soft tissue samples leads to
additional challenges in adapting commercial instrumentation to perform
indents in these complex materials. The next few sections will discuss
particular instrumentation issues that have complicated soft tissue
indentation, including automated methods of surface detection, nonlinear
spring stiffness in the transducer at large displacements, limited
displacement ranges, open-loop load- or displacement-control, and
thermal drift. In addition, the potential errors introduced to indentation
data due to the use of the standard instrumentation will be discussed, as
well as critical areas for future instrumentation development. Hardware
and software vary from commercial instrument to commercial instrument,
and hence instrumentation issues will vary from instrument to instrument.
The critique in the next few sections was written based on experience
with one particular indentation system, which uses a capacitive
transducer for both load application and displacement monitoring.
However, use of other commercial instruments may yield similar
limitations so care should be taken to examine how the instrument
functions.
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