Biomedical Engineering Reference
In-Depth Information
system to correct the applied load or displacement signal to the
transducer to compensate for losses in the system. When used correctly,
indents collected using a trapezoidal load function should show a
perfectly horizontal load-displacement behavior during the hold period,
while indents collected using a trapezoidal displacement profile should
show a perfectly vertical load-displacement behavior during the hold
period. Using closed-loop load- or displacement-control is essential for
performing creep or load-relaxation studies, as it decouples system losses
from displacement or load due to time-dependent properties. In addition,
closed-loop control should eliminate negative loads during unloading in
the absence of adhesion.
4.2.4 . Drift
A common method used to monitor drift in indentation tests involves
holding the tip on the sample surface with a small preload force and
monitoring the change in displacement. It is assumed that any change in
displacement is due to thermal drift. However, in compliant time-
dependent materials the displacement observed while holding on the
surface is a combination of thermal drift, material creep, and sink-in that
cannot be easily separated. Automatic “drift correction” options on
commercial instruments are therefore inaccurate when indenting time-
dependent biological materials, and should not be used. It is likely that
for experiments run over short periods of time ( i.e. less than 1 minute)
the displacement of the indenter tip due to creep will be much higher
than the thermal drift rate, so thermal drift can be considered negligible.
Thermal drift may become a more significant issue when performing
load-relaxation or creep tests to characterize time-dependent properties,
as indent times can be much longer than 1 minute and thermal drift may
no longer be negligible. In addition, some indentation systems use data
collected from the thermal drift monitoring method described above to
set up the feedback loop for displacement- or load-control, which can
make it difficult to achieve accurate displacement-control in time-
dependent biological materials. Clearly this is an issue that will need to
be addressed in future instrumentation development to ensure accurate
characterization of time-dependent biological materials.
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