Biomedical Engineering Reference
In-Depth Information
Figure 9-4. Comparison of idealized load-displacement behavior for indents in
(A) traditional indentation samples using load function (a) in Chapter 5 , Fig. 5-2 , and
(B) soft tissue samples using loading function (b) in Chapter 5 , Fig. 5-2 . Soft tissues tend
to have larger displacements at lower applied loads, due to their lower elastic moduli
(E < 1 MPa vs. E >> 10 GPa). In addition, soft tissues typically exhibit time-dependent
behavior, which is often most easily visualized as a creep response, or a change in
displacement with a constant applied load. Adhesion can also complicate data collection
and analysis in soft tissues, and is most easily visualized as a measured negative load in a
load-displacement curve upon unloading. Note that creep and adhesion interpretation are
further complicated by data acquisition without closed-loop load- or displacement-
control. Under open-loop conditions, even when a “constant” load is applied to the
sample, part of that load goes into deforming the springs that support the indenter tip. At
large displacements, or in the presence of large amounts of creep, this is observed as a
change in load applied to the sample as a function of tip displacement during the hold
period in a trapezoidal load function. These open-loop effects can be observed both
during the hold period and after load is removed from the sample, and can also lead to a
negative load during unloading.
To introduce the challenges associated with indentation of soft tissues
using traditional indentation methods, consider the idealized load-
displacement curves for indents in traditional indentation samples ( e.g. ,
engineering materials with elastic moduli >> 10 GPa) and soft tissue
samples ( e.g ., compliant, hydrated samples with moduli < 1 MPa) using
a trapezoidal load function presented in Fig. 9-4 . Notice both the changes
in the shape and the magnitude of the load-displacement curves. As
illustrated in the figure, the sample compliance, adhesion and time-
dependent properties ( e.g ., creep behavior) account for some of the
 
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