Mechanical Behavior and Properties of Adipose Tissue - The Mechanobiology of Obesity and Related Diseases - page 5

a harmonic excitation with a shear strain of 0.001 and an angular frequency of
10 rad/s for a period of at least 45 min. All samples showed a rapid increase of
roughly 1.5-15 kPa in the dynamic modulus G d after a period of almost constant
stiffness. This rapid change usually occurred between 250-1,200 s after the start of
the harmonic excitation. By repeating the time sweeps after several hours of rest it
became clear that this stiffness change is a reversible effect. Figure 3 is a typical
result. In the first experiment G 0 rises from 1.5 to 10 kPa in a few minutes after a
period of cyclic loading of 500 s. After 3 h of rest G 0 has returned back to the
original value of around 1.5-2.0 kPa and again after 400 s the quick rise in G 0

is

found.
The rapid increase in stiffness points in the direction of a structural change in
the material, which is reversible, but it is not clear what kind of change this is. It is
also not clear, if this phenomenon that is measured in vitro can also be observed
in vivo. All 13 samples that were tested showed more or less the same behavior,
although the magnitude of the stiffness change and the time at which it took place
differed.

4 Large Strain Behavior of Adipose Tissue

Geerligs et al. [ 4 ] performed a series of tests to study the large strain mechanical
behavior of adipose tissues, but for the present chapter we focus on one test, which
is the test with constant shear rate but with increasing total shear strain. The
protocol consisted of range of loading/unloading cycles with constant shear rate
(1 s -1 ) and an increasing amplitude from 0.01 up to 0.5. In between the cycles a
recovery time of 100 s was used.
The shear strain as a function of time is given in Fig. 4 a. Figure 4 b illustrates
that three different response regimes can be distinguished. Up to a strain of 0.15
the loading curves overlap more or less and the loading/unloading curves can be
described with Mooney-Rivlin type models. Geerligs et al. [ 4 ] performed several
other types of tests in this regime up to 15 %, i.e. relaxation tests and tests with
different strain rates. All these tests gave highly reproducible results and the
behavior was consistent with nonlinear visco-elastic models previously published
on other biological materials [ 7 , 19 ].
When the shear strain is higher than 0.15 the stress decreases in each following
loading cycle, pointing in the direction of a structural change in the material (see
Fig. 4 b). However, at a strain above 0.3 the stress/strain curves start to overlap
again. So apparently there is a structural change, but for strains higher than 0.3
there is no more progression in this structural change. Similar experiments going
from the high strain range to the low range showed that after a sufficient recovery
time of the tissue specimen the fat recovers to its original behavior.
This kind of reversible structural change that is strain induced is described in
literature as (anti-) thixotropic behavior depending on whether the viscosity