Mechanical Behavior and Properties of Adipose Tissue - The Mechanobiology of Obesity and Related Diseases - page 3

tissue and performed confined compression tests (creep) to study longterm and
short term behavior. They also found a strong strain rate dependency and went up
to strains of 10 %. Neither of these studies is comprehensive enough to derive a
general constitutive description of adipose tissue. All biological tissues change
properties with age and there is a large variability between species and between
different locations on the body. Other variables like body mass index or pathology
play a major role. But, apart from all this, there is also the observation that adipose
tissue has a strange micro-structure and behaves rather extraordinary. It is the latter
aspect that we like to focus on in this chapter. It is our opinion that for most
applications currently described in literature the very complex behavior of fat is
completely ignored, either because it is too complicated and not the focus of the
authors or, more likely, they are unaware of the problem. We think that ignoring
this behavior in many applications will lead to erroneous results, severally inter-
fering with the validity of conclusions. So our main objective of this chapter is to
create awareness for the ''exotic'' behavior of adipose tissue. All studies were
performed in shear. For a full description of the experimental method and tech-
niques used the reader is referred to the papers by Geerligs et al. [ 3 , 4 ].
The chapter comprises three main sections. The first section briefly describes
the used experimental method and the results in the linear visco-elastic regime.
The second section discusses the temporal shift in properties we found in long term
oscillatory strain tests. The third section is focused on the (anti)-thyxotropic
properties that were observed at large strains. The chapter ends with some con-
sequences of our observations for f.e. pressure ulcer research and problems related
to complications with obesity.

2 Linear Visco-elastic Behavior at Small Strains

All tests were done on porcine subcutaneous fat tissue obtained from a local
slaughterhouse. The middle layer of the back fat has a comparable structure as fat
from the abdominal region of humans [ 9 ]. Cylindrical samples with a diameter of
8 mm and a thickness of 1.5-2 mm were cut and tested in a rotational rheometer
with a controlled strain mode, parallel plate geometry and temperature and
humidity control. To prevent slip, the plates were sand-blasted and a small axial
compression force of 1 mN was applied. To find the linear visco-elastic region,
strain sweeps were applied from 0.04-10 % at different frequencies. Figure 1
shows that the storage and loss moduli are almost constant up to a strain of 0.2 %,
so it was decided to do the frequency sweeps at 0.1 %.
In this small strain regime the fat behavior could be described with linear visco-
elastic theory, as is illustrated by Fig. 2 , where the storage and loss moduli are
given as a function of frequency. The storage modulus is always a factor 5 higher
than the loss modulus (almost elastic material behavior). At an angular frequency

q

ð G 0 Þ 2 þð G 00 Þ 2 G d ¼ x = 10 rad/s the dynamic modulus was found to be