Mechanical Characterization of the Human Liver - Computer Models in Biomechanics - page 361

which drastically improved the data quality and makes possible to repeat an identical
experiment between different measuring sites and subjects. Actually, the possibil-
ity to conduct locally-confined measurements independent of the far-field is a key
strength of the aspiration test. Unknown initial and boundary conditions is one of
the major issues with other invasive measurement techniques: for the analysis of
large indentation, for example, knowledge not only of the contact condition and the
indentation-depth (near-field) are required, but similarly the organ geometry and its
embedding in the abdomen (far-field) are relevant, but these latter are typically not
available. For comparison, the coefficients of variation of data reported in the litera-
ture for the liver are as follows: for indentation Zheng et al. ( 2000 ), Tay et al. ( 2006 )
and Samur et al. ( 2007 ) show data with coefficients of variation of 15 %, 20 %, and
29 %, while as to dynamic elastography Sandrin et al. ( 2003 ), Huwart et al. ( 2006 ),
and Rouvière et al. ( 2006 ) have coefficients of variation of 3 . 2 % (intra-operator
cv), 10 . 3 %, and 5 . 6 % (on a tissue phantom), respectively.
26.4.2 Mechanical Behavior of Glisson's Capsule
Conventional uniaxial tensile tests with simple inflation experiments were combined
to provide information on the multiaxial mechanical behavior of liver capsule. The
inflation test complements well the uniaxial tension test in that, for a homogeneous
and isotropic membrane, it subjects the membrane to different modes of distortion
from 'uniaxial strain' at the clamping interface to 'equibiaxial strain' at the sam-
ple apex over a large strain range; these modes are all of high stress-biaxiality and
typical for physiologically relevant deformations of biological membranes. Thus
uniaxial tension and inflation test together characterize tissues response over a wide
region of the strain space.
Besides the physiologically relevant loading, advantages of the inflation test are
the easy sample preparation and test realization, and the well-defined boundary and
initial conditions. However, the test is restricted in the control of the test kinematics
and kinetics, the applicable nominal strain rate does usually not exceed 10 %/s, the
tension and strain values are typically derived from less-precise video extensome-
ter data and the total measurement of the distension pressure. Furthermore, sample
inhomogeneities, irregular deformations, and surface imperfections influence the in-
terpretation of the test.
Uniaxial and biaxial data were analyzed in order to determine corresponding
constitutive model equations able to describe both experiments. Good predictions
are obtained when the model formulation is based on the second invariant I 2 of
the right Cauchy-Green tensor. As illustrated in Fig. 26.9 , the invariant I 1 can be
interpreted as the average (squared) stretch of the sides of an infinitesimal volume
element. For an incompressible material I 2 is proportional to the average area stretch
of the faces of an infinitesimal volume element. It is thus interesting to note that
Glisson's capsule behaves as a I 2 material. This might be associated with a network
arrangement of collagen fibers in the capsule, leading to a response closer to the one