Biomedical Engineering Reference
In-Depth Information
visible defects. In this way, the underside of the capsule was prepared according to
the desired number of samples (Fig.
26.5
b). Subsequently, the capsule was turned
to its other side and sandpaper rings were glued onto the upside opposing the rings
on the underside. This procedure resulted in capsule samples sandwiched between
two sandpaper rings with their rough side facing away from the capsule. Finally, the
sandwiched samples were cut out with surgical scissors along the outer margin of
the sandpaper rings.
26.3 Results
A representative constitutive model for Glisson's capsule is required in order to
analyze the aspiration experiments on human liver. Correspondingly, the mechanical
characterization of liver capsule is presented first, Sect.
26.3.1
. The capsule model
is then used in the analysis of the sensitivity of the aspiration experiment described
in Sect.
26.3.2
. Next we present results from more than 200 measurements during
open surgery.
26.3.1 Mechanical Behavior of Glisson's Capsule
Uniaxial test results provided qualitatively similar results for human and bovine
samples. The characteristics of all stress-strain curves are consistent with the typ-
ical J-shaped behavior of soft tissues. They exhibit large scatter in the initial stiff-
ness and in the toe- and heel-part of the curves. This scatter can be considerably
reduced by increasing the reference preload. In contrast, the stiffness in the part
of the curves at larger strains is fairly consistent. The high-strain tangent stiffness
in uniaxial stress was evaluated with reference to the free gauge length at 0.2 N
preload, see Fig.
26.6
. The uniaxial mechanical data indicate that human capsule
is about a factor of 3 more compliant as compared to bovine tissue. Corresponding
microstructural analysis (Hollenstein,
2011
) yield a similar factor for the thickness
of these membranes, as well as for the collagen content, or the diameter of collagen
fibers bundles. The human tissue available was not sufficient in quantity for per-
forming corresponding inflation experiments. The observations from inflation tests
on bovine tissue were used as a basis for determination of a human capsule model,
scaled with the factor of 3.
A typical uniaxial and a typical equibiaxial tension-strain characteristic, as de-
rived from the uniaxial tension and the inflation test, are compared in Fig.
26.7
.To
this end, the equibiaxial characteristic was adjusted for the preload of the inflation
test to be consistent with the uniaxial tensile test in terms of pretension. The respec-
tive high strain slopes show that liver capsule is 2-3 times stiffer in an equibiaxial
than in an uniaxial state of stress; note that the uniaxial characteristic, in contrast
to the equibiaxial characteristic, is based on a preconditioned sample. Analytical
