fetal membranes, amnion and chorion, form a ~0.5 mm thin bilayer that serves as
barrier and container throughout gestation. Rupture of
membranes is part of the natural sequence of events for
term delivery, but has serious complications when it
occurs prior to term; spontaneous preterm premature rupture (sPPROM) before 37
weeks of gestation in the absence of labor affects 1-4% of pregnancies and
accounts for 30-40% of preterm deliveries [18p].
In addition, introduction of needles or fetoscopes into the
intrauterine cavity for diagnostic or operative purposes pose a significant risk
for persisting membrane leakage and subsequent membrane failure (iatrogenic
Understanding the deformation and rupture behavior of fetal membranes and the
structural response of the membrane bilayer is a necessary foundation for
attempts to prevent or to repair PPROM. We perform uniaxial and biaxial
experiments on fetal membranes in order to determine constitutive model
parameters, and to examine their relation to molecular correlates for mechanical
Different mechanical tests are applied to evaluate the deformation behavior of
human fetal membranes under different loading conditions. The test results are
analyzed within the realm of nonlinear continuum mechanics and representative
scalar parameters are extracted. The experimental observations serve as basis
for the development of suitable constitutive model formulation.
Biochemical assays are used for the determination of the amount of
microstructural constituents, i.e. collagen, elastin and collagen cross-links.
Comparison of these data with the corresponding parameters from mechanical tests
enables the evaluation of the correlation between mechanical behavior and
Our data show an inverse proportionality between uniaxial
small strain stiffness and the elastin content within a uniaxial stress state [18p].
For a biaxial state of stress we found that the collagen content and its
cross-links correlate with high strain stiffness as well as membrane strength
Mechanical response in different kinematic configurations is considered for
selection of corresponding constitutive model formulations. Information on the
architecture of the material, stiff collagen fibers embedded in a soft
extracellular matrix, can be included in the model formulation [Holzapfel et al,
J Elasticity 2000].
Phenomenological models are used for numerical simulations , for example to
study and analyze different experimental setups or to investigate and optimize
the puncture procedure of amniocentesis (to minimize the risk of PPROM).
Prof. Zimmermann, USZ, Obstetrics and Gynecology Dr. Martin Ehrbar, USZ,
Obstetrics and Gynecology
MD Nicole Ochsenbein, USZ, Obstetrics and Gynecology