Uniaxial and biaxial mechanical behavior of human amnion

Chorioamnion, the membrane surrounding a fetus during gestation (the "amniotic sac"), is a structural soft tissue for which the mechanical behavior is poorly understood-despite its critical role in maintaining a successful pregnancy and delivery. Preterm rupture of the chorioamnion accounts for one third of all premature births. The structural component of chorioamnion is the amnion sublayer, which provides the membrane's mechanical integrity via a dense collagen network. Amnion uniaxial and planar equi-biaxial tension testing was performed using monotonie loading, cyclic loading and stress-relaxation. The prefailure material behavior was highly nonlinear, exhibiting an approximately quadratic response. Cyclic testing, both uniaxial and biaxial, exhibited dramatic energy dissipation in the first cycle followed by less hysteresis on subsequent cycles and an eventual stable hysteresis response with approximately 20% energy dissipation per cycle. Stress-relaxation testing, both uniaxial and biaxial, demonstrated a load dependent response and continued relaxation after long hold times. A nonlinear viscoelastic (separable) hereditary integral approach was used to model the amnion stress-strain-time response during relaxation. The mechanical results are discussed within the context of the in vivo clinical performance of amnion, and the potential for membrane repair.