On the deformation behavior of human amnion

Recently renewed interest for the mechanical behavior of fetal membranes is related to the problem of iatrogenic preterm rupture, limiting the effectiveness and applicability of minimally invasive fetal surgery. This study aimed at characterizing and modeling the deformation behavior of the amnion layer, the highly deformable and tough membrane that surrounds the amniotic fluid and the growing fetus in the uterine cavity. Uniaxial tension tests have been performed on samples obtained immediately after cesarean section, and the deformation field has been analyzed by digital image correlation. The results show that the kinematic response of human amnion is highly reproducible and that the incremental Poisson's ratio is, with a value of up to 8, higher than any previously reported value for biological or synthetic materials. This unique behavior is related to the characteristic architecture of amnion's microstructure and can be rationalized by mechanisms of rotation, stretching and buckling of collagen fibers. Simple constitutive equations have been selected based on this interpretation, which lead to a model with excellent predictive capabilities for the uniaxial and equibiaxial mechanical response of human amnion. Relevant insights were gained on the role of collagen fibers in determining the deformability and toughness of soft biological tissue.