Uniaxial stress-relaxation and stress-strain responses of human amnion.

The mechanical behavior of human amnion is examined under uniaxial tensile loading conditions. Monotonic strain-to-failure and stress-relaxation tests are described for membrane strip samples of amnion obtained by removing the chorion cell layer from specimens of whole chorioamnion. The monotonic behavior of the amnion is characterized by a large stress-free strain (approximately 10%) prior to a quadratic load-displacement response. Substantial stress relaxation behavior (ranging from 20-80%) is observed, described by a two time-constant exponential decay. The effects of the application of a topical antiseptic and of prior straining and relaxation on subsequent monotonic failure properties are examined. The results suggest that while amnion is a remarkably resilient tissue material, its mechanical behavior is typical of nonlinear viscoelastic materials, and depends strongly on its history.