Time-dependent mechanical behavior of human amnion: Macroscopic and microscopic characterization

Characterizing the mechanical response of the human amnion is essential to understand and to eventually prevent premature rupture of fetal membranes. In this study, a large set of macroscopic and microscopic mechanical tests have been carried out on fresh unfixed amnion to gain insight into the time-dependent material response and the underlying mechanisms. Creep and relaxation responses of amnion were characterized in macroscopic uniaxial tension, biaxial tension and inflation configurations. For the first time, these experiments were complemented by microstructural information from nonlinear laser scanning microscopy performed during in situ uniaxial relaxation tests. The amnion showed large tension reduction during relaxation and small inelastic strain accumulation in creep. The short-term relaxation response was related to a concomitant in-plane and out-of-plane contraction, and was dependent on the testing configuration. The microscopic investigation revealed a large volume reduction at the beginning, but no change of volume was measured long-term during relaxation. Tension–strain curves normalized with respect to the maximum strain were highly repeatable in all configurations and allowed the quantification of corresponding characteristic parameters. The present data indicate that dissipative behavior of human amnion is related to two mechanisms: (i) volume reduction due to water outflow (up to ∼20 s) and (ii) long-term dissipative behavior without macroscopic deformation and no systematic global reorientation of collagen fibers

A model for the compressible, viscoelastic behavior of human amnion addressing tissue variability through a single parameter

A viscoelastic, compressible model is proposed to rationalize the recently reported response of human amnion in multiaxial relaxation and creep experiments. The theory includes two viscoelastic contributions responsible for the short- and long-term time- dependent response of the material. These two contributions can be related to physical processes: water flow through the tissue and dissipative characteristics of the collagen fibers, respectively. An accurate agreement of the model with the mean tension and kinematic response of amnion in uniaxial relaxation tests was achieved. By variation of a single linear factor that accounts for the variability among tissue samples, the model provides very sound predictions not only of the uniaxial relaxation but also of the uniaxial creep and strip-biaxial relaxation behavior of individual samples. This suggests that a wide range of viscoelastic behaviors due to patient-specific variations in tissue composition

Mechanical and microstructural investigation of the cyclic behavior of human amnion

The structural and mechanical integrity of amnion is essential to prevent preterm premature rupture (PPROM) of the fetal membrane. In this study, the mechanical response of human amnion to repeated loading and the microstructural mechanisms determining its behavior were investigated. Inflation and uniaxial cyclic tests were combined with corresponding in situ experiments in a multiphoton microscope (MPM). Fresh unfixed amnion was imaged during loading and changes in thickness and collagen orientation were quantified. Mechanical and in situ experiments revealed differences between the investigated configurations in the deformation and microstructural mechanisms. Repeated inflation induces a significant but reversible volume change and is characterized by high energy dissipation. Under uniaxial tension, volume reduction is associated with low energy, unrecoverable in-plane fiber reorientation

Analysis of the uniaxial and multiaxial mechanical response of a tissue-engineered vascular graft

Tissue engineering is aimed at the fabrication of autologous cardiovascular implants, for example, heart valves or vascular grafts. To date, the mechanical characterization of tissue-engineered vascular grafts (TEVGs) has focused mainly on the material's strength and not on the deformation behavior. A total of 31 samples obtained from 3 mature grafts (out of the cells of a single donor) were tested in uniaxial stress and uniaxial strain configurations to characterize their stiffness under uniaxial and biaxial stress states, respectively. Corresponding measurements were carried out on samples of an ovine artery. A physiological stiffness parameter was defined for data analysis and the uniaxial and multiaxial response compared, also in terms of anisotropy. The tension-strain curve of uniaxial stress tests is highly nonlinear, whereas the results show a more gradual deformation response of the material under a uniaxial strain configuration, which better represents the physiological state of biaxial stress. Stiffness parameters and anisotropy factors are significantly influenced by the selection of the testing configuration. Tangent stiffness of a TEVG at physiological loading conditions is significantly (p<0.05) higher for uniaxial stress as compared to uniaxial strain. The same is observed for the ovine tissue. The anisotropy of the scaffold is shown to partially transfer to the mature TEVG. The results of this study show that for a TEVG characterization, a physiological biaxial testing configuration should be preferred to the commonly used uniaxial stress