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 can be represented by the model without the need of recalibration and parameter identification.

A discrete network model to represent the deformation behavior of human amnion.

A discrete network model (DNM) to represent the mechanical behavior of the human amnion is proposed. The amnion is modeled as randomly distributed points interconnected with connector elements representing collagen crosslinks and fiber segments, respectively. This DNM is computationally efficient and allows simulations with large domains. A representative set of parameters has been selected to reproduce the uniaxial tension-stretch and kinematic responses of the amnion. Good agreement is found between the predicted and measured equibiaxial tension-stretch curves. Although the model represents the amnion phenomenologically, model parameters are physically motivated and their effect on the tension-stretch and in-plane kinematic responses is discussed. The model is used to investigate the local response in the near field of a circular hole, revealing that the kinematic response at the circular free boundaries leads to compaction and strong alignment of the network at the border of the defect.

Deformation mechanisms of human amnion: Quantitative studies based on second harmonic generation microscopy.

Multiphoton microscopy has proven to be a versatile tool to analyze the three-dimensional microstructure of the fetal membrane and the mechanisms of deformation on the length scale of cells and the collagen network. In the present contribution, dedicated microscopic tools for in situ mechanical characterization of tissue under applied mechanical loads and the related methods for data interpretation are presented with emphasis on new stepwise monotonic uniaxial experiments. The resulting microscopic parameters are consistent with previous ones quantified for cyclic and relaxation tests, underlining the reliability of these techniques. The thickness reduction and the substantial alignment of collagen fiber bundles in the compact and fibroblast layer starting at very small loads are highlighted, which challenges the definition of a reference configuration in terms of a force threshold. The findings presented in this paper intend to inform the development of models towards a better understanding of fetal membrane deformation and failure, and thus of related problems in obstetrics and other clinical conditions.

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.

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.

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.

Engineering of living autologous human umbilical cord cell-based septal occluder membranes using composite PGA-P4HB matrices.

Interventional closure of intracardiac wall defects using occluder devices has evolved as a highly attractive treatment option. However, incomplete and delayed healing reactions often result in a major risk of residual defects, thromboembolism, or device fractures. Biodegradable living tissue engineered occluder membranes (TEOMs) could provide autologous thromboresistant implants with growth and remodeling capacities. PGA-P4HB composite matrices were seeded with human umbilical cord-derived cells or vascular-derived control cells and exposed to static (n = 19) or dynamic (n = 13) conditioning. Harvested TEOMs were integrated into occluder frameworks, exposed to crimping and delivered into pre-formed defects of juvenile porcine hearts. Dynamically conditioned TEOM constructs showed higher collagen formation in histology than static constructs with significantly higher stiffness moduli in uniaxial tensile testing. Grating interferometry revealed substantial but inhomogeneous cone-like degradation of the composite matrices in dynamic conditioning. The crimping and delivery procedures resulted in no significant changes in macroscopy, histo-morphology, cellular viability, DNA or hydroxyproline content, and scanning electron microscopy findings. Here, we present the in vitro fabrication, crimping and experimental delivery of living human umbilical cord-cell derived TEOMs based on composite matrices as a potential future autologous therapy of intracardiac wall defects.