Quantifying nonlinear anisotropic elastic material properties of biological tissue by use of membrane inflation.

Determination of material parameters for soft tissue frequently involves regression of material parameters for nonlinear, anisotropic constitutive models against experimental data from heterogeneous tests. Here, parameter estimation based on membrane inflation is considered. A four parameter nonlinear, anisotropic hyperelastic strain energy function was used to model the material, in which the parameters are cast in terms of key response features. The experiment was simulated using finite element (FE) analysis in order to predict the experimental measurements of pressure versus profile strain. Material parameter regression was automated using inverse FE analysis; parameter values were updated by use of both local and global techniques, and the ability of these techniques to efficiently converge to a best case was examined. This approach provides a framework in which additional experimental data, including surface strain measurements or local structural information, may be incorporated in order to quantify heterogeneous nonlinear material properties.

Stress and strain in rat pulmonary artery material during a biaxial bubble test.

A biaxial bubble test has been designed to ascertain the mechanical properties of rat pulmonary arteries. The analytical procedure used to estimate stress and strain from the resulting test data is presented along with some analytical results. The bubble test was performed by loading a flat piece of rat pulmonary artery into a test fixture beneath a circular opening; the material was subsequently pressurized from below, producing a "bubble" of deformed material. Due to the anisotropy of the rat pulmonary artery, the resulting bubble was ellipsoidal in shape. Test results were recorded in the form of side-view images taken from various angles at incremental values of pressure. Average strains were estimated with the use of image analysis to measure changes in the bubble perimeter during inflation. Formulations for isotropic materials were applied to estimate stresses based on the anisotropic geometry of the bubbles produced during testing; some results of this preliminary analysis are presented here. Results from this analysis show differences in mechanical properties of the rat pulmonary artery from those of healthy versus hypertensive rats.