Biaxial stress-strain properties of canine pericardium

Interpretation of the material properties of biological tissues under biaxial loading conditions has been hampered by wide variability in the coefficients. This variability has been attributed to experimental noise, numerical instability of the fitting algorithms, or to history dependence of the tissue. We have recently described quantitative methods for addressing some of these difficulties in interpretation. This study describes the application of these methods to anterior sheets of mongrel canine pericardium studied in vitro. Each specimen was stretched biaxially with three different combinations of strains and the resulting stress-strain relationships were fitted with a 5-parameter pseudo strain-energy function. Results show that each specimen is highly nonlinear, anisotropic, and history dependent. In addition there was wide inter-specimen variability in the material coefficients. In some cases the direction of the anisotropy was also history dependent. Possible explanations for this unexpected behaviour are discussed.