Bioprosthetic heart valves still have poor long‐term durability due to calcification and mechanical failure. The function and performance of bioprostheses is known to depend on the collagen architecture and mechanical behavior of the target tissue. So it is necessary to select an appropriate tissue for such prostheses. In this study, porcine, equine, and bovine pericardia were compared histologically and mechanically. The specimens were analyzed under light microscopy. The planar biaxial tests were performed on the tissue samples by applying synchronic loads along the axial (fiber direction) and perpendicular directions. The measured biaxial data were then fitted into both the modified Mooney‐Rivlin model and the anisotropic four parameter Fung‐type model. The modified Mooney‐Rivlin model was applied to the modeling of the bovine, equine, and porcine pericardia using finite element analysis. The equine pericardium illustrated a wavy collagen bundle architecture similar to bovine pericardium, whereas the collagen bundles in the porcine pericardium were thinner and structured. Wavy pericardia may be preferable candidates for transcutaneous aortic valves because they are less likely to be delaminated during crimping. Based on the biaxial tensile test, the specimens indicated some degree of anisotropy; the anisotropy rates of the equine specimens were almost identical, and higher than the other two specimens. In general, porcine pericardium appeared stiffer, based on the greater strain energy magnitude and the average slope of the stress–stretch curves. Moreover, it was less distensible (due to lower areal strain) than the other two pericardial tissues. Furthermore, the porcine model induced localized high stress regions during the systolic and diastolic phases of the cardiac cycle. However, increased mechanical stress on the bioprosthetic leaflets may cause tissue degeneration and reduce the long‐term durability of the valve. Based on our observations, the pericardial specimens behaved as anisotropic and nonlinear tissues—well‐characterized by both the modified Mooney‐Rivlin and the Fung‐type models. The results indicate that, compared to bovine pericardium, equine tissue is mechanically and histologically more appropriate for manufacturing heart valve prostheses. The results of this study can be used in the design and manufacture of bioprosthetic heart valves.

The benefit of bioprosthetic aortic valve over mechanical valve replacements is the release of thromboembolism and digression of long-term anticoagulation treatment. The function of bioprostheses and their efficiency is known to depend on the mechanical properties of the leaflet tissue. So it is necessary to select a suitable tissue for the bioprosthesis. The purpose of the present study is to clarify the viscoelastic behavior of bovine, equine, and porcine pericardium. In this study, pericardiums were compared mechanically from the viscoelastic aspect. After fixation of the tissues in glutaraldehyde, first uniaxial tests with different extension rates in the fiber direction were performed. Then, the stress relaxation tests in the fiber direction were done on these pericardial tissues by exerting 20, 30,40, and 50% strains. After evaluation of viscoelastic linearity, the Prony series, quasilinear viscoelastic (QLV) and modified superposition theory were applied to the stress relaxation data. Finally, the parameters of these constitutive models were extracted for each pericardium tissue. All three tissues exhibited a decrease in relaxation rate with elevating strain, indicating the nonlinear viscoelastic behavior of these tissues. The three-term Prony model was selected for describing the linear viscoelasticity. Among different models, the QLV model was best able to capture the relaxation behavior of the pericardium tissues. More stiffness of porcine pericardium was observed in comparison to the two other pericardium tissues. The relaxation percentage of porcine pericardium was less than the two others. It can be concluded that porcine pericardium behaves more as an elastic and less like a viscous tissue in comparison to the bovine and equine pericardium.

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Cardiovascular diseases are one of the major causes of death in the world and are closely related to blood dynamics and wall mechanical properties of vessels. This makes the study of mechanical properties of arteries and veins essential. In this regard, study on common vessels used in bypass grafting operations is of special importance due to the frequency of this surgery. Human saphenous vein is one of the vessels used for bypass surgeries. The objective of the present study is to characterize the behavior of human saphenous vein using uniaxial and biaxial planar tests. Forty human saphenous samples were obtained after coronary artery bypass surgery (CABG). The planar tensile tests were performed on the tissue specimens by applying loads along two directions. These tests provided the force-displacement curves. The stress-strain curves from uniaxial tests were modeled with a mathematical function and the Young modulus was obtained in both longitudinal and circumferential directions. Measured data of uniaxial tests were then fitted into a hyperelastic four-parameter Fung-type model and also an isotropic Mooney–Rivlin model. The biaxial stress-stretch curves were fitted to a hyperelastic anisotropic four-parameter Fung-type model and a five-parameter Mooney–Rivlin model. The specimens showed some degrees of anisotropy. In both uniaxial and biaxial tests, specimens showed stiffer behaviour in longitudinal as opposed to circumferential directions. The stretch ratio in the circumferential direction was much higher than in the longitudinal orientation.