A layer-specific hyperelastic and viscoelastic characterization of human descending thoracic aortas was experimentally performed. Healthy aortas from twelve beating heart donors with an average age of 49.4 years, were received from Transplant Quebec. Axial and circumferential strips were prepared from the specimens. They were dissected into intima, media and adventitia layers. Measurements of the opening angles were used to identify the circumferential residual stresses. Uniaxial tensile tests on axial and circumferential strips, together with the Gasser-Ogden-Holzapfel material model, were used to characterize the hyperelastic behaviour of the three aortic layers for each donor. Uniaxial harmonic excitations at different frequency, superimposed to initial stretch values, were used to characterize the viscoelastic behaviour. The storage modulus and the loss tangent were obtained for each layer in both directions; comparison to intact aortic wall was also performed. The generalized Maxwell model, within the framework of nonlinear viscoelasticity with internal variables, was used to obtain the constitutive material parameters. Results showed a positive correlation (p < 0.05 for circumferential media and adventitia) between stiffness and donor age for the three layers of the aorta in both axial and circumferential directions. A significant increase (around 50%) of the storage modulus (i.e. dynamic stiffness) was observed between the quasi-static value and loading at 1 Hz frequency, while further increase in frequency marginally affected its value. The loss tangent was only slightly influenced by the stretch value, which justified the use of the viscoelastic model adopted. Finally, similar loss tangent values were found for the three aortic layers.
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