Viscoelastic characterization of the porcine aorta under combined normal and torsional loadings

Abstract

Understanding the mechanical properties of aortic tissue is crucial for improving future medical interventions related to cardiovascular diseases. In this study, we seek to experimentally explore the stress relaxation response of the porcine aorta under constant combined torsional shearing and normal compressive load. Circular samples, measuring 0.5 inches in diameter, were extracted from the side wall of excised porcine thoracic aortas. Experiments were performed on a parallel plate rheometer applying a constant compressive strain ranging from 5 to 25% and a constant shear strain ranging from 10 to 50%. The combined loading was maintained for 1000 seconds, and the stress response of the tissue was recorded. A continuum mechanics model describing the combined axial compression and torsional shearing of an incompressible, viscoelastic solid cylinder was developed. Expressions for the normal stress and shear stress on the top surface of the cylinder were derived. A nonlinear least squares method was applied to fit the derived expressions to the rheometer’s corresponding output signals. The solved time scale parameters varied between the normal and shear directions indicating time-dependent anisotropy. Varying the normal compressive strain or the torsional shear strain did not noticeably affect the time scale of the relaxation by significant amounts in either the normal or shear directions. The magnitude of the stress relaxation response in the normal direction shows dramatic dependency on input compressive strain. Varying the input shear strain did not noticeably affect the time scale of the relaxation in either direction, but did modulate the magnitude of the viscous response.