Nonlinear Dynamics of Human Aortas for Viscoelastic Mechanical Characterization

The mechanical characterization and simulation of human aortas is very relevant to prevent diseases and design aortic grafts for aorta repair. Eleven donors of both genders, with an average age of 51.5 ± 17.3 years were selected for the study. A custom mock circulatory loop (MCL) was built to study the nonlinear dynamics of human descending thoracic aortas. Once the preparation was completed, the aortic segment was fixed to rigid cylindrical supports whose diameters were about the size of the inner diameter of the specimen. The MCL aimed to reproduce the pulsatile pumping of the heart, the Windkessel effect and peripheral resistance of the portion of the arterial tree. The loop was built with 3/4” ID Tygon tubing leading to the tank; a larger tube was used for the return to the pump. To simulate the compliance of the arterial tree upstream and downstream the aorta, two expansion chambers (manufactured by BDC Laboratories) were placed before and after the aorta. Four Polytec laser Doppler vibrometers (model OFV 505/PSV 500) equipped with displacement decoders, were installed at 90 degrees from one another along the circumference of the aorta specimen. The experiments in the MCL were performed at different pulse rates, from roughly 60 bpm (resting condition) to 180 bpm (intense sport activity). Results show cyclic axisymmetric diameter changes, which are satisfactorily compared to in-vivo measurements at a resting heart rate. An increase of the dynamic stiffness with age is also observed. This is accompanied by a strong reduction with age of the cyclic diameter change during the heart pulsation at rest and by a significant reduction of the energy dissipation. Large damping is observed at higher pulse rates due to the combined effects of fluid-structure interaction and viscoelasticity of the aortic wall. The aims of the experiment were successfully met: the viscoelastic material parameters for the human descending thoracic aorta were identified by using a MCL specifically conceived to reproduce physiological pulsatile flow. The loss factors reported were representative of both the aorta and the flow. They would be difficult to measure in-vivo at higher pulse rates. Furthermore, the local pressure measurement would be invasive. This presses on the importance of ex-vivo studies. The projected outcome of this work is creating innovative biomaterials that better reproduce the aortic dynamic behavior. The findings complement expanding avenues in advanced materials, with the aim of creating improved and mechanically compatible cardiovascular devices, like grafts and stents.