Passive biaxial mechanics and viscoelastic relaxation of the thoracic aorta in intermittent hypobaric hypoxia

A CellScale BioTester 5000 was used to perform planar biaxial mechanical testing and stress-relaxation characterization of descending thoracic aorta specimens from Control, acute intermittent hypobaric hypoxia, and chronic intermittent hypobaric hypoxia adult rats. Following dissection, each aorta was cut longitudinally and approximately 6 × 6 mm square specimens were prepared with the circumferential and longitudinal orientations identified. Specimens were mounted on the BioTester using rakes along all four edges and were kept fully hydrated in calcium-free Krebs-Ringer solution at 37 °C throughout mechanical testing. The system used 2.5 N load cells to record circumferential and longitudinal forces while equal actuator displacements were applied simultaneously along both axes. The BioTester-integrated camera captured the speckled tissue surface for digital image correlation, and DaVis software was used to calculate local circumferential and longitudinal stretches within a central region of interest while minimizing rake-boundary effects. For monotonic biaxial characterization, specimens were loaded simultaneously along both axes at 1.5 mm/min. Stress-stretch curves were used to calculate initial stiffness E1, high-stretch stiffness E2, knee stretch, and knee stress. For viscoelastic characterization, samples underwent 10 biaxial load-unload preconditioning cycles followed by a stress-relaxation test at a nominal strain rate of 15%/s to a target stretch of λ = 1.60, which was held constant for 400 seconds. Stress decay, characteristic relaxation time, peak stress, and accumulated dissipated energy were quantified in both anatomical directions. BioTester testing showed that most elastic properties remained similar among groups, while chronic intermittent hypobaric hypoxia significantly reduced longitudinal high-stretch stiffness. Viscoelastic measurements showed non-significant trends toward greater stress relaxation and energy dissipation following chronic hypoxic exposure, particularly circumferentially.