Hardening of the Porcine Thoracic Aorta Subjected to Freezing and Thawing: Experimental Evaluation and Mathematical Modeling

Identifying changes in the mechanical behavior of blood vessels subjected to freezing and thawing, such as occur with cryopreservation, are of key importance. Excising pairs of fresh ring specimens from identical porcine thoracic aortas (n = 8 for each cooling rate), we carried out uniaxial tensile loading and unloading tests over the physiological stress range (first and second tests) and performed a loading test until the breaking point within the range of a load cell (third test). After the first test, one specimen of the pair was frozen at −80°C at a cooling rate of −1°C or −50°C/min and thawed, while the other was held at 5°C as a control. At both cooling rates, for the specimens subjected to freezing, the ratios of the tangential modulus in the stress-strain curve (between 130 and 150 kPa) in the second test to that in the first test differed significantly (p < 0.01) from the respective ratios of the control specimens. We formulated a mathematical model of the stress–strain relationship considering elastic and collagen fibers and an incompressible fluid phase. We evaluated the working hypothesis that collagen fibers reduce their extensibility either by hardening as a mechanical change or by shortening as a geometric change. We attributed this response to the formation of dehydration-induced cross-linking in collagen molecules at the microscopic level.