Mechanical Behavior of Blood Vessels: Elastic and Viscoelastic Contributions

SIMPLE SUMMARY: A frequent type of injuries in traffic collisions and falls from a moderate height is associated with subdural hematomas caused by the mechanical failure of cerebral bridging veins, which link the superior sagittal sinus to the brain. For this reason, both to design safe restraint systems for motor vehicles and to study how these injuries occur, it is important to study the mechanical properties of the bridging veins. Although the mechanical properties of bridging veins have been studied for the last half century, some viscoelastic effects in these vessels that alter their mechanical response have not been analyzed in detail until now. This is the first study that measures, quantifies, and models these viscoelastic effects, thus improving our knowledge of the mechanical response of cerebral bridging veins. ABSTRACT: The mechanical properties of the cerebral bridging veins (CBVs) were studied using advanced microtensile equipment. Detailed high-quality curves were obtained at different strain rates, showing a clearly nonlinear stress–strain response. In addition, the tissue of the CBVs exhibits stress relaxation and a preconditioning effect under cyclic loading, unequivocal indications of viscoelastic behavior. Interestingly, most previous literature that conducts uniaxial tensile tests had not found significant viscoelastic effects in CBVs, but the use of more sensitive tests allowed to observe the viscoelastic effects. For that reason, a careful mathematical analysis is presented, clarifying why in uniaxial tests with moderate strain rates, it is difficult to observe any viscoelastic effect. The analysis provides a theoretical explanation as to why many recent studies that investigated mechanical properties did not find a significant viscoelastic effect, even though in other circumstances, the CBV tissue would clearly exhibit viscoelastic behavior. Finally, this study provides reference values for the usual mechanical properties, as well as calculations of constitutive parameters for nonlinear elastic and viscoelastic models that would allow more accurate numerical simulation of CBVs in Finite Element-based computational models in future works.