A finite strain integral model for the creep behavior of vaginal tissue

Vaginal delivery causes significant stretching of the vagina and surrounding muscles, potentially leading to the development of pelvic floor disorders and other maternal morbidities. Despite the extended duration of labor, little experimental and theoretical work has been done to characterize the long-term viscoelastic behavior of vaginal tissue. To mathematically capture the creep response of rat vaginal tissue measured, this study presents a new anisotropic finite-strain constitutive model within the single integral Pipkin-Rogers viscoelastic framework. The constitutive parameters are computed by curve-fitting the model to strain versus time data collected from ex vivo inflation testing along the two primary anatomical directions of the vagina, the longitudinal and circumferential directions. The results showed good agreement between theory and experiments, suggesting that the proposed model could be used to advance our understanding of the time-dependent deformations that are experienced by the vagina during delivery. This modeling framework represents a first step toward the development of accurate computational tools that can predict the safety of vaginal deliveries, reducing unnecessary Cesarean sections and their related complications.