Time-dependent deformations of vaginal tissue: Experiments and constitutive modeling

Abstract

The vagina is a complex organ that undergoes significant deformations emph{in vivo} in fulfilling multiple crucial biomechanical roles in the female reproductive system, notably acting as the birth canal during parturition. In spite of this, the creep properties of the organ have been insufficiently characterized and warrant further study. Here, we provide a detailed review of experimental characterization of the passive mechanical properties of the vagina, and then present the results of our experimental and theoretical work designed to fill gaps in knowledge on the creep properties of vaginal tissue. In our first study, we subjected rat vaginal canals to three consecutive creep tests, each at progressively increasing constant luminal pressures, without recovery, and measured the resulting deformations using the digital image correlation (DIC) method. Under inflation, the vagina was found to deform significantly more in the circumferential direction (CD) than the longitudinal direction (LD), and the change in deformation over time was significantly higher during the first creep test than during the second and third creep tests at higher loads. In our second study, we proposed a new anisotropic finite-strain constitutive model for the creep of vaginal tissue within the single integral Pipkin-Rogers viscoelastic framework. We assumed the vagina's axes of material symmetry coincide with its main natural anatomical directions, the LD and CD. We validated the model using the strain versus time data collected from the first study, modeling the vaginal canal under inflation as a thin-walled cylindrical pressure vessel. The results of our model showed good agreement between theory and experiments, representing a significant step towards the development of accurate computational tools that can predict the deformations of the vagina to loading. In our third and ultimate study, we once again experimentally characterized creep in vaginal tissue. We performed testing on tissue collected from mice at multiple points in pregnancy: non-pregnant (NP), mid-pregnant (MP), and late-pregnant (LP). Vaginal canals were subjected to two creep tests of increasing pressure, with recovery between creep tests, and strains in the LD and CD were measured using the DIC technique. The resulting strains in the MP vagina were significantly lower than the strains in the NP and LP vagina, indicating the vagina may stiffen during early-mid pregnancy. The LP vagina experienced greater changes in strain during creep than the MP and NP vagina during the first and second creep test respectively. This adaptation may facilitate the ability of the LP vagina to accommodate parturition. Taken together, these studies represent significant steps towards developing comprehensive theoretical and computational techniques for describing the mechanical behavior of the vagina throughout gestation. These results will guide the development of future experimental work in the field of vaginal tissue biomechanics, and later new standards of practice in maternal health for monitoring the healthy progression of pregnancy and informing the administration of medical care.