Mechanical properties of the mammalian vas deferens. I. In the passive state

Knowledge of the mechanical properties of the vas deferens is important in order to understand the mechanical interaction between an intravasal device (IVD) and the vas deferens--a necessary step for successful long-term implantation. It is equally important in order to understand the mechanism of sperm transport through the vas, with or without an IVD implant, by means of quantitative mechanical models. Experiments were performed, in vitro, on vas deferens from rat, bull, and rabbit, to determine its mechanical properties in the passive state. The data consist of (1) load response to simple extension and cyclic extension, (2) extensional response to cyclic loading, and (3) stress relaxation response at constant extensions. The load-elongation behavior is characterized by Fung's exponential model T = (T* + beta)e alpha(lambda-lambda*) - beta quantitatively, where T is the Lagrangian or engineering stress (current force in the specimen divided by the original area of cross section) (dyn/cm2), T* is a convenient stress value (dyn/cm2), alpha is a parameter characterizing material elasticity (dimensionless), beta is a second material parameter (dyn/cm2), lambda is a stretch ratio (dimensionless) equal to l/l0, where l is the instantaneous length of the specimen (cm) and l0 is its reference length measured at 2-gram-force (1 gram force = 981 dyn) applied load (cm), and lambda* is the stretch level corresponding to T* (dimensionless). The vas appears to behave as a viscoelastic material and its reduced relaxation function may be dependent on the initial level of stretch. The cyclic-loading and cyclic-extension data give evidence of internal damping mechanisms, which make the loading curves different from the unloading curves (hysteresis). Also, the mechanical behavior of the vas is found to be altered by repeated loadings in quick succession. It is likely that cyclic loadings, in vivo, occur at much lower levels of stress and thus cause negligible damage, or that there are natural mechanisms which repair the damage. The behavior of tissue from different species of animals are qualitatively similar, although the tissue from the larger-size animal is likely to be stronger and stiffer. Due to very little interweaving between the muscle fibers of the longitudinal and circumferential layers, the data reported reflect the properties of the longitudinal layers only. Because the muscular structure of the three layers is very similar, the properties of the circumferential layer may be extrapolated.