The urinary bladder is a hollow organ that undergoes significant deformation as it receives, stores, and releases urine. To understand the organ mechanics, it is necessary to obtain information about the material properties of the tissues involved. In displacement-controlled tensile tests, tissue samples are mounted on a device that applies stretches to the tissue in one or more directions, resulting in a specific stress response. For this study, we performed uniaxial and biaxial stretch experiments on tissue samples from the body region of the porcine urinary bladder. We analyzed the stress-relaxation, activation dynamics, and passive and active stretch-stress response. Main findings of our experiments are: (1) For uniaxial and biaxial stretching, the time constants for stress-relaxation depend on the stretch amplitude, (2) biaxially stretched samples experienced slower activation with increasing by 163% compared to uniaxial stretching, (3) biaxial tests are characterized by reduced optimum stretches by -18%, and (4) biaxial and uniaxial tests showed no significant difference in maximum active stresses. To interpret the results, we present a continuum mechanical model based on a viscoelastic, isotropic solid extended by a set of active muscle fibers. Model predictions show that results (3) and (4) can be explained by a uniform distribution of fiber orientations and a specific shape of the active fiber stress-stretch relationship. This study highlights how deformation modes during tensile testing affects smooth muscle mechanics, proving insights for interpreting experimental data and improving organ modeling.
J. Geldner, S. Papenkort, S. Kiem, M. Böl, T. Siebert
Active and passive material response of urinary bladder smooth muscle tissue in uniaxial and biaxial tensile testing
Acta Biomaterialia, (2024) [Link]