Active and passive biaxial mechanical properties of urinary bladder wall

Urinary bladder augmentation surgeries are common to restore lost function due to trauma and neurogenic disorder, however not all procedures are successful, suggesting that the materials used for the repair may be inadequate. To create a more appropriate material, rigorous mechanical behavior characterization of the native bladder wall is required, which is not currently available. As a first step towards the development of tissue engineered bladder repair material, the authors performed the first biaxial testing on both passive and active properties of urinary bladder. Similar to other soft tissues, the bladder demonstrated an anisotropic nonlinear stress-strain response. The circumferential strain became negative during the test where the circumferential load was twice the longitudinal load while no strain reversal was seen in the longitudinal direction. This complex axial “coupling” is due to the preferential alignment of the smooth muscle fibers along the circumferential direction. Interestingly, contraction of the tissue resulted in a higher increase in circumferential load than longitudinal load but an almost isotropic decrease in stretch ratio in the two axes (27% circumferential and 33% longitudinal). These results indicate important components to the function of the bladder and must be further investigated before designing a repair material