The urinary bladder undergoes significant deformations during different physiological functions (e.g. filling, emptying, storage). Displacement-controlled tensile tests are commonly employed to characterize the mechanical properties of tissue. Therefore, tissue samples are mounted in a testing apparatus and stretched in one or multiple directions, with the resulting stress response being recorded. However, it is unclear how the biomechanical behavior and tissue properties differ under uniaxial versus biaxial tensile testing conditions. To determine the mechanical properties of the bladder tissue, uniaxial and biaxial tensile tests [1] were performed on 36 tissue samples taken from the body region of the porcine bladder. Tissue relaxation, activation dynamics, and active and passive stress-stretch relations were determined using specialized protocols involving ramps, holding phases, and electrical stimulation. The main findings of the experiments are: (1) Stress relaxation depends on the extent of the applied stretch. (2) Biaxially stretched samples show a 63% slower activation than uniaxially stretched samples when considering the activation time constant. (3) Biaxial tensile tests show an 18% reduction in the tissue optimal stretch compared to uniaxial tests, while (4) there is no significant difference in maximum active stress between the uniaxial and biaxial tensile tests. Based on the experimentally determined properties, a continuum mechanical model describing a viscoelastic, isotropic solid with embedded active fibers was developed. This model is based on uniform fiber orientation and the typical active fiber stress–stretch curve. It helps to interpret and explain results (3) and (4) and improves our understanding of the bladder's physiological processes during deformation. Furthermore, these findings contribute to the interpretation of experimental data and provide mechanical parameters for organ-level modelling. References [1] 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. 193 (2025) 255–266.