Strain-rate dependent material properties of the porcine and human kidney capsule

This study was performed to characterize the mechanical properties of the kidney capsular membrane at strain-rates associated with blunt abdominal trauma. Uniaxial quasi-static and dynamic tensile experiments were performed on fresh, unfrozen porcine and human renal capsules at deformation rates ranging from 0.0001 to 7 m/s (strain-rates of 0.005-250 s−1). Single stroke, dynamic tests were performed on samples of porcine renal capsule at strain-rates of 0.005 s−1 (n=33), 0.05 s−1 (n=17), 0.5 s−1 (n=38), 2 s−1 (n=10), 4 s−1 (n=10), 50 s−1 (n=21), 100 s−1 (n=18), 150 s−1 (n=17), 200 s−1 (n=10), and 250 s−1 (n=17). Due to limited availability of human tissues, only quasi-static tests were performed (0.005 s−1, n=25). Porcine renal capsule properties were found to match the material properties of human capsular tissue sufficiently well such that porcine tissue material can be used as a human test surrogate. The apparent elastic modulus and breaking stress of the porcine renal capsule were observed to increase significantly with increasing strain-rate (P<0.01). Breaking strain was inversely related to strain-rate (P<0.01). The effect of increasing strain-rate on material properties diminished appreciably at rates exceeding 150 s−1. Empirically derived mathematical models of constitutive behavior were developed using a hyperelastic/viscoelastic Ogden formulation, as well as a Cowper-Symonds law material curve multiplication.