Visco-elasto-plastic modeling of small intestinal submucosa (SIS) for application as a vascular graft.

In developing new tissue engineered for vascular grafts, the mechanical properties of the material and its evolution once implanted are of utmost importance because they determine the regeneration of the vessel and the blood flow through the conduit. In fact, compliance mismatch is considered the main determinant of graft failure. In this research, we analyze the dynamic properties of the small intestinal submucosa (SIS), and propose and validate a constitutive model to fit the material's behavior. A uniaxial creep and recovery test was performed on SIS tubes to find the constitutive parameters. The model was composed by an elastic element in series with two Kelvin-Voigt solid elements and a plastic slider. The first elastic component was defined using Mooney-Rivlin strain energy function, while the plastic component was defined using a third-degree polynomial function of the plastic stress. The viscoelastic behavior was defined using the creep compliance formulation for the Kelvin-Voigt model. The parameters for the plastic and non-linear elastic elements followed a normal distribution, while the spring and dashpot constants of the visco-elastic element had a linear dependence on the load applied. The constitutive model was then used to simulate the SIS under the geometrical and pressure conditions found in native vessels for 1000 cycles at a frequency of 60 cycles per minute. From the cases simulated, performance curve charts were obtained in terms of the compliance of the material. These curve charts can be used as a predictive tool of the graft's behavior based on its geometry.