MEAM Ph.D. Thesis Defense: “A Study of Hydrogel Mechanics with Application on the Fracture of Human Blood Clots”

Loading of biological and synthetic hydrogels involves large deformations, and there exists a large literature devoted to their experimental characterization. Analytical investigations have recognized the importance of contributions originating from the liquid phase, and experiments have verified them. The liquid flux fields in these materials usually exhibit fully three-dimensional profiles and are time-dependent. This coupled mechanical-diffusional poroelastic problem is studied here within the framework of continuum poro-elasticity and presents an abundance of interesting phenomena. One such interesting observation in many experiments is the tendency of some hydrogel materials to expel liquid under tension. This behavior is well-documented in biologically swollen tissues, but it appears to be absent from a majority of synthetic hydrogels which exhibit the more common behavior of absorbing liquid under tension. In this thesis the poro-elastic fracture of hydrogel materials is studied and the energy release rate, a fundamental quantity of fracture mechanics, is computed. Liquid flow is shown to contribute significantly to fracture, and it can be utilized to design tough hydrogels.

Beyond the theoretical investigations, continuum poroelasticity is applied to the fracture behavior of human blood clots whose main component is a fibrin gel. Fibrin is a blood clotting protein and the main structural components of clots and thrombi. Different fibrin(ogen) concentrations, types of loading (tension and shear), and geometries are used to study the dependencies of the toughness on the fibrin(ogen), showing that fracture toughness increases with fibrin(ogen) concentration. The poroelastic constitutive model used, incorporating the intricate fibrin fiber mechanics, captures well the experimental data. Insights for the microstructural process happening during fracture are provided through a combination of finite element results and microscopy imaging.