MEAM Seminar: “Hydrogel (fracture) Mechanics with Applications to the Study of Blood Clots”

Blood clotting is primarily responsible for stemming bleeding in vessel injury, comprising a protective mechanism called hemostasis. However, if a blood clot forms within a blood vessel when or where it is not needed, this can result in death and disability. Blood clots and thrombi are composed of blood cells embedded into a polymeric fibrin network. The network is highly swollen with liquid (<1% solid volume fraction); biphasic porous media of such kind are called hydrogels. Both the solid and liquid phase, as well as their combined behavior, determine the properties of the hydrogel material. In this talk, we endeavor to understand the behavior of blood clots and thrombi, which are essential for hemostasis and thrombosis, by utilizing a continuum mechanics framework for hydrogels.

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, while experiments have verified it. The liquid flux fields usually exhibit fully three-dimensional profiles and are time dependent. This coupled mechanical-diffusional poroelastic problem presents an abundance of interesting phenomena. One such interesting observation in many experiments is the tendency of the hydrogel material to expel liquid under tension. This behavior is well-documented in biological swollen tissues, but it appears to be absent from the vast majority of synthetic hydrogels. On the contrary synthetic gels swell during stretching, provided there is liquid to absorb from the surrounding environment. This motivates three types of boundary conditions in the coupled mechanical-diffusion problem for hydrogels: 1) gel immersed in a bath (permeable boundary conditions), 2) gel insulated from bath (impermeable), or 3) gel in an environment with a specific ambient moisture. In this talk all these boundary conditions and their implications will be discussed. The solid volume fraction, loading rate, type of loading, as well as geometry, influence the response of hydrogels dramatically. These factors are studied separately with some highlights from this study being presented. The energy release rate, a fundamental quantity of fracture mechanics, is computed using a poroelastic modified path (or surface)-independent J-integral. Liquid flow is shown to contribute an important amount to the energy release rate. Intriguingly, this contribution can be either positive or negative, the meaning of which will be discussed. To accommodate better understanding of the liquid contribution on the fracture energetics of these materials, a critical stretch criterion is adopted which is motivated from experimental evidence. Under critical conditions, the energy release rate is the fracture toughness of the material and its relation to the coupled response will be explained. These observations can be utilized to design toughening mechanisms for hydrogels based solely on the liquid phase of the material. The methods that will be presented can be utilized to analyze a wide variety of problems from mechano-chemical loading of biological and synthetic hydrogels to failure/damaging of batteries and large-scale soil mechanics.