Viscoporoelasticity of coagulation blood clots

A blood clot is an essential biological tissue in physiological hemostasis and pathological thrombus in human bodies. In both physiological and pathological activities, the blood clots are often under external force from the blood flow. Quantifying the mechanical properties of the blood clot is important for the diagnosis and treatment of bleeding disorders and thrombotic diseases. In this work, the blood clot is shown to be viscoporoelastic and its properties are quantitatively characterized through combined rheology and indentation measurements. First, the viscoelasticity of the blood clot is determined by the shear rheology measurement because shear deformation does not induce volume change, and thus does not invoke the poroelastic behavior (i.e., coupled solvent migration and deformation) of the blood clot. Subsequently, an indentation test is carried out. Using a micromechanical testing machine, a spherical bead is pressed onto the blood clot to a certain depth and held for a period, meanwhile, the force is measured as a function of time. The force relaxation is contributed by both viscoelastic and poroelastic effects. Since the viscoelasticity of the blood clots has been fully characterized in shear rheology, the indentation data is used to determine the poroelastic properties of the blood clots. A general viscoporoelastic model is formulated and based on this model the relaxation indentation problem is simulated. By fitting the experimental force relaxation curves with the simulation results, the poroelastic parameters including permeability and drained Poisson’s ratio are obtained. Interestingly, the value of the drained Poisson’s ratio of the blood clot is very close to zero. To verify this behavior, we also carry out unconfined compression measurements to directly observe the volume evolution of the blood clot. The small value of drained Poisson’s ratio is validated by the unconfined compression result. Using the viscoporoelastic parameters determined in this work, we discuss the contribution of solvent migration and viscoelastic effect on the apparent modulus of the blood clot in physiologically relevant conditions. The high poroelasticity indicates the blood clot can withstand high instantaneous loads in the normal direction by resistance to solvent migration. Both poroelastic and viscoelastic processes dissipate energy, and thus could delay the fracture process of the material. This study systematically measures the viscoporoelastic properties of the blood clot and provides an understanding of the contributions of the two mechanisms to the time-dependent responses of coagulation blood clots.