Structure-function relationships of interstitial transport and mechanics in blood clots

Abstract

Despite major advances in our understanding of the biochemical pathways of thrombus formation, our knowledge of the biophysical mechanisms that regulate thrombus growth remains limited. Explicitly, there is a paucity of data on how hemodynamics affects the physical properties of thrombi. The objective of this thesis is to measure and model the physical properties of thrombi as a function of the relative and absolute density of the blood cell platelets and the biopolymer fibrin. A series of thrombi were made with known fibrin and platelet densities. The permeability was calculated by measuring the interstitial fluid velocity through the thrombi at a constant pressure gradient. Hindered diffusivity was measured using a diaphragm diffusion cell and fluorescence recovery after photobleaching. The elastic regimes of fibrin gels was measured as a function of strain in both shear and extensional geometries and compared to nonaffine and affine network models. The composition and structure of thrombi formed under flow using microfluidics was quantified using confocal and focused ion beam scanning electron microscopy. The permeability of thrombi formed with a solid fraction of 0.02 to 0.61 ranged from 10e-1 to 10e-5 squared microns and was best modeled as an array of disordered cylinders or as a Brinkman medium for low and high platelet densities, respectively. The diffusivity of coagulation zymogen-sized solutes was hindered by up to 93% over the same range and was best described by models accounting for both steric and hydrodynamic hindrance. Low density fibrin gels transition from a thermal network regime to a nonaffine mechanical to an affine mechanical regime with increasing strain. High density fibrin gels transition directly from a thermal to an affine mechanical regime. Within the thermal regime, the pore size can be predicted by the elastic modulus using semiflexible polymer theory providing a link between mechanics and transport. Flow-formed thrombi were found be heterogeneous structures that contained up to 65% solids by volume. Ultimately, these results enable the prediction of the properties of flow-formed thrombi. Importantly, all properties were found to be directly related to thrombus microstructure, thus providing a link between mechanics and transport that should prove useful in developing therapeutic approaches for thrombosis.