Material strengths of shear-induced platelet aggregation clots and coagulation clots.

Arterial occlusion by thrombosis is the immediate cause of some strokes, heart attacks, and peripheral artery disease. Most prior studies assume that coagulation creates the thrombus. However, a contradiction arises as whole blood (WB) clots from coagulation are too weak to stop arterial blood pressures (> 150 mmHg). We measure the material mechanical properties of elasticity and ultimate strength for Shear-Induced Platelet Aggregation (SIPA) type clots, that form under stenotic arterial hemodynamics in comparison with coagulation clots. The ultimate strength of SIPA clots averaged 4.6 ± 1.3 kPa, while WB coagulation clots had a strength of 0.63 ± 0.3 kPa (p < 0.05). The elastic modulus of SIPA clots was 3.8 ± 1.5 kPa at 1 Hz and 0.5 mm displacement, or 2.8 times higher than WB coagulation clots (1.3 ± 1.2 kPa, p < 0.0001). This study shows that the SIPA thrombi, formed quickly under high shear hemodynamics, is seven-fold stronger and three-fold stiffer compared to WB coagulation clots. A force balance calculation shows a SIPA clot has the strength to resist arterial pressure with a short length of less than 2 mm, consistent with coronary pathology.

Structure of shear-induced platelet aggregated clot formed in an in vitro arterial thrombosis model.

The structure of occlusive arterial thrombi is described herein. Macroscopic thrombi were made from whole blood in a collagen-coated, large-scale stenosis model with high shear flow similar to an atherosclerotic artery. The millimeter-sized thrombi were harvested for histology and scanning electron microscopy. Histological images showed 3 distinctive structures of the thrombus. (1) The upstream region showed string-like platelet aggregates growing out from the wall that protrude into the central lumen, with red blood cells trapped between the strings. The strings were >10 times as long as they were wide and reached out to join the strings from the opposite wall. (2) Near the apex, the platelet strings coalesced into a dense mass with microchannels that effectively occluded the lumen. (3) In the expansion region, the thrombus ended abruptly with an annulus of free blood in the flow-separation zone. Scanning electron microscopy showed dense clusters of spherical platelets upstream and downstream, with amorphous platelets in the occluded throat consistent with prior activation. The total clot is estimated to contain 1.23 billion platelets with pores 10 to 100 µm in diameter. The results revealed a complex structure of arterial thrombi that grow from their tips under high shear stress to bridge the 2.5-mm lumen quickly with von Willebrand factor platelet strings. The occlusion leaves many microchannels that allow for some flow through the bulk of the thrombus. This architecture can create occlusion or hemostasis rapidly with minimal material, yet can remain porous for potential delivery of lytic agents to the core of the thrombus.

Platelet α-granules are required for occlusive high-shear-rate thrombosis.

von Willebrand factor (VWF) is essential for the induction of arterial thrombosis. In this study, we investigated the critical role of platelet VWF in occlusive thrombosis formation at high shear in mice that do not express platelet VWF (Nbeal2-/-). Using in silico modeling, in vitro high-shear microfluidics, and an in vivo Folts model of arterial thrombosis we reproduced the platelet dynamics that occur under pathological flow in a stenosed vessel. Computational fluid dynamics (CFDs) simulated local hemodynamics in a stenosis based on arterial geometries. The model predicted shear rates, time course of platelet adhesion, and time to occlusion. These predictions were validated in vitro and in vivo. Occlusive thrombosis developed in wild-type control mice that had normal levels of plasma VWF and platelet VWF in vitro and in vivo. Occlusive thrombosis did not form in the Nbeal2-/- mice that had normal plasma VWF and an absence of platelet VWF. Occlusive thrombosis was corrected in Nbeal2-/- microfluidic assays by the addition of exogenous normal platelets with VWF. Combining model and experimental data, we demonstrated the necessary requirement of platelet VWF in α-granules in forming an occlusive thrombus under high shear. These results could inspire new pharmacological targets specific to pathological conditions and prevent arterial thrombosis.