Session G18: Biological fluid dynamics: Aneurysms and Blood Clots

Experimental Study of Flow Behavior in Aneurysm with Varying Bottleneck Factor

Aneurysms are irregular enlargement of weakened blood vessels and their rupture can be fatal in some cases.  Earlier clinical studies have shown that the morphology of an aneurysm is correlated with the risk of rupture.  The objective of this investigation was to study the impact of morphology on flow behavior in an aneurysm sac. Two idealized saccular aneurysm models with different bottle-neck factors were used and  an in-house experimental setup was developed to enable inflow conditions such as Womersley number and Reynolds number to be controlled as needed. Particle Image Velocimetry measurements were performed on the mid-vertical planes inside the aneurysm sac. The flow evolution, vortex path, and vortex strength were determined using the velocity field data. Proper Orthogonal Decomposition was used to identify the temporal/spatial behavior of the large-scale structures inside the aneurysm. A low-order velocity field was used to analyze the large-scale structures inside the aneurysm. The results showed that the vortex path was primarily influenced by the change in Womersley number, while the formation of secondary vortices was related to the change in Reynolds number.     

Embolus Transport Simulations with Fully Resolved Particle Surfaces

It is generally agreed that the transport of emboli in the large blood vessels is important in determining its fate and its risk for stroke. It is difficult, however, to calculate a trajectory of arbitrarily large particles. The commonly used “small” particle assumption, i.e. forces acting on the particle may be modeled with a drag law, is justified if the particle length scale is much smaller than the flow length scale. In this work, we simulate the motion of embolus-like particles through the blood using an overset mesh technique in conjunction with a Navier-Stokes solver for the blood flow. This allows the fluid stresses on the particle surface to be fully resolved. The particles can consequently be of arbitrary size and shape. We simulate the trajectory of 150 resolved particles through a patient-specific carotid artery bifurcation, with particles of 0.5 and 1.0 mm diameter spheres and 1.0 mm x 0.5 mm spheroids. We also simulate the trajectory of about 10000 particles using the “small” assumption. The proportions of small and non-small particles entering the internal carotid artery are treated as the outcome of the particle fate, and statistical comparisons are made to ascertain the importance of non-small particle effects.   

A Microfluidic Approach to Model Deep Vein Thrombosis

Deep vein thrombosis (DVT)  is a dangerous and painful condition in which blood clots form in deep veins. Mechanisms of clot development remain unclear, however specific flow patterns in veins, especially around the valve flaps play a fundamental role (Bovill and van der Vliet, 2011). To study DVT, we fabricated valves made of polyethylene glycol diacrylate in-situ in a microfluidic device achieving control over geometry and elasticity using stop-flow lithography (Dendukuri et al., 2007; Wexler et al., 2013). We independently varied the elasticity of each valve’s leaflet to obtain symmetrical or asymmetrical characteristics. To analyse the velocity profiles we exploit ghost particle velocimetry (Buzzaccaro et al., 2013; Riccomi et al., 2018) and we study particle accumulation, mimicking clot formation, by flowing polystyrene particles. We found that particles tend to accumulate at the tip of symmetric leaflet valves whereas in an asymmetrical valve a predominant accumulation behind the more flexible valve leaflet happens. This is important as a major risk factor for DVT in the elderly is a reduced elasticity of their valves.   

Mesoscopic Protofibril Aggregation Model for Numerical Analysis of Fibrin Clot Formation

A fibrin clot is main component of thrombus. The formation of the fibrin clot consists of several multi-scale processes. Fibrinogen, which is a plasma protein, is activated by thrombin and polymerized into a filament, the so-called protofibril. The protofibrils aggregate and form network structures called as fibrin clots. According to previous studies, the network structure varies depending on aggregation conditions, e.g., concentrations of fibrinogen and solvent flow, and alters the mechanical and chemical characteristics of the fibrin clot. However, the formation mechanisms of various network structures have not been understood in detail yet because of the spatiotemporal multi-scale of the formation process. In this study, we develop a numerical mesoscopic protofibril aggregation model to describe the fibrin clot formation from the mechanical point of view. We model protofibril as discrete elements consisting of material points bounded by linear springs. Each material point follows an overdamped Langevin equation. Our model can demonstrate the network structure observed in experiments. Additionally, we discuss the relationship between the characteristics of the network structure and each model parameter, to clarify why the various network structures appear in experiments.   

The effect of wall shear stress topology and magnitude on mass transport in atherosclerosis

Atherosclerosis is the major form of cardiovascular disease. The near-wall transport of certain biochemicals and cells influences atherosclerosis. Wall shear stress (WSS) contributes to these transport processes in a complex manner. It has been recently shown that Lagrangian WSS structures, topological features hidden in WSS vector field, govern near-wall transport in high Schmidt and Peclet number flows. Furthermore, the endothelial cells sense and respond to WSS magnitude, which determines the biochemical flux at the vessel wall. Shear-enhanced and shear-reduced biochemical flux boundary conditions were considered at the vessel wall in a coronary artery stenosis and carotid artery model. The advection-diffusion-reaction equation was solved to study biochemical transport. The intertwined relationship between WSS topology, WSS magnitude, and biochemical surface concentration was revealed. A threshold was identified in the wall flux model where WSS topology governed surface concentration patterns when the flux was closer to a homogeneous flux, whereas WSS magnitude determined surface concentration when the flux was sufficiently heterogeneous. Our results shed light on the important, yet complicated role that WSS plays in shear-dependent cardiovascular mass transport problems.    

Cross-Stream Distribution and Dynamics of Red Blood Cells in Sickle Cell Disease

Diseased red blood cells (RBCs) in patients suffering from sickle cell disease (SCD) display substantially different physical properties than do healthy RBCs. Direct numerical simulations are performed for pressure-driven flow in a planar slit containing a binary mixture of deformable biconcave discoids and stiff curved prolate spheroids that represent healthy and sickle RBCs, respectively. The key observation in such suspensions is that the sickles exhibit a strong margination towards the walls. The deformable biconcave discoids tend to undergo a so-called tank-treading motion, while the stiff sickles behave like rigid bodies and undergo a Jeffery-like tumbling or kayaking motion. A systematic investigation of the dynamics of single sickles in free and confined shear flows further reveals that the steady-state motion of the sickles is independent of their initial orientations. The margination behavior coupled with the Jeffery-like motions of the sickles near the walls may help substantiate a recently proposed hypothesis of a mechanism for chronic complications of sickle cell disease.