Simulation of red blood cells in microcapillaries : on the study of a deformable particle in steady and oscillating Poiseuille flow

Red blood cells (RBCs) are the major cellular component of blood (about 98%). Therefore, they are the principal responsible for blood dynamics. At the scale of cells, the inertial forces are negligible and the blood flow is modeled with the Stokes equation. In this thesis, we present a two-dimensional numerical study of RBC behavior under flow using the capsule and the vesicle model. First, in a shear flow, we compare the motion and deformation of the shape in both models. Next, we investigate the behavior of a single, and a pair of vesicles in a steady and oscillating Poiseuille flow. For the steady Poseuille flow the shape of the vesicle depends on the flow strength, the mechanical properties of the membrane, and the width of the channel as reported in the past. The oscillation of the flow is introduced using amplitude modulation of the Poiseuille flow to mimic the pulsatile flow in the human circulatory system. We found that the flow oscillation can accelerate the transition of the vesicle from its initial to its final shape. We also observed shape transition of the Snaking shape (a shape where the vesicle shows an oscillatory motion like a swimmer flagella) to parachute or unconfined slipper shapes. For the pair of vesicles, the flow oscillation also decreases the distance between the vesicles. The influence of the oscillation flow was only observed for low flow rate. While for a higher rate, as the shape transition becomes instantaneous the influence of flow oscillation is then insignificant.Rote Blutzellen (RBCs, engl. Red Blood Cells) sind der zelluläre Hauptbestandteil des Blutes (ca. 98%). Aufgrunddessen sind sie hauptverantwortlich für die dynamischen Eigenschaften des Blutes. Auf zellulärer Ebene sind die Inertialkräfte vernachlässigbar und die Blutströmung wird durch die Stokes-Gleichung modelliert. In der vorliegenden Arbeit präsentieren wir eine zweidimensionale numerische Studie des Verhaltens von RBC in Strömung mithilfe des Kapsel- sowie des Vesikelmodells. Als Erstes wird die Bewegung sowie die Deformation in beiden Modellen im Scherfluss verglichen. Im nächsten Schritt untersuchen wir das Verhalten eines einzelnen sowie eines Vesikelpaars in stationärer sowie oszillierender Poiseuilleströmung. Für stationäre Poiseuilleströmung wurde in der Vergangenheit bereits aufgezeigt, dass die Form des Vesikels von der Flussstärke, den mechanischen Eigenschaften der Membran sowie der Kanalbreite abhängt. Die Oszillation der Strömung wird mittels Amplitudenmodulation des Poiseuilleflusses erreicht und ahmt die pulsierende Strömung im menschlichen Kreislaufsystem nach. Es zeigte sich, dass die Strömungsoszillation den Übergang des Vesikels von seinem Anfangs- zu seinem Endzustand beschleunigen kann. Wir beobachteten auch den Übergang der «Snaking»-Form (ein Zustand, bei dem das Vesikel eine oszillierende Bewegung ähnlich einem Flagellum vollführt) zur «parachute»- oder «unconfined slipper»-Form. Für ein Vesikelpaar führt die Oszillation auch zu einer Verringerug des Abstandes zwischen den Vesikeln. Der Einfluss der oszillierenden Strömung wurde nur für niedrige Flussraten beobachtet. Für höhere Flussraten ist der Einfluss der Oszillation unerheblich, da der Übergang zwischen den Formen instantan erfolgt

Vesicle dynamics in confined steady and harmonically modulated Poiseuille flows

We present a numerical study of the time-dependent motion of a two-dimensional vesicle in a channel under an imposed flow. In a Poiseuille flow the shape of the vesicle depends on the flow strength, the mechanical properties of the membrane, and the width of the channel as reported in the past. This study is focused on the centered snaking (CSn) shape, where the vesicle shows an oscillatory motion like a swimmer flagella even though the flow is stationary. We quantify this behavior by the amplitude and frequency of the oscillations of the vesicle's center of mass. We observe regions in parameter space, where the CSn coexists with the parachute or the unconfined slipper. The influence of an amplitude modulation of the imposed flow on the dynamics and shape of the snaking vesicle is also investigated. For large modulation amplitudes transitions to static shapes are observed. A smaller modulation amplitude induces a modulation in amplitude and frequency of the center of mass of the snaking vesicle. In a certain parameter range we find that the center of mass oscillates with a constant envelope indicating the presence of at least two stable states.Comment: 10 pages, 7 figure

Dextran adsorption onto red blood cells revisited: Single cell quantification by laser tweezers combined with microfluidics

The aggregation of red blood cells (RBC) is of importance for hemorheology, while its mechanism remains debatable. The key question is the role of the adsorption of macromolecules on RBC membranes, which may act as “bridges” between cells. It is especially important that dextran is considered to induce “bridge”-less aggregation due to the depletion forces. We revisit the dextran-RBC interaction on the single cell level using the laser tweezers combined with microfluidic technology and fluorescence microscopy. An immediate sorption of ~104 molecules of 70 kDa dextran per cell was observed. During the incubation of RBC with dextran, a gradual tenfold increase of adsorption was found, accompanied by a moderate change in the RBC deformability. The obtained data demonstrate that dextran sorption and incubation-induced changes of the membrane properties must be considered when studying RBC aggregation in vitro. © 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreemen

Visualization.avi

<p>The procedure of moving the RBC from the microchannel chamber filled with fluorescent dextran to the chamber with with PBS.</p> <p>Upon the transition, the detection mode is switched from reflection to fluorescence, and the emission signal from the cell is clearly observed.</p