Bulletin of the American Physical Society
2006 59th Annual Meeting of the APS Division of Fluid Dynamics
Sunday–Tuesday, November 19–21, 2006; Tampa Bay, Florida
Session EB: Biofluid Dynamics VI: Cells |
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Chair: Juan Lasheras, University of California, San Diego Room: Tampa Marriott Waterside Hotel and Marina Grand Salon F |
Sunday, November 19, 2006 4:15PM - 4:28PM |
EB.00001: Characterization of endothelial cell remodeling using Multiple Particle Tracking Microrheology. Juan C. del Alamo, Gador Canton, Yi-Shuan Li, Gerrard Norwich, Shu Chien, Juan C. Lasheras The cytoskeleton of endothelial cells remodels itself in response to external mechanical stimuli, causing changes in the proliferation and orientation of stress fibers. We speculate that these changes modify the magnitude and isotropy of the viscoelastic cytoplasmatic shear moduli to minimize the cell's internal deformation energy. To assess this idea, we have applied Multiple Particle Tracking Microrheology (MPTM) to vascular endothelial cells subjected to different stress protocols. This technique is based on the observation of the Brownian dynamics of small intracellular markers (either endogenous or exogenous), and has been preferred to the more standard Magnetic Twisting Cytometry because it is more inocuous for the cytoskeleton. We have extended the MPTM technique to provide the anisotropy of the shear moduli and to account for local effects of cell crawling or spreading. [Preview Abstract] |
Sunday, November 19, 2006 4:28PM - 4:41PM |
EB.00002: Traction Forces exerted by crawling cells Baldomero Alonso-Latorre, Juan C. del Alamo, Javier Rodriguez-Rodriguez, Alberto Aliseda, Rudolph Meili, Richard Firtel, Juan C. Lasheras We measure the forces exerted by Dictyostelium discoideum cells crawling over a deformable substrate from the displacements of fluorescent beads embedded in it. A particle tracking technique similar to PIV is used to obtain the displacements. From them, forces are computed by solving the elasto-static equation in a finite thickness slab. We will show that the finite thickness of the substrate and the distance of the beads to its surface affect substantially the results, although previous traction cytometry techniques neglected them. The measured forces are correlated to the different stages of the crawling cycle for various cell strains. It has been observed that a large fraction of the forces measured on the substrate are originated by the cell's internal tension through all the stages of motion, including the protrusion of pseudopods. This result suggests that the viscous drag exerted by the fluid in which the cells are immersed is very small compared to the forces applied by the cytoskeleton on the substrate. [Preview Abstract] |
Sunday, November 19, 2006 4:41PM - 4:54PM |
EB.00003: Red blood cells and platelet aggregation in small blood vessels via Dissipative Particle Dynamics. Igor Pivkin, Peter Richardson, George Karniadakis Blood is composed of a liquid component and cellular components. The cellular components include red blood cells and platelets. Explicit simulations of the cellular components require computational methods capable of tracking time-varying fluid-solid interface. The Dissipative Particle Dynamics (DPD) is an inherently adaptive method and potentially very effective in simulating complex fluid systems. In DPD, the fluid and solid objects are represented as a collection of interacting points, each representing a group of atoms or molecules. We will present results on the effects of the red blood cells on the platelet aggregation in a small blood vessel using DPD. [Preview Abstract] |
Sunday, November 19, 2006 4:54PM - 5:07PM |
EB.00004: In vitro measurements of pulsatile flow over endothelial cells Chia Min Leong, Timothy Wei, Gary Nackman Alterations in mechanotransduction by endothelial cells to underlying smooth muscle cells is a key factor in human arterial diseases such as atherosclerosis and initmal hyperplasia. The goal of this study was to determine the relative importance of the spatially and temporally varying pressure field over the cells relative to the local wall shear stress under different flow conditions. In vitro high resolution micro-PIV measurements were made over cultured endothelial cells flush mounted in a small rectangular channel. Using multiple measurement planes, local surface height, surface pressure, and wall shear stress could be extracted from the measurements. For the steady laminar flow case, data clearly indicate that surface pressure is on the order of wall shear. A comparison of the steady flow case with a pulsatile flow case will also be reported. Consequently, the total stress tensor acting on ECs needs to be considered in examining bio- chemical activity in vascular diseases. [Preview Abstract] |
Sunday, November 19, 2006 5:07PM - 5:20PM |
EB.00005: A Discrete-Element Approach for Blood Cell Adhesion Jennifer Chesnutt, Jeffrey Marshall An efficient computational model for simulation of the individual dynamics of adhering blood cells is discussed. Each cell is represented as a discrete particle so that the model can extend existing discrete-element approaches for dense particulate fluid flows to account for receptor-ligand binding of particles, elliptical particle shape, and deformation of the particles due to shear forces. Capabilities of the method in simulating large numbers of particles are illustrated through simulations of the formation of red blood cell rouleaux in shear flow. The effects of several factors, such as aspect ratio of the elliptical particle, shear rate, strength of the cell adhesion force, and hematocrit are investigated. Comparison of the discrete-element results with results of a level-set approach which computes the entire flow field about a small number of cells is used to develop an improved model of the effect of nearby red blood cells on the cell drag force expression. The method is also being applied to examine the influence of red blood cells on other components of the blood, such as platelet dispersion and activation in high shear regions. [Preview Abstract] |
Sunday, November 19, 2006 5:20PM - 5:33PM |
EB.00006: Leukocyte Margination in a Model Microvessel Jonathan Freund In the inflammation response, multi-body interactions of blood cells in the microcirculation bring leukocytes (white blood cells) to the vessel walls. We investigated the fluid mechanics of this using numerical simulations of 29 red blood cells and one leukocyte flowing in a two-dimensional microvessel. The cells are modeled as linearly elastic shell membranes. Though obviously simplified, this model reproduced the increasingly blunted velocity profiles and increased leukocyte margination observed at lower shear rates. To study its effect, we varied the relative stiffness of the red cells by over a factor of ten, but the margination was found to be much less correlated with this than to the bluntness of the mean velocity profile. The detailed velocity field around near-wall leukocyte was sensitive to the red cell stiffness, but it changed little for strongly versus weakly marginating cases. In the more strongly marginating cases, however, a red cell is typically leaning on the upstream side of the leukocyte and appears to stabilize it. A well-known feature of the microcirculation is a near-wall cell-free layer. We observed that the leukocyte's most probable position was at the edge of this layer, whose thickness increased following a lubrication scaling. The leukocyte's near-wall position is observed to be less stable with increasing mean stand-off distance, but this distance would have potentially greater effect on adhesion since the range of the molecular binding is so short. [Preview Abstract] |
Sunday, November 19, 2006 5:33PM - 5:46PM |
EB.00007: Multiparticle Simulations of Deformable Red Blood Cells using Lattice-Boltzmann Method Robert MacMeccan, Jonathan Clausen, Sheila Rezak, G. Paul Neitzel, Cyrus Aidun The rheology of blood flow is largely dependent on red blood cell (RBC) membrane deformation. Furthermore, simulating blood at physiologic hematocrit requires inclusion of RBC deformation. A new method is developed by coupling the finite-element method for RBC's to the lattice-Boltzmann method for fluid flow. An elastic finite element model provides easy incorporation into the lattice-Boltzmann framework and enough computational efficiency to simulate suspensions at high volume fractions. Red blood cell deformation and surface stress distribution are discussed for three dimensional multiparticle simulations in wall-bounded shear flow. Due to the versatility of finite-element in describing the geometry and deformation of solid particles, this method may be applied to any suspensions of deformable and rigid particles. [Preview Abstract] |
Sunday, November 19, 2006 5:46PM - 5:59PM |
EB.00008: Deformation-induced Lift on Receptor-Ligand Mediated Cell Adhesion to Substrates Explored by a 3-D Computational Fluid Dynamics approach Xiaoyi Li, Kausik Sarkar The adhesion of cells to substrates is a critical step in plenty of biological events. The effects of cell deformation on the adhesion process have been investigated using a direct fluid dynamics simulation based on front-tracking method. A model including membrane elasticity and stochastic receptor-ligand binding has been developed. The study reveals a surprising effect of cell deformation. An asymmetry in upstream-downstream flow field due to cell deformation results in a hydrodynamic lift. The lift force counterbalances the shear torque and causes reduced contact area and reduced number of bond formed, and leads to cell detachment at relatively \textit{low} shear rate. The finding of lift could be used to partially explain the shear threshold phenomenon occurring at small shear stresses. [Preview Abstract] |
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