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The syncopated groove of red blood cells : an ode to their discreet diversity.

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Red blood cells, which make up almost half of the blood volume and are the vehicle for oxygen transport in the body, have a subtle mechanics that allows them to function throughout the networks of the bloodstream. If their structure is relatively well known, there is still no satisfactory model of red blood cell mechanics to predict or simulate its behavior in complex flows or to understand the disorders caused by certain pathologies. LIPhy researchers, in collaboration with the Université Libre de Bruxelles, carried out an extensive experimental study of red blood cell dynamics in a shear flow, which accounts for a wide range of behaviors within a single blood sample and constitutes a reference work for the validation of new models.

Despite their relative simplicity compared to other living cells, red blood cells exhibit surprisingly complex and varied behavior depending on the flow conditions in which they occur at different levels of the circulatory network, from arteries to capillaries. Their deformability is essential to ensure their passage in the finest capillaries but also to limit the increase in viscosity due to their high concentration in blood. This deformability, which results from the deflated shape of cells (compared to a spherical cell), is governed by the mechanical properties of the membrane and of its elastic cytoskeleton. Fundamental questions persist about the properties of the cytoskeleton, its configuration at rest and its biomechanical links with the membrane, which are reflected in the global response of the red blood cell to mechanical and hydrodynamic stresses.
The researchers conducted an experimental study of the dynamics of a very large number of red cells in single shear flow, a reference configuration for the validation of theoretical and numerical models in which many modes of deformation and rotation are observed. A holographic microscopy technique developed by the ULB partner and adapted to the study of biological cell suspensions in flow allowed by an analysis of the distributions of orientation and apparent aspect ratio of cells in flow to distinguish and quantify different populations. In this way, statistically significant and representative results of the multiplicity of dynamic states (rotations, orientations and deformations) observable within the same blood sample were obtained in contrast with previous results obtained on a limited number of cells. This diversity within a given blood sample is due to the dispersion of mechanical properties that determine the transitions between the different motions as a function of flow velocity.

A remarkable result is the observation of a strong hysteresis in the dynamics (for the same shear rate, red blood cells behave differently depending on whether this shear rate increases or decreases), which raises interesting questions on the nonlinear physics of red blood cells.
This work should lead to progress in understanding the details of red cell mechanics and its fine modeling, as these parameters can also be significantly altered by pathologies with potentially severe circulatory consequences.

Voir en ligne : Dynamics of a large population of red blood cells under shear flow, C. Minetti, V. Audemar, T. Podgorski, G. Coupier, J. Fluid Mech. 864, 408-448 (2019)