Hematology & Blood Disorders

Biography

Biography: Annie Viallat

Abstract

Red blood cell (RBC) deformability is postulated to be a major determinant of impaired perfusion, increase of blood viscosity and occlusion in micro-vessels. Deformability refers to the ability for the RBC shape to change in response to external mechanical stresses. For instance, current deformability tests such as ektacytometry1 measure global parameters related to shape changes at the whole cell scale. But for large surface area-to-volume ratios, many shapes can be found for a given set of values of cell surface area and cell volume. Shape change is obtained by redistribution of the inner cytoplasmic volume of the cell and involves only a small perturbation of the membrane energy due to local changes of the membrane curvature. However, strong shear deformation of membrane elements can occur without shape change, for instance after a displacement of the membrane elements along the discocyte shape. These local deformations are typically involved during the tanktreading motion of RBCs in blood flow. They are governed by the viscoelastic properties of both cell membrane and cytoplasm. Finally, the term deformability is also often used to evoke the propensity of the cell membrane to rupture under a tension, for example in a vessel constriction or in the spleen. Despite strong advances in our understanding of the molecular organization of RBCs, the relationships between the global deformability tests, the cell behavior both in micro-flows and in vivo and the rheology of each element of RBCs composite structure are still not elucidated.We propose microfluidic tools to assess the mechanical parameters of RBCs at both suspension and single-cell level. We believe that these tools can be exploited to probe cellular-scale changes to environmental factors. We first show that i) the critical shear rate of the tanktreading-to-tumbling transition and ii)the variation of the swinging frequency with the shear rate of RBCs in shear flow enable the determination of two mechanical parameters2. First, an effective viscosity that accounts both for the cytoplasm and the membrane viscosity, and, second, an effective shear elasticity that accounts both for the shear modulus of the membrane and the stress-free shape of the cell. The stress-free cell shape is the shape for which the membrane elements are not deformed. Recently it was suggested that this shape is close to the sphere for healthy cells. The existence of other possible stress-free shapes in pathological cells has never been investigated and is still an open question. We also show that, in less viscous fluids, the orientation of RBCs versus the shear rate is a signature of the cell shear modulus. We then show that the entrance of RBCs in narrow channels enables the characterization of the fragility of RBCs to membrane rupture. Finally, we show that microfluidics enables to subject flowing RBCs to various environmental factors (such as oxidative stress or variation of dioxygenation) and to measure their mechanical response in-situ.