Abstract-Cells and organelles are shaped by the chemical and physical forces that bend cell membranes. The human red blood cell (RBC) is a model system for studying how such forces determine cell morphology. It is thought that RBCs, which are typically biconcave discoids, take the shape that minimizes their membrane-bending energies, subject to the constraints of fixed area and volume. However, recently it has been hypothesized that shear elasticity arising from the membrane-associated cytoskeleton (MS) is necessary to account for shapes of real RBCs, especially ones with highly curved features such as echinocytes. In this work we tested this hypothesis by following RBC shape changes using spherical harmonic series expansions of theoretical cell surfaces and those estimated from 3D confocal microscopy images of live cells. We found (i) quantitative agreement between shapes obtained from the theoretical model including the MS and real cells, (ii) that weakening the MS, by using urea (which denatures spectrin), leads to the theoretically predicted gradual decrease in spicule number of echinocytes, (iii) that the theory predicts that the MS is essential for stabilizing the discocyte morphology against changes in lipid composition, and that without it, the shape would default to the elliptocyte (a biconcave ellipsoid), (iv) that we were able to induce RBCs to adopt the predicted elliptocyte morphology by treating healthy discocytes with urea. The latter observation is consistent with the known connection between the blood disease hereditary elliptocytosis and spectrin mutations that weaken the cell cortex. We conclude that while the discocyte, in absence of shear, is indeed a minimum energy shape, its stabilization in healthy RBCs requires the MS, and that elliptocytosis can be explained based on purely mechanical considerations.

The viscous drag and mechanical deformation of a single vesicle under hydrodynamics flow during the phase transition of a lipid bilayer is determined by optical tweezers experiments with the aid of computational fluid dynamics simulations. Based on the experimental geometry of the vesicle under hydrodynamics flow, the surface stresses and drag force are numerically calculated. It is found that the vesicle is less rigid and the viscous drag force of the vesicle decreases with the increase of temperature at low Reynolds number flow during sample heating. Interestingly, these mechanical properties are reversible and depend strongly on the liposome's thermotropic phase transition temperature. Overall, this study provides new insights into highly coupled thermal and hydrodynamics effects on the biomechanical properties of model membrane vesicle in physiological flow systems.

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The flow-induced mechanical deformation of a human red blood cell (RBC) during thermal transition between room temperature and 42.0 degrees C is interrogated by laser tweezer experiments. Based on the experimental geometry of the deformed RBC, the surface stresses are determined with the aid of computational fluid dynamics simulation. It is found that the RBC is more deformable while heating through 37.0 degrees C to 42.0 degrees C, especially at a higher flow velocity due to a thermal-fluid effect. More importantly, the degree of RBC deformation is irreversible and becomes softer, and finally reaches a plateau (at a uniform flow velocity U > 60 microm s(-1)) after the heat treatment, which is similar to a strain-hardening dominated process. In addition, computational simulated stress is found to be dependent on the progression of thermotropic phase transition. Overall, the current study provides new insights into the highly coupled temperature and hydrodynamic effects on the biomechanical properties of human erythrocyte in a model hydrodynamic flow system.

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