Blood viscosity decreases with shear stress, a property essential for an efficient perfusion of the vascular tree. Shear thinning is intimately related to the dynamics and mutual interactions of RBCs, the major component of blood. Because of the lack of knowledge about the behavior of RBCs under physiological conditions, the link between RBC dynamics and blood rheology remains unsettled. We performed experiments and simulations in microcirculatory flow conditions of viscosity, shear rates, and volume fractions, and our study reveals rich RBC dynamics that govern shear thinning. In contrast to the current paradigm, which assumes that RBCs align steadily around the flow direction while their membranes and cytoplasm circulate, we show that RBCs successively tumble, roll, deform into rolling stomatocytes, and, finally, adopt highly deformed polylobed shapes for increasing shear stresses, even for semidilute volume fractions of the microcirculation. Our results suggest that any pathological change in plasma composition, RBC cytosol viscosity, or membrane mechanical properties will affect the onset of these morphological transitions and should play a central role in pathological blood rheology and flow behavior.

A recent study of red blood cells (RBCs) in shear flow [Lanotte et al., Proc. Natl. Acad. Sci. U.S.A. 113, 13289 (2016)PNASA60027-842410.1073/pnas.1608074113] has demonstrated that RBCs first tumble, then roll, transit to a rolling and tumbling stomatocyte, and finally attain polylobed shapes with increasing shear rate, when the viscosity contrast between cytosol and blood plasma is large enough. Using two different simulation techniques, we construct a state diagram of RBC shapes and dynamics in shear flow as a function of shear rate and viscosity contrast, which is also supported by microfluidic experiments. Furthermore, we illustrate the importance of RBC shear elasticity for its dynamics in flow and show that two different kinds of membrane buckling trigger the transition between subsequent RBC states.

Red blood cell (RBC) flow-induced clustering, which is a physiopathologically-relevant process in microcirculation, is usually attributed to slower RBCs acting to create a train of trailing cells. Here, based on the first systematic investigation of collective RBC flow behavior in microcapillaries in vitro by high-speed video microscopy and 2D boundary integral numerical simulations, we show that RBC clustering is elicited by hydrodynamical interactions only, even if the effect of polydispersity is taken into account. Furthermore, lower applied pressure drops and longer residence times favor larger RBC clusters. The insight on the mechanism of RBC clustering provided by this work can be applied to further our understanding of RBC aggregability, which is a key parameter implicated in clotting and thrombus formation

We report on the design of microchannels made of glass capillary coated with polymer brushes elaborated by the so-called "grafting-from" technique. We present measurements of velocity profiles for pressure-driven flows of water in such "hairy" capillaries. We show that the flow reduction induced by the presence of the brush is unexpectedly greater than what could be anticipated from simple geometric arguments on the reduction of the effective capillary diameter or from predictions by models describing the brush layer as a poro-elastic boundary.

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Composite material constituted by Fe micro-particles homogeneously dispersed in a silicone matrix, at a volume concentration slightly above the percolation threshold but separated by a thin silicone layer, was produced. The particle magnetic softness and their average size, have been properly improved with respect to previous investigations in order to maximize the piezo-resistive and the piezo-magnetic effects. The optimal combination of magneto-elasticity and piezo-resistivity enables to achieve a record value of magneto-piezo-resistivity sensitivity. An analytical model is proposed to simulate the theoretically expected behavior of electric resistance vs. the applied induction field gradient, so to predict the magneto-piezoresistive response and explain the obtained material tailoring. The experimental results have been in good agreement with the theoretically predicted behaviors, so validating the employed model and the interpretation of the phenomenon. A simple basic application in position sensing is also reported. The analytical model presented in this paper has demonstrated its potentiality to project further improvements, while the experimental results allow for different innovative applications

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The confined flow of red blood cells (RBCs) in microvasculature is essential for oxygen delivery to body tissues and has been extensively investigated in the literature, both in vivo and in vitro. One of the main problems still open in microcirculation is that flow resistance in microcapillaries in vivo is higher than that in vitro. This discrepancy has been attributed to the glycocalyx, a macromolecular layer lining the inner walls of vessels in vivo, but no direct experimental evidence of this hypothesis has been provided so far. Here, we investigate the flow behavior of RBCs in glass microcapillaries coated with a polymer brush (referred to as "hairy" microcapillaries as opposed to "bare" ones with no coating), an experimental model system of the glycocalyx. By high-speed microscopy imaging and image analysis, a velocity reduction of RBCs flowing in hairy microcapillaries as compared to bare ones is indeed found at the same pressure drop. Interestingly, such slowing down is larger than expected from lumen reduction due to the polymer brush and displays an on-off trend with a threshold around 70 nm of polymer brush dry thickness. Above this threshold, the presence of the polymer brush is associated with an increased RBC deformation, and RBC velocity is independent on polymer brush thickness (at the same pressure drop). In conclusion, this work provides direct support to the hypothesis that the glycocalyx is the main factor responsible of the higher flow resistance found in microcapillaries in vivo.