Start-up shape dynamics of red blood cells in microcapillary flow.

Red blood cell (RBC) deformability plays a key role in oxygen exchange between blood and tissues in microcirculation by allowing RBCs to flow in vessels of diameter even smaller than cell size. Hence, RBC flow in microcapillaries has been widely studied in vitro, mostly under steady-state conditions. Here, we provide the first quantitative investigation of the transient behavior of RBC shape in confined Poiseuille flow in vitro. Our approach is based on high-speed video microscopy imaging of RBCs flowing in silica microcapillaries and quantitative data processing by image analysis techniques. In start-up flow, RBCs undergo a complex transition from the biconcave shape to a parachute-like configuration through membrane folding and cytoplasm reorganization. The time scale of this transient process is independent on the applied pressure drop and the measured value for healthy cells (around 0.1s) is in agreement with previous micropipette data from the literature. Glutaraldehyde (GA)-hardened RBCs exhibit a faster shape evolution at higher GA concentration, thus showing that the corresponding time scale becomes shorter at increasing cytoskeleton elasticity. Our results provide a novel microfluidics methodology to measure the RBC characteristic time which is a potential diagnostic parameter of altered cell deformability.