Shape transitions of red blood cells in time-dependent microfluidic flows

The cardiovascular system is a complex network of branching vessels that transport and distribute blood through our bodies. Blood is mainly comprised of highly deformable red blood cells (RBCs) suspended in plasma that determine the unique flow properties of blood in the pulsatile flow of the circulation. In microfluidic experiments under stationary flow conditions, RBCs exhibit characteristic cell shapes, such as parachutes and slippers, depending on the flow rate and the channel confinement. However, knowledge about the cell shape transitions in unsteady flows, as it is in the circulation, is still lacking. Therefore, we probe the flow behavior of RBCs in time-dependent flow fields using microfluidic experiments in combination with optical microscopy. Employing a customized feedback-control mechanism allows us to track and follow single RBCs along the channel flow direction during the flow modulation. We analyze the time-dependent shapes and dynamics of the cells as a function of the pressure waveform, amplitude, and frequency to understand how the time scale of the flow couples with the characteristic time scale of single RBCs in capillaries.