Microfluidic cell-tracking technique in a comoving frame during pulsatile capillary flow*

One of the most intriguing features of red blood cells (RBCs) is their high deformability, which allows them to squeeze through microvessels smaller than the RBC size. Additionally, this flexibility results in a broad range of RBC shapes in microfluidic flows, depending on their confinement and flow velocity. RBCs are often recorded at a fixed channel position using optical microscopy to study their flow and shapes in microfluidic devices. However, this approach limits investigations of the RBC flow behavior and shape transitions to merely a few hundred micrometers, depending on the used camera and objective.

This study presents a microfluidic technique that allows us to track RBCs over several centimeters along the channel flow direction. The microfluidic setup consists of an inverted microscope equipped with a motorized stage, a 20x objective, a highspeed camera, and a high-precision pressure device to generate a time-dependent driving of the flow. We use a customized feedback-control mechanism between the stage and camera to obtain the comoving frame. RBCs are instantaneously tracked, and the observation frame follows the cells in the microfluidic device with a velocity of up to 10 mm/s. Our method enables us to probe the flow behavior and shape transitions of individual RBCs along the channel flow direction under steady and pulsatile flow. Thus, the presented technique opens new pathways to study cell dynamics and interactions in microfluidic devices under physiologically relevant flow conditions.