A passive microfluidic device for continuous buffer exchange

Generally, dielectrophoresis (DEP) analysis of biological cell samples relies on the differing electrical parameters between the cells and the surrounding fluid medium. To achieve effective positive DEP manipulation and sorting of mammalian cells in suspension, it is required to resuspend the cells into a low-conductivity fluid buffer. The use of a low conductivity buffer also aids in minimizing the effects of Joule heating, which can cause cell death and ineffective cell trapping. The common method to prepare the sample relies on centrifugation of sensitive cells, a time-consuming and tedious process that may result in decreased sample viability. Herein is presented a microfluidic device that passively moves cells from a high-conductivity growth media into a low-conductivity DEP buffer. It is comprised of con- verging rows of pillars and uses mechanical filtration to force cells into the new buffer while allowing for the old fluid to flow through the posts and out of separate outlets. Because this device is intended to be used upstream of a contactless dielectrophoresis (cDEP) device, the buffer exchange device must have an outlet flow rate that is within the range necessary for direct integration with the cDEP device, maintain a low shear stress that will not affect the integrity of the sample and achieve sufficiently high cell recovery. Methods of this project included optimizing the shape, size, and orientation of the posts, determining the flow rate for maintaining an ideal DEP buffer conductivity, numerical modeling of shear stress, and determining the cell recovery rate. It is anticipated that this device can be extended to physiological media sample processing such as for liquid biopsy.