Abstract
Microfluidic systems require fluid pumping. Here, we describe microfluidic devices that utilize gravity—which is free, steady, and ubiquitous on earth—as a driving force for generation of constant fluid flow as well as periodically switching flows. We also describe applications of such devices. Gravity-driven steady flow devices are described that enable microfluidic sperm sorting based on mobility and for cell culture where cells within different regions of a channel are treated with different concentrations of chemicals to analyze gradient responses. Gravity-driven fluidic oscillator devices are also described where a single device enabled the screening of the effect of three different shear stresses at four different frequencies (12 shearing conditions total) to reveal that there is an optimal frequency that endothelial cells respond to in terms of cell shape change.