Open microfluidic systems, which have at least one air-liquid interface, are used for a variety of biological assays such as monitoring cell cultures. They are also robust and simple to fabricate. But their very openness makes flow through them difficult to control. David J. Beebe of the University of Wisconsin, Madison, and coworkers have developed a new method, called suspended microfluidics, to establish control of such open systems (Proc. Natl. Acad. Sci. USA 2013, DOI: 10.1073/pnas.1302566110). In suspended microfluidics, surface tension fills and maintains fluid flow in channels that have neither floors nor ceilings. The phenomenon relies on spontaneous capillary flow, which depends on the wall height, the channel width, and the fluid contact angle. Beebe and coworkers derived an analytical model that allows them to design suspended microfluidic systems of various geometries. Their model also predicts known phenomena such as capillary flow in tubes. They used suspended microfluidics to create an array of collagen membranes in a plane between two fluidic chambers. They showed that this array could be used as a system for three-dimensional screening of cell cultures. They also made a device for extracting small molecules from cultured cells for metabolite analysis.