Young 4ever--the use of capillarity for passive flow handling in lab on a chip devices.

When we do our everyday fluid handling in the laboratory such as filling, emptying, metering and cleaning, we often don’t realize that we use a set of everyday forces that are ‘always there’ because they just go without saying. These forces are gravity, suction and the capillary force, for the application of which we need no sophisticated equipment. Apart from their use for fluid handling they were therefore also the first forces to be used for liquid propulsion in separation and analysis systems in the 19th and 20th century laboratory, e.g., the capillary force in thin layer chromatography or in test strips (from pH paper to clinical test strips) and gravity in liquid chromatography. To these everyday forces for fluid handling we could also add the centrifugal force (e.g., in hand-operated bench top centrifuges) and evaporation. The ready availability of these everyday forces makes them particularly suited for fluid handling and propulsion in diagnostics and point-of-care (POC) devices. When such systems are miniaturized, the capillary force scales the best because it is exerted at the contact perimeter of liquid and channel wall, and the perimeter becomes larger relative to the channel cross-sectional area on downscaling. The capillarity-induced pressure, which is the capillary force per unit channel area, is therefore inversely proportional to the characteristic channel dimension as shown in Table 1. Classically, paper, fabric, membranes or mesh is used for capillary propulsion and flow control, for example in glucose strips. Precise flow control and metering however can be problematic in such devices and the past few years have therefore seen a surge in interest in microfluidic POC devices employing capillarity. Also outside the area of POC devices the interest in the use of capillarity in LOC devices has recently increased. This Focus article attempts to give an overview of this developing field, and will show that there is still a lot of potential in the capillary phenomena described in 1805 by Young and de Laplace.