Microfluidic SystemsThe ready availability on the market of porous membranes with cylindrical pores of 15 to 200 nm and a thickness of 6 to 10 ìm facilitates the development of devices for operating three-dimensional analytical units at a liter scale. Using these membranes as gates at the interface of two crossed microfluidic channels, the rate and direction of fluid exchange can be controlled with the electrical potential, polarity, ionic strength of the solution or the diameter of the nanocapillary1. Microfluidic channels, fabricated by soft lithography, have been used for a decade. Dr. Paul W. Bohn, 100-year-old professor of chemical sciences at the University of Illinois at Urbana-Champaign, sees advances toward multilayer liquid chromatography as a key step in the development of micrototal analysis systems (ìTAS). , which would involve such new applications as injection, collection, mixing, switching and sensing. Recently, he has studied the responses of analytes to various stresses applied to the system and its behavior deviations compared to that of a similar system at the macro scale. Microfluidic channels are a convenient and durable means of fluid transport made of poly(dimethylsiloxane) (PDMS), a common polymer with nonpolar side groups. PDMS is durable, very flexible and elastic, oxygen permeable and very hydrophobic2. It also has a negative surface charge density at pH 81. The soft lithography method allows the rapid deposition of complex, intersecting two-dimensional fluid pathways on a silicon wafer. The membrane containing these nanopores is a nuclear membrane etched in polycarbonate 6 to 10 microns thick. (PCTE) which has been coated with poly(vinylpyrrolidone) (PVP) to make it hydrophilic. This coating gives a pH of 8 in the system3. The pores of the membrane are cylindrical and have a diameter between 15 and 200 nm. The size of these pores is of the same order of magnitude as the Debye length (ê-1) of ionic interactions in solution (1 nm < ê-1 < 50 nm) when the ionic force is of the order of millimmolar1. The physical character of the nanopore allows that a modification of the ionic strength of the solution is sufficient to modify the interaction between the solution and the nanopore. By simply changing the concentration, the nature of the flow induced by the electric potential can be switched between electrophoresis and electro-osmosis1. The flow direction can be controlled by the size of the nanopore. For large pores, the negative surface charge density on the microfluidic channel caused by the slightly basic pH of the system