Microfluidic Systems The ready availability on the market of porous membranes with cylindrical pores of 15-200 nm and a thickness of 6-10 ìm facilitates the development of operational devices for three-dimensional analytical units on an attaLiter scale. By using these membranes as gates at the interface of two crossed microfluidic channels, the speed and direction of fluid exchange can be controlled with electrical potential, polarity, ionic strength of the solution, or diameter of the nanocapillary1. Microfluidic channels, fabricated by soft lithography, have been used for a decade. Dr. Paul W. Bohn, a centennial professor of chemical sciences at the University of Illinois at Urbana-Champaign, sees progress toward multilayer liquid chromatography as a critical step in the development of micro total assay systems (ITAS), which it would involve such new applications as injection, collection, mixing, switching and sensing. He recently studied the responses of analytes to various constraints applied to the system and its deviations in behavior from that of a similar system on a 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, highly flexible and elastic, permeable to oxygen and very hydrophobic2. It also has a negative surface charge density at pH 81. The soft lithography method enables the rapid deposition of complex two-dimensional crisscrossed fluid paths onto a silicon wafer. The membrane containing these nanopores is a 6 - 10 micron thick polycarbonate (PCTE) nuclear track etched membrane coated with poly(vinylpyrrolidone) (PVP) to make it hydrophilic. This coating results in a pH of 8 in the system3. The pores in the membrane are cylindrical and between 15 and 200 nm in diameter. The dimensions of these pores are 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 strength is in the millimmolar range1. The physical character of the nanopore allows a change in the ionic strength of the solution to be sufficient to alter the interaction between the solution and the nanopore. By simply changing the concentration, the nature of the flow induced by the electrical potential can be switched between electrophoresis and electroosmosis1. The flow direction can be controlled by the size of the nanopore. With large pores, the negative surface charge density on the microfluidic channel is caused by the slightly basic pH of the system