Microfluidic Systems :: essays research papers

Microfluidic Systems The ready market availability of porous membranes with cylindrical pores of 15-200 nm and a thickness of 6-10 à ¬m facilitates the development of three dimensional analytical unit operation devices on an attaLiter scale. By employing these membranes as gates at the interface of two crossed microfluidic channels, the rate and direction of the fluid exchange can be controlled with electrical potential, polarity, solution ionic strength or diameter of the nanocapillary1. The microfluidic channels, fabricated by soft lithography, have been used for a decade. Dr. Paul W. Bohn, Centennial Professor of Chemical Sciences at the University of Illinois at Urbana-Champaign, sees the advance to multilayered liquid chromatography as a key step in the development of micro total analysis systems (à ¬TAS), which would involve such new applications as injection, collection, mixing, switching and detection. Recently he has been studying the analyte responses to various constraints applied to the sys tem and its deviations in behavior from that of a similar system on the macro scale.   Ã‚  Ã‚  Ã‚  Ã‚  Microfluidic channels are a convenient and durable means of fluid transport made of poly(dimethylsiloxane) (PDMS), a common polymer with non-polar side groups. PDMS is durable, highly flexible and elastic, oxygen permeable and very hydrophobic2. It also has negative surface charge density at pH 81. The method of soft lithography allows for rapid deposition of complex crossed two dimensional fluid pathways on a silicon wafer.   Ã‚  Ã‚  Ã‚  Ã‚  The membrane containing these nanopores is a 6 – 10 micron thick polycarbonate nuclear track-etched membrane (PCTE) that has been 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 of diameters in the range of 15 – 200 nm. The size of these pores are of the same order of magnitude of the Debye length (à ª-1) of the ionic interactions in solution (1 nm < à ª-1 < 50 nm) when the ionic strength is in the millimmolar range1.   Ã‚  Ã‚  Ã‚  Ã‚  The small physical character of the nanopore allows for a change in ionic strength of the solution to be sufficient to alter the interaction between the solution and the nanopore. By merely changing the concentration, the nature of the flow induced by electrical potential can be switched between electrophoresis and electro osmosis1.   Ã‚  Ã‚  Ã‚  Ã‚  The direction of the flow can be controlled by the size of the nanopore. At large pore sizes, the negative surface charge density on the microfluidic channel caused by the slightly basic pH of the system