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Microfabrication in the FLO
Optical micrographs of the stepwise fabrication of a three electrode system inside a 200-um-wide channel. (Adapted with permission. Copyright 1999 American Association for the Advancement of Science.)
Miniaturized mixer Improved performance and faster analysis times are two key benefits of miniaturization. At such small scales, however, the mixing time is often as long as or longer than the reaction time, which leads to insufficient mixing. To overcome this problem, Andreas Manz and co-workers at Imperial College of Science, Technology, and Medicine (U.K.) have developed a microstructure for continuous-flow mixing based on laminar flow theory. The micromixer follows distributive mixing principles, in which the fluid path is physically split into smaller segments and redistributed. Such an arrangement reduces the striation thickness, enhancing diffusion and leading to faster mixing times. The entire device, which is made from a glass/silicon/glass sandwich, measures only
Microstructures inside capillaries are usually fabricated by conventional photolithography with masks, which can be expensive and time-consuming. George M. Whitesides and co-workers at Harvard University let chemistry do the work for them in a technique that they call "fabrication using laminar flow" or FLO. In FLO, multiple liquid streams are mixed laminarly in a channel, so there is no turbulent mixing. Reactions occur at the interface between the liquids or between the liquid and the channel surface. The only mechanism for mixing is diffusion across the interfaces. Whitesides and his group fabricated a variety of structures using FLO. For example, silver was deposited in a capillary by the parallel laminar flow of two components of a commercial electroless silver plating solution. The "wire" was electrically continuous, significantly narrower than the channel, and smoother than the capillary wall. The researchers also fabricated a more complicated system of an array of three microelectrodes inside a 200-um capillary. A polydimethylsiloxane membrane contain-
ing the capillary network was placed on a glass with the main channel oriented perpendicularly to a gold strip that had been deposited. Flowing an aqueous gold etchant as the middle phase of a three-phase laminar flow system created a two-electrode system. The size of the electrodes was determined by controlling the relative volumes of the three phases. The reference electrode was generated between the two gold electrodes by depositing a silver wire similar to the one described previously and then treating it with 1% HC1 tt form AgCl on the surface. The performance of the electrode system was demonstrated with cyclic voltammetry of Ru(NH) Cl in water. The authors point out that FLO can generate structures with features
