DESIGNER CHANNELS FOR MICROFLUIDIC DEVICES.

NIST scientists are using ultrathin polymer layers, polyelectrolyte multilayers (PEMs), to control fluid flow in microfluidic devices. Microfluidic, or so-called "lab-on-a-chip," devices are miniaturized chemical and biochemical analysis systems that may one day replace conventional benchtop instruments. Plastics are emerging as ideal materials for these devices because of their low cost, chemical compatibility, and good optical properties. A given plastic, however, may not possess the right surface properties to sustain a desired electroosmostic flow (EOF). EOF is the most common-method used to pump liquids through microchannels and EOF behavior depends strongly on the surface characteristics of a material. The researchers find that coating channels with PEMs yields greater reproducibility and, with judicious selection of coatings, tailored channels for specific chemical analysis problems. The PEMs coatings are alternating positively and negatively charged polymer layers that are held in place by electrostat ic interactions. Because the direction of EOF depends on whether a surface is positively or negatively charged, the researchers find they are able to control the fluid flow in a device by coating the various arms of a channel with different PEMs. If the right coatings are chosen for the four arms of an X-shaped channel, for example, a single applied voltage will bring all four fluids to the center for mixing. In a more dramatic demonstration, the researchers coated the two halves of a channel with different PEMs and observed simultaneous bidirectional flow. Although a specific application for bidirectional flow hasnt been identified yet, it might be useful for studying reactions at the interface of two fluids or for separations. Two articles describing this work have appeared recently in the journal Analytical Chemistry and an application for a patent has been filed.