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RESEARCH PROFILE
posts created a non-uniform electric ration and concentration of live and In today’s world, there is a need to field across the channel. dead bacteria. Fintschenko and her colworry about not only naturally occurWhen live and dead bacteria were inleagues also demonstrated that the selecring water pathogens but also the threat troduced into the microchannel, they retive trapping of bacterial cells occurred of a bioterrorist attack on water supsponded to the non-uniform electric field even with a concentrated background of plies, which looms ever larger. Never by displaying different dielectrophoretic polystyrene particles. before has there been such a critical need to detect mi(c) (a) (b) crobes in water quickly and efficiently. Key to such analyses, say water quality experts, are good separation and concentration steps. In the March 15 issue of Anal. Chem. (pp 1571– Live (green) and dead (red) bacterial cells get trapped in different locations in a non-uniform electric field created 1579), Yolanda by an array of insulating posts in a microchannel device. (a) At low voltages only live bacteria are trapped. (b) and Fintschenko and (c) Trapping is more pronounced as the voltage increases. her colleagues at the Sandia National Laboratories describe behavior because of the dissimilar memThe use of insulator-based dielectrobrane conductivities. Bacterial cell mema method to separate and concentrate phoresis for separation and concentrabranes become permeable upon death, dead and live E. coli in water in a single tion is not just limited to live and dead step. The new approach, which uses insu- causing the membrane conductivity to bacteria. Fintschenko’s group has tested increase by a factor of 104 or more. In lator-based dielectrophoresis (iDEP), is the microchannel device on Gram negamore attractive than traditional techtive and positive bacteria and found that the microchannel, the dead bacteria had niques, such as centrifugation and filtrathe two types of bacteria can be distintion, because it can be automated, miniaguished. Fintschenko says three distinct turized, and arranged in parallel arrays. species of Bacillus can also be differentiThe genesis of the project came from ated in the microchannel device. conversations Fintschenko had with SanThe potential of the new device is dia’s Water Focus Group to find out its only starting to be explored. Fintschenko greatest need. Fintschenko says, “What and her colleagues are developing a pro[they] kept on coming back to was, ‘We totype that can be scaled up to accomneed a concentrator for pathogens.’” A modate as much as a liter of water with concentrator is necessary to separate live a flow rate of 20 mL/min, allowing bacteria from all the other inert and dead pathogens to be collected from drinking particles in water. Fintschenko adds, “I water. The researchers are also investigatactually talked to a guy from a water ing the possibility of producing the mianalysis company who told me, ‘If you crochannel device in plastic. If the device could, at the very least, separate the live can be stamped out on plastic sheets that out from the dead so we know we’re resemble Saran Wrap, it can be made looking at something that’s alive, instead more portable and disposable and can of a bunch of dead particles, then that be mass-produced. One of Fintschenko’s would be enormously helpful.’” colleagues, Eric Cummings, points out, significantly smaller dielectrophoretic With this in mind, Fintschenko and “If you sandwich a hundred layers of emmobilities than live ones, so the two her colleagues designed a microchannel bossed Saran Wrap, you can get macrotypes of cells ended up trapped in disthat contained an array of insulating scopic throughput in a cheap system that tinctly different locations in the miposts etched in glass. They then applied is no larger than most microfluidic syscrochannel’s non-uniform electric field. a dc electric field across the microchantems right now.” a The differential trapping took
