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Microfluidic device for creating gradients
the number of inputs. It’s basically an (n –1) polynomial. If you have six inputs, you’ll be able to generate a very steep gradient. If you have 2 inputs and 24 outputs, you would just generate a linear gradient,” he says.
analyzed. A gradient of 19 different concentrations over 4 orders of magnitude was generated in the device, and barium was gradually replaced by potasProducing a large set of serially diluted sium. As the cell was scanned over the solutions can be painstaking, time-condifferent concentrations, responses of suming work. To speed up the process, the ion channels were Owe Orwar and colleagues at recorded. With one the Chalmers University of patch-clamped cell and Technology, Göteborg Univerone stock solution, the sity, and Cellectricon AB (all in investigators were able Sweden) have developed a mito acquire a matrix of 19 crofluidic device that can gen 11 data points within erate concentration gradients 30 min. over ~5 orders of magnitude The GABAA receptors from a single stock solution. The device, which is dewere studied in a similar scribed in this issue of Analytifashion. The device genercal Chemistry (pp 3897–3903), ated a gradient of GABA was initially conceptualized for with 23 concentrations ion-channel research. Ionranging from 0.22 to channel behavior is widely 1000 µM. A cell expressstudied by the pharmaceutical ing the receptors was exindustry for drug development posed to each concentrain which the response of protion for 100 ms, and this A microfluidic gradient-generating device, loaded with three fluoroteins to drugs is measured over allowed the investigators phores, displays the resulting concentration gradients. a large concentration range for to collect an entire dose– The investigators used the device to target validation, optimization, and saferesponse curve in ~2 s. ty. Generating solutions for a given drug profile the response of two ion chanThe conventional method of studynels—voltage-gated hERG K+ channels that cover concentrations of several oring GABAA receptors usually involves ders in magnitude is key in dose–reand ligand-gated GABAA receptors. Bewash steps between exposures to differsponse measurements. ent GABA concentrations. Orwar and cause the voltage-gated hERG K+ chanTo produce the necessary set of sericolleagues demonstrated that the wash nels help to repolarize the heart after ally diluted solutions, Johan Pihl, the steps were unnecessary in their system contraction, the channels are studied first author on the paper, says, “You during drug-safety tests to ensure against because of the short exposure times to routinely mix things over 3 or 4 orders controlled solution environments. undesirable side effects. GABAA recepof magnitude, typically using normal The device is not limited to dose– tors are found in the central nervous micropipettes. If you do it over that response measurements of ion channels. system and are activated by -aminokind of range for serial dilutions, it’s rel- n-butyric acid (GABA), an inhibitory It could, in principle, be used to analyze atively easy to get systematic errors.” any type of biological or physical sensor. neurotransmitter. The new microfluidic device replaces Pihl says, “You can basically [use] it for To analyze the behavior of the ion the micropipettes. The device, fabricatanything that is hard to incorporate into channels, the investigators held a cell, ed from silicon on insulator wafers, has a microchannel. Then it can be a very expressing either hERG K+ channels or six input wells. The wells are connected good system to expose sensors to very GABAA receptors, by patch-clamping. to an open-volume area via a microfluThe patch-clamped cell was then exposed well defined solution environments.” idic dilution network. The network con- to a concentration gradient of an analyte He also points out that the device tains 18 generations of dilution and could be used for synthesis of molein the device. “We translate the micromixing in which the mixing occurs by cules on a surface. “For instance, if scope table with the chip so the cell is diffusion. At the exit to the open-volstationary and the chip is translated. This you’re going to synthesize DNA, then ume area, the network expands into 24 is done using a computer-controlled mo- you want to control the amount of channels. time you expose the little piece you’re torized scanning stage,” says Pihl. Pihl explains the rationale for using The response of voltage-gated hERG starting with in a very well defined six input wells. “The steepness of the di- K+ channels to barium—a known block- manner.” a lution gradient will be dependent on —Rajendrani Mukhopadhyay er of the channel—and potassium was J U LY 1 , 2 0 0 5 / A N A LY T I C A L C H E M I S T R Y
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