385 research outputs found
Dynamics of Coalescence of Plugs with a Hydrophilic Wetting Layer Induced by Flow in a Microfluidic Chemistrode
No full textThis manuscript analyzes the dynamics of coalescence of an incoming aqueous plug with a wetting layer above a hydrophilic surface in the chemistrode. The chemistrode is a recently described (Chen, D.; Du, W.; Liu, Y.; Liu, W.; Kuznetsov, A.; Mendez, F. E.; Philipson, L. H.; Ismagilov, R. F. Proc. Natl. Acad. Sci. U.S.A. 2008, 105, 16843-16848) microfluidic analogue of an electrode, but operating at the chemical rather than electrical level, developed with the aim of capturing local stimulus-response processes in chemistry and biology. The chemistrode consists of open-ended V-shaped microfluidic channels that can be brought into contact with a chemical or biological hydrophilic substrate. The chemistrode relies on multiphase aqueous/fluorous flow and uses plugs to achieve high temporal resolution of stimulation and sampling. Coalescence of the incoming plugs, containing the stimuli, with the liquid in the wetting layer is required for chemical exchange to take place in the chemistrode. Here, we investigate the system with triethyleneglycol mono1H, 1H-perfluorooctyl ether RfOEG as the surfactant. This surfactant was necessary to prevent nonspecific absorption of proteins to the aqueous fluorous interface and to ensure biocompatibility of the system, but too much surfactant increased the barrier for coalescence. In this system, coalescence was controlled by the capillary number. At a higher value of the capillary number, coalescence took more time, and deformation of the interface of the incoming plug and the wetting layer was more significant. Above a critical capillary number, coalescence did not occur between the incoming plug and the wetting layer. The critical capillary number was an increasing function of surface tension but was independent of viscosity ratio. Coalescence was surprisingly reproducible, presumably because film rupture during coalescence was reliably initiated at the hydrophilic substrate. These results are useful in rational operation of the chemistrode and also provide an experimental description of deformation, film drainage, and coalescence of surfactant-coated droplets in an external flow field
Millisecond kinetics on a microfluidic chip using nanoliters of reagents
No full textThis paper describes a microfluidic chip for performing kinetic measurements with better than millisecond resolution. Rapid kinetic measurements in microfluidic systems are complicated by two problems: mixing is slow and dispersion is large. These problems also complicate biochemical assays performed in microfluidic chips. We have recently shown (Song, H.; Tice, J. D.; Ismagilov, R. F. Angew. Chem., Int. Ed. 2003, 42, 768-772) how multiphase fluid flow in microchannels can be used to address both problems by transporting the reagents inside aqueous droplets (plugs) surrounded by an immiscible fluid. Here, this droplet-based microfluidic system was used to extract kinetic parameters of an enzymatic reaction. Rapid single-turnover kinetics of ribonuclease A (RNase A) was measured with better than millisecond resolution using sub-microliter volumes of solutions. To obtain the single-turnover rate constant (k = 1100 +/- 250 s(-1)), four new features for this microfluidics platform were demonstrated: (i) rapid on-chip dilution, (ii) multiple time range access, (iii) biocompatibility with RNase A, and (iv) explicit treatment of mixing for improving time resolution of the system. These features are discussed using kinetics of RNase A. From fluorescent images integrated for 2-4 s, each kinetic profile can be obtained using less than 150 nL of solutions of reagents because this system relies on chaotic advection inside moving droplets rather than on turbulence to achieve rapid mixing. Fabrication of these devices in PDMS is straightforward and no specialized equipment, except for a standard microscope with a CCD camera, is needed to run the experiments. This microfluidic platform could serve as an inexpensive and economical complement to stopped-flow methods for a broad range of time-resolved experiments and assays in chemistry and biochemistry
Characterization of the local temperature in space and time around a developing Drosophila embryo in a microfluidic device
No full textThis paper characterizes a microfluidic platform that differentially controls the temperature of each half of a living Drosophila melanogaster fruitfly embryo in space and time (E. M. Lucchetta, J. H. Lee, L. A. Fu, N. H. Patel and R. F. Ismagilov, Nature, 2005, 434, 1134-1138). This platform relies on laminar flow of two streams of liquid with different temperature, and on rapid prototyping in polydimethylsiloxane (PDMS). Here, we characterized fluid flow and heat transport in this platform both experimentally and by numerical simulation, and estimated the temperature distribution around and within the embryo by numerical simulation, to identify the conditions for creating a sharper temperature difference (temperature step) over the embryo. Embryos were removed from the device and immunostained histochemically for detection of Paired protein. Biochemical processes are sensitive to small differences in environmental temperature. The microfluidic platform characterized here could prove useful in understanding dynamics of biochemical networks as they respond to changes in temperature
Ismagilov, "Microgram-scale testing of reaction conditions in solution using nanoliter plugs in microfluidics with detection by MALDI-MS
No full textThis paper describes a microfluidic system to screen and optimize organic reaction conditions on a submicrogram scale. Optimization of reaction conditions is required to achieve high efficiency and selectivity in organic reactions. Combinatorial methods1 and high-throughput screening2 are powerful tools for optimization. To perform solution-phase synthesis, typical microtiter plates or reaction blocks for parallel synthesis run reactions on the scale of mL/reaction1 and are less applicable to precious substrates (e.g., products of long synthetic sequences and natural products that can be isolated only in small quantities). To address this problem, one approach used arrayed micro-wells in combination with a robotic liquid sampler on the scale of 125 nL per reaction.3 To reduce the use of robotics and to minimize evaporation, others used microchannels4-6 to perform reactions, including synthesis of pyrazoles with UV detection (5 íL per reaction)6 and optimizatio
Mathematical description of kinetic resolution with an enantiomerically impure catalyst and nonracemic substrate
No full textKinetic resolution is an important method in organic chemistry; catalytic kinetic resolution is especially attractive because a smaller amount of the optically active material is required. In principle, a naturally occurring catalyst (enzyme) or a synthetic catalyst can be employed. Possible disadvantages of an enzymatic catalyst include limited scope of reactions and substrates; therefore, there is an increasing effort targeted at the design of chemical catalysts for kinetic resolution. The most useful parameter in comparing different catalysts is the selectivity factor S, which is the ratio of the rate constants for the reaction of the catalyst with the two enantiomers of the substrate (S = (k_1/k_2), see below). Mathematical treatment of kinetic resolution should provide a way to calculate S from experimental observables, and for enantiomerically pure catalysts the equations are well-known from the work of Kagan and others
Integrated microfluidic systems
No full textMicrochips: Multistep reactions may be catalyzed by enzymes immobilized on microbeads trapped inside microfluidic channels. Large integrated microfluidic circuits can be created by using multilayer soft lithography in poly(dimethylsiloxane). These circuits (see schematic representation) can be used to perform hundreds of different reactions simultaneously, and can be applied to a range of problems, from enzymatic assays to crystallization of proteins
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