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Results for “Microfluidics”

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At least 937 records · Page 52Linked to original sources

PubMed2000

Trapping of bead-based reagents within microfluidic systems: on-chip solid-phase extraction and electrochromatography

A 330-pL chromatographic bed was fabricated on a glass substrate as part of an electroosmotically pumped microfluidic system. Two weirs within a sample channel formed a cavity in which octadecylsilane (ODS) coated silica beads (1.5-4 microns diameter) were trapped. ODS beads were mobilized into and out of the cavity using electroosmotic flow through a bead-introduction channel which accessed the cavity. This procedure allowed the beads in the cavity to be repeatedly exchanged. A 1 nM solution of a nonpolar analyte (BODIPY 493/503) in buffer was loaded onto the beads for different lengths of time using an electroosmotic flow of 1.2 nL/s. The material retained on the ODS phase was then eluted by electroosmotic flow of acetonitrile with a concentration enhancement of up to 500 times. Capillary electrochromatography was conducted using a similar device. BODIPY and fluorescein were loaded onto a 200-micron-long chromatographic bed and then separated in an isocratic CEC run with an acetonitrile/buffer mobile phase. Complete separation was achieved in less than 20 s with a 2-micron plate height.

Journal Article↗

PubMed2000

Computer simulations of electrokinetic injection techniques in microfluidic devices

Computer simulations are used to study electrokinetic injections on microfluidic devices (microchips). The gated and pinched injection techniques are considered. Each injection technique uses a unique sequence of steps with different electric field distributions and field magnitudes in the channels to effectuate a virtual valve. The goal of these computer simulations is to identify operating parameters providing optimal valve performance. In the pinched injection, the conditions of both loading and dispensing steps were analyzed to reach a compromise between the sample plug spatial extent and its concentration. For the gated injection, the condition of leakage free valve operation was found for the sample loading step. The simulation results for the gated valve are compared with experimental data.

Journal Article↗

PubMed2002

Poly(dimethylsiloxane) as a material for fabricating microfluidic devices.

This Account summarizes techniques for fabrication and applications in biomedicine of microfluidic devices fabricated in poly(dimethylsiloxane) (PDMS). The methods and applications described focus on the exploitation of the physical and chemical properties of PDMS in the fabrication or actuation of the devices. Fabrication of channels in PDMS is simple, and it can be used to incorporate other materials and structures through encapsulation or sealing (both reversible and irreversible).

Coated Materials, Biocompatible↗

PubMed2006

Application of a microfluidic reactor for screening cancer prodrug activation using silica-immobilized nitrobenzene nitroreductase.

The nitroreductase-catalyzed conversion of a strong electron-withdrawing nitro group to the corresponding electron-donating hydroxylamine is useful in a variety of biotechnological applications. Activation of prodrugs for cancer treatments or antibiotic therapy are the most common applications. Here, we show that a bacterial nitrobenzene nitroreductase (NbzA) from Pseudomonas pseudoalcaligenes JS45 activates the dinitrobenzamide cancer prodrug CB1954 and the proantibiotic nitrofurazone. NbzA was purified by affinity chromatography and screened for substrate specificity with respect to prodrug activation. To facilitate screening of alternate potential prodrugs, polyethyleneimine-mediated silica formation was used to immobilize NbzA with high immobilization yields and high loading capacities. Greater than 80% of the NbzA was immobilized, and enzyme activity was significantly more stable than NbzA in solution. The resulting silica-encapsulated NbzA was packed into a microfluidic microreactor that proved suitable for continuous operation using nitrobenzene, CB1954, and the proantibiotic nitrofurazone. The flow-through system provides a rapid and reproducible screening method for determining the NbzA-catalyzed activation of prodrugs and proantibiotics.

Antineoplastic Agents↗

PubMed2002

Magnetic field-controlled microfluidic transport.

Several new forms of magnetohydrodynamic (MHD) flow occurring in the solution gap between two 250-microm-diameter Pt microdisk electrodes, oriented in a face-to-face geometry and immersed in a uniform magnetic field (1 T), are described. The MHD flow results from the Lorentz force generated by diffusion of electrochemically generated molecular ions through the magnetic field. Individual microscopic flow tubes ( approximately 50-microm radius) spanning the gap between the face-to-face electrodes are observed during the 1-e(-) reduction of nitrobenzene in acetonitrile solutions. The flow tubes extend up to approximately 2 cm in length and are stable for indefinite periods. Directional transport of the electrogenerated nitrobenzene radical anion over macroscopic distances within the flow tubes, with minimal diffusional broadening, is demonstrated using an ultramicroelectrode probe to map the convective flux of redox species. Pulsed MHD transport of small packets of molecules and the formation of large area (approximately 3 cm(2)), microscopically thin (25 microm) rotating sheets of solution are also demonstrated. The results suggest that electrochemical methods, in combination with magnetohydrodynamic principles, may be useful for external field-controlled microfluidic systems.

Journal Article↗

PubMed2002

Electrochemical sensing in microfluidic systems using electrogenerated chemiluminescence as a photonic reporter of redox reactions.

This paper describes a microfluidics-based sensing system that relies on electrochemical detection and electrogenerated chemiluminescent (ECL) reporting. The important result is that the ECL reporting reaction is chemically decoupled from the electrochemical sensing reaction. That is, the electrochemical sensing reaction does not participate directly in the ECL process, but because electrochemical cells require charge balance, the sensing and ECL reactions are electrically coupled. This provides a convenient and sensitive means for direct photonic readout of electrochemical reactions that do not directly participate in an ECL reaction and thus broadens the spectrum of redox compounds that can be detected by ECL. The approach can be implemented in either a two-electrode or bipolar (single-electrode) configuration. By manipulating the placement and dimensions of the conductors, the photonic response can be enhanced. The system is used to electrochemically detect benzyl viologen present in solution and report its presence via Ru(bpy)(3)(2+) (bpy = 2,2'-bipyridine) luminescence.

Journal Article↗

PubMed2003

Shell cross-linked cylinders of polyisoprene-b-ferrocenyldimethylsilane: formation of magnetic ceramic replicas and microfluidic channel alignment and patterning.

Stable organometallic cylindrical nanostructures have been prepared by shell cross-linking of PI320-b-PFS53 wormlike micelles in hexane through metal-catalyzed hydrosilylation at 23 degrees C. The cross-linked structures permit the formation of cylindrical ceramic replicas containing size- and separation-tunable arrays of Fe nanoclusters. In addition, microfluidic alignment of the cross-linked cylinders was possible.

Journal Article↗

PubMed2005

Polymer particles with various shapes and morphologies produced in continuous microfluidic reactors.

We report a novel approach to continuous and scalable production of core-shell droplets and polymer capsules in microfluidic devices. The described method is also useful in the synthesis of polymer particles with nonspherical shapes. We used capillary instability-driven break-up of a liquid jet formed by two immiscible fluids. Precise control of emulsification of each liquid allowed for the production of highly monodisperse core-shell droplets with a predetermined diameter of cores and thickness of shells. We also achieved control over the number of cores per droplet and the location of cores in the droplet. We carried out fast throughput photopolymerization of the monomeric shells and obtained polymer particles with various shapes and morphologies, including spheres, truncated spheres and, hemispheres, and single and multicore capsules.

Journal Article↗

PubMed2004

Actively controlled self-assembly of colloidal crystals in microfluidic networks by electrocapillary forces.

We report a simple approach to actively control the formation of the self-assembled colloidal crystals in the microfluidic networks using a combination of electrocapillary forces and evaporation-induced self-assembly. Using this scheme, we can not only selectively fabricate the colloidal crystals in the desired channels, but we can also build colloidal crystals with different optical properties in different channels or in the same channel.

Journal Article↗

PubMed2005

Interfacial polymerization within a simplified microfluidic device: capturing capsules.

A simple approach to a microfluidic device is described. The device is composed of flexible tubing and a needle inserted orthogonal to the long axis of the tubing. This design is well suited to creating oil-water interfaces allowing the formation of laminar flows and monodisperse emulsions. The system is characterized by mapping the phases observed as a function of organic phase flow and Reynolds number. In addition, the device allows interfacial polymerization reactions to capture low coefficient of variation capsules. The shell structure and surface are examined by scanning electron microscopy.

Journal Article↗

PubMed2005

Air-breathing laminar flow-based microfluidic fuel cell.

This communication reports the design and characterization of an air-breathing laminar flow-based microfluidic fuel cell (LFFC). The performance of previous LFFC designs was cathode-limited due to the poor solubility and slow transport of oxygen in aqueous media. Introduction of an air-breathing gas diffusion electrode as the cathode addresses these mass transfer issues. With this design change, the cathode is exposed to a higher oxygen concentration, and more importantly, the rate of oxygen replenishment in the depletion boundary layer on the cathode is greatly enhanced as a result of the 4 orders of magnitude higher diffusion coefficient of oxygen in air as opposed to that in aqueous media. The power densities of the present air-breathing LFFCs are 5 times higher (26 mW/cm2) than those for LFFCs operated using formic acid solutions as the fuel stream and an oxygen-saturated aqueous stream at the cathode ( approximately 5 mW/cm2). With the performance-limiting issues at the cathode mitigated, these air-breathing LFFCs can now be further developed to fully exploit their advantages of direct control over fuel crossover and the ability to individually tailor the chemical composition of the cathode and anode media to enhance electrode performance and fuel utilization, thus increasing the potential of laminar flow-based fuel cells.

Journal Article↗

PubMed2006

Janus and ternary particles generated by microfluidic synthesis: design, synthesis, and self-assembly.

This paper reports a microfluidic method for fast continuous synthesis of Janus particles and three-phase particles with narrow size distribution. Synthesis of particles included emulsification of monomer liquids and in-situ photoinitiated polymerization of multiphase droplets. We show the strategy for precise control over the structure of Janus particles and their structure-dependent assembly in clusters. We demonstrate an asymmetric chemical modification of the surface of JPs by conjugating them with protein molecules. The Janus and ternary particles were synthesized from largely immiscible liquids and had a sharp interface between the constituent phases.

Journal Article↗

PubMed2006

Rapid self-assembly of core-shell organosilicon microcapsules within a microfluidic device.

The preparation of hierarchically structured organosilicon microcapsules from commercially available starting materials is described. Using a microfluidic device, an emulsion of dichlorodiphenylsilane is formed in a continuous phase of aqueous glycerol. The silane droplets undergo hydrolysis, condensation, and crystallization within minutes to form self-assembled, core-shell microcapsules. The microparticles have been characterized with light and electron microscopy, nuclear magnetic resonance spectroscopy (NMR), diffusion-ordered NMR spectroscopy (DOSY), Fourier transform infrared spectroscopy (FTIR), differential scanning calorimetry (DSC), and powder X-ray diffraction (XRD). The characterization data show that the microcapsule walls consist of amorphous, oligomeric poly(diphenylsiloxane) surrounded by a spiny layer of crystalline diphenylsilanediol. Glycerol is occluded within the wall material but is not covalently bound to the silicon components. Glycerol is a crucial element for producing low-dispersity microcapsules with well-ordered surface spines, as the use of methyl cellulose as viscomodifier yields amorphous surfaces.

Journal Article↗

PubMed2006

A microfluidic channel flow cell for electrochemical ESR.

The design, fabrication, and characterization of microfluidic channel flow devices for in situ simultaneous hydrodynamic electrochemical ESR is reported. The microelectrochemical reactors consist of gold film electrodes situated within rectangular ducts of height 350 microm and widths in the range 500-2000 microm. The small dimensions of the channels result in minimal dielectric loss when centralized within a cylindrical TE011 resonant cavity, leading to a high level of sensitivity. This is demonstrated by using the one-electron oxidation of N,N,N',N'-tetramethyl-p-phenylenediamine (TMPD) in acetonitrile as a model system, wherein the ESR spectra obtained for the corresponding stable radical cation are of a high signal-to-noise ratio. Signal intensity is measured as a function of flow rate for this system, and the behavior is validated by means of 3-dimensional numerical modeling of the hydrodynamic flow profile.

Journal Article↗

PubMed2005

Two-dimensional ordered arrays of aligned lipid tubules on substrates with microfluidic networks.

Microfluidic networks is a powerful tool for aligning one-dimensional materials over a large area on solid substrates. Here we show that lipid nano- and microtubules can be assembled into two-dimensional (2-D) parallel arrays with controlled separations by combining fluidic alignment with dewetting, which occurs within microchannels. We also demonstrate that lipid tubules can be bent into a well-defined shape at the entrance of the channels by the capillary force. Atomic force microscopy is used to study the structure and stability of the aligned lipid tubules on substrates. The deposition experiments with silica colloidal particles show that the 2-D parallel-aligned tubules can be used as a template to synthesize silica films with controlled morphologies and patterns on substrates in a single-step process.

Journal Article↗

PubMed2005

Controlled synthesis of nonspherical microparticles using microfluidics.

The controlled synthesis of nonspherical microparticles using microfluidics processing is described. Polymer droplets, formed by shearing a photopolymer using a continuous water phase at a T-junction, were constrained to adopt nonspherical shapes by confining them using appropriate microchannel geometries. Plugs were obtained by shearing the polymer phase at low shear rates, while disks were obtained by flattening droplets using a channel of low height. The nonspherical shapes formed were permanently preserved by photopolymerizing the constrained droplets in situ using ultraviolet light. Monodisperse plugs and disks of different lengths and diameters were obtained by varying the flow rates of the two phases.

Journal Article↗

PubMed2005

On the surface conductance, flow rate, and current continuities of microfluidics with nonuniform surface potentials.

The characteristics of electrokinetic flow in a microchannel depend on both the nature of surface potentials, that is, whether it is uniform or nonuniform, and the electrical potential distribution along the channel. In this paper, the nonlinear Poisson-Boltzmann equation is used to model the electrical double layer and the lattice Boltzmann model coupled with the constraint of current continuity is used to simulate the microfluidic flow field in a rectangular microchannel with a step variation of surface potentials. This current continuity, including surface conduction, convection, and bulk conduction currents, has often been neglected in the literature for electroosmotic flow with nonuniform (heterogeneous) microchannels. Results show that step variation of ion distribution caused by step variation surface potential will influence significantly the electrical potential distribution along the channel and volumetric flow rate. For the system considered, we showed that the volumetric flow rate could have been overestimated by as much as 70% without consideration of the current continuity constraint.

Journal Article↗

PubMed2004

Controlled production of monodisperse double emulsions by two-step droplet breakup in microfluidic devices.

A microfluidic device having both hydrophobic and hydrophilic components is exploited for production of multiple-phase emulsions. For producing water-in-oil-in-water (W/O/W) dispersions, aqueous droplets ruptured at the upstream hydrophobic junction are enclosed within organic droplets formed at the downstream hydrophilic junction. Droplets produced at each junction could have narrow size distributions with coefficients of variation in diameter of less than 3%. Control of the flow conditions produces variations in internal/external droplet sizes and in the internal droplet number. Both W/O/W emulsions (with two types of internal droplets) and oil-in-water-in-oil emulsions were prepared by varying geometry and wettability in microchannels.

Journal Article↗