Rapid Prototyping of Microfluidic Systems in Poly(dimethylsiloxane)

Analysis of amino acids and proteins using a poly(methyl methacrylate) microfluidic system

Plastic microchips are very promising analytical devices for the high-speed analysis of biological compounds. However, due to its hydrophobicity, their surface strongly interacts with nonpolar analytes or species containing hydrophobic domains, resulting in a significant uncontrolled adsorption on the channel walls. This paper describes the migration of fluorescence-labeled amino acids and proteins using the poly(methyl methacrylate) microchip. A cationic starch derivative significantly decreases the adsorption of analytes on the channel walls. The migration time of the analytes was related to their molecular weight and net charge or pI of the analytes. FITC-BSA migrated within 2 min, and the theoretical plate number of the peak reached 480,000 plates/m. Furthermore, proteins with a wide range of pI values and molecular weights migrated within 1 min using the microchip.

Applications of capillary electrophoresis on microchip

CE on microchip is an emerging separation technique that has attracted wide attention and gained considerable popularity. Because of miniaturization of the separation format, CE on chip typically offers shorter analysis time and lower reagent consumption with potential development of portable analytical instrumentation. This review with 143 references is focused on proteins and peptides analysis, DNA separation including fragment sizing, genotyping, mutation detection and sequencing, and also the analysis of low-molecular-weight compounds, namely explosive residues and warfare agents, pharmaceuticals and drugs of abuse, and various small molecules in body fluids.

Microfluidic Device for Rapid (<15 min) Automated Microarray Hybridization

Background: Current hybridization protocols on microarrays are slow and need skilled personnel. Microfluidics is an emerging science that enables the processing of minute volumes of liquids to perform chemical, biochemical, or enzymatic analyzes. The merging of microfluidics and microarray technologies constitutes an elegant solution that will automate and speed up microarray hybridization.

Methods: We developed a microfluidic flow cell consisting of a network of chambers and channels molded into a polydimethylsiloxane substrate. The substrate was aligned and reversibly bound to the microarray printed on a standard glass slide to form a functional microfluidic unit. The microfluidic units were placed on an engraved, disc-shaped support fixed on a rotational device. Centrifugal forces drove the sample and buffers directly onto the microarray surface.

Results: This microfluidic system increased the hybridization signal by ∼10fold compared with a passive system that made use of 10 times more sample. By means of a 15–min automated hybridization process, performed at room temperature, we demonstrated the discrimination of 4 clinically relevant Staphylococcus species that differ by as little as a single-nucleotide polymorphism. This process included hybridization, washing, rinsing, and drying steps and did not require any purification of target nucleic acids. This platform was sensitive enough to detect 10 PCR-amplified bacterial genomes.

Conclusion: This removable microfluidic system for performing microarray hybridization on glass slides is promising for molecular diagnostics and gene profiling.

Functionality and stability of heparin immobilized onto poly(dimethylsiloxane)

Poly(dimethylsiloxane) (PDMS) has become an attractive material when working in the field of microfluidics, mainly because of the rapid prototyping process it involves. The increased surface volume ratio in microchannels makes the interaction between sample and material surface highly important, evident when handling complex biological samples such as plasma or blood. This study demonstrates a new grade of non-covalent heparin surface that adds efficient anticoagulant property to the PDMS material. The surface modification is a simple and fast one-step process performed at neutral pH, optimal when working with closed microsystems. The heparin formed a uniform and functional coating on hydrophobic PDMS with comparatively high level of antithrombin-binding capacity. In addition, long-term studies revealed that the immobilized heparin was more or less stable in the microchannels over a time of three weeks. Recalcified plasma in contact with native PDMS showed complete coagulation after 1h, while no fibrin formation was detected in plasma incubated on heparin-coated PDMS within the same time. In conclusion, we see the heparin coating developed and evaluated in this study as a tool that greatly facilitates the use of PDMS in microfluidics dealing with plasma or blood samples.

Single cell manipulation, analytics, and label-free protein detection in microfluidic devices for systems nanobiology

Single cell analytics for proteomic analysis is considered a key method in the framework of systems nanobiology which allows a novel proteomics without being subjected to ensemble-averaging, cell-cycle, or cell-population effects. We are currently developing a single cell analytical method for protein fingerprinting combining a structured microfluidic device with latest optical laser technology for single cell manipulation (trapping and steering), free-solution electrophoretical protein separation, and (label-free) protein detection. In this paper we report on first results of this novel analytical device focusing on three main issues. First, single biological cells were trapped, injected, steered, and deposited by means of optical tweezers in a poly(dimethylsiloxane) microfluidic device and consecutively lysed with SDS at a predefined position. Second, separation and detection of fluorescent dyes, amino acids, and proteins were achieved with LIF detection in the visible (VIS) (488 nm) as well as in the deep UV (266 nm) spectral range for label-free, native protein detection. Minute concentrations of 100 fM injected fluorescein could be detected in the VIS and a first protein separation and label-free detection could be achieved in the UV spectral range. Third, first analytical experiments with single Sf9 insect cells (Spodoptera frugiperda) in a tailored microfluidic device exhibiting distinct electropherograms of a green fluorescent protein-construct proved the validity of the concept. Thus, the presented microfluidic concept allows novel and fascinating single cell experiments for systems nanobiology in the future.

Microwave plasma treatment of polymer surface for irreversible sealing of microfluidic devices

Microwave plasma was generated in a glass bottle containing 2-3 Torr of oxygen for plasma treatment of a polymer surface. A "kitchen microwave oven" and a dedicated microwave digestion oven were used as the power source. Poly(dimethylsiloxane)(PDMS) slabs treated by a 30 W plasma for 30-60 s sealed irreversibly to form microfluidic devices that can sustain solution flow of an applied pressure of 42 psi without leaking. Experimental set up and conditions for the production of a homogeneous plasma to activate the PDMS surface for irreversible sealing are described in detail. The surface of a microwave plasma-treated PDMS slab was characterized using atomic force microscopy (AFM) and attenuated total reflection-Fourier Transform infrared spectroscopy (ATR-FTIR). The plasma-treated surface bears silica characteristics.

Patterned Solvent Delivery and Etching for the Fabrication of Plastic Microfluidic Devices

A very simple method for micropatterning flat plastic substrates that can be used to build microfluidic devices is demonstrated. Patterned poly(dimethylsiloxane) elastomer is used as a template to control the flow path of an etching solvent through a channel design to be reproduced on the plastic substrate. The etching solvent was a acetone/ethanol mixture for poly(methyl methacrylate) substrates or a dimethylformamide/acetone mixture for polystyrene. The method is extremely fast in that duplicate plastic substrates can be patterned in just a few minutes each. We identified conditions that lead to smooth channel surfaces and characterized the rate of etching under these conditions. We determined that, for sufficiently short etching times (shallow channel depths), the etch rate is independent of the linear flow rate. This is very important since it means that the etch depth is approximately constant even in complex channel geometries where there will be a wide range of etchant flow rates at different positions in the pattern to be reproduced. We also demonstrate that the method can be used to produce channels with different depths on the same substrate as well as channels that intersect to form a continuous fluid junction. The method provides a nice alternative to existing methods to rapidly fabricate microfluidic devices in rigid plastics without the need for specialized equipment.

Microfluidic system for dielectrophoretic separation based on a trapezoidal electrode array

This paper presents a novel microfluidic device for dielectrophoretic separation based on a trapezoidal electrode array (TEA). In this method, particles with different dielectric properties are separated by the device composed of the TEA for the dielectrophoretic deflection of particles under negative dielectrophoresis (DEP) and poly(dimethylsiloxane)(PDMS) microfluidic channel with a sinuous and expanded region. Polystyrene microparticles are exposed to an electric field generated from the TEA in the microfluidic channel and are dielectrophoretically focused to make all of them line up to one sidewall. When these particles arrive at the region of another TEA for dielectrophoretic separation, they are separated having different positions along the perpendicular direction to the fluid flow due to their different dielectrophoretic velocities. To evaluate the separation process and performance, both the effect of the flow rate on dielectrophoretic focusing and the influence of the number of trapezoidal electrodes on dielectrophoretic separation are investigated. Now that this method utilizes the TEA as a source of negative DEP, non-specific particle adhering to the electrode surface can be prevented; conventional separation approaches depending on the positive DEP force suffer from this problem. In addition, since various particle types are continuously separated, this method can be easily applicable to the separation and analysis of various dielectric particles with high particle recovery and selectivity.

Hydrodynamic fabrication of polymeric barcoded strips as components for parallel bio-analysis and programmable microactuation

In this paper, we report a novel technique for the manufacture of polymeric barcoded strips having diverse characteristics such as biocatalyst-based sensing and pH-responsive actuation. The fabrication involves the use of both a microfluidic platform and in-situ photopolymerization. It is expected that this method, which is a very simple, cost-effective, and environment-friendly means for mass production, will facilitate the stable immobilization of diverse biological substances such as enzymes, DNA, and antigens. Therefore, we fabricated an enzyme-immobilized barcoded strip for multiple bio-analysis. On the basis of this technology, we adopted different materials and thereby fabricated programmable microactuation components that are responsive to pH variation.

Microfluidic technologies in drug discovery

Microfluidic systems are increasingly used as tools in various stages of the drug discovery process. Microscale systems offer several obvious advantages, such as low sample consumption and significantly reduced analysis or experiment time. These technologies raise the possibility of massive parallelization and concomitant reduction in cost per acquired data point. In addition, fluids in confined spaces display unique behaviors that can be used to acquire information not accessible using macroscopic systems. This article will focus on the implementation of microfluidic systems and technologies in the process of drug discovery.

Laminar flow-based electrochemical microreactor for efficient regeneration of nicotinamide cofactors for biocatalysis

One of the long-standing challenges in biocatalysis is the search for methods to continuously regenerate essential cofactors such as NADH that would enable a wide range of enzymes to be used in the more environmentally friendly synthesis of chiral fine chemicals including pharmaceuticals, cosmetics, and food additives. This communication reports a microreactor-based cofactor regeneration method that exploits the microfluidic phenomenon of laminar flow: a reactant stream and a buffer stream are introduced in a microchannel and continue to flow side by side without turbulent mixing between two electrodes that cover opposing channel walls. Adjustment of the flow rate ratio of the two streams in laminar flow enables focusing of the reactant stream close to the cathode, thereby reversing a normally unfavorable reaction equilibrium essential for cofactor regeneration. The absence of a bulk phase in these microreactors prevents the undesired reverse reaction to take place, which has prevented the use of electrochemical cofactor regeneration in macroscale processes. Here, we demonstrate the regeneration of NADH with conversion efficiencies as high as 31%. We also show the subsequent in situ conversion of an achiral substrate, pyruvate, into a chiral product, l-lactate, within this microreactor.

Fabrication of Discontinuous Surface Patterns within Microfluidic Channels Using Photodefinable Vapor-Based Polymer Coatings

In this report, we introduce a surface modification method for the fabrication of discontinuous surface patterns within microfluidic systems. The method is based on chemical vapor deposition (CVD) of a photodefinable coating, poly(4-benzoyl-p-xylylene-co-p-xylylene), onto the luminal surface of a microfluidic device followed by a photopatterning step to initiate spatially controlled surface binding. During photopatterning, light-reactive groups of the CVD polymer spontaneously react with molecules adjunct to the surface, such as poly(ethylene oxide). We demonstrate the potential of these reactive polymers for surface modification by preventing nonspecific protein adsorption on different substrates including silicon and poly(dimethylsiloxane) as measured by fluorescence microscopy. More importantly, three-dimensional patterns have successfully been created within polymer-based microfluidic channels, establishing spatially controlled, bioinert surfaces. The herein reported surface modification method addresses a critical challenge with respect to surface engineering of microfluidic devices, namely, the fabrication of discontinuous patterns within microchannels.

Profiling pH Gradients Across Nanocapillary Array Membranes Connecting Microfluidic Channels

Nanocapillary array membranes (NCAMs), comprised of thin (d approximately 5-10 microm) nuclear track-etched polycarbonate sheets containing approximately 10(8) cm(-2) nearly parallel nanometer-diameter capillaries, may act to gate fluid transport between microfluidic channels to effect, for example, sample collection. There is interest in H+-transport across these NCAMs because there is significant practical interest in being able to process analyte-containing samples under different pH conditions in adjacent layers of an integrated microfluidic circuit and because protons, with their inherently high mobility, present a challenge in separating microfluidic environments with different properties. To evaluate the capability of NCAMs to support pH gradients, the proton transport properties of NCAMs were studied using laser scanning confocal fluorescence microscopy (LSCFM). Spatiotemporal maps of [H+] in microfluidic channels adjacent to the NCAMs yield information regarding diffusive and electrokinetic transport of protons. The NCAMs studied here are characterized by a positive zeta potential, zeta > 0, so at small nanocapillary diameters, the overlap of electrical double layers associated with opposite walls of the nanocapillary establish an energy barrier for either diffusion or electrokinetic transport of cations through the nanometer-diameter capillaries due to the positive charge on the nanocapillary surface. Proton transfer through an NCAM into microchannels is reduced for pore diameters, d < or = 50 nm and ionic strengths I < or = 50 mM, while for large pore diameters or solution ionic strengths, the incomplete overlap of electric double layer allows more facile ionic transfer across the membranes. These results establish the operating conditions for the development of multilevel integrated nanofluidic/microfluidic architectures which can support multidimensional chemical analysis of mass-limited samples requiring sequential operations to be implemented at different pH values.

Surface-Reactive Acrylic Copolymer for Fabrication of Microfluidic Devices

A surface-reactive acrylic polymer, poly(glycidyl methacrylate-co-methyl methacrylate) (PGMAMMA), was synthesized and evaluated for suitability as a substrate for fabrication of microfluidic devices for chemical analysis. This polymer has good thermal and optical properties and is mechanically robust for cutting and hot embossing. A key advantage of this polymeric material is that the surface can be easily modified to control inertness and electroosmotic flow using a variety of chemical procedures. In this work, the procedures for aminolysis, photografting of linear polyacrylamide, and atom-transfer radical polymerization on microchannel surfaces in PGMAMMA substrates were developed, and the performance of resultant microfluidic electrophoresis devices was demonstrated for the separation of amino acids, peptides, and proteins. Separation efficiencies as high as 4.6 x 10(4) plates for a 3.5-cm-long separation channel were obtained. The results indicate that PGMAMMA is an excellent substrate for microfabricated fluidic devices, and a broad range of applications should be possible.

Focused electrophoretic motion and selected electrokinetic dispensing of particles and cells in cross-microchannels

The electrokinetic focusing and the resultant accelerated electrophoretic motion of polystyrene particles and red blood cells were visualized in microfluidic cross-channels. The experimentally measured width of the focused stream and the measured velocity increase of particles and cells at different voltage ratios follow the proposed analytical formula within the experimental error. The attained velocity increase is insensitive to the particle size, particle property (i.e., particle or cell), and particle trajectory. By solving the electrical potential field in the cross-channel at the experimental conditions, we demonstrate that the squeezed electrical field lines in the channel intersection determine the shape of the focused stream, and the nonuniform distribution of axial electrical field strength underlies the variation of particle/cell electrophoretic velocity through the focusing region. However, the dielectrophoretic force resulting from the nonuniform electrical field in the intersection seems to push the acceleration region of particles and cells slightly in the downstream direction. We have also achieved the single particle/cell dispensing by instantly triggering an electrical pulse perpendicular to the focused particulate flow in a double-cross microchannel. The electrokinetic manipulation of particle/cell in microchannels demonstrated in this work can be used for developing integrated lab-on-a-chip devices for studies of cells.

Continuous-Flow Fractionation of Animal Cells in Microfluidic Device Using Aqueous Two-Phase Extraction

Monitoring of live cells is important in the field of medical science, diagnostics, biology, and the pharmaceutical industry. In this study, live and dead CHO-K1 (Chinese Hamster Ovary) cells were fractionated by continuous-flow extraction in a microfluidic device using immiscible aqueous two-phase extraction technique. The polymer solutions offered stable two-phase flows in microchannel without diffusive mixing. The fundamentals of aqueous two-phase extraction can support stable and reproducible recovery and separation of biomolecules in microfluidic devices. Polyethylene glycol 8000 (PEG 8000, 4%) and dextran T 500 (5%) were selected as model polymer solutions. The appropriate flow rates of polymer and cell solutions were suggested. The fractionation efficiency of live and dead CHO K-1 cells from the culture broth was compared in normal macroscale system and microfluidic device. The optimum pH for the fractionation was 6.6 in both the normal and micro-scale systems. The loss of target live cells by sedimentation was circumvented in microfluidic device because of the negligible effect of gravity on the sedimentation. Most live cells were distributed to PEG-rich phase, while dead cells were found at the interface of two polymer solutions in microchannel. In this case, the recovery and fractionation efficiency of live cells in the PDMS-based microfluidic device was 100% and 97.0%, respectively.

Mini-electrochemical detector for microchip electrophoresis

This paper presents the development of a mini-electrochemical detector for microchip electrophoresis. The small size (3.6 x 5.0 cm2, W x L) of the detector is compatible with the dimension of the microchip. The use of universal serial bus (USB) ports facilitates installation and use of the detector, miniaturizes the detector, and makes it ideal for lab-on-a-chip applications. A fixed 10 M ohm feedback resistance was chosen to convert current of the working electrode to voltage with second gain of 1, 2, 4, 8, 16, 32, 64 and 128 for small signal detection instead of adopting selectable feedback resistance. Special attention has been paid to the power support circuitry and printed circuit board (PCB) design in order to obtain good performance in such a miniature size. The working electrode potential could be varied over a range of +/-2.5 V with a resolution of 0.01 mV. The detection current ranges from -0.3 x 10(-7) A to 2.5 x 10(-7) A and the noise is lower than 1 pA. The analytical performance of the new system was demonstrated by the detection of epinephrine using an integrated PDMS/glass microchip with detection limit of 2.1 microM (S/N = 3).

Fabrication of poly(dimethylsiloxane) microfluidic system based on masters directly printed with an office laser printer

Applications of poly(dimethylsiloxane) (PDMS)-based microfluidic systems are more popular nowadays. Previous fabrication methods of the masters for PDMS microchannels require complicated steps and/or special device. In this paper, we demonstrated that the toner printed on the transparency film with the office laser printer (1200 dpi) can be used as the positive relief of the masters. The transparency film was printed in two steps in order to obtain the same printing quality for the crossed lines. With the laser-printed master, the depth of the fabricated PDMS microchannels was ca. 10 microm and the smallest width was ca. 60 microm. Surface characteristics of the PDMS/PDMS microchannels were performed with SEM. Their electrokinetic properties were investigated by the aids of the measurement of electroosmotic flow (EOF) and the Ohm's curve. Using the PDMS/PDMS microchip CE systems, electroactive biological molecules and non-electroactive inorganic ions were well separated, respectively. This simple approach could make it easy to carry out the studies of PDMS microfluidic systems in more general labs without special devices.

Isoelectric focusing in cyclic olefin copolymer microfluidic channels coated by polyacrylamide using a UV photografting method

As an alternative material to glass or silicon, microfluidic devices made from a cyclic olefin copolymer (COC) were fabricated. This material is of interest because of the relative ease of fabrication, low costs, and solvent resistance. However, as a result of the strong hydrophobic interactions normally present, COC surfaces are not suitable for protein separations. To reduce the protein adsorption and make COC suitable for protein separations, UV-initiated grafting of polyacrylamide was used to coat the surface of COC devices. The change in surface properties caused by different graft times was studied. The surface hydrophilicity and electroosmotic mobility were characterized by contact angle and electroosmosis measurements. Isoelectric focusing was performed to test protein separations in polyacrylamide-coated COC microchannels. A single protein, carbonic anhydrase, was used to analyze the focusing effects and peak capacities in uncoated and polyacrylamide-coated COC devices. Peak capacities ranging from 75 to 190 were achieved with a polyacrylamide-coated surface. A mixture of two proteins, conalbumin labeled with Alexa Fluor 488 and beta-lactoglobulin A labeled with Alexa Fluor 546, was used to test protein separations. Linear and rapid separation of proteins was achieved in the polyacrylamide-coated COC microfluidic device.

Microoxen: microorganisms to move microscale loads

It is difficult to harness the power generated by biological motors to carry out mechanical work in systems outside the cell. Efforts to capture the mechanical energy of nanomotors ex vivo require in vitro reconstitution of motor proteins and, often, protein engineering. This study presents a method for harnessing the power produced by biological motors that uses intact cells. The unicellular, biflagellated algae Chlamydomonas reinhardtii serve as "microoxen." This method uses surface chemistry to attach loads (1- to 6-microm-diameter polystyrene beads) to cells, phototaxis to steer swimming cells, and photochemistry to release loads. These motile microorganisms can transport microscale loads (3-microm-diameter beads) at velocities of approximately 100-200 microm.sec(-1) and over distances as large as 20 cm.

Poly(oxyethylene) based surface coatings for poly(dimethylsiloxane) microchannels

Control of surface properties in microfluidic systems is an indispensable prerequisite for successful bioanalytical applications. Poly(dimethylsiloxane) (PDMS) microfluidic devices are hampered from unwanted adsorption of biomolecules and lack of methods to control electroosmotic flow (EOF). In this paper, we propose different strategies to coat PDMS surfaces with poly(oxyethylene) (POE) molecules of varying chain lengths. The native PDMS surface is pretreated by exposure to UV irradiation or to an oxygen plasma, and the covalent linkage of POE-silanes as well as physical adsorption of a triblock-copolymer (F108) are studied. Contact angle measurements and atomic force microscopy (AFM) imaging revealed homogeneous attachment of POE-silanes and F108 to the PDMS surfaces. In the case of F108, different adsorption mechanisms to hydrophilic and hydrophobic PDMS are discussed. Determination of the electroosmotic mobilities of these coatings in PDMS microchannels prove their use for electrokinetic applications in which EOF reduction is inevitable and protein adsorption has to be suppressed.

Development of a microfluidic biosensor module for pathogen detection

The development of a microfluidic biosensor module with fluorescence detection for the identification of pathogenic organisms and viruses is presented in this article. The microfluidic biosensor consists of a network of microchannels fabricated in polydimethylsiloxane (PDMS) substrate. The microchannels are sealed with a glass substrate and packed in a Plexiglas housing to provide connection to the macro-world and ensure leakage-free flow operation. Reversible sealing permits easy disassembly for cleaning and replacing the microfluidic channels. The fluidic flow is generated by an applied positive pressure gradient, and the module can be operated under continuous solution flow of up to 80 microL min(-1). The biosensor recognition principle is based on DNA/RNA hybridization and liposome signal amplification. Superparamagnetic beads are incorporated into the system as a mobile solid support and are an essential part of the analysis scheme. In this study, the design, fabrication and the optimization of concentrations and amounts of the different biosensor components are carried out. The total time required for an assay is only 15 min including sample incubation time. The biosensor module is designed so that it can be easily integrated with a micro total analysis system, which will combine sample preparation and detection steps onto a single chip.

Electrokinetic generation of temporally and spatially stable concentration gradients in microchannels

Generating stable microscale concentration gradients is key to numerous biological and chemical analyses. Microfluidic systems offer the ability to maintain laminar fluid diffusion interfaces ideal for the production of temporally stable concentration gradients. Previous efforts have focused on pressure driven flows and have relied on networks of branching channels to create streams of varying concentrations which can subsequently be combined to form the desired gradients. In this study, we numerically and experimentally demonstrate a novel electrokinetic technique which utilizes applied voltages and surface charge heterogeneity in simpler channel geometries to control and manipulate microscale concentration gradients without the need for parallel lamination. Flow rates ranged from 30 to 460 nl min(-1) for Peclet numbers between 70 and 1100. Spatial stability of 0.6 mm or greater was obtained for a wide range of gradient shapes and magnitudes over lateral dimensions of 400-450 microm. Sensitivity analysis determined that this technique is largely independent of channel depth and species electrophoretic mobility, however channel width and the diffusion coefficient of the analyte are critical. It was concluded that by adjusting applied voltages and/or channel width, this approach to concentration gradient generation can be adapted to a wide range of applications.

Evaluation of microfluidic biosensor development using microscopic analysis of molecular beacon hybridization kinetics

Molecular beacons, oligonucleotide probes that fluoresce upon hybridization to a target nucleic acid, can be used in microfluidic devices to detect and quantify nucleic acids in solution as well as inside bacterial cells. Three essential steps towards the development of such devices as integrated microfluidic biosensors using molecular beacons were investigated in the present study. First, experiments using real-time confocal microscopy indicated that diffusion of DNA molecular beacons across a 100-mum diameter microfluidic channel took less than one minute after the flow of reagents was stopped. Second, experiments to evaluate hybridization kinetics of DNA molecular beacons with target nucleic acids in solution showed that DNA molecular beacons can be used to characterize hybridization kinetics in real time in microfluidic channels and that hybridization signals approached their maximum in approximately three minutes. Finally, it was demonstrated that peptide nucleic acid molecular beacons can be used to detect bacterial cells in microfluidic devices. These results suggest that the use of microfluidic devices to detect nucleic acids in solution and in bacterial cells is promising and that further development of an integrated microfluidic biosensor for bacterial detection based on this concept is warranted.

Integrated Light Collimating System for Extended Optical-Path-Length Absorbance Detection in Microchip-Based Capillary Electrophoresis

We have developed an integrated light collimating system with a microlens and a pair of slits for extended optical path length absorbance detection in a capillary electrophoresis (CE) microchip. The collimating system is made of the same material as the chip, poly(dimethylsiloxane) (PDMS), and it is integrated into the chip during the molding of the CE microchannels. In this microchip, the centers of an extended 500-microm detection cell and two optical fibers are self-aligned, and a planoconvex microlens (r = 50 microm) for light collimation is placed in front of a light-delivering fiber. To block stray light, two rectangular apertures, realized by a specially designed three-dimensional microchannel, are made on each end of the detection cell. In comparison to conventional extended detection cell having no collimator, the percentage of stray radiation readout fraction in the collimator integrated detection cell is significantly reduced from 31.6 to 3.8%. The effective optical path length is increased from 324 to 460 microm in the collimator integrated detection cell. The detection sensitivity is increased by 10 times in the newly developed absorbance detection cell as compared to an unextended, 50-microm-long detection cell. The concentration detection limit (S/N = 3) for fluorescein in the collimator integrated detection cell is 1.2 microM at the absorbance detection limit of 0.001 AU.

Fabrication of a Poly(dimethylsiloxane)-Based Electrochemiluminescence Detection Cell for Capillary Electrophoresis

An easy but effective technique is described here for quick fabrication of low-cost electrochemiluminescence detection cells for capillary electrophoresis. The technique is based on molding of poly(dimethylsiloxane) (PDMS) with a capillary column inserted into a pipet tip. Two access holes are left in the PDMS slab; they provide neat accommodations for the separation capillary and the working electrode made with the same type of tip. Since the access holes are well-aligned, the electrode and the capillary are automatically aligned; thus, end-column detection is easily obtained. Fabrication of the detection cell is straightforward; no micromechanical operation is included. Also the principle for the procedure makes it possible to efficiently batch production detection cells with good reproducibility. Because of the end-column scheme, the cell can be adopted for electrophoresis with electrochemical detection as well.

Stable Permanently Hydrophilic Protein-Resistant Thin-Film Coatings on Poly(dimethylsiloxane) Substrates by Electrostatic Self-Assembly and Chemical Cross-Linking

Poly(dimethylsiloxane) (PDMS) is a biomaterial that presents serious surface instability characterized by hydrophobicity recovery. Permanently hydrophilic PDMS surfaces were created using electrostatic self-assembly of polyethyleneimine and poly(acrylic acid) on top of a hydrolyzed poly(styrene-alt-maleic anhydride) base layer adsorbed on PDMS. Cross-linking of the polyelectrolyte multilayers (PEMS) by carbodiimide coupling and covalent attachment of poly(ethylene glycol) (PEG) chains to the PEMS produced stable, hydrophilic, protein-resistant coatings, which resisted hydrophobicity recovery in air. Attenuated total reflection Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy revealed that the thin films had excellent chemical stability and resisted hydrophobicity recovery in air over 77 days of measurement. The spectra also showed a dense coverage for PEG dialdehyde and excellent resistance to protein adsorption from undiluted rat serum. Atomic force microscopy revealed dense coverage with PEG dialdehyde and PEG diamine. Contact angle measurements showed that all films were hydrophilic and that the PEG dialdehyde-topped thin film had a virtually constant contact angle (approximately 20 degrees ) over the five months of the study. Electrokinetic analysis of the coatings in microchannels always exposed to air also gave good protein separation and constant electroosmotic flow during the five months that the measurements were done. We expect that the stable, hydrophilic, protein-resistant thin-film coatings will be useful for many applications that require long-term surface stability.

Microfluidic Gradient-Generating Device for Pharmacological Profiling

We describe an on-chip microfluidic gradient-generating device that generates concentration gradients spanning nearly 5 orders of magnitude starting from a single concentration. The exiting stream of drugs held at different concentrations remains laminar in a recording chamber and can be presented as 24 discrete solutions to a cell-based sensor. The high-performance characteristics of the device are demonstrated by pharmacological screening of voltage-gated K+ channels (hERG) and ligand-gated GABA(A) receptors using scanning-probe patch-clamp measurements. Multiple data point dose-response curves and IC50 and EC50 values were rapidly obtained, typically in less than 30 min, through its combined functionality of gradient generation and open-volume laminar flow. The device facilitates rapid pharmacological profiling of ion channel and GPCR effectors and enables the acquisition of large numbers of data points with minute sample consumption and handling.

Electrokinetically Based Approach for Single-Nucleotide Polymorphism Discrimination Using a Microfluidic Device

In this work, we describe and implement an electrokinetic approach for single-nucleotide polymorphism (SNP) discrimination using a PDMS/glass-based microfluidic chip. The technique takes advantage of precise control of the coupled thermal (Joule heating), shear (electroosmosis), and electrical (electrophoresis) energies present at an array of probes afforded by the application of external electrical potentials. Temperature controllers and embedded thermal devices are not required. The chips can be easily and inexpensively fabricated using standard microarray printing methods combined with soft-lithography patterned PDMS fluidics, making these systems easily adaptable to applications using higher density arrays. Extensive numerical simulations of the coupled flow and thermal properties and microscale thermometry experiments are described and used to characterize the in-channel conditions. It was found that optimal conditions for SNP detection occur at a lower temperature on-chip than for typical microarray experiments, thereby revealing the importance of the electrical and shear forces to the overall process. To demonstrate the clinical utility of the technique, the detection of single-base pair mutations in the survival motor neuron gene, associated with the childhood disease spinal muscular atrophy, is conducted.

Thermoplastic Elastomer Gels: An Advanced Substrate for Microfluidic Chemical Analysis Systems

We demonstrate the use of thermoplastic elastomer gels as advanced substrates for construction of complex microfluidic networks suitable for use in miniaturized chemical analysis systems. These gels are synthesized by combining inexpensive polystyrene-(polyethylene/polybutylene)-polystyrene triblock copolymers with a hydrocarbon extender oil for which the ethylene/butylene midblocks are selectively miscible. The insoluble styrene end blocks phase separate into localized nanodomains, resulting in the formation of an optically transparent, viscoelastic, and biocompatible gel network that is melt-processable at temperatures in the vicinity of 100 degrees C. This unique combination of properties allows microfluidic channels to be fabricated in a matter of minutes by simply making impressions of the negative relief structures on heated master molds. Melt processability allows multiple impressions to be made against different masters to construct complex geometries incorporating multi-height features within the same microchannel. Intricate interconnected multilayered structures are also easily fabricated owing to the ability to bond and seal multiple layers by briefly heating the material at the bond interface. Thermal and mechanical properties are tunable over a wide range through proper selection of gel composition.

A modular microfluidic architecture for integrated biochemical analysis

Microfluidic laboratory-on-a-chip (LOC) systems based on a modular architecture are presented. The architecture is conceptualized on two levels: a single-chip level and a multiple-chip module (MCM) system level. At the individual chip level, a multilayer approach segregates components belonging to two fundamental categories: passive fluidic components (channels and reaction chambers) and active electromechanical control structures (sensors and actuators). This distinction is explicitly made to simplify the development process and minimize cost. Components belonging to these two categories are built separately on different physical layers and can communicate fluidically via cross-layer interconnects. The chip that hosts the electromechanical control structures is called the microfluidic breadboard (FBB). A single LOC module is constructed by attaching a chip comprised of a custom arrangement of fluid routing channels and reactors (passive chip) to the FBB. Many different LOC functions can be achieved by using different passive chips on an FBB with a standard resource configuration. Multiple modules can be interconnected to form a larger LOC system (MCM level). We demonstrated the utility of this architecture by developing systems for two separate biochemical applications: one for detection of protein markers of cancer and another for detection of metal ions. In the first case, free prostate-specific antigen was detected at 500 aM concentration by using a nanoparticle-based bio-bar-code protocol on a parallel MCM system. In the second case, we used a DNAzyme-based biosensor to identify the presence of Pb(2+) (lead) at a sensitivity of 500 nM in <1 nl of solution.

Disposable microfluidic devices: fabrication, function, and application

This review article describes recent developments in microfluidics, with special emphasis on disposable plastic devices. Included is an overview of the common methods used in the fabrication of polymer microfluidic systems, including replica and injection molding, embossing, and laser ablation. Also described are the different methods by which on-chip operations--such as the pumping and valving of fluid flow, the mixing of different reagents, and the separation and detection of different chemical species--have been implemented in a microfluidic format. Finally, a few select biotechnological applications of microfluidics are presented to illustrate both the utility of this technology and its potential for development in the future.

Microfabrication and Laser Manipulation of Functional Microtool Using In-Situ Photofabrication

Single cell experiments have become very important for investigating unknown cell properties. We developed a novel technique to study individual cell properties on a chip using newly developed cell manipulation by laser tweezers with the photo-crosslinkable resin, using this resin to developed functional colored, fluorescent, cell binding, rotation free, and rope shaped microtools on a chip. Colored and fluorescent microtools are for cell manipulation using inexpensive image processing. Cell binding microtools are for high-speed transport of target cells. Rotation free microtools are for attitude control and precise force measurement of cells and DNA. Rope shaped microtools are for versatile manipulation. Laser tweezers is used to position-control microtools. We used a mercury lamp for UV illumination at the local area for combining microtools and fabricated our functional microtools and manipulated cells on the microchip.

Autonomous Polymer Loading and Sample Injection for Microchip Electrophoresis

We have developed an extremely simple method for microchip electrophoresis. Loading of a sieving polymer solution and injection of a sample solution are autonomously executed by a microchip fabricated in poly(dimethylsiloxane) (PDMS). In advance, the energy for the fluid pumping is stored in bulk PDMS by evacuating air dissolved in PDMS, and the information for the sample plug regulation is coded into the microchannel design. Besides the simplicity, our method brings about an advantageous effect: sample compaction due to the discontinuous electrophoretic mobility at the sample/polymer interface. The sample compaction effect was moderate in ordinary size-dependent separation for double-stranded DNA and was extreme in affinity electrophoresis for single-stranded DNA (ssDNA). In the latter separation mode, ssDNA components were sequence-specifically separated by difference in affinity to a probe oligonucleotide immobilized to the sieving polymer matrix. We separated up to 60-mer ssDNA mixtures based on single-base substitutions. The separation processes included typically 100-fold sample compaction and were completed within 15-30 s. This technology provides easy, simple, and sensitive method for detection of gene point mutations and typing of single-nucleotide polymorphisms.

Integration of a carbon microelectrode with a microfabricated palladium decoupler for use in microchip capillary electrophoresis/electrochemistry

A method to integrate a carbon microelectrode with a microfabricated palladium decoupler for use in microchip capillary electrophoresis (CE) is detailed. As opposed to previous studies with decouplers for microchip CE, the working electrode material, which is made by micromolding of a carbon ink, is different from the decoupling electrode material (palladium). The manner in which the working electrode is made does not add additional etching or lithographic steps to the fabrication of the glass electrode plate. The hybrid poly(dimethylsiloxane)/glass device was characterized with fluorescence microscopy and by monitoring the CE-based separation of dopamine. Hydrodynamic voltammograms exhibited diffusion-limited currents occurring at potentials above +1.0 V. It was also shown that the half-wave potential does not shift as the separation potential is changed, as is the case in nondecoupled systems. Gated injections of dopamine in a 25 mM boric acid buffer (pH 9.2) showed a linear response from 200 to 5 microM (r2 = 0.9992), with a sensitivity of 5.47 pA/microM and an estimated limit of detection of 2.3 microM (0.621 fmol, S/N = 3). This is the first report of coupling a carbon electrode with a decoupler in microchip CE.

Comparison of surfactants for dynamic surface modification of poly(dimethylsiloxane) microchips

In the present report, the use of negatively charged surfactants as modifiers of the background electrolyte is reported using poly(dimethylsiloxane) (PDMS) microchips. In particular, the use of anionic surfactants, such as sodium dodecyl sulfate, phosphatidic acid, and deoxycholate, was studied. When surfactants were present in the run buffer, an increase in the electroosmotic flow (EOF) was observed. Two additional effects were also observed: (i) stabilization of the run-to-run EOF, (ii) an improvement in the electrochemical response for several biomolecules. In order to characterize the analysis conditions, the effects of different surfactant, electrolyte, and pH were studied. EOF measurements were performed using either the current monitoring method or by detection of a neutral molecule. The first adsorption/desorption kinetics studies are also reported for different surfactants onto PDMS. The separation of biologically important analytes (glucose, penicillin, phenol, and homovanillic acid) was improved decreasing the analysis time from 200 to 125 s. However, no significant changes in the number of theoretical plates were observed.

Effects of flow and diffusion on chemotaxis studies in a microfabricated gradient generator

An understanding of chemotaxis at the level of cell-molecule interactions is important because of its relevance in cancer, immunology, and microbiology, just to name a few. This study quantifies the effects of flow on cell migration during chemotaxis in a microfluidic device. The chemotaxis gradient within the device was modeled and compared to experimental results. Chemotaxis experiments were performed using the chemokine CXCL8 under different flow rates with human HL60 promyelocytic leukemia cells expressing a transfected CXCR2 chemokine receptor. Cell trajectories were separated into x and y axis components. When the microchannel flow rates were increased, cell trajectories along the x axis were found to be significantly affected (p < 0.05). Total migration distances were not affected. These results should be considered when using similar microfluidic devices for chemotaxis studies so that flow bias can be minimized. It may be possible to use this effect to estimate the total tractile force exerted by a cell during chemotaxis, which would be particularly valuable for cells whose tractile forces are below the level of detection with standard techniques of traction-force microscopy.

Accelerated Particle Electrophoretic Motion and Separation in Converging−Diverging Microchannels

Accelerated particle electrophoretic motions were visualized in converging-diverging microchannels on poly(dimethylsiloxane) chips. The accelerated particle electrophoretic separation is highly desirable in on-chip flow cytometry and high-speed electrophoresis. The effects of electric field, particle size, particle trajectory, and channel structure on the particle electrophoretic motion are examined. We find that the ratio of the particle velocity in the throat to that in the straight channel is significantly lower than their cross-sectional area ratio. This discrepancy may be attributed to the locally higher electric field around the two poles of a particle, as compared to other regions inside the microchannel. We also find that the particle velocity ratio is increased for smaller particles moving through symmetric converging-diverging channels under lower electric fields. These variations may be attributed to the negative dielectrophoretic force that is generated by the nonuniform electric field in the converging-diverging section. In addition, we find that particle trajectory has insignificant influences on the maximum velocity ratio obtained in the throat.

Selective filling for patterning in microfluidic channels

The ability to pattern different polymers in microfluidic channels is indispensable for the development of multifunctional, "lab-on-a-chip" devices. A simple method, based on the concept of selective filling, is described for introducing different polymers at defined locations in microfluidic channels. Selective filling is based on the difference in the free energy of filling between an open and a covered part of the channel. It is implemented by covering part of the channel, along its length, with a temporary poly(dimethylsiloxane) (PDMS) slab. Preferential filling is related to the contact angle of the liquid solution on the chip surface. An expression for the critical contact angle is derived, and its dependence on the geometry of the channel is established. It is further shown that a trapezoidal geometry of the cross-section of the channel is optimal for selective filling. Experimental verification of the applicability of the critical contact angle in predicting selective filling is demonstrated by introducing liquid prepolymer solutions of different contact angles in the glass channel that was etched using photolithography and wet etching. Finally, patterning of two different polymers along the axial direction of the microfluidic channel is demonstrated using this selective filling technique.

MICROFLUIDIC TOOLS FOR STUDYING THE SPECIFIC BINDING, ADSORPTION, AND DISPLACEMENT OF PROTEINS AT INTERFACES

A combination of temperature and concentration gradient microfluidic devices were employed to study the mechanistic details of biomacromolecule interactions at oxide interfaces. These lab-on-a-chip techniques allowed high-throughput, multiplexed data collection using only nanoliters of analyte. The three examples presented demonstrate rapid data acquisition relative to standard methods. First, we show ligand-receptor binding data for multivalent binding between membrane-bound ligands and incoming aqueous proteins with several binding pockets. A model is described for obtaining both the first and second dissociation constant for the reaction. The second example employs temperature gradient microfluidics to study the thermoresponsive properties of polymers and proteins. Both the folding mechanism and subsequent formation of an aqueous two-phase system were followed. Finally, these microfluidic techniques were combined with fluorescence microscopy and nonlinear optical spectroscopy to elucidate the mechanism of fibrinogen displacement from silica surfaces. This combination of methods enabled both direct and indirect observation of protein conformational changes.

In situ oligonucleotide synthesis on poly(dimethylsiloxane): a flexible substrate for microarray fabrication

In this paper, we demonstrate in situ synthesis of oligonucleotide probes on poly(dimethylsiloxane) (PDMS) microchannels through use of conventional phosphoramidite chemistry. PDMS polymer was moulded into a series of microchannels using standard soft lithography (micro-moulding), with dimensions <100 microm. The surface of the PDMS was derivatized by exposure to ultraviolet/ozone followed by vapour phase deposition of glycidoxypropyltrimethoxysilane and reaction with poly(ethylene glycol) spacer, resulting in a reactive surface for oligonucleotide coupling. High, reproducible yields were achieved for both 6mer and 21mer probes as assessed by hybridization to fluorescent oligonucleotides. Oligonucleotide surface density was comparable with that obtained on glass substrates. These results suggest PDMS as a stable and flexible alternative to glass as a suitable substrate in the fabrication and synthesis of DNA microarrays.

A reusable microfluidic plate with alternate-choice architecture for assessing growth preference in tissue culture

We present the design of a chamber to evaluate in vitro how species and concentrations of soluble molecules control features of cell growth-potentially including cell proliferation, cell motility, process extension, and process termination. We have created a reusable cell culture plate that integrates a microfluidic media delivery network with standard cell culture environment. The microfluidic network delivers a stream of cell culture media with a step-like concentration gradient down a 50-100 microm wide microchannel called the presentation region. Migrating cells or growing cell processes freely choose between the two distinct chemical environments in the presentation region, but they are forced to exclusively choose either one environment or the other when they grow past a physical barrier acting as a decision point. Our fabrication technique requires little specialized equipment, and can be carried out in approximately 4 days per plate. We demonstrate the effectiveness of our plates as neurites from spiral ganglion explants preferentially grow in media containing neurotrophin-3 (NT-3) as opposed to media without NT-3. Our design could be used without modification to study dissociated cell responses to soluble growth cues, and for behavioral screening of small motile organisms.

A polymeric master replication technology for mass fabrication of poly(dimethylsiloxane) microfluidic devices

A protocol of producing multiple polymeric masters from an original glass master mold has been developed, which enables the production of multiple poly(dimethylsiloxane) (PDMS)-based microfluidic devices in a low-cost and efficient manner. Standard wet-etching techniques were used to fabricate an original glass master with negative features, from which more than 50 polymethylmethacrylate (PMMA) positive replica masters were rapidly created using the thermal printing technique. The time to replicate each PMMA master was as short as 20 min. The PMMA replica masters have excellent structural features and could be used to cast PDMS devices for many times. An integration geometry designed for laser-induced fluorescence (LIF) detection, which contains normal deep microfluidic channels and a much deeper optical fiber channel, was successfully transferred into PDMS devices. The positive relief on seven PMMA replica masters is replicated with regard to the negative original glass master, with a depth average variation of 0.89% for 26-microm deep microfluidic channels and 1.16% for the 90 mum deep fiber channel. The imprinted positive relief in PMMA from master-to-master is reproducible with relative standard deviations (RSDs) of 1.06% for the maximum width and 0.46% for depth in terms of the separation channel. The PDMS devices fabricated from the PMMA replica masters were characterized and applied to the separation of a fluorescein isothiocyanate (FITC)-labeled epinephrine sample.

Fabrication of DNA microarrays onto poly(methyl methacrylate) with ultraviolet patterning and microfluidics for the detection of low-abundant point mutations

We have developed a simple ultraviolet (UV)-photomodification protocol using poly(methyl methacrylate) and polycarbonate to produce functional scaffolds consisting of carboxylic groups that allow covalent attachment of amine-terminated oligonucleotide probes to these surface groups through carbodiimide coupling. Use of the photomodification procedure coupled to microfluidics allowed for the rapid generation of medium-density DNA microarrays. The method reported herein involves the use of poly(dimethylsiloxane) microchannels reversibly sealed to photomodified poly(methyl methacrylate) surfaces to serve as stencils for patterning the oligonucleotide probes. After array construction, the poly(dimethylsiloxane) stencil is rotated 90 degrees to allow interrogation of the array using microfluidics. The photomodification process for array fabrication involves only three steps: (1) broadband UV exposure of the polymer surface, (2) carbodiimide coupling of amine-terminated oligonucleotide probes to the surface (via an amide bond), and (3) washing of the surface. The density of probes attached to this activated surface was found to be approximately 41pmolcm(-2), near the steric-saturation limit for short oligonucleotide probes. We demonstrate the use of this procedure for screening multiple KRAS2 mutations possessing high diagnostic value for colorectal cancers. A ligase detection reaction/universal array assay was carried out using parallel detection of two different low-abundant DNA point mutations in KRAS2 oncogenes with the allelic composition evaluated at one locus. Four zip code probes immobilized onto the poly(methyl methacrylate) surface directed allele-specific ligation products containing mutations in the KRAS2 gene (12.2D, 12.2A, 12.2V, and 13.4D) to the appropriate address of a universal array with minimal amounts of cross-hybridization or misligation.