Rapid Prototyping of Microfluidic Systems in Poly(dimethylsiloxane)

Phase-Changing Sacrificial Materials for Solvent Bonding of High-Performance Polymeric Capillary Electrophoresis Microchips

A new method for solvent bonding polymeric substrates to form microfluidic systems has been demonstrated. Prior to device sealing, channels in an embossed poly(methyl methacrylate) (PMMA) piece are filled with a heated liquid (paraffin wax) that forms a solid sacrificial layer at room temperature. The sacrificial material prevents the bonding solvent (acetonitrile) and softened PMMA from filling the channels. Once the sealing step is complete, the sacrificial layer is melted and removed, leaving enclosed microfluidic channels. We found that PMMA substrates welded together using this method could withstand internal pressures of >2250 psi, more than 1 order of magnitude higher than their thermally bonded counterparts. To demonstrate the usefulness of this method, microchip capillary electrophoresis (CE) devices in PMMA were created and tested. Amino acid and peptide mixtures were separated in <15 s, with >40,000 theoretical plates in a 2.5-cm separation distance. Electric fields as high as 1.5 kV/cm were applied in these microchips, and >300 CE runs were performed on a single device with no degradation of separation performance. The simplicity of the methods presented here and the improved robustness of the resulting devices should facilitate the broader implementation of polymer microchips in microfluidic analyses.

Wall coating for capillary electrophoresis on microchips

This review article with 116 references describes recent developments in the preparation of wall coatings for capillary electrophoresis (CE) on a microchip. It deals with both dynamic and permanent coatings and concentrates on the most frequently used microchip materials including glass, poly(methyl methacrylate), poly(dimethyl siloxane), polycarbonate, and poly(ethylene terephthalate glycol). Characterization of the channel surface by measuring electroosmotic mobility and water contact angle of the surface is included as well. The utility of the microchips with coated channels is demonstrated by examples of CE separations on these chips.

Electrokinetic control of fluid in plastified laser-printed poly(ethylene terephthalate)-toner microchips

The application of plastified laser-printed poly(ethylene terephthalate)(PET)-toner microchips to capillary electrophoresis was investigated. Electroosmotic flow was observed in the direction of the cathode for the buffer system studied (phosphate, pH 3-10). Average electroosmotic mobilities of 1.71 x 10(-4) to 4.35 x 10(-4) cm(2) V(-1) s(-1) were observed from pH 3 to 10. This variation suggests that silica fillers in the toner and on the surface of the polymer dominate the zeta potential of the material, which is also confirmed by XPS measurements. Dopamine and catechol were used as model analytes for microchip electrophoresis in combination with electrochemical detection. Results show that these two analytes can be efficiently separated and detected electrochemically with the plastified laser-printed PET-toner microchips.

Human neural stem cell growth and differentiation in a gradient-generating microfluidic device

This paper describes a gradient-generating microfluidic platform for optimizing proliferation and differentiation of neural stem cells (NSCs) in culture. Microfluidic technology has great potential to improve stem cell (SC) cultures, whose promise in cell-based therapies is limited by the inability to precisely control their behavior in culture. Compared to traditional culture tools, microfluidic platforms should provide much greater control over cell microenvironment and rapid optimization of media composition using relatively small numbers of cells. Our platform exposes cells to a concentration gradient of growth factors under continuous flow, thus minimizing autocrine and paracrine signaling. Human NSCs (hNSCs) from the developing cerebral cortex were cultured for more than 1 week in the microfluidic device while constantly exposed to a continuous gradient of a growth factor (GF) mixture containing epidermal growth factor (EGF), fibroblast growth factor 2 (FGF2) and platelet-derived growth factor (PDGF). Proliferation and differentiation of NSCs into astrocytes were monitored by time-lapse microscopy and immunocytochemistry. The NSCs remained healthy throughout the entire culture period, and importantly, proliferated and differentiated in a graded and proportional fashion that varied directly with GF concentration. These concentration-dependent cellular responses were quantitatively similar to those measured in control chambers built into the device and in parallel cultures using traditional 6-well plates. This gradient-generating microfluidic platform should be useful for a wide range of basic and applied studies on cultured cells, including SCs.

Replica multichannel polymer chips with a network of sacrificial channels sealed by adhesive printing method

Replica microchips for capillary array electrophoresis containing 10 separation channels (50 microm width, 50 microm depth and 100 microm pitch) and a network of sacrificial channels (100 microm width and 50 microm depth) were successfully fabricated on a poly(methyl methacrylate) (PMMA) substrate by injection molding. The strategy involved development of moving mask deep X-ray lithography to fabricate an array of channels with inclined channel sidewalls. A slight inclination of channel sidewalls, which can not be fabricated by conventional deep X-ray lithography, is highly required to ensure the release of replicated polymer chips from a mold. Moreover, the sealing of molded PMMA multichannel chips with a PMMA cover film was achieved by a novel bonding technique involving adhesive printing and a network of sacrificial channels. An adhesive printing process enables us to precisely control the thickness of an adhesive layer, and a network of sacrificial channels makes it possible to remove air bubbles and an excess adhesive, which are crucial to achieving perfect sealing of replica PMMA chips with well-defined channel and injection structures. A CCD camera equipped with an image intensifier was used to simultaneously monitor electrophoretic separations in ten micro-channels with laser-induced fluorescence detection. High-speed and high-throughput separations of a 100 bp DNA ladder and phi X174 Hae III DNA restriction fragments have been demonstrated using a 10-channel PMMA chip. The current work establishes the feasibility of mass production of PMMA multichannel chips at a cost-effective basis.

Preparation of Z-L-Phe-OH-NBD imprinted microchannel and its molecular recognition study

An integrated microchip was presented for selective recognition of Z-L-Phe-OH-NBD, using molecular imprinting technique. Molecularly imprinted polymer (MIP) were prepared by copolymerization in the presence of template molecule Z-L-Phe-OH-NBD, in which methacrylic acid and 4-vinylpyridine were used as functional monomers and ethylene dimethacrylate used as crosslinker. Imprinted polymer particles were introduced into a microchannel fabricated with a new material i.e. poly(methylvinylsiloxane) by simply rapid prototyping method. Imprinted effects were evaluated by laser-induced fluorescence (LIF) detection where the results indicated that good selective recognition for Z-L-Phe-OH-NBD in the imprinted polymer was obtained; the adsorption percentage of Z-L-Phe-OH-NBD was 61%. In contrast to conventional molecular imprinting analysis, integration shortened overall analysis time from 4h to 10 min.

Stable and Fluid Ethylphosphocholine Membranes in a Poly(dimethylsiloxane) Microsensor for Toxin Detection in Flooded Waters

Highly stable and fluid supported bilayer membranes were fabricated by fusion of positively charged ethylphosphocholine (DOPC+) vesicles into poly(dimethylsiloxane) (PDMS) microchannels for immunosensing of cholera toxin (CT) in flooded waters. Compared to phosphatidylcholine (PC) layers in the microchannels, DOPC+ membranes show exceptionally strong resistance to air-dry damage, as demonstrated by fluorescence recovery after photobleaching (FRAP) measurements and protein adsorption studies. In FRAP experiments, the mobile fraction of PC membranes was found to decrease by 10% upon drying/rehydration and the lateral diffusion coefficient decreased from 2.2 to 1.6 microm(2)/s, whereas the mobile fraction and diffusion coefficient for DOPC+ membranes remain virtually unchanged during this process. Characterization by confocal microscopy reveals that only 1% of the DOPC+ membrane in the microchannels was removed by the drying/rehydration process, as compared to 11% for PC. Protein adsorption trends indicate that the charge of DOPC+ membranes allows for tuning of solution conditions to enable the desired protein-membrane interaction to predominate at the interface. A flow-based immunoassay for bacterial toxin was developed with 5% GM1/DOPC+ membranes in PDMS channels, and a detection limit of 250 amol for CT was obtained from the calibration curves. The assay was successfully applied to detection of CT spiked in water samples from the Santa Ana River, with nearly identical response and sensitivity.

Recirculation of nanoliter volumes within microfluidic channels

A microfluidic device is described, capable of recirculating nanoliter volumes in restricted microchannel segments. The device consists of a PDMS microfluidic structure, reversibly sealed to a glass substrate with integrated platinum electrodes. The integrated electrodes generate electroosmotic flow locally, which results in a cycling flow in the channel segment between the two electrodes in case one channel exit is closed (dead-end channel). This cycling flow is a consequence of the counterbalancing hydrodynamic pressure against the electroosmotically generated flow. Acid-base indicators were employed to study the formation of H(+) and OH(-) at both the in-channel electrodes. The formation of acid can locally change the zeta-potential of the channel wall, which will affect the flow profile. Using this method, small analyte volumes can be mixed for prolonged times within well-defined channel segments and/or exposed to in-channel sensor surfaces.

Microfabricated systems for nucleic acid analysis

High throughput and automation of nucleic acid analysis are required in order to exploit the information that has been accumulated from the Human Genome Project. Microfabricated analytical systems enable parallel sample processing, reduced analysis-times, low consumption of sample and reagents, portability, integration of various analytical procedures and automation. This review article discusses miniaturized analytical systems for nucleic acid amplification, separation by capillary electrophoresis, sequencing and hybridization. Microarrays are also covered as a new analytical tool for global analysis of gene expression. Thus. instead of studying the expression of a single gene or a few genes at a time we can now obtain the expression profiles of thousands of genes in a single experiment.

Towards single molecule analysis in PDMS microdevices: from the detection of ultra low dye concentrations to single DNA molecule studies

In this paper, we report on the performance of electrophoretical separation and laser-induced fluorescence (LIF) detection of dyes and fluorescently labeled biomolecules in poly(dimethylsiloxane) (PDMS) microdevices. The dyes fluorescein and fluorescein isothiocyanate (FITC) have been separated effectively in nM concentrations. Fluorescein injections gave linear concentration response in the range from 4 to 100 pM. As ultimate detection sensitivity, 100 fM injected fluorescein was obtained. Further, 100 fM injected fluorescein could be detected. This is to our knowledge the smallest electrokinetically injected dye concentration detected on a microchip. Injection studies of fluorescently labeled avidin revealed a theoretical detection limit of 25 nM for laser-induced fluorescence detection in good agreement with separations in glass chips. Furthermore, the injection of several and even one single DNA molecule using a PDMS cross injector has been demonstrated as well as free solution separation of lambda- and T2-DNA (60 pM each) in periodically structured channels.

Isoelectric Focusing in a Poly(dimethylsiloxane) Microfluidic Chip

This paper reports the application of ampholyte-based isoelectric focusing in poly(dimethylsiloxane) (PDMS) using methylcellulose (MC) to reduce electroosmosis and peak drift. Although the characteristics of PDMS make it possible to fabricate microfluidic chips using soft lithography, unstable electroosmotic flow (EOF) and cathodic drift are significant problems when this medium is used. This paper demonstrates that EOF is greatly reduced in PDMS by applying a dynamic coat of MC to the channel walls and that higher concentrations of MC can be used to increase the viscosity of the electrode solutions in order to suppress pH gradient drift and reduce "compression"of the pH gradient. To illustrate the effect of MC on performance, several fluorescent proteins were focused in microchip channels 5 microm deep by 300 microm wide by 2 cm long in 3-10 min using broad-range ampholytes at electric field strengths ranging from 25 to 100 V/cm.

Microfluidic solid phase suspension transport with an elastomer-based, single piezo-actuator, micro throttle pump

We report a Micro Throttle Pump (MTP) which has been shown to pump 5 microm diameter polystyrene beads at a concentration of 4.5 x 10(7) beads ml(-1). This new MTP design is constructed in a straightforward manner and actuated by a single piezoelectric (PZT) element. Maximum flow rates at 800 Hz drive frequency of 132 microl min(-1) with water and 108 microl min(-1) with a bead suspension were obtained. Maximum back-pressures of 6 kPa were observed in both cases. The reported MTP employs specific location of distinct internal microfluid structures cast in a single compliant elastomeric substrate to exploit the opposing directions of flexure of regions of a piezoelectric-glass composite bonded to the elastomer. By this novel means, distinct flexural regions, exhibiting compressive and tensile stresses respectively, allow both the pump's integrated input and output throttles and its pump chamber to be actuated concurrently by a single PZT. To support MTP design we also report the characterisation of an individual throttle's resistance as a function of actuator deflection and discuss the underlying mechanism of the throttling effect.

Microfluidic operations using deformable polymer membranes fabricated by single layer soft lithography

We show that it is possible to use single layer soft lithography to create deformable polymer membranes within microfluidic chips for performing a variety of microfluidic operations. Single layer microfluidic chips were designed, fabricated, and characterized to demonstrate pumping, sorting, and mixing. Flow rates as high as 0.39 microl min(-1) were obtained by peristaltic pumping using pneumatically-actuated membrane devices. Sorting was attained via pneumatic actuation of membrane units placed alongside the branch channels. An active mixer was also demonstrated using single-layer deformable membrane units.

Sol−Gel Modified Poly(dimethylsiloxane) Microfluidic Devices with High Electroosmotic Mobilities and Hydrophilic Channel Wall Characteristics

Using a sol-gel method, we have fabricated poly(dimethylsiloxane) (PDMS) microchips with SiO2 particles homogeneously distributed within the PDMS polymer matrix. These particles are approximately 10 nm in diameter. To fabricate such devices, PDMS (Sylgard 184) was cast against SU-8 molds. After curing, the chips were carefully removed from the mold and sealed against flat, cured pieces of PDMS to form enclosed channel manifolds. These chips were then solvated in tetraethyl orthosilicate (TEOS), causing them to expand. Subsequently, the chips were placed in an aqueous solution containing 2.8% ethylamine and heated to form nanometer-sized SiO2 particles within the cross-linked PDMS polymer. The water contact angle for the PDMS-SiO2 chips was approximately 90.2 degrees compared to a water contact angle for Sylgard 184 of approximately 108.5 degrees . More importantly, the SiO2 modified PDMS chips showed no rhodamine B absorption after 4 h, indicating a substantially more hydrophilic and nonabsorptive surface than native PDMS. Initial electroosmotic mobilities (EOM) of (8.3+/-0.2)x10(-4) cm2/(V.s) (RSD=2.6% (RSD is relative standard deviation); n=10) were measured. This value was approximately twice that of native Sylgard 184 PDMS chips (4.21+/-0.09)x10(-4) cm2/(V.s) (RSD=2.2%; n=10) and 55% greater than glass chips (5.3+/-0.4)x10(-4) cm2/(V.s) (RSD=7.7%; n=5). After 60 days of dry storage, the EOM was (7.6+/-0.3)x10(-4) cm2/(V.s) (RSD=3.9%; n=3), a decrease of only 8% below that of the initially measured value. Separations performed on these devices generated 80,000-100,000 theoretical plates in 6-14 s for both tetramethylrhodamine succidimidyl ester and fluorescein-5-isothiocyanate derivatized amino acids. The separation distance was 3.5 cm. Plots of peak variance vs analyte migration times gave diffusion coefficients which indicate that the separation efficiencies are within 15% of the diffusion limit.

Label-free molecular interaction determinations with nanoscale interferometry

Quantification of protein-protein and ligand-substrate interactions is central to understanding basic cellular function and for evaluating therapeutics. To mimic biological conditions, such studies are best executed without modifying the proteins or ligands (i.e., label-free). While tools for label-free assays exist, they have limitations making them difficult to fully integrate into microfluidic devices. Furthermore, it has been problematic to reduce detection volumes for on-channel universal analyte quantification without compromising sensitivity, as needed in label-free methods. Here we show how backscattering interferometry in rectangular channels (BIRC) facilitates label-free studies within picoliter volumes. The simple and unique optical train was based on rectangular microfluidic channels molded in poly(dimethylsiloxane) and low-power coherent radiation. Quantification of irreversible streptavidin-biotin binding and reversible protein A-human IgG Fc molecular interactions in a 225 pL detection volume was carried out label-free and noninvasively. Detection limits of 47 x 10(-15) mol of biotin reacted with surface-immobilized streptavidin were achieved. In the case of reversible interactions of protein A and the Fc fragment of human IgG, detection limits were determined to be 2 x 10(-15) mol of IgG Fc. These experiments demonstrate for the first time that (1) high-sensitivity universal solute quantification is possible using interferometry performed within micrometer-sized channels formed in inexpensive PDMS chips, (2) label-free reversible molecular interaction can be studied with femtomoles of solute, and (3) BIRC has the potential to quantify binding affinities in a high-throughput format.

Capillary zone electrophoresis of amino acids on a hybrid poly(dimethylsiloxane)-glass chip

Poly(dimethylsiloxane) (PDMS)-PDMS and hybrid PDMS-glass devices have been characterized and compared in terms of current-voltage linearity, contact angle, electroosmotic velocity, electroosmotic mobility, and electrokinetic potential in dependence on the surface treatment. The hybrid PDMS-glass microfluidic devices have further been tested as on-chip capillary electrophoresis systems for the separation of fluorescently labeled amino acids. It has been demonstrated that different methods of surface pretreatment of the PDMS-glass devices result in significantly different separation performance, with plate numbers varying from 650 to 57 000 in dependence on the surface state and the nature of the amino acids. Electrophoretic separations of amino acids have been achieved within tens of seconds with detection limits of less than 2 microM (approximately 2 x 10(-16) to 2.5 x 10(-16) mol quantities at injection volumes of 110-120 pL). The detected amounts of fluorescein isothiocyante (FITC)-amino acids are at least ten times lower, since the amino acid:FITC ratio is 10:1 mol. The results demonstrate the perspective of such hybrid PDMS-glass microfluidic systems and the methods to modify their surfaces for on-chip separation methods for biomolecules.

Development of a microfabricated palladium decoupler/electrochemical detector for microchip capillary electrophoresis using a hybrid glass/poly(dimethylsiloxane) device

The fabrication and evaluation of a palladium decoupler and working electrode for microchip capillary electrophoresis (CE) with electrochemical detection is described. The use of the Pd decoupler allows the working electrode to be placed directly in the separation channel and eliminates the band-broadening characteristic of the end-channel configuration. The method used for fabrication of the decoupler and working electrode was based on thin-layer deposition of titanium followed by palladium onto a glass substrate. When employed as the cathode in CE, palladium absorbs the hydrogen gas that is generated by the hydrolysis of water. The effect of the decoupler size on the ability to remove hydrogen was evaluated with regard to reproducibility and longevity. Using boric acid and TES buffer systems, 500 microm was determined to be the optimum decoupler size, with effective voltage isolation lasting for approximately 6 h at a constant field strength of 600 V/cm. The effect of distance between the decoupler and working electrode on noise and resolution for the separation of dopamine and epinephrine was also investigated. It was found that 250 microm was the optimum spacing between the decoupler and working electrode. At this spacing, laser-induced fluorescence detection at various points around the decoupler established that the band broadening due to pressure-induced flow that occurs after the decoupler did not significantly affect the separation efficiency of fluorescein. Limits of detection, sensitivity, and linearity for dopamine (500 nM, 3.5 pA/microM, r(2) = 0.9996) and epinephrine (2.1 microM, 2.6 pA/microM, r(2) = 0.9996) were obtained using the palladium decoupler in combination with a Pd working electrode.

Chemical force titrations of amine- and sulfonic acid-modified poly(dimethylsiloxane)

Chemical force titrations-measurements of the adhesive interaction between a pair of suitably chemically modified atomic force microscopy (AFM) tip and sample surfaces as a function of pH-have been carried out for various combinations of silanol, amine, carboxylic acid, and sulfonic acid functional groups on both tip and sample. The primary surface material studied was poly(dimethylsiloxane) (PDMS). Surface modification was carried out using a plasma oxidation process to form silanol sites; further modification with amine or sulfonic acid sites was carried out by reaction of the silanol sites with the appropriate trialkoxysilane derivative. AFM tips were also modified using trialkoxysilane compounds. In the cases of tip/sample combinations with the same functional group on each, surface pK(1/2) values could be determined. In several "mixed" tip/sample combinations, a peak appeared in the titration curve midway between the surface pK(1/2) values of the tip and sample, consistent with an ionic H-bonding model for the interactions. The amine/sulfonic acid pair showed more complex behavior; the amine-terminated tip/sulfonic acid-terminated PDMS surface force titration curve consisted of two peaks centered at pH 4 and pH 8. Reversing the tip/sample pair resulted in the peak positions being shifted upward by 1.0 pH unit. The peak appearing at lower pH is assigned to electrostatic interactions between the two oppositely charged surfaces, whereas the higher pH peak is believed to arise due to ionic H-bonding interactions. AFM images show the effects on surface patterning of amine- and sulfonic acid-modified PDMS surfaces that have undergone two different oxidation methods (air plasma oxidation and Tesla coil oxidation). The surface morphologies of freshly prepared and 24 h aged air plasma oxidized PDMS are also discussed in this study.

Microfluidics for flow cytometric analysis of cells and particles

This review describes recent developments in microfabricated flow cytometers and related microfluidic devices that can detect, analyze, and sort cells or particles. The high-speed analytical capabilities of flow cytometry depend on the cooperative use of microfluidics, optics and electronics. Along with the improvement of other components, replacement of conventional glass capillary-based fluidics with microfluidic sample handling systems operating in microfabricated structures enables volume- and power-efficient, inexpensive and flexible analysis of particulate samples. In this review, we present various efforts that take advantage of novel microscale flow phenomena and microfabrication techniques to build microfluidic cell analysis systems.

Aggregation of paramagnetic particles in the presence of a hydrodynamic shear

We present an experimental study of the aggregation of paramagnetic particles, in the presence of controlled laminar shear flow, conducted in microchannels subjected to an external magnetic field. The microfluidic channels are made of either glass/silicon or polydimethylsiloxane. In ranges of time up to hundreds of seconds, the growth mechanism of the linear chain consists of the accumulation of isolated particles or small clusters onto existing chains, which are all moving at different speeds. In this time regime the chain length increases linearly and has a growth rate that increases as a power law with the shear. At longer times the chain lengths saturate. The Smoluchovski model, which assumes single particle-chain interactions only, closely reproduces the observations both qualitatively and quantitatively. In particular, the evolution of the growth rate of the mean chain length with respect to the shear rate S, predicted as S1/4, is found to be consistent with the experiments.

Fabrication of poly(methyl methacrylate) microfluidic chips by atmospheric molding

A greatly simplified method for fabricating poly(methyl methacrylate) (PMMA) separation microchips is introduced. The new protocol relies on UV-initiated polymerization of the monomer solution in an open mold under ambient pressure. Silicon microstructures are transferred to the polymer substrate by molding a methyl methacrylate solution in a sandwich (silicon master/Teflon spacer/glass plate) mold. The chips are subsequently assembled by thermal sealing of the channel and cover plates. The new fabrication method obviates the need for specialized replication equipment and reduces the complexity of prototyping and manufacturing. Variables of the fabrication process were assessed and optimized. The new method compares favorably with common fabrication techniques, yielding high-quality devices with well-defined channel and injection-cross structures, and highly smoothed surfaces. Nearly 100 PMMA chips were replicated using a single silicon master, with high chip-to-chip reproducibility (relative standard deviations of 1.5 and 4.7% for the widths and depths of the replicated channels, respectively). The relatively high EOF value of the new chips (2.12 x 10(-4) cm(2) x V(-1) x s(-1)) indicates that the UV polymerization process increases the surface charge and hence enhances the fluidic transport. The attractive performance of the new CE microchips has been demonstrated in connection with end-column amperometric and contactless-conductivity detection schemes. While the new approach is demonstrated in connection with PMMA microchips, it could be applied to other materials that undergo light-initiated polymerization. The new approach brings significant simplification of the process of fabricating PMMA devices and should lead to a widespread low-cost production of high-quality separation microchips.

Determination of levoglucosan from smoke samples using microchip capillary electrophoresis with pulsed amperometric detection

Separation and detection of native anhydrous carbohydrates derived from the combustion of biomass using an electrophoretic microchip with pulsed amperometric detection (PAD) is described. Levoglucosan represents the largest single component of the water extractable organics in smoke particles and can be used to trace forest fires or discriminate urban air pollution sources. Detection of levoglucosan and other sugar anhydrides in both source and ambient aerosol samples is typically performed by gas chromatographic (GC) separation with mass spectrometric (MS) detection. This method is cost, time, and labor intensive, typically involving a multistep solvent extraction, chemical derivatization, and finally analysis by GC/MS. However, it provides a rich wealth of chemical information as the result of the combination of a separation method and MS and exhibits good sensitivity. In contrast, microchip capillary electrophoresis offers the possibility of performing simpler, less expensive, and faster analysis. In addition, integrated devices can be fabricated and incorporated with an aerosol collection system to perform semicontinuous, onsite analysis. In the present report, the effect of the separation potential, buffer pH and composition, injection time, and pulsed amperometric detection parameters were studied in an effort to optimize both the separation and detection of anhydrous sugars. Using the optimized conditions, the analysis can be performed in less than a minute, with detection limits ranging from 22 fmol (16.7 microM) for levoglucosan to 336 fmol (258.7 microM) for galactosan. To demonstrate the capabilities of the device, a comparison was made between GC/MS and microchip electrophoresis using an aerosol source sample generated in a wood-burning chamber. A second example utilizing an ambient aerosol sample illustrates a matrix interference necessitating additional method development for application to samples not dominated by wood smoke.

Development of an integrated optic oxygen sensor using a novel, generic platform

This paper describes the development of a generic platform for enhanced, integrated optic sensors based on fluorescence detection. The platform employs a novel optical configuration in order to achieve enhanced performance and has inherent multianalyte detection capability. The sensor element comprises a multimode ridge waveguide that has been patterned with an analyte-sensitive fluorescent spot, which is excited directly using a LED. The platform was applied to the detection of gaseous oxygen as a proof of principle. The sol-gel-derived sensor spots were doped with an oxygen-sensitive fluorescent dichlororuthenium dye complex and intensity-based calibration data were generated from the oxygen-dependent waveguide output. The sensor achieved a LOD of 0.62% and a resolution of less than 0.96% gaseous oxygen, which compares favourably with a similar, recently reported system. This device highlights the combination of inexpensive rapid prototyping techniques and a dedicated sensor enhancement strategy that together facilitate the production of an effective prototype sensor platform.

Microfluidic Immunoassay for Bacterial Toxins with Supported Phospholipid Bilayer Membranes on Poly(dimethylsiloxane)

We report a heterogeneous immunoassay for cholera toxin (CT) using supported bilayer membranes (SBMs) in a poly(dimethylsiloxane) (PDMS) microfluidic device. Phosphatidylcholine membranes assembled on plasma-oxidized PDMS by vesicle fusion bring about favorable surface properties, such as improved wettability and protein resistance. Contact angle measurements show that the lipid membranes can preserve hydrophilic surfaces for hours, whereas untreated substrates rapidly undergo hydrophobic recovery. Fluorescence recovery after photobleaching performed in situ reveals that the membranes have relatively high lateral mobility. Experimental data-fitting to theoretical models yields diffusion coefficients of 1.8 +/- 0.7 microm(2)/s on PDMS and 3.4 +/- 0.8 microm(2)/s on glass. Fluorescence studies utilizing tagged proteins show that SBMs reduce nonspecific adsorption of avidin and BSA on PDMS by 2-3 orders of magnitude, as compared to that on plasma oxidized surfaces. SBMs and their protein-resistant properties are not significantly affected by long flow times, indicating good membrane stability. These studies increase our understanding of the relationship between molecular level interactions and membrane properties, allowing for development of a rapid heterogeneous immunoassay for CT in PDMS microchips with cell surface receptor molecules. Using optimized sample injection and buffer washing conditions, microfluidic immunoassay of CT is complete within 25 min, and a dynamic range over 3 orders of magnitude with a detection limit of 8 fmol of toxin is achieved.

Rapid fabrication of poly(dimethylsiloxane)-based microchip capillary electrophoresis devices using CO2 laser ablation

The use of CO(2) laser ablation for the patterning of capillary electrophoresis (CE) microchannels in poly(dimethylsiloxane)(PDMS) is described. Low-cost polymer devices were produced using a relatively inexpensive CO(2) laser system that facilitated rapid patterning and ablation of microchannels. Device designs were created using a commercially available software package. The effects of PDMS thickness, laser focusing, power, and speed on the resulting channel dimensions were investigated. Using optimized settings, the smallest channels that could be produced averaged 33 microm in depth (11.1% RSD, N= 6) and 110 microm in width (5.7% RSD, N= 6). The use of a PDMS substrate allowed reversible sealing of microchip components at room temperature without the need for cleanroom facilities. Using a layer of pre-cured polymer, devices were designed, ablated, and assembled within minutes. The final devices were used for microchip CE separation and detection of the fluorescently labeled neurotransmitters aspartate and glutamate.

Reagent-Loaded Cartridges for Valveless and Automated Fluid Delivery in Microfluidic Devices

An important problem in the life sciences and in health care is simple and rapid detection of biomarkers. Although microfluidic devices are potentially useful in addressing this problem, current techniques for automating fluid delivery--which include valves and electroosmosis--require sophisticated microfabrication of the chip, bulky instrumentation, or both. In this paper, we describe a simple and reliable technique for storing and delivering a sequence of reagents to a microfluidic device. The technique is low-cost, requires minimal user intervention, and can be performed in resource-poor settings (e.g., outside of a laboratory) in the absence of electricity and computer-controlled equipment. In this method, cartridges made of commercially available tubing are filled by sequentially injecting plugs of reagents separated by air spacers. The air spacers prevent the reagents from mixing with each other during cartridge preparation, storage, and usage. As an example, we used this "plug-in cartridge" technology to complete a solid-phase immunoassay in a microchannel in 2 min with low-nanomolar sensitivity and demonstrate the diagnosis of HIV in 13 min.

Immobilization of individual cells by local photo-polymerization on a chip

A novel separation method for random screening of target cells from a large heterogeneous population by using a local photo-polymerization is developed. A photo-crosslinkable resin solution is mixed with the sample liquid and we controlled the state from sol to gel by irradiating the near ultraviolet (UV) light with the mercury lamp and He-Cd laser near the target cell. We applied three types of immobilization methods such as direct immobilization method, caging method, and direct immobilization with position control method. The selected cell is immobilized in the cured resin directly or inside the cage of the cured resin. In the position control method, laser tweezers are employed to manipulate the target cell indirectly by using the droplet of the resin as a microtool. The cell is positioned properly by the laser manipulation system and is immobilized in the polymerized resin. After the selected cells are immobilized we can easily remove the other objects by the cleaning flow in the microchannel since the polymerized resin strongly binds with the cover glass and resists more than 466 mm s(-1) flow speed in the microchannel (microchannel size: width is 500 micron and depth is 100 micron). We tested the mercury lamp as well as the He-Cd laser for UV-light irradiation at the local area and confirmed improvement of resolution of the cured area by using the He-Cd laser (from 7 micron to 5 micron). Based on this method, we succeeded in single cell immobilization and basic experiments such as culture and fluorescent dyeing of immobilized yeast cells.

Multilayer poly(vinyl alcohol)-adsorbed coating on poly(dimethylsiloxane) microfluidic chips for biopolymer separation

A poly(dimethylsiloxane) (PDMS) microfluidic chip surface was modified by multilayer-adsorbed and heat-immobilized poly(vinyl alcohol) (PVA) after oxygen plasma treatment. The reflection absorption infrared spectrum (RAIRS) showed that 88% hydrolyzed PVA adsorbed more strongly than 100% hydrolyzed one on the oxygen plasma-pretreated PDMS surface, and they all had little adsorption on original PDMS surface. Repeating the coating procedure three times was found to produce the most robust and effective coating. PVA coating converted the original PDMS surface from a hydrophobic one into a hydrophilic surface, and suppressed electroosmotic flow (EOF) in the range of pH 3-11. More than 1,000,000 plates/m and baseline resolution were obtained for separation of fluorescently labeled basic proteins (lysozyme, ribonuclease B). Fluorescently labeled acidic proteins (bovine serum albumin, beta-lactoglobulin) and fragments of dsDNA phiX174 RF/HaeIII were also separated satisfactorily in the three-layer 88% PVA-coated PDMS microchip. Good separation of basic proteins was obtained for about 70 consecutive runs.

Electrotransfection of Mammalian Cells Using Microchannel-Type Electroporation Chip

Transfection of DNA molecules into mammalian cells with electric pulsations, which is so-called electroporation, is a powerful and widely used method that can be directly applied to gene therapy. However, very little is known about the basic mechanisms of DNA transfer and cell response to the electric pulse. We developed a microelectroporation chip with poly(dimethylsiloxane) (PDMS) to investigate the mechanism of electroporation as a first step of DNA transfer and to introduce the benefits of miniaturization into the genetic manipulation. The microelectroporation chip has a microchannel with a height of 20 microm and a length of 2 cm. Owing to the transparency of PDMS, we could in situ observe the uptake process of propidium iodide (PI) into SK-OV-3 cells, which shows promise in visualization of gene delivery in living cells. We also noticed the geometric effect on the degree of electroporation in microchannels with diverse channel width. This experimental result shows that the geometry can be another parameter to be considered for the electroporation when it is performed in microchannels with an exponential decaying pulse generator. Cell culturing is possible within the microelectroporation chip, and we also successfully transfected SK-OV-3 cells with enhanced green fluorescent protein genes, which demonstrates the feasibility of the microelectroporation chip in genetic manipulation.

Electrokinetically Controlled DNA Hybridization Microfluidic Chip Enabling Rapid Target Analysis

Biosensors and more specifically biochips exploit the interactions between a target analyte and an immobilized biological recognition element to produce a measurable signal. Systems based on surface nucleic acid hybridization, such as microarrays, are particularly attractive due to the high degree of selectivity in the binding interactions. One of the drawbacks of this reaction is the relatively long time required for complete hybridization to occur, which is often the result of diffusion-limited reaction kinetics. In this work, an electrokinetically controlled DNA hybridization microfluidic chip will be introduced. The electrokinetic delivery technique provides the ability to dispense controlled samples of nanoliter volumes directly to the hybridization array (thereby increasing the reaction rate) and rapidly remove nonspecific adsorption, enabling the hybridization, washing, and scanning procedures to be conducted simultaneously. The result is that all processes from sample dispensing to hybridization detection can be completed in as little as 5 min. The chip also demonstrates an efficient hybridization scheme in which the probe saturation level is reached very rapidly as the targets are transported over the immobilized probe site enabling quantitative analysis of the sample concentration. Detection levels as low as 50 pM have been recorded using an epifluorescence microscope.

Investigation into the Applicability of the Centrifugal Microfluidics Platform for the Development of Protein−Ligand Binding Assays Incorporating Enhanced Green Fluorescent Protein as a Fluorescent Reporter

The incorporation of a protein-ligand binding assay into a centrifugal microfluidics platform is described. The platform itself is a disc-shaped polymer substrate, upon which a series of microfluidic channels and reservoirs have been machined. Centrifugal microfluidics platforms require no internal moving parts, and fluid propulsion is achieved solely through rotation of the disc. Fluid flow is controlled by passive valves, the opening of which is dependent on the angular frequency of the rotating platform, the channel dimensions, and the physical properties of the fluid. To evaluate the effectiveness of incorporating a protein-based assay onto the centrifugal microfluidics analytical platform, a class-selective, homogeneous assay for the detection of phenothiazine antidepressants was employed. This class of drugs is known to bind to calmodulin, a calcium binding protein. Specifically, a fusion protein between calmodulin and enhanced green fluorescent protein was utilized. Calmodulin undergoes a conformational change upon binding to phenothiazines that alters the fluorescence properties of the attached fluorescent protein, which can be correlated to the concentration of the drug present. Another important aspect of this work was to study the efficacy of the platform to perform reconstitution assays. To do this, the biological reagent was dried on the platform and rehydrated to carry out the assay. The ability to prealiquot reagents on the platform should enhance its versatility and portability. The integration of protein-based assays in this platform should be useful in the design of analytical systems for high-throughput screening of pharmaceuticals and clinical diagnostics.

Titania and Alumina Sol−Gel-Derived Microfluidics Enzymatic-Reactors for Peptide Mapping: Design, Characterization, and Performance

The design and characterization of titania-based and alumina-based Poly(dimethylsiloxane) (PDMS) microfluidics enzymatic-reactors along with their analytical features in coupling with MALDI-TOF and ESI-MS were reported. Microfluidics with microchannel and stainless steel tubing (SST) were fabricated using PDMS casting and O(2)-plasma techniques, and were used for the preparation of an enzymatic-reactor. Plasma oxidation for the PDMS microfluidic system enabled the channel wall of the microfluidics to present a layer of silanol (SiOH) groups. These SiOH groups act as anchors onto the microchannel wall linked covalently with the hydroxyl groups of trypsin-encapsulated sol matrix. As a result, the trypsin-encapsulated gel matrix was anchored onto the wall of the microchannel, and the leakage of gel matrix from the microchannel was effectively prevented. A feature of the microfluidic enzymatic-reactors is the feasibility of performing on-line protein analysis by attached SST electrode and replaceable tip. The success of trypsin encapsulation was investigated by AFM imaging, assay of enzymatic activity, CE detection, and MALDI-TOF and ESI-MS analysis. The lab-made devices provide an excellent extent of digestion even at a fast flow rate of 7.0 microL/min, which affords the very short residence time of ca. 2 s. With the present device, the digestion time was significantly shortened compared to conventional tryptic reaction schemes. In addition, the encapsulated trypsin exhibits increased stability even after continuous use. These features are required for high-throughput protein identification.

Frontal photopolymerization for microfluidic applications

Frontal photopolymerization (FPP) offers numerous advantages for the rapid prototyping of microfluidic devices. Quantitative utilization of this method, however, requires a control of the vertical dimensions of the patterned resist material. To address this fundamental problem, we study the ultraviolet (UV) photopolymerization of a series of multifunctional thiolene resists through a combination of experiments and analytical modeling of the polymerization fronts. We describe this nonlinear spatio-temporal growth process in terms of a "minimal" model involving an order parameter phi(x, t) characterizing the extent of monomer-to-polymer conversion, the optical attenuation T(x, t), and the solid front position h(t). The latter exhibits an induction time (or equivalent critical UV dose) characterizing the onset of frontal propagation. We also observe a novel transition between two logarithmic rates of growth, determined by the Beer-Lambert attenuation constants mu(0) and mu(infinity) of the monomer and fully polymerized material, respectively. The measured frontal kinetics and optical transmission of the thiolene resist materials are consistent with our photopolymerization model, exhibiting both "photodarkening" and "photoinvariant" polymerization. This is apparently the first observation of photodarkening reported in FPP. On the basis of these results, multilevel fluidic devices with controlled height are readily fabricated with modulated illumination. A representative two-level microfluidic device, incorporating a chaotic mixer, a T junction, and a series of controlled flow constrictions, illustrates the practical versatility of this fabrication method.

Computerized microfluidic cell culture using elastomeric channels and Braille displays

Computer-controlled microfluidics would advance many types of cellular assays and microscale tissue engineering studies wherever spatiotemporal changes in fluidics need to be defined. However, this goal has been elusive because of the limited availability of integrated, programmable pumps and valves. This paper demonstrates how a refreshable Braille display, with its grid of 320 vertically moving pins, can power integrated pumps and valves through localized deformations of channel networks within elastic silicone rubber. The resulting computerized fluidic control is able to switch among: (i) rapid and efficient mixing between streams, (ii) multiple laminar flows with minimal mixing between streams, and (iii) segmented plug-flow of immiscible fluids within the same channel architecture. The same control method is used to precisely seed cells, compartmentalize them into distinct subpopulations through channel reconfiguration, and culture each cell subpopulation for up to 3 weeks under perfusion. These reliable microscale cell cultures showed gradients of cellular behavior from C2C12 myoblasts along channel lengths, as well as differences in cell density of undifferentiated myoblasts and differentiation patterns, both programmable through different flow rates of serum-containing media. This technology will allow future microscale tissue or cell studies to be more accessible, especially for high-throughput, complex, and long-term experiments. The microfluidic actuation method described is versatile and computer programmable, yet simple, well packaged, and portable enough for personal use.

Transport, location, and quantal release monitoring of single cells on a microfluidic device

A novel microfluidic device has been developed for on-chip transport, location, and quantal release monitoring of single cells. The microfluidic device consists of a plate of PDMS containing channels for introducing cells and stimulants and a glass substrate into which a cell micro-chamber was etched. The two tightly reversibly sealed plates can be separated for respective cleaning, which significantly extends the lifetime of the microchip that is frequently clogged in cell analysis experiments. Using hydraulic pressure, single cells were transported and located on the microfluidic chip. After location of a single PC12 cell on the microfluidic chip, the cell was stimulated by nicotine that was also introduced through the micro-channels, and the quantum release of dopamine from the cell was amperometricly detected with our designed carbon fiber microelectrode. The results have demonstrated the convenience and efficiency of using the microfluidic chip for monitoring of quantal release from single cells and have offered a facile method for the analysis of single cells on microfluidic devices.

Dynamic interfacial effect of electroosmotic slip flow with a moving capillary front in hydrophobic circular microchannels

Miniaturization of chemical analysis using microfabrication is an emerging technology. The use of polymeric materials as opposed to conventional glass substrate is also a promising alternative. As most polymeric materials are hydrophobic relative to glass, we describe here the implication for the loading process of electroosmotic flow (EOF) when a three-phase (solid-liquid-vapor) contact line exists. The presence of these interfaces can result in a large Laplace pressure that resists EOF and hence hinders its flow performance. This effect depends on the phenomenological contact angle at the solid-liquid interface. In our model for EOF, we considered simultaneously the presence of an electric double layer, liquid slips via a weaker solid-liquid interaction and Laplace pressure across a liquid-vapor interface.

Cationic Starch Derivatives as Dynamic Coating Additives for Analysis of Amino Acids and Peptides Using Poly(methyl methacrylate) Microfluidic Devices

Plastic microchips are very promising analytical devices because they are less fragile and are suitable for mass production. However, due to their hydrophobicity, the surface strongly interacts with nonpolar analytes or species containing hydrophobic domains, resulting in significant uncontrolled adsorption on channel walls. This paper describes the poly(methyl methacrylate) surface treatment by dynamic coating additives that considerably decreases adsorption of analytes to channel walls. Among the additives studied, quaternary ammonium starch derivatives suppressed the adsorption of fluorescently labeled amino acids and peptides most effectively. The effect was valid over the wide pH range from 2.5 to 8.0. Using a 10 mM phosphate buffer (pH 7.0) with 3% (w/v) quaternary ammonium starch as the running buffer, Asp and Glu, respectively, migrated at 54.6 and 57.6 s with efficiencies of 380 000 and 370 000 plates/m. In addition, this cationic starch derivative was found to possess good solubility and low viscosity.

Application of on-chip cell cultures for the detection of allergic response

In this report, the development of a microfluidic cell chip for monitoring allergic response is described. A rat basophilic leukemia cell line (RBL-2H3), a tumor analog of rat mucosal mast cells, has been used as a model to observe its allergic response upon antigen stimulus. The cells were cultivated on a poly(dimethylsiloxane) (PDMS) chip, the surface of which was modified by several methods. The PDMS chip, which comprised a cell cultivation chamber and microfluidic channels, was fabricated by conventional molding methods. In order to detect the allergic response, a fluorescent dye, quinacrine, was introduced inside the cell compartment that included histamine. The cells were stimulated with dinitrophenylated bovine serum albumin (DNP-BSA) after incubation with anti-DNP IgE. When exocytosis events occurred, the microfluidic system detected the fluorescent signal of quinacrine, which was released from RBL-2H3 cells by using a photomultiplier tube (PMT) fitted onto a microscope.

Application of fluorescence correlation spectroscopy for velocity imaging in microfluidic devices

In this paper we present and demonstrate a technique for mapping fluid flow rates in microfluidic systems with sub-micrometer resolution using confocal microscopy in conjunction with fluorescence correlation spectroscopy (FCS). Flow velocities ranging from approximately 50 microm/s to approximately 10 cm/s can be recorded using fluorescent polymer nanospheres as fluid motion tracers. Velocity profiles and images of the flow in poly(dimethylsiloxane)-glass microchannels are presented and analyzed. Using the method, velocity images along the horizontal (top view) and vertical planes within a microdevice can be obtained. This is, to our knowledge, the first report of FCS for producing velocity maps. The high-resolution velocity maps can be used to characterize and optimize microdevice performance and to validate simulation efforts.

Electrochemical detector based on sol-gel-derived carbon composite material for capillary electrophoresis microchips

An on-chip disk electrode based on sol-gel-derived carbon composite material could be easily and reproducibly fabricated. Unlike other carbon-based electrodes reported previously, this detector is rigid, convenient to fabricate, and amenable to chemical modifications. Based on the stable and reproducible characters of this detector, a copper particle-modified detector was developed for the detection of carbohydrates which extends the application of the carbon-based electrode. In our experiments, the performance of the new integrated detector for rapid on-chip measurement of epinephrine and glucose was illustrated. Experimental procedures including the fabrication of this detector, the configuration of separation channel outlet and electrode verge, and the performance characteristics of this new electrochemical detector were investigated.

Microfluidic glucose stimulation reveals limited coordination of intracellular Ca2+ activity oscillations in pancreatic islets

The pancreatic islet is a functional microorgan involved in maintaining normoglycemia through regulated secretion of insulin and other hormones. Extracellular glucose stimulates insulin secretion from islet beta cells through an increase in redox state, which can be measured by NAD(P)H autofluorescence. Glucose concentrations over approximately 7 mM generate synchronous oscillations in beta cell intracellular Ca2+ concentration ([Ca2+]i), which lead to pulsatile insulin secretion. Prevailing models assume that the pancreatic islet acts as a functional syncytium, and the whole islet [Ca2+]i response has been modeled in terms of islet bursting and pacemaker models. To test these models, we developed a microfluidic device capable of partially stimulating an islet, while allowing observation of the NAD(P)H and [Ca2+]i responses. We show that beta cell [Ca2+]i oscillations occur only within regions stimulated with more than approximately 6.6 mM glucose. Furthermore, we show that tolbutamide, an antagonist of the ATP-sensitive K+ channel, allows these oscillations to travel farther into the nonstimulated regions of the islet. Our approach shows that the extent of Ca2+ propagation across the islet depends on a delicate interaction between the degree of coupling and the extent of ATP-sensitive K+-channel activation and illustrates an experimental paradigm that will have utility for many other biological systems.

Improved surface chemistries, thin film deposition techniques, and stamp designs for nanotransfer printing

Nanotransfer printing represents an additive approach for patterning thin layers of solid materials with nanometer resolution. The surface chemistries, thin film deposition techniques, and stamp designs are all important for the proper operation of this method. This paper presents some details concerning processing procedures and other considerations needed for patterning two- and three-dimensional nanostructures with low density of defects and minimal distortions.

Combinatorial mixing of microfluidic streams

We have devised a microfluidic mixer design that produces all the mixture combinations of a given number of dilutions of the input compounds. As proof of the concept, we present a device that generates four titrations of two dye solutions, blue and yellow, and combinatorially mixes the blue titrations with the yellow titrations to deliver the sixteen mixture combinations in separate outlet microchannels. Our device features four different flow levels made by stacking nine laser-cut Mylar laminates. The fluidic network has a symmetric design that guarantees that the flow rates are the same at all the outlets, with deviations attributable to imperfections in the fabrication, assembly, or perfusion processes. Design rules for scaling up the number of compounds and/or dilutions are presented. The mixing scheme has broad applicability in high-throughput combinatorial testing applications such as drug screening, cell-based biochemical assays, lab-on-a-chip devices, and biosensors.

A buckling-based metrology for measuring the elastic moduli of polymeric thin films

As technology continues towards smaller, thinner and lighter devices, more stringent demands are placed on thin polymer films as diffusion barriers, dielectric coatings, electronic packaging and so on. Therefore, there is a growing need for testing platforms to rapidly determine the mechanical properties of thin polymer films and coatings. We introduce here an elegant, efficient measurement method that yields the elastic moduli of nanoscale polymer films in a rapid and quantitative manner without the need for expensive equipment or material-specific modelling. The technique exploits a buckling instability that occurs in bilayers consisting of a stiff, thin film coated onto a relatively soft, thick substrate. Using the spacing of these highly periodic wrinkles, we calculate the film's elastic modulus by applying well-established buckling mechanics. We successfully apply this new measurement platform to several systems displaying a wide range of thicknessess (nanometre to micrometre) and moduli (MPa to GPa).

Rapid Prototyping of Thermoset Polyester Microfluidic Devices

This paper presents a simple procedure for the fabrication of thermoset polyester (TPE) microfluidic systems and discusses the properties of the final devices. TPE chips are fabricated in less than 3 h by casting TPE resin directly on a lithographically patterned (SU-8) silicon master. Thorough curing of the devices is obtained through the combined use of ultraviolet light and heat, as both an ultraviolet and a thermal initiator are employed in the resin mixture. Features on the order of micrometers and greater are routinely reproduced using the presented procedure, including complex designs and multilayer features. The surface of TPE was characterized using contact angle measurements and X-ray photoelectron spectroscopy (XPS). Following oxygen plasma treatment, the hydrophilicity of the surface of TPE increases (determined by contact angle measurements) and the proportion of oxygen-containing functional groups also increases (determined by XPS), which indicates a correlated increase in the charge density on the surface. Native TPE microchannels support electroosmotic flow (EOF) toward the cathode, with an average electroosmotic mobility of 1.3 x 10(-4) cm(2) V(-1) s(-1) for a 50-microm square channel (20 mM borate at pH 9); following plasma treatment (5 min at 30 W and 0.3 mbar), EOF is enhanced by a factor of 2. This enhancement of the EOF from plasma treatment is stable for days, with no significant decrease noted during the 5-day period that we monitored. Using plasma-treated TPE microchannels, we demonstrate the separation of a mixture of fluorescein-tagged amino acids (glycine, glutamic acid, aspartic acid). TPE devices are up to 90% transparent (for approximately 2-mm-thick sample) to visible light (400-800 nm). The compatibility of TPE with a wide range of solvents was tested over a 24-h period, and the material performed well with acids, bases, alcohols, cyclohexane, n-heptane, and toluene but not with chlorinated solvents (dichloromethane, chloroform).

Immobilization of DNA onto poly(dimethylsiloxane) surfaces and application to a microelectrochemical enzyme-amplified DNA hybridization assay

This paper describes immobilization of DNA onto the interior walls of poly(dimethylsiloxane) (PDMS) microsystems and its application to an enzyme-amplified electrochemical DNA assay. DNA immobilization was carried out by silanization of the PDMS surface with 3-mercaptopropyltrimethoxysilane to yield a thiol-terminated surface. 5'-acrylamide-modified DNA reacts with the pendant thiol groups to yield DNA-modified PDMS. Surface-immobilized DNA oligos serve as capture probes for target DNA. Biotin-labeled target DNA hybridizes to the PDMS-immobilized capture DNA, and subsequent introduction of alkaline phosphatase (AP) conjugated to streptavidin results in attachment of the enzyme to hybridized DNA. Electrochemical detection of DNA hybridization benefits from enzyme amplification. Specifically, AP converts electroinactive p-aminophenyl phosphate to electroactive p-aminophenol, which is detected using an indium tin oxide interdigitated array (IDA) electrode. The IDA electrode eliminates the need for a reference electrode and provides a steady-state current that is related to the concentration of hybridized DNA. At present, the limit of detection of the DNA target is 1 nM in a volume of 20 nL, which corresponds to 20 attomoles of DNA.