Integration of isoelectric focusing with multi-channel gel electrophoresis by using microfluidic pseudo-valves

Induced cell migration based on a bioactive hydrogel sheet combined with a perfused microfluidic system

Endothelial cell migration is a crucial step in the process of new blood vessel formation-a necessary process to maintain cell viability inside thick tissue constructs. Here, we report a new method for maintaining cell viability and inducing cell migration using a perfused microfluidic platform based on collagen gel and a gradient hydrogel sheet. Due to the helpful role of the extracellular matrix components in cell viability, we developed a hydrogel sheet from decellularized tissue (DT) of the bovine heart and chitosan (CS). The results showed that hydrogel sheets with an optimum weight ratio of CS/DT = 2 possess a porosity of around 75%, a mechanical strength of 23 kPa, and display cell viability up to 78%. Then, we immobilized a radial gradient of vascular endothelial growth factor (VEGF) on the hydrogel sheet to promote human umbilical vein endothelial cell migration. Finally, we incorporated the whole system as an entirety on the top of the microfluidic platform and studied cell migration through the hydrogel sheet in the presence of soluble and immobilized VEGF. The results demonstrated that immobilized VEGF stimulated cell migration in the hydrogel sheet at all depths compared with soluble VEGF. The results also showed that applying a VEGF gradient in both soluble and immobilized states had a significant effect on cell migration at limited depths (<100 μm). The main finding of this study is a significant improvement in cell migration using an in vivo imitating, cost-efficient and highly reproducible platform, which may open up a new perspective for tissue engineering applications.

Experimental and numerical study of band-broadening effects associated with analyte transfer in microfluidic devices for spatial two-dimensional liquid chromatography created by additive manufacturing

Conventional one-dimensional column-based liquid chromatographic (LC) systems do not offer sufficient separation power for the analysis of complex mixtures. Column-based comprehensive two-dimensional liquid chromatography offers a higher separation power, yet suffers from instrumental complexity and long analysis times. Spatial two-dimensional liquid chromatography can be considered as an alternative to column-based approaches. The peak capacity of the system is ideally the product of the peak capacities of the two dimensions, yet the analysis time remains relatively short due to parallel second-dimension separations. Aspects affecting the separation efficiency of this type of systems include flow distribution to homogeneously distribute the mobile phase for the second-dimension (²D) separation, flow confinement during the first-dimension (¹D) separation, and band-broadening effects during analyte transfer from the ¹D separation channel to the ²D separation area. In this study, the synergy between computational fluid dynamics (CFD) simulations and rapid prototyping was exploited to address band broadening during the ²D development and analyte transfer from ¹D to ²D. Microfluidic devices for spatial two-dimensional liquid chromatography were designed, simulated, 3D-printed and tested. The effects of presence and thickness of spacers in the ²D separation area were addressed and leaving these out proved to be the most efficient solution regarding band broadening reduction. The presence of a stationary-phase material in the ¹D channel had a great effect on the analyte transfer from the ¹D to the ²D and the resulting band broadening. Finally, pressure limit of the fabricated devices and printability are discussed.

Protein separation under a microfluidic regime

Lab-on-a-Chip (LoC), or micro-Total Analysis Systems (μTAS), is recognized as a powerful analytical technology with high capabilities, though end-user products for protein purification are still far from being available on the market. Remarkable progress has been achieved in the separation of nucleic acids and proteins using electrophoretic microfluidic devices, while pintsize devices have been developed for protein isolation according to miniaturized chromatography principles (size, charge, affinity, etc.). In this work, we review the latest advances in the fabrication of components, detection methods and commercial implementation for the separation of biological macromolecules based on microfluidic systems, with some critical remarks on the perspectives of their future development towards standardized microfluidic systems and protocols. An outlook on the current needs and future applications is also presented.

Advances in capillary electrophoresis and the implications for drug discovery

INTRODUCTION

Many screening platforms are prone to assay interferences that can be avoided by directly measuring the target or enzymatic product. Capillary electrophoresis (CE) and microchip electrophoresis (MCE) have been applied in a variety of formats to drug discovery. CE provides direct detection of the product allowing for the identification of some forms of assay interference. The high efficiency, rapid separations, and low volume requirements make CE amenable to drug discovery. Areas covered: This article describes advances in capillary electrophoresis throughput, sample introduction, and target assays as they pertain to drug discovery and screening. Instrumental advances discussed include integrated droplet microfluidics platforms and multiplexed arrays. Applications of CE to assays of diverse drug discovery targets, including enzymes and affinity interactions are also described. Expert opinion: Current screening with CE does not fully take advantage of the throughputs or low sample volumes possible with CE and is most suitable as a secondary screening method or for screens that are inaccessible with more common platforms. With further development, droplet microfluidics coupled to MCE could take advantage of the low sample requirements by performing assays on the nanoliter scale at high throughput.

Sequential phosphorylation analysis using dye-tethered peptides and microfluidic isoelectric focusing electrophoresis

We report a simple method for analyzing sequential phosphorylation by protein kinases using fluorescent peptide substrates and microfluidic isoelectric focusing (μIEF) electrophoresis. When a dye-labeled peptide substrate was sequentially phosphorylated by two consecutive protein kinases (mitogen-activated protein kinase (MAPK) and glycogen synthase kinase 3 (GSK3)), its differently phosphorylated forms were easily separated and visualized by fluorescent focusing zones in the μIEF channel based on a change in the isoelectric point (pI) by phosphorylation. As a result, ratiometric and quantitative analysis of the fluorescent focusing regions shifted by phosphorylation enabled the analysis of phosphorylation efficiency and the relevant inhibition of protein kinases (MAPK and GSK3) with high simplicity and selectivity. Furthermore, the GSK3 activity in the cell lysates was elucidated by μIEF electrophoresis in combination with immunoprecipitation. Our results suggest that this method has great potential for analyzing the sequential phosphorylation of multiple protein kinases that are implicated in cellular signaling pathways.

Use of vacuum bagging for fabricating thermoplastic microfluidic devices

In this work we present a novel thermal bonding method for thermoplastic microfluidic devices. This simple method employs a modified vacuum bagging technique, a concept borrowed from the aerospace industry, to produce conventional thick substrate microfluidic devices, as well as multi-layer film devices. The bonds produced using this method are superior to those obtained using conventional thermal bonding methods, including thermal lamination, and are capable of sustaining burst pressures in excess of 550 kPa. To illustrate the utility of this method, thick substrate devices were produced, as well as a six-layer film device that incorporated several complex features.

Microchip electrophoresis for fast and interference-free determination of trace amounts of glyphosate and glufosinate residues in agricultural products

Fast screening of herbicide residues is becoming important to ensure food safety, but traditional chromatographic methods may not be suitable for rapid on-site analysis of samples with complicated matrices. Here, we describe a method for rapid and sensitive determination of glyphosate (GLYP) and glufosinate (GLUF) residues in agricultural products by electrophoresis on disposable microchips with laser-induced fluorescence detection. With this method, quantitative analysis of trace amounts of GLYP and GLUF can be achieved with relatively simple sample preparation.

Microchamber integration unifies distinct separation modes for two-dimensional electrophoresis

By combining isoelectric focusing (IEF) with subsequent gel electrophoresis, two-dimensional electrophoresis (2DE) affords more specific characterization of proteins than each constituent unit separation. In a new approach to integrating the two assay dimensions in a microscope slide-sized glass device, we introduce microfluidic 2DE using photopatterned polyacrylamide (PA) gel elements housed in a millimeter-scale, 20-μm-deep chamber. The microchamber minimizes information loss inherent to channel network architectures commonly used for microfluidic 2DE. To define the IEF axis along a "lane" at the top of the chamber, we used free solution carrier ampholytes and immobilized acrylamido buffers in the PA gels. This approach yielded high-resolution (0.1 pH unit) and rapid (<20 min) IEF. Next, protein transfer to the second dimension was accomplished by chemical mobilization perpendicular to the IEF axis. Mobilization drove focused proteins off the IEF lane and into a region for protein gel electrophoresis. Using fluorescently labeled proteins, we observed transfer-induced band broadening factors ~7.5-fold lower than those observed in microchannel networks. Both native polyacrylamide gel electrophoresis (PAGE) and pore-limit electrophoresis (PLE) were studied as the second assay dimension and completed in <15 min. PLE yields protein molecular mass information without the need for ionic surfactant or reducing agents, simplifying device design and operation. Microchamber-based 2DE unifies two independent separation dimensions in a single device with minimal transfer-associated information losses. Peak capacities for the total assay ranged from 256 to 35 with <1 h assay duration. The rapid microchamber 2DE assay has the potential to bridge an existing gap in targeted proteomics for protein biomarker validation and systems biology that may complement recent innovation in mass spectrometry.

Fast and interference-free determination of glyphosate and glufosinate residues through electrophoresis in disposable microfluidic chips

With an increasing concern on food safety, fast screening of residues of widespread herbicides becomes necessary. Herein we report a microchip electrophoresis system with laser induced fluorescence (LIF) detection for rapid and sensitive analysis of glyphosate (GLYP) and glufosinate (GLUF) residues. Disposable cyclic olefin copolymer microchips and a low-cost LIF detector were employed to minimize the cost of the analysis. Systematic optimization of experimental conditions was performed to achieve highly efficient analysis. Under the selected condition, GLYP and GLUF were efficiently resolved from sample matrices with a buffer containing 10 mmol/L borax and 2.0% (m/v) hydroxypropyl cellulose at pH 9.0. The number of theoretical plates of 1.0×10(6) m(-1) was attained for both analytes. Derivatization at lower concentrations (<10 μg/L) was also examined, successful detection of 0.34 μg/L GLYP and 0.18 μg/L GLUF was confirmed. The system was applied for the determination of both analytes in real samples without any preconcentration involved. Recoveries of GLYP and GLUF spiked in these samples were 84.0-101.0% and 90.0-103.0%, respectively.

Chip-capillary hybrid device for automated transfer of sample preseparated by capillary isoelectric focusing to parallel capillary gel electrophoresis for two-dimensional protein separation

In this article, we introduce a chip-capillary hybrid device to integrate capillary isoelectric focusing (CIEF) with parallel capillary sodium dodecyl sulfate- polyacrylamide gel electrophoresis (SDS-PAGE) or capillary gel electrophoresis (CGE) toward automating two-dimensional (2D) protein separations. The hybrid device consists of three chips that are butted together. The middle chip can be moved between two positions to reroute the fluidic paths, which enables the performance of CIEF and injection of proteins partially resolved by CIEF to CGE capillaries for parallel CGE separations in a continuous and automated fashion. Capillaries are attached to the other two chips to facilitate CIEF and CGE separations and to extend the effective lengths of CGE columns. Specifically, we illustrate the working principle of the hybrid device, develop protocols for producing and preparing the hybrid device, and demonstrate the feasibility of using this hybrid device for automated injection of CIEF-separated sample to parallel CGE for 2D protein separations. Potentials and problems associated with the hybrid device are also discussed.

Microscale 2D separation systems for proteomic analysis

Microscale 2D separation systems have been implemented in capillaries and microfabricated channels. They offer advantages of faster analysis, higher separation efficiency and less sample consumption than the conventional methods, such as liquid chromatography (LC) in a column and slab gel electrophoresis. In this article, we review their recent advancement, focusing on three types of platforms, including 2D capillary electrophoresis (CE), CE coupling with capillary LC, and microfluidic devices. A variety of CE and LC modes have been employed to construct 2D separation systems via sophistically designed interfaces. Coupling of different separation modes has also been realized in a number of microfluidic devices. These separation systems have been applied for the proteomic analysis of various biological samples, ranging from a single cell to tumor tissues.

Characterization of bonding between poly(dimethylsiloxane) and cyclic olefin copolymer using corona discharge induced grafting polymerization

Thermoplastics have been increasingly used for fabricating microfluidic devices because of their low cost, mechanical/biocompatible attributes, and well-established manufacturing processes. However, there is sometimes a need to integrate such a device with components made from other materials such as polydimethylsiloxane (PDMS). Bonding thermoplastics with PDMS to produce hybrid devices is not straightforward. We have reported our method to modify the surface property of a cyclic olefin copolymer (COC) substrate by using corona discharge and grafting polymerization of 3-(trimethoxysilyl)propyl methacrylate; the modified surface enabled strong bonding of COC with PDMS. In this paper, we report our studies on the surface modification mechanism using attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR), X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM) and contact angle measurement. Using this bonding method, we fabricated a three-layer (COC/PDMS/COC) hybrid device consisting of elastomer-based valve arrays. The microvalve operation was confirmed through the displacement of a dye solution in a fluidic channel when the elastomer membrane was pneumatically actuated. Valve-enabled microfluidic handling was demonstrated.

Stability of DNA origami nanoarrays in cell lysate

Scaffolded DNA origami, a method to create self-assembled nanostructures with spatially addressable features, has recently been used to develop water-soluble molecular chips for label-free RNA detection, platforms for deterministic protein positioning, and single molecule reaction observatories. These applications highlight the possibility of exploiting the unique properties and biocompatibility of DNA nanostructures in live, cellular systems. Herein, we assembled several DNA origami nanostructures of differing shape, size and probes, and investigated their interaction with lysate obtained from various normal and cancerous cell lines. We separated and analyzed the origami-lysate mixtures using agarose gel electrophoresis and recovered the DNA structures for functional assay and subsequent microscopic examination. Our results demonstrate that DNA origami nanostructures are stable in cell lysate and can be easily separated from lysate mixtures, in contrast to natural, single- and double-stranded DNA. Atomic force microscope (AFM) and transmission electron microscope (TEM) images show that the DNA origami structures are fully intact after separation from cell lysates and hybridize to their targets, verifying the superior structural integrity and functionality of self-assembled DNA origami nanostructures relative to conventional oligonucleotides. The stability and functionality of DNA origami structures in cell lysate validate their use for biological applications, for example, as programmable molecular rafts or disease detection platforms.

Thermoplastic microfluidic devices and their applications in protein and DNA analysis

Microfluidics is a platform technology that has been used for genomics, proteomics, chemical synthesis, environment monitoring, cellular studies, and other applications. The fabrication materials of microfluidic devices have traditionally included silicon and glass, but plastics have gained increasing attention in the past few years. We focus this review on thermoplastic microfluidic devices and their applications in protein and DNA analysis. We outline the device design and fabrication methods, followed by discussion on the strategies of surface treatment. We then concentrate on several significant advancements in applying thermoplastic microfluidic devices to protein separation, immunoassays, and DNA analysis. Comparison among numerous efforts, as well as the discussion on the challenges and innovation associated with detection, is presented.

Chemical-assisted bonding of thermoplastics/elastomer for fabricating microfluidic valves

Thermoplastics such as cyclic olefin copolymer (COC) and polymethylmethacrylate (PMMA) have been increasingly used in fabricating microfluidic devices. However, the state-of-the-art microvalve technology is a polydimethylsiloxane (PDMS)-based three-layer structure. In order to integrate such a valve with a thermoplastics-based microfluidic device, a bonding method for thermoplastics/PDMS must be developed. We report here a method to bond COC with PDMS through surface activation by corona discharge, surface modification using 3-(trimethoxysilyl)propyl methacrylate (TMSPMA), and thermal annealing. The method is also applicable to PMMA. The bonding strength between thermoplastics and PDMS was represented by the peeling force, which was measured using a method established by the International Organization for Standardization (ISO). The bonding strength measurement offered an objective and quantitative indicator for protocol optimization, as well as comparison with other PDMS-associated bonding methods. Using optimized bonding conditions, two valve arrays were fabricated in a COC/PDMS/COC device and cyclic operations of valve closing/opening were successfully demonstrated. The valve-containing devices withstood 100 psi (∼689 KPa) without delamination. Further, we integrated such valve arrays in a device for protein separation and demonstrated isoelectric focusing in the presence of valves.

2D separations on a 1D chip: gradient elution moving boundary electrophoresis-chiral capillary zone electrophoresis

A new method is described for two-dimensional (2D) separations using a microfluidic chip normally employed for single dimension electrophoresis. The method employs a combination of gradient elution moving boundary electrophoresis (GEMBE) and chiral capillary zone electrophoresis (CZE). The simplicity of the first dimension GEMBE method enables its implementation in the injection channel of a conventional electrophoresis chip, simplifying the design and operation of the device. The method was used for high resolution 2D chiral separations of a mixture of amino acids considered as possible signatures of extant or extinct life for solar system exploration. The enantiomers of aspartic acid, glutamic acid, serine, alanine, and valine were all resolved as well as glycine (achiral) and several unidentified impurities, giving an estimated peak capacity of 35 for the region between valine and glycine. The results highlight the need for high peak capacity separations for chiral amino acid analysis if accurate enantiomeric ratios are to be determined.

Automated microfluidic protein immunoblotting

This protocol describes regional photopatterning of polyacrylamide gels in glass microfluidic devices as a platform for seamless integration of multiple assay steps. The technology enables rapid, automated protein immunoblotting, demonstrated in this study for native western blotting. The fabrication procedure is straightforward and requires approximately 3 h from the start of gel photopatterning to completion of native protein western blotting, a substantial time savings over slab-gel immunoblotting. The assay itself requires less than 5 min. Importantly, all assay stages are programmably controlled by a high-voltage power supply and monitored by an epifluorescence microscope equipped with a charge-coupled device camera. Our approach overcomes severe limitations associated with conventional immunoblotting, including multiple steps requiring manual intervention, low throughput and substantial consumption of reagents. We also describe a simple chemical recycling protocol so that glass chips can be reused. The fabrication technique described forms the basis for a diverse suite of bioanalytical tools, including DNA/RNA blotting and multidimensional separations.

Mixed-mode electrokinetic and chromatographic peptide separations in a microvalve-integrated polymer chip

A cycloolefin polymer chip supporting the concatenation of isoelectric focusing (IEF) and reversed-phase liquid chromatography (RPLC) is demonstrated for high throughput two dimensional peptide separations. A unique benefit of the mixed-mode platform is the ability of IEF to act as a highly concentrating electrokinetic separation mode for effective isolation of sample components prior to RPLC. The thermoplastic chip contains integrated high pressure microvalves, enabling uniform sample transfer from the IEF channel to multiple parallel RPLC channels, gradient elution from each RPLC column, and hydrodynamic isolation between the separation dimensions. The reusable system is shown to provide efficient 2-D separations together with facile interfacing with MALDI-MS, suggesting a new path towards effective peptide analysis from complex samples.

Ultrashort separation length homogeneous electrophoretic immunoassays using on-chip discontinuous polyacrylamide gels

To realize efficient homogeneous electrophoretic immunoassays, we introduce discontinuous polyacrylamide gels that enable quantitative assay completion in separation lengths as short as 350 mum in <10 s. The discontinuous cross-linked gels reduce the required electrophoretic separation lengths and thereby significantly reduce the required applied electrical potentials needed to achieve 100's V/cm electric field strengths for rapid electrophoresis. To optimize the discontinuous polyacrylamide gel assay format, we demonstrate development of a two-color homogeneous electrophoretic immunoassay for concurrent quantitation of C reactive protein (CRP) and tumor necrosis factor-alpha (TNF-alpha) for monitoring inflammatory response. To achieve necessary pore-size control at the gel discontinuity, an optimized mask-based fabrication protocol is introduced. The fabrication approach improves electrophoretic separations using the discontinuous separation gels by eliminating two confounding phenomena: (1) smaller than desired pores at the discontinuity which result in undesired physical exclusion of large-species and (2) an associated transition from small to large pores aft of the interface which acts to "destack" analyte bands during the separation. With the use of the optimized discontinuous separation gels, both assays were linear and quantitative over a two-log detection range, with a lower limit of detection of 11 ng/mL for CRP and 40 ng/mL for TNF-alpha. An optimal single-point detector location was identified by balancing the separation resolution and assay duration constraints. The ultrashort separation distance electrophoretic assays developed here provide flexibility in chip and instrument design by relaxing electrical potential requirements and expanding the possibilities for assay multiplexing, therefore addressing important design considerations when developing field-portable diagnostic assays for near-patient environments.

A cyclo olefin polymer microfluidic chip with integrated gold microelectrodes for aqueous and non-aqueous electrochemistry

This paper presents an entirely polymeric microfluidic system, made of cyclo olefin polymer (COP), with integrated gold microband electrodes for electrochemical applications in organic media. In the present work, we take advantage of the COP's high chemical stability to polar organic solvents in two different ways: (i) to fabricate gold microelectrodes using COP as a substrate by standard lithographic and lift-off techniques; and (ii) to perform electrochemical experiments in organic media. In particular, fourteen parallel gold microelectrodes with a width of 14 microm and separated from their closest neighbour by 16 microm were fabricated by lithographic and lift-off techniques on a 188 microm thick COP sheet. A closed channel configuration was obtained by pressure-assisted thermal bonding between the COP sheet containing the microelectrodes and a microstructured COP sheet, where a 3 cm long, 50 microm wide and 24 microm deep channel was fabricated via hot embossing. Cyclic voltammetric measurements were carried out in aqueous and organic media, using a solution consisting of 5 mM ferrocyanide/ferricyanide in 0.5 M KNO(3) and 5 mM ferrocene in 0.1 M TBAP/acetonitrile, respectively. Experimental currents obtained for different flow rates ranging from 1 to 10 microL min(-1) were compared to the theoretical steady state currents calculated by the Levich equation for a band electrode (R. G. Compton, A. C. Fisher, R. G. Wellington, P. J. Dobson and P. A. Leigh, J. Phys. Chem., 1993, 97, 10410-10415). In both cases, the difference between the experimental and the predicted data is less than 5%, thus validating the behaviour of the fabricated device. This result opens the possibility to use a microfluidic system made entirely from COP with integrated microband electrodes in organic electroanalysis and in electrosynthesis.

Microchip isoelectric focusing with monolithic immobilized pH gradient materials for proteins separation

Monolithic immobilized pH gradient (M-IPG) materials were prepared in microchannles by photoinitiated polymerization of acrylamide, glycidylmethacrylate and Bis, followed by the attachment of focused Ampholine onto the surface of porous monoliths via epoxide groups. With M-IPG materials as matrix, FITC-labeled ribonuclease B, myoglobin and alpha-casein were well separated by microchip isoelectric focusing (muCIEF) without carrier amphocytes (CAs) added in the buffer. Both chemical and pressure mobilization were applied to drive focused zones for LIF detection. Our experimental results showed that pressure mobilization was preferable with neglectable band broadening, and good peak shape and high detection sensitivity were obtained. All these results demonstrate that muCIEF with M-IPG materials is not only an efficient mode for protein enrichment and separation but also attractive to couple with other CE modes to achieve multi-dimensional separation or MS for further identification, without the interference of mobile CAs.

On-chip technologies for multidimensional separations

We review microfluidic devices designed for multidimensional sample analysis, with a primer on relevant theory, an emphasis on protein analysis, and an eye towards future improvements and challenges to the field. Image shows results of an on-chip IEF-CE separation of a protein mixture; unpublished surface plot data from A. E. Herr.

Microfluidic devices with photodefinable pseudo-valves for protein separation

Plastic microfluidic devices are fabricated with an array of pseudo-valves for two-dimensional (2D) protein separation. The devices are made by compression molding; the mold is created by electroplating on a glass master fabricated by photolithography. Each device consists of one channel for isoelectric focusing (IEF) and multiple parallel channels for polyacrylamide gel electrophoresis (PAGE). The IEF channel (first dimension) is orthogonal to the PAGE channels (second dimension). Microfluidic pseudo-valves are created at the intersections of orthogonal channels by photodefinable, in situ gel polymerization. These valves enable the introduction of two types of separation media into orthogonal channels for performing 2D protein separation in the device. The presence of the pseudo-valves prevents one separation medium from being contaminated by the other medium, although proteins are allowed to transfer from the first to the second dimension under an electric field. Two-dimensional protein separation is achieved in less than 10 min, an improvement of two orders of magnitude compared with the conventional 2D gel electrophoresis using an IEF strip and a PAGE slab.

Capillary electrophoresis applied to proteomic analysis

In the postgenomic era, proteomics has become a dominant field for identifying and quantifying the complex protein machinery of the cell. The expression levels, posttranslational modifications, and specific interactions of proteins control the biology of such processes as development, differentiation, and signal transduction. Studies of the proteins involved in these processes often lead to a better understanding of biology and of human disease. Powerful separation techniques and sensitive detection methods enable researchers to untangle these complicated networks of processes. CE coupled with either MS or LIF are two of the techniques that make this possible. This review will cover proven CE-based methods for proteomics on the cell and tissue level and their application in biological and clinical studies, relevant new developments in enabling technology such as microfluidic CE-MS demonstrated on model systems, and comment on the future of CE in proteomics.

Microfluidic 2-D PAGE using multifunctional in situ polyacrylamide gels and discontinuous buffers

A two-dimensional microfluidic system is presented for intact protein separations combining isoelectric focusing (IEF) and sodium dodecyl sulfate (SDS) polyacrylamide gel electrophoresis (PAGE) employing in situ photopolymerized polyacrylamide (PAAm) gels. The PAAm gels are used for multiple functions. In addition to serving as a highly-resolving separation medium for gel electrophoresis, discrete polyacrylamide gel plugs are used to enable the efficient isolation of different on-chip media including anolyte, catholyte, and sample/ampholyte solutions for IEF. The gel plugs are demonstrated as on-chip reagent containers, holding defined quantities of SDS for on-chip SDS-protein complexation, and enabling the use of a discontinuous buffer system for sample band sharpening during SDS-PAGE. The 2-D chip also employs several unique design features including an angled isoelectric focusing channel to minimize sample tailing, and backbiasing channels designed to achieve uniform interdimensional sample transfer. Separation results using E. coli cell lysate are presented using a 10-channel chip with and without the discontinuous buffer system, with resolving power more than doubled in the former case. Further improvements in separation resolution are demonstrated using a 20-channel chip design.

Optimization of sample transfer in two-dimensional microfluidic separation systems

Multidimensional microfluidic separation systems combining a first dimension microchannel with an array of parallel second dimension microchannels can suffer from non-uniform sample transfer between the dimensions, sample leakage, and injection plug tailing within the second dimension array. These factors can significantly reduce overall two-dimensional separation performance. In this paper, numerical and analytical models reveal an optimized chip design which combines multidimensional backbiasing and an angled channel geometry to ensure leakage-free and uniform interdimensional sample transfer, while also minimizing injected sample plug lengths. The optimized design is validated experimentally using a multidimensional chip containing five second dimension channels.