Nanoliter capillary electrochromatography columns based on collocated monolithic support structures molded in poly(dimethyl siloxane)

Design and Fabrication of Organ-on-Chips: Promises and Challenges

The advent of the miniaturization approach has influenced the research trends in almost all disciplines. Bioengineering is one of the fields benefiting from the new possibilities of microfabrication techniques, especially in cell and tissue culture, disease modeling, and drug discovery. The limitations of existing 2D cell culture techniques, the high time and cost requirements, and the considerable failure rates have led to the idea of 3D cell culture environments capable of providing physiologically relevant tissue functions in vitro. Organ-on-chips are microfluidic devices used in this context as a potential alternative to in vivo animal testing to reduce the cost and time required for drug evaluation. This emerging technology contributes significantly to the development of various research areas, including, but not limited to, tissue engineering and drug discovery. However, it also brings many challenges. Further development of the technology requires interdisciplinary studies as some problems are associated with the materials and their manufacturing techniques. Therefore, in this paper, organ-on-chip technologies are presented, focusing on the design and fabrication requirements. Then, state-of-the-art materials and microfabrication techniques are described in detail to show their advantages and also their limitations. A comparison and identification of gaps for current use and further studies are therefore the subject of the final discussion.

A practical guide to rapid-prototyping of PDMS-based microfluidic devices: A tutorial

Micro total analytical systems (μTAS) are attractive to multiple fields that include chemistry, medicine and engineering due to their portability, low power usage, potential for automation, and low sample and reagent consumption, which in turn results in low waste generation. The development of fully-functional μTAS is an iterative process, based on the design, fabrication and testing of multiple prototype microdevices. Typically, microfabrication protocols require a week or more of highly-skilled personnel time in high-maintenance cleanroom facilities, which makes this iterative process cost-prohibitive in many locations worldwide. Rapid-prototyping tools, in conjunction with the use of polydimethylsiloxane (PDMS), enable rapid development of microfluidic structures at lower costs, circumventing these issues in conventional microfabrication techniques. Multiple rapid-prototyping methods to fabricate PDMS-based microfluidic devices have been demonstrated in literature since the advent of soft-lithography in 1998; each method has its unique advantages and drawbacks. Here, we present a tutorial discussing current rapid-prototyping techniques to fabricate PDMS-based microdevices, including soft-lithography, print-and-peel and scaffolding techniques, among other methods, specifically comparing resolution of the features, fabrication processes and associated costs for each technique. We also present thoughts and insights towards each step of the iterative microfabrication process, from design to testing, to improve the development of fully-functional PDMS-based microfluidic devices at faster rates and lower costs.

Fabrication of immobilized enzyme reactors with pillar arrays into polydimethylsiloxane microchip

This paper demonstrates the design, efficiency and applicability of a simple and inexpensive microfluidic immobilized enzymatic reactor (IMER) for rapid protein digestion. The high surface-to-volume ratio (S/V) of the reactor was achieved by forming pillars in the channel. It was found that pillar arrays including dimensions of 40 μm × 40 μm as pillar diameter and interpillar distance can provide both relatively high S/V and flow rate in the PDMS chip, the fabrication of which was performed by means of soft lithography using average research laboratory infrastructure. CZE peptide maps of IMER-based digestions were compared to peptide maps obtained from standard in-solution digestion of proteins. The peak patterns of the electropherograms and the identified proteins were similar, however, digestion with the IMER requires less than 10 min, while in-solution digestion takes 16 h.

Thermally robust and biomolecule-friendly room-temperature bonding for the fabrication of elastomer-plastic hybrid microdevices

Here, we introduce a simple and fast method for bonding a poly(dimethylsiloxane) (PDMS) silicone elastomer to different plastics. In this technique, surface modification and subsequent bonding processes are performed at room temperature. Furthermore, only one chemical is needed, and no surface oxidation step is necessary prior to bonding. This bonding method is particularly suitable for encapsulating biomolecules that are sensitive to external stimuli, such as heat or plasma treatment, and for embedding fracturable materials prior to the bonding step. Microchannel-fabricated PDMS was first oxidized by plasma treatment and reacted with aminosilane by forming strong siloxane bonds (Si-O-Si) at room temperature. Without the surface oxidation of the amine-terminated PDMS and plastic, the two heterogeneous substrates were brought into intimate physical contact and left at room temperature. Subsequently, aminolysis occurred, leading to the generation of a permanent seal via the formation of robust urethane bonds after only 5 min of assembling. Using this method, large-area (10 × 10 cm) bonding was successfully realized. The surface was characterized by contact angle measurements and X-ray photoelectron spectroscopy (XPS) analyses, and the bonding strength was analyzed by performing peel, delamination, leak, and burst tests. The bond strength of the PDMS-polycarbonate (PC) assembly was approximately 409 ± 6.6 kPa, and the assembly withstood the injection of a tremendous amount of liquid with the per-minute injection volume exceeding 2000 times its total internal volume. The thermal stability of the bonded microdevice was confirmed by performing a chamber-type multiplex polymerase chain reaction (PCR) of two major foodborne pathogens - Escherichia coli O157:H7 and Salmonella typhimurium - and assessing the possibility for on-site direct detection of PCR amplicons. This bonding method demonstrated high potential for the stable construction of closed microfluidic systems socketed with biomolecule-immobilized surfaces such as DNA, antibody, enzyme, peptide, and protein microarrays.

Adhesion of MRC-5 and A549 cells on poly(dimethylsiloxane) surface modified by proteins

PDMS is a very popular material used for fabrication of Lab-on-a-Chip systems for biological applications. Although PDMS has numerous advantages, it is a highly hydrophobic material, which inhibits adhesion and proliferation of the cells. PDMS surface modifications are used to enrich growth of the cells. However, due to the fact that each cell type has specific adhesion, it is necessary to optimize the parameters of these modifications. In this paper, we present an investigation of normal (MRC-5) and carcinoma (A549) human lung cell adhesion and proliferation on modified PDMS surfaces. We have chosen these cell types because often they are used as models for basic cancer research. To the best of our knowledge, this is the first presentation of this type of investigation. The combination of a gas-phase processing (oxygen plasma or ultraviolet irradiation) and wet chemical methods based on proteins' adsorption was used in our experiments. Different proteins such as poly-l-lysine, fibronectin, laminin, gelatin, and collagen were incubated with the activated PDMS samples. To compare with other works, here, we also examined how ratio of prepolymer to curing agent (5:1, 10:1, and 20:1) influences PDMS hydrophilicity during further modifications. The highest adhesion of the tested cells was observed for the usage of collagen, regardless of PDMS ratio. However, the MRC-5 cell line demonstrated better adhesion than A549 cells. This is probably due to the difference in their morphology and type (normal/cancer).

Cellobiose dehydrogenase functionalized urinary catheter as novel antibiofilm system

Urinary catheters expose patients to a high risk of acquiring nosocomial infections. To prevent this risk of infection, cellobiose dehydrogenase (CDH), an antimicrobial enzyme able to use various oligosaccharides as electron donors to produce hydrogen peroxide using oxygen as an electron acceptor, was covalently grafted onto plasma-activated urinary polydimethylsiloxane (PDMS) catheter surfaces. Successful immobilization of CDH on PDMS was confirmed by Fourier transformed-infrared spectrometry and production of H2 O2 . The CDH functionalized PDMS surfaces reduced the amount of viable Staphylococcus aureus by 60%, total biomass deposited on the surface by 30% and 70% of biofilm formation. The immobilized CDH was relatively stable in artificial urine over 16 days, retaining 20% of its initial activity. The CDH coated PDMS surface did not affect the growth and physiology of HEK 239 and RAW 264,7 mammalian cells. Therefore this new CDH functionalized catheter system shows great potential for solving the current problems associated with urinary catheters. © 2015 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 104B: 1448-1456, 2016.

Nonplanar conductive surfaces via "bottom-up" nanostructured gold coating

Development of technologies for the construction of bent, curved, and flexible conductive surfaces is among the most important albeit challenging goals in the promising field of "flexible electronics". We present a generic solution-based "bottom-up" approach for assembling conductive gold nanostructured layers on nonplanar polymer surfaces. The simple two-step experimental scheme is based upon incubation of an amine-displaying polymer [the abundantly used poly(dimethylsiloxane) (PDMS), selected here as a proof of concept] with Au(SCN)4(-), followed by a brief treatment with a conductive polymer [poly(3,4-thylenedioxythiophene)/poly(styrenesulfonate)] solution. Importantly, no reducing agent is co-added to the gold complex solution. The resultant surfaces are conductive and exhibit a unique "nanoribbon" gold morphology. The scheme yields conductive layers upon PDMS in varied configurations: planar, "wrinkled", and mechanically bent surfaces. The technology is simple, inexpensive, and easy to implement for varied polymer surfaces (and other substances), opening the way for practical applications in flexible electronics and related fields.

Simple surface modification of poly(dimethylsiloxane) for DNA hybridization

Here, we present a simple chemical modification of poly(dimethylsiloxane) (PDMS) by curing a mixture of 2 wt% undecylenic acid (UDA) in PDMS prepolymer on a gold-coated glass slide. This gold slide had been previously pretreated with a self-assembled hydrophilic monolayer of 3-mercaptopropionic acid (MPA). During curing of the UDA∕PDMS prepolymer, the hydrophilic UDA carboxyl moieties diffuses toward the hydrophilic MPA carboxyl moieties on the gold surface. This diffusion of the UDA within the PDMS prepolymer to the surface is a direct result of surface energy minimization. Once completely cured, the PDMS is peeled off the gold substrate, thereby exposing the interfacial carboxyl groups. These groups are then available for subsequent attachment of 5(')-amino terminated DNA oligonucleotides via amide linkages. Our results show that the covalently tethered oligonucleotides can successfully capture fluorescein-labeled complementary oligonucleotides via hybridization, which are visualized using fluorescence microscopy.

Liquid chromatography on chip

LC is one of the most powerful separation techniques as illustrated by its leading role in analytical sciences through both academic and industrial communities. Its implementation in microsystems appears to be crucial in the development of mu-Total Analysis System. If electrophoretic techniques have been widely used in miniaturized devices, LC has faced multiple challenges in the downsizing process. During the past 5 years, significant breakthroughs have been achieved in this research area, in both conception and use of LC on chip. This review emphasizes the development of novel stationary phases and their implementation in microchannels. Recent instrumental advances are also presented, highlighting the various driving forces (pressure, electrical field) that have been selected and their respective ranges of applications.

Design and evaluation of flow distributors for microfabricated pillar array columns

Five different flow distributors have been compared as a function of the flow rate for their ability to distribute small sample volumes over the entire width of flat rectangular microfabricated pillar array columns. The investigated designs can be divided in two major categories: (1) bifurcating, radially non-interconnecting distributors and (2) radially interconnecting distributors consisting of diamond-shaped pillars, elongated in the direction perpendicular to the flow, providing a high ratio of radial permeability over axial permeability. The quality of the flow distribution was evaluated experimentally by injecting equal volumes of fluorescent tracer into each of the tested designs and calculating the obtained peak variances using the method of moments. Purely bifurcating distributors perform less well than the best possible radially interconnected distributors, because the former inevitably require the use of wide open channels (d > 10 microm), wherein a lot of band broadening can occur. By doubling the aspect ratio of the radially stretched pillars from 5 to 10, the measured peak variance drops to 1/8 of the original value. The best results were obtained with a distributor in which the flow is distributed by a bed of anisotropic pillars with an aspect ratio of 10, but our results indicate that a substantial improvement can still be made by increasing the aspect ratio and adding gradually diverging sidewalls to the inlet.

Rapid prototyping of microstructures by soft lithography for biotechnology

This chapter describes the methods and specific procedures used to fabricate microstructures by soft lithography. These techniques are useful for the prototyping of devices useful for applications in biotechnology. Fabrication by soft lithography does not require specialized or expensive equipment; the materials and facilities necessary are found commonly in biological and chemical laboratories in both academia and industry. The combination of the fact that the materials are low-cost and that the time from design to prototype device can be short (< 24 h) makes it possible to use and to screen rapidly devices that also can be disposable. Here we describe the procedures for fabricating microstructures with lateral dimensions as small as 1 mum. These types of microstructures are useful for microfluidic devices, cell-based assays, and bioengineered surfaces.

Development of an acrylate monolith in a cyclo-olefin copolymer microfluidic device for chip electrochromatography separation

An acrylate monolith has been synthesized into a cyclic olefin copolymer microdevice for reversed-phase electrochromatography purposes. Microchannels, designed by hot embossing, were filled up with an acrylate monolith to serve as a hydrophobic stationary phase. A lauryl acrylate monolith was formulated to suit the hydrophobic material, by implementing 100% organic porogenic solvent. This new composition was tested in capillary prior to its transfer into the microfluidic device. Surface functionalization of the cyclic olefin copolymer surface was applied using UV-grafting technique to improve the covalent attachment of this monolith to the plastic walls of the microfluidic chip. The on-chip performances of this monolith were evaluated in detail for the reversed-phase electrochromatographic separation of polycyclic aromatic hydrocarbons, with plate heights reaching down to 10 microm when working at optimal velocity.

Multidimensional liquid phase separations for mass spectrometry

Large part of the current research in biology, medicine, and biotechnology depends on the analysis of DNA (genomics), proteins (proteomics), or metabolites (metabolomics). The advances in biotechnology also command development of adequate analytical instrumentation capable to analyze minute amounts of samples. The analysis of the content of single cells may serve as an example of ultimate analytical applications. Most of the separation techniques have been developed in the last three decades and alternative approaches are being investigated. At present, the main protocols for analyses of complex mixtures include 2-DE (IEF) followed by electrophoresis in SDS polyacrylamide gel (SDS-PAGE) and chromatographic techniques. Information-rich techniques such as MS and NMR are essential for the identification and structure analysis of the analyzed compounds. High resolution separation of the individual sample components is often a prerequisite for success. High resolution proteomic analysis in the majority of laboratories still relies on the time consuming and laborious offline methods. This review highlights some of the important aspects of 2-D separations including microfluidics.

Integration of porous layers in ordered pillar arrays for liquid chromatography

The present paper describes a method for the production of partly porous micro-pillars in columns suitable for use in liquid chromatography. These layers increase the available surface at least two orders of magnitude without destroying the huge benefits of the ordered nature of the system. A process flow was developed that enabled us to create a 550 nm thick porous layer on the pillar array in a sealed channel configuration, withstanding pressures up to at least 70 bar. Measuring band broadening under non-retained conditions, only a modest increase in plate height was observed in the porous pillar array as compared to that in a non-porous pillar array. The homogeneity of the layers was demonstrated using an optical microscope and SEM pictures and by monitoring peak velocities at constant pressures. The internal porosity was determined using particles with a diameter larger than the mesopores in combination with a dye that could penetrate into the pores.

Experimental investigation of the band broadening originating from the top and bottom walls in micromachined nonporous pillar array columns

We report on the experimental investigation of the effect of the top and bottom wall plates in micromachined nonporous pillar array columns. It has been found that their presence yields an additional c-term type of band broadening that can make up a significant fraction of the total band broadening (at least if considering nonporous pillars and a nonretained tracer). Their presence also induces a clear (downward) shift of the optimal velocity. These observations are, however in excellent quantitative agreement with the theoretical expectations obtained from a computational fluid dynamics study. The presently obtained experimental results, hence, demonstrate that the employed high aspect ratio Bosch etching process can be used to fabricate micromachined pillar arrays that are sufficiently refined to achieve the theoretical performance limit.

An evaluation of the experimental approaches to detection of small ions in CE

This review points out some important trends in the development of the detection techniques for small ions in CE. On the basis of selected literature references it briefly discusses some general requirements on detection techniques in CE. Various optical measurements, mass spectrometric approaches and electrochemical detection techniques are dealt with. Some specific features of microchip CE separation and detection are pointed out and possibilities of dual detection are mentioned. The principal parameters of the above detection techniques are then briefly compared.

Capillary electrochromatography of proteins and peptides

This review summarizes applications of CEC for the analysis of proteins and peptides. This "hybrid" technique is useful for the analysis of a broad spectrum of proteins and peptides and is a complementary approach to liquid chromatographic and capillary electrophoretic analysis. All modes of CEC are described--granular packed columns, monolithic stationary phases as well as open-tubular CEC. Attention is also paid to pressurized CEC and the chip-based platform.

On the 3-dimensional effects in etched chips for high performance liquid chromatography-separations

In an attempt to quantify the potential of photolithographically etched micro-pillar arrays as a perfectly ordered alternative for the packed bed of spheres, the additional band broadening originating from the top and bottom plate has been investigated using computational fluid dynamics simulations. These calculations provide insight in the theoretical expectations that can be made for the experimental work that is currently being conducted by a number of groups. The calculations show that the additional band broadening contribution can be expected to go through a transient regime as a function of the axial distance along the array. In its fully developed regime and in the most relevant velocity range, the top and bottom wall contribution almost doubles the band broadening compared to the band broadening in a perfectly ordered array of non-porous, non-retentive pillars without top and bottom wall. Compared to the band broadening in an array of porous, retentive pillars on the other hand, the top and bottom wall-effect can be expected to become negligible. A simplified, phenomenological model yielding a first principles prediction of both the transient and the steady-state top and bottom wall band broadening as a function of the inter-pillar distance and the pillar height is proposed and shows good qualitative agreement with the exact calculations.

Recent advances in enhancing the sensitivity of electrophoresis and electrochromatography in capillaries and microchips

Poor sensitivity is considered to be one of the major limitations of electrophoretic separation methods, particularly when compared to traditional liquid chromatographic techniques. To address this issue, various in-line preconcentration techniques have been developed over the past 15 years, ranging in power and complexity, and there are now a number of well understood approaches routinely capable of providing a 10,000- to 100,000-fold increase in sensitivity, as well as several that can be pushed above a million. Furthermore, these have been achieved with particularly troublesome and often difficult samples, such as those having high salinity from a biological or environmental origin. This review will discuss the most common methods for improving the sensitivity of CE, CEC and microchip version of these, with particular attention to those approaches developed over the last five years.

Preparation and application of monolithic beds in the separation of selected natural biologically important compounds

The importance of monolithic (continuous) beds is connected with their easy preparation and the far-reaching possibilities of modification of their surface and porous properties. These properties make them particularly attractive for the analysis of biologically important compounds characterized by a wide spectrum of physicochemical properties. This review summarizes their preparation methods as well as their application as continuous beds for determination of such biologically important compounds as catecholamines, vitamins, flavonoids, amino acids, peptides, and proteins.

Microchip-based CEC of nitroaromatic and nitramine explosives using silica-based sol–gel stationary phases from methyl- and ethyl-trimethoxysilane precursors

Microchip-based CEC of nitroaromatic and nitramine explosives with UV absorbance detection is described. The stationary phase was deposited in the microchip using the sol-gel process. Unique to this work, is the exclusive use of alkylated-trimethoxysilane precursors in the gel solution. Using alkylated precursors allows for the synthesis of a hydrophobic stationary phase in a single step. Three sol-gel formulations of increasing hydrophobicity and suitable for the separation of explosives are established from methyl- and ethyl-trimethoxysilane precursors. Increasing the alkyl-chain length improved the resolution significantly, allowing for the separation of up to seven analytes. Direct injection onto the head of the stationary phase for long injection times, results in sub-mg/L detection limits with little effect on separation efficiency.

Selection of comparison criteria and experimental conditions to evaluate the kinetic performance of monolithic and packed-bed columns

The present study concerns the problem of finding appropriate experimental conditions and comparison criteria to assess the kinetic performance of LC supports with different sizes or morphologies. A general procedure, based on evaluating each support for its own optimal mobile-phase composition, is proposed. The practical elaboration of the procedure is illustrated using the specific case of a capillary LC separation of a series of polycyclic aromatic test compounds employing silica-monolith capillary columns and capillary columns packed with 6-microm porous particles. To compare the systems for their ability to yield the fastest critical-pair separation, plate-height measurements are transformed into an effective plate number kinetic plot, i.e., a plot of the extrapolated retention time divided by the square of the extrapolated effective plate number (t(R)/N(eff)(2)) versus N(eff). This type of data representation provides a direct and universal basis to compare the kinetic performance of different LC supports and it corrects for differences in retention strength arising from different phase ratios.

A microfluidic fluorous solid-phase extraction chip for purification of amino acids

An electrokinetically-driven microfluidic chip was developed to realize beads-based solid-phase extraction (SPE) of amino acids. This chip uses a two-level (deep/shallow) poly(dimethylsiloxane) (PDMS) microchannel network to confine the fluorous reversed-phase silica beads within the SPE chamber. The mixture of fluorous tagged and non-tagged amino acids was carried into the fluorous solid-phase extraction (F-SPE) chamber by electrokinetic pumping and was successfully separated and extracted. By adding a reference material to the sample, the extraction efficiency of the eluted fluorous-tagged amino acid was calculated using the detection results from mass spectrometry (MS). The F-SPE microchips showed good reproducibility and efficiency, yielding an average extraction efficiency of 55% with a RSD of 10.6% under the typical experimental conditions.

Cationic amylopectin derivatives as additives for analysis of proteins in capillary electrophoresis

Positively charged amylopectin, which is a major constituent of cationic starch, was used to modify the inner surface of fused-silica capillaries by addition to the running solution, which was subsequently employed in CE. Capillaries filled with cationic amylopectin derivatives were shown to generate a stable reversed EOF in the investigated range of pH 4-8. Among the additives studied, quaternary ammonium amylopectin derivatives with high amino and low hydroxypropyl groups showed fast electroosmotic mobility and very effectively suppressed the adsorption of proteins. The run-to-run and batch-to-batch repeatability of the procedures were satisfactory with RSDs of 0.5% and 2.4%, respectively. A basic protein, alpha-chymotrypsinogen, migrated within 6 min and the theoretical plate number of it reached 560 000 plates/m.

Monolithic continuous beds as a new generation of stationary phase for chromatographic and electro-driven separations

Monolithic supports are a new generation of stationary phases which are typically prepared using a simple molding process carried out within the confines of the capillary. They provide high rates of mass transfer at lower pressure drops, enable much faster separations and the nature of the pores allows easy permeability for large molecules. This review summarizes the current achievements and application of organic polymer-based monolithic columns, silica-based monolithic columns and their application in bioaffinity processing, modern biotechnology, determination of microorganisms and chiral separations. Special attention is paid to microfabricated devices with monolithic supports because their fabrication of particles directly in the channel eliminates the need for a frit and also creates a unique homogeneity of packing.

Development of acrylate-based monolithic stationary phases for electrochromatographic separations

Organic monolithic stationary phases were synthesized in fused-silica capillaries. They were prepared by in situ polymerization under UV irradiation of various alkyl acrylates, 1,3-butanediol diacrylate, and 2-acrylamido-2-methyl-1-propanesulfonic acid in a ternary porogenic solvent. The resulting stationary phases were tested in CEC. The influence of UV irradiation energy on the resulting separative performances of the monoliths was studied. It was thus demonstrated that the use of hexyl acrylate rather than butyl acrylate and lauryl methacrylate gives highly efficient monoliths (more than 300 000 plates per meter) with optimized EOF. It was also confirmed that the mobile phase ionic strength may affect significantly the separation efficiency. The influence of the nature of the mobile phase organic modifier (ACN or methanol) on EOF, retention, efficiency, and selectivity was studied and differences were observed. Finally, the performances of monolithic stationary phases developed and optimized for CEC separations were evaluated in nanoLC.

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

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

A first principles explanation for the experimentally observed increase in A-term band broadening in small domain silica monoliths and other chromatographic supports

The present computational study illustrates how the existence of a residual lower limit on the variance of the skeleton and through-pore size of monolithic columns can be expected to severely compromise the possibility to prepare well-performing small domain monolithic columns. Adopting rather conservative estimates for the minimal standard deviation on the pore and the skeleton size (0.2 and 0.04 microm, respectively), the presented calculations show that, if such a fixed lower limit on the size variance exists, it will be impossible to decrease the A-term band broadening below a given critical value, no matter how small the domain size is made. From a given critical domain size value on, any attempt to further decrease the domain size without being able to co-reduce the size variance can be expected to be counterproductive and leads to an increase instead of to a further decrease of the plate heights.

Accelerated Particle Electrophoretic Motion and Separation in Converging−Diverging Microchannels

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

On the optimisation of the bed porosity and the particle shape of ordered chromatographic separation media

We report on a theoretical study wherein we considered a large number of ordered two-dimensional porous pillar arrays with different pillar shapes and widely varying external porosity and calculated the flow resistance and the band broadening (under retentive conditions) over the complete range of practical velocities using a commercial computational fluid dynamics software package. It is found that the performance of the small porosity systems is very sensitive to the exact pillar shape, whereas this difference gradually disappears with increasing porosity. The obtained separation impedances are very small in comparison to packed bed and monolithic columns and decrease with increasing porosity. If accounting for the current micromachining limitations, a proper selection of the exact shape and porosity even becomes more critical, and different design rules are obtained depending on whether porous or non-porous pillars are considered.

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.