Rapid Prototyping of Microfluidic Systems in Poly(dimethylsiloxane)

Prototyping of microfluidic devices in poly(dimethylsiloxane) using solid-object printing

A solid-object printer was used to produce masters for the fabrication of microfluidic devices in poly(dimethylsiloxane) (PDMS). The printer provides an alternative to photolithography for applications where features of > 250 microm are needed. Solid-object printing is capable of delivering objects that have dimensions as large as 250 x 190 x 200 mm (x, y, z) with feature sizes that can range from 10 cm to 250 microm. The user designs a device in 3-D in a CAD program, and the CAD file is used by the printer to fabricate a master directly without the need for a mask. The printer can produce complex structures, including multilevel features, in one unattended printing. The masters are robust and inexpensive and can be fabricated rapidly. Once a master was obtained, a PDMS replica was fabricated by molding against it and used to fabricate a microfluidic device. The capabilities of this method are demonstrated by fabricating devices that contain multilevel and tall features, devices that cover a large area (approximately 150 cm2), and devices that contain nonintersecting, crossing channels.

A high-throughput continuous sample introduction interface for microfluidic chip-based capillary electrophoresis systems

The development of efficient sample introduction and pretreatment systems for microfluidic chip-based analytical systems is important for their application to real-life samples. In this work, world-to-chip interfacing was achieved by a novel flow-through sampling reservoir featuring a guided overflow design. The flow-through reservoir was fabricated on a 30 x 60 x 3 mm planar glass chip of crossed-channel design used for capillary electrophoresis separations. The 20-microL sample reservoir was produced from a section of plastic pipet tip and fixed at one end of the sampling channel. Sample change was performed by pumping 80-microL samples sandwiched between air segments at approximately 0.48 mL/min flow rate through the flow-through reservoir, introduced from an access hole on the bottom side of the chip. A filter paper collar wrapped tightly around the reservoir guided the overflowing sample solution into a plastic trough surrounding the reservoir and then to waste. The performance of the system was demonstrated in the separation and determination of FITC-labeled arginine, glycine, phenylalanine, and glutamic acid with LIF detection, by continuously introducing a train of different samples through the system without electrical interruption. Employing a separation channel of 4 cm (2-cm effective separation length) and 1.4-kV separation voltage, maximum throughputs of 80/h were achieved with <4.1% carryover and precisions ranging from 1.5% for arginine to 2.6% RSD (n = 11) for glycine. The sampling system was tested in the continuous monitoring of the derivatizing process of amino acids by FITC over a period of 4 h, involving 166 analytical cycles. An outstanding overall precision of 4.8% RSD (n = 166) was achieved for the fluorescein internal standard.

Microfluidic chips for clinical and forensic analysis

This review gives an overview of developments in the field of microchip analysis for clinical diagnostic and forensic applications. The approach chosen to review the literature is different from that in most microchip reviews to date, in that the information is presented in terms of analytes tested rather than microchip method. Analyte categories for which examples are presented include (i) drugs (quality control, seizures) and explosives residues, (ii) drugs and endogenous small molecules and ions in biofluids, (iii) proteins and peptides, and (iv) analysis of nucleic acids and oligonucleotides. Few cases of microchip analysis of physiological samples or other "real-world" matrices were found. However, many of the examples presented have potential application for these samples, especially with ongoing parallel developments involving integration of sample pretreatment onto chips and the use of fluid propulsion mechanisms other than electrokinetic pumping.

Polymer microchips bonded by O2-plasma activation

This paper presents a fabrication of polymer microchips with homogeneous material technique due to surface treatment by plasma before sealing. UV laser photoablation was used for fast prototyping of microstructures, and oxygen plasma was used as a surface treatment for both the microfabricated substrate and the polymer cover. It was found that with an oxidative plasma treatment, successful bonding could be achieved without adhesive material between polymer sheets substantially below the glass transition temperature of the polymer. Homogeneous polyethylene terephthalate (PET) microstructures were characterized by scanning electron microscopy (SEM) and analyzed by X-ray photoelectron spectroscopy (XPS) surface analyses after different surface treatments. The electroosmotic flow characteristics including the velocity and the stability over 20 days have been tested and compared to composite channels, in which the cover presents a polyethylene (PE) adhesive layer. Capillary zone electrophoresis in both homogeneous and composite microanalytical devices were then performed and compared in order to evaluate the separation efficiency. In preliminary experiments, a plate height of 0.6 microm has been obtained with homogenous microchannels. The surface analysis pointed out that the surface chemistry is of prime importance for the performance of microfluidic separation.

Biomolecular recognition on well-characterized beads packed in microfluidic channels

We describe a new approach for the analysis of biomolecular recognition in microfluidic channels. The method involves real-time detection of soluble molecules binding to receptor-bearing microspheres, sequestered in affinity column format inside a microfluidic channel. Identification and quantitation of analytes occurs via direct fluorescence measurements or fluorescence resonance energy transfer (FRET). We establish a model system that detects the FLAG epitope. The assay can potentially detect subfemtomole quantities of antibody with a high signal-to-noise ratio and a large dynamic range spanning nearly 4 orders of magnitude in analyte concentration in microliter-to-submicroliter volumes of analyte fluid. Kinetic and equilibrium constants for the reaction of this receptor-ligand pair are obtained through modeling of kinetic responses of the affinity microcolumn and are consistent with those obtained by flow cytometry. Because of the correlation between kinetic and equilibrium data obtained for the microcolumns, quantitative analysis can be done prior to the steady-state end point of the recognition reaction. This method has the promise of combining the utility of affinity chromatography with the advantage of direct, quantitative, and real-time analysis and the cost-effectiveness of microanalytical devices. The approach has the potential to be generalized to a host of bioaffinity assay methods including analysis of protein complexes and molecular assembly and microsystem-based multianalyte determinations.

The analysis of uric acid in urine using microchip capillary electrophoresis with electrochemical detection

Clinical studies have linked irregular concentrations of uric acid in urine to several diseases. Conventional methods for the measurement of uric acid are however temperature-dependent, expensive, and require labile reagents. The miniaturization of analytical techniques, specifically capillary electrophoresis, offers an ideal alternative for clinical analyses such as uric acid determination. The added benefits include reduced reagent and analyte consumption, decreased maintenance costs, and increased throughput and portability. A microchip capillary electrophoresis-electrochemical system for the analysis of uric acid in urine is described. The poly(dimethylsiloxane) (PDMS)/glass microchip utilizes amperometric detection via an off-chip platinum working electrode. Linear responses from 1 to 165 microM and 15 to 110 microM were found for dopamine and uric acid, respectively. The limit of detection for both compounds was 1 microM. Once characterized, the system was used to measure the concentration of uric acid in a dilute urine sample in less than 30 s. The measured uric acid concentration was verified with the uricase reaction and found to be acceptable. Six additional urine samples were evaluated with the microchip device and the uric acid concentration for each sample was found to be in the expected clinical concentration range.

Soft lithography in biology and biochemistry

Soft lithography, a set of techniques for microfabrication, is based on printing and molding using elastomeric stamps with the patterns of interest in basrelief. As a technique for fabricating microstructures for biological applications, soft lithography overcomes many of the shortcomings of photolithography. In particular, soft lithography offers the ability to control the molecular structure of surfaces and to pattern the complex molecules relevant to biology, to fabricate channel structures appropriate for microfluidics, and to pattern and manipulate cells. For the relatively large feature sizes used in biology (> or = 50 microns), production of prototype patterns and structures is convenient, inexpensive, and rapid. Self-assembled monolayers of alkanethiolates on gold are particularly easy to pattern by soft lithography, and they provide exquisite control over surface biochemistry.

Towards disposable lab-on-a-chip: poly(methylmethacrylate) microchip electrophoresis device with electrochemical detection

A fully disposable microanalytical device based on combination of poly(methylmethacrylate) (PMMA) capillary electrophoresis microchips and thick-film electrochemical detector strips is described. Variables influencing the separation efficiency and amperometric response, including separation voltage or detection potential are assessed and optimized. The versatility, simplicity and low-cost advantages of the new design are coupled to an attractive analytical performance, with good precision (relative standard deviation RSD = 1.68% for n = 10). Applicability for assays of mixtures of hydrazine, phenolic compounds, and catecholamines is demonstrated. Such coupling of low-cost PMMA-based microchips with thick-film electrochemical detectors holds great promise for mass production of single-use micrototal analytical systems.

An integrated fritless column for on-chip capillary electrochromatography with conventional stationary phases

A new polymer device for use with conventional particulate stationary phases for on-chip, fritless, capillary electrochromatography (CEC) has been realized. The structure includes an injector and a tapered column in which the particles of the stationary phase are retained and stabilized. The chips were easily fabricated in poly(dimethylsiloxane) using deep-reactive-ion-etched silicon masters, and tested using a capillary electrophoretic separation of FITC-labeled amino acids. To perform CEC, the separation channel was packed using a vacuum with 3-microm, octadecylsilanized silica microspheres. The packing was stabilized in the column by a thermal treatment, and its stability and quality were evaluated using in-column indirect fluorescence detection. The effects of voltage on electro-osmotic flow and on efficiency were investigated, and the separation of two neutral compounds was achieved in less than 15 s.

Towards disposable lab-on-a-chip: poly(methylmethacrylate) microchip electrophoresis device with electrochemical detection

A fully disposable microanalytical device based on combination of poly(methylmethacrylate) (PMMA) capillary electrophoresis microchips and thick-film electrochemical detector strips is described. Variables influencing the separation efficiency and amperometric response, including separation voltage or detection potential are assessed and optimized. The versatility, simplicity and low-cost advantages of the new design are coupled to an attractive analytical performance, with good precision (relative standard deviation RSD = 1.68% for n = 10). Applicability for assays of mixtures of hydrazine, phenolic compounds, and catecholamines is demonstrated. Such coupling of low-cost PMMA-based microchips with thick-film electrochemical detectors holds great promise for mass production of single-use micrototal analytical systems.

Bioanalysis in microfluidic devices

Microfabricated bioanalytical devices (also referred to as laboratory-on-a-chip or micro-TAS) offer highly efficient platforms for simultaneous analysis of a large number of biologically important molecules, possessing great potential for genome, proteome and metabolome studies. Development and implementation of microfluidic-based bioanalytical tools involves both established and evolving technologies, including microlithography, micromachining, micro-electromechanical systems technology and nanotechnology. This article provides an overview of the latest developments in the key device subject areas and the basic interdisciplinary technologies. Important aspects of DNA and protein analysis, interfacing issues and system integration are all thoroughly discussed, along with applications for this novel "synergized" technology in high-throughput separations of biologically important molecules. This review also gives a better understanding of how to utilize these technologies as well as to provide appropriate technical solutions to problems perceived as being more fundamental.

Microfabrication in biology and medicine

Microfabrication uses integrated-circuit manufacturing technology supplemented by its own processes to create objects with dimensions in the range of micrometers to millimeters. These objects can have miniature moving parts, stationary structures, or both. Microfabrication has been used for many applications in biology and medicine. These applications fall into four domains: tools for molecular biology and biochemistry, tools for cell biology, medical devices, and biosensors. Microfabricated device structures may provide significantly enhanced function with respect to a conventional device. Sometimes microfabrication can enable devices with novel capabilities. These enhancing and enabling qualities are conferred when microfabrication is used appropriately to address the right types of problems. Herein, we describe microfabrication technology and its application to biology and medicine. We detail several classes of advantages conferred by microfabrication and how these advantages have been used to date.

Toward a fully integrated positive-pressure driven microfabricated liquid analyzer

A versatile integrated analyzer with a flow-programmed injection strategy and multiwavelength detection is described with applications toward sampling, flow injection analysis, and capillary separations. Continuous near-real-time sampling is a major benefit of the flow-programmed injection technique. Injection volumes ranging from 250 pL to several microliters were made without electrophoretic flow. Multiwavelength grating light reflection spectroscopy (GLRS) and transmission absorbance spectroscopy were performed simultaneously in a detection volume of 150 pL. The utility of these detection methods for refractive index (RI) and absorbance detection in capillary channels is demonstrated through analysis of salt, indicator, and dyes. GLRS is a unique, selective, and path-length-independent technique for probing RI, absorbance, and other optical properties. A limit of detection (LOD) of 170 microM was achieved for GLRS interferometric detection of FD&C Red #3, which corresponded to 2.6 fmol of analyte in the 150-pL detection volume. A LOD of 2 mM for phosphate buffer, or 3 fmol in the 150-pL detection volume will also be demonstrated. A siloxane coating on the GLRS grating was employed as a sensing layer to probe interactions between the sample and stationary phase. The combined GLRS interferometric response provided insight into both optical and chromatographic properties of samples. Open tubular capillary liquid chromatography with multidimensional multiwavelength detection is demonstrated for the analysis of three food dyes. Separation efficiency, N, of 16,000 was achieved for an unretained dye peak eluting at 12 min. Integration of novel sampling and detection schemes makes this a broadly applicable liquid analyzer.

Separation of 100-kilobase DNA molecules in 10 seconds

Long double-stranded DNA molecules were separated in microfabricated hexagonal arrays in less than 1 min, several orders of magnitude faster than by using conventional technology. DNA samples were first concentrated at the entrance to the array in a thin band by entropic focusing. They were then separated by pulsed field electrophoresis. T4 (168.9 kbp) and lambda (48.5 kbp) DNAs could be resolved into two clearly separated bands in approximately 10 s in these experiments. This corresponds to a mass resolution of 6% in 11 min in a 1-cm-long array.

Fabrication of a configurable, single-use microfluidic device

This paper describes microfluidic devices that contain connections that can be opened by the user after fabrication. The devices are fabricated in poly(dimethylsiloxane) (PDMS) and comprise disconnected fluidic channels that are separated by 20 microm of PDMS. Applying voltages above the breakdown voltage of PDMS (21 V/microm) opened pathways between disconnected channels. Fluids could then be pumped through the openings. The voltage used and the ionic strength of the buffer in the channels determined the size of the opening. Opening connections in a specific order provides the means to control complex reactions on the device. A device for ELISA was fabricated to demonstrate the ability to store and deliver fluids on demand.

Polymer based micro-reactors

In this paper, we describe the fabrication technologies necessary for the production of polymer-based micro-fluidic devices. These technologies include hot embossing as a micro-structuring method as well as so-called back-end processes to complete the micro-devices. Applications such as capillary electrophoresis, micro-mixers and nanowell plates are presented.

Trends in miniaturized total analysis systems for point-of-care testing in clinical chemistry

A currently emerging approach enables more widespread monitoring of health parameters in disease prevention and biomarker monitoring. Miniaturisation provides the means for the production of small, fast and easy-to-operate devices for reduced-cost healthcare testing at the point-of-care (POC) or even for household use. A critical overview is given on the present state and requirements of POC testing, on microTAS elements suited for implementation in future microTAS devices for POC testing and microTAS systems for the determination of clinical parameters.

Flow-through polymerase chain reactions in chip thermocyclers

The miniaturization of analytical devices by micromachining technology is destined to have a major impact on medical and bioanalytical fields. To meet the current demands for rapid DNA amplification, various instruments and innovative technologies have been introduced by several groups in recent years. The development of the devices was extended in different directions and adapted to corresponding applications. In this review the development of a variety of devices and components for performing DNA amplification as well as the comparison of batch-process thermocyclers with reaction chambers and flow-through devices for different purposes are discussed. The main attention is turned to a flow device concept for thermocycling using microfabricated elements for local heat flow management, for which simulations and considerations for further improvement regarding design, material choice and applied technology were performed. The present review article mainly discusses and compares thermocycling devices for rapid thermocycling made of silicon or of silicon and glass with a short excursion to the possibility of plastic chip devices. In order to perform polymerase chain reactions (PCRs) in the microreactors, special attention must be paid to the conditions of the internal surfaces. For microchips, surface effects are generally pronounced because the surface to volume ratio increases upon miniaturization. Solutions for solving this problem are presented. We propose an overview of layouts for batch-process thermocyclers with different parallelization of reaction chambers and also of different designs of continuous flow thermocycling chips, paying particular attention to the parameters which influence the efficiency of such chip devices. Finally we point out some recent issues for applications in the field of clinical diagnostics.

Modular approach to fabrication of three-dimensional microchannel systems in PDMS-application to sheath flow microchips

A modular approach to fabrication of three-dimensional microchannel systems in polydimethylsiloxane (PDMS) is presented. It is based on building blocks with microstructuring on up to three faces. The assembled 3D-microchip consists of three building blocks in two layers. For assembly of the bottom layer two building blocks are joined horizontally, whereby the side structuring of the first is sealed against the flat side surface of the other. This results in the formation of a vertical interconnection opening between the building blocks to supplement the microstructuring on the lower faces. The 3D microchannel system is completed by placing a third building block, with microstructuring only on its lower face, on top of the assembled layer. While plasma assisted bonding is used between the two building blocks of the bottom layer, inherent adhesion is sufficient between the layers and for attaching the assembled 3D-microchip to a substrate. This modular approach was applied to the fabrication of a 3D-sheath flow microchip. It comprises a 20 microm deep microchannel system with sample inlet, open sensing area and outlet in the bottom layer and sheath flow inlet in the top layer. 100 microM fluorescein at 6 microL min(-1) was used as sample flow and water at increasing flow rates as sheath flow. With ratios of sheath to sample flow up to 20:1 sample layers down to 1 microm thickness could be generated. Sample layer thickness was determined via volume detection on an epi-fluorescence microscope followed by image analysis.

SPR imaging measurements of 1-D and 2-D DNA microarrays created from microfluidic channels on gold thin films

Microfluidic channels fabricated from poly(dimethylsiloxane) (PDMS) are employed in surface plasmon resonance imaging experiments for the detection of DNA and RNA adsorption onto chemically modified gold surfaces. The PDMS microchannels are used to (i) fabricate "1-D" single-stranded DNA (ssDNA) line arrays that are used in SPR imaging experiments of oligonucleotide hybridization adsorption and (ii) create "2-D" DNA hybridization arrays in which a second set of PDMS microchannels are placed perpendicular to a 1-D line array in order to deliver target oligonucleotide solutions. In the 1-D line array experiments, the total sample volume is 500 microL; in the 2-D DNA array experiments, this volume is reduced to 1 microL. As a demonstration of the utility of these microfluidic arrays, a 2-D DNA array is used to detect a 20-fmol sample of in vitro transcribed RNA from the uidA gene of a transgenic Arabidopsis thaliana plant. It is also shown that this array fabrication method can be used for fluorescence measurements on chemically modified gold surfaces.

Microfluidic arrays of fluid-fluid diffusional contacts as detection elements and combinatorial tools

This paper describes microfluidic systems that can be used to investigate multiple chemical or biochemical interactions in a parallel format. These three-dimensional systems are generated by crossing two sets of microfluidic channels, fabricated in two different layers, at right angles. Solutions of the reagents are placed in the channels; in different modes of operation, these solutions can be either flowing or stationary-the latter is important when one set of channels is filled with viscous gels with immobilized reagents. At every crossing, the channels are separated either by a single membrane or by a composite separator comprising a membrane, a microwell, and a second membrane. These components allow diffusive mass transport and minimize convective transport through the crossing. Polycarbonate membranes with 0.1-1-microm vertical pores were used to fabricate the devices. Each crossing of parallel channels serves as an element in which chemical or biochemical interactions can take place; interactions can be detected by monitoring changes in fluorescence and absorbance. These all-organic systems are straightforward to fabricate and to operate and may find applications as portable microanalytical systems and as tools in combinatorial research.

Electroosmotic properties of microfluidic channels composed of poly(dimethylsiloxane)

Microfluidic devices fabricated from polymers exhibit great potential in biological analyses. Poly(dimethylsiloxane) (PDMS) has shown promise as a substrate for rapid prototyping of devices. Despite this, disagreement exists in the literature as to the ability of PDMS to support electroosmotic (EO) flow and the stability of that flow over time. We demonstrate that in low ionic strength solutions near neutral in pH. oxidized PDMS had a four-fold greater EO mobility (mu(eo)) compared to native PDMS. The greater mu(eo) was maintained irrespective of whether glass or PDMS was used as a support forming one side of the channel. This enhanced mu(eo) was preserved as long as the channels were filled with an aqueous solution. Upon exposure of the channels to air, the mobility decreased by a factor of two with a half-life of 9 h. The EO properties of the air-exposed, oxidized PDMS were regenerated by exposure to strong base. High ionic strength, neutral in pH buffers compatible with living eukaryotic cells diminished the EO flow in the oxidized PDMS devices to a much greater extent than in the native PDMS devices. For analyses utilizing intact and living cells, oxidation of PDMS may not be an effective strategy to substantially increase the mu(eo).

Microelectromechanical systems and neurosurgery: a new era in a new millennium

MICROMACHINES AND MICROELECTROMECHANICAL SYSTEMS (MEMS) are terms that are new to neurosurgeons but certain to become "household terms" in neurosurgery in the near future. These new terms serve as an introduction to a new world of sensors, actuators, and "smart systems" that will change the ways in which neurosurgeons interact with their environment. Through the use of microelectronics and micromachining technologies, MEMS will allow neurosurgeons to perform familiar tasks with greater precision, perform tasks that previously were not done at all, and monitor physiological and biochemical parameters more accurately and with greater safety. This review provides the information necessary to understand the fundamental concepts of MEMS and their application to the neurosurgical arena. It defines the relevant terms and describes the history behind the "micromachine revolution," the capabilities and limitations of MEMS technology, and how this revolution is germane to neurosurgery and to neurosurgeons.

Pressure-driven laminar flow in tangential microchannels: an elastomeric microfluidic switch

This paper describes laminar fluid flow through a three-dimensional elastomeric microstructure formed by two microfluidic channels, fabricated in layers that contact one another face-to-face (typically at a 90 degree angle), with the fluid flows in tangential contact. There are two ways to control fluid flow through these tangentially connected microchannels. First, the flow profiles through the crossings are sensitive to the aspect ratio of the channels; the flow can be controlled by applying external pressure and changing this aspect ratio. Second, the flow direction of an individual laminar stream in multiphase laminar flow depends on the lateral position of the stream within the channel; this position can be controlled by injecting additional streams of fluid into the channel. We describe two microfluidic switches based on these two ways for controlling fluid flow through tangential microchannels and present theoretical arguments that explain the observed dependence of the flow profiles on the aspect ratio of the channels.

Field-effect flow control in a polydimethylsiloxane-based microfluidic system

The application of the field-effect for direct control of electroosmosis in a polydimethylsiloxane (PDMS)-based microfluidic system, constructed on a silicon wafer with a 2.0 microm electrically insulating layer of silicon dioxide, is demonstrated. This microfluidic system consists of a 2.0 cm open microchannel fabricated on a PDMS slab, which can reversibly adhere to the silicon wafer to form a hybrid microfluidic device. Aside from mechanically serving as a robust bottom substrate to seal the channel and support the microfluidic system, the silicon wafer is exploited to achieve field-effect flow control by grounding the semiconductive silicon medium. When an electric field is applied through the channel, a radial electric potential gradient is created across the silicon dioxide layer that allows for direct control of the zeta potential and the resulting electroosmotic flow (EOF). By configuring this microfluidic system with two power supplies at both ends of the microchannel, the applied electric potentials can be varied for manipulating the polarity and the magnitude of the radial electric potential gradient across the silicon dioxide layer. At the same time, the longitudinal potential gradient through the microchannel, which is used to induce EOF, is held constant. The results of EOF control in this hybrid microfluidic system are presented for phosphate buffer at pH 3 and pH 5. It is also demonstrated that EOF control can be performed at higher solution pH of 6 and 7.4 by modifying the silicon wafer surface with cetyltrimethylammonium bromide (CTAB) prior to assembly of the hybrid microfluidic system. Results of EOF control from this study are compared with those reported in the literature involving the use of other microfluidic devices under comparable solution conditions.

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

Following current trends in miniaturization of analytical chemistry, an inexpensive disposable analytical tool in the form of a liquid chromatography column fabricated on a poly(dimethyl siloxane) (PDMS) chip was created. Ease of fabricating the chromatography column was demonstrated by molding collocated monolithic support structures (COMOSS) directly in the column. Positive photo-resist, SPR 220, was used to create column structures on a negative relief master providing final channel dimensions of 2.7-5.2 microm wide by 10.0 microm deep, while monolithic dimensions were 9.8 x 9.8 x 10.0 microm - 12.3 x 12.3 x 10.0 microm. The ability to separate biological samples such as peptides from a tryptic digest of fluorescein isothiocyanate labeled bovine serum albumin (FITC-BSA) was shown. Separations in capillary electrochromatographic (CEC) mode were performed yielding column efficiencies of 4.0 x 10(5) plates/m.

An integrated fluorescence detection system in poly(dimethylsiloxane) for microfluidic applications

This paper describes a prototype of an integrated fluorescence detection system that uses a microavalanche photodiode (microAPD) as the photodetector for microfluidic devices fabricated in poly(dimethylsiloxane) (PDMS). The prototype device consisted of a reusable detection system and a disposable microfluidic system that was fabricated using rapid prototyping. The first step of the procedure was the fabrication of microfluidic channels in PDMS and the encapsulation of a multimode optical fiber (100-microm core diameter) in the PDMS; the tip of the fiber was placed next to the side wall of one of the channels. The optical fiber was used to couple light into the microchannel for the excitation of fluorescent analytes. The photodetector, a prototype solid-state microAPD array, was embedded in a thick slab (1 cm) of PDMS. A thin (80 microm) colored polycarbonate filter was placed on the top of the embedded microAPD to absorb scattered excitation light before it reached the detector. The microAPD was placed below the microchannel and orthogonal to the axis of the optical fiber. The close proximity (approximately 200 microm) of the microAPD to the microchannel made it unnecessary to incorporate transfer optics; the pixel size of the microAPD (30 microm) matched the dimensions of the channels (50 microm). A blue light-emitting diode was used for fluorescence excitation. The microAPD was operated in Geiger mode to detect the fluorescence. The detection limit of the prototype (approximately 25 nM) was determined by finding the minimum detectable concentration of a solution of fluorescein. The device was used to detect the separation of a mixture of proteins and small molecules by capillary electrophoresis; the separation illustrated the suitability of this integrated fluorescence detection system for bioanalytical applications.

Surface biopassivation of replicated poly(dimethylsiloxane) microfluidic channels and application to heterogeneous immunoreaction with on-chip fluorescence detection

Poly(dimethylsiloxane) (PDMS) appeared recently as a material of choice for rapid and accurate replication of polymer-based microfluidic networks. However, due to its hydrophobicity, the surface strongly interacts with apolar analytes or species containing apolar domains, resulting in significant uncontrolled adsorption on channel walls. This contribution describes the application and characterization of a PDMS surface treatment that considerably decreases adsorption of low and high molecular mass substances to channel walls while maintaining a modest cathodic electroosmotic flow. Channels are modified with a three-layer biotin-neutravidin sandwich coating, made of biotinylated IgG, neutravidin, and biotinylated dextran. By replacing biotinylated dextran with any biotinylated reagent, the modified surface can be readily patterned with biochemical probes, such as antibodies. Combination of probe immobilization chemistry with low nonspecific binding enables affinity binding assays within channel networks. The example of an electrokinetic driven, heterogeneous immunoreaction for human IgG is described.

Portable cell-based biosensor system using integrated CMOS cell-cartridges

The use of cell-based biosensors outside of the laboratory has been limited due to many issues including preparation of the sample, maintenance of the biological environment, and integration of the electronics for data collection and analysis. This paper describes a system that addresses several of these issues with the development of an integrated silicon-polydimethylsiloxane cell-cartridge. The cell-cartridge contains a CMOS silicon chip that incorporates a digital interface, temperature control system, microelectrode electrophysiology sensors, and analog signal buffering. Additionally, the cell-cartridge supports two separate cell populations in two 10 microl sealed chambers that have independent fluidic channels for sample injection. A portable, microcontroller-based electronics system capable of monitoring the action potential (AP) activity within the cell-cartridges was also developed. The AP activities of cardiomyocyte syncytia in the two chambers differentially responded to the flow of a control medium versus the flow of a biochemical agent. The cell-cartridges and portable electronics system were used to successfully record AP activity from cardiomyocytes outside of the laboratory under realistic application conditions.

Rapid fabrication of microfluidic devices in poly(dimethylsiloxane) by photocopying

A very simple and fast method for the fabrication of poly(dimethylsiloxane) (PDMS) microfluidic devices is introduced. By using a photocopying machine to make a master on transparency instead of using lithographic equipment and photoresist, the fabrication process is greatly simplified and speeded up, requiring less than 1.5 h from design to device. Through SEM characterization, any micro-channel network with a width greater than 50 microm and a depth in the range of 8-14 microm can be made by this method. After sealing to a Pyrex glass plate with micromachined platinum electrodes, a microfluidic device was made and the device was tested in FIA mode with on-chip conductometric detection without using either high voltage or other pumping methods.

Nanowire nanosensors for highly sensitive and selective detection of biological and chemical species

Boron-doped silicon nanowires (SiNWs) were used to create highly sensitive, real-time electrically based sensors for biological and chemical species. Amine- and oxide-functionalized SiNWs exhibit pH-dependent conductance that was linear over a large dynamic range and could be understood in terms of the change in surface charge during protonation and deprotonation. Biotin-modified SiNWs were used to detect streptavidin down to at least a picomolar concentration range. In addition, antigen-functionalized SiNWs show reversible antibody binding and concentration-dependent detection in real time. Lastly, detection of the reversible binding of the metabolic indicator Ca2+ was demonstrated. The small size and capability of these semiconductor nanowires for sensitive, label-free, real-time detection of a wide range of chemical and biological species could be exploited in array-based screening and in vivo diagnostics.

Photomodification of polymer microchannels induced by static and dynamic excimer ablation: effect on the electroosmotic flow

This paper presents a study of polymer surfaces modified by laser ablation using poly(ethylene terephthalate) (PET) as a model system. The surface properties induced by static and dynamic ablation with the 193-nm pulsed radiation of an ArF excimer laser (4 x 10(7) W/cm2) in air have been successfully used to control the electroosmotic flow (EOF) in photoablated PET microchannels. Through the creation of well-defined static ablation patterns onto the walls of a trapezoidal channel, it was found that the resulting reduction in the EOF could be controlled. For example, a reduction of 25% in the EOF was observed in 42-microm-deep microchannels when using a static ablation pattern treating 50% of the total wall surface area. A numerical study describing the fluidic behavior induced by a static pattern is also presented. Moreover, X-ray photoelectron spectroscopy has been used to point out surface changes between static and dynamic ablation, thereby demonstrating an ability to create new functionalities in microchannels by laser treatment.

Electroosmotically induced hydraulic pumping with integrated electrodes on microfluidic devices

Electroosmotic manipulation of fluids was demonstrated using thin metal electrodes integrated within microfluidic channels at the substrate and cover plate interface. Devices were fabricated by photolithographically patterning electrodes on glass cover plates that were then bonded to polymeric substrates into which the channels were cast. Polymeric substrates were used to provide a permeable membrane for the transport and removal of gaseous electrolysis products generated at the electrodes. Electroosmotic flow between interdigitated electrodes was demonstrated and provided electric field-free pumping of fluids in sections of the channel outside of the electrode pairs. The resultant pumping velocities were shown to be dependent on the applied voltage, not on the applied field strength, and independent of the length of the electroosmotically pumped region.

Development of a fully integrated analysis system for ions based on ion-selective optodes and centrifugal microfluidics

A fully integrated, miniaturized analysis system for ions based on a centrifugal microfluidics platform and ion-selective optode membranes is described. The microfluidic architecture is composed of channels, five solution reservoirs, a measuring chamber, and a waste reservoir manufactured onto a disk-shaped substrate of poly(methyl methacrylate). Ion-selective optode membranes, composed of plasticized poly(vinyl chloride) impregnated with an ionophore, a proton chromoionophore, and a lipophilic anionic additive, were cast, with a spin-on device, onto a support layer and then immobilized on the disk. Fluid propulsion is achieved by the centrifugal force that results from spinning the disk, while a system of valves is built onto the disk to control flow. These valves operate based on fluid properties and fluid/substrate interactions and are controlled by the angular frequency of rotation. With this system, we have been able to deliver calibrant solutions, washing buffers, or "test" solutions to the measuring chamber where the optode membrane is located. An analysis system based on a potassium-selective optode has been characterized. Results indicate that optodes immobilized on the platform demonstrate theoretical responses in an absorbance mode of measurement. Samples of unknown concentration can be quantified to within 3% error by fitting the response function for a given optode membrane using an acid (for measuring the signal for a fully protonated chromoionophore), a base (for fully deprotonated chromoionophore), and two standard solutions. Further, the ability to measure ion concentrations by employing one standard solution in conjunction with acid and base and with two standards alone were studied to delineate whether the current architecture could be simplified. Finally, the efficacy of incorporating washing steps into the calibration protocol was investigated.

Microchip capillary electrophoresis/electrochemistry

Microfabricated fluidic devices have generated considerable interest over the past ten years due to the fact that sample preparation, injection, separation, derivatization, and detection can be integrated into one miniaturized device. This review reports progress in the development of microfabricated analytical systems based on microchip capillary electrophoresis (CE) with electrochemical (EC) detection. Electrochemical detection has several advantages for use with microchip electrophoresis systems, for example, ease of miniaturization, sensitivity, and selectivity. In this review, the basic components necessary for microchip CEEC are described, including several examples of different detector configurations. Lastly, details of the application of this technique to the determination of catechols and phenols, amino acids, peptides, carbohydrates, nitroaromatics, polymerase chain reaction (PCR) products, organophosphates, and hydrazines are described.

Microchip electrophoretic separation systems for biomedical and pharmaceutical analysis

The application of microchip capillary electrophoresis (CE) systems to biomedical and pharmaceutical analysis is described and reviewed. Fabrication, instrumentation, and operation of the systems are discussed. An overview of applications is presented, covering four main areas: DNA sequencing, genetic analysis, immunoassays, and protein and peptide analysis. These systems have the potential to dramatically change the way that biochemical analyses are performed.

Microchip capillary electrophoresis/electrochemistry

Microfabricated fluidic devices have generated considerable interest over the past ten years due to the fact that sample preparation, injection, separation, derivatization, and detection can be integrated into one miniaturized device. This review reports progress in the development of microfabricated analytical systems based on microchip capillary electrophoresis (CE) with electrochemical (EC) detection. Electrochemical detection has several advantages for use with microchip electrophoresis systems, for example, ease of miniaturization, sensitivity, and selectivity. In this review, the basic components necessary for microchip CEEC are described, including several examples of different detector configurations. Lastly, details of the application of this technique to the determination of catechols and phenols, amino acids, peptides, carbohydrates, nitroaromatics, polymerase chain reaction (PCR) products, organophosphates, and hydrazines are described.

On-line coupling of micro-enzyme reactor with micro-membrane chromatography for protein digestion, peptide separation, and protein identification using electrospray ionization mass spectrometry

To miniaturize high-performance membrane chromatography, a poly(vinylidene fluoride) membrane medium, employed as the stationary phase, is sandwiched between two poly(dimethylsiloxane) substrates containing the microchannels. The microchannels are fabricated by the capillary molding technique, involving the use of capillaries as the channel template and the fluid inlet/outlet. The micro(micro)-membrane chromatography system is coupled with a micro-enzyme reactor containing immobilized trypsins for performing rapid protein digestion, peptide separation, and protein identification using electrospray ionization mass spectrometry. Separation performance of cytochrome c digest in micro-membrane chromatography is compared with the results obtained from a regular reversed-phase micro-liquid chromatography. The efficacy and the potentials of micro-membrane chromatography in tryptic mapping are reported. On-line integration of the micro-enzyme reactor with micro-chromatographic separation techniques and electrospray ionization mass spectrometry clearly provides a microanalytical platform for automated sample handling, minimized sample loss, and reduced sample consumption. It also provides enhanced detection sensitivity and dynamic range for the analysis of complex protein mixtures such as cell lysates in proteomics research.

A method for filling complex polymeric microfluidic devices and arrays

This paper describes an improved method for filling microfluidic structures with aqueous solutions. The method, channel outgas technique (COT), is based on a filling procedure carried out at reduced pressures. This procedure is compared with previously reported methods in which microfluidic channels are filled either by using capillary forces or by applying a pressure gradient at one or more empty reservoirs. The technique has proven to be > 90% effective in eliminating the formation of bubbles within microfluidic networks. It can be applied to many devices, including those containing PDMS-terminated channel features, a single channel inlet, and three-dimensional arrays.

Microfabricated PDMS multichannel emitter for electrospray ionization mass spectrometry

A novel microfabricated multichannel emitter for electrospray ionization mass spectrometry (ESI-MS) was implemented with polydimethylsiloxane (PDMS) using a soft lithography technique. The emitters are formed as electrospray tips along a thin membrane on the edge of the device with channels of 100 microm x 30 microm dimensions. The electrospray performance of the PDMS emitters for a single channel device and a four channel device interfaced with a time-of-flight mass spectrometer was evaluated for detecting the molecular weight of reference peptides (angiotensin I and bradykinin). The emitters were durable at the flow rate of 1-20 microL min(-1) for more than 30 h of continuous electrospray with limit of detection of 1 microM (S/N 18). This microfabrication method for a PDMS multichannel emitter as an integral part of a microfluidic device will facilitate development of more complex microfluidic analysis systems using ESI-MS.

Integrated Plastic Microfluidic Devices with ESI-MS for Drug Screening and Residue Analysis

For this work, two different plastic microfluidic devices are designed and fabricated for applications in high-throughput residue analysis of food contaminants and drug screening of small-molecule libraries. Microfluidic networks on copolyester and poly(dimethylsiloxane) substrates are fabricated by silicon template imprinting and capillary molding techniques. The first device is developed to perform affinity capture, concentration, and direct identification of targeted compounds using electrospray ionization mass spectrometry. Poly(vinylidene fluoride) membranes sandwiched between the imprinted copolyester microchannels in an integrated platform provide continuous affinity dialysis and concentration of a reaction mixture containing aflatoxin B1 antibody and aflatoxins. The second microfluidic device is composed of microchannels on the poly(dimethylsiloxane) substrates. The device is designed to perform miniaturized ultrafiltration of affinity complexes of phenobarbital antibody and barbiturates, including the sequential loading, washing, and dissociation steps. These microfabricated devices not only significantly reduce dead volume and sample consumption but also increase the detection sensitivity by at least 1-2 orders of magnitude over those reported previously. Improvements in detection sensitivity are attributed to analyte preconcentration during the affinity purification step, limited analyte dilution in the microdialysis junction, minimal sample loss, and the amenability of ESI-MS to nanoscale sample flow rates.