Microfluidic technologies for studying synthetic circuits

Advances in synthetic biology have augmented the available toolkit of biomolecular modules, allowing engineering and manipulation of signaling in a variety of organisms, ranging in complexity from single bacteria and eukaryotic cells to multi-cellular systems. The richness of synthetic circuit outputs can be dramatically enhanced by sophisticated environmental control systems designed to precisely pattern spatial-temporally heterogeneous environmental stimuli controlling these circuits. Moreover, the performance of the synthetic modules and 'blocks' needed to assemble more complicated networks requires more complete characterization as a function of arbitrarily complex environmental inputs. Microfluidic technologies are poised to meet these needs through a variety of innovative designs capitalizing on the unique benefits of manipulating fluids on the micro-scales and nano-scales. This review discusses the utility of microfluidics for the study of synthetic circuits and highlights recent work in the area.

Integrated liquid chromatography-heated nebulizer microchip for mass spectrometry

A new integrated microchip for liquid chromatography-mass spectrometry (LC-MS) is presented. The chip is made from bonded silicon and glass wafers with structures for a packed LC column channel, a micropillar frit, a channel for optional optical detection, and a heated vaporizer section etched in silicon and platinum heater elements on the glass cover. LC eluent is vaporized and mixed with nebulizer gas in the vaporizer section and the vapor is sprayed out from the chip. Nonpolar and polar analytes can be efficiently ionized in the gas phase by atmospheric pressure photoionization (APPI) as demonstrated with polycyclic aromatic hydrocarbons (PAHs) and selective androgen receptor modulators (SARMs). This is not achievable with present LC-MS chips, since they are based on electrospray ionization, which is not able to ionize nonpolar compounds efficiently. The preliminary quantitative performance of the new chip was evaluated in terms of limit of detection (down to 5 ng mL(-1)), linearity (r>0.999), and repeatability of signal response (RSD=2.6-4.0%) and retention time (RSD=0.3-0.5%) using APPI for ionization and PAHs as standard compounds. Determination of fluorescent compounds is demonstrated by using laser-induced fluorescence (LIF) for detection in the optical detection channel before the vaporizer section.

Fabrication of a multichannel PDMS/glass analytical microsystem with integrated electrodes for amperometric detection

The fabrication process of novel multichannel microfluidic devices with integrated electrodes for amperometric detection is described. Soft-lithography, lift-off and O(2) plasma surface activation sealing techniques were employed for rapid prototyping of cost effective PDMS/glass microchips. The capabilities of the proposed microdevices were demonstrated by the electrooxidation of hydroquinone and N-acetyl-p-aminophenol (APAP) on a Au working electrode at +800 mV and +700 mV, respectively, against a Au pseudo reference electrode, and of thiocyanate on a Cu working electrode at +700 mV against a Ag/AgCl (KCl saturated) reference electrode. Linear response over the range up to 1.0 mmol L(-1) for APAP and up to 4.0 mmol L(-1) for hydroquinone and thiocyanate were verified through calibration curves with correlation coefficients greater than 0.97 (minimum of five data points). The sensitivities for hydroquinone, thiocyanate, and APAP were 28, 19, and 78 microA mol(-1) L, respectively. Under the experimental conditions used, the estimated limits of detection were 0.21, 0.95, and 0.12 mmol L(-1) for hydroquinone, thiocyanate and APAP, respectively. The geometries of the devices were designed to allow fast calibration procedures and reliable results for in-field applications. Exerting a strong influence over the device performance, the sealing process was greatly enhanced by depositing auxiliary TiSiO(2) thin-films. The general performance of the system was verified by amperometric assays of N-acetyl-p-aminophenol standard solutions, and the influences exerted by the present fabrication methods regarding reproducibility and reliability are addressed. The proposed device was successfully applied in the determination of the concentration of APAP in two commercial formulations.

Electrically actuated, pressure-driven liquid chromatography separations in microfabricated devices

Electrolysis-based micropumps integrated with microfluidic channels in micromachined glass substrates are presented. Photolithography combined with wet chemical etching and thermal bonding enabled the fabrication of multi-layer devices containing electrically actuated micropumps interfaced with sample and mobile phase reservoirs. A stationary phase was deposited on the microchannel walls by coating with 10% (w/w) chlorodimethyloctadecylsilane in toluene. Pressure-balanced injection was implemented by controlling the electrolysis time and voltage applied in the two independent micropumps. Current fluctuations in the micropumps due to the stochastic formation of bubbles on the electrode surfaces were determined to be the main cause of variation between separations. On-chip electrochemical pumping enabled the loading of pL samples with no dead volume between injection and separation. A mobile phase composed of 70% acetonitrile and 30% 50 mM acetate buffer (pH 5.45) was used for the chromatographic separation of three fluorescently labeled amino acids in <40 s with an efficiency of >3000 theoretical plates in a 2.5 cm-long channel. Our results demonstrate the potential of electrochemical micropumps integrated with microchannels to perform rapid chromatographic separations in a microfabricated platform. Importantly, these devices represent a significant step toward the development of miniaturized and fully integrated liquid chromatography systems.

A simple and efficient approach for calculating permeability coefficients and HETP for rectangular columns

There had been little progress in development of the theoretical basis of rectangular chromatography columns until Spangler made great progress by using a more exact model than Golay's. Unfortunately, there was a deficiency in his calculations, which led to a conclusion inconsistent with the previous theories. In this paper, a simpler formula with defined variables was first established to calculate the mean permeability coefficient for a rectangular GC column. A formula was also established to calculate the height equivalent to a theoretical plate (HETP) for a rectangular column based on this work and the correction of Spangler's theory. By comparing both our predictions and Spangler's predictions with Golay's, respectively, we could demonstrate that our theory is more exact. Further, one parameter (A) was found to be not monotonous. This finding leads to the conclusion that the square column has the highest performance among all the rectangular-shaped columns used for chromatography, and that a width/depth ratio of around three is desirable if the column is used for mixing reactants in lab-on-a-chip systems, instead of for chromatography. The conclusions are applicable not only for gas but also for liquid chromatography columns.

Recent applications in nanoliquid chromatography

Since its first introduction by Karlsson and Novotny in 1988 nano-LC has emerged as a complementary and/or competitive separation method to conventional HPLC, offering several advantages such as higher efficiency, ability to work with minute sample sizes and lower consumption of mobile phases, and better compatibility with MS, etc. Although its use was not so extended initially, in the last years new and interesting applications have appeared which deserve to be carefully considered. The aim of this review is therefore to provide an updated and critical survey of different nano-LC applications in analytical chemistry.

Pressure-driven reverse-phase liquid chromatography separations in ordered nonporous pillar array columns

Building upon the micromachined column idea proposed by the group of Regnier in 1998, we report on the first high-resolution reversed-phase separations in micromachined pillar array columns under pressure-driven LC conditions. A three component mixture could be separated in 3 s using arrays of nonporous silicon pillars with a diameter of approximately 4.3 microm and an external porosity of 55%. Under slightly retained component conditions (retention factor k' = 0.65-1.2), plate heights of about H = 4 microm were obtained at a mobile phase velocity around u = 0.5 mm/s. In reduced terms, such plate heights are as low as hmin = 1. Also, since the flow resistance of the column is much smaller than in a packed column (mainly because of the higher external porosity of the pillar array), the separation impedance of the array was as small as E = 150, i.e., of the same order as the best currently existing monolithic columns. At pH = 3, yielding very low retention factors (k' = 0.13 and 0.23), plate heights as low as H = 2 microm were realized, yielding a separation of the three component mixture with an efficiency of N = 4000-5000 plates over a column length of 1 cm. At higher retention factors, significantly larger plate heights were obtained. More experimental work is needed to investigate this more in depth. The study is completed with a discussion of the performance limits of the pillar array column concept in the frame of the current state-of-the-art in microfabrication precision.

Microfabricated devices: A new sample introduction approach to mass spectrometry

Instrument miniaturization is one way of addressing the issues of sensitivity, speed, throughput, and cost of analysis in DNA diagnostics, proteomics, and related biotechnology areas. Microfluidics is of special interest for handling very small sample amounts, with minimal concerns related to sample loss and cross-contamination, problems typical for standard fluidic manipulations. Furthermore, the small footprint of these microfabricated structures leads to instrument designs suitable for high-density, parallel sample processing, and high-throughput analyses. In addition to miniaturized systems designed with optical or electrochemical detection, microfluidic devices interfaced to mass spectrometry have also been demonstrated. Instruments for automated sample infusion analysis are now commercially available, and microdevices utilizing chromatographic or capillary electrophoresis separation techniques are under development. This review aims at documenting the technologies and applications of microfluidic mass spectrometry for the analysis of proteomic samples.

On-chip temperature gradient interaction chromatography

This paper reports the first integrated microelectromechanical system (MEMS) HPLC chip that consists of a parylene high-pressure LC column, an electrochemical sensor, a resistive heater and a thermal-isolation structure for on-chip temperature gradient interaction chromatography application. The separation column was 8 mm long, 100 microm wide, 25 microm high and was packed with 5 microm sized, C18-coated beads using conventional slurry-packing technique. A novel parylene-enhanced, air-gap thermal isolation technology was used to reduce heater power consumption by 58% and to reduce temperature rise in the off-column area by 67%. The fabricated chip consumed 400 mW when operated at 100 degrees C. To test the chromatography performance of the fabricated system, a mixture of derivatized amino acids was chosen for separation. A temporal temperature gradient scanning from 25 to 65 degrees C with a ramping rate of 3.6 degrees C/min was applied to the column during separation. Successful chromatographic separation of derivatized amino acids was carried out using our chip. Compared with conventional temperature gradient HPLC system which incorporates "macro oven" to generate temporal temperature gradient on the column, our chip's thermal performance, i.e., power consumption and thermal response, is greatly improved without sacrificing chromatography quality.

A microfluidic SELEX prototype

Aptamers are nucleic acid binding species capable of recognizing a wide variety of targets ranging from small organic molecules to supramolecular structures, including organisms. They are isolated from combinatorial libraries of synthetic nucleic acid by an iterative process referred to as SELEX (Systematic Evolution of Ligands by Exponential Enrichment). Here we describe an automated microfluidic, microline-based assembly that uses LabView-controlled actuatable valves and a PCR machine, and which is capable of the selection and synthesis of an anti-lysozyme aptamer as verified by sequence analysis. The microfluidic prototype described is 1) a simple apparatus that is relatively inexpensive to assemble, making automated aptamer selection accessible to many investigators, and 2) useful for the continued "morphing" of macro-->meso-->microfabricated structures until a convergence to a few functional systems evolves and emerges, partly or completely achieving simpler, smaller and more rapid SELEX applications.

DNA arrays, electronic noses and tongues, biosensors and receptors for rapid detection of toxigenic fungi and mycotoxins: a review

This paper presents an overview of how microsystem technology tools can be applied to the development of rapid, out-of-laboratory measurement capabilities for the determinations of toxigenic fungi and mycotoxins in foodstuffs. Most of the topics discussed are all under investigation within the European Commission-sponsored project Good-Food (FP6-IST). These are DNA arrays, electronic noses and electronic tongues for the detection of fungal contaminants in feed, and biosensors and chemical sensors based on microfabricated electrode systems, antibodies and novel synthetic receptors for the detection of specific mycotoxins. The approach to resolution of these difficult measurement problems in real matrices requires a multidisciplinary approach. The technology tools discussed can provide a route to the rapid, on-site generation of data that can aid the safe production of high-quality foodstuffs.

A New Synthetic Method for Controlled Polymerization Using a Microfluidic System

While many parallel synthesis methods developed by the pharmaceutical and life science communities are being applied to polymer synthesis, there remains a need to construct "libraries" of polymeric materials that explore a wider range of polymer structures with accuracy, flexibility, and rapid, often small, changes. We report the use of microfluidics to create an environment for continuous controlled radical polymerization. Varying either the flow rate or the relative concentrations of reactants (i.e., stoichiometry) controls the molecular properties of the products. Molecular variables, here molecular weight, can then be varied continuously. Well-defined materials with narrow molecular weight distributions are produced inside the microfluidic reactor and are available for processing, such as further mixing, deposition, or coating on surfaces. Preliminary kinetic data appear to agree well with literature values reported for larger-scale reactions.

On-Chip Hydrodynamic Chromatography Separation and Detection of Nanoparticles and Biomolecules

For the first time, on-chip planar hydrodynamic chromatography is combined with UV absorption detection. This technique is suitable for size characterization of synthetic polymers, biopolymers, and particles. Possible advantages of an on-chip hydrodynamic chromatography system over conventional techniques, such as size exclusion chromatography, and field-flow fractionation are fast analysis, high efficiency, reduced solvent consumption, and easy temperature control. The hydrodynamic separations are performed in a planar configuration realized in fused silica using a mixture of fluorescent and nonfluorescent polystyrene particles with sizes ranging from 26 to 155 nm. The planar chip configuration consists of a 1-microm-high, 0.5-mm-wide, and 69-mm-long channel, an integrated 150-pL injection structure, and a 30-microm-deep and 30-microm-wide detection cell, suitable for UV absorption detection. By combination of the separation data obtained in the new fused-silica chip with those obtained using a previously presented planar hydrodynamic chromatography chip, which was realized using silicon and glass microtechnology, a description of the retention and dispersion behavior of planar hydrodynamic chromatography is obtained. Especially the influence of the sidewalls on the dispersion is investigated. Furthermore a hydrodynamic separation within 70 s of several biopolymers is shown in the glass-silicon chip.

Introduction to micro-analytical systems: bioanalytical and pharmaceutical applications

This review presents a brief overview of recent developments in miniaturization of analytical instruments utilizing microfabrication technology. The concept 'Micro-Total Analysis Systems micro-TAS)', also termed 'Lab-on-a-chip', and the latest progresses in the development of microfabricated separation devices and on-chip detection techniques are discussed. Applications of micro-analytical methods to bioanalytical and pharmaceutical studies are also described, including chemical reactions, assays, and analytical separations of biomolecules in micro-scale.

Multiple open-channel electroosmotic pumping system for microfluidic sample handling

The development of a novel, fully integrated, miniaturized pumping system for generation of pressure-driven flow in microfluidic platforms is described. The micropump, based on electroosmotic pumping principles, has a multiple open-channel configuration consisting of hundreds of parallel, small-diameter microchannels. Specifically, pumps with microchannels of 1-6 microm in depth, 4-50 mm in length, and an overall area of a few square millimeters, were constructed. Flow rates of 10-400 nL/min were generated in electric-field-free regions in a stable, reproducible and controllable manner. In addition, eluent gradients were created by simultaneously using two pumps. Pressures up to 80 psi were produced with the present pump configurations. The pump can be easily interfaced with other operational elements of a micrototal analysis system (micro-TAS) device with multiplexing capabilities. A new microfluidic valving system was also briefly evaluated in conjunction with these pumps. The micropump was utilized to deliver peptide samples for electrospray ionization-mass spectrometric (ESI-MS) detection.

A chip system for size separation of macromolecules and particles by hydrodynamic chromatography

For the first time, a miniaturized hydrodynamic chromatography chip system has been developed and tested on separation of fluorescent nanospheres and macromolecules. The device can be applied to size characterization of synthetic polymers, biopolymers, and particles, as an attractive alternative to the classical separation methods such as size exclusion chromatography or field-flow fractionation. The main advantages are fast analysis, high separation efficiency, negligible solvent consumption, and easy temperature control. The prototype chip contains a rectangular flat separation channel with dimensions of 1 microm deep and 1000 microm wide, integrated with a 300-pL injector on a silicon substrate. The silicon microtechnology provides precisely defined geometry, high rigidity, and compatibility with organic solvents or high temperature. All flows are pressure driven, and a specific injection system is employed to avoid excessive sample loading times, demonstrating an alternative way of lab-on-a-chip design. Separations obtained in 3 min show the high performance of the device and are also the first demonstration of flat channel hydrodynamic chromatography in practice.

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.

On-chip definition of picolitre sample injection plugs for miniaturised liquid chromatography

The fabrication of components for a miniaturised liquid chromatography system on silicon has recently been reported by our research group [J. Cap. Electrophoresis Microchip Technol. 6 (1999) 33; Analyst 125 (2000) 25]. To date, inlet and outlet connection ports, separation micro-channels (20-200 microm in width, 0.5-10 microm in depth, 15-60 cm in length), and an intersection for picolitre injection have been etched on a silicon wafer and then sealed with a Pyrex cover plate on which platinum electrodes for on-chip detection have been patterned. The platinum electrodes have been used for the amperometric detection of phenol, using 20 nl off-chip injection. In this work we present our latest results obtained with on-chip pressure driven picolitre injection, designed to realize the full capabilities of this micro-LC system. The injection volume is dependent on the micro-channel depth, width, and also on the intersection length, allowing injection in the low nanolitre to picolitre range.

Ion chromatography on-chip

On-chip separation of inorganic anions by ion-exchange chromatography was realized. Micro separation channels were fabricated on a silicon wafer and sealed with a Pyrex cover plate using standard photolithography, wet and dry chemical etching, and anodic bonding techniques. Quaternary ammonium latex particles were employed for the first time to coat the separation channels on-chip. Owing to the narrow depths of the channels on the chip, 0.5-10 microm, there were more interactions of the analytes with the stationary phase on the chip than in a 50-microm I.D. capillary. With off-chip injection (20 nl) and UV detection, NO2-, NO3-, I-, and thiourea were separated using 1 mM KCl as the eluent. The linear ranges for NO2- and NO3- are from 5 to 1000 microM with the detection limits of 0.5 microM.