Electrokinetic injection techniques in microfluidic chips

Improving device design in insulator-based electrokinetic tertiary separations

This study presents a methodology for designing effective insulator-based electrokinetic (iEK) systems for separating tertiary microparticle samples, which can be extended to more complex samples. First, 144 distinct iEK microchannel designs were built considering different shapes and arrangements of the insulating posts. Second, a mathematical model was developed with COMSOL software to predict the retention time of each particle type in the microchannel, this allowed identifying the best channel designs for two distinct types of separations: charge-based and sized-based. Third, the experimental charge-based and size-based separations of the tertiary microparticle mixtures were performed employing the improved designs identified with COMSOL modeling. The experimental results demonstrated successful separation in terms of separation resolution and good agreement with COMSOL predictions. The findings from this study show that the proposed method for device design, which combines mathematical modeling with varying post shape and post arrangement is an effective approach for identifying iEK systems capable of separating complex microparticle samples.

Strategies for capillary electrophoresis: Method development and validation for pharmaceutical and biological applications-Updated and completely revised edition

This review is in support of the development of selective, precise, fast, and validated capillary electrophoresis (CE) methods. It follows up a similar article from 1998, Wätzig H, Degenhardt M, Kunkel A. "Strategies for capillary electrophoresis: method development and validation for pharmaceutical and biological applications," pointing out which fundamentals are still valid and at the same time showing the enormous achievements in the last 25 years. The structures of both reviews are widely similar, in order to facilitate their simultaneous use. Focusing on pharmaceutical and biological applications, the successful use of CE is now demonstrated by more than 600 carefully selected references. Many of those are recent reviews; therefore, a significant overview about the field is provided. There are extra sections about sample pretreatment related to CE and microchip CE, and a completely revised section about method development for protein analytes and biomolecules in general. The general strategies for method development are summed up with regard to selectivity, efficiency, precision, analysis time, limit of detection, sample pretreatment requirements, and validation.

Valveless On-Chip Aliquoting for Molecular Diagnosis

The detection of nucleic acids as specific markers of infectious diseases is commonly implemented in molecular biology laboratories. The translation of these benchtop assays to a lab-on-a-chip format demands huge efforts of integration and automation. The present work is motivated by a strong requirement often posed by molecular assays that combine isothermal amplification and CRISPR/Cas-based detection: after amplification, a 2-8 microliter aliquot of the reaction products must be taken for the subsequent reaction. In order to fulfill this technical problem, we have designed and prototyped a microfluidic device that is able to meter and aliquot in the required range during the stepped assay. The operation is achieved by integrating a porous material that retains the desired amount of liquid after removing the excess reaction products, an innovative solution that avoids valving and external actuation. The prototypes were calibrated and experimentally tested to demonstrate the overall performance (general fluidics, metering, aliquoting, mixing and reaction). The proposed aliquoting method is fully compatible with additional functions, such as sample concentration or reagent storage, and could be further employed in alternative applications beyond molecular diagnosis.

Open-source lab hardware: A versatile microfluidic control and sensor platform

Here we describe a completely integrated and customizable microfluidic control and sensing architecture that can be readily implemented for laboratory or portable chemical or biological control and sensing applications. The compact platform enables control of 32 solenoid valves, a multitude of pumps and motors, a thermo-electric controller, a pressure controller, and a high voltage power supply. It also features two temperature probe interfaces, one relative humidity and ambient temperature sensor, two pressure sensors, and interfaces to an electrical conductivity sensor, flow sensor, and a bubble detector. The platform can be controlled via an onboard microcontroller and requires no proprietary software.

Fine-Tuning Electrokinetic Injections Considering Nonlinear Electrokinetic Effects in Insulator-Based Devices

The manner of sample injection is critical in microscale electrokinetic (EK) separations, as the resolution of a separation greatly depends on sample quality and how the sample is introduced into the system. There is a significant wealth of knowledge on the development of EK injection methodologies that range from simple and straightforward approaches to sophisticated schemes. The present study focused on the development of optimized EK sample injection schemes for direct current insulator-based EK (DC-iEK) systems. These are microchannels that contain arrays of insulating structures; the presence of these structures creates a nonuniform electric field distribution when a potential is applied, resulting in enhanced nonlinear EK effects. Recently, it was reported that the nonlinear EK effect of electrophoresis of the second kind plays a major role in particle migration in DC-iEK systems. This study presents a methodology for designing EK sample injection schemes that consider the nonlinear EK effects exerted on the particles being injected. Mathematical modeling with COMSOL Multiphysics was employed to identify proper voltages to be used during the EK injection process. Then, a T-microchannel with insulating posts was employed to experimentally perform EK injection and separate a sample containing two types of similar polystyrene particles. The quality of the EK injections was assessed by comparing the resolution (Rs) and number of plates (N) of the experimental particle separations. The findings of this study establish the importance of considering nonlinear EK effects when planning for successful EK injection schemes.

Development of a Mobile Analytical Chemistry Workstation Using a Silicon Electrochromatography Microchip and Capacitively Coupled Contactless Conductivity Detector

Capillary electrochromatography (CEC) is a separation technique that hybridizes liquid chromatography (LC) and capillary electrophoresis (CE). The selectivity offered by LC stationary phase results in rapid separations, high efficiency, high selectivity, minimal analyte and buffer consumption. Chip-based CE and CEC separation techniques are also gaining interest, as the microchip can provide precise on-chip control over the experiment. Capacitively coupled contactless conductivity detection (C⁴D) offers the contactless electrode configuration, and thus is not in contact with the solutions under investigation. This prevents contamination, so it can be easy to use as well as maintain. This study investigated a chip-based CE/CEC with C⁴D technique, including silicon-based microfluidic device fabrication processes with packaging, design and optimization. It also examined the compatibility of the silicon-based CEC microchip interfaced with C⁴D. In this paper, the authors demonstrated a nanofabrication technique for a novel microchip electrochromatography (MEC) device, whose capability is to be used as a mobile analytical equipment. This research investigated using samples of potassium ions, sodium ions and aspirin (acetylsalicylic acid).

Reduction in sample injection bias using pressure gradients generated on chip

Sample injection in microchip-based capillary zone electrophoresis (CZE) frequently rely on the use of electric fields which can introduce differences in the injected volume for the various analytes depending on their electrophoretic mobilities and molecular diffusivities. While such injection biases may be minimized by employing hydrodynamic flows during the injection process, this approach typically requires excellent dynamic control over the pressure gradients applied within a microfluidic network. The current article describes a microchip device that offers this needed control by generating pressure gradients on-chip via electrokinetic means to minimize the dead volume in the system. In order to realize the desired pressure-generation capability, an electric field was applied across two channel segments of different depths to produce a mismatch in the electroosmotic flow rate at their junction. The resulting pressure-driven flow was then utilized to introduce sample zones into a CZE channel with minimal injection bias. The reported injection strategy allowed the introduction of narrow sample plugs with spatial standard deviations down to about 45 μm. This injection technique was later integrated to a capillary zone electrophoresis process for analyzing amino acid samples yielding separation resolutions of about 4-6 for the analyte peaks in a 3 cm long analysis channel.

Pulsatile reverse flow actuated microfluidic injector: toward the application for single-molecule chemotropism assay

A localized chemical delivery technique to study cellular responses to signaling molecules.

Electroosmotic Flow in a Rough Nanochannel with Surface Roughness Characterized by Fractal Cantor

Molecular dynamics simulation is applied to study the electroosmotic flow in rough nanochannels, with particular attention given to the fluid–solid interactions. In the simulation, the surface roughness is characterized by a fractal Cantor. The roles of roughness height and fractal dimension on nanoscale electroosmotic flow are examined and analyzed. The concentration distributions, zeta potential and electroosmotic velocity are presented and investigated. The results indicate that surface roughness plays a significant role in the fluid–solid interaction and nanoscale electroosmotic flow. The distribution of dipole angle for water molecules in both the near-wall region and middle region is almost unaffected by surface roughness; however, a significant difference of dipole angle distribution is observed in the fluid region away from the wall. Interestingly, the concentration distributions, electroosmotic velocity and zeta potential are highly affected by the surface fractal dimension, even with the same roughness height.

Instrumentation design for hydrodynamic sample injection in microchip electrophoresis: a review

Reproducible and representative sample injection in microchip electrophoresis has been a bottleneck for quantitative analytical applications. Electrokinetic sample injection is the most used because it is easy to perform. However, this injection method is usually affected by sample composition and the bias effect. On the other hand, these drawbacks are overcome by the hydrodynamic (HD) sample injection, although this injection mode requires HD flow control. This review gives an overview of the basic principles, the instrumentation designs, and the performance of HD sample injection systems for microchip electrophoresis.

Integrated electrokinetic sample focusing and surface plasmon resonance imaging system for measuring biomolecular interactions

Label-free biomolecular binding measurement methods, such as surface plasmon resonance (SPR), are becoming increasingly more important for the estimation of real-time binding kinetics. Recent advances in surface plasmon resonance imaging (iSPR) are emerging for label-free microarray-based assay applications, where multiple biomolecular interactions can be measured simultaneously. However, conventional iSPR microarray systems rely on protein printing techniques for ligand immobilization to the gold imaging surface and external pumps for analyte transport. In this article, we present an integrated microfluidics and iSPR platform that uses only electrokinetic transport and guiding of ligands and analytes and, therefore, requires only electrical inputs for sample transport. An important advantage of this new approach, compared to conventional systems, is the ability to direct a single analyte to a specific ligand location in the microarray, which can facilitate analysis parallelization. Additionally, this simple approach does not require complicated microfluidic channel arrangements, external pumps, or valves. As a demonstration, kinetics and affinity have been extracted from measured binding responses of human IgG and goat antihuman IgG using a simple 1:1 model and compared to responses measured with conventional pressure driven analyte transport. The measured results indicate similar binding kinetics and affinity between the electrokinetic and pressure-driven sample manipulation methods and no cross contamination to adjacent measurement locations has been observed.

Nanofluidic technology for biomolecule applications: a critical review

In this review, we present nanofluidic phenomena, particularly as they relate to applications involving analysis of biomolecules within nanofabricated devices. The relevant length scales and physical phenomena that govern biomolecule transport and manipulation within nanofabricated nanofluidic devices are reviewed, the advantages of nanofabricated devices are presented, and relevant applications are cited. Characteristic length scales include the Debye length, the Van der Waals radius, the action distance of hydrogen bonding, the slip length, and macromolecular dimensions. On the basis of the characteristic lengths and related nanofluidic phenomena, a nanofluidic toolbox will be assembled. Nanofluidic phenomena that affect biomolecule behavior within such devices can include ion depletion and enrichment, modified velocity and mobility, permselectivity, steric hindrance, entropy, adsorption, and hydrodynamic interaction. The complex interactions and coupled physics of such phenomena allow for many applications, including biomolecule separation, concentration, reaction/hybridization, sequencing (in the case of DNA) and detection. Examples of devices for such applications will be presented, followed by a discussion of near-term challenges and future thoughts for the field.

On-chip technologies for multidimensional separations

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

A low-leakage sample plug injection scheme for crossform microfluidic capillary electrophoresis devices incorporating a restricted cross-channel intersection

This study develops a crossform CE microfluidic device in which a single-circular barrier or a double-circular barrier is introduced at the cross-channel intersection. Utilizing a conventional crossform injection scheme, it is shown that these barriers reduce sample leakage and deliver a compact sample band into the separation channel, thereby ensuring an enhanced detection performance. A series of numerical and experimental investigations are performed to investigate the effects of the barrier type and the barrier ratio on the flow streamlines within the microchannel and to clarify their respective effects on the sample leakage ratio and sample plug variance during the injection process. The results indicate that a single-circular barrier injector with a barrier ratio greater than 20% and a double-circular barrier injector with a barrier ratio greater than 40% minimize the sample leakage ratio and produce a compact sample plug. As a result, both injectors have an excellent potential for use in high-quality, high-throughput chemical analysis procedures and in many other applications throughout the micro-total analysis systems field.

Chemical cytometry on microfluidic chips

Chemical cytometry, referring to the analysis of the chemical contents in individual cells, has been in intensive study since Kennedy's first work that was published in Science. The early researches relied on fine-tip capillaries to capture the cells and do the analyses, which were lab- and time-intensive and required high skills of operation. The emergence of microfluidics has greatly spurred this research field and a great number of research papers have been published in the last decades. Highly integrated microfluidic chips have been developed to capture multiple single cells, lyse them, perform chemical reactions in enclosed microchambers, separate contents by CE and detect chemical species in individual cells. This review focuses on the development of relevant components and their integration for on-chip chemical cytometry.

Optimization of sample transfer in two-dimensional microfluidic separation systems

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

Rapid and variable-volume sample loading in sieving electrophoresis microchips using negative pressure combined with electrokinetic force

A rapid and variable-volume sample loading scheme for chip-based sieving electrophoresis was developed by negative pressure combined with electrokinetic force. This was achieved by using a low-cost microvacuum pump and a single potential supply at a constant voltage. Both 12% linear polyacrylamide (LPA) with a high viscosity of 15000 cP and 2% hydroxyethylcellulose (HEC) with a low viscosity of 102 cP were chosen as the sieving materials to study the behavior and the versatility of the proposed method. To reduce the hydrodynamic resistance in the sampling channel, sieving material was only filled in the separation channel between the buffer waste reservoir (BW) to the edge of the crossed intersection. By applying a subambient pressure to the headspace of sample waste reservoir (SW), sample and buffer solution were drawn immediately from sample reservoir (S) and buffer reservoir (B) across the intersection to SW. At the same time, the charged sample in the sample flow was driven across the interface between the sample flow and the sieving matrix into the sieving material filled separation channel by the applied electric field. The injected sample plug length is in proportion with the loading time. Once the vacuum in SW reservoir was released to activate electrophoretic separation, flows from S and B to SW were immediately terminated by the back flow induced by the difference of the liquid levels in the reservoirs to prevent sample leakage during the separation stage. The sample consumption was about 1.7 x 10(2) nL at a loading time of 1 s for each cycle. Only 0.024 s was required to transport bias-free analyte to the injection point. It is easy to freely choose the sample plug volume in this method by simply changing the loading time and to inject high quality sample plug with non-distorted shape into the separation channel. The system has been proved to possess an exciting potential for improving throughput, repeatability, sensitivity and separation performance of chip-based sieving electrophoresis.

Design, simulation, and optimization of a miniaturized device for size-fractioned DNA extraction

Fraction collection following electrophoresis has many applications in biology analysis. These assays typically need to identify specific fractions in the separated sample for further processing and require extraction of one or a group of fragments. Unfortunately, the conventional methods involve cumbersome procedures and are not amenable to integration, automation, and extraction of microscale samples. In this work, we present a scheme of chip-based electroelution for the extraction of DNA fragments, which allows the isolation and collection of target DNA fragments in a single device. Both theoretical analysis and experimental results have shown that there are some cross-contaminations from adjacent bands and a little loss of DNA recovery during target DNA extraction with the prototype miniaturized device. In order to improve the purity and yield of the extracted DNA fragment, an optimal channel configuration for DNA recovery miniaturized device was proposed. The simulations and the experimental results are in good agreement and confirm that with the optimized device design a high-efficiency DNA extraction is achieved.

A virtual valve for smooth contamination-free flow switching

We present a channel geometry that allows for clean switching between different inlets of a microchip without any contamination of the inlets or the downstream flow. We drive this virtual valve with a pneumatic pressure setup that minimizes disturbance of the downstream flow during the switching procedure by simultaneous variation of the pressures applied to the different inlets. We assess the efficiency of the setup by spectroscopic measurement of downstream dye concentrations, and demonstrate its practical utility by sequentially constructing multiple layers of alginate hydrogel. The method is potentially useful for a whole series of further applications, such as changing perfusion liquids for cell culture and cell analysis, metering, chemical-reaction initiation and multi-sample chromatography, to name a few.

Micromixer utilizing electrokinetic instability-induced shedding effect

This paper presents a T-shaped micromixer featuring 45 degrees parallelogram barriers (PBs) within the mixing channel. The presented device obtains a rapid mixing of two sample fluids with conductivity ratio of 10:1 (sample concentration:running buffer concentration) by means of the electrokinetic instability-induced shedding effects which are produced when a direct current (DC) electric field of an appropriate intensity is applied. The presented device uses a single high-voltage power source to simultaneously drive and mix the sample fluids. The effectiveness of the mixer is characterized experimentally as a function of the applied electrical field intensity and the extent to which the PBs obstruct the mixing channel. The experimental results indicate that the mixing performance reaches 91% at a cross-section located 2.3 mm downstream of the T-junction when the barriers obstruct 4/5 of the channel width and an electrical field of 300 V/cm is applied. The micromixing method presented in this study provides a simple low-cost solution to mixing problems in lab-on-a-chip systems.

Experimental and numerical investigation into leakage effect in injectors of microfluidic devices

This paper performs an experimental and numerical investigation into low-leakage injectors designed for electrophoresis microchips. The principal material transport mechanisms of electrokinetic migration, fluid flow, and diffusion are considered in developing a mathematical model of the electrophoresis process. Low-leakage injectors designed with injection channels orientated at various included angles are designed and tested. The numerical and experimental results indicate that the injector with a 30 degrees included angle successfully minimizes sample leakage and has an exciting potential for use in high-quality, high-throughput chemical analysis procedures and in many other applications in the field of micro-total analysis systems.

Multiple volume injection technique for high-resolution DNA sample detection utilizing planar microfluidic chip

This paper presents an experimental and numerical investigation into the use of high-resolution injection techniques to separate DNA fragments within electrophoresis microchips. The study addresses the principal material transport mechanisms such as electrokinetic migration, fluid flow, diffusion, variable-volume injection methods, and gives detail analyses to the double-L injection technique, which employs electrokinetic manipulations to avoid sample leakage within the microchip. We also study the sample leakage effect during sample injection and separation. The standard DNA ladder and the phi/-174 DNA fragments are used to test the performance for this proposed method. Results show that this unique injection system in the current microfluidic chip presented within this paper is capable of simulating the functions of the cross, double-T form through appropriate manipulations of the electric field within its various channels. The proposed double-L injection method confirms its ability to reduce sample leakage effect during operation. The integrated microfluidic chip and double-L injection technique developed in this study has an exciting potential for use in high-quality, high-throughput chemical analysis applications and in many other applications throughout the field of micro-total-analysis systems.

Characterization of electrokinetic gating valve in microfluidic channels

Electrokinetic gating, functioning as a micro-valve, has been widely employed in microfluidic chips for sample injection and flow switch. Investigating its valving performance is fundamentally vital for microfluidics and microfluidics-based chemical analysis. In this paper, electrokinetic gating valve in microchannels was evaluated using optical imaging technique. Microflow profiles at channels junction were examined, revealing that molecular diffusion played a significant role in the valving disable; which could cause analyte leakage in sample injection. Due to diffusion, the analyte crossed the interface of the analyte flow and gating flow, and then formed a cometic tail-like diffusion area at channels junction. From theoretical calculation and some experimental evidences, the size of the area was related to the diffusion coefficient and the velocity of analytes. Additionally, molecular diffusion was also believed to be another reason of sampling bias in gated injection.

Counter-flow gradient electrofocusing

Counter-flow gradient electrofocusing techniques are methods whereby a combination of electrophoresis and a bulk solution counter-flow is used to accumulate or focus analytes at stationary points along a separation column. This review first describes the various forms of counter-flow gradient electrofocusing that have been demonstrated in the literature and then compares figures of merit for counter-flow focusing methods and conventional CE methods. In an effort to compare the concentration enhancement of the various focusing techniques against each other, as well as of stacking methods, the parameter of analyte-accumulation velocity is introduced and employed to normalize the efficacy of the techniques.

Application of electrokinetic instability flow for enhanced micromixing in cross-shaped microchannel

This paper proposes a cross-shaped micromixer featuring a pair barrier within the mixing channel. The proposed device obtains a rapid mixing of two sample fluids by means of the electrokinetic instability-induced shedding effects which are produced when a DC electric field of an appropriate intensity is applied. The proposed device uses a single high-voltage power source to simultaneously drive and mix the sample fluids. The effectiveness of the mixer is characterized experimentally as a function of the applied electric field intensity and the extent to which a pair barrier obstruct the mixing channel. The experimental results indicate that the mixing performance reaches 96% at a cross-section located 1 mm downstream of the cross-junction when an electric field of 300 V/cm is applied. The micromixing method presented in this study provides a simple low-cost solution to mixing problems in lab-on-a-chip systems.

A rapid DNA digestion system

This paper presents a novel microfluidic DNA digestion system incorporating a high performance micro-mixer. Through the appropriate control of fixed and periodic switching DC electric fields, electrokinetic forces are established to mix the DNA and restriction enzyme samples and to drive them through the reaction column of the device. The experimental and numerical results show that a mixing performance of 98% can be achieved within a mixing channel of length 1.6 mm when a 150 V/cm driving voltage and a 5 Hz switching frequency are applied. The relationship between the mixing performance, switching frequency, and main applied electric field is derived. It is found that the optimal switching frequency depends upon the magnitude of the main applied electric field. The successful digestion of lambda-DNA using Eco RI restriction enzyme is demonstrated. The DNA-enzyme reaction is completed within 15 min in the proposed microfluidic system, compared to 50 min in a conventional large-scale system. Hence, the current device provides a valuable tool for rapid lambda-DNA digestion, while its mixer system delivers a simple yet effective solution for mixing problems in the micro-total-analysis-systems field.

Fabrication and characterization of semicircular detection electrodes for contactless conductivity detector – CE microchips

This study uses simple and reliable microfabrication techniques to fabricate CE biochips, integrating a novel contactless conductivity detector in a miniaturized detection system in a microfluidic biochip. The off-channel electrodes are deposited around side channels by Au sputtering and patterned using a standard "lift-off" process. A vacuum fusion bonding process is employed to seal the lower substrate containing the microchannels and the electrodes to an upper glass cover plate. The variations in the capacitance between the semicircular detection electrodes in the side channels are measured as different samples and ions pass through the detection region of the CE separation channel. Samples of Rhodamine B, commercial sports drinks, mineral waters, and a red wine, respectively, are mixed in different buffer solutions, separated, and successfully detected using the developed device. The semicircular detection electrodes for the contactless conductivity detector have microscale dimensions and provide a valuable contribution to the realization of the lab-on-a-chip concept.

Improvements on the electrokinetic injection technique for microfluidic chips

This paper presents a T-form electrokinetic injection system for the discrete time-based loading and dispensing of samples of variable-volume in a microfluidic chip. A novel push-pull effect is produced during the loading and dispensing processes by the application of an appropriate control voltage distribution. The experimental and numerical results show that this push-pull loading technique produces compact sample plugs and hence improves the detection resolution of the microfluidic device. The injection system is integrated with a microflow switch, and a suitable voltage control scheme is proposed to guide the sample to the desired outlet port such that the microfluidic device can function as a microdispenser. The time-based variable-volume T-form injection method presented in this study is performed using a compact geometry and a simple control scheme and can be readily integrated with other microfluidic devices to form a microfluidic system capable of continuous monitoring and analysis of bioreactions in the life science and biochemistry fields.

Single potential electrophoresis microchip with reduced bias using pressure pulse injection

We propose two variants of a new injection technique for use in electrophoresis microchips, called "front gate pressure injection" and "back gate pressure injection", that both enable a controlled and reproducible sample introduction with reduced bias compared to electrokinetic gated injection. A continuous flow of a test solution of fluorescein/rhodamine B in 20 mM Tris/boric acid buffer (pH 8.6) sample test solution is electrokinetically driven near to the entrance of the separation channel, using a single voltage (3 kV) that is constant in time. A sample plug is injected in the separation channel by a pressure pulse of the order of 0.1 s. The latter is generated using the mechanical deflection of a PDMS membrane that is loosely placed on a dedicated chip reservoir. The analysis of the peak area ratio of the separated compounds demonstrates a nearly constant sample composition when using pressure-based injection. A small remaining injection bias for the shortest membrane deflection times can be attributed to a dilution effect of the charged compound due to the presence of an electrical field transverse to the sample flow boundary in the channel junction.

Electrokinetic sample transport in a microchannel with spatial electrical conductivity gradients

Studies of the sample transport in a microchannel with the electrical conductivity gradient are critical to develop techniques for on-chip sample transport control. A numerical model presented in this paper, consisting of the electrical potential equation, full Navier-Stokes equation and species conservation equation, is used to simulate sample transport in a microchannel with the consideration of the conductivity gradient. There are two situations studied here, sample pumping (where sample separation is minimized by employing a high-conductivity buffer in the sample region), and sample stacking (where sample separation is expedited by using a low-conductivity buffer as the sample carrier). The effects of applied electrical potential, sample diffusion coefficient and the ratio of conductivity of the driving buffer over the sample carrying buffer are investigated by using the developed model.

New advances in microchip fabrication for electrochromatography

There is a great demand in separation technologies for faster and more effective analysis processes. Miniaturization is a suitable technique for satisfying this demand as reduction in size gives increased separation speed with higher efficiency. CEC is an electric-field-mediated separation technique where the liquid flow is generated by the electric field itself. The main advantage of using electric field over pressure for flow generation is the flat flow profile of the EOF; thus, CEC is one of the best candidates to construct a novel and high-efficiency microanalytical device. The aim of the present paper is to review the basic fabrication and bonding principles, as well as connection and system integration options for microfluidics-based electrochromatography. The physical structure and fluidic channel formation are critically evaluated, including glass microstructuring and fusion bonding. Recent developments in nanoflow measurements and the application of various flow control units are also extensively discussed.

Development of a new hybrid technique for rapid speciation analysis by directly interfacing a microfluidic chip-based capillary electrophoresis system to atomic fluorescence spectrometry

This paper represents the first study on direct interfacing of microfluidic chip-based capillary electrophoresis (chip-CE) to a sensitive and selective detector, atomic fluorescence spectrometry (AFS) for rapid speciation analysis. A volatile species generation technique was employed to convert the analytes from the chip-CE effluent into their respective volatile species. To facilitate the chip-CE effluent delivery and to provide the necessary medium for subsequent volatile species generation, diluted HCl solution was introduced on the chip as the makeup solution. The chip-CE-AFS interface was constructed on the basis of a concentric "tube-in-tube" design for introducing a KBH4 solution around the chip effluent as sheath flow and reductant for volatile species generation as well. The generated volatile species resulting from the reaction of the chip-CE effluent and the sheath flow were separated from the reaction mixture in a gas-liquid separator and swept into the AFS atomizer by an argon flow for AFS determination. Inorganic mercury (Hg(II)) and methylmercury (MeHg(I)) were chosen as the targets to demonstrate the performance of the present technique. Both mercury species were separated as their cysteine complexes within 64 s. The precision (relative standard deviation, RSD, n = 5) of migration time, peak area, and peak height for 2 mg.L(-1) Hg(II) and 4 mg.L(-1) MeHg(I) (as Hg) ranged from 0.7 to 0.9%, 2.1 to 2.9%, and 1.5 to 1.8%, respectively. The detection limit was 53 and 161 microg.L(-1) (as Hg) for Hg(II) and MeHg(I), respectively. The recoveries of the spikes of mercury species in four locally collected water samples ranged from 92 to 108%.

Microautosamplers for discrete sample injection and dispensation

Microfluidic systems show considerable potential for use in the continuous reaction and analysis of biosamples for various applications, such as drug screening and chemical synthesis. Typically, microfluidic chips are externally connected with large-scale autosamplers to inject specific volumes of discrete samples in the continuous monitoring and analysis of multiple samples. This paper presents a novel microelectromechanical system (MEMS)-based autosampler capable of performing the discrete injection and dispensation of variable-volume samples. This microdevice can be integrated with other microfluidic devices to facilitate the continuous monitoring and analysis of multiple biosamples. By means of electroosmotic focusing and switching controlled by the direct application of electric sources on specific fluid reservoirs, a precise sample volume can be injected into the specified outlet port. Fluorescence dye images verify the performance of the developed device. An injection-and-washing scheme is developed to prevent cross-contamination during the continuous injection of different samples. This approach renders feasible the injection of several discrete samples using a single microchip. Compared to its large-scale counterparts, the developed microautosampler is compact in size, has low fabrication costs, is straightforward to control, and most importantly, is readily integrated with other microfluidic devices (e.g., microcapillary electrophoresis chips) to form a microfluidic system capable of the continuous monitoring and analysis of bioreactions. The proposed microautosampler could be promising towards realizing the micrototal analysis system (mu-TAS) concept.

Denaturing gradient-based two-dimensional gene mutation scanning in a polymer microfluidic network

An integrated two-dimensional (2-D) DNA separation platform, combining standard gel electrophoresis with temperature gradient gel electrophoresis (TGGE) on a polymer microfluidic chip, is reported. Rather than sequentially sampling DNA fragments eluted from standard gel electrophoresis, size-resolved fragments are simultaneously electrokinetically transferred into an array of orthogonal microchannels and screened for the presence of sequence heterogeneity by TGGE in a parallel and high throughput format. A bulk heater assembly is designed and employed to externally generate a temporal temperature gradient along an array of TGGE channels. Extensive finite element modeling is performed to determine the optimal geometries of the microfluidic network for minimizing analyte band dispersion caused by interconnected channels in the network. A pH-mediated on-chip analyte stacking strategy is employed prior to the parallel TGGE separations to further reduce additional band broadening acquired during the electrokinetic transfer of DNA fragments between the first and second separation dimensions. A comprehensive 2-D DNA separation is completed in less than 5 min for positive detection of single-nucleotide polymorphisms in multiplex PCR products that vary in size and sequence.

Numerical simulation of electrokinetic injection techniques in capillary electrophoresis microchips

The effective design and control of a capillary electrophoresis (CE) microchip requires a thorough understanding of the electrokinetic transport phenomena associated with its microfluidic injection system. The present study utilizes a numerical simulation approach to investigate these electrokinetic transport processes and to study the control parameters of the injection process. Injection systems with a variety of different configurations are designed and tested, including the cross-form, T-form, double-T-form, variable-volume focused flow cross-form, and variable-volume triple-T-form configuration. Each injection system cycles through a predetermined series of steps in which the magnitudes and distributions of the applied electric field are precisely manipulated in order to effectuate a virtual valve. This study investigates the sample leakage effect associated with each of the injection configurations and applies the double-L, pullback, and focusing injection techniques to minimize the sample leakage effect. The injection methods presented in this paper have the exciting potential for use in high-quality, high-throughput chemical analysis applications and throughout the micro-total-analysis systems field.

High-resolution DNA separation in microcapillary electrophoresis chips utilizing double-L injection techniques

An experimental and numerical investigation into the use of high-resolution injection techniques to separate DNA fragments within electrophoresis microchips is presented. The principal material transport mechanisms of electrokinetic migration, fluid flow, and diffusion are considered, and several variable-volume injection methods are discussed. A detailed analysis is provided of a double-L injection technique, which employs appropriate electrokinetic manipulations to reduce sample leakage within the microchip. The leakage effect in electroosmotic flow (EOF) is investigated using a sample composed of rhodamine B and Cy3 dye. Meanwhile, the effects of sample leakage in capillary electrophoresis (CE) separation are studied by considering the separation of 100-base pairs (bp) DNA ladders and HaeIII-digested PhiX-174 DNA samples. The present experimental and simulation results indicate that the unique injection system employed in the current microfluidic chip has the ability to replicate the functions of both the conventional cross-channel and the shift-channel injection systems. Furthermore, applying the double-L injection method to these two injection systems is shown to reduce sample leakage significantly. The proposed microfluidic chip and double-L injection technique developed in this study have an exciting potential for use in high-resolution, high-throughput biochemical analysis applications and in many other applications throughout the micrototal analysis systems field.