A novel microfluidic mixer utilizing electrokinetic driving forces under low switching frequency

This paper presents a novel technique in which low-frequency periodic electrokinetic driving forces are utilized to mix electrolytic fluid samples rapidly and efficiently in a double-T-form microfluidic mixer. Without using any additional equipment to induce flow perturbations, only a single high-voltage power source is required for simultaneously driving and mixing the sample fluids which results in a simple and low-cost system for the mixing purpose. The effectiveness of the mixer as a function of the applied electric field and the periodic switching frequency is characterized by the intensity distribution calculated downstream from the mixing zone. The present numerical and experimental results confirm that the proposed double-T-form micromixer has excellent mixing capabilities. The mixing efficiency can be as high as 95% within a mixing length of 1000 microm downstream from the secondary T-junction when a 100 V/cm driving electric field strength and a 2 Hz periodic switching frequency are applied. The results reveal that the optimal switching frequency depends upon the magnitude of the main applied electrical field. The rapid double-T-form microfluidic mixer using the periodic driving voltage switching model proposed in this study has considerable potential for use in lab-on-a-chip systems.     

A miniaturized high-voltage integrated power supply for portable microfluidic applications

In this work a portable microfluidic device with a reusable integrated high voltage power supply is presented, which allows for quick exchange of inexpensive disposable poly(dimethylsiloxane)(PDMS) microfluidic chips on a carrier only slightly larger than a microscope slide. The device is powered by an onboard MN21 cell battery (5 mm radius, 30 mm long) and is demonstrated through the rapid and controlled transport of a fluorescent dye through an expansion chamber geometry. Power consumption experiments demonstrate the device's ability to complete over 40 dispense-flushing cycles on a single battery.     

Radial sample preconcentration

In this paper we present a radial sample preconcentration strategy enabled by axisymmetric concentration polarization in a microfluidic chamber on a uniform nanoporous film. Sample analytes are focused into the centre, creating a concentrated plug that is injected vertically into the microfluidic analysis layer. No balancing pressure driven flows or tangential fields are required, and the process has essentially zero footprint on the analysis layer. An electrokinetic loading scheme enables repeat loading/concentration cycles, and a finned radial chamber geometry dampens instabilities and accommodates larger volumes. Modelling results indicate over 1800-fold concentration increases are possible in 10 s, for high mobility buffers and high applied field strength. At moderate field strength and buffer mobility, experiments demonstrate a 168-fold increase in concentration of FITC-BSA protein in 36 s.     

Numerical study of a novel induced-charge electrokinetic micro-mixer

A novel micro-mixer based on the induced-charge electrokinetic motion of an electrically conducting particle is proposed and numerically demonstrated in this paper. For most microfluidic applications, it is desired to mix different streams of solutions rapidly in a continuous flow mode. Therefore, in this work, we consider a mixing chamber containing an electrically conducting particle and the mixing chamber is located in the middle of a microchannel. Vortices are generated around the electrically conducting particle in an aqueous solution due to the interaction of the applied electric field and the induced surface charge on the particle. These vortices will enhance significantly the mixing of different solutions around the particle. The effectiveness of mixing the two streams entering the mixing chamber is numerically studied as functions of the applied electric field. Excellent mixing can be achieved in this system under two perpendicularly applied electric fields. The proposed micro-mixer is simple and easy to be fabricated for lab-on-a-chip applications.     

Multiple injection techniques for microfluidic sample handling

This paper presents an experimental and numerical investigation into electrokinetic focusing flow injection for bioanalytical applications on 1 x N (i.e., 1 sample inlet port and N outlet ports) and M x N (i.e., M sample inlet ports and N outlet ports) microfluidic chips. A novel device is presented which integrates two important microfluidic phenomena, namely electrokinetic focusing and valveless flow switching within multiported microchannels. The study proposes a voltage control model which achieves electrokinetic focusing in a prefocusing sample injection system and which allows the volume of the sample to be controlled. Using the developed methods, the study shows how the sample may be prefocused electrokinetically into a narrow stream prior to being injected continuously into specified outlet ports. The microfluidic chips presented within this paper possess an exciting potential for use in a variety of techniques, including high-throughput chemical analysis, cell fusion, fraction collection, fast sample mixing, and many other applications within the micrototalanalysis systems field.     

Performance of experimental sample injectors for high-performance liquid chromatography microcolumns

An experimental injector for HPLC microcolumns and a 3-nl conductivity detector connected directly to the injector outlet with a 19-nl tube were used to study injector dispersion, guide the design of improved injectors, and suggest appropriate injection techniques. With regard to the small injection volumes required when no on-column concentration technique is used, we show that in some circumstances: (i) there are two volumes to be considered, the sample volume (that which is intended to be injected) and the effective injection volume (that which contains all the sample after it has completely emerged from the injector). Due to dispersion, the latter is often many times the former. An injector performance factor is defined as the ratio of the two volumes. (ii) A smaller sample chamber volume in an injector does not necessarily produce a proportionately smaller effective injection volume, in which case there is a reduction of peak height that degrades sensitivity without a commensurate reduction in peak width that would improve resolution. (iii) Adjusting the geometry of the sample chamber and stator passage can significantly improve injector performance, as illustrated for sample volumes from 2 nl to 1 microl. (iv) In some cases, reducing the diameter of an injector passageway in an attempt to reduce dispersion actually causes performance to worsen.     

Trapping of DNA by dielectrophoresis

Under suitable conditions, a DNA molecule in solution will develop a strong electric dipole moment. This induced dipole allows the molecule to be manipulated with field gradients, in a phenomenon known as dielectrophoresis (DEP). Pure dielectrophoretic motion of DNA requires alternate current (AC) electric fields to suppress the electrophoretic effect of the molecules net charge. In this paper, we present two methods for measuring the efficiency of DEP for trapping DNA molecules as well as a set of quantitative measurements of the effects of strand length, buffer composition, and frequency of the applied electric field. A simple configuration of electrodes in combination with a microfluidic flow chamber is shown to increase the concentration of DNA in solution by at least 60-fold. These results should prove useful in designing practical microfluidic devices employing this phenomenon either for separation or concentration of DNA.     

Rapid concentration of deoxyribonucleic acid via Joule heating induced temperature gradient focusing in poly-dimethylsiloxane microfluidic channel

This paper reports rapid microfluidic electrokinetic concentration of deoxyribonucleic acid (DNA) with the Joule heating induced temperature gradient focusing (TGF) by using our proposed combined AC and DC electric field technique. A peak of 480-fold concentration enhancement of DNA sample is achieved within 40 s in a simple poly-dimethylsiloxane (PDMS) microfluidic channel of a sudden expansion in cross-section. Compared to a sole DC field, the introduction of an AC field can reduce DC field induced back-pressure and produce sufficient Joule heating effects, resulting in higher concentration enhancement. Within such microfluidic channel structure, negative charged DNA analytes can be concentrated at a location where the DNA electrophoretic motion is balanced with the bulk flow driven by DC electroosmosis under an appropriate temperature gradient field. A numerical model accounting for a combined AC and DC field and back-pressure driven flow effects is developed to describe the complex Joule heating induced TGF processes. The experimental observation of DNA concentration phenomena can be explained by the numerical model.     

Improvement of size-based particle separation throughput in slanted spiral microchannel by modifying outlet geometry

The inertial microfluidic technique, as a powerful new tool for accurate cell/particle separation based on the hydrodynamic phenomenon, has drawn considerable interest in recent years. Despite numerous microfluidic techniques of particle separation, there are few articles in the literature on separation techniques addressing external outlet geometry to increase the throughput efficiency and purity. In this work, we report on a spiral inertial microfluidic device with high efficiency (>98%). Herein, we demonstrate how changing the outlet geometry can improve the particle separation throughput. We present a complete separation of 4 and 6 μm from 10 μm particles potentially applicable to separate microalgae (Tetraselmis suecica from Phaeodactylum tricornutum). Two spiral microchannels with the same cross section dimension but different outlet geometry were considered and tested to investigate the particle focusing behavior and separation efficiency. As compared with particle focusing observed in channels with a simple outlet, the particle focusing in a modified outlet geometry appears in a more successful focusing manner with complete separation. This simple approach of particle separation makes it attractive for lab-on-a-chip devices for continuous extraction and filtration of a wide range of cell/particle sizes.     

Dynamic monitoring of glucagon secretion from living cells on a microfluidic chip

A rapid microfluidic based capillary electrophoresis immunoassay (CEIA) was developed for on-line monitoring of glucagon secretion from pancreatic islets of Langerhans. In the device, a cell chamber containing living islets was perfused with buffers containing either high or low glucose concentration. Perfusate was continuously sampled by electroosmosis through a separate channel on the chip. The perfusate was mixed on-line with fluorescein isothiocyanate-labeled glucagon (FITC-glucagon) and monoclonal anti-glucagon antibody. To minimize sample dilution, the on-chip mixing ratio of sampled perfusate to reagents was maximized by allowing reagents to only be added by diffusion. Every 6 s, the reaction mixture was injected onto a 1.5-cm separation channel where free FITC-glucagon and the FITC-glucagon–antibody complex were separated under an electric field of 700 V cm⁻¹. The immunoassay had a detection limit of 1 nM. Groups of islets were quantitatively monitored for changes in glucagon secretion as the glucose concentration was decreased from 15 to 1 mM in the perfusate revealing a pulse of glucagon secretion during a step change. The highly automated system should be enable studies of the regulation of glucagon and its potential role in diabetes and obesity. The method also further demonstrates the potential of rapid CEIA on microfluidic systems for monitoring cellular function.     

Design of passive mixers utilizing microfluidic self-circulation in the mixing chamber

This paper proposes the design of a passive micromixer that utilizes the self-circulation of the fluid in the mixing chamber for applications in the Micro Total Analysis Systems (microTAS). The micromixer with a total volume of about 20 microL and consisting of an inlet port, a circular mixing chamber and an outlet port was designed. The device was actuated by a pneumatic pump to induce self-circulation of the fluid. The self-circulation phenomenon in the micromixer was predicted by the computational simulation of the microfluidic dynamics. Flow visualization with fluorescence tracer was used to verify the numerical simulations and indicated that the simulated and the experimental results were in good agreement. Besides, an index for quantifying the mixing performance was employed to compare different situations and to demonstrate the advantages of the self-circulation mixer. The mixing efficiencies in the mixer under different Reynolds numbers (Re) were evaluated numerically. The numerical results revealed that the mixing efficiency of the mixer with self-circulation was 1.7 to 2 times higher than that of the straight channel without a mixing chamber at Re= 150. When Re was as low as 50, the mixing efficiency of the mixer with self-circulation in the mixing chamber was improved approximately 30% higher than that in the straight channel. The results indicated that the self-circulation in the mixer could enhance the mixing even at low Re. The features of simple mixing method and fabrication process make this micromixer ideally suitable for microTAS applications.     

Single-cell analysis by electrochemical detection with a microfluidic device

A novel electrochemical method with a microfluidic device was developed for analysis of single cells. In this method, cell injection, loading and cell lysis, and electrokinetic transportation and detection of intracellular species were integrated in a microfluidic chip with a double-T injector coupled with an end-channel amperometric detector. A single cell was loaded at the double-T injector on the microfluidic chip by using electric field. Then, the docked cell was lysed by a direct current electric field strength of 220 V/cm. The analyte of interest inside the cell was electrokinetically transported to the detection end of separation channel and was electrochemically detected. External standardization was used to quantify the analyte of interest in individual cells. Ascorbic acid (AA) in single wheat callus cells was chosen as the model compound. AA could be directly detected at a carbon fiber disk bundle electrode. The selectivity of electrochemical detection made the electropherogram simple. The technique described here could, in principle, be applied to a variety of electroactive species within single cells.     

微流控芯片电泳/电化学检测人单个肝癌细胞中的谷胱甘肽

采用微流控装置结合电化学检测研究了测定人单个血红细胞中谷胱甘肽(GSH)的方法。在该方法中,细胞的进样、定位、溶膜以及细胞中谷胱甘肽的转移和检测都在配有通道端安培检测器的双T形芯片中完成。单个细胞用液压导入到双T的交界面,在电泳缓冲液中毛地黄皂苷的作用下,细胞膜被穿孔。再施加直流电压,细胞被溶膜。释放出来的GSH被此直流电压电迁移至通道端并在Au/Hg电极上被检测。用校正曲线法可以定量测定单个细胞中的GSH。     

Novel electroosmotic micromixer configuration based on ion-selective microsphere

In this paper, a micromixer of a new configuration is presented, consisting of a spherical chamber in the center of which an ion-selective microsphere is placed. Stratified liquid is introduced through the chamber via inlet and outlet holes under an external pressure gradient and an external electric field is directed in such a way that the resulting electroosmotic flow is directed against the pressure-driven flow, resulting in mixing. The investigation is carried out by direct numerical simulation on a super-computer. Optimal values of the applied electric field are determined to yield strong mixing. Above this optimal mixing regime, a number of instabilities and bifurcations are realized, which qualitatively coincide with those occurring during electrophoresis of an ion-selective microgranule. As shown by our calculation, these instabilities do not lead to an enhanced mixing. The resulting electroconvective vortices remain confined near the surface of the microgranule, and do not sufficiently perturb the stratified fluid flow further from the granule. On the other hand, another type of instability caused by the salt concentration gradient can generate sufficiently strong oscillations to enhance mixing. However, this only occurs when the external electric field is sufficiently high that the electroosmotic flow is comparable to the pressure-driven flow. This ultimately leads to creation of reverse flows of the liquid and cessation of the device operation. Thus, it was shown that the best mixing occurs in the absence of electrokinetic instability. Based on the data obtained, it is possible to select the necessary geometric characteristics of the micromixer to achieve the optimal mixing mode for a given set of liquids, which may be ten times more effective than passive mixers at the same flow rates. A comparison with the experimental data of the other authors confirms the effectiveness of this device and its other capabilities. Furthermore, the basic device design can be operated in other modes, for example, an electrohydrodynamic pump, a streaming current generator, or even a micro-reactor, depending on the system parameters and choice of an ion-selective granule.     

High electric field strength two-dimensional peptide separations using a microfluidic device

New instrumentation has been developed to improve the resolution, efficiency, and speed of microfluidic 2D separations using MEKC coupled to high field strength CE. Previously published 2D separation instrumentation [Ramsey, J. D. et al., Anal. Chem. 2003, 75, 3758-3764] from our group was limited to a maximum potential difference of 8.4 kV, resulting in an electric field strength of only approximately 200 V/cm in the first dimension. The circuit described in this report has been designed to couple a higher voltage supply with a rapidly switching, lower voltage supply to utilize the best features of each. Voltages applied in excess of 20 kV lead to high electric field strength separations in both dimensions, increasing the separation resolution, efficiency, and peak capacity while reducing the required analysis time. Detection rates as high as six peptides per second (based on total analysis time) were observed for a model protein tryptic digest separation. Additionally, higher applied voltages used in conjunction with microfluidic chips with longer length channels maintained higher electric field strengths and produced peak capacities of over 4000 for some separations. Total separation time in these longer channel devices was comparable to that obtained in short channels at low field strength; however, resolving power improved approximately threefold.     

A simple, disposable microfluidic device for rapid protein concentration and purification via direct-printing

A facile and disposable microfluidic device for rapid protein concentration was fabricated by using a direct printing process. Two printed V-shaped microchannels in mirror image orientation were separated by a 100 mum wide toner gap. When a high electric field was applied across the two channels, nanofissures were formed by electric breakdown at the junction toner gap. This microfluidic device with nanofissures was used as a concentrator for protein. Negatively charged proteins were observed to concentrate at the anode side of the nanofissures upon application of an electric field across this junction. Using this device, about 10(3)-10(5)-fold protein concentration was achieved within 10 min. Systematic investigation showed that the concentration mechanism could be explained by the ion exclusion-enrichment effect of the nanofissures. In addition, the present microchip device integrated both functions of concentration and purification were confirmed. This simple on chip protein preconcentration and purification device could be a disposable sample preparation component in printed microfluidic systems used for practical biochemical assays.     

Continuous dielectrophoretic bacterial separation and concentration from physiological media of high conductivity

Biological sample processing involves purifying target analytes from various sample matrices and concentrating them to a small volume from a large volume of crude sample. This complex process is the major obstacle for developing a microfluidic diagnostic platform. In this study, we present a microfluidic device that can continuously separate and concentrate pathogenic bacterial cells from complex sample matrices such as cerebrospinal fluid and whole blood. Having overcome critical limitations of dielectrophoretic (DEP) operation in physiological media of high conductivity, we utilized target specific DEP techniques to incorporate cell separation, medium exchange, and target concentration into an integrated platform. The proposed microfluidic device can uptake mL volumes of crude biological sample and selectively concentrate target cells into a submicrolitre volume, providing ~10(4) fold of concentration. We designed the device based on the electrokinetic theory and electric field simulation, and tested the device performance with different sample types. The separation efficiency of the device was as high as 97.0% for a bead mixture in TAE buffer and 94.3% and 87.2% for E. coli in human cerebrospinal fluid and blood, respectively. A capture efficiency of 100% was achieved in the concentration chamber. With a relatively simple configuration, the proposed device provides a robust method of continuous sample processing, which can be readily integrated into a fully automated microfluidic diagnostic platform for pathogen detection and quantification.     

Fabrication of antigen-containing nanoparticles using microfluidics with Tesla structure

The polyethyleneimine (PEI)-based antigen delivery system has been proved valuable for therapeutic vaccines. However, the previous bulk mixing method is not ideal to prepare PEI-base antigen- containing nanoparticles. The wide-size distribution and poor reproducibility limit its further application. In this research, we developed a microfluidic method to prepare nanopolyplexes on chips with Tesla structure to improve the fabrication. The structure and bioactivity of protein were not hampered by the shearing force in microfluidics. Comparison between physiochemical parameters suggested that the polyplexes prepared by Tesla chips were more uniform than those prepared by bulk mixing and non-Tesla chips. The reproducibility was improved obviously. The preparation was not influence much by the operating parameters such as flow rates, reagents concentration, and switching of inlets. The results indicated that it was a robust and reliable method. The data that were obtained from BMDC model demonstrated that the nanopolyplexes with optimal weight ratio had higher antigen cross-presentation efficiency than those in free antigen group. In conclusion, Tesla structured microfluidics offers a better method over bulk mixing in the preparation of PEI-based antigen-containing nanopolyplexes. It greatly expands the scope of our study and increases the potential of PEI-based antigen delivery system.     

Control of flow rate and concentration in microchannel branches by induced-charge electrokinetic flow

This paper presents a numerical study of controlling the flow rate and the concentration in a microchannel network by utilizing induced-charge electrokinetic flow (ICEKF). ICEKF over an electrically conducting surface in a microchannel will generate vortices, which can be used to adjust the flow rates and the concentrations in different microchannel branches. The flow field and concentration field were studied under different applied electric fields and with different sizes of the conducting surfaces. The results show that, by using appropriate size of the conducting surfaces in appropriate locations, the microfluidic system can generate not only streams of the same flow rate or linearly decreased flow rates in different channels, but also different, uniform concentrations within a short mixing length quickly.     

Microfluidic distillation chip for methanol concentration detection

An integrated microfluidic distillation system is proposed for separating a mixed ethanol-methanol-water solution into its constituent components. The microfluidic chip is fabricated using a CO₂ laser system and comprises a serpentine channel, a boiling zone, a heating zone, and a cooled collection chamber filled with de-ionized (DI) water. In the proposed device, the ethanol-methanol-water solution is injected into the microfluidic chip and driven through the serpentine channel and into the collection chamber by means of a nitrogen carrier gas. Following the distillation process, the ethanol-methanol vapor flows into the collection chamber and condenses into the DI water. The resulting solution is removed from the collection tank and reacted with a mixed indicator. Finally, the methanol concentration is inversely derived from the absorbance measurements obtained using a spectrophotometer. The experimental results show the proposed microfluidic system achieves an average methanol distillation efficiency of 97%. The practicality of the proposed device is demonstrated by detecting the methanol concentrations of two commercial fruit wines. It is shown that the measured concentration values deviate by no more than 3% from those obtained using a conventional bench top system.