A simple mechanism for reliable particle sorting in a microdevice with combined electroosmotic and pressure-driven flow

Selective transport and sorting of particles in microfluidic devices by electroosmosis is complicated due to superposition of uncontrolled hydrodynamic pressure contributions on the electroosmotic force. In this paper, we present a microfluidic concept for the reliable and simple separation and sorting of particles in a microchip by electroosmosis combined with pressure-driven flow. The presented device allows fluid quantities to be switched and particles to be sorted within a channel manifold using only a single power supply with fixed voltage and an electric switch. Consequently, chip operation and fluid switching procedure are greatly simplified compared to a situation, in which several independent power sources are used for flow balancing, as is the common procedure. With the triple-T channel design presented, backpressure flow disturbing the electrokinetic fluid and particle separation process is eliminated by introducing controlled opposed hydrodynamic flow of buffer from side channels. This pressure-driven flow is generated on-chip by setting up differences in the reservoir pressures in a defined manner. A detailed flow analysis based on the equivalence of fluid flow and electric current is performed and the conditions for reliable chip function are worked out.     

Study of the electroosmotic flow as a means to propel the mobile phase in capillary electrochromatography in view of further miniaturization of capillary electrochromatography systems

In this paper, we investigate the phenomenon of electroosmosis as a means to propel a mobile phase, in particular in view of an application in microfluidic systems, which are characterized by significantly smaller volumes of the reservoirs and the separation channels compared to conventional instrumentation. In the microfluidic chip, pH changes due to water electrolysis quickly showed an effect on the electroosmotic flow (EOF), which could be counteracted by either regularly exchanging or buffering the mobile phase. Surface treatment was of no effect in regard to EOF stabilization in empty channels but may have an influence in channels filled with a charged monolith. In fused-silica capillaries the EOF was generally found to decrease from 'naked' to surface-treated to monolith-filled capillaries. The EOF tended to be higher when an organic solvent (acetonitrile) was added to the mobile phase and could be further increased by substituting the water with equal amounts of methanol. In addition, the hydrostatic pressure exerted by the EOF was investigated. In a microfluidic chip with empty (cross-)channels such an effect could be responsible for a redirection of the flow. In capillaries partially filled with a noncharged (non-EOF-generating) monolith, a linear relationship could be established between the EOF created in the empty section of the capillary (apparent mobility) and the length of the monolith (backpressure). In capillaries partially filled with a charged (EOF-producing) monolith, flow inhomogeneities must be expected as a consequence of a superimposition of hydrodynamic pressure and EOF as mobile phase driving force.     

Single channel layer, single sheath-flow inlet microfluidic flow cytometer with three-dimensional hydrodynamic focusing

Flow cytometry is a technique capable of optically characterizing biological particles in a high-throughput manner. In flow cytometry, three dimensional (3D) hydrodynamic focusing is critical for accurate and consistent measurements. Due to the advantages of microfluidic techniques, a number of microfluidic flow cytometers with 3D hydrodynamic focusing have been developed in recent decades. However, the existing devices consist of multiple layers of microfluidic channels and tedious fluidic interconnections. As a result, these devices often require complicated fabrication and professional operation. Consequently, the development of a robust and reliable microfluidic flow cytometer for practical biological applications is desired. This paper develops a microfluidic device with a single channel layer and single sheath-flow inlet capable of achieving 3D hydrodynamic focusing for flow cytometry. The sheath-flow stream is introduced perpendicular to the microfluidic channel to encircle the sample flow. In this paper, the flow fields are simulated using a computational fluidic dynamic (CFD) software, and the results show that the 3D hydrodynamic focusing can be successfully formed in the designed microfluidic device under proper flow conditions. The developed device is further characterized experimentally. First, confocal microscopy is exploited to investigate the flow fields. The resultant Z-stack confocal images show the cross-sectional view of 3D hydrodynamic with flow conditions that agree with the simulated ones. Furthermore, the flow cytometric detections of fluorescence beads are performed using the developed device with various flow rate combinations. The measurement results demonstrate that the device can achieve great detection performances, which are comparable to the conventional flow cytometer. In addition, the enumeration of fluorescence-labelled cells is also performed to show its practicality for biological applications. Consequently, the microfluidic flow cytometer developed in this paper provides a practical platform that can be used for routine analysis in biological laboratories. Additionally, the 3D hydrodynamic focusing channel design can also be applied to various applications that can advance the lab on a chip research.     

Fluidic low pass filter for hydrodynamic flow stabilization in microfluidic environments

Fluctuations in flow rate invariably occur in microfluidic devices. This fluidic instability results in a deteriorating performance and the suspension of their unique functions occasionally. In this study, a fluidic-LPF (low pass filter), which is composed of an ACU (air compliance unit) and a FCSP (fluidic channel with high fluidic resistance for sufficient preload), has been proposed for providing the stabilization of hydrodynamic flow in microfluidic devices. To investigate the characteristics of various fluidic networks including our fluidic-LPF, we used a parametric identification method to estimate the time constants via a transient response that was based on a discrete parameter model. In addition, we propose the use of a pulsation index (PI) to quantify the fluctuations in flow rate. We verified the formula for PI derived herein by varying individually both the periods and the air compliance volumes in the ACU, both theoretically and experimentally. We found that the PI depended strongly on either the time constants or the periods of the flow rates at the inlet. Additionally, the normalized differences between the experimental results and the theoretical estimations were less than 6%, which shows that the proposed formula for PI can provide an accurate quantification of the fluctuations in flow, and estimate the parametric effects. Finally, we have successfully demonstrated that our fluidic-LPF can regulate fluctuations in the flow at extremely low flow rates (~ 10 μL h(-1)) and can also control severe fluidic fluctuations (PI = 0.67) with excessively long periods (100 s) via a microfluidic viscometer. We therefore believe that the stabilization of hydrodynamic flow using a fluidic-LPF could be used easily and extensively with a range of microfluidic platforms that require constant flow rates.     

Dual opposite injection electrokinetic chromatography: nonionic microemulsion pseudostationary phase and novel approach to electrokinetic sampling bias

Dual opposite injection capillary electrophoresis (DOI-CE) is a family of CE techniques in which the sample is introduced into both ends of the capillary. For the analysis of compounds with widely varying pKa values using a voltage-driven separation scheme, DOI-CE is superior to conventional CE with sample introduction at only one end of the capillary due to DOI-CE's broader elution window. To enhance the DOI-CE separation, a running buffer with a microemulsion system was developed. Since DOI-CE works best under conditions of low electroosmotic flow (EOF), the suppression of EOF via the addition of a multiply charged cation (e.g., Zn2+) to the buffer was investigated, and was found to suppress the EOF effectively at moderate concentrations (2.5-10 mM). Three different dual opposite injection modes were studied: simultaneous electrokinetic injection, sequential electrokinetic injection, and sequential hydrodynamic injection. The injection bias in the first two electrokinetic injection modes was compared with the sequential hydrodynamic injection. Corrections in the bias of the electrokinetic injections were discussed, and an improved approach was suggested. Finally, the effect of the relative concentration of the multiply charged cation in the sample plug and running buffer on the peak shape of co-electroosmotic and counter-electroosmotic ions was examined, and found to be much more influential on the latter.     

Mapping vortex-like hydrodynamic flow in microfluidic networks using fluorescence correlation spectroscopy

The ability to quickly measure flow parameters in microfluidic devices is critical for micro total analysis system (μTAS) applications. Macrofluidic methods to assess flow suffer from limitations that have made conventional methods unsuitable for the flow behavior profiling. Single molecule fluorescence correlation spectroscopy (FCS) has been employed in our study to characterize the fluidic vortex generating at a T-shape junction of microscale channels. Due to its high spatial and temporal resolution, the corresponding magnitudes relative to different flow rates in the main channel can be quantitatively differentiated using flow time (τ_F) measurements of dye molecules traversing the detection volume in buffer solution. Despite the parabolic flow in the channel upstream, a heterogeneous distribution of flow has been detected across the channel intersection. In addition, our current observations also confirmed the aspect of vortex-shaped flow in low-shear design that was developed previously for cell culture. This approach not only overcomes many technical barriers for examining hydrodynamic vortices and movements in miniature structures without physically integrating any probes, but it is also especially useful for the hydrodynamic studies in polymer-glass based micro -reactor and -mixer.     

A capillary electrophoresis chip with hydrodynamic sample injection for measurements from a continuous sample flow

A microchip-based capillary electrophoresis device supported by a microfluidic network made of poly(dimethylsiloxane), used for measuring target analytes from a continuous sample flow, is presented. The microsystem was fabricated by means of replica molding in combination with standard microfabrication technologies, resulting in microfluidic components and an electrochemical detector. A new hydrodynamic sample injection procedure is introduced, and the maximum number of consecutive measurements that can be made with a poly(dimethylsiloxane) capillary electrophoresis chip with amperometric detection is investigated with respect to reproducibility. The device features a high degree of functional integration, so the benefits associated with miniaturized analysis systems apply to it.     

The influence of surface treatments on the electroosmotic flow in micellar electrokinetic capillary chromatography

Summary An investigation was carried out to study the electroosmotic flow (EOF) in micellar electrokinetic capillary chromatography (MECC). Fused silica capillaries were treated with various reagents to investigate the parameters influencing the magnitude and reproducibility of the EOF. The results for untreated and treated columns were collected and compared under the same experimental conditions. Furthermore, experiments with and without sodium dodecylsulfate (SDS) addition to the buffer were carried out to elucidate the influence of this surfactant on the EOF. The mechanisms behind the effects of these modifications on the EOF and the stability of the coatings on the column wall are discussed.     

Towards high concentration enhancement of microfluidic temperature gradient focusing of sample solutes using combined AC and DC field induced Joule heating

It is challenging to continuously concentrate sample solutes in microfluidic channels. We present an improved electrokinetic technique for enhancing microfluidic temperature gradient focusing (TGF) of sample solutes using combined AC and DC field induced Joule heating effects. The introduction of an AC electric field component services dual functions: one is to produce Joule heat for generating temperature gradient; the other is to suppress electroosmotic flow. Consequently the required DC voltages for achieving sample concentration by Joule heating induced TGF are reduced, thereby leading to smaller electroosmotic flow (EOF) and thus backpressure effects. As a demonstration, the proposed technique can lead to concentration enhancement of sample solutes of more than 2500-fold, which is much higher than the existing literature reported microfluidic concentration enhancement by utilizing the Joule heating induced TGF technique.     

Three-dimensional (3D) hydrodynamic focusing for continuous sampling and analysis of adherent cells

A simple three-dimensional (3D) hydrodynamic focusing microfluidic device integrated with continuous sampling, rapid dynamic lysis, capillary electrophoretic (CE) separation and detection of intracellular content is presented. One of the major difficulties in microfluidic cell analysis for adherent cells is that the cells are prone to attaching to the channel surface. To solve this problem, a cross microfluidic chip with three sheath-flow channels located on both sides of and below the sampling channel was developed. With the three sheath flows around the sample solution-containing cells, the formed soft fluid wall prevents the cells from adhering to the channel surface. Labeled cells were 3D hydrodynamically focused by the sheath-flow streams and smoothly introduced into the cross-section one by one. The introduction of sheath-flow streams not only ensured single-cell sampling but avoided blockage of the sampling channel by adherent cells as well. The maximum rate for introduction of individual cells into the separation channel was about 151 cells min(-1). With electric field applied on the separation channel, the aligned cells were driven into the separation channel and rapidly lysed within 400 ms at the entry of the channel by sodium dodecylsulfate (SDS) added in the sheath-flow solution. The microfluidic system was evaluated by analysis of reduced glutathione (GSH) and reactive oxygen species (ROS) in single HepG2 cells. The average analysis throughput of ROS and GSH in single cells was 16-18 cells min(-1).     

The effect of electroosmotic and hydrodynamic flow profile superposition on band broadening in capillary electrophoresis

The effect of the superposition of electroosmotic flow and pressureinduced hydrodynamic counterflow on efficiency has been investigated for different capillary electrophoretic systems. Results are shown for 50 and 75 μm internal diameter capillaries at several voltage and counterpressure levels. Hydrodynamic counterflows were successfully applied in electrokinetic chromatography in order to delay the entry of a UV-active pseudostationary phase, tetraphenyl porphyrintetrasulfonate, into the detection zone allowing the separation of neutral nitroaromatics. The separations are based on the weak charge-transfer interactions between the porphyrin and the analytes.     

Capillary zone electrophoresis with electroosmotic flow controlled by external radial electric field.

A new way of regulation of electroosmotic flow (EOF) in capillary zone electrophoresis (CZE) by external electric field has been developed. A set of three high-voltage power supplies is used to form a radial electric field across the capillary wall. One power supply is applied in the usual way as a driving force of CZE and EOF to the ends of the inner capillary compartment dipped into the electrode vessels and filled with background electrolyte. Two power supplies are connected to the ends of the outer low-conductivity coating of the capillary which is formed by the dispersion of copolymer of aniline and p-phenylenediamine in polystyrene matrix. The difference between electric potentials on the outer capillary surface and inside the capillary determines the voltage of radial electric field across the capillary wall and affects the electrokinetic potential at the solid-liquid interface inside the capillary. The effect of magnitude and polarity of external radial electric field on the flow rate of EOF, on the migration times of charged analytes and on the separation efficiency and resolution of CZE separations of synthetic oligopeptides, diglycine, triglycine and octapeptide fragments of human insulin was evaluated. Through the EOF control by external electric field the dynamic effective length of the capillary was obtained and the speed of analysis and resolution of CZE separations of peptide analytes could be optimized.     

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.     

Hydrodynamic flow and electroosmotic flow in zirconia-packed capillaries

Fused-silica capillaries were packed with Zirchrom-PBD stationary phase for application in CEC, nanoLC and pseudoelectrochromatography (PEC). Acido-basic properties of zirconia can be used to control the EOF even if the zirconia particles were coated by polybutadiene. As for native zirconia, the EOF is pH-dependent and the pI is close to pH 5. The mixed-mode pressure-voltage technique induced a modulation of the mobile-phase velocity as well as an electrophoretic migration of the solutes in order to improve the resolution of the separation. A significant increase of the flow appeared when both hydrodynamic and EOFs were in the same direction. But an important reduction of the electroosmotic velocity was observed when the hydrodynamic flow and EOF were opposed in Zirchrom-PBD columns. This behaviour has been observed at high or low pH on several columns. Separations of neutral and charged compounds have been performed with these columns in PEC mode.     

Numerical analysis of the thermal effect on electroosmotic flow and electrokinetic mass transport in microchannels

Joule heating is present in electrokinetically driven flow and mass transport in microfluidic systems. Nowadays, there is a trend of replacing costly glass-based microfluidic systems by the disposable, cheap polymer-based microfluidic systems. Due to poor thermal conductivity of polymer materials, the thermal management of the polymer-based microfluidic systems may become a problem. In this study, numerical analysis is presented for transient temperature development due to Joule heating and its effect on the electroosmotic flow (EOF) and mass species transport in microchannels. The proposed model includes the coupling Poisson-Boltzmann (P-B) equation, the modified Navier-Stokes (N-S) equations, the conjugate energy equation, and the mass species transport equation. The results show that the time development for both the electroosmotic flow field and the Joule heating induced temperature field are less than 1 s. The Joule heating induced temperature field is strongly dependent on channel size, electrolyte concentration, and applied electric field strength. The simulations reveal that the presence of the Joule heating can result in significantly different characteristics of the electroosmotic flow and electrokinetic mass transport in microchannels.     

Photobleaching-based flow measurement in a commercial capillary electrophoresis chip instrument

For microfluidic analytical instruments, a facile, fast, and accurate instrument test is highly demanded. The test includes the quantitative verification of the relationship between pressure drop and flow velocity for the hydrodynamic pump, between the electric voltage and electroosmotic flow (EOF) for the high-voltage supply, and the chip quality. The key point for the test is the measurement of the flow velocity. However, most currently available velocimetries cannot be directly used without any instrumental modification or adding extra instruments. We applied a recently developed Laser Induced Fluorescence Photobleaching Anemometer (LIFPA) for the instrument test through measuring fluid flow velocity in a microfluidic instrument with optical measurement without any modification and extra instrument. We have successfully used the method to test Caliper HTS 250 System from Caliper Life Sciences (Hopkinton, MA) with its own light source and detector. The experimental result demonstrates that this single-point method of measuring flow velocity can be easily used for accurate test of a microfluidic instrument in less than 10 min at extremely low cost without any modification and extra instrument.     

Dynamic capillary coatings for electroosmotic flow control in capillary electrophoresis

Control of the electroosmotic flow (EOF) is critical for achieving optimal separations by capillary electrophoresis. For instance, manipulation of the EOF can yield either high resolution separations or rapid analyses. Dynamic capillary coatings are a simple and cost-effective approach to altering the EOF. The normal EOF can be slowed using buffer additives such as Mg²⁺ and hexamethonium which ion exchange onto the surface silanols to lower the effective wall charge. Alternatively, cationic polyelectrolytes or cationic surfactants can be used to establish a cationic coating on the capillary wall, which results in a reversed EOF. Practical considerations such as pH stability and reproducibility obtainable with an EOF modifier will be discussed.     

Numerical analysis of electroosmotic flow in dense regular and random arrays of impermeable, nonconducting spheres

We present a numerical scheme for analyzing steady-state isothermal electroosmotic flow (EOF) in three-dimensional random porous media, involving solution of the coupled Poisson, Nernst-Planck, and Navier-Stokes equations. While traditional finite-difference methods were used to resolve the Poisson-Nernst-Planck problem, the (electro)hydrodynamics has been addressed with high efficiency using the lattice-Boltzmann method. The developed model allows simulation of electrokinetic transport under most general conditions, including arbitrary value and distribution of electrokinetic potential at the solid-liquid interface, electrolyte composition, and pore space morphology. The approach provides quantitative information on a spatial distribution of simulated velocities. This feature was utilized to characterize EOF fields in regular and random, confined and bulk packings of hard (i.e., impermeable, nonconducting) spheres. Important aspects of pore space morphology (sphere size distribution), surface heterogeneity (mismatch in electrokinetic potentials at confining wall and sphere surface), and fluid phase properties (electrical double layer thickness) were investigated with respect to their influence on the EOF dynamics over microscopic and macroscopic spatial domains. Most important is the observation of a generally nonuniform pore-level EOF velocity profile in the sphere packings (even in the thin double layer limit) which is caused by pore space morphology and which is in contrast to the pluglike velocity distribution in a single, straight capillary under the same conditions.     

Influencing electroosmotic flow and selectivity in open tubular electrochromatography by tetrakis(pentafluorophenyl)porphyrin as capillary wall modifier

A physically adsorbed and covalently bonded porphyrin derivative, 5,10,15,20-tetrakis(pentafluorophenyl)porphyrin, H₂TPFPP, has been used as a fused-silica capillary wall modifier in open tubular capillary electrochromatography (OT-CEC), and its influence on the electroosmotic flow (EOF) velocity and on the selectivity of OT-CEC separations of a set of model aromatic carboxylic acids has been tested. Whereas most of the coatings of this category bring about an increase in selectivity with a concomitant slow down of the EOF, H₂TPFPP coating, depending on pH of the background electrolyte used, resulted both in decreasing of EOF at pH 8.5 by 5% and in increasing of EOF by 10–43% at pH 6 and 5, respectively. The separation efficiency and the resolution of aromatic carboxylic acids separation in coated capillaries, namely in that one with covalent coating, were better than in the bare fused-silica capillary. The perspectives of H₂TPFPP as capillary wall modifier are visualized in introducing well defined electroosmotic properties of materials used for miniaturized separation channels preparation in chip-based electromigration devices.