A microfluidic device for rapid concentration of particles in continuous flow by DC dielectrophoresis

This article presents a microfluidic device (so called concentrator) for rapid and efficient concentration of micro/nanoparticles using direct current dielectrophoresis (DC DEP) in continuous fluid flow. The concentrator is composed of a series of microchannels constructed with PDMS-insulating microstructures to focus efficiently the electric field in the flow direction to provide high field strength and gradient. Multiple trapping regions are formed within the concentrator. The location of particle trapping depends on the strength of the electric field applied. Under the experimental conditions, both streaming movement and DEP trapping of particles simultaneously take place within the concentrator at different regions. The former occurs upstream and is responsible for continuous transport of the particles, whereas the latter occurs downstream and rapidly traps the particles delivered from upstream. The observation agrees with the distribution of the simulated electric field and DEP force. The performance of the device is demonstrated by successfully and effectively concentrating fluorescent nanoparticles. At the sufficiently high electric field, the device demonstrates a trapping efficiency of 100%, which means downstream DEP traps and concentrates all (100%) the incoming particles from upstream. The trapping efficiency of the device is further studied by measuring the fluorescence intensity of concentrated particles in the channel. Typically, the fluorescence intensity becomes saturated in Trap 1 by applying the voltage (400 V) for >2 min, demonstrating that rapid concentration of the nanoparticles (10⁷ particles/ml) is achieved in the device. The microfluidic concentrator described can be implemented in applications where rapid concentration of targets is needed such as concentrating cells for sample preparation and concentrating molecular biomarkers for detection.     

Towards a continuous microfluidic rheometer

In a previous paper we presented a way to measure the rheological properties of complex fluids on a microfluidic chip (Guillot et al., Langmuir 22:6438, 2006). The principle of our method is to use parallel flows between two immiscible fluids as a pressure sensor. In fact, in a such flow, both fluids flow side by side and the size occupied by each fluid stream depends only on both flow rates and on both viscosities. We use this property to measure the viscosity of one fluid knowing the viscosity of the other one, both flow rates and the relative size of both streams in a cross-section. We showed that using a less viscous fluid as a reference fluid allows to define a mean shear rate with a low standard deviation in the other fluid. This method allows us to measure the flow curve of a fluid with less than 250 μL of fluid. In this paper we implement this principle in a fully automated set up which controls the flow rate, analyzes the picture and calculates the mean shear rate and the viscosity of the studied fluid. We present results obtained for Newtonian fluids and complex fluids using this set up and we compare our data with cone and plate rheometer measurements. By adding a mixing stage in the fluidic network we show how this set up can be used to characterize in a continuous way the evolution of the rheological properties as a function of the formulation composition. We illustrate this by measuring the rheological curve of four formulations of polyethylene oxide solution with only 1.3 mL of concentrated polyethylene oxide solution. This method could be very useful in screening processes where the viscosity range and the behavior of the fluid to an applied stress must be evaluated.     

A microfluidic platform with integrated flow sensing for focal chemical stimulation of cells and tissue

A microfluidic platform for precise biochemical control of the extracellular microenvironment was developed. A chemical interface was established with cells or tissues through the precise and focal delivery of soluble chemical agents through a pore addressed by a polymer microchannel. Thermal flow sensors were integrated along the length of the microchannel and monitored internal flow rate. Sensor performance was characterized in anticipation of future studies with real-time feedback control of focal delivery. The microfluidic system was characterized by determining the fluid delivery rates through the pores and concentration profiles of agents delivered. Finally, focal delivery to rat retinal tissue was demonstrated.     

Efficient batch-mode mixing and flow patterns in a microfluidic centrifugal platform: a numerical and experimental study

During recent years centrifugal-based microfluidic devices known as Lab-on-a-CD have attracted a lot of attentions. Applications of these CD-based platforms are ubiquitous in numerous biological analyses and chemical syntheses. Mixing of different species in microscale is one of the essential operations in biochemical applications where this seemingly simple task remains a major obstruction. Application of centrifugal force, however, may significantly improve the flow agitation and mixing, especially when it is combined with the Coriolis force which acts perpendicular to centrifugal force. In this study, mixing process in minichambers located on a rotating platform under a periodic acceleration and deceleration angular velocity profile is investigated both numerically and experimentally. We have incorporated various arrangements of obstacles and baffles, which are usually used in stationary mixers, within a batch-mode rotating mixing chamber. Subsequently, the effect of these obstacles on flow field and mixing process has been studied, and among these arrangements four cases have been selected for further experimental analysis. Experimental studies have been performed on a multi-layer CD platform fabricated in polycarbonate plates, and subsequently mixing has been investigated in these minichambers. The quantitative mixing data were obtained after a set of image analyses on the captured images of mixing chamber during the process and the results were compared with the simulation. The results indicate a good resemblance between the two studies both qualitatively and quantitatively. Furthermore, it has been shown that the application of obstacles and baffles together in chamber results in reducing the mixing time more than 50 % as compared to a chamber without any obstacle and/or baffle configuration. Obtaining mixing times less than 10 s in both studies, makes these CD-based platforms an appropriate device for many applications in which a cost-effective device as well as low mixing time is required.

     

Triplex-pumping CD-like microfluidic platform with parabolic microchannels

In this research, we propose a triplex-pumping CD-like multi-channel electrophoresis-based biomedical separation system that is driven by the interactions of the centrifugal force, the electric field force, and the Coriolis force. In this novel platform,the heat-conduction theory is implemented to formulate the relations between the sample velocity and the dimensions of the microchannel. The parabola microchannel that has the least friction force under triplex-pumping forces is determined by computer simulations. The centrifugal force control of this system is realized by the angular velocity of a DC servo motor, while the electric field is governed through multi-stage potential circuits, which are suitably designed and fabricated by sputtering using the metal mask method, and can be adjusted to provide multi-stage voltages. Experimental results demonstrated that the proposed parabola microchannels can effectively eliminate the effects of friction force.     

Triplex-pumping CD-like microfluidic platform with parabolic microchannels

In this research, we propose a triplex-pumping CD-like multi-channel electrophoresis-based biomedical separation system that is driven by the interactions of the centrifugal force, the electric field force, and the Coriolis force. In this novel platform,the heat-conduction theory is implemented to formulate the relations between the sample velocity and the dimensions of the microchannel. The parabola microchannel that has the least friction force under triplex-pumping forces is determined by computer simulations. The centrifugal force control of this system is realized by the angular velocity of a DC servo motor, while the electric field is governed through multi-stage potential circuits, which are suitably designed and fabricated by sputtering using the metal mask method, and can be adjusted to provide multi-stage voltages. Experimental results demonstrated that the proposed parabola microchannels can effectively eliminate the effects of friction force.

     

A microfluidic platform for formation of double-emulsion droplets

A new method for actively controlling the number of internal droplets of water-in-oil-in-water (W/O/W) double-emulsion droplets was demonstrated. A new microfluidic platform for double-emulsion applications has been developed, which integrates T-junction channels, moving-wall structures, and a flow-focusing structure. Inner water-in-oil (W/O) single-emulsion droplets were first formed at a major T-junction. Then the droplets were sub-divided into smaller uniform droplets by passing through a series of secondary T-junctions (branches). The moving-wall structures beside the secondary T-junctions were used to control the number of the sub-divided droplets by selectively blocking the branches. Finally, double-emulsion droplets were formed by using a flow-focusing structure downstream. Experimental data demonstrate that the inner and outer droplets have narrow size distributions with coefficient of variation (CV) of less than 3.5% and 5.7%, respectively. Double-emulsion droplets with 1, 2, 3, and up to 10 inner droplets have been successfully formed using this approach. The size of the inner droplets and outer droplets could be also fine-tuned with this device. The development of this new platform was promising for drug delivery applications involving double emulsions.     

Optical separation of droplets on a microfluidic platform

This paper describes the optical separation of microdroplets according to their refractive indices. The behavior of the droplets was characterized in terms of the optical force and the hydrodynamic effects present upon illumination of the droplets in a direction normal to the flow direction in a rectangular microfluidic channel. The optical forces acting on the droplets and the resultant droplet trajectories were analyzed and compared with the numerically predicted values. The relationship between the drag force and optical force was examined to understand the system performance properties in the context of screening applications involving the removal of unwanted droplets. Two species of droplets were compared for their photophoretic displacements by varying the illumination intensity. Because the optical forces exerted on the droplets were functions of the refractive indices and sizes of the droplets, a variety of chemical species could be separated simultaneously.     

A novel integrated microfluidic platform to perform fluorescence in situ hybridization for chromosomal analysis

The fluorescence in situ hybridization (FISH) technique has been commonly employed to detect the chromosomal abnormalities. However, applications of this technique are limited due to its lengthy process and labor-intensive sample preparation. In this study, a novel integrated microfluidic chip capable of performing the entire FISH protocol automatically was reported. This novel technique can achieve several advantages, including reduce the consumption of bio-samples and reagents, automation and rapid analysis compared to the conventional method. In this study, several functional microfluidic devices were integrated on a single chip to perform automatic FISH on the microfluidic platform. Experimental data demonstrated that the developed microfluidic system successfully provided superior performance for probing the chromosomal abnormality of cells. Furthermore, the novel microfluidic system performed the entire process automatically within 3 h, where the conventional method required 10 h to perform the entire protocol manually. This data indicated superior performance of the novel method. Our findings conclude that the novel integrated FISH protocol is more convenient to perform large quantities of samples, which can be used in clinical trials.     

Continuous-flow in-droplet magnetic particle separation in a droplet-based microfluidic platform

This paper presents a continuous-flow in-droplet magnetic particle separation in a droplet-based microfluidic device for magnetic bead-based bioassays. Two functions, electrocoalescence and magnetic particle manipulation, are performed in this device. A pair of charging metallic needles is inserted into two aqueous channels of the device. By electrostatic force, two different solutions can be merged to be mixed at a junction of droplet generation. The manipulation of magnetic particles is achieved using an externally applied magnetic field. The magnetic particles are separated by the magnetic field to one side of the droplet and extracted by splitting the droplet into two daughter droplets: one contains the majority of the magnetic particles and the other is almost devoid of magnetic particles. The applicability of the continuous-flow in-droplet magnetic particle separation is demonstrated by performing a proof-of-concept immunoassay between streptavidin-coated magnetic beads and biotin labelled with fluorescence. This approach will be useful for various biological and chemical analyses and compartmentalization of small samples.     

Integration of impedance spectroscopy sensors in a digital microfluidic platform

Digital microfluidics combines the advantages of a low consumption of reagents with a high flexibility of processing fluid samples. For applications in life sciences not only the processing but also the characterization of fluids is crucial. In this contribution, a microfluidic platform, combining the actuation principle of electrowetting on dielectrics for droplet manipulations and the sensor principle of impedance spectroscopy for the characterization of the fluid composition and condition, is presented. The fabrication process of the microfluidic platform comprises physical vapor deposition and structuring of the metal electrodes onto a substrate, the deposition of a dielectric isolator and a hydrophobic top coating. The key advantage of this microfluidic chip is the common electric nature of the sensor and the actuation principle. This allows for fabricating digital microfluidic devices with a minimal number of process steps. Multiple measurements on fluids of different composition (including rigid particles) and of different conditions (temperature, sedimentation) were performed and process parameters were monitored online. These sample applications demonstrate the versatile applications of this combined technology.     

A vapor based microfluidic flow regulator

We introduce a flow regulating technology that uses trapped air bubbles in a hydrophobic microfluidic channel. We present basic designs for flow regulators and flow valves using trapped air. Experiments have successfully demonstrated the capability of this technique for delivering constant and varying flow rate, and for on-off valving. This approach to valving provides a simple, yet effective way to monolithically integrate flow and valve control on polymer Lab-on-Chip devices.     

A microfluidic platform for electrical detection of DNA hybridization

Current methods used for detection of DNA hybridization involve the use of DNA microarrays which require overnight incubation times along with bulky and expensive fluorescent scanners. Here, we demonstrate electrical detection of DNA hybridization in an oligonucleotide functionalized microfluidic channel. We use microchannels functionalized with DNA probes integrated with electrodes for measuring conductance across the channel. As beads conjugated with the target DNA passing through the channel are captured on the surface, we are able to electrically detect changes in resistance due to bead capture. Our assay can be completed in less than an hour using less than a microliter of reagent, and has the potential for extensive multiplexing. Such a device can be useful as a handheld platform in a clinical setting where one would need to rapidly genotype a small number of genes rapidly.     

A magnetophoresis-based microfluidic detection platform under a static-fluid environment

Microfluidic cell separations and immunoassays exploit a dynamic flow environment by electrical pumps to manipulate fluids containing biomolecules and microbeads. In particular, the magnetophoresis-based microfluidics requires a delicate flow control of pumps because the flow rate affects the result sensitively. Consequently, the dynamic flow environment requiring pumps prevents the magnetophoresis-based microfluidics from popularization and miniaturization. Herein, we present a magnetophoresis-based microfluidic platform under a static-fluid environment for the detection of microbeads labeled with magnetic nanoparticles (MNPs) by simple manual operation of fluids. To overcome the residual flow caused by the manual operation, we designed a microfluidic device having a pair of microchannels; one for detecting the target and the other for a reference. The deviations due to the residual flow were corrected by comparing the difference between the mean velocities of microbeads in each microchannel where microbeads labeled with five different concentrations of MNPs could be classified. On the basis of the convenience and portability of magnetophoresis under a static-fluidic environment, this new microfluidic platform enabled semiquantitative detection of labeled particles without any complex electrical devices and could thus be used as a portable detection platform.     

Automated microfluidic processing platform for multiplexed magnetic bead immunoassays

A microfluidic platform is presented which fully automates all incubation steps of a three-stage, multiplexed magnetic bead immunoassay, such as the Luminex^? xMAP technology. Magnetic actuation is used to transfer the microbeads between co-infused adjacent laminar flow streams to transport the beads into and out of incubation and wash solutions, with extended incubation channels to allow sufficient bead incubation times (1–30?min, commonly 5?min per stage) to enable high-sensitivity. The serial incubation steps of the immunoassay are completed in succession within the device with no operator interaction, and the continuous flow operation with magnetic bead transfer defines the incubation sequencing requiring no external fluidic controls beyond syringe pump infusion. The binding kinetics of the assay is empirically characterized to determine the required incubation times for specific assay sensitivities in the range 1?pg/ml to 100?ng/ml. By using a Luminex^? xMAP duplex assay, concurrent detection of IL-6 and TNF-α was demonstrated on-chip with a detection range 10?pg/ml to 1?ng/ml. This technology enables rapid automation of magnetic microbead assays, and has the potential to perform continuous concentration monitoring.     

A micromechanical flow sensor for microfluidic applications

We fabricated a microfluidic flow meter and measured its response to fluid flow in a microfluidic channel. The flow meter consisted of a micromechanical plate, coupled to a laser deflection system to measure the deflection of the plate during fluid flow. The 100 /spl mu/m square plate was clamped on three sides and elevated 3 /spl mu/m above the bottom surface of the channel. The response of the flow meter was measured for flow rates, ranging from 2.1 to 41.7 /spl mu/L/min. Several fluids, with dynamic viscosities ranging from 0.8 to 4.5/spl times/10/sup -3/ N/m, were flowed through the channels. Flow was established in the microfluidic channel by means of a syringe pump, and the angular deflection of the plate monitored. The response of the plate to flow of a fluid with a viscosity of 4.5/spl times/10/sup -3/ N/m was linear for all flow rates, while the plate responded linearly to flow rates less than 4.2 /spl mu/L/min of solutions with lower dynamic viscosities. The sensitivity of the deflection of the plate to fluid flow was 12.5/spl plusmn/0.2 /spl mu/rad/(/spl mu/L/min), for a fluid with a viscosity of 4.5/spl times/10/sup -3/ N/m. The encapsulated plate provided local flow information along the length of a microfluidic channel.     

Aliquoting on the centrifugal microfluidic platform based on centrifugo-pneumatic valves

We present a new method for aliquoting liquids on the centrifugal microfluidic platform. Aliquoting is an essential unit operation to perform multiple parallel assays (“geometric multiplexing”) from one individual sample, such as genotyping by real-time polymerase chain reactions (PCR), or homogeneous immunoassay panels. Our method is a two-stage process with an initial metering phase and a subsequent transport phase initiated by switching a centrifugo-pneumatic valve. The method enables aliquoting liquids into completely separated reaction cavities. It includes precise metering that is independent on the volume of pre-stored reagents in the receiving cavities. It further excludes any cross-contamination between the receiving cavities. We characterized the performance for prototypes fabricated by three different technologies: micro-milling, thermoforming of foils, and injection molding. An initial volume of ~90 μl was split into 8 aliquots of 10 μl volume each plus a waste reservoir on a thermoformed foil disk resulting in a coefficient of variation (CV) of the metered volumes of 3.6%. A similar volume of ~105 μl was split into 16 aliquots of 6 μl volume each on micro-milled and injection-molded disks and the corresponding CVs were 2.8 and 2.2%, respectively. Thus, the compatibility of the novel aliquoting structure to the aforementioned prototyping and production technologies is demonstrated. Additionally, the important question of achievable volume precision of the aliquoting structure with respect to the production tolerances inherent to each of these production technologies is addressed experimentally and theoretically. The new method is amenable to low cost mass production, since it does not require any post-replication surface modifications like hydrophobic patches.     

Self-optimizing, thermally adaptive microfluidic flow structures

Although microfluidic devices offer many benefits, high fluid shear stresses in such devices are an undesirable consequence of miniaturization. In the present study, we present an adaptive “smart” design that mitigates the effects of high shear stresses in microfluidic-based devices by autonomously optimizing its internal flow structure. This concept was demonstrated by testing a prototype microscale thermal-fluid device that responded to changes in the local thermal environment. The autonomous, self-optimizing functionality was achieved using poly(N-isopropylacrylamide) hydrogel actuated microvalves, which independently controlled the flow to four distinct regions within the device. The experimental results showed that the device optimized its internal topological flow arrangement such that fluid was delivered only to regions where cooling was required. As a result, a series of spatially distributed thermal loads were dissipated with minimal pumping power consumption.     

Droplet coalescence by geometrically mediated flow in microfluidic channels

Microfluidic flow is geometrically mediated at a trifurcating junction allowing periodically formed, equally spaced out emulsion droplets to redistribute and fuse consistently. This is achieved by controlling the ratio between the droplet transport time across the trifurcating junction and the drainage time of the fluid volume separating the droplets t _r/t _d. Three different microfluidic trifurcation geometries have been designed and compared for their droplet fusion efficiencies. Fusion of up to six droplets has been observed in these devices. The fusion of two droplets occurs when t _r/t _d is equal to 1.25 and the number of fused droplets increases with t _r/t _d. When the junction length (d) is 216 μm fusion of 2–6 six droplets are possible however when the junction length is increased to 360 μm fusion of only two droplets is observed.