Showing papers in "Lab on a Chip in 2004"

An integrated digital microfluidic lab-on-a-chip for clinical diagnostics on human physiological fluids

Abstract: Clinical diagnostics is one of the most promising applications for microfluidic lab-on-a-chip systems, especially in a point-of-care setting. Conventional microfluidic devices are usually based on continuous-flow in microchannels, and offer little flexibility in terms of reconfigurability and scalability. Handling of real physiological samples has also been a major challenge in these devices. We present an alternative paradigm—a fully integrated and reconfigurable droplet-based “digital” microfluidic lab-on-a-chip for clinical diagnostics on human physiological fluids. The microdroplets, which act as solution-phase reaction chambers, are manipulated using the electrowetting effect. Reliable and repeatable high-speed transport of microdroplets of human whole blood, serum, plasma, urine, saliva, sweat and tear, is demonstrated to establish the basic compatibility of these physiological fluids with the electrowetting platform. We further performed a colorimetric enzymatic glucose assay on serum, plasma, urine, and saliva, to show the feasibility of performing bioassays on real samples in our system. The concentrations obtained compare well with those obtained using a reference method, except for urine, where there is a significant difference due to interference by uric acid. A lab-on-a-chip architecture, integrating previously developed digital microfluidic components, is proposed for integrated and automated analysis of multiple analytes on a monolithic device. The lab-on-a-chip integrates sample injection, on-chip reservoirs, droplet formation structures, fluidic pathways, mixing areas and optical detection sites, on the same substrate. The pipelined operation of two glucose assays is shown on a prototype digital microfluidic lab-on-chip, as a proof-of-concept.

Multi-step synthesis of nanoparticles performed on millisecond time scale in a microfluidic droplet-based system

Abstract: This paper reports a plug-based, microfluidic method for performing multi-step chemical reactions with millisecond time-control. It builds upon a previously reported method where aqueous reagents were injected into a flow of immiscible fluid (fluorocarbons) (H. Song et al., Angew. Chem. Int. Ed., 2003, 42, 768). The aqueous reagents formed plugs – droplets surrounded and transported by the immiscible fluid. Winding channels rapidly mixed the reagents in droplets. This paper shows that further stages of the reaction could be initiated by flowing additional reagent streams directly into the droplets of initial reaction mixture. The conditions necessary for an aqueous stream to merge with aqueous droplets were characterized. The Capillary number could be used to predict the behavior of the two-phase flow at the merging junction. By transporting solid reaction products in droplets, the products were kept from aggregating on the walls of the microchannels. To demonstrate the utility of this microfluidic method it was used to synthesize colloidal CdS and CdS/CdSe core-shell nanoparticles.

Design of microfluidic channel geometries for the control of droplet volume, chemical concentration, and sorting

Abstract: Passive microfluidic channel geometries for control of droplet fission, fusion and sorting are designed, fabricated, and tested. In droplet fission, the inlet width of the bifurcating junction is used to control the range of breakable droplet sizes and the relative resistances of the daughter channels were used to control the volume of the daughter droplets. Droplet fission is shown to produce concentration differences in the daughter droplets generated from a primary drop with an incompletely mixed chemical gradient, and for droplets in each of the bifurcated channels, droplets were found to be monodispersed with a less than 2% variation in size. Droplet fusion is demonstrated using a flow rectifying design that can fuse multiple droplets of same or different sizes generated at various frequencies. Droplet sorting is achieved using a bifurcating flow design that allows droplets to be separated base on their sizes by controlling the widths of the daughter channels. Using this sorting design, submicron satellite droplets are separated from the larger droplets.

Transport and reaction in microscale segmented gas–liquid flow

Abstract: We use micro particle image velocimetry (μPIV) and fluorescence microscopy techniques to characterize microscale segmented gas–liquid flow at low superficial velocities relevant for chemical reactions with residence times of up to several minutes. Different gas–liquid microfluidic channel networks of rectangular cross section are fabricated in poly(dimethylsiloxane) (PDMS) using soft lithography techniques. The recirculation motion in the liquid segments associated with gas–liquid flows as well as the symmetry characteristics of the recirculations are quantified for straight and meandering channel networks. Even minor surface roughness effects and the compressibility of the gas phase induce loss of symmetry and enhance mixing across the centerline in straight channels. Mixing is further accelerated in meandering channels by the periodic switching of recirculation patterns across the channel center. We demonstrate a new, piezoelectrically activated flow injection technique for determining residence time distributions (RTDs) of fluid elements in multiphase microfluidic systems. The results confirm a narrowed liquid phase RTD in segmented flows in comparison to their single-phase counterparts. The enhanced mixing and narrow RTD characteristics of segmented gas–liquid flows are applied to liquid mixing and in sol–gel synthesis of colloidal nanoparticles.

A novel in-plane passive microfluidic mixer with modified Tesla structures

Abstract: An innovative in-plane passive micromixer using modified Tesla structures, which are used as passive valves, has been designed, simulated, fabricated and successfully characterized in this paper. Simulation and experimental results of the developed novel micromixer have shown excellent mixing performance over a wide range of flow conditions in the micro scale. The micromixer realized in this work has achieved even better mixing performance at a higher flow rate, and its pressure drop is less than 10 KPa at the flow rate of 100 µl min−1. This micromixer shows characteristics similar to Taylor dispersion, with contributions from both diffusion and convection. The mixer has a diffusion domain region at low flow rate, but it moves to a convection domain region at high flow rate. Due to the simple in-plane structure of the novel micromixer explored in this work, the mixer can be easily realized and integrated with on-chip microfluidic devices and micro total analysis systems (μ-TAS).

Microenvironment design considerations for cellular scale studies

Abstract: In vivo cellular microenvironments are not well-mimicked in present in vitro cell culture systems Microtechnology, and microfluidics in particular, provides the tools to create in vivo-like cellular microenvironments in vitro Features of in vitro cellular microenvironments are discussed and compared to macroscale cell culture environments; the concept of an effective culture volume (ECV) is introduced to facilitate the comparison Current research using microtechnology to investigate in vitro cellular microenvironments is presented and areas where more research is needed in characterizing the in vitro microenvironment are outlined

Acoustic control of suspended particles in micro fluidic chips

Abstract: A method to separate suspended particles from their medium in a continuous mode at microchip level is described. The method combines an ultrasonic standing wave field with the extreme laminar flow properties obtained in a silicon micro channel. The channel was 750 µm wide and 250 µm deep with vertical side walls defined by anisotropic wet etching. The suspension comprised “Orgasol 5µm” polyamide spheres and distilled water. The channel was perfused by applying an under pressure (suction) to the outlets. The channel was ultrasonically actuated from the back side of the chip by a piezoceramic plate. When operating the acoustic separator at the fundamental resonance frequency the acoustic forces were not strong enough to focus the particles into a well defined single band in the centre of the channel. The frequency was therefore changed to about 2 MHz, the first harmonic with two pressure nodes in the standing wave, and consequently two lines of particles were formed which were collected via the side outlets. Two different microchip separator designs were investigated with exit channels branching off from the separation channel at angles of 90° and 45° respectively. The 45° separator displayed the most optimal fluid dynamic properties and 90% of the particles were gathered in 2/3 of the original fluid volume.

An optimised split-and-recombine micro-mixer with uniform ‘chaotic’ mixing

Abstract: A second generation micro-mixer, being a further optimised version of a first prototype, relying on the consequent utilisation of the split-and-recombine principle is presented. We show that the mixing can be characterized by a positive finite-time Lyapunov exponent although being highly regular and uniform. Using computational fluid dynamics (CFD) we investigate the mixing performance for Reynolds numbers in the range of about 1 to about 100. In particular for low Reynolds numbers (Re < 15) the CFD results predict an almost ideal multi-lamination. Thus, the developed mixer is especially suited for efficient mixing of highly viscous fluids. Furthermore, the numerical results are experimentally validated by investigations of mixing of water–glycerol solutions. The experimental results are found to be in excellent agreement with the numerical data and prove the high mixing efficiency.

Microfabrication and microfluidics for tissue engineering : state of the art and future opportunities

Abstract: An introductory overview of the use of microfluidic devices for tissue engineering is presented. After a brief description of the background of tissue engineering, different application areas of microfluidic devices are examined. Among these are methods for patterning cells, topographical control over cells and tissues, and bioreactors. Examples where microfluidic devices have been employed are presented such as basal lamina, vascular tissue, liver, bone, cartilage and neurons. It is concluded that until today, microfluidic devices have not been used extensively in tissue engineering. Major contributions are expected in two areas. The first is growth of complex tissue, where microfluidic structures ensure a steady blood supply, thereby circumventing the well-known problem of providing larger tissue structures with a continuous flow of oxygen and nutrition, and withdrawal of waste products. The second, and probably more important function of microfluidics, combined with micro/nanotechnology, lies in the development of in vitro physiological systems for studying fundamental biological phenomena.

Dielectric spectroscopy in a micromachined flow cytometer: theoretical and practical considerations

Abstract: We propose a model to determine the influence of different cell properties, such as size, membrane capacitance and cytoplasm conductivity, on the impedance spectrum as measured in a microfabricated cytometer. A dielectric sphere of equivalent complex permittivity is used as a simplified model to describe a biological cell. The measurement takes place between a pair of facing microelectrodes in a microchannel filled with a saline solution. The model incorporates various cell parameters, such as dielectric properties, size and position in the channel. A 3D finite element model is used to evaluate the magnitude of the electric field in the channel and the resultant changes in charge densities at the measurement electrode boundaries as a cell flows past. The charge density is integrated on the electrode surface to determine the displacement current and the channel impedance for the computed frequency range. The complete impedance model combines the finite element model, the electrode-electrolyte interface impedance and stray impedance, which are measured from a real device. The modeled dielectric complex spectra for various cell parameters are discussed and a measurement strategy for cell discrimination with such a system is proposed. We finally discuss the amount of noise and measurement fluctuations of the sensor.

Principles of droplet electrohydrodynamics for lab-on-a-chip.

Abstract: Electrically controlled droplet-based labs-on-a-chip operate under the principles of electro-capillarity and dielectrophoresis. The microfluidic mechanics of manipulating electrified droplets are complex and not entirely understood. In this article, we analyse these operating principles, especially electrowetting on dielectric (a form of electro-capillarity) and dielectrophoresis, under a unified framework of droplet electrohydrodynamics. We differentiate them by their electric origins and their energy transduction mechanisms. Our study shows that both electrowetting on dielectric and dielectrophoresis are effective for droplet generation and manipulation. In addition, our study demonstrates: (1) the presence of a wetting contribution to dielectrophoresis; and (2) contact angle reduction is merely an observable consequence of, not a condition for, the occurrence of electrowetting on dielectric. Simulations are used extensively in this article to illustrate device operation, to expose underlying physics, and to validate our conclusions. Simulations of electrically driven droplet generation, droplet translocation, droplet fusion, and droplet fission are presented.

Integration of single cell injection, cell lysis, separation and detection of intracellular constituents on a microfluidic chip.

Abstract: A microfluidic system was developed for the analysis of single biological cells, with functional integration of cell sampling, single cell loading, docking, lysing, and capillary electrophoretic (CE) separation with laser induced fluorescence (LIF) detection in microfabricated channels of a single glass chip. Channels were 12 µm deep and 48 µm wide, with a simple crossed-channel design. The effective separation channel length was 35 mm. During sampling with a cell suspension (cell population 1.2 × 105 cells per mL in physiological salt solution), differential hydrostatic pressure (created by adjusting liquid levels in the four reservoirs) was used to control cell flow exclusively through the channel crossing. Single cell loading into the separation channel was achieved by electrophoretic means by applying a set of potentials at the four reservoirs, counteracting the hydrostatic flow. A special docking (adhering) procedure for the loaded cell was applied before lysis by repeatedly connecting and disconnecting a set of low potentials, allowing precise positioning of the cell within the separation channel. Cell lysis was then effected within 40 ms under an applied CE separation voltage of 1.4 kV (280 V cm−1) within the working electrolyte (pH 9.2 borate buffer) without additional lysates. The docked lysing approach reduced dispersion of released intracellular constituents, and significantly improved the reproducibility of CE separations. Glutathione (GSH) was used as a model intracellular component in single human erythrocyte cells. NDA derivatized GSH was detected using LIF. A throughput of 15 samples h−1, a retention time precision of 2.4% RSD was obtained for 14 consecutively injected cells. The average cellular concentration of GSH in human erythrocytes was found to be 7.2 × 10−4 ± 3.3 × 10−4 M (63 ± 29 amol per cell). The average separation efficiency for GSH in lysed cells was 2.13 × 106 ± 0.4 × 106 plates per m, and was about a factor of 5 higher than those obtained with GSH standards using pinched injection.

Theory and numerical simulation of droplet dynamics in complex flows—a review

Abstract: We review theoretical and numerical studies and methods for droplet deformation, breakup and coalescence in flows relevant to the design of micro channels for droplet generation and manipulation.

Macro-to-micro interfaces for microfluidic devices

Abstract: Since the concept of miniaturized total analysis systems (μTAS) was invented, a great number of microfluidic devices have been demonstrated for a variety of applications. However, an important hurdle that still needs to be cleared is the connection of a microfluidic device with the rest of the world, which is often referred to as the macro-to-micro interface, interconnect, or world-to-chip interface. In this review, we will examine the methods used by pioneers in the field and other investigators, review the approaches for capillary electrophoresis-based devices and those using pneumatic pumping, and present additional discussion on interface standardization and choosing and designing interconnects for your applications.

Power-free poly(dimethylsiloxane) microfluidic devices for gold nanoparticle-based DNA analysis

Abstract: An extremely simple, power-free pumping method for poly(dimethylsiloxane) (PDMS) microfluidic devices is presented. By exploiting the high gas solubility of PDMS, the energy for the pumping is pre-stored in the degassed bulk PDMS, therefore no additional structures other than channels and reservoirs are required. In a Y-shaped microchannel with cross section of 100 µm width × 25 µm height, this method has provided flow rate of 0.5–2 nL s−1, corresponding to linear velocity of 0.2–0.8 mm s−1, with good reproducibility. As an application of the power-free pumping, gold nanoparticle-based DNA analysis, which does not rely on the cross-linking mechanism between nanoparticles, has been implemented in a microchannel with three inlets. Target 15mer DNA has been easily and unambiguously discriminated from its single-base substituted mutant. Instead of colorimetric detection in a conventional microtube, an alternative detection technique suitable for microdevices has been discovered—observation of deposition on the PDMS surfaces. The channel layout enabled two simultaneous DNA analyses at the two interfaces between the three laminar streams.

Optical tweezers applied to a microfluidic system

Abstract: We will demonstrate how optical tweezers can be combined with a microfluidic system to create a versatile microlaboratory. Cells are moved between reservoirs filled with different media by means of optical tweezers. We show that the cells, on a timescale of a few seconds, can be moved from one reservoir to another without the media being dragged along with them. The system is demonstrated with an experiment where we expose E. coli bacteria to different fluorescent markers. We will also discuss how the system can be used as an advanced cell sorter. It can favorably be used to sort out a small fraction of cells from a large population, in particular when advanced microscopic techniques are required to distinguish various cells. Patterns of channels and reservoirs were generated in a computer and transferred to a mask using either a sophisticated electron beam technique or a standard laser printer. Lithographic methods were applied to create microchannels in rubber silicon (PDMS). Media were transported in the channels using electroosmotic flow. The optical system consisted of a combined confocal and epi-fluorescence microscope, dual optical tweezers and a laser scalpel.

Miniaturised nucleic acid analysis

Abstract: The application of micro total analysis systems has grown exponentially over the past few years, particularly diversifying in disciplines related to bioassays. The primary focus of this review is to detail recent new approaches to sample preparation, nucleic acid amplification and detection within microfluidic devices or at the microscale level. We also introduce some applications that have as yet to be explored in a miniaturised environment, but should benefit from improvements in analytical efficiency and functionality when transferred to planar-chip formats. The studies described in this review were published in commonly available journals as well as in the proceedings of three major conferences relevant to microfluidics (Micro Total Analysis Systems, Transducers and The Nanotechnology Conference and Trade Show). Although an emphasis has been placed on papers published since 2002, pertinent articles preceding this publication year have also been included.

Hydrodynamic microfabrication via“on the fly” photopolymerization of microscale fibers and tubes

Abstract: A microfluidic apparatus capable of creating continuous microscale cylindrical polymeric structures has been developed. This system is able to produce microstructures (e.g. fibers, tubes) by employing 3D multiple stream laminar flow and “on the fly” in-situ photopolymerization. The details of the fabrication process and the characterization of the produced microfibers are described. The apparatus is constructed by merging pulled glass pipettes with PDMS molding technology and used to manufacture the fibers and tubes. By controlling the sample and sheath volume flow rates, the dimensions of the microstructures produced can be altered without re-tooling. The fiber properties including elasticity, stimuli responsiveness, and biosensing are characterized. Responsive woven fabric and biosensing fibers are demonstrated. The fabrication process is simple, cost effective and flexible in materials, geometries, and scales.

Generation of dynamic temporal and spatial concentration gradients using microfluidic devices.

Abstract: This paper describes a microfluidic approach to generate dynamic temporal and spatial concentration gradients using a single microfluidic device. Compared to a previously described method that produced a single fixed gradient shape for each device, this approach combines a simple “mixer module” with gradient generating network to control and manipulate a number of different gradient shapes. The gradient profile is determined by the configuration of fluidic inputs as well as the design of microchannel network. By controlling the relative flow rates of the fluidic inputs using separate syringe pumps, the resulting composition of the inlets that feed the gradient generator can be dynamically controlled to generate temporal and spatial gradients. To demonstrate the concept and illustrate this approach, examples of devices that generate (1) temporal gradients of homogeneous concentrations, (2) linear gradients with dynamically controlled slope, baseline, and direction, and (3) nonlinear gradients with controlled nonlinearity are shown and their limitations are described.

Rapid PCR in a continuous flow device

Abstract: Continuous flow polymerase chain reaction (CFPCR) devices are compact reactors suitable for microfabrication and the rapid amplification of target DNAs. For a given reactor design, the amplification time can be reduced simply by increasing the flow velocity through the isothermal zones of the device; for flow velocities near the design value, the PCR cocktail reaches thermal equilibrium at each zone quickly, so that near ideal temperature profiles can be obtained. However, at high flow velocities there are penalties of an increased pressure drop and a reduced residence time in each temperature zone for the DNA/reagent mixture, that potentially affect amplification efficiency. This study was carried out to evaluate the thermal and biochemical effects of high flow velocities in a spiral, 20 cycle CFPCR device. Finite element analysis (FEA) was used to determine the steady-state temperature distribution along the micro-channel and the temperature of the DNA/reagent mixture in each temperature zone as a function of linear velocity. The critical transition was between the denaturation (95 °C) and renaturation (55 °C–68 °C) zones; above 6 mm s−1 the fluid in a passively-cooled channel could not be reduced to the desired temperature and the duration of the temperature transition between zones increased with increased velocity. The amplification performance of the CFPCR as a function of linear velocity was assessed using 500 and 997 base pair (bp) fragments from λ-DNA. Amplifications at velocities ranging from 1 mm s−1 to 20 mm s−1 were investigated. The 500 bp fragment could be observed in a total reaction time of 1.7 min (5.2 s cycle−1) and the 997 bp fragment could be detected in 3.2 min (9.7 s cycle−1). The longer amplification time required for detection of the 997 bp fragment was due to the device being operated at its enzyme kinetic limit (i.e., Taq polymerase deoxynucleotide incorporation rate).

Measurements of scattered light on a microchip flow cytometer with integrated polymer based optical elements

Abstract: Flow cytometry is widely used for analyzing microparticles, such as cells and bacteria. In this paper, we report an innovative microsystem, in which several different optical elements (waveguides, lens and fiber-to-waveguide couplers) are integrated with microfluidic channels to form a complete microchip flow cytometer. All the optical elements, the microfluidic system, and the fiber-to-waveguide couplers were defined in one layer of polymer (SU-8, negative photoresist) by standard photolithography. With only a single mask procedure required, all the fabrication and packaging processes can be finished in one day. Polystyrene beads were measured in the microchip flow cytometer, and three signals (forward scattering, large angle scattering and extinction) were measured simultaneously for each bead. To our knowledge this is the first time forward scattered light and incident light extinction were measured in a microsystem using integrated optics. The microsystem can be applied for analyzing different kinds of particles and cells, and can easily be integrated with other microfluidic components.

Molded polyethylene glycol microstructures for capturing cells within microfluidic channels

Abstract: The ability to control the deposition and location of adherent and non-adherent cells within microfluidic devices is beneficial for the development of micro-scale bioanalytical tools and high-throughput screening systems. Here, we introduce a simple technique to fabricate poly(ethylene glycol) (PEG) microstructures within microfluidic channels that can be used to dock cells within pre-defined locations. Microstructures of various shapes were used to capture and shear-protect cells despite medium flow in the channel. Using this approach, PEG microwells were fabricated either with exposed or non-exposed substrates. Proteins and cells adhered within microwells with exposed substrates, while non-exposed substrates prevented protein and cell adhesion (although the cells were captured inside the features). Furthermore, immobilized cells remained viable and were stained for cell surface receptors by sequential flow of antibodies and secondary fluorescent probes. With its unique strengths in utility and control, this approach is potentially beneficial for the development of cell-based analytical devices and microreactors that enable the capture and real-time analysis of cells within microchannels, irrespective of cell anchorage properties.

Micro magnetic stir-bar mixer integrated with parylene microfluidic channels

Abstract: Previously, we reported a micro magnetic stir-bar mixer driven by an external rotating magnetic field and its rapid mixing performance in polydimethyl-siloxane (PDMS) channels. The PDMS piece with embedded fluid channels were manually aligned to a glass substrate and assembled. In this paper, we report the fabrication and testing results of a micro magnetic stir-bar monolithically integrated in parylene surface-micromachined channels with improved design features, including small tolerance of the stir-bar to channel wall (10 µm). Using of parylene based microchannels with improved design not only provides improved mixing, but also eliminates certain problems associated with PDMS-based channels. For example, porosity of PDMS causes evaporation and absorption of chemicals and thus channels made of PDMS are prone to cross-contamination. We have also demonstrated that the magnetic stir-bar can be used to pump liquid in micro channels.

Droplet-based chemistry on a programmable micro-chip

Abstract: We describe the manipulation of aqueous droplets in an immiscible, low-permittivity suspending medium. Such droplets may serve as carriers for not only air- and water-borne samples, contaminants, chemical reagents, viral and gene products, and cells, but also the reagents to process and characterise these samples. We present proofs-of-concept for droplet manipulation through dielectrophoresis by: (1) moving droplets on a two-dimensional array of electrodes, (2) achieving dielectrically-activated droplet injection, (3) fusing and reacting droplets, and (4) conducting a basic biological assay through a combination of these steps. A long-term goal of this research is to provide a platform fluidic processor technology that can form the core of versatile, automated, micro-scale devices to perform chemical and biological assays at or near the point of care, which will increase the availability of modern medicine to people who do not have ready access to modern medical institutions, and decrease the cost and delays associated with that lack of access.

High-sensitivity miniaturized immunoassays for tumor necrosis factor α using microfluidic systems

Abstract: We use microfluidic chips to detect the biologically important cytokine tumor necrosis factor α (TNF-α) with picomolar sensitivity using sub-microliter volumes of samples and reagents. The chips comprise a number of independent capillary systems (CSs), each of which is composed of a filling port, an appended microchannel, and a capillary pump. Each CS fills spontaneously by capillary forces and includes a self-regulating mechanism that prevents adventitious drainage of the microchannels. Thus, interactive control of the flow in each CS is easily achieved via collective control of the evaporation in all CSs by means of two Peltier elements that can independently heat and cool. Long incubation times are crucial for high sensitivity assays and can be conveniently obtained by adjusting the evaporation rate to have low flow rates of ∼30 nL min−1. The assay is a sandwich fluorescence immunoassay and takes place on the surface of a poly(dimethylsiloxane) (PDMS) slab placed across the microchannels. We precoat PDMS with capture antibodies (Abs), localize the capture of analyte molecules using a chip, then bind the captured analyte molecules with fluorescently-tagged detection Abs using a second chip. The assay results in a mosaic of fluorescence signals on the PDMS surface which are measured using a fluorescence scanner. We show that PDMS is a compatible material for high sensitivity fluorescence assays, provided that detection antibodies with long excitation wavelength fluorophores (≥580 nm) are employed. The chip design, long incubation times, proper choice of fluorophores, and optimization of the detection Ab concentration all combine to achieve high-sensitivity assays. This is exemplified by an experiment with 170 assay sites, occupying an area of ∼0.6 mm2 on PDMS to detect TNF-α in 600 nL of a dendritic cell (DC) culture medium with a sensitivity of ∼20 pg mL−1 (1.14 pM).

Dielectrophoresis-based programmable fluidic processors.

Abstract: Droplet-based programmable processors promise to offer solutions to a wide range of applications in which chemical and biological analysis and/or small-scale synthesis are required, suggesting they will become the microfluidic equivalents of microprocessors by offering off-the-shelf solutions for almost any fluid based analysis or small scale synthesis problem. A general purpose droplet processor should be able to manipulate droplets of different compositions (including those that are electrically conductive or insulating and those of polar or non-polar nature), to control reagent titrations accurately, and to remain free of contamination and carry over on its reaction surfaces. In this article we discuss the application of dielectrophoresis to droplet based processors and demonstrate that it can provide the means for accurately titrating, moving and mixing polar or non-polar droplets whether they are electrically conductive or not. DEP does not require contact with control surfaces and several strategies for minimizing surface contact are presented. As an example of a DEP actuated general purpose droplet processor, we show an embodiment based on a scaleable CMOS architecture that uses DEP manipulation on a 32 × 32 electrode array having built-in control and switching circuitry. Lastly, we demonstrate the concept of a general-purpose programming environment that facilitates droplet software development for any type of droplet processor.

Immunomagnetic T cell capture from blood for PCR analysis using microfluidic systems.

Abstract: A one-step immunomagnetic separation technique was performed on a microfluidic platform for the isolation of specific cells from blood samples. The cell isolation and purification studies targeted T cells, as a model for low abundance cells (about 1∶10,000 cells), with more dilute cells as the ultimate goal. T cells were successfully separated on-chip from human blood and from reconstituted blood samples. Quantitative polymerase chain reaction analysis of the captured cells was used to characterize the efficiency of T cell capture in a variety of flow path designs. Employing many (4–8), 50 µm deep narrow channels, with the same overall cross section as a single, 3 mm wide channel, was much more effective in structuring dense enough magnetic bead beds to trap cells in a flowing stream. The use of 8-multiple bifurcated flow paths increased capture efficiencies from ∼20 up to 37%, when compared to a straight 8-way split design, indicating the value of ensuring uniform flow distribution into each channel in a flow manifold for effective cell capture. Sample flow rates of up to 3 µL min−1 were evaluated in these capture beds.

Continuous cell washing and mixing driven by an ultrasound standing wave within a microfluidic channel

Abstract: Ultrasound standing wave radiation force and laminar flow have been used to transfer yeast cells from one liquid medium to another (washing) by a continuous field-flow fractionation (FFF) approach. Two co-flowing streams, a cell-free suspending phase (flow rate > 50% of the total flow-through volume) and a yeast suspension, were introduced parallel to the nodal plane of a 3 MHz standing wave resonator. The resonator was fabricated to have a single pressure nodal plane at the centre line of the chamber. Laminar flow ensured a stable interface was maintained as the two suspending phases flowed through the sound field. Initiation of the ultrasound transferred cells to the cell-free phase within 0.5 s. This particle transfer procedure circumvents the pellet formation and re-suspension steps of centrifuge based washing procedures. In addition, fluid mixing was demonstrated in the same chamber at higher sound pressures. The channel operates under negligible back-pressure (cross-section, 0.25 × 10 mm) and with only one flow convergence and one flow division step, the channel cannot be easily blocked. The force acting on the cells is small; less than that experienced in a centrifuge generating 100g. The acoustically-driven cell transfer and mixing procedures described may be particularly appropriate for the increasingly complex operations required in molecular biology and microbiology and especially for their conversion to continuous flow processes.

Electroosmotic flow with Joule heating effects

Abstract: Electroosmotic flow with Joule heating effects was examined numerically and experimentally in this work. We used a fluorescence-based thermometry technique to measure the liquid temperature variation caused by Joule heating along a micro capillary. We used a caged-fluorescent dye-based microfluidic visualization technique to measure the electroosmotic velocity profile along the capillary. Sharp temperature drops close to the two ends and a high-temperature plateau in the middle of the capillary were observed. Correspondingly, concave–convex–concave velocity profiles were observed in the inlet–middle–outlet regions of a homogeneous capillary. These velocity perturbations were due to the induced pressure gradients resulting from axial variations of temperature. The measured liquid temperature distribution and the electroosmotic velocity profile along the capillary agree well with the predictions of a theoretical model developed in this paper.

Electrokinetic molecular separation in nanoscale fluidic channels

Abstract: A method for separation of mixtures in fluidic systems through electrokinetic transport by use of nanochannels when the fluidic systems approach the size of an electrical double layer, thereby allowing separation based on charge. The disclosed apparatus comprises a T-chip with a nanochannel section. The method and apparatus are useful for separation of many molecular species, including peptides, proteins, and DNA.