Control of flow direction in microfluidic devices with polyelectrolyte multilayers

Electroosmotic flow (EOF) is commonly utilized in microfluidics. Because the direction of the EOF can be determined by the substrate surface charge, control of the surface chemical state offers the potential, in addition to voltage control, to direct the flow in microfluidic devices. We report the use of polyelectrolyte multilayers (PEMs) to alter the surface charge and control the direction of flow in polystyrene and acrylic microfluidic devices. Relatively complex flow patterns with simple arrangements of applied voltages are realized by derivatization of different arms of a single device with oppositely charged polyelectrolytes. In addition, flow in opposite directions in the same channel is possible. A positively derivatized plastic substrate with a negatively charged lid was used to achieve top-bottom opposite flows. Derivatization of the two sides of a plastic microchannel with oppositely charged polyelectrolytes was used to achieve side-by-side opposite flows. The flow is characterized using fluorescence imaging and particle velocimetry.     

Voltammetric monitoring of transient hydrodynamic flow profiles in microfluidic flow cells

We consider the transition to steady-state flow in the inlet region of a hydrodynamic channel cell and show that a microelectrode positioned within this inlet region allows chronoamperometric results to be recorded, from which information about the extent of the development of the flow profile may be deduced as well as information about the precise dimensions of the microfluidic channel.     

Principles of surface-directed liquid flow in microfluidic channels

To direct liquid flow inside microchannels, surface free energies were patterned by use of self-assembled monolayers (SAMs) in combination with either multistream laminar flow or photolithography. For the photolithographic method, two photocleavable SAMs were designed and synthesized. Carboxylic acid-terminated monolayers were obtained by photodeprotection, which was confirmed by contact angle and X-ray photoelectron spectroscopy. Using either of these patterning methods, we show that aqueous liquids flow only along the hydrophilic pathways when the pressure is maintained below a critical value; the liquids are referred to as being confined by virtual walls. Several principles of liquid flow in surface-patterned channels were derived analytically and verified experimentally. These principles include the maximum pressure that virtual walls can withstand, the critical width of the hydrophilic pathway that can support spontaneous flow, the smallest width of the liquid streams under an external pressure, the critical radius of curvature of turns that can be introduced into the hydrophilic pathway without liquid crossing the hydrophilic-hydrophobic boundary, and the minimal distance for two liquid streams to remain separated under the maximum pressure. Experimental results are in good agreement with the analytical predictions.     

Fluid flow past an aperture in a microfluidic channel

Electroosmotically driven flow in neurotransmitter-based retinal prostheses offers a novel approach to interfacing the nervous system. Here, we show that electroosmotically driven flow in a microfluidic channel can be used either to eject or to withdraw fluid through a small aperture in the channel wall. We study this fluid movement numerically using a finite-element method and experimentally using microfabricated channels and apertures. Two devices are used to test the concept of fluid ejection and withdrawal: (1) a single, large channel with four apertures and (2) a prototype neural interface with four individually addressable apertures. We compared experimental and numerical results in microchannels using the observed pH dependence of the fluorescent dye fluorescein, finding good agreement between the results. Because of the simplicity and rapid response of electroosmotic flow, this technique may be useful for neurotransmitter-based neural interfaces.     

High-throughput and high-resolution flow cytometry in molded microfluidic devices

We describe the design, fabrication, and operation of two types of flow cytometers based on microfluidic devices made of a single cast of poly(dimethylsiloxane). The stream of particles or cells injected into the devices is hydrodynamically focused in both transverse and lateral directions, has a uniform velocity, and has adjustable diameter and shape. The cytometry system built around the first microfluidic device has fluorescence detection accuracy comparable with that of a commercial flow cytometer and can analyze as many as 17 000 particles/s. This high-throughput microfluidic device could be used in inexpensive stand-alone cytometers or as a part of integrated microanalysis systems. In the second device, a stream of particles is focused to a flow layer of a submicrometer thickness that allows imaging the particles with a high numerical aperture microscope objective. To take long-exposure, low-light fluorescence images of live cells, the device is placed on a moving stage, which accurately balances the translational motion of particles in the flow. The achieved resolution is comparable to that of still micrographs. This high-resolution device could be used for analysis of morphology and fluorescence distribution in cells in continuous flow.     

Pneumatic flow switching on centrifugal microfluidic platforms in motion

This paper describes a flow switching technique applicable to centrifugal microfluidic platforms, using a regulated stream of compressed gas. This pneumatic flow switching technique allows for flow control at a T-shaped junction between one inlet channel and two outlet channels. This technique provides a noncontact, robust, and efficient method for switching the direction of fluid flow while a disk is rotating at relatively low frequencies. The switching operation can be implemented reproducibly with applied gas flow rates between 17 and 58 L min(-1) and rotational frequencies between 400 rpm (6.6 Hz) and 1200 rpm (20 Hz).     

Boundary element solution of thermal creep flow in microfluidic devices

Flow in rarefied gases can be caused by a tangential temperature gradient along the contour boundaries (tangential heat flux), without the presence of any other external driven force, inducing a fluid motion from colder to hotter regions. This phenomenon is known as thermal creep and has gained importance in recent years in connection with micro-scale gas flow systems. Prediction of the flow field in micro-systems can be obtained by using continuum based models under appropriate boundary conditions accounting for the slip velocity due to tangential shear rate and heat flux. In this work a boundary integral equation formulation for Stokes slip flow, based on the normal and tangential projection of the Green's integral representational formulae for the velocity field is presented. The tangential heat flux is evaluated in terms of the tangential gradient of the temperature integral representational formulae presenting singularities of the Cauchy type, which are removed by the use of an auxiliary potential field. These formulations are used to evaluate the performance of different microfluidic devices.     

FT-IR spectroscopic imaging of reactions in multiphase flow in microfluidic channels

Rapid, in situ, and label-free chemical analysis in microfluidic devices is highly desirable. FT-IR spectroscopic imaging has previously been shown to be a powerful tool to visualize the distribution of different chemicals in flows in a microfluidic device at near video rate imaging speed without tracers or dyes. This paper demonstrates the possibility of using this imaging technology to capture the chemical information of all reactants and products at different points in time and space in a two-phase system. Differences in the rates of chemical reactions in laminar flow and segmented flow systems are also compared. Neutralization of benzoic acid in decanol with disodium phosphate in water has been used as the model reaction. Quantitative information, such as concentration profiles of reactant and products, can be extracted from the imaging data. The same feed flow rate was used in both the laminar flow and segmented flow systems. The laminar flow pattern was achieved using a plain wide T-junction, whereas the segmented flow was achieved by introducing a narrowed section and a nozzle at the T-junction. The results show that the reaction rate is limited by diffusion and is much slower with the laminar flow pattern, whereas the reaction is completed more quickly in the segmented flow due to better mixing.     

Continuous flow microfluidic device for rapid erythrocyte lysis

Leukocyte isolation from whole blood to study inflammation requires the removal of contaminating erythrocytes. Leukocytes, however, are sensitive to prolonged exposure to hyper/hypoosmotic solutions, temperature changes, mechanical manipulation, and gradient centrifugation. Even though care is taken to minimize leukocyte activation and cell loss during erythrocyte lysis, it is often not possible to completely avoid it. Most procedures for removal of contaminating erythrocytes from leukocyte preparations are designed for bulk processing of blood, where the sample is manipulated for longer periods of time than necessary at the single-cell level. Ammonium chloride-mediated lysis is the most commonly used method to obtain enriched leukocyte populations but has been shown to cause some activation and selective loss of certain cell types. The leukocyte yield and subsequent activation status of residual leukocytes after NH(4)Cl-mediated lysis have been shown to depend on the time of exposure to the lysis buffer. We have developed a microfluidic lysis device that deals with erythrocyte removal at nearly the single-cell level. We can achieve complete lysis of erythrocytes and approximately 100% recovery of leukocytes where the cells are exposed to an isotonic lysis buffer for less than 40 s, after which the leukocytes are immediately returned to physiological conditions. Theoretically, this process can be made massively parallel to process several milliliterss of whole blood to obtain a pure leukocyte population in less than 15 min.     

Detection of kinase translocation using microfluidic electroporative flow cytometry

Directed localization of kinases within cells is important for their activation and involvement in signal transduction. Detection of these events has been largely carried out based on imaging of a low number of cells and subcellular fractionation/Western blotting. These conventional techniques either lack the high throughput desired for probing an entire cell population or provide only the average behaviors of cell populations without information from single cells. Here we demonstrate a new tool, referred to as microfluidic electroporative flow cytometry, to detect the translocation of an EGFP-tagged tyrosine kinase, Syk, to the plasma membrane in B cells at the level of the cell population. We combine electroporation with flow cytometry and observe the release of intracellular kinase out of the cells during electroporation. We found that the release of the kinase was strongly influenced by its subcellular localization. Cells stimulated through the antigen receptor have a fraction of the kinase at the plasma membrane and retain more kinase after electroporation than do cells without stimulation and translocation. We are able to differentiate a cell population with translocation from one without it with the information collected from individual cells of the entire population. This technique potentially allows detection of protein translocation at the single-cell level. Due to the frequent involvement of kinase translocations in disease processes such as oncogenesis, our approach will have utility for kinase-related drug discovery and tumor diagnosis and staging.     

Effect of analyte adsorption on the electroosmotic flow in microfluidic channels

The predictability and constancy over time of the electroosmotic flow in microchannels is an important consideration in microfluidic devices. A common cause for alteration of the flow is the adsorption of analytes to channel walls, for example, during capillary electrophoresis of proteins. It is shown that certain experimental data, published by Towns and Regnier (Towns, J. K; Regnier, F. E. Anal. Chem. 1992, 64, 2473-2478.), on the anomalous elution times for proteins in capillary electrophoresis can be explained using a simple model for analyte adsorption that uses a result first reported by Anderson and Idol (Anderson, J. L.; Idol, W. K Chem. Eng. Commun. 1985, 38, 93-106.) on the electroosmotic flux in capillaries with axial variations in zeta-potential. It is suggested that it might be possible to use such a model to dynamically correct for altered elution times in capillary electrophoretic devices.     

Multiplexed spectral signature detection for microfluidic color-coded bioparticle flow

Here, we report a high-speed photospectral detection technique capable of discriminating subtle variations of spectral signature among fluorescently labeled cells and microspheres flowing in a microfluidic channel. The key component used in our study is a strain-tunable nanoimprinted grating microdevice coupled with a photomultiplier tube (PMT). The microdevice permits acquisition of the continuous spectral profiles of multiple fluorescent emission sources at 1 kHz. Optically connected to a microfluidic flow chamber via a multimode optical fiber, our multiwavelength detection platform allows for cytometric measurement of cell groups emitting nearly identical fluorescence signals with a maximum emission wavelength difference as small as 5 nm. The same platform also allows us to demonstrate microfluidic flow cytometry of four different microsphere types in a wavelength bandwidth as narrow as 40 nm at a high (>85%) confidence level. Our study shows that detection of fluorescent spectral signatures at high speed and high resolution can expand specificity of multicolor flow cytometry. The enhanced capability enables multiplexed analysis of color-coded bioparticles based on single-laser excitation and single-detector spectroscopy in a microfluidic setting. The fluorescence signal discrimination power achieved by the optofluidic technology holds great promise to enable quantification of cellular parameters with higher accuracy as well as enumeration of a larger number of cell types than conventional flow cytometric methods.     

Pressure-driven laminar flow in tangential microchannels: an elastomeric microfluidic switch

This paper describes laminar fluid flow through a three-dimensional elastomeric microstructure formed by two microfluidic channels, fabricated in layers that contact one another face-to-face (typically at a 90 degree angle), with the fluid flows in tangential contact. There are two ways to control fluid flow through these tangentially connected microchannels. First, the flow profiles through the crossings are sensitive to the aspect ratio of the channels; the flow can be controlled by applying external pressure and changing this aspect ratio. Second, the flow direction of an individual laminar stream in multiphase laminar flow depends on the lateral position of the stream within the channel; this position can be controlled by injecting additional streams of fluid into the channel. We describe two microfluidic switches based on these two ways for controlling fluid flow through tangential microchannels and present theoretical arguments that explain the observed dependence of the flow profiles on the aspect ratio of the channels.     

Quantitative microstructural examination of RTM fabrics designed for enhanced flow

The resin transfer moulding (RTM) process involves the long-range flow of resin through a mould packed with dry reinforcement. The process can be considered as similar to the flow of fluids through porous media, and hence the situation can be modelled by the Darcy and Kozeny-Carman equations. The Kozeny-Carman equation predicts the effect of changes in the pore structure of the reinforcement on flow rate, through a parameter known as hydraulic radius, which is itself a function of the wetted surface in any given volume. Laminates have been manufactured, from fabrics which include flow-enhancing tows, in a transparent RTM mould. The cured laminates were sectioned for quantitative microscopy and image analysis. An analysis of the effect of substituting spiral-wound flow-enhancing tows for conventional tows in the reinforcement fabric is presented.     

3D mapping of microfluidic flow in laboratory-on-a-chip structures using optical tweezers

Accurate measurement of flow in microfluidic systems is both challenging and important, providing information that can be used to better understand flow fields within laboratory-on-a-chip devices and validate computational simulations. Here, we use optical tweezers within a microfluidic system to measure the velocity vectors of flow fields in two and three dimensions around a microstructures including both molded features within channels and cells. The experimental results are compared to a complex fluid dynamics model showing an agreement between the two of better than 3 microm/s. This measurement is highly reproducible and minimally invasive, which in the future could be used to provided more in-depth studies of the rheological properties of biological cells and microstructures in laboratory-on-a-chip devices.     

Three-dimensional hydrodynamic lattice-gas simulations of ternary amphiphilic fluids under shear flow

We describe the implementation of shear flow in a three-dimensional lattice-gas model for amphiphilic fluids. We investigate the effect of shear on the morphology of the bicontinuous ternary microemulsion phase, and in particular its effect on the formation and stability of a lamellar phase. Metastable lamellar phases were observed for both 32(3) and 64(3) systems subjected to shear, and the stability of preformed lamellar phases under shear flow established for both 64(3) and 128(3) systems. The effect of micellization on interfacial behaviour in these phases is discussed in the context of the linear non-equilibrium thermodynamic theory of rate processes and nonlinear Becker-D?ring aggregation theory.     

Cell stimulus and lysis in a microfluidic device with segmented gas-liquid flow

We describe a microfluidic device with rapid stimulus and lysis of mammalian cells for resolving fast transient responses in cell signaling networks. The device uses segmented gas-liquid flow to enhance mixing and has integrated thermoelectric heaters and coolers to control the temperature during cell stimulus and lysis. Potential negative effects of segmented flow on cell responses are investigated in three different cell types, with no morphological changes and no activation of the cell stress-sensitive mitogen activated protein kinases observed. Jurkat E6-1 cells are stimulated in the device using alpha-CD3, and the resulting activations of ERK and JNK are presented for different time points. Stimulation of cells performed on chip results in pathway activation identical to that of conventionally treated cells under the same conditions.     

Integrated membrane filters for minimizing hydrodynamic flow and filtering in microfluidic devices

Microfluidic devices have gained significant scientific interest due to the potential to develop portable, inexpensive analytical tools capable of quick analyses with low sample consumption. These qualities make microfluidic devices attractive for point-of-use measurements where traditional techniques have limited functionality. Many samples of interest in biological and environmental analysis, however, contain insoluble particles that can block microchannels, and manual filtration prior to analysis is not desirable for point-of-use applications. Similarly, some situations involve limited control of the sample volume, potentially causing unwanted hydrodynamic flow due to differential fluid heads. Here, we present the successful inclusion of track-etched polycarbonate membrane filters into the reservoirs of poly(dimethylsiloxane) capillary electrophoresis microchips. The membranes were shown to filter insoluble particles with selectivity based on the membrane pore diameter. Electrophoretic separations with membrane-containing microchips were performed on cations, anions, and amino acids and monitored using conductivity and fluorescence detection. The dependence of peak areas on head pressure in gated injection was shown to be reduced by up to 92%. Results indicate that separation performance is not hindered by the addition of membranes. Incorporating membranes into the reservoirs of microfluidic devices will allow for improved analysis of complex solutions and samples with poorly controlled volume.     

Continuous flow microfluidic demixing of electrolytes by induced charge electrokinetics in structured electrode arrays

A continuous flow microfluidic demixing process is realized. It utilizes high external electrical fields that are applied over electrically floating noble metal electrodes in an otherwise straight microchannel. The process converts axial electrical potential gradients into lateral molecular selective transport via a structure oriented ensemble of numerous electrodes. While the individual electrodes locally modify the electrolyte distribution by nonlinear electrokinetic effects and concentration polarization, the directed orientation of the electrode array combines the individual polarization zones to a dedicated molecular enrichment against the generated concentration gradient. A homogeneously concentrated electrolyte can be separated into arbitrarily shaped laminae of increased and depleted concentration by the presented microfluidic demixer.     

Ion flow crossing over a polyelectrolyte diode on a microfluidic chip

Key evidences are reported for the rectification mechanism of an aqueous ion diode based on polyelectrolytic plugs on a microfluidic chip by monitoring the ion flow crossing over the junction. The ion flow penetrating the polyelectrolyte junction is visualized by employing a fluorescent chemodosimeter, rhodamine B hydrazide and the pH-dependent dye, carboxy-fluorescein. How hysteresis phenomena, exhibited through the nonlinear behavior of the polyelectrolyte diode, are affected by a variety of parameters (e.g., switching potential, scan rate, and electrolyte concentration) is also investigated. The insights and knowledge from this study provide a crucial foundation for ion control at the iontronic diode in the aqueous phase, leading to more advanced aqueous organic computing devices and more diverse applications for microfluidic logic devices.