Measurement and analysis of ionic leakage profiles in refrigerated human red blood cells using dielectrophoresis and inductively coupled mass spectroscopy.

Human red blood cells (RBCs) undergo ionic leakage through passive diffusion during refrigerated storage, affecting their quality and health. We investigated the dynamics of ionic leakage in human RBCs over a 20-day refrigerated storage period using extracellular ion quantification and dielectrophoresis (DEP). Four type O- human blood donors were examined to assess the relationship between extracellular ion concentrations (Na+, K+, Mg2+, Ca2+, and Fe2+), RBC cytoplasm conductivity, and membrane conductance. A consistent negative correlation between RBC cytoplasm conductivity and membrane conductance, termed the "ionic leakage profile" (ILP), was observed across the 20-day storage period. Specifically, we noted a gradual decline in DEP-measured RBC cytoplasm conductivity alongside an increase in membrane conductance. Further examination of the electrical origins of this ILP using inductively coupled plasma mass spectrometry revealed a relative decrease in extracellular Na+ concentration and an increase in K+ concentration over the storage period. Correlation of these extracellular ion concentrations with DEP-measured RBC electrical properties demonstrated a direct link between changes in the cytoplasmic and membrane domains and the leakage and transport of K+ and Na+ ions across the cell membrane. Our analysis suggests that the inverse correlation between RBC cytoplasm and membrane conductance is primarily driven by the passive diffusion of K+ from the cytoplasm and the concurrent diffusion of Na+ from the extracellular buffer into the membrane, resulting in a conductive reduction in the cytoplasmic domain and a subsequent increase in the membrane. The ILP's consistent negative trend across all donors suggests that it could serve as a metric for quantifying blood bank storage age, predicting the quality and health of refrigerated RBCs.

High-Throughput Continuous Free-Flow Dielectrophoretic Trapping of Micron-Scale Particles and Cells in Paper Using Localized Nonuniform Pore-Scale-Generated Paper-Based Electric Field Gradients.

Dielectrophoresis (DEP) utilizes a spatially varying nonuniform electrical field to induce forces on suspended polarizable soft matter including particles and cells. Such nonuniformities are conventionally created using 2D or 3D micrometer-scale electrode arrays. Alternatively, insulator-based dielectrophoresis (iDEP) uses small micrometer-scale insulating structures to spatially distort and generate regions of localized field gradients to selectively trap, isolate, and concentrate bioparticles, including bacteria, viruses, red blood cells, and cancer cells from a suspending electrolyte solution. Despite significant advances in the microfabrication technology, the commercial adoption of DEP devices for soft matter manipulation remains elusive. One reason for low market penetration is a lack of low-cost and scalable fabrication methods to quickly microfabricate field-deforming structures to generate localized DEP-inducing electric field gradients. We propose here that paper-based devices can offer a low-cost and easy-to-use alternative to traditional iDEP devices. In this article, we demonstrate for the first time the ability to perform iDEP-style particle trapping using the naturally occurring micrometer-scale insulating porous structures of paper. In particular, we use polymeric laminated nonwoven fiberglass paper channels as a source of insulating structures for iDEP. We apply a flow of polarizable microparticles directly within the nonwoven channel and simultaneously drop an electric field perpendicular to the flow direction to induce DEP. We show the ability to readily trap and concentrate particles in paper by DEP with an applied voltage as low as 2 V using two different flow mechanisms: a constant fluid flow rate using an external pump and passive fluid flow by capillary wicking. Using a combination of micro computed tomography and finite element analysis, we then present a computational model to probe the microscale DEP force formation dynamics within the paper structure. This new paper-based iDEP platform enables the development of robust, low-cost, and portable next-generation iDEP systems for a wide variety of sample purification and liquid handling applications.

Free-flow biomolecular concentration and separation of proteins and nucleic acids using teíchophoresis.

The ability to preconcentrate, separate, and purify biomolecules, such as proteins and nucleic acids, is an important requirement for the next generation of portable diagnostic tools for environmental monitoring and disease detection. Traditionally, such pretreatment has been accomplished using large, centralized liquid- or solid-phase extraction equipment, which can be time-consuming and requires many processing steps. Here, we present a newly developed electrokinetic concentration technique, teíchophoresis (TPE), to concentrate and separate proteins, and to concentrate nucleic acids. In TPE, a free-flowing sample is exposed to a perpendicular electric field in the vicinity of a mass-impermeable conductive wall and a conductive terminating electrolyte (TE), which creates a high electric field strength zone between the lower mobility sample and the no-flux barrier. Unlike a similar electrokinetic concentration method, isotachophoresis (ITP), TPE does not require a leading electrolyte (LE), yet still enables a continuous field-driven electrophoretic ion migration across the channel and a free-flowing biomolecular concentration at the conductive wall. Here, we demonstrate the use of free-flow TPE (FFTPE) to manipulate biomolecular samples containing proteins or nucleic acids. We first use TPE to drive a 6.6-fold concentration increase of avidin-FITC, and also demonstrate protein separation and stacking between ovalbumin-fluorescein and BSA-AlexaFluor 555, both without the use of a conventional LE. Further, we utilize TPE to perform a 21-fold concentration increase of nucleic acids. Our results show that TPE is biocompatible with both proteins and nucleic acids, requires only 10 V DC, produces no significant sample pH changes during operation, and demonstrates that this method can be used as an effective sample pretreatment to prepare biological samples for downstream analysis in a continuous free-flowing microfluidic channel.

Continuous Molecular Concentration and Separation Using Pulsed-Field Conductive-Wall Single-Buffer Teíchophoresis.

We present an experimental study of a novel continuous electrokinetic molecular concentration and separation technique termed teíchophoresis (TPE). We demonstrate here that TPE can serve as a potential alternative to the electrokinetic method isotachophoresis (ITP). In ITP, an electric field serves to focus charged species between a low-mobility terminating electrolyte (TE) and a high-mobility leading electrolyte (LE). Similarly, TPE serves to focus charged species between a low-mobility TE; however, the LE is conveniently replaced with a no-flux boundary generated by a conductive wall. The electric field can still penetrate this no-flux region due to the wall's finite conductivity, but ion migration is impeded due to the physicality of the wall. We perform detailed concentration and separation experiments across varying electric potentials, flow rates, and TE concentrations. We also show that TPE can achieve a 60,000-fold concentration factor continuously without an LE, using only 10 V DC. In comparison with conventional batch-driven ITP, continuous free-flow wall TPE (FFTPE) has the potential to serve as a simplified alternative method. FFTPE offers a high concentration power at a fraction of the required voltage, does not require an LE, and has the increased throughput potential of a continuous process.

Dielectrophoretic detection of electrical property changes of stored human red blood cells.

The ability to transport and store a large human blood inventory for transfusions is an essential requirement for medical institutions. Thus, there is an important need for rapid and low-cost characterization tools for analyzing the properties of human red blood cells (RBCs) while in storage. In this study, we investigate the ability to use dielectrophoresis (DEP) for measuring the storage-induced changes in RBC electrical properties. Fresh human blood was collected, suspended in K2-EDTA anticoagulant, and stored in a blood bank refrigerator for a period of 20 days. Cells were removed from storage at 5-day intervals and subjected to a glutaraldehyde crosslinking reaction to "freeze" cells at their ionic equilibrium at that point in time and prevent ion leakage during DEP analysis. The DEP behavior of RBCs was analyzed in a high permittivity DEP buffer using a three-dimensional DEP chip (3DEP) and also compared to measurements taken with a 2D quadrupole electrode array. The DEP analysis confirms that RBC electrical property changes occur during storage and are only discernable with the use of the cell crosslinking reaction above a glutaraldehyde fixation concentration of 1.0 w/v%. In particular, cytoplasm conductivity was observed to decrease by more than 75% while the RBC membrane conductance was observed to increase by more than 1000% over a period of 20 days. These results show that the presented combination of chemical crosslinking and DEP can be used as rapid characterization tool for monitoring electrical properties changes of human RBCs while subjected to refrigeration in blood bank storage.

Faradaic-free electrokinetic nucleic acid amplification (E-NAAMP) using localized on-chip high frequency Joule heating.

We present a novel Faradaic reaction-free nucleic acid amplification (NAA) method for use with microscale liquid samples. Unlike previous Joule heating methods where the electrodes produce electrolysis gaseous by-products and require both the electrodes be isolated from a sample and the venting of produced electrolysis gas, our electrokinetic Nucleic Acid Amplification (E-NAAMP) method alleviates these issues using a radio frequency (RF) alternating current electric field. In this approach, a pair of microscale thin film gold electrodes are placed directly in contact with a nucleic acid reaction mixture. A high frequency (10-40 MHz) RF potential is then applied across the electrode pair to induce a local Ohmic current within the sample and drive the sample temperature to increase by Joule heating. The temperature increase is sustainable in that it can be generated for several hours of constant use without generating any pH change to the buffer or any microscopically observable gaseous electrolysis by-products. Using this RF Joule heating approach, we demonstrate successful direct thermal amplification using two popular NAA biochemical reactions: loop-mediated isothermal amplification and polymerase chain reaction. Our results demonstrate that a simple microscale electrode structure can be used for thermal regulation for NAA reactions without observable electrolytic reactions, minimal enzyme activity loss and sustained (>50 h use per device) continuous operations without electrode delamination. As such, E-NAAMP offers substantial miniaturization of the heating elements for use in microfluidic or miniaturized NAA reaction systems.

Piezoresistive Conductive Microfluidic Membranes for Low-Cost On-Chip Pressure and Flow Sensing.

Over the last two decades, the field of microfluidics has received significant attention from both academia and industry. Each year, researchers report thousands of new prototype devices for use in a broad range of environmental, pharmaceutical, and biomedical engineering applications. While lab-on-a-chip fabrication costs have continued to decrease, the hardware required for monitoring fluid flows within the microfluidic devices themselves remains expensive and often cost-prohibitive for researchers interested in starting a microfluidics project. As microfluidic devices become capable of handling complex fluidic systems, low-cost, precise, and real-time pressure and flow rate measurement capabilities have become increasingly important. While many labs use commercial platforms and sensors, these solutions can often cost thousands of dollars and can be too bulky for on-chip use. Here we present a new inexpensive and easy-to-use piezoresistive pressure and flow sensor that can be easily integrated into existing on-chip microfluidic channels. The sensor consists of PDMS-carbon black conductive membranes and uses an impedance analyzer to measure impedance changes due to fluid pressure. The sensor costs several orders of magnitude less than existing commercial platforms and can monitor local fluid pressures and calculate flow rates based on the pressure gradient.

Microfluidic pressure in paper (µPiP): rapid prototyping and low-cost liquid handling for on-chip diagnostics.

Paper-based microfluidics was initially developed for use in ultra-low-cost diagnostics powered passively by liquid wicking. However, there is significant untapped potential in using paper to internally guide porous microfluidic flows using externally applied pressure gradients. Here, we present a new technique for fabricating and utilizing low-cost polymer-laminated paper-based microfluidic devices using external pressure. Known as microfluidic pressure in paper (µPiP), devices fabricated by this technique are capable of sustaining a pressure gradient for use in precise liquid handling and manipulation applications similar to conventional microfluidic open-channel designs, but instead where fluid is driven directly through the porous paper structure. µPiP devices can be both rapidly prototyped or scalably manufactured and deployed at commercial scale with minimal time, equipment, and training requirements. We present an analysis of continuous pressure-driven flow in porous paper-based microfluidic channels and demonstrate broad applicability of this method in performing a variety of different liquid handling applications, including measuring red blood cell deformability and performing continuous free-flow DNA electrophoresis. This new platform offers a budget-friendly method for performing microfluidic operations for both academic prototyping and large-scale commercial device production.

A flow-based microfluidic device for spatially quantifying intracellular calcium ion activity during cellular electrotaxis.

How a cell senses, responds, and moves toward, or away from an external cue is central to many biological and medical phenomena including morphogenesis, immune response, and cancer metastasis. Many eukaryotic cells have internal sensory mechanisms that allow them to sense these cues, often in the form of gradients of chemoattractant, voltage, or mechanical stress, and bias their motion in a specific direction. In this study, a new method for using microfluidics to study the electrotactic migration of cells is presented. Electrotaxis (also known as galvanotaxis) is the phenomenon by which cells bias their motion directionally in response to an externally applied electrical field. In this work, we present a new flow-based, salt bridge-free microfluidic device for imaging and quantifying cell motility and intracellular ion activity during electrotaxis. To eliminate salt bridges, we used a low nanoliter flow rate to slowly drive Faradaic waste products away from and out of the electrotaxis zone. This cell migration zone consisted of an array of fluidic confinement channels approximately 2 µm in thickness. This confined height served to insulate the migrating cells from the electric field at the top and bottom of the cell, such that only the two-dimensional perimeter of the cells interacted with the electrical source. We demonstrate the ability to quantify the electrotactic velocity of migrating Dictyostelium discoideum cells and show how this confined design facilitates the imaging and quantification of the ion activity of electrotaxing cells. Finally, by spatially imaging the calcium concentration within these cells, we demonstrate that intracellular calcium preferentially translocates to the leading edge of migrating Dictyostelium cells during electrotaxis but does not exhibit this behavior during migration by chemotaxis in a gradient of cyclic adenosine 3',5'-monophosphate or when cells freely migrate in the absence of an external cue.

Microfluidic platform for the real time measurement and observation of endothelial barrier function under shear stress.

Electric cell-substrate impedance sensing (ECIS) is a quickly advancing field to measure the barrier function of endothelial cells. Most ECIS systems that are commercially available use gold electrodes, which are opaque and do not allow for real-time imaging of cellular responses. In addition, most ECIS systems have a traditional tissue culture Petri-dish set up. This conventional set-up does not allow the introduction of physiologically relevant shear stress, which is crucial for the endothelial cell barrier function. Here, we created a new ECIS micro-bioreactor (MBR) that incorporates a clear electrode made of indium tin oxide in a microfluidic device. Using this device, we demonstrate the ability to monitor the barrier function along culture of cells under varying flow rates. We show that while two cell types align in the direction of flow in responses to high shear stress, they differ in the barrier function. Additionally, we observe a change in the barrier function in response to chemical perturbation. Following exposure to EDTA that disrupts cell-to-cell junctions, we could not observe distinct morphological changes but measured a loss of impedance that could be recovered with EDTA washout. High magnification imaging further demonstrates the loss and recovery of the barrier structure. Overall, we establish an ECIS MBR capable of real-time monitoring of the barrier function and cell morphology under shear stress and allowing high-resolution analysis of the barrier structure.

Microfluidic free-flow zone electrophoresis and isotachophoresis using carbon black nano-composite PDMS sidewall membranes.

We present a new type of free-flow electrophoresis (FFE) device for performing on-chip microfluidic isotachophoresis and zone electrophoresis. FFE is performed using metal gallium electrodes, which are isolated from a main microfluidic flow channel using thin micron-scale polydimethylsiloxane/carbon black (PDMS/CB) composite membranes integrated directly into the sidewalls of the microfluidic channel. The thin membrane allows for field penetration and effective electrophoresis, but serves to prevent bubble generation at the electrodes from electrolysis. We experimentally demonstrate the ability to use this platform to perform on-chip free-flow electrophoretic separation and isotachophoretic concentration. Due to the small size and simple fabrication procedure, this PDMS/CB platform could be used as a part of an on-chip upstream sample preparation toolkit for portable microfluidic diagnostic applications.

Cell Blebbing in Confined Microfluidic Environments.

Migrating cells can extend their leading edge by forming myosin-driven blebs and F-actin-driven pseudopods. When coerced to migrate in resistive environments, Dictyostelium cells switch from using predominately pseudopods to blebs. Bleb formation has been shown to be chemotactic and can be influenced by the direction of the chemotactic gradient. In this study, we determine the blebbing responses of developed cells of Dictyostelium discoideum to cAMP gradients of varying steepness produced in microfluidic channels with different confining heights, ranging between 1.7 µm and 3.8 µm. We show that microfluidic confinement height, gradient steepness, buffer osmolarity and Myosin II activity are important factors in determining whether cells migrate with blebs or with pseudopods. Dictyostelium cells were observed migrating within the confines of microfluidic gradient channels. When the cAMP gradient steepness is increased from 0.7 nM/µm to 20 nM/µm, cells switch from moving with a mixture of blebs and pseudopods to moving only using blebs when chemotaxing in channels with confinement heights less than 2.4 µm. Furthermore, the size of the blebs increases with gradient steepness and correlates with increases in myosin-II localization at the cell cortex. Reduction of intracellular pressure by high osmolarity buffer or inhibition of myosin-II by blebbistatin leads to a decrease in bleb formation and bleb size. Together, our data reveal that the protrusion type formed by migrating cells can be influenced by the channel height and the steepness of the cAMP gradient, and suggests that a combination of confinement-induced myosin-II localization and cAMP-regulated cortical contraction leads to increased intracellular fluid pressure and bleb formation.

Microfluidics made easy: A robust low-cost constant pressure flow controller for engineers and cell biologists.

Over the last decade, microfluidics has become increasingly popular in biology and bioengineering. While lab-on-a-chip fabrication costs have continued to decrease, the hardware required for delivering controllable fluid flows to the microfluidic devices themselves remains expensive and often cost prohibitive for researchers interested in starting a microfluidics project. Typically, microfluidic experiments require precise and tunable flow rates from a system that is simple to operate. While many labs use commercial platforms or syringe pumps, these solutions can cost thousands of dollars and can be cost prohibitive. Here, we present an inexpensive and easy-to-use constant pressure system for delivering flows to microfluidic devices. The controller costs less than half the price of a single syringe pump but can independently switch and deliver fluid through up to four separate fluidic inlets at known flow rates with significantly faster fluid response times. It is constructed of readily available pressure regulators, gauges, plastic connectors and adapters, and tubing. Flow rate is easily predicted and calibrated using hydraulic circuit analysis and capillary tubing resistors. Finally, we demonstrate the capabilities of the flow system by performing well-known microfluidic experiments for chemical gradient generation and emulsion droplet production.

Microfluidic bubbler facilitates near complete mass transfer for sustainable multiphase and microbial processing.

A microfluidic device (channels <70 µm) was utilized to create micro-scale bubbles to significantly increase mass transfer efficiency at low flow rates. The convergence of one gas and two liquid channels at a Y-junction generates bubbles via cyclic changes in pressure. At low flow rates, the bubbles had an average diameter of 110 µm, corresponding to a volumetric mass transfer KL a of 1.43 h(-1) . Values of KL a normalized per flow rate showed that the microbubbler had a 100-fold increased transfer efficiency compared to four other commonly used bubblers. The calculated percentage of oxygen transferred was approximately 90%, which was consistent with a separate off-gas analysis. The improved mass transfer was also tested in an algae bioreactor in which the microbubbler absorbed approximately 90% of the CO2 feed compared to 2% in the culture with an alternative needle bubbling method. The microbubbler yielded a cell density 82% of the cell density for the alternative needle tip with an 800-fold lower flow rate (0.5 mL/min versus 400 mL/min) and a 700-fold higher ratio of biomass to fed carbon dioxide. The application of microfluidics may transform interfacial processing in order to increase mass transfer efficiencies, minimize gas feeding, and provide for more sustainable multiphase processes. Biotechnol. Bioeng. 2016;113: 1924-1933. © 2016 Wiley Periodicals, Inc.

Microfluidic Mixing and Analog On-Chip Concentration Control Using Fluidic Dielectrophoresis.

Microfluidic platforms capable of complex on-chip processing and liquid handling enable a wide variety of sensing, cellular, and material-related applications across a spectrum of disciplines in engineering and biology. However, there is a general lack of available active microscale mixing methods capable of dynamically controlling on-chip solute concentrations in real-time. Hence, multiple microfluidic fluid handling steps are often needed for applications that require buffers at varying on-chip concentrations. Here, we present a novel electrokinetic method for actively mixing laminar fluids and controlling on-chip concentrations in microfluidic channels using fluidic dielectrophoresis. Using a microfluidic channel junction, we co-flow three electrolyte streams side-by-side so that two outer conductive streams enclose a low conductive central stream. The tri-laminar flow is driven through an array of electrodes where the outer streams are electrokinetically deflected and forced to mix with the central flow field. This newly mixed central flow is then sent continuously downstream to serve as a concentration boundary condition for a microfluidic gradient chamber. We demonstrate that by actively mixing the upstream fluids, a variable concentration gradient can be formed dynamically downstream with single a fixed inlet concentration. This novel mixing approach offers a useful method for producing variable on-chip concentrations from a single inlet source.

Label-free biomolecular detection at electrically displaced liquid interfaces using interfacial electrokinetic transduction (IET).

Biosensors require a biorecognition element that specifically binds to a target analyte, and a signal transducer, which converts this targeted binding event into a measurable signal. While current biosensing methods are capable of sensitively detecting a variety of target analytes in a laboratory setting, there are inherent difficulties in developing low-cost portable biosensors for point-of-care diagnostics using traditional optical, mass, or electroanalytical-based signal transducers. It is therefore important to develop new biosensing transducer elements for recognizing binding events at low cost and in portable environments. Here, we demonstrate a novel electrokinetic liquid biosensing method for the sensitive label-free detection of a model biomolecule against a background of serum protein. The biosensor is based on the motion of a microfluidic-generated electrical liquid interface when subjected to an external alternating current electrical field. We demonstrate that the electric field-induced motion of the interface can be used as a sensitive and specific transducer for the detection of avidin at femtomolar concentrations in solution. This new detection strategy does not require surface functionalization or fluorescent labels, and has the potential to serve as a sensitive low-cost method for portable biomarker detection.

Contactless microfluidic pumping using microchannel-integrated carbon black composite membranes.

The ability to pump and manipulate fluid at the micron-scale is a basic requirement for microfluidic platforms. Many current manipulation methods, however, require expensive and bulky external supporting equipment, which are not typically compatible for portable applications. We have developed a contactless metal electro-osmotic micropump capable of pumping conductive buffers. The pump operates using two pairs of gallium metal electrodes, which are activated using an external voltage source and separated from a main flow channel by a thin micron-scale polydimethylsiloxane (PDMS) membrane. The thin contactless membrane allows for field penetration and electro-osmotic flow within the microchannel, but eliminates electrode damage and sample contamination commonly associated with traditional DC electro-osmotic pumps that utilize electrodes in direct contact with the working fluid. Our previous work has demonstrated the effectiveness of this method in pumping deionized water. However, due to the high resistivity of PDMS, this method proved difficult to apply towards manipulating conductive buffers. To overcome this limitation, we fabricated conductive carbon black (CB) powder directly into the contactless PDMS membranes. The increased electrical conductivity of the contactless PDMS membrane significantly increased micropump performance. Using a microfluidic T-channel device and an electro-osmotic flow model, we determined the influence that CB has on pump pressure for CB weight percents varying between 0 and 20. The results demonstrate that the CB increases pump pressure by two orders of magnitude and enables effective operations with conductive buffers.

Correction: Microfluidic pumping, routing and metering by contactless metal-based electro-osmosis.

Correction for 'Microfluidic pumping, routing and metering by contactless metal-based electro-osmosis' by Xiaotong Fu et al., Lab Chip, 2015, DOI: 10.1039/c5lc00504c.

Microfluidic pumping, routing and metering by contactless metal-based electro-osmosis.

Over the past decade, many microfluidic platforms for fluid processing have been developed in order to perform on-chip fluidic manipulations. Many of these methods, however, require expensive and bulky external supporting equipment, which are not typically applicable for microsystems requiring portability. We have developed a new type of portable contactless metal electro-osmotic micropump capable of on-chip fluid pumping, routing and metering. The pump operates using two pairs of gallium metal electrodes, which are activated using an external voltage source, and separated from a main flow channel by a thin micron-scale PDMS membrane. The thin contactless membrane allows for field penetration and electro-osmotic (EO) flow within the microchannel, but eliminates electrode damage and sample contamination commonly associated with traditional DC electro-osmotic pumps that utilize electrodes in direct contact with the working fluid. The maximum flow rates and pressures generated by the pump using DI water as a working buffer are 10 nL min(-1) and 30 Pa, respectively. With our current design, the maximum operational conductivity where fluid flow is observed is 0.1 mS cm(-1). Due to the small size and simple fabrication procedure, multiple micropump units can be integrated into a single microfluidic device for automated on-chip routing and sample metering applications. We experimentally demonstrated the ability to quantify micropump electro-osmotic flowrate and pressure as a function of applied voltage, and developed a mathematical model capable of predicting the performance of a contactless micropump for a given external load and internal hydrodynamic microchannel resistance. Finally, we showed that by activating specific pumps within a microchannel network, our micropumps are capable of routing microchannel fluid flow and generating plugs of solute.

Fluidic dielectrophoresis: The polarization and displacement of electrical liquid interfaces.

Traditional particle-based dielectrophoresis has been exploited to manipulate bubbles, particles, biomolecules, and cells. In this work, we investigate analytically and experimentally how to utilize Maxwell-Wagner polarization to initiate fluidic dielectrophoresis (fDEP) at electrically polarizable aqueous liquid-liquid interfaces. In fDEP, an AC electric field is applied across a liquid electrical interface created between two coflowing fluid streams with different electrical properties. When potentials as low as 2 volts are applied, we observe a frequency-dependent interfacial displacement that is dependent on the relative differences in the electrical conductivity (Δσ) and dielectric constant (Δɛ) between the two liquids. At low frequency this deflection is independent of dielectric constant, while at high frequency it is independent of electrical conductivity. At intermediate frequencies, we observe an fDEP cross-over frequency that is independent of applied voltage, sensitive to both fluid electrical properties, and where no displacement is observed. An analytical fDEP polarization model is presented that accurately predicts the liquid interfacial cross-over frequency, the dependence of interfacial displacement on liquid electrical conductivity and dielectric constant, and accurately scales the measured fDEP displacement data. The results show that miscible aqueous liquid interfaces are capable of polarizing under AC electric fields, and being precisely deflected in a direction and magnitude that is dependent on the applied electric field frequency.