AC Electroosmosis Research Articles

Dielectrophoretic direct assembling of Mxene flakes at the level of screen-printed interdigitated microelectrodes and their evaluation in gas sensing applications

Abstract Controlling, orientating, and assembling 2D materials is critical for their successful exploitation as active elements in various applications, particularly for sensors. Despite the increased interest in exploiting the properties of MXenes, to date, the direct assembly of this category of materials based on an electric field has not been reported. Thus, this work presents an experimental study on dielectrophoretic assembly of pristine MXene flakes at the level of screen-printed interdigitated microelectrodes. The development of MXene uniaxial "bridges" across electrode micro-gaps can be controlled by convenient dielectrophoretic parameters such as voltage and frequency, which are thoroughly discussed. Moreover, appropriate frequencies for avoiding parasitic electrokinetic phenomena (AC electro-osmosis, electrothermal effect) that hamper the application of dielectrophoresis were identified. Finally, the proposed methodology for assembling MXene flakes demonstrates its feasibility of being used for development of chemiresistors as of satisfactory response of pristine MXene “bridges” to ethanol atmosphere.

Study of particle equilibrium based on the combination of light-actuated AC electroosmosis and light-actuated dielectrophoresis.

Optoelectronic tweezers (OETs) represent a flexible, high-throughput method for manipulating micro/nano particles or cells. This technique involves not only light-actuated dielectrophoresis (LDEP) but also light-actuated AC electroosmosis (LACE), which occurs concurrently in OETs devices. Despite this, the combination of negative LDEP and LACE has been relatively unexplored in previous research. To this end, particle equilibrium in OETs devices under the combined influence of negative LDEP and LACE was hereby proposed for what we believe is the first time. The findings revealed that particles experiencing negative dielectrophoresis encountered opposing forces from LDEP and LACE, reaching equilibrium near the light pattern. The location of the equilibrium point was frequency-dependent. The research further demonstrated the rapid differentiation between individual particles and adherent particles by leveraging the distinct equilibrium point positions. These phenomena were corroborated through numerical simulations, which showed a strong correlation between the theoretical analysis results and the experimental data. Overall, the particle equilibrium phenomenon in OET systems exhibits high stability and holds promising potential for future applications in particle or cell sorting and patterning two-dimensional structures.

Rapid and Sensitive Detection by Combining Electric Field Effects and Surface Plasmon Resonance: A Theoretical Study.

Surface plasmon resonance (SPR) has been extensively employed in biological sensing, environmental detection, as well as chemical industry. Nevertheless, the performance possessed by conventional surface plasmon resonance (SPR) biosensors can be further limited by the transport of analyte molecules to the sensing surface, noteworthily when small molecules or low levels of substances are being detected. In this study, a rapid and highly sensitive SPR biosensor is introduced to enhance the ability of the target analytes' collection by integrating AC electroosmosis (ACEO) and dielectrophoresis (DEP). Both the above-mentioned phenomena principally arise from the generation of the AC electric fields. This generation can be tailored by shaping the interdigitated electrodes (IDEs) that also serve as the SPR biomarker sensing area. The effects exerted by different parameters (e.g., the frequency and voltage of the AC electric field as well as microelectrode structures) are considered in the iSPR (interdigitated SPR) biosensor operation, and the iSPR biosensors are optimized with the sensitivity. The results of this study confirm that the iSPR can efficiently concentrate small molecules into the SPR sensing area, such that SPR reactions achieve an order of magnitude increase, and the detection time is shortened. The rapid and sensitive sensor takes on critical significance in the development of on-site diagnostics in a wide variety of human and animal health applications.

Optimization of upstream particle concentration from flow using AC electro-osmosis and dielectrophoresis.

There are many applications where upstream sample processing is required to concentrate dispersed particles in flow; this may be to increase the concentration (e.g., to enhance biosensor accuracy) or to decrease it (e.g., by removing contaminants from flow). The AC electrokinetic phenomenon, dielectrophoresis (DEP), has been used widely for particle trapping for flow, but the magnitude of the force drops reduces rapidly with distance from electrode edges, so that nm-scale particles such as viruses and bacteria are only trapped when near the electrode surface. This limits the usable flow rate in the device and can render the final device unusable for practical applications. Conversely, another electrokinetic phenomenon, AC electro-osmosis (ACEO), can be used to move particles to electrode surfaces but is unable to trap them from flow, limiting their ability for sample cleanup or trap-and-purge concentration. In this paper, we describe the optimization of ACEO electrodes aligned parallel to pressure-driven flow as a precursor/preconditioner to capture particles from a flow stream and concentrate them adjacent to the channel wall to enhance DEP capture. This is shown to be effective at flow rates of up to 0.84 ml min-1. Furthermore, the analysis of the 3D flow structure in the ACEO device by both simulation and confocal microscopy suggests that while the system offers significant benefits, the flow structure in the volume near the channel lid is such that while substantial trapping can occur, particles in this part of the chamber cannot be trapped, independent of the chamber height.

Screen-printed interdigitated microelectrodes employment in dielectrophoretic manipulation of MWCNTs.

As key enablers of Industry 4.0 and Internet of Things, sensors are among the first devices which are to encounter fast physical transformation (from rigid to flexible) as of large-scale utilization of printing technologies. In order to step-up this process, adaptation of conventional fabrication technologies (based on metallization) employed in sensors' development should be tested and demonstrated. Within thispaper, we are reporting the functionality of dielectrophoresis (DEP)for electromanipulation of multi-walled carbon nanotubes (MWCNTs) as sensing element, at the level of printed interdigitated electrodes. First, we present the flatbed screen-printed process of interdigitated microelectrodes on flexible substrate with tailored geometries employed afterwards for generating convenient dielectrophoretic forces of optimal magnitude and frequency for trapping MWCNTs. Successful dielectrophoresis operability of MWCNTs across silver-based screen-printed μIDE (interdigitated microelectrodes) provided with electrode gaps of ≈ 150μm was validated and suitable values of the signal frequencies for avoiding parasitic electrokinetic phenomena (AC electro-osmosis, electrothermal effect) occurring simultaneously with DEP were identified. Time-dependent effect of DEPover MWCNTs bridges formation is discussed, as well as voltage magnitude contribution.

Three-dimensional AC electroosmotic micropump with high power density

This paper presents a novel three-dimensional alternating current electroosmosis (ACEO) micropump with high power density. ACEO is a phenomenon to generate a flow of a liquid such as water by applying an AC voltage to the liquid. Using a specific configuration, a net flow due to ACEO becomes unidirectional and an ACEO micropump can be constructed. Many ACEO micropumps have been proposed; however, most of them are of the planar type, and their output power per device volume is not sufficiently high for applications such as micro fluid power sources for micro total analysis systems, lab-on-a-chips, biomedical microelectromechanical systems (bio-MEMSs), soft microactuators, and soft microrobots. To achieve a higher output power density, in this study, we proposed an ACEO micropump using an array of plate-cylinder electrodes, which is called PC-ACEO-MP. To induce unidirectional flow efficiently, a square pole-slit electrodes ACEO micropump called SS-ACEO-MP has been proposed using asymmetrical pillar-shaped electrodes. PC-ACEO-MP is an extension of SS-ACEO-MP with a three-dimensional structure. First, the finite element method simulations were performed and the results showed that PC-ACEO-MP has the potential to realize high output power with a 1 cm3 effective pump volume using large numbers of plate-cylinder electrodes connected in parallel and in series. Second, a fabrication method using MEMS fabrication process, including electroplating, was proposed and developed for a micro-holed electrode plate that forms parallel cylinder electrodes. The validity of the proposed process was confirmed by fabricating large models and micro-models of the micro-holed electrode plate. The diameter and number of microholes were 10 μm and 36 100, respectively, for the large models and 3 μm and 65 500, respectively, for the micro models. Finally, PC-ACEO-MPs were constructed and their promising pumping characteristics were clarified through experiments using deionized water. The estimated power density was approximately 400 times higher than that of the former high output power ACEO micropump.

Long-Range ACEO Phenomena in Microfluidic Channel

Microfluidic devices are increasingly utilized in numerous industries, including that of medicine, for their abilities to pump and mix fluid at a microscale. Within these devices, microchannels paired with microelectrodes enable the mixing and transportation of ionized fluid. The ionization process charges the microchannel and manipulates the fluid with an electric field. Although complex in operation at the microscale, microchannels within microfluidic devices are easy to produce and economical. This paper uses simulations to convey helpful insights into the analysis of electrokinetic microfluidic device phenomena. The simulations in this paper use the Navier–Stokes and Poisson Nernst–Planck equations solved using COMSOL to determine the maximum attainable fluid velocity with an electric potential applied to the microchannel and the most suitable frequency or voltage to use for transporting the fluid. Alternating current electroosmosis (ACEO) directs and provides velocity to the ionized fluid. ACEO can also mix the fluid at low frequencies for the purpose of dispersing particles. DC electroosmosis (DCEO) applies voltage along the microchannel to create an electric field that ionizes fluid within the microchannel, making it a cost-effective method for transporting fluid. This paper explores a method for an alternate efficient utilization of microfluidic devices for efficient mixing and transportation of ionized fluid and analyzes the electrokinetic phenomena through simulations using the Navier–Stokes and Poisson Nernst–Planck equations. The results provide insights into the parameters at play for transporting the fluid using alternating current electroosmosis (ACEO) and DC electroosmosis (DCEO).

Simulation and experimental validation of the interplay between dielectrophoretic and electroosmotic behavior of conductive and insulator particles for nanofabrication and lab-on-chip applications

In this article, we investigated the different roles of dielectrophoresis (DEP) and AC electroosmosis (ACEO) phenomena in manipulating conductive and insulator particles as future fabrication and deposition technique. A computational model was implemented using the finite element method (FEM) to better understand the interplay and velocities caused by these two electrokinetic phenomena. The simulation results showed that Carbon Nanotubes (CNTs) experience positive DEP (pDEP) up to frequencies of 1 GHz, while polystyrene (PS) particles only showed negative DEP (nDEP) regardless of the frequency. The velocity due to the ACEO was significant at the electrodes’ level at low frequencies, especially between 100 Hz and 10 kHz. The simulation results were validated experimentally, where CNTs and PS were dispersed in a medium and subjected to non-uniform electric fields. The behavior and patterns the particles formed on planner interdigitated electrodes (IDEs) match with their trajectories from the simulation. At a frequency of 1 MHz, CNTs were attracted to regions with strong electric fields, while PS particles were repelled to regions with low electric fields indicating strong pDEP and nDEP, respectively. A cloud of particles was observed at 5 kHz, indicating strong ACEO at low frequencies. This study provides a simplified and reliable technique that can be applied to manipulate different types of particles for future multilayer fabrication, thin film deposition, and lap-on-chip applications.

Effects of 3D electrodes arrangement in a novel AC electroosmotic micropump: Numerical modeling and experimental validation.

To date, a comprehensive systematic optimization framework, capable of accurately predicting an efficient electrode geometry, is not available. Here, different geometries, including 3D step electrodes, have been designed in order to fabricate AC electroosmosis micropumps. It is essential to optimize both geometrical parameters of electrode, such as width and height of steps on each base electrode and their location in one pair, the size of each base electrode (symmetric or asymmetric), the gap of electrode pairs, and nongeometrical parameters such as fluid flow in a channel and electrical characteristics (e.g., frequency and voltage). The governing equations comprising of electric domain and fluid domain have been coupled using finite element method. The developed model was employed to investigate the effect of electrode geometric parameters on electroosmotic slip velocity and its subsequent effect on pressure and flow rate. Numerical simulation indicates that the optimal performance can be achieved using a design with varying step height and displacement, at a given voltage (2.5V) and frequency (1kHz). Finally, in order to validate the numerical simulation, the optimal microchip was fabricated using a combination of photolithography, electroplating, and a polydimethylsiloxane microchannel. Our results indicate that our micropump is capable of generating a pressure, velocity, and flow rate of 74.2Pa, 1.76mm/s, and 14.8µl/min, respectively. This result reveals that our proposed geometry outperforms the state-of-the-art micropumps previously reported in the literature by improving the fluid velocity by 32%, with 80% less electrodes per unit length, and whereas the channel length is ∼80% shorter.

Back Cover: Efficient nanoparticle focusing utilizing cascade AC electroosmotic flow

DOI: 10.1002/elps.202200054 The cover picture shows continuous focusing of nanoscale samples using a cascade AC electroosmosis (cACEO) flow. ACEO flow is produced by applying AC signal to the coplanar integrated electrode. The cascade electrode, combining chevron and double-gap geometry, was developed to perform efficient 3D nanoparticle focusing. Chevron electrode was used to generate inward flow from the sidewall to the center area to accumulate particles from the scattering area; then, the DG electrode provides the necessary ACEO to concentrate nanoparticles. The study shows that the cACEO concentrator holds significant potential to concentrate nanoscale samples, such as particles, rare samples, and biomaterials.

AC electrokinetic immobilization of influenza virus.

The use of alternating current (AC) electrokinetic forces, like dielectrophoresis and AC electroosmosis, as a simple and fast method to immobilize sub-micrometer objects onto nanoelectrode arrays is presented. Due to its medical relevance, the influenza virus is chosen as a model organism. One of the outstanding features is that the immobilization of viral material to the electrodes can be achieved permanently, allowing subsequent handling independently from the electrical setup. Thus, by using merely electric fields, we demonstrate that the need of prior chemical surface modification could become obsolete. The accumulation of viral material over time is observed by fluorescence microscopy. The influences of side effects like electrothermal fluid flow, causing a fluid motion above the electrodes and causing an intensity gradient within the electrode array, are discussed. Due to the improved resolution by combining fluorescence microscopy with deconvolution, it is shown that the viral material is mainly drawn to the electrode edge and to a lesser extent to the electrode surface. Finally, areas of application for this functionalization technique are presented.

AC electroosmosis micromixing on a lab-on-a-foil electric microfluidic device

Efficient mixing of fluids in lab-on-a-chip devices is very important for many biomedical and biochemical applications. Lab-on-a-foil as a novel concept provides a method for fast prototyping or mass production of microfluidic devices based on thin and flexible films materials. In this article, electroosmosis micromixing is conducted in a lab-on-a-foil microfluidic device. With the electroosmotic flow (EOF), an efficient micromixing is realized inside a microchannel by tooth-shaped planar electrodes. The mixing performance is evaluated based on intensity measurement, and frequency sweeping is used to identify optimal performance. Furthermore, according to local intensity profiles, the EOF pattern is analyzed to provide a deep understanding on the influence of frequency and flow rate. The amplitude of voltage and the number of pairs of electrode tooth are also investigated to find the optimal conditions of the device. To the best of our knowledge, this is the first demonstration of the AC EOF in a lab-on-a-foil electric device and the exploration of EOF pattern vertically and horizontally in the microchannels. This study provides a method to optimize mixing performance in EOF-based micromixer. Furthermore, the fabrication method cast the potential for mass production of low-cost flexible electric microfluidic devices.

AC-Electroosmosis-Assisted Surface Plasmon Resonance Sensing for Enhancing Protein Signals with a Simple Kretschmann Configuration.

A surface plasmon resonance (SPR) sensor chip fabricated with a comb-shaped microelectrode array to supply alternating current (AC) voltage is reported. The chip induces circulating flow near the surface (i.e., AC electroosmosis). The circulating flow provides a mixing effect, which enhances the binding of the analyte molecules. We evaluated the SPR characteristics of the chip and demonstrated an improvement in protein binding to the chip surface. SPR sensor chips with comb-shaped microelectrodes were fabricated using standard UV lithography. Sensing experiments were conducted using a standard Kretschmann-type SPR measurement system. To demonstrate the mixing effect of AC electroosmosis, we evaluated the binding of immunoglobulin G molecules onto the sensor surface where anti-immunoglobulin G antibodies were covalently immobilized. The result indicates that the amount of binding increases by a factor of 1.7 above that achieved by using a conventional chip, suggesting enhancement of the protein signal.

Front Cover: Enhancing affinity‐based electroanalytical biosensors by integrated AC electrokinetic enrichment—A mini review

DOI: 10.1002/elps.202100168 The cover picture shows how affinity-based electroanalytical biosensors can be enhanced by integrated AC electrokinetic (ACEK) effects. ACEK effects, including AC electrothermal (ACET) effect, AC electroosmosis (ACEO) effect and dielectrophoretic (DEP) effect, are produced by applying AC signals to the interdigitated electrode arrays. ACEK effects induce directed movement of microflows and particles (e.g., proteins, nucleic acids, bacteria, small molecules, etc.) in the solution, so that the analytes in the solution can be quickly routed toward the surface of biosensors, thus improving the binding efficiency between the analytes and the probes on the surface of the sensor. Studies have shown that the method holds great promise for achieving high sensitivity, low detection limit and rapid turnaround.

Passive solute separation in AC electroosmosis including surface charge-coupled hydrodynamic slip effects.

Separation of electrically neutral, mutually noninteracting passive solutes via AC electroosmotic slit channel flows is investigated for general asymmetric wall surface zeta potentials and apparent hydrodynamic slip lengths. We consider the nontrivial coupling between the surface potentials (or charge densities) and the apparent slip lengths, and focus our attention on the occurrence of a so called "crossover phenomenon" for separating out the slow diffusers when both slow and fast diffusers are present. Results show that regardless of the potential-slip coupling, wider bandwidths become available for crossover phenomenon to occur when the electroosmotic velocity gradient (magnitude) is greater. Contrarily, plug-like velocity profiles inhibit crossover phenomenon, and the potential-slip parametric combinations leading to such profiles can be easily identified by the conditions for minimal transport enhancement reported in recent literature. When separating out the slow diffuser or crossover phenomenon is desired, we recommend incorporating significant asymmetry in the surface potential and apparent slip boundary conditions such that the operating frequency and flow oscillation amplitude may be lowered to more practical values. Our results also agree with and strengthen the physical picture for explaining crossover phenomenon in macroscopic pressure-driven oscillatory flows.

A 3D-ACEK/SERS system for highly efficient and selectable electrokinetic bacteria concentration/detection/ antibiotic-susceptibility-test on whole blood

This study demonstrates a novel multi-functional microfluidic system, designated three dimensional Alternative Current Electrokinetic/Surface Enhanced Raman Scattering (3D-ACEK/SERS), which can concentrate bacteria from whole blood, identify bacterial species, and determine antibiotic susceptibilities of the bacteria rapidly. The system consists of a hybrid electrokinetic mechanism, integrating AC-electroosmosis (AC-EO) and dielectrophoresis (DEP) that allows thousand-fold concentration of bacteria, including S. aureus, Escherichia coli, and Chryseobacterium indologenes, in the center of an electrode with a wide range of working distance (hundreds to thousands of μm), while exclusion of blood cells through negative DEP forces. This microchip employs SERS assay to determine the identity of the concentrated bacteria in approximately 2 min with a limit of detection of 3 CFU/ml, 5 orders of magnitude lower than that using standard centrifugation−purification process. Finally, label-free antibiotic susceptibility testing has been successfully demonstrated on the platform using both antibiotic-sensitive and multidrug-resistant bacterial strains illustrating a potential utility of the system to clinical applications.

Self-powered AC electrokinetic microfluidic system based on triboelectric nanogenerator

The AC electrokinetic technology, such as AC electroosmosis, induced-charged electroosmosis, dielectrophoresis, etc., can precisely manipulate trace fluids and has been widely used in lab on a chip. However, the power supplies used in AC electrokinetic experiments are mostly function generators or other large-scare instruments, which is an obstacle to portability. Encouragingly, triboelectric nanogenerator (TENG) with the characteristics of portable, green, and simple structure is a potential opportunity for AC electrokinetic microfluidic operation. We have designed a self-powered AC electrokinetic microfluidic system based on vertical contact-separation mode TENG for the first time. The TENG, as a power supply, costs only 2 cents and maintains excellent output performance after millions of working cycles. Through the combination of TENG and microfluidic chip, this system successfully realizes AC electroosmotic flow and induced-charged electroosmotic flow in the microchannel and completes the effective mixing of two fluids at low frequencies (<10 Hz). Furthermore, we have achieved precise control of particle behavior, including focusing, deflection, and separation under manual operation. Therefore, the self-powered AC electrokinetic microfluidic system designed in this work not only expands the application fields of TENG but also provides a new power solution for AC electrokinetic microfluidic operation. This system is expected to get rid of the need for large-scale power supply in traditional methods and has broad application prospects in the fields of portable detection, micro-reaction, and micro-bioanalysis.

Mixing Mechanism of Microfluidic Mixer with Staggered Virtual Electrode Based on Light-Actuated AC Electroosmosis.

In this paper, we present a novel microfluidic mixer with staggered virtual electrode based on light-actuated AC electroosmosis (LACE). We solve the coupled system of the flow field described by Navier–Stokes equations, the described electric field by a Laplace equation, and the concentration field described by a convection–diffusion equation via a finite-element method (FEM). Moreover, we study the distribution of the flow, electric, and concentration fields in the microchannel, and reveal the generating mechanism of the rotating vortex on the cross-section of the microchannel and the mixing mechanism of the fluid sample. We also explore the influence of several key geometric parameters such as the length, width, and spacing of the virtual electrode, and the height of the microchannel on mixing performance; the relatively optimal mixer structure is thus obtained. The current micromixer provides a favorable fluid-mixing method based on an optical virtual electrode, and could promote the comprehensive integration of functions in modern microfluidic-analysis systems.

AC electrokinetic isolation and detection of extracellular vesicles from dental pulp stem cells: Theoretical simulation incorporating fluid mechanics.

Extracellular vesicles (EVs) are cell-derived nanoscale vesicles involved in intracellular communication and the transportation of biomarkers. EVs released by mesenchymal stem cells have been recently reported to play a role in cell-free therapy of many diseases. However, the demand for better research tools to replace the tedious conventional methods used to study EVs is getting stronger. EVs' manipulation using alternating current (AC) electrokinetic forces in a microfluidic device has appeared to be a reliable and sensitive diagnosis and trapping technique. Given that different AC electrokinetic forces may contribute to the overall motion of particles and fluids in a microfluidic device, EVs' electrokinetic trapping must be examined considering all dominant forces involved depending on the experimental conditions. In this paper, AC electrokinetic trapping of EVs using an interdigitated electrode arrays is investigated. A 2D numerical simulation incorporating the two significant AC electrokinetic phenomena (Dielectrophoresis and AC electroosmosis) has been performed. Theoretical predictions are then compared with experimental results and allow for a plausible explanation of observations inconsistent with DEP theory. It is demonstrated that the inconsistencies can be attributed to a significant extent to the contribution of the AC electroosmotic effect.

Theory and simulation of electroosmotic suppression of acoustic streaming.

Acoustic handling of nanoparticles in resonating acoustofluidic devices is often impeded by the presence of acoustic streaming. For micrometer-sized acoustic chambers, this acoustic streaming is typically driven by viscous shear in the thin acoustic boundary layer near the fluid-solid interface. Alternating current (ac) electroosmosis is another boundary-driven streaming phenomenon routinely used in microfluidic devices for the handling of particle suspensions in electrolytes. Here, we study how streaming can be suppressed by combining ultrasound acoustics and ac electroosmosis. Based on a theoretical analysis of the electrokinetic problem, we are able to compute numerically a form of the electrical potential at the fluid-solid interface, which is suitable for suppressing the typical acoustic streaming pattern associated with a standing acoustic half-wave. In the linear regime, we even derive an analytical expression for the electroosmotic slip velocity at the fluid-solid interface and use this as a guiding principle for developing models in the experimentally more relevant nonlinear regime that occurs at elevated driving voltages. We present simulation results for an acoustofluidic device, showing how implementing a suitable ac electroosmosis results in a suppression of the resulting electroacoustic streaming in the bulk of the device by 2 orders of magnitude.