Continuous-Flow Nanoparticle Trapping Driven by Hybrid Electrokinetics in Microfluidics

A Novel Micromixer That Exploits Electrokinetic Vortices Generated on a Janus Droplet Surface

Micromixers play a crucial role as essential components in microfluidic analysis systems. This paper introduces a novel micromixer designed by harnessing electrokinetic vortices arising on the surface of a Janus droplet within a microchannel. The Janus droplet is characterized by different polarities of charges on its two sides (upstream part and downstream part). In the presence of a direct current electric field, the droplet's surface generates electroosmotic flows in opposite directions, resulting in the formation of vortices and facilitating solution mixing. Results from numerical simulations suggest that a better mixing performance of the micromixer is associated with both a higher absolute value of the zeta potential ratio between the downstream and upstream surfaces of the Janus droplet and a larger downstream surface area. Additionally, this study reveals that microchannel dimensions significantly influence the performance of the micromixer. Smaller microchannel widths and heights correspond to a larger mixing index for the micromixer. The micromixer presented in this study features a simple structure, easy fabrication, and holds promising application potential.

Electrokinetic Effect of a Two-Liquid Interface within a Slit Microchannel

This paper presents an investigation of the electrokinetic effect at a two-liquid (immiscible liquid-aqueous solution) interface within a slit microchannel using a three-dimensional (3D) numerical model, with a particular focus on the impact of the surface ζ-potential and liquid phase height on the interface electrokinetic velocity. The findings indicate that the direction of the interface movement depends on the ζ-potentials at the two-liquid interface and the microchannel wall. When the absolute value of the negative ζ-potential at the interface is smaller than that at the wall, the interface moves toward the negative pole of the applied direct current (DC) electric field; conversely, it moves in the opposite direction. The velocity of interface motion decreases as the height of the aqueous phase and the dynamic viscosity ratio between the immiscible liquid and the aqueous solution increase. Conversely, the velocity increases with an elevation in the height of the immiscible liquid phase and the DC electric field intensity. This study holds significant importance in elucidating the patterns of change in fluid interface electrokinetic effects and their potential applications in manipulating and separating particulate pollutants within water systems.

Electropatterning-Contemporary developments for selective particle arrangements employing electrokinetics

The selective positioning and arrangement of distinct types of multiscale particles can be used in numerous applications in microfluidics, including integrated circuits, sensors and biochips. Electrokinetic (EK) techniques offer an extensive range of options for label-free manipulation and patterning of colloidal particles by exploiting the intrinsic electrical properties of the target of interest. EK-based techniques have been widely implemented in many recent studies, and various methodologies and microfluidic device designs have been developed to achieve patterning two- and three-dimensional (3D) patterned structures. This review provides an overview of the progress in electropatterning research during the last 5 years in the microfluidics arena. This article discusses the advances in the electropatterning of colloids, droplets, synthetic particles, cells, and gels. Each subsection analyzes the manipulation of the particles of interest via EK techniques such as electrophoresis and dielectrophoresis. The conclusions summarize recent advances and provide an outlook on the future of electropatterning in various fields of application, especially those with 3D arrangements as their end goal.

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 (2.5 V) and frequency (1 kHz). 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 74.2 Pa, 1.76 mm/s, and 14.8 µl/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.

Electromagnetic Forces and Torques: From Dielectrophoresis to Optical Tweezers

Electromagnetic forces and torques enable many key technologies, including optical tweezers or dielectrophoresis. Interestingly, both techniques rely on the same physical process: the interaction of an oscillating electric field with a particle of matter. This work provides a unified framework to understand this interaction both when considering fields oscillating at low frequencies─dielectrophoresis─and high frequencies─optical tweezers. We draw useful parallels between these two techniques, discuss the different and often unstated assumptions they are based upon, and illustrate key applications in the fields of physical and analytical chemistry, biosensing, and colloidal science.

Separation of Cells and Microparticles in Insulator-Based Electrokinetic Systems

Presented here is the first continuous separation of microparticles and cells of similar characteristics employing linear and nonlinear electrokinetic phenomena in an insulator-based electrokinetic (iEK) system. By utilizing devices with insulating features, which distort the electric field distribution, it is possible to combine linear and nonlinear EK phenomena, resulting in highly effective separation schemes that leverage the new advancements in nonlinear electrophoresis. This work combines mathematical modeling and experimentation to separate four distinct binary mixtures of particles and cells. A computational model with COMSOL Multiphysics was used to predict the retention times (t_R,p) of the particles and cells in iEK devices. Then, the experimental separations were carried out using the conditions identified with the model, where the experimental retention time (t_R,e) of the particles and cells was measured. A total of four distinct separations of binary mixtures were performed by increasing the level of difficulty. For the first separation, two types of polystyrene microparticles, selected to mimic Escherichia coli and Saccharomyces cerevisiae cells, were separated. By leveraging the knowledge gathered from the first separation, a mixture of cells of distinct domains and significant size differences, E. coli and S. cerevisiae, was successfully separated. The third separation also featured cells of different domains but closer in size: Bacillus cereus versus S. cerevisiae. The last separation included cells in the same domain and genus, B. cereus versus Bacillus subtilis. Separation results were evaluated in terms of number of plates (N) and separation resolution (R_s), where R_s values for all separations were above 1.5, illustrating complete separations. Experimental results were in agreement with modeling results in terms of retention times, with deviations in the 6-27% range, while the variation between repetitions was between 2 and 18%, demonstrating good reproducibility. This report is the first prediction of the retention time of cells in iEK systems.

Efficient nanoparticle focusing utilizing cascade AC electroosmotic flow

This study presents on‐chip continuous accumulation and concentration of nanoscale samples using a cascade alternating current electroosmosis (cACEO) flow. ACEO can generate flow motion caused by ion movement due to interactions between the AC electric field and the induced charge layer on the electrode surface, with the potential to accumulate particles, especially in low‐conductive liquid. However, the intrinsic particle diffusive motion, which is sensitive to particle size, is an essential element influencing accumulation efficiency. In this study, an electrode combining chevron and double‐gap geometry embedded in a microfluidic channel was developed to perform efficient three‐dimensional (3D) nanoparticle focusing using ACEO. The chevron electrode pattern was introduced upstream of the focusing zone to overcome particle accumulation in scattering zones near the channel sidewall. To demonstrate the efficiency of the proposed device for particle accumulation, three nanoparticle types were used: latex, metal, and biomaterial. Continuous 3D concentration of 50‐nm polystyrene particles was confirmed. The concentration factor, determined based on image processing, became quite high when 50‐nm gold nanoparticles were used. Moreover, nanoparticles with a 20‐nm diameter were accumulated using cACEO. Finally, we used the concentrator chip to accumulate 50‐nm liposome particles, confirming that the device could also successfully concentrate biomaterials.

Unique Properties of Surface-Functionalized Nanoparticles for Bio-Application: Functionalization Mechanisms and Importance in Application

This review tries to summarize the purpose of steadily developing surface-functionalized nanoparticles for various bio-applications and represents a fascinating and rapidly growing field of research. Due to their unique properties—such as novel optical, biodegradable, low-toxicity, biocompatibility, size, and highly catalytic features—these materials are considered superior, and it is thus vital to study these systems in a realistic and meaningful way. However, rapid aggregation, oxidation, and other problems are encountered with functionalized nanoparticles, inhibiting their subsequent utilization. Adequate surface modification of nanoparticles with organic and inorganic compounds results in improved physicochemical properties which can overcome these barriers. This review investigates and discusses the iron oxide nanoparticles, gold nanoparticles, platinum nanoparticles, silver nanoparticles, and silica-coated nanoparticles and how their unique properties after fabrication allow for their potential use in a wide range of bio-applications such as nano-based imaging, gene delivery, drug loading, and immunoassays. The different groups of nanoparticles and the advantages of surface functionalization and their applications are highlighted here. In recent years, surface-functionalized nanoparticles have become important materials for a broad range of bio-applications.

Integrated Nanomaterials and Nanotechnologies in Lateral Flow Tests for Personalized Medicine Applications

The goal of personalized medicine is to target the right treatments to the right patients at the right time. Patients with a variety of cancers and other complex diseases are regularly tested as part of patient care, enabling physicians to personalize patient monitoring and treatment. Among the sought-after diagnostic tools, there is an increasing interest and need for those based on a low-cost, easy, rapid, and accurate method for the detection of specific circulating biomarkers above a detection threshold. Lateral flow tests (LFTs), enhanced by nanotechnology, can fulfil these requirements, providing a significant support to personalized patient monitoring. In this review, after a short historical synopsis of membrane-based lateral flow assays, including a description of a typical configuration of a LFT strip, a careful collection is presented of the best characterized nanotechnology approaches previously reported for the enhancement of target detection performance. The attempt is to offer an overview of currently integrated nanotechnologies in LFTs, fostering the actual future development of advantageous diagnostic devices for patient monitoring.

Enhancing affinity-based electroanalytical biosensors by integrated AC electrokinetic enrichment-A mini review

Biosensors play a central role in moving diagnostics to being on-site or decentralized. Affinity biosensor, an important category of biosensors, has important applications in clinical diagnosis, pharmaceuticals, immunology, and other fields. Affinity biosensors rely on specific binding between target analytes and biological ligands such as antibodies, nucleic acids, or other receptors to generate measurable signals. Oftentimes the target analytes in practical samples are of low abundance in a complex matrix. Traditional affinity biosensors mainly rely on random diffusion of analytes in solution to conjugate with biorecognition elements on the sensor surface of electrodes. The process may take hours or even days, which is not conducive to rapid and sensitive detection of biosensors. Therefore, it is strongly desired to incorporate an enrichment mechanism for target analytes into biosensor-based detection. AC electrokinetic (ACEK) effect can realize rapid enrichment of analytes by application of AC electric fields, which holds great promise for achieving high sensitivity, low detection limit, and rapid turnaround. This article reviews the studies of affinity biosensors integrated with ACEK enrichment in the past decade, and summarizes the latest detection methods, detection devices and applications, hoping to provide some insights and references for researchers in related fields.

Microparticle transport along a planar electrode array using moving dielectrophoresis

We present a device that can achieve controlled transport of colloidal microparticles using an array of micro-electrodes. By exciting the micro-electrodes in regular sequence with an AC voltage, a time-varying moving dielectrophoretic force-field is created. This force propels colloidal microparticles along the electrode array. Using this method, we demonstrate bidirectional transport of polystyrene micro-spheres. Electromagnetic simulation of the device is performed, and the dielectrophoretic force profile around the electrode array is mapped. We develop a Brownian dynamics model of the trajectory of a particle under the influence of the time-varying force-field. Numerical and experimental results showing controlled particle transport are presented. The numerical model is found to be in good agreement with experimental data. The developed numerical framework can be useful in designing and modeling lab-on-a-chip devices that employ external non-contact forces for micro-/nanoparticle manipulation.

Adsorption-Reaction Processes Between Gelatin and PDMS-E Emulsion Droplets

In this work, the effects of droplet size and reaction time on the adsorption-reaction processes between gelatin and α-[3-(2,3-epoxypropoxy) propyl]-ω-butyl-polydimethylsiloxane (PDMS-E) emulsion droplets were studied. Gelatin molecules were only physically adsorbed on the surface of the PDMS-E droplet in the 0-75 min range, which was unrelated to the droplet size (100-1000 nm). For the small-size droplets (<410 nm), the physical adsorption proceeded over 75 min followed by agglomeration. For middle-size droplets (410-680 nm), the physical adsorption finished at 125 min; subsequently, the nucleophilic reaction between the primary amino group and the epoxy group began to happen, and globular-like or core-shell colloidal particles were formed. For large-size droplets (>680 nm), the nucleophilic reaction occurred at 75 min and produced core-shell or multi-layered colloidal particles. In a word, the physical absorption or nucleophilic reaction between gelatin and PDMS-E emulsion droplets could be controlled by controlling the droplet size and reaction time. Furthermore, the soft tissue paper coated with large-size droplets exhibited excellent resistance to water permeability and flame-resistant performance, which were carried out by water resistance and flammability tests.

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.

Design and 3D modeling investigation of a microfluidic electrode array for electrical impedance measurement of single yeast cells

High-resolution microscopic imaging may cause intensive image processing and potential impact of light irradiation on yeast replicative lifespan (RLS). Electrical impedance spectroscopy (EIS) could be alternatively used to perform high-throughput and label-free yeast RLS assays. Prior to fabricating EIS-integrated microfluidic devices for yeast RLS determination, systematic modeling and theoretical investigation are crucial for device design and optimization. Here, we report three-dimensional (3D) finite-element modeling and simulations of EIS measurement in a microfluidic single yeast in-situ impedance array (SYIIA), which is designed by patterning an electrode matrix underneath a cell-trapping array. SYIIA was instantiated and modeled as a 5 × 5 sensing array comprising 25 units for cell immobilization, culturing, and time-lapse EIS recording. Simulations of yeast growing and budding in a sensing unit demonstrated that EIS signals enable the characterization of cell growth and daughter-cell dissections. In the 5 × 5 sensing array, simulation results indicated that when monitoring a target cell, daughter dissections in its surrounding traps may induce variations of the recorded EIS signals, which could cause mistakes in identifying target daughter-cell dissections. To eliminate the mis-identifications, electrode array pitch was optimized. Therefore, the results could conduct the design and optimization of microfluidic electrode-array-integrated devices for high-throughput and accurate yeast RLS assays.