Open and closed microfluidics for biosensing

Biosensing is vital for many areas like disease diagnosis, infectious disease prevention, and point-of-care monitoring. Microfluidics has been evidenced to be a powerful tool for biosensing via integrating biological detection processes into a palm-size chip. Based on the chip structure, microfluidics has two subdivision types: open microfluidics and closed microfluidics, whose operation methods would be diverse. In this review, we summarize fundamentals, liquid control methods, and applications of open and closed microfluidics separately, point out the bottlenecks, and propose potential directions of microfluidics-based biosensing.

Dielectrophoretic assembly and separation of particles and cells in continuous flow

Dielectrophoretic (DEP) separation has been recognized as a practical tool in the separation of cells and particles for clinical diagnosis, the pharmaceutical industry and environmental monitoring. Assembly of particles and cells under DEP force is a common phenomenon and has an influence on their separation but has not been understood fully. Encouraged by these aspects, we developed a microfluidic device with a bipolar electrode array to investigate the assembly and separation of particles and cells at a large scale. First, we studied the assembly and evolution mechanisms of particles of one type under an AC electric field. Then, we investigated the interaction and assembly of multiple particles with dissimilar properties under DEP force. Depending on the development of microfluidic devices, we visualize the assembly process of yeast cells at the electrode rims and of polystyrene particles at the channel centers, and explore the influence of pearl chain formation on their separation. With increasing flow velocity from 288 to 720 μL h-1, the purity of 5 μm polystyrene particles surpasses 94.9%. Furthermore, we studied the DEP response of Scenedesmus sp. and C. vulgaris, and explored the influence of cell chains on the isolation of C. vulgaris. The purity of Scenedesmus sp. and C. vulgaris witnessed a decrease from 95.7% to 90.8% when the flow rate increased from 288 to 864 μL h-1. Finally, we investigated the extension of the electric field under chains of Oocystis sp. at the electrode rims by studying chain formation and capture of C. vulgaris, and studied its effect on cell chain length, recovered cell purity and cell concentration. When chains of Oocystis sp. were formed, the purity of C. vulgaris kept unchanged and the concentration decreased from 2793 cells per μL to 2039 cells per μL. This work demonstrates continuous DEP-based assembly and separation of particles and cells, which facilitates high-efficiency isolation of targeted cells.

pH Gradients in Spatially Non-Uniform AC Electric Fields around the Charging Frequency; A Study of Two Different Geometries and Electrode Passivation

Dielectrophoresis (DEP), a precision nonlinear electrokinetic tool utilized within microfluidic devices, can induce bioparticle polarization that manifests as motion in the electric field; this phenomenon has been leveraged for phenotypic cellular and biomolecular detection, making DEP invaluable for diagnostic applications. As device operation times lengthen, reproducibility and precision decrease, which has been postulated to be caused by ion gradients within the supporting electrolyte medium. This research focuses on characterizing pH gradients above, at, and below the electrode charging frequency (0.2-1.4 times charging frequency) in an aqueous electrolyte solution in order to extend the parameter space for which microdevice-imposed artifacts on cells in clinical diagnostic devices have been characterized. The nonlinear alternating current (AC) electric fields (0.07 Vpp/μm) required for DEP were generated via planar T-shaped and star-shaped microelectrodes overlaid by a 70 μm high microfluidic chamber. The experiments were designed to quantify pH changes temporally and spatially in the two microelectrode geometries. In parallel, a 50 nm hafnium oxide (HfO2) thin film on the microelectrodes was tested to provide insights into the role of Faradaic surface reactions on the pH. Electric field simulations were conducted to provide insights into the gradient shape within the microelectrode geometries. Frequency dependence was also examined to ascertain ion electromigration effects above, at, and below the electrode charging frequency. The results revealed Faradaic reactions above, at, and below the electrode charging frequency. Comparison experiments further demonstrated that pH changes caused by Faradaic reactions increased inversely with frequency and were more pronounced in the star-shaped geometry. Finally, HfO2 films demonstrated frequency-dependent properties, impeding Faradaic reactions.

Microplastics in aquatic environments: A comprehensive review of toxicity, removal, and remediation strategies

The occurrence of microplastics (MPs) in aquatic environments has been a global concern because they are toxic and persistent and may serve as a vector for many legacies and emerging pollutants. MPs are discharged to aquatic environments from different sources, especially from wastewater plants (WWPs), causing severe impacts on aquatic organisms. This study mainly aims to review the Toxicity of MPs along with plastic additives in aquatic organisms at various trophic compartments and available remediation methods/strategies for MPs in aquatic environments. Occurrences of oxidative stress, neurotoxicity, and alterations in enzyme activity, growth, and feeding performance were identical in fish due to MPs toxicity. On the other hand, growth inhibition and ROS formation were observed in most of the microalgae species. In zooplankton, potential impacts were acceleration of premature molting, growth retardation, mortality increase, feeding behaviour, lipid accumulation, and decreased reproduction activity. MPs togather with additive contaminants could also exert some toxicological impacts on polychaete, including neurotoxicity, destabilization of the cytoskeleton, reduced feeding rate, growth, survivability and burrowing ability, weight loss, and high rate of mRNA transcription. Among different chemical and biological treatments for MPs, high removal rates have been reported for coagulation and filtration (>86.5 %), electrocoagulation (>90 %), advanced oxidation process (AOPs) (30 % to 95 %), primary sedimentation/Grit chamber (16.5 % to 58.84 %), adsorption removal technique (>95 %), magnetic filtration (78 % to 93 %), oil film extraction (>95 %), and density separation (95 % to 100 %). However, desirable extraction methods are required for large-scale research in MPs removal from aquatic environments.

Dielectrophoretic Colloidal Levitation by Electrode Polarization in Oscillating Electric Fields

Controlled colloidal levitation is key to many applications. Recently, it was discovered that polymer microspheres were levitated to a few micrometers in aqueous solutions in alternating current (AC) electric fields. A few mechanisms have been proposed to explain this AC levitation such as electrohydrodynamic flows, asymmetric rectified electric fields, and aperiodic electrodiffusiophoresis. Here, we propose an alternative mechanism based on dielectrophoresis in a spatially inhomogeneous electric field gradient extending from the electrode surface micrometers into the bulk. This field gradient is derived from electrode polarization, where counterions accumulate near electrode surfaces. A dielectric microparticle is then levitated from the electrode surface to a height where the dielectrophoretic lift balances gravity. The dielectrophoretic levitation mechanism is supported by two numerical models. One model assumes point dipoles and solves for the Poisson-Nernst-Planck equations, while the second model incorporates a dielectric sphere of a realistic size and permittivity and uses the Maxwell-stress tensor formulation to solve for the electrical body force. In addition to proposing a plausible levitation mechanism, we further demonstrate that AC colloidal levitation can be used to move synthetic microswimmers to controlled heights. This study sheds light on understanding the dynamics of colloidal particles near an electrode and paves the way to using AC levitation to manipulate colloidal particles, active or passive.

Steering Micromotors via Reprogrammable Optoelectronic Paths

Steering micromotors is important for using them in practical applications and as model systems for active matter. This functionality often requires magnetic materials in the micromotor, taxis behavior of the micromotor, or the use of specifically designed physical boundaries. Here, we develop an optoelectronic strategy that steers micromotors with programmable light patterns. In this strategy, light illumination turns hydrogenated amorphous silicon conductive, generating local electric field maxima at the edge of the light pattern that attracts micromotors via positive dielectrophoresis. As an example, metallo-dielectric Janus microspheres that self-propelled under alternating current electric fields were steered by static light patterns along customized paths and through complex microstructures. Their long-term directionality was also rectified by ratchet-shaped light patterns. Furthermore, dynamic light patterns that varied in space and time enabled more advanced motion controls such as multiple motion modes, parallel control of multiple micromotors, and the collection and transport of motor swarms. This optoelectronic steering strategy is highly versatile and compatible with a variety of micromotors, and thus it possesses the potential for their programmable control in complex environments.

Microdynamics of active particles in defect-rich colloidal crystals

HYPOTHESIS

Because they are self-propulsive, active colloidal particles can interact with their environment in ways that differ from passive, Brownian particles. Here, we explore how interactions in different microstructural regions may contribute to colloidal crystal annealing.

EXPERIMENTS

We investigate active particles propagating in a quasi-2D colloidal crystal monolayer produced by alternating current electric fields (active-to-passive particle ratio ∼ 1:720). The active particle is a platinum Janus sphere propelled by asymmetric decomposition of hydrogen peroxide. Crystals are characterized for changes in void properties. The mean-squared-displacement of Janus particles are measured to determine how active microdynamics depend on the local microstructure, which is comprised of void regions, void-adjacent regions (defined as within three particle diameters of a void), and interstitial regions.

FINDINGS

At active particle energy E_A = 2.55 k_BT, the average void size increases as much as three times and the average void anisotropy increases about 40% relative to the passive case. The average microdynamical enhancement, <δ(t)>, of Janus particles in the crystal relative to an equivalent passive Janus particle is reduced compared to that of a free, active particle (<δ(t) > is 1.88 ± 0.04 and 2.66 ± 0.08, respectively). The concentration of active particles is enriched in void and void-adjacent regions. Active particles exhibit the greatest change in dynamics relative to the passive control in void-adjacent regions (<δ(t)> = 2.58 ± 0.06). The results support the conjecture that active particle microdynamical enhancement in crystal lattices is affected by local defect structure.

Directed Assembly of Nanomaterials for Making Nanoscale Devices and Structures: Mechanisms and Applications

Nanofabrication has been utilized to manufacture one-, two-, and three-dimensional functional nanostructures for applications such as electronics, sensors, and photonic devices. Although conventional silicon-based nanofabrication (top-down approach) has developed into a technique with extremely high precision and integration density, nanofabrication based on directed assembly (bottom-up approach) is attracting more interest recently owing to its low cost and the advantages of additive manufacturing. Directed assembly is a process that utilizes external fields to directly interact with nanoelements (nanoparticles, 2D nanomaterials, nanotubes, nanowires, etc.) and drive the nanoelements to site-selectively assemble in patterned areas on substrates to form functional structures. Directed assembly processes can be divided into four different categories depending on the external fields: electric field-directed assembly, fluidic flow-directed assembly, magnetic field-directed assembly, and optical field-directed assembly. In this review, we summarize recent progress utilizing these four processes and address how these directed assembly processes harness the external fields, the underlying mechanism of how the external fields interact with the nanoelements, and the advantages and drawbacks of utilizing each method. Finally, we discuss applications made using directed assembly and provide a perspective on the future developments and challenges.

Customized metallodielectric colloids and their behavior in dielectrophoretic fields

A synthetic strategy for fabricating colloidal particles with spatially segregated amine-functionalized lobes enables regioselective coating with gold to afford metallodielectric particles with a variety of shapes and lobe sizes. This approach can produce either dissymmetric dumbbell-shaped two-lobed Au-TPM particles (Au-T) or dissymmetric or symmetric three-lobed particles with gold coating on one (Au-T-T and T-Au-T) or two lobes (Au-T-Au). Dielectrophoretic (DEP) forces exerted by an AC field confined between two opposing electrodes generate aggregates ranging from 1D chains to 2D close-packed lattices, depending on the particle shape and lobe arrangement. The aggregate structures reflect the lowest energy configurations resulting from the induced dipole moments created in particle lobes within the confined electric field.

Theoretical and experimental analysis of negative dielectrophoresis‐induced particle trajectories

Many biomedical analysis applications require trapping and manipulating single cells and cell clusters within microfluidic devices. Dielectrophoresis (DEP) is a label‐free technique that can achieve flexible cell trapping, without physical barriers, using electric field gradients created in the device by an electrode microarray. Little is known about how fluid flow forces created by the electrodes, such as thermally driven convection and electroosmosis, affect DEP‐based cell capture under high conductance media conditions that simulate physiologically relevant fluids such as blood or plasma. Here, we compare theoretical trajectories of particles under the influence of negative DEP (nDEP) with observed trajectories of real particles in a high conductance buffer. We used 10‐µm diameter polystyrene beads as model cells and tracked their trajectories in the DEP microfluidic chip. The theoretical nDEP trajectories were in close agreement with the observed particle behavior. This agreement indicates that the movement of the particles was highly dominated by the DEP force and that contributions from thermal‐ and electroosmotic‐driven flows were negligible under these experimental conditions. The analysis protocol developed here offers a strategy that can be applied to future studies with different applied voltages, frequencies, conductivities, and polarization properties of the targeted particles and surrounding medium. These findings motivate further DEP device development to manipulate particle trajectories for trapping applications.

Induction and suppression of cell lysis in an electrokinetic microfluidic system

The ability to strategically induce or suppress cell lysis is critical for many cellular-level diagnostic and therapeutic applications conducted within electrokinetic microfluidic platforms. The chemical and structural integrity of sub-cellular components is important when inducing cell lysis. However, metal electrodes and electrolytes participate in undesirable electrochemical reactions that alter solution composition and potentially damage protein, RNA, and DNA integrity within device microenvironments. For many biomedical applications, cell viability must be maintained even when device-imposed cell-stressing stimuli (e.g., electrochemical reaction byproducts) are present. In this work, we explored a novel and tunable method to accurately induce or suppress device-imposed artifacts on human red blood cell (RBC) lysis in non-uniform AC electric fields. For precise tunability, a dielectric hafnium oxide (HfO₂ ) layer was used to prevent electron transfer between the electrodes and the electric double layer and thus reduce harmful electrochemical reactions. Additionally, a low concentration of Triton X-100 surfactant was explored as a tool to stabilize cell membrane integrity. The extent of hemolysis was studied as a function of time, electrode configuration (T-shaped and star-shaped), cell position, applied non-uniform AC electric field, with uncoated and HfO₂ coated electrodes (50 nm), and absence and presence of Triton X-100 (70 µM). Tangible outcomes include a parametric analysis relying upon literature and this work to design, tune, and operate electrokinetic microdevices to intentionally induce or suppress cellular lysis without altering intracellular components. Implications are that devices can be engineered to leverage or minimize device-imposed biological artefacts extending the versatility and utility of electrokinetic diagnostics.

2D colloids in rotating electric fields: A laboratory of strong tunable three-body interactions

Many-body forces play a prominent role in structure and dynamics of matter, but their role is not well understood in many cases due to experimental challenges. Here, we demonstrate that a novel experimental system based on rotating electric fields can be utilised to deliver unprecedented degree of control over many-body interactions between colloidal silica particles in water. We further show that we can decompose interparticle interactions explicitly into the leading terms and study their specific effects on phase behaviour. We found that three-body interactions exert critical influence over the phase diagram domain boundaries, including liquid-gas binodal, critical and triple points. Phase transitions are shown to be reversible and fully controlled by the magnitude of external rotating electric field governing the tunable interactions. Our results demonstrate that colloidal systems in rotating electric fields are a unique laboratory to study the role of many-body interactions in physics of phase transitions and in applications, such as self-assembly, offering exciting opportunities for studying generic phenomena inherent to liquids and solids, from atomic to protein and colloidal systems.

Directional and Reconfigurable Assembly of Metallodielectric Patchy Particles

Colloidal particles with surface patches can self-assemble with high directionality, but the resulting assemblies cannot reconfigure unless the patch arrangement (number, symmetry, etc.) is altered. While external fields with tunable inputs can guide the assembly of dynamic structures, they encourage particle alignment relative to its shape rather than the surface patterns. Here, we report on the synthesis of metallodielectric patchy particles and their assembly under the AC electric field, which gives rise to a series of structures including two-layer alternating chains, open-brick walls, staggering stacks, and vertical chains that are directed by the patches yet reconfigurable by the field. The configurations of the assemblies (e.g., the chains) can be further switched between a rigid and a flexible state emulating the conformations of polymers. Our work suggests that, for directed colloidal assembly, the particle complexities (patches and shapes) can be coupled with the external manipulations in a cooperative manner for creating materials with precise yet reconfigurable structures.

The Energy Conversion behind Micro-and Nanomotors

Inspired by the autonomously moving organisms in nature, artificially synthesized micro-nano-scale power devices, also called micro-and nanomotors, are proposed. These micro-and nanomotors that can self-propel have been used for biological sensing, environmental remediation, and targeted drug transportation. In this article, we will systematically overview the conversion of chemical energy or other forms of energy in the external environment (such as electrical energy, light energy, magnetic energy, and ultrasound) into kinetic mechanical energy by micro-and nanomotors. The development and progress of these energy conversion mechanisms in the past ten years are reviewed, and the broad application prospects of micro-and nanomotors in energy conversion are provided.

Ions in an AC Electric Field: Strong Long-Range Repulsion between Oppositely Charged Surfaces

Two oppositely charged surfaces separated by a dielectric medium attract each other. In contrast we observe a strong repulsion between two plates of a capacitor that is filled with an aqueous electrolyte upon application of an alternating potential difference between the plates. This long-range force increases with the ratio of diffusion coefficients of the ions in the medium and reaches a steady state after a few minutes, which is much larger than the millisecond timescale of diffusion across the narrow gap. The repulsive force, an order of magnitude stronger than the electrostatic attraction observed in the same setup in air, results from the increase in osmotic pressure as a consequence of the field-induced excess of cations and anions due to lateral transport from adjacent reservoirs.

Synergistic Speed Enhancement of an Electric-Photochemical Hybrid Micromotor by Tilt Rectification

A hybrid micromotor is an active colloid powered by more than one power source, often exhibiting expanded functionality and controllability than those of a singular energy source. However, these power sources are often applied orthogonally, leading to stacked propulsion that is just a sum of two independent mechanisms. Here, we report that TiO₂-Pt Janus micromotors, when subject to both UV light and AC electric fields, move up to 90% faster than simply adding up the speed powered by either source. This unexpected synergy between light and electric fields, we propose, arises from the fact that an electrokinetically powered TiO₂-Pt micromotor moves near a substrate with a tilted Janus interface that, upon the application of an electric field, becomes rectified to be vertical to the substrate. Control experiments with magnetic fields and three types of micromotors unambiguously and quantitatively show that the tilting angle of a micromotor correlates positively with its instantaneous speed, reaching maximum at a vertical Janus interface. Such "tilting-induced retardation" could affect a wide variety of chemically powered micromotors, and our findings are therefore helpful in understanding the dynamics of micromachines in confinement.

Directed Assembly of Liquid Metal-Elastomer Conductors for Stretchable and Self-Healing Electronics

Growing interest in soft robotics, stretchable electronics, and electronic skins has created demand for soft, compliant, and stretchable electrodes and interconnects. Here, dielectrophoresis (DEP) is used to assemble, align, and sinter eutectic gallium indium (EGaIn) microdroplets in uncured poly(dimethylsiloxane) (PDMS) to form electrically conducting microwires. There are several noteworthy aspects of this approach. 1) Generally, EGaIn droplets in silicone at loadings approaching 90 wt% remain insulating and form a conductive network only when subjected to sintering. Here, DEP facilitates assembly of EGaIn droplets into conductive microwires at loadings as low as 10 wt%. 2) DEP is done in silicone for the first time, enabling the microwires to be cured in a stretchable matrix. 3) Liquid EGaIn droplets sinter during DEP to form a stretchable metallic microwire that retains its shape after curing the silicone. 4) Use of liquid metal eliminates the issue of compliance mismatch observed in soft polymers with solid fillers. 5) The silicone-EGaIn "ink" can be assembled by DEP within the crevices of severely damaged wires to create stretchable interconnects that heal the damage mechanically and electrically. The DEP process of this unique set of materials is characterized and the interesting attributes enabled by such liquid microwires are demonstrated.

A practical guide to active colloids: choosing synthetic model systems for soft matter physics research

Synthetic active colloids that harvest energy stored in the environment and swim autonomously are a popular model system for active matter. This emerging field of research sits at the intersection of materials chemistry, soft matter physics, and engineering, and thus cross-talk among researchers from different backgrounds becomes critical yet difficult. To facilitate this interdisciplinary communication, and to help soft matter physicists with choosing the best model system for their research, we here present a tutorial review article that describes, in appropriate detail, six experimental systems of active colloids commonly found in the physics literature. For each type, we introduce their background, material synthesis and operating mechanisms and notable studies from the soft matter community, and comment on their respective advantages and limitations. In addition, the main features of each type of active colloid are summarized into two useful tables. As materials chemists and engineers, we intend for this article to serve as a practical guide, so those who are not familiar with the experimental aspects of active colloids can make more informed decisions and maximize their creativity.

Buoyancy-Free Janus Microcylinders as Mobile Microelectrode Arrays for Continuous Microfluidic Biomolecule Collection within a Wide Frequency Range: A Numerical Simulation Study

We numerically study herein the AC electrokinetic motion of Janus mobile microelectrode (ME) arrays in electrolyte solution in a wide field frequency, which holds great potential for biomedical applications. A fully coupled physical model, which incorporates the fluid-structure interaction under the synergy of induced-charge electroosmotic (ICEO) slipping and interfacial Maxwell stress, is developed for this purpose. A freely suspended Janus cylinder free from buoyancy, whose main body is made of polystyrene, while half of the particle surface is coated with a thin conducting film of negligible thickness, will react actively on application of an AC signal. In the low-frequency limit, induced-charge electrophoretic (ICEP) translation occurs due to symmetric breaking in ICEO slipping, which renders the insulating end to move ahead. At higher field frequencies, a brand-new electrokinetic transport phenomenon called "ego-dielectrophoresis (e-DEP)" arises due to the action of the localized uneven field on the inhomogeneous particle dipole moment. In stark contrast with the low-frequency ICEP translation, the high-frequency e-DEP force tends to drive the asymmetric dipole moment to move in the direction of the conducting end. The bidirectional transport feature of Janus microspheres in a wide AC frequency range can be vividly interpreted as an array of ME for continuous loading of secondary bioparticles from the surrounding liquid medium along its direction-controllable path by long-range electroconvection. These results pave the way for achieving flexible and high-throughput on-chip extraction of nanoscale biological contents for subsequent on-site bioassay based upon AC electrokinetics of Janus ME arrays.

Surfactant concentration modulates the motion and placement of microparticles in an inhomogeneous electric field

This study examined the effects of surfactants on the motion and positioning of microparticles in an inhomogeneous electric field. The microparticles were suspended in oil with a surfactant and the electric field was generated using sawtooth-patterned electrodes. The microparticles were trapped, oscillating, or attached to the electrodes. The proportion of microparticles in each state was defined by the concentration of surfactant and the voltage applied to the electrodes. Based on the trajectory of the microparticles in the electric field, we developed a new physical model in which the surfactant adsorbed on the microparticles allowed the microparticles to be charged by contact with the electrodes, with either positive or negative charges, while the non-adsorbed surfactant micellizing in the oil contributed to charge relaxation. A simulation based on this model showed that the charging and charge relaxation, as modulated by the surfactant concentration, can explain the trajectories and proportion of the trapped, oscillating, and attached microparticles. These results will be useful for the development of novel self-assembly and transport technologies and colloids sensitive to electricity.

Photocatalytic TiO2 Micromotors for Removal of Microplastics and Suspended Matter

Environmental contamination is a major global challenge, and the effects of contamination are found in most habitats. In recent times, the pollution by microplastics has come to the global attention and their removal displays an extraordinary challenge with no reasonable solutions presented so far. One of the new technologies holding many promises for environmental remediation on the microscale are self-propelled micromotors. They present several properties that are of academic and technical interest, such as the ability to overcome the diffusion limitation in catalytic processes, as well as their phoretic interaction with their environment. Here, we present two novel strategies for the elimination of microplastics using photocatalytic Au@Ni@TiO₂-based micromotors. We show that individual catalytic particles as well as assembled chains show excellent collection and removal of suspended matter and microplastics from environmental water samples.

Confined 1D Propulsion of Metallodielectric Janus Micromotors on Microelectrodes under Alternating Current Electric Fields

There is mounting interest in synthetic microswimmers ("micromotors") as microrobots as well as a model system for the study of active matters, and spatial navigation is critical for their success. Current navigational technologies mostly rely on magnetic steering or guiding with physical boundaries, yet limitations with these strategies are plenty. Inspired by an earlier work with magnetic domains on a garnet film as predefined tracks, we present an interdigitated microelectrodes (IDE) system where, upon the application of AC electric fields, metallodielectric (e.g., SiO₂-Ti) Janus particles are hydrodynamically confined and electrokinetically propelled in one dimension along the electrode center lines with tunable speeds. In addition, comoving micromotors moved in single files, while those moving in opposite directions primarily reoriented and moved past each other. At high particle densities, turbulence-like aggregates formed as many-body interactions became complicated. Furthermore, a micromotor made U-turns when approaching an electrode closure, while it gradually slowed down at the electrode opening and was collected in large piles. Labyrinth patterns made of serpentine chains of Janus particles emerged by modifying the electrode configuration. Most of these observations can be qualitatively understood by a combination of electroosmotic flows pointing inward to the electrodes, and asymmetric electrical polarization of the Janus particles under an AC electric field. Emerging from these observations is a strategy that not only powers and confines micromotors on prefabricated tracks in a contactless, on-demand manner, but is also capable of concentrating active particles at predefined locations. These features could prove useful for designing tunable tracks that steer synthetic microrobots, as well as to enable the study of single file diffusion, active turbulence, and other collective behaviors of active matters.

An Experimental Study of 3D Electrode-Facilitated Particle Traffic Flow-Focusing Driven by Induced-Charge Electroosmosis

In this paper we present a novel microfluidic approach for continuous, rapid and switchable particle concentration, using induced-charge electroosmosis (ICEO) in 3D electrode layouts. Field-effect control on non-linear electroosmosis in the transverse direction greatly facilitates a selective concentration of biological yeast cells from a straight main microchannel into one of the three downstream branch channels in our microfluidic device. For the geometry configuration of 3D driving electrode plates on sidewalls and a 2D planar gate electrode strip on the channel bottom surface, we briefly describe the underlying physics of an ICEO-based particle flow-focusing method, and provide relevant simulation results to show how gate voltage amplitude can be used to guide the motion trajectory of the concentrated particle stream. With a relatively simple geometrical configuration, the proposed microfluidic device provides new possibilities to controllably concentrate micro/nanoparticles in continuous flow by using ICEO, and is suitable for a high-throughput front-end cell concentrator interfacing with various downstream biosensors.

Macro-/Micro-Controlled 3D Lithium-Ion Batteries via Additive Manufacturing and Electric Field Processing

This paper presents a new concept for making battery electrodes that can simultaneously control macro-/micro-structures and help address current energy storage technology gaps and future energy storage requirements. Modern batteries are fabricated in the form of laminated structures that are composed of randomly mixed constituent materials. This randomness in conventional methods can provide a possibility of developing new breakthrough processing techniques to build well-organized structures that can improve battery performance. In the proposed processing, an electric field (EF) controls the microstructures of manganese-based electrodes, while additive manufacturing controls macro-3D structures and the integration of both scales. The synergistic control of micro-/macro-structures is a novel concept in energy material processing that has considerable potential for providing unprecedented control of electrode structures, thereby enhancing performance. Electrochemical tests have shown that these new electrodes exhibit superior performance in their specific capacity, areal capacity, and life cycle.

Separation of neural stem cells by whole cell membrane capacitance using dielectrophoresis

Whole cell membrane capacitance is an electrophysiological property of the plasma membrane that serves as a biomarker for stem cell fate potential. Neural stem and progenitor cells (NSPCs) that differ in ability to form neurons or astrocytes are distinguished by membrane capacitance measured by dielectrophoresis (DEP). Differences in membrane capacitance are sufficient to enable the enrichment of neuron- or astrocyte-forming cells by DEP, showing the separation of stem cells on the basis of fate potential by membrane capacitance. NSPCs sorted by DEP need not be labeled and do not experience toxic effects from the sorting procedure. Other stem cell populations also display shifts in membrane capacitance as cells differentiate to a particular fate, clarifying the value of sorting a variety of stem cell types by capacitance. Here, we describe methods developed by our lab for separating NSPCs on the basis of capacitance using several types of DEP microfluidic devices, providing basic information on the sorting procedure as well as specific advantages and disadvantages of each device.

Motion Manipulation of Micro- and Nanomotors

Inspired by the self-migration of microorganisms in nature, artificial micro- and nanomotors can mimic this fantastic behavior by converting chemical fuel or external energy into mechanical motion. These self-propelled micro- and nanomotors, designed either by top-down or bottom-up approaches, are able to achieve different applications, such as environmental remediation, sensing, cargo/sperm transportation, drug delivery, and even precision micro-/nanosurgery. For these various applications, especially biomedical applications, regulating on-demand the motion of micro- and nanomotors is quite essential. However, it remains a continuing challenge to increase the controllability over motors themselves. Here, we will discuss the recent advancements regarding the motion manipulation of micro- and nanomotors by different approaches.

Approaches to self-assembly of colloidal monolayers: A guide for nanotechnologists

Self-assembly of quasi-spherical colloidal particles in two-dimensional (2D) arrangements is essential for a wide range of applications from optoelectronics to surface engineering, from chemical and biological sensing to light harvesting and environmental remediation. Several self-assembly approaches have flourished throughout the years, with specific features in terms of complexity of the implementation, sensitivity to process parameters, characteristics of the final colloidal assembly. Selecting the proper method for a given application amidst the vast literature in this field can be a challenging task. In this review, we present an extensive classification and comparison of the different techniques adopted for 2D self-assembly in order to provide useful guidelines for scientists approaching this field. After an overview of the main applications of 2D colloidal assemblies, we describe the main mechanisms underlying their formation and introduce the mathematical tools commonly used to analyse their final morphology. Subsequently, we examine in detail each class of self-assembly techniques, with an explanation of the physical processes intervening in crystallization and a thorough investigation of the technical peculiarities of the different practical implementations. We point out the specific characteristics of the set-ups and apparatuses developed for self-assembly in terms of complexity, requirements, reproducibility, robustness, sensitivity to process parameters and morphology of the final colloidal pattern. Such an analysis will help the reader to individuate more easily the approach more suitable for a given application and will draw the attention towards the importance of the details of each implementation for the final results.

High-Throughput Selective Capture of Single Circulating Tumor Cells by Dielectrophoresis at a Wireless Electrode Array

We demonstrate continuous high-throughput selective capture of circulating tumor cells by dielectrophoresis at arrays of wireless electrodes (bipolar electrodes, BPEs). The use of BPEs removes the requirement of ohmic contact to individual array elements, thus enabling otherwise unattainable device formats. Capacitive charging of the electrical double layer at opposing ends of each BPE allows an AC electric field to be transmitted across the entire device. Here, two such designs are described and evaluated. In the first design, BPEs interconnect parallel microchannels. Pockets extruding from either side of the microchannels volumetrically control the number of cells captured at each BPE tip and enhance trapping. High-fidelity single-cell capture was achieved when the pocket dimensions were matched to those of the cells. A second, open design allows many non-targeted cells to pass through. These devices enable high-throughput capture of rare cells and single-cell analysis.

In situ assembly of active surface-enhanced Raman scattering substrates via electric field-guided growth of dendritic nanoparticle structures

Surface-enhanced Raman scattering (SERS) can provide ultrasensitive detection of chemical and biological analytes down to the level of a single molecule. The need for costly, nanostructured, noble-metal substrates, however, poses a major obstacle in the widespread application of the method. Here we present for the first time a novel type of metallic nanostructured substrates that, not only exhibit a remarkable SERS activity, but are also produced in a facile, cost-effective and nanofabrication-free manner. The substrates are formed through an electric field-guided assembly process of silver nanocolloids, which results in extended and interconnected dendritic nanoparticle structures with a high density of "hot spots". A unique and significant performance attribute of these nanostructures is their ability to also function as concentration amplification devices, thereby further enhancing their analyte detection efficiency. This major advantage against conventional SERS substrates is illustrated experimentally here with the concentration and detection of proteins from solution. Low limits of detection for illicit drugs, food contaminants and pesticides in relevant matrices are also demonstrated. The SERS-active dendrites are reusable and can be removed and replaced in a few minutes. The SERS substrates presented herein constitute a significant advance towards more effective and less expensive diagnostic tools.

Tuning electric field aligned CNT architectures via chemistry, morphology, and sonication from micro to macroscopic scale

Electric-field alignment of carbon nanotubes (CNT) is widely used to produce composite materials with anisotropic mechanical, electrical, and optical properties. Nevertheless, consistent results are difficult to achieve, and even under identical electric field conditions the resulting aligned morphologies can vary over μm to cm length scales. In order to improve reproducibility, this study addresses (1) how solution processing steps (oxidation, sonication) affect CNT properties, and (2) how CNT chemistry, morphology, and dispersion influence alignment. Aligned CNT were deposited onto PVDF membranes using a combination of electric-field alignment and vacuum-filtration. At each step in solution processing, the CNT chemistry (oxygen content) and morphology (length/diameter) were characterized and compared to the final aligned morphology. Well-dispersed CNT with high oxygen content (>8.5%O) yielded uniform membrane coatings and microscopically aligned CNT, whereas CNT with low oxygen CNT (<2.2%O) produced aligned bundles visible at a macroscopic level, but microscopically the individual CNT remained disordered. Based on regression analysis, CNT with larger mean length and diameter, smaller length and diameter variation, and higher oxygen content yielded increased electrical anisotropy, and bath sonication was slightly preferable to probe sonication for initial dispersion.

Remarkable Conductive Anisotropy of Metallic Microcoil/PDMS Composites Made by Electric Field Induced Alignment

We successfully fabricated a highly anisotropic electrical conductive microcoil/polydimethylsiloxane (PDMS) composite based on helical Spirulina-templated metallic particles using an electric field-induced alignment method. The optimized AC electric field (2 kV/cm, 1 kHz) could efficiently assemble the lightweight conductive microcoils into continuous long chains and form unique end-to-end physical contacts between adjacent particles in the alignment direction, leading to highly conductive channels. Furthermore, the electrical conductivity in the alignment direction reached up to ∼10 S/m for 1 wt % loading and exhibited almost 7-8 orders of magnitude higher than that in perpendicular directions, which is by far the most remarkable conductive anisotropy for anisotropic conductive composites (ACCs). In addition, the anisotropic composites exhibit excellent current-carrying capability in a functional light emitting diode (LED) circuit. Therefore, due to the superior conductive anisotropy and high conductivity, the composites have promising potential in high reliability electrical interconnections and subminiature integrated circuits.

Advancements in microfluidics for nanoparticle separation

Nanoparticles have been widely implemented for healthcare and nanoscience industrial applications. Thus, efficient and effective nanoparticle separation methods are essential for advancement in these fields. However, current technologies for separation, such as ultracentrifugation, electrophoresis, filtration, chromatography, and selective precipitation, are not continuous and require multiple preparation steps and a minimum sample volume. Microfluidics has offered a relatively simple, low-cost, and continuous particle separation approach, and has been well-established for micron-sized particle sorting. Here, we review the recent advances in nanoparticle separation using microfluidic devices, focusing on its techniques, its advantages over conventional methods, and its potential applications, as well as foreseeable challenges in the separation of synthetic nanoparticles and biological molecules, especially DNA, proteins, viruses, and exosomes.

Self-Assembly of Single-Sized and Binary Colloidal Particles at Air/Water Interface by Surface Confinement and Water Discharge

We present an innovative apparatus allowing self-assembly at air/water interface in a smooth and reproducible way. The combination of water discharge and surface confinement of the area over which self-assembly takes place allows transfer of the assembled monolayer without any risk of damage to the colloidal crystal. As we demonstrate, the designed approach offers remarkable advantages in terms of cost and robustness compared to state-of-the art methods and is suitable for the fabrication of highly ordered monolayers even for more challenging assembly experiments such as transfer on rough substrates or assembly of binary colloids. Hence, our apparatus represents a significant headway toward high scale production of large area colloidal crystals. For the binary colloid assembly experiments, we also report the first experimental demonstration of a morphology based on the alternation of three and four small particles in the interstices between large particles.

Bending of Responsive Hydrogel Sheets Guided by Field-Assembled Microparticle Endoskeleton Structures

Hydrogel composites that respond to stimuli can form the basis of new classes of biomimetic actuators and soft robotic components. Common latex microspheres can be assembled and patterned by AC electric fields within a soft thermoresponsive hydrogel. The field-oriented particle chains act as endoskeletal structures, which guide the macroscopic bending pattern of the actuators.

Teslaphoresis of Carbon Nanotubes

This paper introduces Teslaphoresis, the directed motion and self-assembly of matter by a Tesla coil, and studies this electrokinetic phenomenon using single-walled carbon nanotubes (CNTs). Conventional directed self-assembly of matter using electric fields has been restricted to small scale structures, but with Teslaphoresis, we exceed this limitation by using the Tesla coil's antenna to create a gradient high-voltage force field that projects into free space. CNTs placed within the Teslaphoretic (TEP) field polarize and self-assemble into wires that span from the nanoscale to the macroscale, the longest thus far being 15 cm. We show that the TEP field not only directs the self-assembly of long nanotube wires at remote distances (>30 cm) but can also wirelessly power nanotube-based LED circuits. Furthermore, individualized CNTs self-organize to form long parallel arrays with high fidelity alignment to the TEP field. Thus, Teslaphoresis is effective for directed self-assembly from the bottom-up to the macroscale.

Manipulation and assembly behavior of Spirulina-templated microcoils in the electric field

Manipulation and assembly of complicated metallic Spirulina-templated microcoils can be achieved through alternating electric fields.

Ultra-capacitor flexible films with tailored dielectric constants using electric field assisted assembly of nanoparticles

In this study, the chaining and preferential alignment of barium titanate nanoparticles (100 nm) through the thickness direction of a polymer matrix in the presence of an electric field is shown. Application of an AC electric field in a well-dispersed solution leads to the formation of chains of nanoparticles in discrete rows oriented with their primary axis in the E-field direction due to dielectrophoresis. The change in the orientation of these chains was quantified through statistical analysis of SEM images and was found to be dependent on E-field, frequency and viscosity. When a DC field is applied a distinct layer consisting of dense particles was observed with micro-computed tomography. These studies show that the increase in DC voltage leads to increase in the thickness of the particle rich layer along with the packing density also increasing. Increasing the mutual interactions between particles due to the formation of particle chains in the "Z"-direction decreases the critical percolation concentration above which substantial enhancement of properties occurs. This manufacturing method therefore shows promise to lower the cost of the products for a range of applications including capacitors by either enhancing the dielectric properties for a given concentration or reduces the concentration of nanoparticles needed for a given property.

Trapping and chaining self-assembly of colloidal polystyrene particles over a floating electrode by using combined induced-charge electroosmosis and attractive dipole-dipole interactions

We propose a novel low-frequency strategy to trap 10 μm colloidal polystyrene (PS) particles of small buoyancy velocity on the surface of a floating electrode, on the basis of combined induced-charge electroosmotic (ICEO) flow and dipole-dipole chaining phenomenon. For field frequencies of 5-50 Hz, much lower than the reciprocal RC time scale, double-layer polarization makes electric field lines pass around the 'insulating' surface of the ideally polarizable floating electrode. Once the long-range ICEO convective micro-vortexes transport particles quickly from the bulk fluid to the electrode surface, neighbouring particles aligned along the local horizontal electric field attract one another by attractive dipolar interactions, and form arrays of particle chains that are almost parallel with the applied electric field. Most importantly, this low-frequency trapping method takes advantage of the dielectrophoretic (DEP) particle-particle interaction to enhance the downward buoyancy force of this dipolar chaining assembly structure, in order to overcome the upward ICEO fluidic drag and realize stable particle trapping around the flow stagnation region. For the sake of comparison, the field frequency is further raised far above the DC limit. At the intermediate frequencies of 200 Hz-2 kHz, this trapping method fails to work, since the normal electric field component emanates from the conducting electrode surface. Besides, at high field frequencies (>3 kHz), particles can be once again effectively trapped at the electrode center, though with a compact (3 kHz) or disordered (10 kHz) 2D packing state on the electrode surface and mainly governed by the short-range negative DEP force field, resulting in requiring a much longer trapping time. To gain a better interpretation of the various particle behaviours observed in experiments, we develop a theoretical framework that takes into account both Maxwell-Wagner interfacial charge relaxation at the particle/electrolyte interface and the field-induced double-layer polarization at the electrode/electrolyte interface, and apply it to quantify the particle-particle electrokinetic interactions. With this simple geometrical configuration of a floating electrode, our results provide a new way to realize trapping of colloidal particles with a small buoyancy velocity under the combined action of ICEO flow and an attractive dipole-dipole interaction.

Multiple-Cylindrical Electrode System for Rotational Electric Field Generation in Particle Rotation Applications

Lab-on-a-chip micro-devices utilizing electric field-mediated particle movement provide advantages over current cell rotation techniques due to the flexibility in configuring micro-electrodes. Recent technological advances in micro-milling, three-dimensional (3D) printing and photolithography have facilitated fabrication of complex micro-electrode shapes. Using the finite-element method to simulate and optimize electric field induced particle movement systems can save time and cost by simplifying the analysis of electric fields within complex 3D structures. Here we investigated different 3D electrode structures to obtain and analyse rotational electric field vectors. Finite-element analysis was conducted by an electric current stationary solver based on charge relaxation theory. High-resolution data were obtained for three-, four-, six- and eight-cylindrical electrode arrangements to characterize the rotational fields. The results show that increasing the number of electrodes within a fixed circular boundary provides larger regions of constant amplitude rotational electric field. This is a very important finding in practice, as larger rotational regions with constant electric field amplitude make placement of cells into these regions, where cell rotation occurs, a simple task – enhancing flexibility in cell manipulation. Rotation of biological particles over the extended region would be useful for biotechnology applications which require guiding cells to a desired location, such as automation of nuclear transfer cloning.

Dielectrophoretic stretching of DNA tethered to a fiber tip

In this work, we studied the stretching of λ phage DNA molecules immobilized on an optical fiber tip attached to a force sensitive tuning fork under ac electric fields. We designed a two electrodes stretching system in a small chamber: one is a gold-coated optical fiber tip electrode, and the other is a gold-coated flat electrode. By applying a dielectrophoretic (DEP) force, the immobilized λ DNA molecules on the tip are stretched and the stretching process is monitored by a fluorescent microscope. The DNA stretching in three-dimensional space is optimized by varying electrode shape, electrode gap distance, ac frequency, and solution conductivity. By observing the vibrational amplitude change of a quartz tuning fork, we measured the effects due to Joule heating and the DEP force on the tethered DNA molecules in solution. This work demonstrates a method to manipulate and characterize immobilized λ DNA molecules on a probe tip for further study of single DNA molecules.

Particle clustering during pearl chain formation in a conductive-island based dielectrophoretic assembly system

Particle clustering during pearl chain formation in a conductive-island based dielectrophoretic assembly system.

Diverse transformations of liquid metals between different morphologies

Transformation from a film into a sphere, rapid merging of separate objects, controlled self-rotation, and planar locomotion are the very unusual phenomena observed in liquid metals under application of an electric field to a liquid metal immersed in or sprayed with water. A mechanism for these effects is suggested and potential applications - for example the recovery of liquid metal previously injected into the body for therapeutic purposes - are outlined.

Rapid identification of bacteria utilizing amplified dielectrophoretic force-assisted nanoparticle-induced surface-enhanced Raman spectroscopy

Dielectrophoresis (DEP) has been widely used to manipulate, separate, and concentrate microscale particles. Unfortunately, DEP force is difficult to be used in regard to the manipulation of nanoscale molecules/particles. For manipulation of 50- to 100-nm particles, the electrical field strength must be higher than 3 × 10(6) V/m, and with a low applied voltage of 10 Vp-p, the electrode gap needs to be reduced to submicrons. Our research consists of a novel and simple approach, using a several tens micrometers scale electrode (low cost and easy to fabricate) to generate a dielectrophoretic microparticle assembly to form nanogaps with a locally amplified alternating current (AC) electric field gradient, which is used to rapidly trap nanocolloids. The results show that the amplified DEP force could effectively trap 20-nm colloids in the nanogaps between the 5-μm particle aggregates. The concentration factor at the local detection region was shown to be approximately 5 orders of magnitude higher than the bulk solution. This approach was also successfully used in bead-based surface-enhanced Raman spectroscopy (SERS) for the rapid identification of bacteria from diluted blood.

Electric field alignment of nanofibrillated cellulose (NFC) in silicone oil: impact on electrical properties

This work aims to study how the magnitude, frequency, and duration of an AC electric field affect the orientation of two kinds of nanofibrillated cellulose (NFC) dispersed in silicone oil that differ by their surface charge density and aspect ratio. In both cases, the electric field alignment occurs in two steps: first, the NFC makes a gyratory motion oriented by the electric field; second, NFC interacts with itself to form chains parallel to the electric field lines. It was also observed that NFC chains become thicker and longer when the duration of application of the electric field is increased. In-situ dielectric properties have shown that the dielectric constant of the medium increases in comparison to the randomly dispersed NFC (when no electric field is applied). The optimal parameters of alignment were found to be 5000 Vpp/mm and 10 kHz for a duration of 20 min for both kinds of NFC. The highest increase in dielectric constant was achieved with NFC oxidized for 5 min (NFC-O-5 min) at the optimum conditions mentioned above.