Electrowetting-on-dielectric System Research Articles

A Novel EWOD Platform for Freely Transporting Droplets in Double and Single-Plate Structures.

This study developed a novel dielectric wetting microfluidic operation platform combining parallel-plate and coplanar-plate regions with a curved surface structure as the connection structure. With the new electrowetting on dielectric (EWOD) platform, "droplet pull-out" has been successfully achieved and viewed as an essential new operation for microfluidics with the dielectric wetting technique. The EWOD system is divided into a PDMS substrate top plate and an indium tin oxide (ITO) glass substrate as a bottom layer on this chip. In the parallel-plate region, the droplets can be generated and transported through the square parallel electrodes; in the single-plate area, the droplets can be pulled out from the parallel structure, transported and mixed through the common grounded coplanar electrodes. In dielectric wetting performance testing, coplanar electrodes can apply a maximum driving force of 31.22 µN to DI water and 13.38 µN to propylene carbonate (PC). This driving force is sufficient to detach the sample from the top cover and pull the sub-droplet from the parallel plate structure for DI water, PC and polyethylene glycol diacrylate (PEGDA) buffer. The novel EWOD system also possesses the advantage of precise volume control for liquid samples; the volume error of the generated droplet can be controlled within 0.1% to 2%.

A Review of Nanostructures in Electrowetting-on-dielectric Systems: From Nanostructured Dielectric Layers to Nanofluids

Abstract: The extensive interest in electrowetting-on-dielectric (EWOD) as a key in advancing the efficiency and controllability of fluid-based microelectromechanical and actuator systems has resulted in a deluge of technological research, especially in the area of microfluidics, liquid lenses, and fluid-based lab-on-chips. More recently, the integration of nanostructures into EWOD-driven devices has shown promising improvement in these devices’ performance, design, and miniaturization. Due to the exceptional properties, availability, versatility, and tunability of nanostructures, they are being utilized as components of EWOD systems for various applications. Utilization ranges from fabricating nanodimensional dielectric layers to incorporating nanoparticles in fluid droplets. With the current trend in improving the performance and functionality of EWOD-driven devices at low voltage operations, it is timely to revisit the fundamental principle of EWOD phenomena and how it is extended experimentally using nanostructures. In this paper, we present the different nanostructures investigated as dielectric materials in various EWOD experiments focusing on metal oxide and silicon nitride layers. Notes on the structure of these dielectric layers are also presented. Furthermore, various EWOD experiments employing nanofluid droplets are also described. This paper provides a clear picture of nanostructures’ diverse impact on the advancement of EWOD technology. The insights presented in this paper may also serve as a guidepost for future exploration and development of the role of nanostructures in EWOD-driven devices.

Microfluidic platform using focused ultrasound passing through hydrophobic meshes with jump availability.

Applications in chemistry, biology, medicine, and engineering require the large-scale manipulation of a wide range of chemicals, samples, and specimens. To achieve maximum efficiency, parallel control of microlitre droplets using automated techniques is essential. Electrowetting-on-dielectric (EWOD), which manipulates droplets using the imbalance of wetting on a substrate, is the most widely employed method. However, EWOD is limited in its capability to make droplets detach from the substrate (jumping), which hinders throughput and device integration. Here, we propose a novel microfluidic system based on focused ultrasound passing through a hydrophobic mesh with droplets resting on top. A phased array dynamically creates foci to manipulate droplets of up to 300 L. This platform offers a jump height of up to 10 cm, a 27-fold improvement over conventional EWOD systems. In addition, droplets can be merged or split by pushing them against a hydrophobic knife. We demonstrate Suzuki-Miyaura cross-coupling using our platform, showing its potential for a wide range of chemical experiments. Biofouling in our system was lower than in conventional EWOD, demonstrating its high suitability for biological experiments. Focused ultrasound allows the manipulation of both solid and liquid targets. Our platform provides a foundation for the advancement of micro-robotics, additive manufacturing, and laboratory automation.

Electrowetting-on-dielectric powered by triboelectric nanogenerator

Electrowetting-on-dielectric (EWOD) is a versatile technique for controlling the liquid-wetting behavior of solid surfaces, but usually relies on bulky and complex power suppliers for high driving voltages, which largely limits the practical applications toward miniaturization, portability, and multifunctional integration. Here, a mechanical stimuli-controlled EWOD strategy powered by a triboelectric nanogenerator (TENG) with tunable high-voltage output is demonstrated, which allows for manipulating liquid wettability by mechanical stimuli. The physical behaviors of the voltage transfer and contact angle response during the TENG-driven EWOD process are experimentally investigated, and the underlying mechanisms are revealed by a circuit model, which can give general guidance for the design and optimization of the mechanical stimuli-controlled EWOD systems. As a derived application of this strategy to the microfabrication field, a self-powered electrocapillary infilling and micromolding method is further developed, which allows for fabricating the originally difficult-to-mold microstructures in a handy way. This mechanical stimuli-controlled TENG-driven EWOD strategy also shows general applicability for different liquid systems of not only the liquid prepolymer but also the DI water and ionic liquid, which can potentially extend to more functional applications involving tunable wettability or liquid infilling/permeation, such as wearable microfluidics or lab-on-chip, electrolyte infilling for porous electrodes, and on-demand drug permeation systems.

Virtual Stencil for Patterning and Modeling in a Quantitative Volume Using EWOD and DEP Devices for Microfluidics.

Applying microfluidic patterning, droplets were precisely generated on an electrowetting-on-dielectric (EWOD) chip considering these parameters: number of generating electrodes, number of cutting electrodes, voltage, frequency and gap between upper and lower plates of the electrode array on the EWOD chip. In a subsequent patterning experiment, an environment with three generating electrodes, one cutting electrode and a gap height 10 μm, we obtained a quantitative volume for patterning. Propylene carbonate liquid and a mixed colloid of polyphthalate carbonate (PPC) and photosensitive polymer material were manipulated into varied patterns. With support from a Z-axis lifting platform and a UV lamp, a cured 3D structure was stacked. Using an EWOD system, a multi-layer three-dimensional structure was produced for the patterning. A two-plate EWOD system patterned propylene carbonate in a quantitative volume at 140 Vpp/20 kHz with automatic patterning.

Design of a Hand-Held and Battery-Operated Digital Microfluidic Device Using EWOD for Lab-on-a-Chip Applications.

Microfluidics offer many advantages to Point of Care (POC) devices through lower reagent use and smaller size. Additionally, POC devices offer the unique potential to conduct tests outside of the laboratory. In particular, Electro-wetting on Dielectric (EWOD) microfluidics has been shown to be an effective way to move and mix liquids enabling many PoC devices. However, much of the research surrounding these microfluidic systems are focused on a single aspect of the system capability, such as droplet control or a specific new application at the device level using the EWOD technology. Often in these experiments the supporting systems required for operation are bench top equipment such as function generators, power supplies, and personal computers. Although various aspects of how an EWOD device is capable of moving and mixing droplets have been demonstrated at various levels, a complete self-contained and portable lab-on-a-chip system based on the EWOD technology has not been well demonstrated. For instance, EWOD systems tend to use high voltage alternating current (AC) signals to actuate electrodes, but little consideration is given to circuitry size or power consumption of such components to make the entire system portable. This paper demonstrates the feasibility of integrating all supporting hardware and software to correctly operate an EWOD device in a completely self-contained and battery-powered handheld unit. We present results that demonstrate a complete sample preparation flow for deoxyribonucleic acid (DNA) extraction and isolation. The device was designed to be a field deployable, hand-held platform capable of performing many other sample preparation tasks automatically. Liquids are transported using EWOD and controlled via a programmable microprocessor. The programmable nature of the device allows it to be configured for a variety of tests for different applications. Many considerations were given towards power consumption, size, and system complexity which make it ideal for use in a mobile environment. The results presented in this paper show a promising step forward to the portable capability of microfluidic devices based on the EWOD technology.

Electrowetting-on-dielectric (EWOD): Current perspectives and applications in ensuring food safety.

Digital microfluidic (DMF) platforms have contributed immensely to the development of multifunctional lab-on-chip systems for performing complete sets of biological and analytical assays. Electrowetting-on-dielectric (EWOD) technology, due to its outstanding flexibility and integrability, has emerged as a promising candidate for such lab-on-chip applications. Triggered by an electrical stimulus, EWOD devices allow precise manipulation of single droplets along the designed electrode arrays without employing external pumps and valves, thereby enhancing the miniaturization and portability of the system towards transcending important laboratory assays in resource-limited settings. In recent years, the simple fabrication process and reprogrammable architecture of EWOD chips have led to their widespread applications in food safety analysis. Various EWOD devices have been developed for the quantitative monitoring of analytes such as food-borne pathogens, heavy metal ions, vitamins, and antioxidants, which are significant in food samples. In this paper, we reviewed the advances and developments in the design of EWOD systems for performing versatile functions starting from sample preparation to sample detection, enabling rapid and high-throughput food analysis.

Microfluidic patterning using a digital microfluidic system

There are some issues in printing technology, such as stencils having a brief life span and limitations in patterning; therefore, we propose microfluidic patterning using an electrowetting-on-dielectric (EWOD) and liquid-dielectrophoresis (L-DEP) system. Propylene carbonate liquid was generated, moved, separated, and positioned, and was patterned into various English alphabetic characters. Unlike traditional printing techniques, patterns become limited and fixed, so the shape of the desired pattern cannot be modified arbitrarily. Each single English alphabetic pattern can be formed with a volume of droplet of about 100 nl; the width of the pattern can be decreased to about 0.15 mm or less. Using an EWOD system, we have implemented an open-plate L-DEP system and a two-plate L-DEP system, for the patterning of all English alphabetic characters with the same volume of liquid at a fixed voltage.

Analysis of voltage distribution in electrowetting on Dielectric (EWOD) system

Electrowetting on Dielectric (EWOD) is one of the most popular digital microfluidic techniques, deployed in Lab-on-a-Chip (LoC) systems for analyzing samples of biomedical relevance. Despite the popularity of EWOD for testing of biochemical and biomedical samples, the technique faces two major challenges: preserving the cells of biosample and electrodes due to the application of high voltage and ensuring the droplets stay on tracks in the electrode system.A detailed study of the electrostatic properties like capacitance and distribution of voltage for EWOD systems has not been performed till date, which otherwise may prove fruitful in designing future LoC platforms for efficient sampling and analysis. Therefore for effective study of digital micro fluidic devices, accurate value of potential difference with variation in dielectric layer of EWOD can be helpful. In the current paper, potential distribution around the sample is analyzed by carrying out voltage analysis of the droplet using COMSOL. Comparative analysis of effect of the three dielectric materials on EWOD was performed by comparing the voltage drop across the dielectric layer.Since thickness and nature of dielectric material governs the operational voltage of EWOD system, comparative study of voltage drop of the device was done for promising dielectric materials like PDMS, PMMA and SiO2 for dielectric thickness of 10 μm, 20 μm, 30 μm, 40 μm and 50 μm. The capacitance for all the respective design parameters was also evaluated using COMSOL. Young Lipmann’s equation was used to establish the validity of the results. The dependency of EWOD system on electrolyte conductivity was also studied for NaCl solution for concentrations varying from 0.43 mM to 100 mM. The capacitance study of all the electrolyte concentration was performed as well to identify the electrolyte change.

A medical innovation: a new and improved method of DNA extraction with electrowetting-on-dielectric of genetic testing in-vitro fertilization (IVF)

The current technique for the detection of fetus genotype by in-vitro fertilization (IVF) is quite expensive and is unsuitable/ inconvenient for all patients and even affects the embryonic growth cycle. We thus propose a new method to extract small amount of cell-free DNA (cf-DNA) from embryo culture medium. We introduce a new format for DNA extraction depending on digital microfluidics (DMF), electrowetting-on-dielectric (EWOD) and magnetic forces to separate, suspend DNA-coated magnetic particles. Nano-droplets (of nano-liter size) are electrostatically controlled on an array of insulated electrodes by EWOD system. By applying voltage to the various electrodes, multiple nano-droplets can be manipulated precisely in the same device. This feature enables one to extract DNA at ultra-small concentrations (fg/μL to pg/μL) from an embryo culture medium with an EWOD system. We applied our EWOD platform to extract a small amount of cf-DNA from a clinical embryo culture medium of mouse. In the future, we will distinguish the DNA in the real embryo buffer according to the length of the telomere. This feature would be valuable for the selection of an embryo before an implantation when the gene information can be recognized.

Time-resolved measurements of DNA interactions in an electrowetting-on-dielectric system using confocal microscopy

To identify new drug candidates a deep and profound knowledge of molecule interactions is needed. In the current work, a combination of an electrowetting-on-dielectric (EWOD) system with a confocal microscopy is presented for the first time. The aim of this research is to attain time-resolved information about nucleic acid interactions at a single molecule level. Confocal microscopy is a promising technique for reaction analysis on a molecular scale. But the liquid handling of the needed tiny volumes of highly diluted solutions is very challenging. An EWOD based system for droplet handling can address this demand. In this paper the development of an EWOD system for droplet handling in nanoliter scale is discussed and the combination of the EWOD system with a confocal microscope, to investigate nucleic acid interactions, is evaluated.

Effect of electrode geometry on droplet velocity in open EWOD based device for digital microfluidics applications

Electro wetting-on-dielectric (EWOD) is an emerging method for handling droplet motion by applying an electric field to an array of electrodes. The dependence of droplet velocities on different electrode configuration in open EWOD system has been investigated in this work. In this paper, open configured EWOD devices with different geometries of electrodes, polydimethylsiloxane (PDMS) as a base layer are designed and fabricated. The electrowetting force is computed by analytical methods as well as by numerical methods and its effect on droplet velocity is studied in detail. The velocity of the droplet is measured by using open image processing tool.

Low-Cost Silver Screen-Printed Electrowetting on Dielectrics Structure for Optofluidic Switches

This work demonstrates a low-cost fabrication method for Electrowetting-on-Dielectric (EWOD) structures for microsystem and Lab-on-a-chip applications. The electrowetting effect allows droplet movement by manipulating the surface tension equilibrium in an EWOD system. A voltage-induced imbalance of the contact angle (CA) at the triple contact line leads to droplet actuation into the direction of the smaller CA. The presented low-cost EWOD stack consists of a suitable substrate, actuation electrodes and dielectric/hydrophobic layers. Screen printed silver electrodes on glass substrates were investigated and characterized. Indium Tin Oxide (ITO) glass slides with a top coated Teflon fluoropolymer were used as electrical counter electrodes. As an application, a demonstrator of a simple optofluidic (OF) switch is simulated in terms of optical path switching time and fabricated using the silver screen printed setup. The incident beam of a laser light source becomes deflected depending on the position of an EWOD actuated droplet inside the device.

Impedance spectroscopy and electrical modeling of electrowetting on dielectric devices

Using impedance spectroscopy, we have determined models for the elements which determine the ac electrical behavior in electrowetting on dielectric (EWOD) systems. Three commonly used EWOD electrode configurations were analyzed. In each case, the impedance can be modeled by a combination of elements, including the solution resistance, the capacitance of the dielectric layer, and the constant phase impedance of the electrode double layers. The sensitivity of the system’s impedance to variations in the electrowetted area is also analyzed for these common configurations. We also demonstrate that the impedance per unit area of typical EWOD systems is invariant to bias voltage.

Improving the dielectric properties of an electrowetting-on-dielectric microfluidic device with a low-pressure chemical vapor deposited Si3N4 dielectric layer.

Dielectric breakdown is a common problem in a digital microfluidic system, which limits its application in chemical or biomedical applications. We propose a new fabrication of an electrowetting-on-dielectric (EWOD) device using Si3N4 deposited by low-pressure chemical vapor deposition (LPCVD) as a dielectric layer. This material exhibits a greater relative permittivity, purity, uniformity, and biocompatibility than polymeric films. These properties also increase the breakdown voltage of a dielectric layer and increase the stability of an EWOD system when applied in biomedical research. Medium droplets with mouse embryos were manipulated in this manner. The electrical properties of the Si3N4 dielectric layer-breakdown voltage, refractive index, relative permittivity, and variation of contact angle with input voltage-were investigated and compared with a traditional Si3N4 dielectric layer deposited as a plasma-enhanced chemical vapor deposition to confirm the potential of LPCVD Si3N4 applied as the dielectric layer of an EWOD digital microfluidic system.

Polydimethylsiloxane material as hydrophobic and insulating layer in electrowetting-on-dielectric systems

Open and closed electrowetting-on-dielectric (EWOD) systems based on a spin coated polydimethylsiloxane (PDMS) layer are presented. The PDMS layer acts as both insulation and hydrophobic material. Characterization, through sessile drop experiments, shows the hydrophobic behaviors of the PDMS and saturation of the contact angle at negative bias voltage applied to the droplet. This behavior is ascribed to trapped carrier in the PDMS layer and explains the movement of the droplet toward the grounded electrode found in the EWOD experiments.An electronic board controls all the signals needed for the actuation and sensing functionalities of the EWOD systems. Detection of drop position along the electrode array is successfully achieved by implementing the time-constant method, which evaluates the variation of electrode capacitance induced by the droplet presence on the PDMS surface corresponding to the metal electrode.The microfluidic operations (movement, dispensing and splitting) in both open and closed configurations have been verified and accomplished at voltages around 200V.

Amorphous silicon photosensors for on-chip detection in digital microfluidic system

In this paper we present the integration, on a single glass substrate, of amorphous silicon photodiodes with an electrowetting-on-dielectric (EWOD) system as a technological demonstrator for achieving a compact, stand alone Lab-on-Chip (LoC) system. The EWOD system comprises a thin film of indium tin oxide (ITO) acting as actuation electrodes and a 1μm-thick polydimethylsiloxane (PDMS) layer acting as both insulation and hydrophobic layer. The a-Si:H photosensors are ITO/p-type/intrinsic/n-type/metal stacked structures, aligned with the transparent EWOD electrodes, to detect optical signals generated inside (or modulated by) the liquid droplets handled by the digital microfluidic system.The fabrication process has been designed and performed taking into account the compatibility of all the technological steps of the photosensor and EWOD structures fabrication. The successful integration has been demonstrated checking the correct geometry of EWOD electrodes and measuring the optoelectronic performances of the a-Si:H photosensors at the end of the system fabrication. The correct operation and potentiality of the presented device has been assessed monitoring a photodiode current when a water droplet is moved forward and backward over the EWOD electrodes aligned with the photosensors.

Optimum thickness of hydrophobic layer for operating voltage reduction in EWOD systems

This paper presents the optimum thickness of a hydrophobic layer for operating voltage reduction in electrowetting-on-dielectric (EWOD) systems by investigating the effects of thickness on the surface wettability and hysteresis using atomic force microscopy (AFM). To investigate the surface wettability, the thicknesses of Cytop and Teflon layers coated with different weight percentages of Cytop and Teflon solutions are precisely measured by AFM. The contact angles of deionized water droplets on the Cytop and Teflon layers of different thicknesses are measured using an optical microscope from the side. The results of the contact angle measurements of deionized water droplets on Cytop layers indicate that layers thicker than 3nm maintain an angle of approximately 110°; the contact angles of the drops on Cytop layers thinner than 3nm abruptly drop and decrease as the layer becomes thinner. The Teflon layers show a similar trend to the Cytop layers, except that the critical thickness is larger (7nm). The contact angle hysteresis for different thicknesses of Cytop and Teflon layers is investigated by the tilting plate method. The results reveal that for thin hydrophobic layers, the contact angle hysteresis is over 10° but decreases as the layer thickness increases. When the layers are thicker than 12nm, the contact angle hysteresis is reduced by approximately 4° for Cytop and 7° for Teflon and becomes saturated. To investigate the effect of aging on the surface wettability and hysteresis, the film stability of Cytop and Teflon layers is separately tested. The initial contact angles are reduced about 1.2% for Cytop and 1.6% for Teflon within 1 day and then maintained up to 10 days, while the initial contact angle hysteresis is increased about 117% for Cytop and 39% for Teflon within a day and then maintained up to 10 days, regardless of the thickness of the Cytop and Teflon layers. Based on the results of the surface wettability and hysteresis, an optimum hydrophobic layer thickness of 12nm is established. Finally, the thickness effects of the hydrophobic layer on the operating voltage in EWOD actuation are tested. As expected, the operating voltage for thin hydrophobic layers is lower than that for thick layers. For the Cytop layer, the saturation voltages for 12nm, 500nm, and 1500nm are 80V, 100V, and 120V, respectively. Similarly, for the Teflon layer, the saturation voltages for 12nm, 600nm, and 1600nm are 90V, 110V, and 130V, respectively. The relative differences in the saturation voltages for both materials with respect to thickness are approximately the same.

Applications of EWOD Systems for DNA Reaction and Analysis

A digital microfluidic system based on electrowetting-on-dielectric (EWOD) technology has attracted considerable attention because of its promising potential in biological, biochemical and biomedical applications. The digital microfluidic (EWOD) system enables a discrete droplet to be manipulated through reconfig-urable paths or channels defined by electrode actuation. Minute sample and reagent volume, precise droplet control, low energy consumption, low cost, rapid analysis, portablility for point-of-care diagnosis, flexibility for integrating with detector, multiple-step and simultaneous reaction processes are all advantages of EWOD system. Thus, it has become a promising and popular tool for bio-applications such as cell assays, proteomics, enzyme assays, immunoassays, DNA extraction, DNA repair and DNA amplification. In DNA-based applications, ordinarily, the DNA replication (e.g., DNA amplification) is necessary. Among the DNA replication techniques, Polymerase Chain Reaction (PCR) because of its simplicity and cost-effectiveness is the most common laboratory technique. Therefore, the development of EWOD-based microfluidic PCR system combined with other novel techniques for DNA amplification or DNA-based assays has extensively been reported. In this paper, the recent developments and studies of EWOD microfluidic system for DNA-based applications published in the last five years have been reviewed.

Mixed finite element method for electrowetting on dielectric with contact line pinning

We present a mixed finite element method for a model of the flow in a Hele-Shaw cell of 2-D fluid droplets surrounded by air driven by surface tension and actuated by an electric field. The application of interest regards a micro-fluidic device called ElectroWetting on Dielectric (EWOD). Our analysis first focuses on the time discrete (continuous in space) problem and is presented in a mixed variational framework, which incorporates curvature as a natural boundary condition. The model includes a viscous damping term for interface motion, as well as contact line pinning (sticking of the interface) and is captured in our formulation by a variational inequality. The semi-discrete problem uses a semiimplicit time discretization of curvature. We prove the well-posedness of the semi-discrete problem and fully discrete problem when discretized with iso-parametric finite elements. We derive a priori error estimates for the space discretization. We also prove the convergence of an Uzawa algorithm for solving the semi-discrete EWOD system with inequality constraint. We conclude with a discussion about experimental orders of convergence.