Electrowetting-on-dielectric Device Research Articles

Contact Angle Hysteresis and Sliding Angle of Commercial Hydrophobic Materials for Electrowetting Actuating Surface

Hydrophobic materials are one of the main components in electrowetting-ondielectric (EWOD) devices, acting as the actuating surface upon which droplets move. EWOD technology has been extensively utilised as a platform for particle detection due to its versatility, simple design, and minimal waste. However, the technology could be more widely adopted if the materials and fabrication costs could be reduced. This study presents an investigation into two off-the-shelf hydrophobic materials as alternatives to conventional materials for the actuating surface component of the device. The surfaces were prepared using two different deposition methods, which were spin coating and spray coating. For each method, samples with two thicknesses were prepared, consisting of one layer and two layers. The contact angle hysteresis and sliding angle of the commercial hydrophobic surfaces were measured. One of the surfaces exhibited good performance with a contact angle hysteresis and sliding angles of 28° and 34°, respectively. These promising findings suggest the potential use of commercial hydrophobic materials for the future development of disposable and low-cost EWOD devices.

Replaceable Dielectric Film for Low-Voltage and High-Performance Electrowetting-Based Digital Microfluidics.

Electrowetting-on-dielectric (EWOD) technology has been considered as a promising candidate for digital microfluidic (DMF) applications due to its outstanding flexibility and integrability. The dielectric layer with a hydrophobic surface is the key element of an EWOD device, determining its driving voltage, reliability, and lifetime. Hereby, inspired by the ionic-liquid-filled structuring polymer with high capacitance independent on thickness, namely ion gel (IG), we develop a polymer (P)-ion gel-amorphous fluoropolymer, namely, PIGAF, composite film as a replaceable hydrophobic dielectric layer for fabrication of a high-efficiency and stable EWOD-DMF device at relatively low voltage. The results show that the proposed EWOD devices using the PIGAF-based dielectric layer can achieve a large contact angle (θ) change of ∼50° and excellent reversibility with a contact angle hysteresis of ≤5° at a relatively low voltage of 30 Vrms. More importantly, the EWOD actuation voltage did not change obviously with the PIGAF film thickness in the range of several to tens of microns, enabling the thickness of the film to be adjusted according to the demand within a certain range while keeping the actuation voltage low. An EWOD-DMF device can be prepared by simply stacking a PIGAF film onto a PCB board, demonstrating stable droplet actuation (motion) at 30 Vrms and 1 kHz as well as a maximum moving velocity of 69 mm/s at 140 Vrms and 1 kHz. The PIGAF film was highly stable and reliable, maintaining excellent EWOD performance after multiple droplet manipulations (≥50 cycles) or long-term storage of 1 year. The proposed EWOD-DMF device has been demonstrated for digital chemical reactions and biomedical sensing applications.

Closed EWOD-based low-cost portable thermal detection system for point-of-care applications

Electrowetting-on-Dielectric (EWOD) is a powerful technology for biological and chemical diagnostics. It offers advantages of low sample volume, high sensitivity, high throughput and the possibility of automation. Of special interest is a miniaturization of a thermal detection system by utilizing the EWOD technique since it can minimize effort while reducing cost and the system size. The present study describes the development of a closed EWOD device for droplet actuation that is integrated with thermal cycling and optical diagnostics. Important steps, including sample and reagent transportation, merging and mixing, and color detection, are performed on the integrated platform. The thermal module uses heating as well as Peltier elements and K-type thermocouples with a PID controller to perform precise thermal cycling needed in clinical and medical diagnostics. Experimentally recorded temperature distribution that is realized in the closed EWOD configuration is numerically validated using the COMSOL simulator. The efficacy of the integrated system is experimentally demonstrated by performing Fehling’s test for glucose detection in a glycerin solution. Furthermore, the device is used to demonstrate the transportation of a liquid drop of blood mimicking liquid at a relatively high temperature of 90 °C. The system thus developed is reliable, providing real-time proof-of-concept and the feasibility of building a low-cost point-of-care EWOD device. • A portable closed EWOD based medical diagnostic system with thermal cycling is demonstrated. • Thermal management using heating and cooling element is integrated with the closed EWOD device. • Temperature distribution within the device is measured experimentally and validated against simulation. • Droplet motion is successfully realized over the temperature range studied, providing real-time proof-of-concept. • The system developed is tested by performing Fehling’s test for glucose detection.

Distinct condensation droplet distribution patterns under low- and high-frequency electrowetting-on-dielectric (EWOD) effect

Distinct droplet distribution patterns of open-plate grid-wire-patterned EWOD devices under low (1 Hz, 5 Hz) and high (500 Hz, 1000 Hz) frequencies of AC-EWOD were discussed in this paper. Under low frequencies, droplets stayed inside the grid; under high frequencies, droplets adhered to the grid wires. Single droplet (1 μL) dynamics were studied with a discovery of contact-angle-fully-recover and contact-angle-partially-recover oscillation modes. These two modes induced distinct droplet coalescence mechanisms and distribution patterns. The natural response frequency was introduced to validate these two modes. Visualization experiments of droplet distribution were carried out on 0.5 mm, 1 mm, and 2 mm grid samples under 2 Hz, 50 Hz, 100 Hz, and 200 Hz. Condensate was collected to investigate the grid size (0.5 mm, 1 mm, 2 mm) and frequency effect (1 Hz, 100 Hz, 200 Hz) on condensation rate. Higher frequency demonstrated a larger condensation rate than the lower ones; the 1 mm grid samples showed more inspiring results compared with the 0.5 mm and 2 mm grid samples. EWOD provided an active and reversible way to interfere with droplet dynamics, and promote droplet growth and coalescence, which demonstrated great potential for future applications in droplet control and condensation enhancement.

A Wearable Electrowetting on Dielectrics Sensor for Real‐Time Human Sweat Monitor by Triboelectric Field Regulation

AbstractWearable sweat sensors can analyze the abundant composition of solutes and metabolites in sweat to reflect the health state of the wearers in real time. The realization of active motion control for sweat droplets is significant for a multifunctional sweat monitoring device with several analysis chambers. Here, a wearable droplet‐based human sweat monitoring platform (WSMP), by combining an electrowetting on dielectrics (EWOD) device and a triboelectric nanogenerator (TENG), is demonstrated. It allows to collect and transport sweat droplets in different chambers by dielectric wetting effect and eventually merge and react with a pH indicator. The mechanical and electrical model of WSMP is introduced to describe the relationship between the open‐circuit voltage of the TENG and the voltage applied on the EWOD device. The high‐voltage electrical field generated by the TENG can change the wettability of solid–liquid interfaces and realize the controlling of droplet motion. The contact angle of electrolyte droplets changes over 30% with the triboelectric voltage of 5 kV. The driving, merging, and color reaction can be realized by actively controlling the motion of droplets. Finally, a wearable WSMP worn on the shank successfully demonstrates the preliminary detection of the pH level of human sweat.

Intelligent resistive-switching EWOD device based on the Fe doped ZnO memristor

Multi-functionalization and intellectualization are the main goal for the development of various excellent device. Resistive-switching memristor is an emerging artificial intelligence device. Combining resistive switching with electrowetting-on-dielectric (EWOD) is a pioneering idea to fulfill the multifunctional and intelligent resistive-switching EWOD device. In the paper, Fe doped ZnO memristors have been successfully prepared by a simple hydrothermal method, and exhibit reversible electronic resistive switching behavior, which is mainly attributed to the electron trapping and detrapping from the introduced oxygen vacancies by Fe doping. The surface structure of hierarchical dual-layer nanorod array endows the prepared ZnO memristors with reversible EWOD response of low energy consumption, which enables the device to satisfy the development of excellent EWOD device. Through the in-depth study, the stable high and low resistive states of prepared Fe doped ZnO memristors have distinct surface wetting properties, and the liquid droplet on the memristors can be intelligently controlled, and realize the resistive-switching EWOD intelligent device. The work gives a new way to the development of excellent artificial intelligence liquid switch and devices.

Programmable High Integration and Resolution Digital Microfluidic Device Driven by Thin Film Transistor Arrays

A digital microfluidic (DMF) system based on electrowetting on dielectrics (EWOD) is a new technology that is able to manipulate microlitre droplets on the plane. Conventional EWOD devices have difficulty achieving high integration and a large number of electrodes due to the limitation of electrical connections. To realize the precise control of multiple droplets in a large-area EWOD device, we propose a programmable high integration and resolution DMF device based on a thin film transistor array (TFTA) &#x2013; EWOD in this paper. The TFTA backplane with thousands of individually addressable electrodes used in electrowetting displays (EWDs) is integrated into this device. A source integrated circuit (IC) and a gate IC are bonded on the substrate glass of the device as the driver chip. A TFTA-EWOD device with <inline-formula> <tex-math notation="LaTeX">$\mathrm {320 \times 240}$ </tex-math></inline-formula> electrodes is designed, fabricated and tested in our experiments. Furthermore, a matching droplet control system with detection and feedback functions was developed and implemented. This system is capable of detecting and analysing individual droplet status while concurrently continuing with other droplet manipulations on the TFTA-EWOD device. The experimental results show that different droplet manipulations, including dispensing, merging and transport, can be carried out successfully on this device. We also successfully applied the proposed device and system to a colorimetric enzymatic assay to show that the proposed scheme is capable of chemical analysis experiments. There are no additional errors introduced by the proposed TFTA-EWOD device. The designed device and matching system proposes a programmable high integration, high resolution, and universal solution for complex DMF applications. We hope that the proposed device and system can provide an experimental platform to other research teams who focus on DMF applications.

Paper-based electrowetting devices fabricated with cellulose paper and paraffin wax

Paper-based microfluidic devices based on electrowetting principle, more specifically electrowetting on dielectric (EWOD), have attracted increasing attention due to their powerful liquid handling capability as compared to continuous-flow paper-based analytical devices (μPAD) that use capillary effects to control liquids. Here, we demonstrate fabrication of paper-based EWOD devices using cellulose paper as substrate and paraffin wax as dielectric hydrophobic layer by a simple pressing treatment for the cellulose paper and melting or spray-coating of paraffin wax. We optimized the key fabrication parameters according to the surface morphology and hydrophobicity of the obtained wax film, including the pressing pressure, thermal annealing temperature and amount of paraffin wax. Moreover, the EWOD devices fabricated by the spray-coating method demonstrate larger contact angle changes but with smaller contact angle hysteresis under DC electric field when compared with the devices fabricated by the melting method. Our study opens up potential fabrication of hybrid paper-based microfluidic device combining advantages of continuous-flow and electrowetting using cellulose paper and paraffin wax.

A transparent electrowetting-on-dielectric device driven by triboelectric nanogenerator for extremely fast anti-fogging

Fogging widely occurs in industrial processes, surgical treatment, and human daily lives. Due to the reduction in transmittance, fogging induces adverse effects or even disaster. Traditional anti-fogging methods face the limitations of large energy consumption and sensitive to contamination. This paper proposes a method of using rotating freestanding triboelectric nanogenerator (RF-TENG) which is capable of converting arbitrary mechanical energy into electricity to drive an electrowetting-on-dielectric (EWOD) device to achieve anti-fogging. The RF-TENG could generate variable open-circuit voltage (0.73–5.26 kV) and frequency (20–100 Hz) by changing the contact area and the rotating speed of the input mechanical energy. The fog droplets coalescence and shedding processes were captured by a high-speed camera, revealing that the droplet oscillation and lateral movement on the EWOD device mainly contribute to the anti-fogging phenomenon. The effects of open-circuit voltage and frequency of the RF-TENG on anti-fogging performance are discussed. The experimental results show that when the open-circuit voltage is greater than ± 1 kV and a frequency larger than 60 Hz is able to achieve a large dry area fraction from 34% to 98% in a very short time (< 0.5 s). Finally, a practical application of using a RF-TENG for window glass anti-fogging was achieved. The results indicate that the EWOD glass show a good visible light transmittance, only 3% lower than the ordinary glass. This study paves a way for utilizing the renewable mechanical energy (e.g. wind) for indoor anti-fogging.

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.

Using office inkjet printer to develop paper-based electrowetting-on-dielectric micromixer based on capillary wave-induced droplet vibration mixing for the reproducibility improvement of chemiluminescence assays

BackgroundWe use inkjet printing and film adhesion methods to economically develop paper-based electrowetting-on-dielectric (EWOD) mixing device. Because the periodic shrinking and stretching of droplet contact line induces surface capillary wave, the coalesced droplet on EWOD device vibrates to accelerate solute mixing on the merged spot. MethodsAn office inkjet printer fabricates pads of conducting silver nanoparticles to compose EWOD activation electrodes on a piece of glossy label paper, covered with one oil-coated dielectric film. Using the triggering relays of each pad, two droplets of thirty microliter individually loaded on the electrode ends are driven to coalesce. Alloy nanorod-catalyzed chemiluminescence assay can be performed on this mixing device. FindingsWhen ac voltage of 100 Hz is applied on the electrode flakes, compared with solute transport efficiency in static condition, solute diffusivity is enhanced by 4 times using this mixing device. Linear relation (R2 = 0.99) between luminescence intensity acquired with smartphone camera and luminol concentration of aforementioned chemiluminescence assay is obtained, demonstrating reproducibility improvement for accurate determination.

Automatic radioisotope manipulation for small amount of nuclear medicine using an EWOD device with a dimple structure.

Delicate animal experiments and microdose clinical trials using short-lived radioisotopes require rapid preparation with high accuracy and careful attention to safety within a limited timeframe. We have developed an open-style electrowetting on dielectric (EWOD) device containing dimple structures for the rapid preparation of radiolabelled reagents. The device was demonstrated by automatic preparation of a technetium diethylenetriamine pentaacetate (99mTc-DTPA) with high chelation efficiency (99.7 ± 0.13%). Additionally, we demonstrated the single-photon emission computed tomography/computed tomography imaging of mouse kidney using the 99mTc-DTPA prepared with the EWOD device. The obtained organ tomographic images were sufficient for the evaluation of mouse models for specific diseases. These results indicate that manual radiolabelling for a small amount of nuclear medicine can be replaced by a process using the proposed EWOD device as a human error reduction technique.

Sensitivity Validation of EWOD Devices for Diagnosis of Early Mortality Syndrome (EMS) in Shrimp Using Colorimetric LAMP-XO Technique.

Electrowetting-on-dielectric (EWOD) is a microfluidic technology used for manipulating liquid droplets at microliter to nanoliter scale. EWOD has the ability to facilitate the accurate manipulation of liquid droplets, i.e., transporting, dispensing, splitting, and mixing. In this work, EWOD fabrication with suitable and affordable materials is proposed for creating EWOD lab-on-a-chip platforms. The EWOD platforms are applied for the diagnosis of early mortality syndrome (EMS) in shrimp by utilizing the colorimetric loop-mediated isothermal amplification method with pH-sensitive xylenol orange (LAMP–XO) diagnosis technique. The qualitative sensitivity is observed by comparing the limit of detection (LOD) while performing the LAMP–XO diagnosis test on the proposed lab-on-a-chip EWOD platform, alongside standard LAMP laboratory tests. The comparison results confirm the reliability of EMS diagnosis on the EWOD platform with qualitative sensitivity for detecting the EMS DNA plasmid concentration at 102 copies in a similar manner to the common LAMP diagnosis tests.

Modeling, simulation, and optimization of electrowetting-on-dielectric (EWOD) devices.

With widespread research studies on electrowetting-on-dielectric (EWOD) for droplet manipulation in the field of lab-on-a-chip, how to improve the driving capability of droplets has increasingly attracted enormous interest. Aiming to decrease driving voltages and improve driving effectiveness, this paper studies the modeling, simulation, and optimization of EWOD devices. The theoretical model is refined mainly in consideration of the saturation effect of the contact angle and then verified by both simulation and experiments. As a design guide to decrease the driving voltage, a theoretical criterion of droplet splitting, the most difficult one among four basic droplet manipulations, is developed and then verified by experimental results. Moreover, a novel sigmoid electrode shape is found by the optimization method based on finite element analysis and achieves better driving effectiveness and consistent bidirectional driving capability, compared with the existing electrode shapes. Taken together, this paper provides an EWOD analysis and optimization method featuring a lower voltage and a better effectiveness and opens up opportunities for optimization designs in various EWOD-based applications.

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.

Stick\u2013slip behavior during electrowetting-on-dielectric: polarization and substrate effects

A novel triple-line stick–slip behavior, manifested by “sawtooth oscillations” of the contact angle (CA), was observed during sessile droplet advance by electrowetting-on-dielectric (EWOD) for DC voltages and during droplet retreat for AC voltages. The onset of stick–slip occurred on polished substrate surfaces when the applied potential approached the EWOD saturation voltage and at lower voltages on rougher surfaces. Stick–slip was reduced at higher AC frequencies (> 1 kHz), not significantly influenced by pH or voltage polarity and did not occur with AC polarization on substrates with a Parylene coating but no hydrophobic top-layer. The different triple-line pinning behaviors under DC and AC polarization are shown to be consistent with heterogeneous wetting associated with immobilization of charged species—referred to as charge trapping—near the triple-line at saturation. These experiments and insights offer a new approach for understanding and addressing EWOD device limitations related to CA saturation and charged species trapping leading to improved performance in micro-/nanofluidic pumps, digital microfluidic chips, and electret devices.Graphical

Rapid Fabrication of Close-Typed Electrowetting on Dielectric Devices

Electrowetting on Dielectric (EWOD) devices have becomes a common device for manipulating liquid droplets in chemical, electrochemical, disease diagnosis and biomaterial detection processes. EWOD devices are prefect platform for Lab-on-a-Chip devices due to their simplicity with no moving parts and high manoeuvrability precision. In this paper, the rapid fabrication procedure of close-type EWOD devices is proposed. The EWOD electrodes was designed to manipulate droplets as small as 8 microlitre. The experimental test reveals that the droplet velocity increases with the magnitude of applied voltage. The further experiment also confirms the capability of the fabricated EWOD on droplet dispersion, which is common requirement for Lab-on-a-chip applications.

Low-cost and low-topography fabrication of multilayer interconnections for microfluidic devices

Multilayer interconnections are needed for microdevices with a large number of independent electrodes. A multi-level photolithographic process is commonly employed to provide multilayer interconnections in integrated circuit devices, but it is often too expensive for large-area or disposable devices frequently needed for microfluidics. The printed circuit board (PCB) can provide multilayer interconnection at low cost, but its rough topography poses a challenge for small droplets to slide over. Here we report a low-cost fabrication of low-topography multilayer interconnects by selective and controlled anodization of thin-film metal layers. The process utilizes anodization of metal (tantalum in this paper) or, more specifically, repetitions of a partial anodization to form insulation layers between conductive layers and a full anodization to form isolating regions between electrodes, replacing the usual process of depositing, planarizing, and etching insulation layers. After verifying the electric connections and insulations as intended, the developed method is applied to electrowetting-on-dielectric (EWOD), whose complex microfluidic products are currently built on PCB or thin-film transistor substrates. To demonstrate the utility, we fabricate a three metal-layer EWOD device with steps (surface topography) less than 1 micrometer (vs. > 10 micrometers of PCB EWOD devices) and confirm basic digital microfluidic operations.

2D large-scale EWOD devices with honeycomb electrodes for multiplexed multidirectional driving of micro-droplets

Chemical and biological applications could strongly benefit from large-scale droplet manipulation of electrowetting-on-dielectric (EWOD). However, the large number of driving electrodes of EWOD chips has been one of the most significant obstacles for these applications. This paper unveils a compact printed circuit board based EWOD chip with a hexagonal electrode array, which uses only seven signals to control an unlimited number of driving electrodes (169 in this paper). Simulation and experimental results illustrate the device’s ability to transport multiple droplets synchronously with a reconfigurable driving route and a velocity of up to 10.0 mm/s, which is beneficial for complex or high-throughput EWOD applications.