Electrowetting-on-dielectric Actuation Research Articles

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.

Self-powered digital microfluidics driven by rotational triboelectric nanogenerator

Digital microfluidic (DMF) has emerged as one of the most popular microfluidic platforms for sample-preparation in biochemical analysis and lab-on-a-chip applications. Operated with electrowetting on dielectric (EWOD) mechanism, DMF conventionally requires an external power source to provide the actuation voltage, which limited its portability and broader applications in point-of-care testing (POCT) environment. Herein, a DMF device, self-powered by triboelectric nanogenerator (TENG) is presented. TENG possesses a number of unique characteristics, and is very attractive to be integrated with DMF. It only requires a simple configuration with low-cost fabrication that can improve the DMF portability, but it also provides high voltage, low current output characteristics that are consistent with the EWOD actuation requirements. Basic droplet manipulations, including transportation, split, merge, dispense, and even elongate to follow the electrode patterns of alphabets, on a DMF device powered with manually-rotated Disk-TENG are demonstrated for the first time. Further, droplets containing samples and reagents are transported and mixed on the programmed electrode patterns on the chip to conduct chemical reactions, including nucleic acid amplification and phenol red test, showing that Disk-TENG can serve as the power source for DMF chips in POCT applications.

Low-Cost and Disposable Electrowetting-on-Dielectric Lab on a Chip With an Integrated Electrochemical Detector Fabricated by Screen-Printing Process

In this paper, a low-cost and disposable electrowetting-on-dielectric (EWOD) digital microfluidic chip with an integrated electrochemical detector was fabricated based on a screen printing process. T-junction EWOD electrodes for merging buffer and analyte droplets were formed by screen-printing of silver paste before covering with spin-coated polydimethylsiloxane dielectric and Teflon AF hydrophobic layers, respectively. The electrochemical detector comprising three electrodes, including carbon working, carbon counter, and silver/silver chloride reference electrodes, was subsequently deposited on the dielectric layer by two additional screen-printing steps. The system was tested for rapid electrochemical analyses of nicotinamide adenine dinucleotide and L-ascorbic acid. The analyte and buffer droplets were blended under the EWOD actuation with varying concentrations and detected at the electrochemical detector using cyclic voltammetry. The results showed good detection performances with medium sensitivity, moderate detection limits, and short total analysis times of less than 14 s. Therefore, the screen-printed EWOD chip with integrated screen-printed carbon electrodes could be a promising alternative for low-cost automated chemical analyses.

The numerical study of the effect of design parameters on EWOD actuation in microchannels of rectangular cross section

Electrowetting on Dielectric (EWOD) method is widely used to actuate microdroplets sandwiched between parallel plates in microfluidic chips. However, utilization of this method to transport liquid plugs in conduits has not been comprehensively studied to date. In this paper, the EWOD actuation of a liquid plug confined in a microchannel of rectangular cross section is numerically studied using OpenFoam. The effect of various parameters including electrowetting dimensionless number ( $$\eta $$ ), contact angle hysteresis (CAH), dynamic viscosity and surface tension coefficient on transport velocity is investigated by employing an interactive simulation approach. The transport velocity can be considered a gauge to evaluate the actuation efficiency. The results show that, increasing $$\eta $$ has a greater impact on velocity for larger values of CAH. In addition, it is shown that the average velocity increases linearly by increasing the surface tension coefficient. An inverse relationship between velocity and dynamic viscosity is also observed. These trends are in accordance with the theoretical model developed for a similar case in the literature. To estimate the maximum possible gap size between electrodes (MPGS), non-actuated distance travelled before the liquid plug comes to stop is evaluated. The results show that by decreasing the values of $$\eta $$ , the effect of changing CAH on MPGS becomes more substantial.

Multifunctional liquid lens for variable focus and zoom

This paper presents a novel multifunctional liquid lens (MLL) for miniature camera modules integrated with smart devices such as smartphones and pads. MLL firstly offers concurrent variable-focus and variable-magnification functions in a single liquid lens system using two different actuation schemes: electrowetting-on-dielectric (EWOD) actuation for controlling the curvature of liquid interface and electromagnetic actuation for controlling the height of the liquid column. Imaging focus test is carried out using the prototype of MLL. When a target electrical voltage is applied to an electrode covered by a hydrophobic dielectric layer on the inside wall of a glass cylindrical lens chamber, the curvature of liquid interface is changed according to the principle of EWOD and the focal length of the lens is changed. As a result, an initially blurred check pattern image is clearly focused. Imaging magnification test is separately conducted. When an electrical voltage is applied to an electromagnetic system located beneath the prototype of MLL, a ring-shaped magnet presses an elastic membrane down. As a consequence, liquid residing in the actuation chamber is pumped into the lens chamber through open walls of a supporting frame, and the height of the liquid column in the lens chamber rises. The height change of the liquid column as a function of the applied voltage is measured by using a CCD camera. The height of the liquid column is linearly increased from 0 mm to 1.2 mm with the rise of applied voltage from 0 V to 50 V with 5 V increment. The average height variation per volt is about 24 μm. For the proof of concept, the sequential imaging focus and magnification test of MLL is demonstrated using simultaneous EWOD and electromagnetic actuation. Finally, the response time of MLL is measured by using a high-speed camera. The proposed MLL offers a simple design structure to be easily miniaturized but covers a wide range of imaging focus and magnification operations for high optical performance.

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.

Monolithic Fabrication of EWOD Chips for Picoliter Droplets

We report monolithic fabrication of parallel-plate electrowetting-on-dielectric (EWOD) chips for digital micro-fluidics of picoliter droplets. Instead of assembling a second substrate to form a top plate-the common practice with all previous parallel-plate EWOD chips-the top plate is surface micromachined as a transparent thin-film membrane that forms a monolithic cavity having a gap height on the order of micrometers with excellent accuracy and uniformity. The membrane is embedded with EWOD driving electrodes and confines droplets against the device substrate to perform digital microfluidic operations. Two main attributes of the monolithic architecture that distinguish it from tradition methods are: (i) it enables excellent control of droplet dimensions down to the micrometer scale, and (ii) it does not require the typical alignment and assembly steps. Basic device functions such as creation and splitting are verified by EWOD actuation of ~100 picoliter droplets surrounded by air or oil inside a 10 μm-high cavity. Additionally, flow focusing of droplets containing 5.3 μm beads demonstrates one example of the utilities afforded by monolithic fabrication.

Bubble actuation by electrowetting-on-dielectric (EWOD) and its applications: A review

This paper reviews the principles, operations, and applications of bubble-based electrowetting-on-dielectric (EWOD). EWOD has proved to be an efficient tool in digital microfluidics that employs discrete droplets, and various applications that use the principles of EWOD have been developed from lab-on-a-chip to optical systems. Similar to its use with droplets, EWOD can also be applied to gaseous bubbles. This review begins with a discussion of the principles of EWOD for a bubble on an electrode covered with a hydrophobic dielectric layer. It then addresses EWOD actuation and the transportation of a bubble in an aqueous medium, along with a physical explanation of bubble motion. The operation of EWOD is then extended to the on-chip creation/elimination and splitting of bubbles. In particular, micro-mixers and pumps are discussed as potential applications of these operations. Unlike droplets, bubbles can be easily oscillated by external excitation, which provides additional functionalities. By integrating EWOD with external excitation, a number of new advanced applications are introduced, including the capture/separation of particles and the propulsion of objects. In these advanced operations, cavitational microstreaming flows and acoustic radiation forces are mainly responsible for the physical mechanisms. This paper also discusses these advanced operations along with their underlying physics. It is expected that in addition to bubble oscillation, other bubble actuation modes will create new functionalities and new potential applications.

Electrowetting-Based Actuation of Liquid Droplets for Micro Transportation Systems

This paper describes the electrowetting-on-dielectric (EWOD) actuation of liquid droplets for transportation of micromechanical parts. As dimensions of mechanical parts decrease, surface tension and electrostatic force become dominant compared to gravity force. Recent studies show that these forces can be utilized for actuation of liquid droplets and transportation of micromechanical elements. In this study, behavior of liquid droplet actuated by EWOD on a single substrate is analyzed using a high speed camera. Effects of applied voltage, switching frequency, and volume of the droplet are investigated experimentally. Furthermore, application of EWOD actuation to micro transportation systems is proposed. Surface properties for the conveyer pad are discussed.

Dielectric materials for electrowetting-on-dielectric actuation

The fundamental building blocks of typical electrowetting-on-dielectric (EWOD) actuation and their importance in the EWOD mechanism are introduced and reviewed, respectively. The emphasis in this experimental study of EWOD is on dielectric materials, upon which the performance of EWOD devices is heavily dependent. Dielectric breakdown of several typical polymeric and inorganic insulators employed as dielectrics for EWOD has been analytically investigated, which is forced to occur between the electrodes and conductive liquids under certain threshold potential. The electric breakdown occurring in both dielectric layer and surrounding medium (air or silicon oil) has been studied to build up a mathematical model of breakdown voltage as a function of dielectric thickness. Contact angle measurement of some polymeric materials and self-assembled monolayer using pure water has been carried out to demonstrate the contact angle tunability and reversibility, respectively, upon EWOD actuation.

Droplet manipulation and microparticle sampling on perforated microfilter membranes

This paper describes droplet manipulation and microparticle sampling where droplets driven by the electrowetting-on-dielectric (EWOD) principle are transported on perforated microfilter membranes and pick up microparticles in their path. Three designs of microfilter membranes that have different hole shapes (rounded rectangle versus circle holes), sizes (⩾6 µm) and opening area ratios (5, 9 and 20%) are microfabricated and tested along with a commercial membrane (rounded rectangle holes of 60 µm × 128 µm, 17% opening area ratio). All the tested perforated membranes are embedded with a linear array of electrodes for EWOD actuation. Reversible EWOD actuations and droplet transportations are successfully achieved on the perforated filter membranes. Particle sampling is examined against glass particles (8 µm diameter and ∼14° contact angle) and polystyrene particles (8 µm diameter and ∼66° contact angle). It is demonstrated that as droplets are moved on the microfilter surfaces by EWOD actuation, they efficiently pick up the microparticles in their path, showing high sampling efficiencies: over 95% for the glass particles and over 85% for the polystyrene particles. This particle sampling method uses a small liquid volume (microliters or smaller) and has the capability of fully automatic handling. Thus, it is expected to be highly compatible and easily integrated with lab-on-a-chip systems for follow-up biological and chemical analyses.

Digital microfluidic design and optimization of classic and new fluidic functions for lab on a chip systems

This paper deals with microfluidic studies for lab-on-a-chip development. The first goal was to develop microsystems immediately usable by biologists for complex protocol integrations. All fluid operations are performed on nano-liter droplet independently handled solely by electrowetting on dielectric (EWOD) actuation. A bottom-up architecture was used for chip design due to the development and validation of elementary fluidic designs, which are then assembled. This approach speeds up development and industrialization while minimizing the effort in designing and simplifying chip-fluidic programming. Dispensing reproducibility for 64 nl droplets obtained a CV below 3% and mixing time was only a few seconds. Ease of the integration was demonstrated by performing on chip serial dilutions of 2.8-folds, four times. The second part of this paper concerns the development of new innovative fluidic functions in order to extend EWOD-actuated digital fluidics’ capabilities. Experiments of particle dispensing by EWOD droplet handling are reported. Finally, work is shown concerning the coupling of EWOD actuation and magnetic fields for magnetic bead manipulation.

Concentration and binary separation of micro particles for droplet-based digital microfluidics

This paper describes a concept of concentration and binary separation of particles and its experimental confirmations for digital microfluidics where droplets are driven by the mechanism of electrowetting-on-dielectric (EWOD). As a fundamental separation unit, a binary separation scheme is developed, separating two different types of particles in one droplet into two droplets, one type each. The separation scheme consists of three distinctive steps, each with their own challenges: (1) isolate two different types of particles by electrophoresis into two regions inside a mother droplet, (2) physically split the mother droplet into two daughter droplets by EWOD actuation so that each type of particle is concentrated in each daughter droplet, and (3) free the daughter droplets from the separation site by EWOD to ready them for follow-up microfluidic operations. By applying a similar procedure to a droplet containing only one type of particle, two daughter droplets of different particle concentrations can be created. Using negatively charged carboxylate modified latex (CML) particles, 83% of the total particles are concentrated in a daughter droplet. Successful binary separation is also demonstrated using negatively charged CML particles and no-charge-treated polystyrene particles. Despite the undesired vortex developed inside the mother droplet, about 70% of the total CML particles are concentrated in one daughter droplet while about 70% of the total polystyrene particles are concentrated in the other daughter droplet.

Characterization of electrowetting actuation on addressable single-side coplanar electrodes

This paper studies and characterizes electrowetting-on-dielectric (EWOD) actuations on coplanar electrodes with an electrode-free cover plate or no cover plate. By arranging driving and reference electrodes on one plate, such an EWOD configuration can accommodate more sensing mechanisms from above and thus allows increased flexibility for system development. Various coplanar electrodes are tested for contact angle changes by EWOD with a focus on the effect of the percentage gap between electrodes and are found to be in good agreement with a simple analytical model. The droplet-moving devices demonstrate the successful moving, cutting and merging of droplets (∼0.1 µl) in a parallel-plate configuration, i.e., between the driving plate with coplanar electrodes and the passive plate with no electrode. EWOD actuation in single plate configuration, i.e. no cover plate, is also demonstrated, adding additional flexibility for the system design.

Using EWOD (electrowetting-on-dielectric) actuation in a micro conveyor system

This paper reports the development of a novel application of electrowetting-on-dielectric (EWOD) actuation in a micro conveyor system. Using the movement of liquid droplets by EWOD actuation as a part of active mechanical moving components, a simple and effective mechanism of the micro conveyor system can be realized. Now, we are expanding the use of EWOD actuation as a part of mechanical moving components of the micro conveyor system for the first time. The concept, fabrication and test of the initial devices are reported and various possible applications are discussed.