Achromatic doublet electrowetting prism array for beam steering device in foveated display

A foveated display is a technology that can solve the problem of insufficient angular resolution (relative to the human eye) for near-eye display. In a high-resolution foveated display, a beam steering element is required to track the human gaze. An electrowetting prism array is a transmissive non-mechanical beam steering device, that allows a light and compact optical system to be configured and a large aperture possible. However, the view is obstructed by the sidewall of the prism array. When the size of the cell prism is 7mm, the prism array has an 87% fill-factor. To push the fill-factor to 100%, the cell prisms were magnified using a lens array. Image processing was performed such that the image produced by the lens array was identical to the original. Beam steering by refraction is accompanied by chromatic dispersion, which causes chromatic aberration, making colors appear blurry. The refractive index condition to reduce chromatic dispersion was obtained using the doublet structure of the electrowetting prism. The chromatic dispersion was reduced by 70% on average.

PortaDrop: A portable digital microfluidic platform providing versatile opportunities for Lab-On-A-Chip applications

Electrowetting-on-dielectric is a decent technique to manipulate discrete volumes of liquid in form of droplets. In the last decade, electrowetting-on-dielectric systems, also called digital microfluidic systems, became more frequently used for a variety of applications because of their high flexibility and reconfigurability. Thus, one design can be adapted to different assays by only reprogramming. However, this flexibility can only be useful if the entire system is portable and easy to use. This paper presents the development of a portable, stand-alone digital microfluidic system based on a Linux-based operating system running on a Raspberry Pi, which is unique. We present "PortaDrop" exhibiting the following key features: (1) an "all-in-one box" approach, (2) a user-friendly, self-explaining graphical user interface and easy handling, (3) the ability of integrated electrochemical measurements, (4) the ease to implement additional lab equipment via Universal Serial Bus and the General Purpose Interface Bus as well as (5) a standardized experiment documentation. We propose that PortaDrop can be used to carry out experiments in different applications, where small sample volumes in the nanoliter to picoliter range need to be handled an analyzed automatically. As a first application, we present a protocol, where a droplet is consequently exchanged by droplets of another medium using passive dispensing. The exchange is monitored by electrical impedance spectroscopy. It is the first time, the media exchange caused by passive dispensing is characterized by electrochemical impedance spectroscopy. Summarizing, PortaDrop allows easy combination of fluid handling by means of electrowetting and additional sensing.

Liquid Combination with High Refractive Index Contrast and Fast Scanning Speeds for Electrowetting Adaptive Optics

Electrowetting adaptive optical devices are versatile, with applications ranging from microscopy to remote sensing. The choice of liquids in these devices governs its tuning range, temporal response, and wavelength of operation. We characterized a liquid system, consisting of 1-phenyl-1-cyclohexene and deionized water, using both lens and prism devices. The liquids have a large contact angle tuning range, from 173 to 60°. Measured maximum scanning angle was realized at ±13.7° in a two-electrode prism, with simulation predictions of ±18.2°. The liquid's switching time to reach 90° contact angle from rest, in a 4 mm diameter device, was measured at 100 ms. Steady-state scanning with a two-electrode prism showed linear and consistent scan angles of ±4.8° for a 20 V differential between the two electrodes, whereas beam scanning using the liquid system achieved ±1.74° at 500 Hz for a voltage differential of 80 V.

Self-Powered Microfluidic Transport System Based on Triboelectric Nanogenerator and Electrowetting Technique

Electrowetting technique is an actuation method for manipulating position and velocity of fluids in the microchannels. By combining electrowetting technique and a freestanding mode triboelectric nanogenerator (TENG), we have designed a self-powered microfluidic transport system. In this system, a mini vehicle is fabricated by using four droplets to carry a pallet (6 mm × 8 mm), and it can transport some tiny object on the track electrodes under the drive of TENG. The motion of TENG can provide both driving power and control signal for the mini vehicle. The maximum load for this mini vehicle is 500 mg, and its highest controllable velocity can reach 1 m/s. Freestanding TENG has shown excellent capability to manipulate microfluid. Under the drive of TENG, the minimum volume of the droplet can reach 70-80 nL, while the tiny droplet can freely move on both horizontal and vertical planes. Finally, another strategy for delivering nanoparticles to the designated position has also been demonstrated. This proposed self-powered transport technique may have great applications in the field of microsolid/liquid manipulators, drug delivery systems, microrobotics, and human-machine interactions.

A 3D microblade structure for precise and parallel droplet splitting on digital microfluidic chips

Existing digital microfluidic (DMF) chips exploit the electrowetting on dielectric (EWOD) force to perform droplet splitting. However, the current splitting methods are not flexible and the volume of the droplets suffers from a large variation. Herein, we propose a DMF chip featuring a 3D microblade structure to enhance the droplet-splitting performance. By exploiting the EWOD force for shaping and manipulating the mother droplet, we obtain an average dividing error of <2% in the volume of the daughter droplets for a number of fluids such as deionized water, DNA solutions and DNA-protein mixtures. Customized droplet splitting ratios of up to 20 : 80 are achieved by positioning the blade at the appropriate position. Additionally, by fabricating multiple 3D microblades on one electrode, two to five uniform daughter droplets can be generated simultaneously. Finally, by taking synthetic DNA targets and their corresponding molecular beacon probes as a model system, multiple potential pathogens that cause sepsis are detected rapidly on the 3D-blade-equipped DMF chip, rendering it as a promising tool for parallel diagnosis of diseases.

Two-phase microfluidic flow modeling in an electrowetting display microwell

Digital microfluidics provides precise control of a single microdroplet, producing more opportunities for bio-molecule studies, chemical reaction and optofluidics applications. By manipulating the surface of droplets, light can be focused, scattered, or reflected toward different positions. We build a model of electro-responsive optical microfluidic system, operated based on the electrowetting mechanism, which can split or push droplets moving within a microwell. The initial close state and operated open state in a single microwell displays the color of a dye oil droplet and the substrate, respectively, represented as the dark and bright pixel in the display board. Our results indicate that the microdroplets interface could be successfully deformed and moved towards different directions within a short period of time.

Fertilization of Mouse Gametes in Vitro Using a Digital Microfluidic System

We demonstrated in vitro fertilization (IVF) using a digital microfluidic (DMF) system, so-called electrowetting on dielectric (EWOD). The DMF device was proved to be biocompatible and the DMF manipulation of a droplet was harmless to the embryos. This DMF platform was then used for the fertilization of mouse gametes in vitro and for embryo dynamic culture based on a dispersed droplet form. Development of the embryos was instantaneously recorded by a time-lapse microscope in an incubator. Our results indicated that increasing the number of sperms for IVF would raise the rate of fertilization. However, the excess of sperms in the 10 μL culture medium would more easily make the embryo dead during cell culture. Dynamic culture powered with EWOD can manipulate a single droplet containing mouse embryos and culture to the eight-cell stage. The fertilization rate of IVF demonstrated by DMF system was 34.8%, and about 25% inseminated embryos dynamically cultured on a DMF chip developed into an eight-cell stage. The results indicate that the DMF system has the potential for application in assisted reproductive technology.

Digital Microfluidic Dynamic Culture of Mammalian Embryos on an Electrowetting on Dielectric (EWOD) Chip

Current human fertilization in vitro (IVF) bypasses the female oviduct and manually inseminates, fertilizes and cultivates embryos in a static microdrop containing appropriate chemical compounds. A microfluidic microchannel system for IVF is considered to provide an improved in-vivo-mimicking environment to enhance the development in a culture system for an embryo before implantation. We demonstrate a novel digitalized microfluidic device powered with electrowetting on a dielectric (EWOD) to culture an embryo in vitro in a single droplet in a microfluidic environment to mimic the environment in vivo for development of the embryo and to culture the embryos with good development and live births. Our results show that the dynamic culture powered with EWOD can manipulate a single droplet containing one mouse embryo and culture to the blastocyst stage. The rate of embryo cleavage to a hatching blastocyst with a dynamic culture is significantly greater than that with a traditional static culture (p<0.05). The EWOD chip enhances the culture of mouse embryos in a dynamic environment. To test the reproductive outcome of the embryos collected from an EWOD chip as a culture system, we transferred embryos to pseudo-pregnant female mice and produced live births. These results demonstrate that an EWOD-based microfluidic device is capable of culturing mammalian embryos in a microfluidic biological manner, presaging future clinical application.

Detaching droplets in immiscible fluids from a solid substrate with the help of electrowetting

The detachment (or removal) of droplets from a solid surface is an indispensable process in numerous practical applications which utilize digital microfluidics, including cell-based assay, chip cooling, and particle sampling. When a droplet that is fully stretched by impacting or electrowetting is released, the conversion of stored surface energy to kinetic energy can lead to the departure of the droplet from a solid surface. Here we firstly detach sessile droplets in immiscible fluids from a hydrophobic surface by electrowetting. The physical conditions for droplet detachment depend on droplet volume, viscosity of ambient fluid, and applied voltage. Their critical conditions are determined by exploring the retracting dynamics for a wide range of driving voltages and physical properties of fluids. The relationships between physical parameters and dynamic characteristics of retracting and jumping droplets, such as contact time and jumping height, are also established. The threshold voltage for droplet detachment in oil with high viscosity is largely reduced (~70%) by electrowetting actuations with a square pulse. To examine the applicability of three-dimensional digital microfluidic (3D-DMF) platforms to biological applications such as cell culture and cell-based assays, we demonstrate the detachment of droplets containing a mixture of human umbilical vein endothelial cells (HUVECs) and collagen (concentration of 4 × 10(4) cells mL(-1)) in silicone oil with a viscosity of 0.65 cSt. Furthermore, to complement the technical limitations due to the use of a needle electrode and to demonstrate the applicability of the 3D-DMF platform with patterned electrodes to chemical analysis and synthesis, we examine the transport, merging, mixing, and detachment of droplets with different pH values on the platform. Finally, by using DC and AC electrowetting actuations, we demonstrate the detachment of oil droplets with a very low contact angle (<~13°) in water on a hydrophobic surface.

Nanophotonic implementation of optoelectrowetting for microdroplet actuation

The development and ultimate operation of a nanocomposite high-aspect-ratio photoinjection (HARP) device is presented in this work. The device makes use of a nanocomposite material as the optically active layer and the device achieves a large optical penetration depth with a high aspect ratio which provides a strong actuation force far away from the point of photoinjection. The nanocomposite material can be continuously illuminated and the position of the microdroplets can, therefore, be controlled to diffraction limited resolution. The nanocomposite HARP device shows great potential for future on-chip applications.

Design and analysis of a low actuation voltage electrowetting-on-dielectric microvalve for drug delivery applications

This paper presents a low actuation voltage microvalve with optimized insulating layers that manipulates a conducting ferro-fluid droplet by the principle of electrowetting-on-dielectric (EWOD). The proposed EWOD microvalve contains an array of chromium (Cr) electrodes on the soda-lime glass substrate, covered by both dielectric and hydrophobic layers. Various dielectric layers including Su-8 2002, Polyvinylidenefluoride (PVDF) and Cyanoethyl pullulan (CEP), and thin (50 nm) hydrophobic Teflon and Cytonix are used to analyze the EWOD microvalves at different voltages. The Finite Element Method (FEM) based software, Coventorware is used to carry out the simulation analysis. It is observed that the EWOD microvalve having a CEP dielectric layer with dielectric constant of about 20 and thickness of 1 μm, and a Cytonix hydrophobic layer with thickness of 50 nm operated the conducting ferro-fluid droplet at the actuation voltage as low as 7.8 V.

Electrowetting-on-dielectrics for manipulation of oil drops and gas bubbles in aqueous-shell compound drops

We present the manipulation of oil, organic and gaseous chemicals by electrowetting-on-dielectric (EWOD) technology using aqueous-shell compound drops. We demonstrate that the transport and coalescence of viscous oil drops, the reaction of bromine with styrene in benzene solution, and the reaction of red blood cells with carbon monoxide bubbles can be accomplished using this method.

An EWOD-based microfluidic chip for single-cell isolation, mRNA purification and subsequent multiplex qPCR

Single cell analysis circumvents the need to average data from large populations by observing each cell individually, thus enabling the analysis of cell-to-cell variability. The ability to work on this scale presents many new opportunities for the life sciences and biomedical applications. Microfluidics has become a tool of choice for such studies and electrowetting on dielectric (EWOD) technology is well adapted for samples with reduced size and biological studies at the single cell level. In the present manuscript, for the first time, we present an integrated and automated system based on EWOD that can process the complete workflow on a single device, from the isolation of a single cell to mRNA purification and gene expression analysis.

Electrocapillarity of an electrolyte solution in a nanoslit with overlapped electric double layer: continuum approach

A nanoslit is a long narrow opening between two parallel plates that are nanometers apart from each other. When an electrolyte solution is present inside a nanoslit, an overlapped electrical double layer (EDL) is formed and there exist distributions of the osmotic pressure and the Maxwell stress across the nanoslit. It is well known that the total normal stress (osmotic pressure contribution + Maxwell stress contribution) in the direction normal to the nanoslit surface is uniform and the value is the same as the osmotic pressure at the centerline. On the other hand, it is not well known that the total normal stress in the direction parallel to the slit surface is not uniform. When there is an electrolyte-gas interface inside a nanoslit, this total normal stress in the direction parallel to the slit surface generates the electrocapillarity effect. In the present work, the electromechanical approach is adopted to estimate the electrocapillarity effect in terms of the slit surface potential (or the surface charge density), the gap size, and the bulk ion concentrations. In order to handle the problem analyically, it is assumed that the nanoslit problem is in the continuum range and the interface is initially flat. The deformation of the interface due to the nonuniform total normal stress along the interface is also obtained by using the first order perturbation method. The significance of the present work can be manifested by the fact that external voltage is frequently used in nanoscaled systems and the electrocapillarity effect should be considered in addition to the intrinsic capillarity due to surface tension.

Active digital microfluidic paper chips with inkjet-printed patterned electrodes

Active, paper-based, microfluidic chips driven by electrowetting are fabricated and demonstrated for reagent transport and mixing. Instead of using the passive capillary force on the pulp to actuate a flow of a liquid, a group of digital drops are transported along programmed trajectories above the electrodes printed on low-cost paper, which should allow point-of-care production and diagnostic activities in the future.

Direct writing of flexible electronics through room temperature liquid metal ink

Background

Conventional approaches of making a flexible circuit are generally complex, environment unfriendly, time and energy consuming, and thus expensive. Here, we describe for the first time the method of using high-performance GaIn₁₀-based electrical ink, a significantly neglected room temperature liquid metal, as both electrical conductors and interconnects, for directly writing flexible electronics via a rather easy going and cost effective way.

Methods

The new generation electric ink was made and its wettability with various materials was modified to be easily written on a group of either soft or rigid substrates such as epoxy resin board, glass, plastic, silica gel, paper, cotton, textiles, cloth and fiber etc. Conceptual experiments were performed to demonstrate and evaluate the capability of directly writing the electrical circuits via the invented metal ink. Mechanisms involved were interpreted through a series of fundamental measurements.

Results

The electrical resistivity of the fluid like GaIn₁₀-based material was measured as 34.5 µΩ·cm at 297 K by four point probe method and increased with addition of the oxygen quantity, which indicates it as an excellent metal ink. The conductive line can be written with features that are approximately 10 µm thick. Several functional devices such as a light emitting diode (LED) array showing designed lighting patterns and electrical fan were made to work by directly writing the liquid metal on the specific flexible substrates. And satisfactory performances were obtained.

Conclusions

The present method opens the way to directly and quickly writing flexible electronics which can be as simple as signing a name or drawing a picture on the paper. The unique merit of the GaIn₁₀-based liquid metal ink lies in its low melting temperature, well controlled wettability, high electrical conductivity and good biocompability. The new electronics writing strategy and basic principle has generalized purpose and can be extended to more industrial areas, even daily life.

Integration of field effect transistor-based biosensors with a digital microfluidic device for a lab-on-a-chip application

A new platform for lab-on-a-chip system is suggested that utilizes a biosensor array embedded in a digital microfluidic device. With field effect transistor (FET)-based biosensors embedded in the middle of droplet-driving electrodes, the proposed digital microfluidic device can electrically detect avian influenza antibody (anti-AI) in real time by tracing the drain current of the FET-based biosensor without a labeling process. Digitized transport of a target droplet enclosing anti-AI from an inlet to the embedded sensor is enabled by the actuation of electrowetting-on-dielectrics (EWOD). A reduction of the drain current is observed when the target droplet is merged with a pre-existing droplet on the embedded sensor. This reduction of the drain current is attributed to the specific binding of the antigen and the antibody of the AI. The proposed hybrid device consisting of the FET-based sensor and an EWOD device, built on a coplanar substrate by monolithic integration, is fully compatible with current fabrication technology for control and read-out circuitry. Such a completely electrical manner of inducing the transport of bio-molecules, the detection of bio-molecules, the recording of signals, signal processing, and the data transmission process does not require a pump, a fluidic channel, or a bulky transducer. Thus, the proposed platform can contribute to the construction of an all-in-one chip.

Electric-field-induced wetting and dewetting in single hydrophobic nanopores

The behaviour of water in nanopores is very different from that of bulk water. Close to hydrophobic surfaces, the water density has been found to be lower than in the bulk, and if confined in a sufficiently narrow hydrophobic nanopore, water can spontaneously evaporate. Molecular dynamics simulations have suggested that a nanopore can be switched between dry and wet states by applying an electric potential across the nanopore membrane. Nanopores with hydrophobic walls could therefore create a gate system for water, and also for ionic and neutral species. Here, we show that single hydrophobic nanopores can undergo reversible wetting and dewetting due to condensation and evaporation of water inside the pores. The reversible process is observed as fluctuations between conducting and non-conducting ionic states and can be regulated by a transmembrane electric potential.

Biofunctionalization of electrowetting-on-dielectric digital microfluidic chips for miniaturized cell-based applications

In this paper we report on the controlled biofunctionalization of the hydrophobic layer of electrowetting-on-dielectric (EWOD) based microfluidic chips with the aim to execute (adherent) cell-based assays. The biofunctionalization technique involves a dry lift-off method with an easy to remove Parylene-C mask and allows the creation of spatially controlled micropatches of biomolecules in the Teflon-AF(®) layer of the chip. Compared to conventional methods, this method (i) is fully biocompatible; and (ii) leaves the hydrophobicity of the chip surface unaffected by the fabrication process, which is a crucial feature for digital microfluidic chips. In addition, full control of the geometry and the dimensions of the micropatches is achieved, allowing cells to be arrayed as cell clusters or as single cells on the digital microfluidic chip surface. The dry Parylene-C lift-off technique proves to have great potential for precise biofunctionalization of digital microfluidic chips, and can enhance their use for heterogeneous bio-assays that are of interest in various biomedical applications.

A unified platform for optoelectrowetting and optoelectronic tweezers

A platform capable of seamlessly unifying both optoelectrowetting and optoelectronic tweezers is presented. This enables the user to manipulate aqueous droplets (with electrowetting) as well as individual particles within those droplets (with dielectrophoresis). The device requires no photolithography and droplet/particle manipulation can occur continuously over the entire surface of the device. Droplet and 10 µm polystyrene particle speeds of up to 8 mm s(-1) and 60 µm s(-1), respectively, are demonstrated. Particle concentration within, and subsequent splitting of, a droplet is performed resulting in average concentration efficiencies of 93%. Serial concentration is also demonstrated resulting in exponentially increasing particle concentrations and a 10× concentration increase. Finally, the platform is used to select a single cell out of a cohort and subsequently encapsulate it in its own aqueous droplet.

Droplet-on-a-wristband: chip-to-chip digital microfluidic interfaces between replaceable and flexible electrowetting modules

We present a long (204 mm), curved (curvature of 0.04 mm(-1)), and closed droplet pathway in "droplet-on-a-wristband" (DOW) with the designed digital microfluidic modular interfaces for electric signal and droplet connections based on the study of electrowetting-on-dielectric (EWOD) in inclined and curved devices. Instead of using sealed and leakage-proof pipes to transmit liquid and pumping pressure, the demonstrated modular interface for electrowetting-driven digital microfluidics provides simply electric and fluidic connections between two adjacent parallel-plate modules which are easy-to-attach/detach, showing the advantages of using droplets for microfluidic connections between modules. With the previously reported digital-to-channel interfaces (Abdelgawad et al., Lab Chip, 2009, 9, 1046-1051), the chip-to-chip interface presented here would be further applied to continuous microfluidics. Droplet pumping across a single top plate gap and through a modular interface with two gaps between overlapping plates are investigated. To ensure the droplet transportation in the DOW, we actuate droplets against gravity in an inclined or curved device fabricated on flexible PET substrates prepared by a special razor blade cutter and low temperature processes. Pumping a 2.5 μl droplet at a speed above 105 mm s(-1) is achieved by sequentially switching the entire 136 driving electrodes (1.5 mm × 1.5 mm) along the four flexible modules of the DOW fabricated by 4-inch wafer facilities.

An empirically validated analytical model of droplet dynamics in electrowetting on dielectric devices

Explicit analytical models that describe the capillary force on confined droplets actuated in electrowetting on dielectric devices and the reduction in that force by contact angle hysteresis as a function of the three-dimensional shape of the droplet interface are presented. These models are used to develop an analytical model for the transient position and velocity of the droplet. An order of magnitude analysis showed that droplet motion could be modeled using the driving capillary force opposed by contact angle hysteresis, wall shear, and contact line friction. Droplet dynamics were found to be a function of gap height, droplet radius, surface tension, fluid density, the initial and deformed contact angles, contact angle hysteresis, and friction coefficients pertaining to viscous wall friction and contact line friction. The first four parameters describe the device geometry and fluid properties; the remaining parameters were determined experimentally. Images of the droplet during motion were used to determine the evolution of the shape, position, and velocity of the droplet with time. Comparisons between the measured and predicted results show that the proposed model provides good accuracy over a range of practical voltages and droplet aspect ratios.

Towards an electrowetting-based digital microfluidic platform for magnetic immunoassays

We demonstrate ElectroWetting-On-Dielectric (EWOD) transport and SQUID gradiometer detection of magnetic nanoparticles (MNPs) suspended in a 2 microl de-ionized water droplet. This proof-of-concept methodology constitutes the first development step towards a highly sensitive magnetic immunoassay platform with SQUID readout and droplet-based sample handling. Magnetic AC-susceptibility measurements were performed on MNPs with a hydrodynamic diameter of 100 nm using a high-Tc dc Superconducting Quantum Interference Device (SQUID) gradiometer as detector. We observed that the signal amplitude per unit volume is 2.5 times higher for a 2 microl sample droplet compared to a 30 microl sample volume.