Rotating flow within a droplet actuated with AC EWOD

Electrowetting-on-dielectric (EWOD) is now used in numerous microsystems like digital lab-on-chips. This paper deals with a characteristic hydrodynamic flow appearing in droplets actuated by EWOD with AC voltage. In the coplanar electrode configuration, two pairs of vortex flows are observed to form in a droplet centred on the electrode gap. All experiments are performed in oil as ambient phase and flows in the droplet are analysed using fluorescent beads. At the same time, droplet oscillations induced by AC EWOD are also revealed under stroboscopic lighting. These experiments show that vortex location can be controlled by frequency actuation with fair degree of reproducibility.     

EWOD microfluidic systems for biomedical applications

As the technology advances, a growing number of biomedical microelectromechanical systems (bio-MEMS) research involves development of lab-on-a-chip devices and micrototal analysis systems. For example, a portable instrument capable of biomedical analyses (e.g., blood sample analysis) and immediate recording, whether the patients are in the hospital or home, would be a considerable benefit to human health with an excellent commercial viability. Digital microfluidic (DMF) system based on the electrowetting-on-dielectric (EWOD) mechanism is an especially promising candidate for such point-of-care systems. The EWOD-based DMF system processes droplets in a thin space or on an open surface, unlike the usual microfluidic systems that process liquids by pumping them in microchannels. Droplets can be generated and manipulated on EWOD chip only with electric signals without the use of pumps or valves, simplifying the chip fabrication and the system construction. Microfluidic operations by EWOD actuation feature precise droplet actuation, less contamination risk, reduced reagents volume, better reagents mixing efficiency, shorter reaction time, and flexibility for integration with other elements. In addition, the simplicity and portability make the EWOD-based DMF system widely popular in biomedical or chemical fields as a powerful sample preparation platform. Many chemical and biomedical researches, such as DNA assays, proteomics, cell assays, and immunoassays, have been reported using the technology. In this paper, we have reviewed the recent developments and studies of EWOD-based DMF systems for biomedical applications published mostly during the last 5 years.     

Creating, transporting, cutting, and merging liquid droplets by electrowetting-based actuation for digital microfluidic circuits

Reports the completion of four fundamental fluidic operations considered essential to build digital microfluidic circuits, which can be used for lab-on-a-chip or micro total analysis system (/spl mu/TAS): 1) creating, 2) transporting, 3) cutting, and 4) merging liquid droplets, all by electrowetting, i.e., controlling the wetting property of the surface through electric potential. The surface used in this report is, more specifically, an electrode covered with dielectrics, hence, called electrowetting-on-dielectric (EWOD). All the fluidic movement is confined between two plates, which we call parallel-plate channel, rather than through closed channels or on open surfaces. While transporting and merging droplets are easily verified, we discover that there exists a design criterion for a given set of materials beyond which the droplet simply cannot be cut by EWOD mechanism. The condition for successful cutting is theoretically analyzed by examining the channel gap, the droplet size and the degree of contact angle change by electrowetting on dielectric (EWOD). A series of experiments is run and verifies the criterion.     

Kinematics and deformation of ferrofluid droplets under magnetic actuation

This paper reports the experimental results on kinematics and deformation of ferrofluid droplets driven by planar coils. Ferrofluid droplets act as liquid magnets, which can be controlled and manipulated by an external magnetic field. In our experiments, the magnetic field was generated by two pairs of planar coils, which were fabricated on a double-sided printed circuit board. The first pair of coils constrains the ferrofluid droplet to a one-dimensional motion. The second pair generates the magnetic gradient needed for the droplet motion. The direction of the motion can be controlled by changing the sign of the gradient or of the driving current. Kinematic characteristics of the droplet such as the velocity–position diagram and the aspect ratio of the droplet are investigated. The analysis and discussion are based on the different parameters such as the droplet size, the viscosity of the surrounding medium, and the driving current. This simple actuation concept would allow the implementation of lab-on-a-chip platforms based on ferrofluid droplets.     

A Microcantilever-Based Picoliter Droplet Dispenser With Integrated Force Sensors and Electroassisted Deposition Means

This paper introduces a picoliter droplet dispenser relying on an array of silicon microcantilevers. The microcantilevers bear fluidic channels, and liquid transfer is achieved by a direct contact of the cantilever tip and the surface. A high degree of control over the location and geometry of the fabricated patterns is ensured by incorporating force sensors and electroassisted deposition means, i.e., electrowetting actuation and electrospotting, to the devices. The cantilever array, a PC-controlled stage, and an electronic circuit dedicated to the piezoresistance measurements form a closed-loop system that enables the automatic displacement of the array and the control of the deposition parameters. By using an external loading chip, different liquids are loaded onto the cantilevers, enabling the parallel deposition of several entities in a single spotting run. This paper details the design of the cantilevers assisted by finite-element modeling, the fabrication of the cantilever array, and the closed-loop operation. Moreover, proof-of-concept experiments are presented to demonstrate the versatility of our deposition system in terms of deposited materials and spot sizes. The control of the spotting process, the versatility of the printed materials, and the added electroassisted features prove that this tool has a real potential for research work and industrial applications.     

Monolithic fabrication of electro-fluidic polymer microchips

Integration of electronic wiring with microfluidic chips is an important process as it allows electrical interactions with the fluidic media, for example required for resistive and capacitive sensing. It is also necessary in order to implement various actuation and control mechanisms such as pumping, electrophoresis and temperature control. Typically electrical wire traces are added to microfabricated fluidic chips using metal deposition processes that are carried out after the fluidic chip has been fabricated. The process for adding the wiring is complicated and is limited to select metals that can be deposited by evaporation or sputtering. We present a single step method for integrating electrical wires into polymer microfluidic chips that are fabricated by a hot embossing process. This process can flexibly embed any kind of commercially available metal wire with a microfluidic chip and the wiring may be integrated to come into surface contact with the fluid or may be embedded in close proximity to (but insulated from)the fluid paths for example for local heating purposes. This method significantly reduces total processing time and is thus a valuable method for wire integration into polymer chips. We demonstrate two applications—a microelectrolysis chip and a heater chip that were fabricated using this methodology. The design, fabrication process and the initial test results are presented.     

Driving characteristics of the electrowetting-on-dielectric device using atomic-layer-deposited aluminum oxide as the dielectric

Electrowetting on dielectric (EWOD) is useful in manipulating droplets for digital (droplet-based) microfluidics, but its high driving voltage over several tens of volts has been a barrier to overcome. This article presents the characteristics of EWOD device with aluminum oxide (Al₂O₃, ε _r  ≈ 10) deposited by atomic layer deposition (ALD), for the first time as the high-k dielectric for lowering the EWOD driving voltage substantially. The EWOD device of the single-plate configuration was fabricated by several steps for the control electrode array of 1 mm × 1 mm squares with 50 μm space, the dielectric layer of 1,270 Å thick ALD Al₂O₃, the reference electrode of 20 μm wide line electrode, and the hydrophobic surface treatment by Teflon-AF coating, respectively. We observed the movement of a 2 μl water droplet in an air environment, applying a voltage between one of the control electrodes and the reference electrode in contact with the droplet. The droplet velocity exponentially depending on the applied voltage below 15 V was obtained. The measured threshold voltage to move the droplet was as low as 3 V which is the lowest voltage reported so far in the EWOD researches. This result opens a possibility of manipulating droplets, without any surfactant or oil treatment, at only a few volts by EWOD using ALD Al₂O₃ as the dielectric.     

Meniscus-Assisted High-Efficiency Magnetic Collection and Separation for EWOD Droplet Microfluidics

This paper describes a technique to increase the efficiency of magnetic concentration on an electrowetting-on-dielectric (EWOD)-based droplet (digital) microfluidic platform operated in air, i.e., on dry surface. Key differences in the force scenario for droplet microfluidics vis-a-vis the conventional continuous microfluidic systems are identified to explain the rationale behind the proposed idea. In particular, the weakness of the magnetic force relative to the bead-substrate adhesion and the liquid-air interfacial tension is highlighted, and a new technique to achieve high-efficiency magnetic collection with the assistance of the interfacial force is proposed. An improvement in collection efficiency (e.g., from ~ 73% to ~ 99%) is observed with the new technique of ldquomeniscus-assisted magnetic bead collectionrdquo. In addition, isolation of the magnetic species from a mixed sample of magnetic and nonmagnetic beads is demonstrated. Comparison with other related reports is also presented.     

Computational and performance analysis of a continuous magnetophoretic bioseparation chip with alternating magnetic fields

This paper presents the modeling and optimization of a magnetophoretic bioseparation chip for isolating cells, such as circulating tumor cells from the peripheral blood. The chip consists of a continuous-flow microfluidic platform that contains locally engineered magnetic field gradients. The high-gradient magnetic field produced by the magnets is spatially non-uniform and gives rise to an attractive force on magnetic particles flowing through a fluidic channel. Simulations of the particle–fluid transport and the magnetic force are performed to predict the trajectories and capture lengths of the particles within the fluidic channel. The computational model takes into account key forces, such as the magnetic and fluidic forces and their effect on design parameters for an effective separation. The results show that the microfluidic device has the capability of separating various cells from their native environment. An experimental study is also conducted to verify and validate the simulation results. Finally, to improve the performance of the separation device, a parametric study is performed to investigate the effects of the magnetic bead size, cell size, number of beads per cell, and flow rate on the cell separation performance.     

Direct-Referencing Two-Dimensional-Array Digital Microfluidics Using Multilayer Printed Circuit Board

Digital (i.e., droplet-based) microfluidics, by the electrowetting-on-dielectric (EWOD) mechanism, has shown great potential for a wide range of applications, such as lab-on-a-chip. While most reported EWOD chips use a series of electrode pads essentially in 1D line pattern designed for specific tasks, the desired universal chips allowing user-reconfigurable paths would require the electrode pads in 2D pattern. However, to electrically access the electrode pads independently, conductive lines need to be fabricated underneath the pads in multiple layers, raising a cost issue particularly for disposable chip applications. In this paper, we report the building of digital microfluidic plates based on a printed circuit board (PCB), in which multilayer electrical access lines were created inexpensively using the mature PCB technology. However, due to its surface topography and roughness and resulting high resistance against droplet movement, the as-fabricated PCB surfaces require high (~500 V) voltages unless coated with or immersed in oil. Our goal is the EWOD operations of droplets not only on oil-covered surfaces but also on dry ones. To meet the varying levels of performances, three types of gradually complex post-PCB microfabrication process are developed and evaluated. By introducing land-grid-array sockets in the packaging, a scalable digital microfluidic system with a reconfigurable and low-cost chip is also demonstrated.     

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.     

Electrostatic force calculation for an EWOD-actuated droplet

This paper examines the electrostatic force on a microdroplet transported via electrowetting on dielectric (EWOD). In contrast with previous publications, this article details the force distribution on the advancing and receding fluid faces, in addition to presenting simple algebraic formulae for the net force in terms of system parameters. Dependence of the force distribution and its integral on system geometry, droplet location, and material properties is described. The consequences of these theoretically and numerically obtained results for design and fabrication of EWOD devices are considered.     

MIMO regulation control design for magnetic steering of a ferromagnetic particle inside a fluidic environment

As an important development of medical instrumentation, minimally invasive therapeutic operations have been recently introduced. The foremost element of minimally invasive techniques is navigating a micro-device through human body, especially inside blood vessels. A remote actuation over the micro-device is normally provided by electromagnetic actuators. In most applications, a control scheme is also required to initiate the actuation force, the magnetic propulsion, such that at every time step, the micro-device moves towards or along a given path. This paper contributes in development of the electromagnetic system model mostly used in magnetic navigation systems to be representable in control affine form. Next, a multi-input multi-output (MIMO) trajectory tracking controller is designed to conduct the auto-navigation of the device along a given path. This method is a generalised version of a ‘semi-global nonlinear output regulation’ introduced for single-input single-output (SISO) systems. Finally, the proposed scheme is examined for an iron particle moving in a fluidic environment. The simulation results show fast decay in deviation of the particle position from the reference path under some assumptions. This shows that the proposed scheme can be offered for medical applications.     

A Liquid–Solid Direct Contact Low-Loss RF Micro Switch

This paper reports the design, fabrication, and testing of a liquid-metal (LM) droplet-based radio-frequency microelectromechanical systems (RF MEMS) shunt switch with dc-40 GHz performance. The switch demonstrates better than 0.3 dB insertion loss and 20 dB isolation up to 40 GHz, achieving significant improvements over previous LM-based RF MEMS switches. The improvement is attributed to use of electrowetting on dielectric (EWOD) as a new actuation mechanism, which allows design optimized for RF switching. A two-droplet design is devised to solve the biasing problem of the actuation electrode that would otherwise limit the performance of a single-droplet design. The switch design uses a microframe structure to accurately position the liquid-solid contact line while also absorbing variations in deposited LM volumes. By sliding the liquid-solid contact line electrostatically through EWOD, the switch demonstrates bounceless switching, low switch-on time (60 mus), and low power consumption (10 nJ per cycle).     

基于静电力的微液滴驱动芯片

为了精确操控微流体,设计并制作了一种基于静电力的微液滴驱动芯片.介绍了驱动原理和工艺流程,搭建了驱动检测实验平台.芯片采用开放式的结构,只需单层共平面控制电极,以硅作衬底,氧化硅和多晶硅作绝缘层,重掺杂多晶硅为驱动电极阵列,高质量Si3N4作介质层,碳氟聚合物为疏水层.实验表明:微液滴可在芯片上按程序设定方式在二维平面...     

SAW nanopump for handling droplets in view of biological applications

This paper reports on the fabrication and development of a surface acoustic waves (SAW) platform dedicated to digital micro fluidics for biological applications. SAW at about 20 MHz are generated by InterDigital Transducers (IDT) laid on a LiNbO₃ piezoelectric substrate. An electrical characterization of the IDT is reported and first results related to droplet handling with the SAW platform are given. We show that accurate droplet displacement is the result of both a radio frequency (RF) pulsed excitation and a chemical pre-treatment of the platform surface. For biological applications the droplet carrying the biomaterial is squeezed between the platform and a cover to increase the surface exchange between the droplet and hydrophilic functionalized areas. Out of such areas the free displacement is significantly improved by a surface hydrophobic pre-treatment. Moreover, the fabrication of additional hydrophilic micro tracks is shown to be a solution for crossing these areas without droplet splitting. This result is a key point to validate the structure of the novel micro fluidic platform proposed in this paper.     

A particle swarm optimization method for fault localization and residue removal in digital microfluidic biochips

Second generation microfluidic biochip is known as digital microfluidic biochip (DMFB). DMFB performs different clinical pathological experimentation, DNA sequencing, air contamination detection and many other bio-chemical experiments based on appropriate bio-assay protocols. DMFB comprises two dimensional array of electrodes fabricated over two parallel glass plates, which is capable of performing miniaturized (nano/ pico liter volume) droplet dispensing, transportation and mixing through electro-wetting of dielectrics (EWOD). Droplet operations through EWOD are mostly managed by droplet scheduling algorithms which are NP hard in nature. At present reliability is a major concern for commercialization of operational DMFB. Reliable output of DMFB is mostly affected by different faults within electrodes. This further suffers from cross-contamination issues among different droplets due to repeated use of DMFB boards for different assay operations. Present work proposes a novel test droplet routing method based on adaptive weighted particle swarm optimization (PSO) model. This test droplet circulation method aims to identify defective electrodes and simultaneously performs residue removal. Experimental findings of the proposed model on some standard test benches and real life bio-assay samples reveal operational supremacy in terms of overall computational time and operational accuracy over some existing best known models.     

Droplet transport through dielectrophoretic actuation using line electrode

We explore a novel transverse line electrode configuration for droplet transport through dielectrophoretic actuation with potential lab-on-chip applications. Using a lumped electromechanical model, we show a weak dependence of DEP actuation force on electrode spacing in this configuration. The configuration successfully triggers translational drop motion with minimal changes in contact angle at considerably low voltages. Two sessile, deionized water drops placed horizontally apart on a indium-tin–oxide-coated glass with additional coatings of polydimethylsiloxane, and a thin layer of Teflon is merged by applying an AC field (88 V_rms at 150 kHz) through a common horizontal wire electrode. A lateral motion of two drops is induced along the horizontal electrode, eventually leading to coalescence. The drop motion is unique compared to electrowetting in its near-constant dynamic contact angle, and irreversibility on withdrawal of electric field. The effect of frequency on the drop behavior is examined through a parametric study on single drops within the range of 2–200 kHz. It is interesting to observe a switch-over from DEP behavior at high frequency to EWOD behavior at low frequency around a critical frequency (Jones in Langmuir 18:4437–4443, 2002).     

Flow rate analysis of an EWOD-based device: how important are wetting-line pinning and velocity effects?

An electrowetting on dielectric (EWOD)-based micropump was used as a platform to study the contribution of the pinning and wetting-line velocity effects on its flow rate. In this micropump, a droplet is driven into a microchannel using EWOD to manipulate a meniscus in the channel. An interesting observation was that the shrinking input droplet changes its shape in two modes: (1) in the first mode, droplet contact angle decreases while its wetting area remains constant (pinning) and (2) in the second mode, droplet wetting line recedes while its contact angle changes as a function of its velocity (dynamic contact angle). Unexpectedly, the micropump flow rate was found to be constant in spite of the changes in the droplet radius. The pump performance was studied to unravel the physical concept behind its constant flow rate. A detailed characterization of variation in contact angle due to pinning, wetting-line velocity, and EWOD was carried out. Dynamic contact angles were used to accurately calculate the pressure gradient between the droplet and the meniscus for flow rate estimation. It was shown that neglecting either the wetting-line energy or the velocity effect results in not only a considerable gap between the predicted and the measured flow rates but also an unphysical instability in flow rate analysis. However, when these effects were fully taken into account, an excellent agreement between the predicted and the measured flow rates was obtained.     

Multiphysics modeling of an IPMC microfluidic control device

A microfluidic control device that uses an electroactive polymer for actuation has been recently proposed. This design has potential to control temperature sensitive particle-laden liquids. The electro-mechanical characteristics of ionic polymer metal composite (IPMC) actuators have been studied both theoretically and empirically. However, very little data has been published on the thermal behavior of IPMC actuators. To realize the proposed fluidic control device, it is essential to understand the thermal properties of the device under actuation conditions. This paper discusses the theoretical basis for developing a multiphysics model describing electroactive polymer actuation. In addition, experimental results are presented that give insight to the thermal characteristics of IPMC actuation.