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.     

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.     

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.     

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.     

Detection of microdroplet size and speed using capacitive sensors

Detection of the presence, size and speed of microdroplets in microfluidic devices is presented using commercially available capacitive sensors which make the droplet based microfluidic systems scalable and inexpensive. Cross-contamination between the droplets is eliminated by introducing a passivation layer between the sensing electrodes and droplets. A simple T-junction generator is used to generate droplets in microchannels. Coplanar electrodes are used to form a capacitance through the microfluidic channel. The change in capacitance due to the presence of a droplet in the sensing area is detected and used to determine the size and speed of the droplet. The design of a single pair of electrodes is used to detect the presence of a droplet and the interdigital finger design is used to detect the size and speed of the droplet. An analytical model is developed to predict the detection signal and guide the experimental optimization of the sensor geometry. The measured droplet information is displayed through a Labview interface in real-time. The use of capacitance sensors to monitor droplet sorting at a T-junction is also presented. The discussions in this paper can be generalized to any droplet detection application and can serve as a guideline in sensor selection.     

Numerical modeling of microscale droplet dispensing in parallel-plate electrowetting-on-dielectric (EWOD) devices with various reservoir designs

Water droplet dispensing in microfluidic parallel-plate electrowetting-on-dielectric (EWOD) devices with various reservoir designs has been numerically studied. The Navier–Stokes equations are solved using a finite-volume formulation with a two-step projection method on a fixed grid. The free surface of the liquid is tracked by a coupled level set and volume-of-fluid method with the surface tension force determined by the continuum surface force model. Contact angle hysteresis which is an indispensible element in EWOD modeling has been implemented. A simplified model is adopted for the viscous stresses exerted by the parallel plates at the solid–liquid interface. Good agreement has been achieved between the numerical results and the corresponding experimental data. The dispensing mechanism has been carefully examined, and droplet volume inconsistency for each design has been investigated. It has been discovered that the pressure distribution on the cutting electrode at the beginning of the cutting stage is of considerable significance for the inconsistency of droplet volumes. Several key elements which directly affect the pressure distribution and volume inconsistency have been identified.     

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.     

Thin-film electrode based droplet detection for microfluidic systems

We report on a droplet-producing microfluidic system with electrical impedance-based detection. The microfluidic devices are made of polydimethylsiloxane (PDMS) and glass with thin film electrodes connected to an impedance-monitoring circuit. Immiscible fluids containing the hydrophobic and hydrophilic phases are injected with syringe pumps and spontaneously break into water-in-oil droplet trains. When a droplet passes between a pair of electrodes in a medium having different electrical conductivity, the resulting impedance change signals the presence of the particle for closed-loop feedback during processing. The circuit produces a digital pulse for input into a computer control system. The droplet detector allows estimation of a droplet's arrival time at the microfluidic chip outlet for dispensing applications. Droplet detection is required in applications that count, sort, and direct microfluidic droplets. Because of their low cost and simplicity, microelectrode-based droplet detection techniques should find applications in digital microfluidics and in three-dimensional printing technology for rapid prototyping and biotechnology.     

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.     

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.     

Characterization and modeling of a microfluidic dielectrophoresis filter for biological species

Microfabricated interdigitated electrode array is a convenient form of electrode geometry for dielectrophoretic trapping of particles and biological entities such as cells and bacteria within microfluidic biochips. We present experimental results and finite element modeling of the holding forces for both positive and negative dielectrophoretic traps on microfabricated interdigitated electrodes within a microfluidic biochip fabricated in silicon with a 12-/spl mu/m-deep chamber. Anodic bonding was used to close the channels with a glass cover. An Experimental protocol was then used to measure the voltages necessary to capture different particles (polystyrene beads, yeast cells, spores and bacteria) against destabilizing fluid flows at a given frequency. The experimental results and those from modeling are found to be in close agreement, validating our ability to model the dielectrophoretic filter for bacteria, spores, yeast cells, and polystyrene beads. This knowledge can be very useful in designing and operating a dielectrophoretic barrier or filter to sort and select particles entering the microfluidic devices for further analysis.     

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.     

Development of an integrated CMOS DNA detection biochip

This paper presents a CMOS DNA detection biochip using an electrical detection method with self-assembly multilayer gold nanoparticles (AuNPs). Each measuring spot of this biochip consists of three major parts; a pair of electrodes with a nanogap, a current amplifier circuit, and a heater with an embedded temperature sensor. The biochip is first fabricated by a TSMC (Taiwan Semiconductor Manufacturing Company Ltd.) 0.35 μm 2P4M standard CMOS process. Then, post-CMOS micromachining etch processes are used to expose the surface of the nanogap to test samples for the establishment of multilayer AuNPs through hybridization between single strand DNAs in the samples. The gap distance between a pair of electrodes is 350 nm. Before taking DNA detection measurements, self-assembly monolayer AuNPs is established on the nanogap surface between two microelectrodes. Multilayer AuNPs can be observed if hybridization between single strand DNAs occurs. An approximately 1000-fold increase in electric current between the multilayer AuNPs over the monolayer AuNPs serves an indication of the presence of target DNA in test samples. After integrating the electrodes with an embedded current amplifier, the electric current of multilayer AuNPs is amplified to the order of mA that can be easily measured by a commercial Volt-Ohm-Milliammeter. The heating system with a heating element and a temperature sensor can be used to distinguish single base-pair mismatch hybridization from complementary hybridization for the establishment of multilayer AuNPs. The lowest detectable concentration of target DNA on this biochip is 0.1 nM.     

数字微流控生物芯片测试诊断过程分析和优化

针对数字微流控生物芯片的测试和诊断过程进行建模和分析,并根据并行测试的分块数和单元出错概率为相应的测试和诊断成本建立函数。通过Matlab对测试诊断成本函数的分析表明：随着并行测试分块数的增大,测试诊断成本的变化趋势不明显,也就是说,并行测试的分块数对测试诊断成本的影响不大;而随着单元出错概率p的增加,测试成本呈明显的增加趋势,且增加的幅度较大。另外,诊断过程中,根据单元出错概率对出错的子阵列再进行诊断,诊断过程必须持续若干次,直到所有故障定位后才能结束。在这些诊断中,针对最后一次定位的诊断成本是最大的,而且与其他次的诊断过程的成本相差几十个数量级,决定了总成本的大小。这些结论为数字微流控生物芯片的测试和诊断过程优化提供重要的理论依据,并为测试诊断方法的设计提供指导。     

Design of a flow-controlled asymmetric droplet splitter using computational fluid dynamics

In this work the design of a segmented flow microfluidic device is presented that allows droplet splitting ratios from 1:1 up to 20:1. This ratio can be dynamically changed on chip by altering an additional oil flow. The design was fabricated in PDMS chips using the standard SU-8 mold technique and does not require any valves, membranes, optics or electronics. To avoid a trial and error approach, fabricating and testing several designs, a computational fluid dynamics model was developed and validated for droplet formation and splitting. The model was used to choose between several variations of the splitting T-junction with the extra oil inlet, as well to predict the additional flow rate needed to split the droplets in various ratios. Experimental and simulated results were in line, suggesting the model’s suitability to optimize future designs and concepts. The resulting asymmetric droplet splitter design opens possibilities for controlled sampling and improved magnetic separation in bio-assay applications.     

Numerical modeling of electrowetting processes in digital microfluidic devices

We use a three-dimensional multiphase lattice-Boltzmann model to study basic operations such as transport, merging and splitting of nanoliter water droplets actuated by electrowetting in digital microfluidic devices. In a first step, numerical and analytical predictions for the droplet transport velocity are compared and very good agreement is obtained for a wide range of contact angles. The same algorithm is employed then to study the dynamics of the splitting processes at different contact angles and different geometries of the cell. The configuration of the liquid droplet involved in a splitting process and the dependence of the splitting time on the transport velocity are also investigated and phenomenological laws describing these processes are also proposed.     

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.     

MEDA芯片菊花链的自动容错设计

微点阵生物芯片MEDA biochip将微电子和微流体结合,是最近出现的一种新型数字微流控芯片,它基于微点阵思想,每个微点阵单元都包含驱动电路和检测电路,从而实现对生化实验的实时监测。为减少外部引脚,MEDA芯片的所有微单元由菊花链串联,以实现对芯片的准确控制。作为MEDA芯片的关键数据通路,菊花链上即使只有一个单元发生故障,也会导致整条链失效,因此必须对菊花链进行有效的容错设计。为MEDA芯片设计了具有自测试和容错功能的菊花链结构,该结构由测试响应触发菊花链故障单元的自动容错。当菊花链的某个单元出现故障时,其测试响应产生异常,从而触发故障单元的自动修复,若修复失败,则异常的测试响应会再次触发该单元的旁路,从而实现自动容错。实验结果表明,该结构可以在测试和诊断故障的同时进行有效的容错,并在容错失败时将其永久旁路。     

新型电子装备软件故障诊断方法研究

操作程序(软件)已成为新型电子装备的重要的组成部分．软件中存在的缺陷影响电子装备的战术与技术性能。为了检测和诊断电子装备操作软件故障．针对软件特点和故障特征．建立了软件故障诊断模型，分析了故障隔离方法，提出了基于故障报告信息的诊断策略。实践证明．该方法较好地解决了电子装备操作软件的故障诊断问题。