Digital Microfluidic Devices Research Articles

Appendix C: Automated Vitrification of Mammalian Embryos on a Digital Microfluidic Device.

This chapter introduces a digital microfluidic device that automates sample preparation for mammalian embryo vitrification. Individual microdroplets manipulated on the microfluidic device were used as microvessels to transport a single mouse embryo through a complete vitrification procedure. Advantages of this approach, compared to manual operation and channel-based microfluidic vitrification, include automated operation, cryoprotectant concentration gradient generation, and feasibility of loading and retrieval of embryos.

Finger-Powered Electro-Digital-Microfluidics.

Portable microfluidic devices are promising for point-of-care (POC) diagnosis and bio- and environmental surveillance in resource-constrained or non-laboratory environments. Lateral-flow devices, some built off paper or strings, have been widely developed but the fixed layouts of their underlying wicking/microchannel structures limit their flexibility and present challenges in implementing multistep reactions. Digital microfluidics can circumvent these difficulties by addressing discrete droplets individually. Existing approaches to digital microfluidics, however, often require bulky power supplies/batteries and high voltage circuits. We present a scheme to drive digital microfluidic devices by converting mechanical energy of human fingers to electrical energy using an array of piezoelectric elements. We describe the integration our scheme into two promising digital microfluidics platforms: one based on the electro-wetting-on-dielectric (EWOD) phenomenon and the other on the electrophoretic control of droplet (EPD). Basic operations of droplet manipulations, such as droplet transport, merging and splitting, are demonstrated using the finger-powered digital-microfluidics.

Single-Sided Digital Microfluidic (SDMF) Devices for Effective Coolant Delivery and Enhanced Two-Phase Cooling

Digital microfluidics (DMF) driven by electrowetting-on-dielectric (EWOD) has recently been attracting great attention as an effective liquid-handling platform for on-chip cooling. It enables rapid transportation of coolant liquid sandwiched between two parallel plates and drop-wise thermal rejection from a target heating source without additional mechanical components such as pumps, microchannels, and capillary wicks. However, a typical sandwiched configuration in DMF devices only allows sensible heat transfer, which seriously limits heat rejection capability, particularly for high-heat-flux thermal dissipation. In this paper, we present a single-sided digital microfluidic (SDMF) device that enables not only effective liquid handling on a single-sided surface, but also two-phase heat transfer to enhance thermal rejection performance. Several droplet manipulation functions required for two-phase cooling were demonstrated, including continuous droplet injection, rapid transportation as fast as 7.5 cm/s, and immobilization on the target hot spot where heat flux is locally concentrated. Using the SDMF platform, we experimentally demonstrated high-heat-flux cooling on the hydrophilic-coated hot spot. Coolant droplets were continuously transported to the target hot spot which was mitigated below 40 K of the superheat. The effective heat transfer coefficient was stably maintained even at a high heat flux regime over ~130 W/cm2, which will allow us to develop a reliable thermal management module. Our SDMF technology offers an effective on-chip cooling approach, particularly for high-heat-flux thermal management based on two-phase heat transfer.

Dynamics of magnetic modulation of ferrofluid droplets for digital microfluidic applications

Active control of droplet generation in a microfluidic platform attracts interest for development of digital microfluidic devices ranging from biosensors to micro-reactors to point-of-care diagnostic devices. The present paper characterizes, through an unsteady three-dimensional Volume of Fluid (VOF) simulation, the active control of ferrofluid droplet generation in a microfluidic T-junction in presence of a non-uniform magnetic field created by an external magnetic dipole. Two distinctly different positions of the dipole were considered – one upstream of the junction and one downstream. While keeping the ferrofluid flow rate fixed, a parametric variation of the continuous phase capillary number, dipole strength, and dipole position was carried out. Differences in the flow behaviour in terms of dripping or jetting and the droplet characteristics in terms of droplet formation time period and droplet size were studied. The existence of a threshold dipole strength, below which the magnetic force was not able to influence the flow behaviour, was identified. It was also observed that, for dipoles placed upstream of the junction, droplet formation was suppressed at some higher dipole strengths, and this value was found to increase with increasing capillary number. Droplet time period was also found to increase with increasing dipole strength, along with droplet size, i.e. an increase in droplet volume.

Fluid Transport in Partially Shielded Electrowetting on Dielectric Digital Microfluidic Devices

Theoretical and experimental approaches verifying the fluidic operation of a partially shielded digital microfluidics device are presented in this paper. This paper is motivated by recent demand from the synthetic biology community for electrowetting on dielectric (EWD) enabled in-droplet electroporation, but is generalizable to a range of EWD applications that require shielding structures to be patterned on the EWD. An electrode patterned in an additional metal layer on the insulator that supports EWD actuation reduces the effective strength of the EW force due to dielectric shielding at the droplet contact line. A numerical model was developed to predict the impact of the partially shielding electrode on threshold voltage, EW force, fluid velocity, and droplet transport time. Compared with a batch of devices lacking the extra electrode, the presence of the added metal layer resulted in a 29% increase in threshold voltage, an 82% increase in transport time, and a 44% decrease in average transport velocity. Each trend agrees with the simulation results obtained from the fluid transport model. These results support the development of design rules for microfluidic devices that require partially shielding metal layers to integrate with EWD device architectures. [2016-0034]

Dimensionless model for impedimetric sensing of particle laden droplets in digital microfluidic devices

Microfluidic lab-on-a-chip (LOC) devices miniaturize and automate biological protocols while reducing labor and equipment costs. An automated LOC can be used by operators with little training, provided the device incorporates real-time autonomous feedback to the user. Digital microfluidic (DMF) devices are a LOC platform that can create, move, merge, and mix discrete droplets of fluid through the application of asymmetric electric fields. The geometry of DMF devices also allows for integration of real-time impedimetric feedback without integration of additional sensing components. This investigation presents an analytical model for impedimetric sensing of particle laden droplets in DMF devices. The proposed model is based on an equivalent circuit that contains particles of a general size and vertical distribution. Since the proposed model provides an analytical formulation, it can be used to identify dimensionless parameters that govern the impedimetric response of the system. These parameters include: measurement frequency, local area fraction, dimensionless particle size, and the ratios of electrical properties of the particles and the surrounding fluid. The model was used to predict impedimetric response of particle laden droplets in DMF devices. Impedance increased monotonically with volume fraction regardless of particle orientation. This increase was more rapid for horizontally distributed particles than vertically distributed particles. The model also predicts that particle concentration and measurement frequency can be used to independently control sensitivity and saturation of the impedance measurements to changes in electrical properties of the fluid that may occur due to chemical reactions in the droplet.

A digital microfluidic interface between solid-phase microextraction and liquid chromatography–mass spectrometry

We introduce a method to couple solid-phase microextraction (SPME) with HPLC-MS using digital microfluidics (DMF). In the new system, SPME fibers are used to extract analytes from complex sample solutions, after which the analytes are desorbed into solvent droplets in a DMF device. The open geometry of DMF allows straightforward insertion of SPME fibers without requiring a complicated interface, and automated droplet manipulation enables multiplexed processing of the fibers. In contrast to other multiplexed SPME elution interfaces, the low volumes inherent to DMF allow for pre-concentration of analytes prior to analysis. The new SPME-DMF-HPLC-MS method was applied to the quantification of pg/mL-level free steroid hormones in urine. We propose that this new method will be useful for a wide range of applications requiring cleanup and pre-concentration with convenient coupling to high-performance analytical techniques.

An inkjet printed, roll-coated digital microfluidic device for inexpensive, miniaturized diagnostic assays.

The diagnosis of infectious disease is typically carried out at the point-of-care (POC) using the lateral flow assay (LFA). While cost-effective and portable, LFAs often lack the clinical sensitivity and specificity required for accurate diagnoses. In response to this challenge, we introduce a new digital microfluidic (DMF) platform fabricated using a custom inkjet printing and roll-coating process that is scalable to mass production. The performance of the new devices is on par with that of traditional DMF devices fabricated in a cleanroom, with a materials cost for the new devices of only US $0.63 per device. To evaluate the usefulness of the new platform, we performed a 13-step rubella virus (RV) IgG immunoassay on the inkjet printed, roll-coated devices, which yielded a limit of detection of 0.02 IU mL-1, well below the diagnostic cut-off of 10 IU mL-1 for RV infection and immunity. We propose that this represents a breakthrough for DMF, lowering the costs to a level such that the new platforms will be an attractive alternative to LFAs for the diagnosis of infectious disease at the POC.

Polymerase chain reaction in microfluidic devices.

The invention of the polymerase chain reaction (PCR) has caused a revolution in molecular biology, giving access to a method of amplifying deoxyribonucleic acid (DNA) molecules across several orders of magnitude. Since the first application of PCR in a microfluidic device was developed in 1998, an increasing number of researchers have continued the development of microfluidic PCR systems. In this review, we introduce recent developments in microfluidic-based space and time domain devices as well as discuss various designs integrated with multiple functions for sample preparation and detection. The development of isothermal nucleic acid amplification and digital PCR microfluidic devices within the last five years is also highlighted. Furthermore, we introduce various commercial microfluidic PCR devices.

Sub-7-second genotyping of single-nucleotide polymorphism by high-resolution melting curve analysis on a thermal digital microfluidic device.

We developed a thermal digital microfluidic (T-DMF) device enabling ultrafast DNA melting curve analysis (MCA). Within 7 seconds, the T-DMF device succeeded in differentiating a melting point difference down to 1.6 °C with a variation of 0.3 °C in a tiny droplet sample (1.2 μL), which was 300 times faster and with 20 times less sample spending than the standard MCA (35 minutes, 25 μL) run in a commercial qPCR machine. Such a performance makes it possible for a rapid discrimination of single-nucleotide mutation relevant to prompt clinical decision-making. Also, aided by electronic intelligent control, the T-DMF device facilitates sample handling and pipelining in an automatic serial manner. An optimized oval-shaped thermal electrode is introduced to achieve high thermal uniformity. A device-sealing technique averts sample contamination and permits uninterrupted chemical/biological reactions. Simple fabrication using a single chromium layer fulfills both the thermal and typical transport electrode requirements. Capable of thermally modulating DNA samples with ultrafast MCA, this T-DMF device has the potential for a wide variety of life science analyses, especially for disease diagnosis and prognosis.

Motorized actuation system to perform droplet operations on printed plastic sheets.

We developed an open microfluidic system to dispense and manipulate discrete droplets on planar plastic sheets. Here, a superhydrophobic material is spray-coated on commercially-available plastic sheets followed by the printing of hydrophilic symbols using an inkjet printer. The patterned plastic sheets are taped to a two-axis tilting platform, powered by stepper motors, that provides mechanical agitation for droplet transport. We demonstrate the following droplet operations: transport of droplets of different sizes, parallel transport of multiple droplets, merging and mixing of multiple droplets, dispensing of smaller droplets from a large droplet or a fluid reservoir, and one-directional transport of droplets. As a proof-of-concept, a colorimetric assay is implemented to measure the glucose concentration in sheep serum. Compared to silicon-based digital microfluidic devices, we believe that the presented system is appealing for various biological experiments because of the ease of altering design layouts of hydrophilic symbols, relatively faster turnaround time in printing plastic sheets, larger area to accommodate more tests, and lower operational costs by using off-the-shelf products.

Offline Washing Schemes for Residue Removal in Digital Microfluidic Biochips

A digital microfluidic biochip (DMB) is often deployed for multiplexing several assays in space and in time. The residue left by one assay may contaminate the droplets used for subsequent assays. Biochemical assays involving cell culture and those based on particle microfluidics also require sweeping of residual media from an active droplet on-chip. Thus, fluidic operations such as washing or residue removal need to be performed routinely either to clean contamination from the droplet pathways or to rinse off certain droplets on the chip. In this work, several graph-based techniques are presented for offline washing of biochips that may have either a regular geometry (e.g., a 2D array of electrodes), or an irregular geometry (e.g., an application-specific layout). The schemes can be used for total washing, that is, for cleaning the entire biochip or for selective washing of sites or pathways located sparsely on the chip. The problem of reducing the path length and washing time of the droplets is investigated with or without capacity constraints. The proposed algorithms for offline washing make use of several techniques such as graph traversal, integer linear programming (ILP) modeling, and customized heuristics based on the nature of the geometric distribution of the contamination profile. The contaminated pathways are assumed to be Manhattan or curved, and hence the techniques are applicable to the conventional field-actuated DMBs as well as to the emerging classes of light-actuated and active-matrix DMBs. These techniques will be useful in enhancing the reliability of a wide class of emerging digital microfluidic healthcare devices

Digital Microfluidic System with Vertical Functionality

Digital (droplet) microfluidics (DµF) is a powerful platform for automated lab-on-a-chip procedures, ranging from quantitative bioassays such as RT-qPCR to complete mammalian cell culturing. The simple MEMS processing protocols typically employed to fabricate DµF devices limit their functionality to two dimensions, and hence constrain the applications for which these devices can be used. This paper describes the integration of vertical functionality into a DµF platform by stacking two planar digital microfluidic devices, altering the electrode fabrication process, and incorporating channels for reversibly translating droplets between layers. Vertical droplet movement was modeled to advance the device design, and three applications that were previously unachievable using a conventional format are demonstrated: (1) solutions of calcium dichloride and sodium alginate were vertically mixed to produce a hydrogel with a radially symmetric gradient in crosslink density; (2) a calcium alginate hydrogel was formed within the through-well to create a particle sieve for filtering suspensions passed from one layer to the next; and (3) a cell spheroid formed using an on-chip hanging-drop was retrieved for use in downstream processing. The general capability of vertically delivering droplets between multiple stacked levels represents a processing innovation that increases DµF functionality and has many potential applications.

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.

Electrochemiluminescence on digital microfluidics for microRNA analysis.

Electrochemiluminescence (ECL) is a sensitive analytical technique with great promise for biological applications, especially when combined with microfluidics. Here, we report the first integration of ECL with digital microfluidics (DMF). ECL detectors were fabricated into the ITO-coated top plates of DMF devices, allowing for the generation of light from electrically excited luminophores in sample droplets. The new system was characterized by making electrochemical and ECL measurements of soluble mixtures of tris(phenanthroline)ruthenium(II) and tripropylamine (TPA) solutions. The system was then validated by application to an oligonucleotide hybridization assay, using magnetic particles bearing 21-mer, deoxyribose analogues of the complement to microRNA-143 (miRNA-143). The system detects single nucleotide mismatches with high specificity, and has a limit of detection of 1.5 femtomoles. The system is capable of detecting miRNA-143 in cancer cell lysates, allowing for the discrimination between the MCF-7 (less aggressive) and MDA-MB-231 (more aggressive) cell lines. We propose that DMF-ECL represents a valuable new tool in the microfluidics toolbox for a wide variety of applications.

Droplet Necking and Morphology Variations Induced by Changing the Gap Height During Transport in Digital Microfluidic Systems

In this paper, we show that the droplet morphology varies significantly during the transport process in digital microfluidic (DMF) systems depending on the gap height between the top and bottom plates. The experimental results elucidate the effect of changing the gap height on the initiation and progression of droplet motion. For example, extreme necking is observed during transport from one electrode to another at low gap heights. In essence, this necking is unlike the previously reported necking occurring during the splitting process over three electrodes. Pronounced elongation of droplets at certain stages during the droplet motion was examined. The effect of changing the gap height on the droplet velocity profile is illustrated. We found that prolonged actuation time is needed to transport the droplet successfully at low gap heights. This paper entails detailed experimental and numerical characterization of droplet morphology to insinuate useful design and prototyping tips for enhanced DMF devices. Such deformation, elongation, and necking corroborate that the existing analytical models may be inadequate for certain gap heights as they typically assume a circular shape for the droplet during motion. [2014-0292]

Design, fabrication and characterization of low cost printed circuit board based EWOD device for digital microfluidics applications

Digital microfluidics (DMF) is an emerging liquid-handling technology that enables individual control over droplets on array of electrodes. In this paper a low cost approach for development and characterization of open configured electro wetting on dielectric (EWOD) based digital microfluidic device is presented. The array of electrodes pattern with 155 μm gap is realized on copper plated printed circuit board (PCB). Biocompatible polydimethylsiloxane (PDMS) is used as dielectric as well as hydrophobic layer which is a competitive substitute of expensive materials like Teflon-AF and parylene-C. The device is tested by the in-house developed low cost droplet handling and characterization system. A contact angle is measured by curve fitting; the volume of a droplet is calculated using a novel approach based on mathematical modeling of droplet as a spherical cap. The droplet transporting and merging are successfully performed on a fabricated device and velocities for forward and reverse direction on square and interdigitated pattern are analyzed. The device is also successfully demonstrated for the mixing applications using direct statistical method.

On-chip aqueous two-phase system (ATPS) formation, consequential self-mixing, and their influence on drop-to-drop aqueous two-phase extraction kinetics

Aqueous two-phase systems (ATPSs) allow an advantageous aqueous two-phase extraction process (ATPE), a special type of liquid–liquid extraction. Compared with conventional liquid–liquid extraction using aqueous/organic extraction media, ATPE is known to provide relatively easy mass transfer and a gentle environment for biological separation applications. Considering the recent interest in microscale ATPE, we aimed to study (i) the potential of preparing ATPS droplets on a digital microfluidic device, and (ii) the influence of the fluidic dynamics created during the formation of ATPS, with the goal of enhancing on-chip ATPE process. On-chip ATPS formation was evaluated by preparing a series of ATPSs on electrowetting on dielectric digital microfluidic chips and comparing their characteristics with the same ATPSs prepared at macroscale using conventional procedures. An enhanced on-chip drop-to-drop ATPE process was achieved by incorporating a self-mixing condition created during ATPSformation. Results indicate a successful on-chip ATPS preparation as well as enhanced extraction performance by self-mixing in the absence of forced mixing. Findings of this research suggest an alternative, simple, yet adequate technique to provide mixing for on-chip applications, such as sample preparation in portable microfluidics, for which it is unfavorable to implement complicated mixing sequences or complex device geometries.

A guiding light: spectroscopy on digital microfluidic devices using in-plane optical fibre waveguides.

We present a novel method for in-plane digital microfluidic spectroscopy. In this technique, a custom manifold (.stl file available online as ESM) aligns optical fibres with a digital microfluidic device, allowing optical measurements to be made in the plane of the device. Because of the greater width vs thickness of a droplet on-device, the in-plane alignment of this technique allows it to outperform the sensitivity of vertical absorbance measurements on digital microfluidic (DMF) devices by ∼14×. The new system also has greater calibration sensitivity for thymol blue measurements than the popular NanoDrop system by ∼2.5×. The improvements in absorbance sensitivity result from increased path length, as well as from additional effects likely caused by liquid lensing, in which the presence of a water droplet between optical fibres increases fibre-to-fibre transmission of light by ∼2× through refraction and internal reflection. For interrogation of dilute samples, stretching of droplets using digital microfluidic electrodes and adjustment of fibre-to-fibre gap width allows absorbance path length to be changed on-demand. We anticipate this new digital microfluidic optical fibre absorbance and fluorescence measurement system will be useful for a wide variety of analytical applications involving microvolume samples with digital microfluidics.

Characterization of electrode alignment for optimal droplet charging and actuation in droplet-based microfluidic system.

The actuation method using electric force as a driving force is utilized widely in droplet-based microfluidic systems. In this work, the effects of charging electrode alignment on direct charging of a droplet on electrified electrodes and a subsequent electrophoretic control of the droplet are investigated. The charging characteristics of a droplet according to different electrode alignments are quantitatively examined through experiments and systematic numerical simulations with varying distances and angles between the two electrodes. The droplet charge acquired from the electrified electrode is directly proportional to the distance and barely affected by the angle between the two electrodes. This implies that the primary consideration of electrode alignment in microfluidic devices is the distance between electrodes and the insignificant effect of angle provides a great degree of freedom in designing such devices. Not only the droplet charge acquired from the electrode but also the force exerted on the droplet is analyzed. Finally, the implications and design guidance for microfluidic systems are discussed with an electrophoresis of a charged droplet method-based digital microfluidic device.