Digital Microfluidics Technology Research Articles

The application and reflection of emerging molecular detection technologies in Salmonella detection

Salmonella is an important foodborne pathogen and one of the main causes of diarrhea. Every year, about 550 million people suffer from diarrhea due to Salmonella infection, of which about 230 000 die. It has become a major global public safety issue. The application fields of Salmonella detection involve food safety, water quality monitoring, animal husbandry, public health monitoring, and medical diagnosis. The detection requirements mainly come from three aspects: pathogen identification, serotype identification, drug resistance and virulence identification. In recent years, the detection technology for Salmonella has made rapid progress, especially the emergence and development of emerging molecular detection technologies, providing new perspectives for Salmonella detection in different scenarios. However, due to the diversity of Salmonella serotypes and the complexity of detection scenarios, existing detection technologies still have some pain points (such as long detection time, cumbersome operation steps, low scene adaptability, etc.). This article will elaborate on the application of several emerging molecular detection technologies with distinct characteristics, such as CRISPR Cas technology, digital PCR technology, sequencing technology, and microfluidic technology, in Salmonella detection. It aims to provide a reference for the development and improvement of Salmonella detection technology and the establishment of infection warning and control systems.

AI-powered modular and general-purpose droplet processing system based on single-sided continuous optoelectrowetting chip

Single-sided continuous optoelectrowetting (SCOEW) is a versatile light-controlled digital microfluidic technology. However, the absence of automated algorithms for generating light patterns to manipulate droplets has constrained SCOEW chips to employing only pre-recorded sequences. This limitation hinders SCOEW platforms in effectively addressing common experimental challenges like droplet pinning. To overcome this issue, we have developed a set of algorithms for the automatic generation of droplet-driving light patterns. By integrating these algorithms with machine vision (using YOLOv3) to detect, locate, and classify droplets through video camera input, and employing a path-planning algorithm (A*) to optimize droplet trajectories, our system achieves intelligent functionalities. These include simultaneous manipulation of multiple droplets, automated merging based on color-coded droplet composition, obstacle avoidance, and resolution of potential droplet manipulation failures (such as droplet pinning). In a practical application, we implemented an intelligent loop-mediated isothermal amplification (LAMP) assay on the platform. This modular approach to droplet processing is easily adaptable to other SCOEW hardware, offering potential integration of customizable functionality. The result is a promising suite of portable tools for a variety of biochemical analysis application.

Advances in electrowetting-on-dielectric digital microfluidics technology for disease diagnosis and prevention applications

Rapid and accurate diagnostic technologies are crucial for early detection and diagnosis of diseases. Electrowetting-on-dielectric digital microfluidics, with its high-precision detection and high-throughput screening capabilities, significantly enhances the accuracy and efficiency in early disease diagnosis and personalized treatment, enabling swift disease detection and widespread screening. This article provides a comprehensive review of the working principles and fabrication processes of digital microfluidic chips based on electrowetting on dielectric method. It details the latest research progress in the areas of nucleic acids, proteins, and cells, organizes the commercialization of digital microfluidics technology, and finally discusses the current challenges and future directions of digital microfluidic chips.

Surface acoustic wave digital microfluidics with surface wettability gradient.

This paper reports a digital microfluidic technology that combines surface wettability gradient and surface acoustic waves. The technology enables selection of the driven object, facilitating reactions among multiple droplets and improving the precision of droplet control. Octagonal patterns with a wetting gradient and orthogonally distributed interdigital transducers were created on the surface of a LiNbO3 wafer by photolithography. Leveraging the propagation characteristics of surface acoustic waves on different wetting models, the latter serve as a switch for microfluidic motion, successfully achieving selection of the driven object and demonstrating sequential reactions among multiple droplets. Also, under the excitation of standing surface acoustic waves, droplets on the wetting gradient surface move slightly in the direction of wetting gradient descent, significantly enhancing the positional accuracy of the droplets to the micrometer level. With the advantages of surface acoustic wave digital microfluidics, this technology addresses the challenges of multi-droplet digital manipulation and improved droplet positional accuracy.

The Long and Winding Road: On-Demand DNA Synthesis in High Demand

The Long and Winding Road: On-Demand DNA Synthesis in High Demand

DropLab: an automated magnetic digital microfluidic platform for sample-to-answer point-of-care testing—development and application to quantitative immunodiagnostics

In point-of-care testing (POCT), tests are performed near patients and results are given rapidly for timely clinical decisions. Immunodiagnostic assays are one of the most important analyses for detecting and quantifying protein-based biomarkers. However, existing POCT immunodiagnostics mainly rely on the lateral flow assay (LFA), which has limited sensitivity or quantification capability. Although other immunodiagnostic assays, such as enzyme-linked immunosorbent assays (ELISAs), offer more sensitive and quantitative results, they require complex liquid manipulations that are difficult to implement in POCT settings by conventional means. Here, we show the development of DropLab, an automated sample-in-answer-out POCT immunodiagnostic platform based on magnetic digital microfluidic (MDM) technology. DropLab performs microbead-based ELISA in droplets to offer more sensitive and quantitative testing results. The intricate liquid manipulations required for ELISA are accomplished by controlling droplets with magnetic microbeads using MDM technology, which enables us to achieve full automation and easy operations with DropLab. Four ELISAs (the sample in triplicates and a negative control) can be run in parallel on the thermoformed disposable chip, which greatly improves the throughput and accuracy compared to those of other POCT immunodiagnostic devices. DropLab was validated by measuring two protein targets and one antibody target. The testing results showed that the limit of detection (LOD) of DropLab matched that of the conventional ELISA in a microwell plate. DropLab brings MDM one step closer to being a viable medical technology that is ready for real-world POCT applications.

Toward Suppressing Charge Trapping Based on a Combined Driving Waveform with an AC Reset Signal for Electro-Fluidic Displays.

Digital microfluidic technology based on the principle of electrowetting is developing rapidly. As an extension of this technology, electro-fluidic displays (EFDs) have gradually become a novel type of display devices, whose grayscales can be displayed by controlling oil film in pixels with a microelectromechanical system (MEMS). Nevertheless, charge trapping can occur during EFDs' driving process, which will produce the leakage current and seriously affect the performance of EFDs. Thus, an efficient driving waveform was proposed to resolve these defects in EFDs. It consisted of a driving stage and a stabilizing stage. Firstly, the response time of oil film was shortened by applying an overdriving voltage in the driving stage according to the principle of the electrowetting. Then, a direct current (DC) voltage was designed to display a target luminance by analyzing leakage current-voltage curves and a dielectric loss factor. Finally, an alternating current (AC) reset signal was applied in the stabilizing stage to suppress the charge trapping effect. The experiment results indicated that compared with a driving waveform with a reset signal and a combined driving waveform, the average luminance was improved by 3.4% and 9.7%, and the response time was reduced by 29.63% and 51.54%, respectively.

A digital microfluidic system with integrated electrochemical impedance detection arrays

Electrochemical sensing provides a new way for miniaturization and low cost of equipment. Meanwhile, digital microfluidic (DMF) technology is making lab-on-chip a reality. The two technologies are essential in the point-of-care area. Electrochemical detection on electrodes and DMF-based droplet driving electrode realize their respective functions by means of electrical signals. In terms of working mode, it seems to have certain similarity, and electrochemical sensing seems to be more suitable for integrating digital microfluidic systems. We thus integrated electrochemical sensing on an active matrix (AM) DMF system. As a common transparent conductive glass, indium tin oxide (ITO) has been widely used as the top plate of the digital microfluidic chip. In this work, we used ITO glass with patterned interdigital electrodes as the top plate of the AM-DMF chip. We achieved on-chip droplet volume detection by electrochemical impedance spectroscopy (EIS). We also proposed a novel sample dilution method on the DMF chip. It takes only 30 seconds to dilute solutions into 4 different concentrations. We then analyzed droplet samples of different concentrations with EIS method. And the correlation coefficient of the fitted curve is as high as 0.9964. In conclusion, EIS is a powerful detection technology for fast, high-integration and low-cost detection on DMF system. The integration of EIS and DMF can be used in various biochemical analyses in the future.

An automated nucleic acid detection platform using digital microfluidics with an optimized Cas12a system.

Outbreaks of both influenza virus and the novel coronavirus SARS-CoV-2 are serious threats to human health and life. It is very important to establish a rapid, accurate test with large-scale detection potential to prevent the further spread of the epidemic. An optimized RPA-Cas12a-based platform combined with digital microfluidics (DMF), the RCD platform, was established to achieve the automated, rapid detection of influenza viruses and SARS-CoV-2. The probe in the RPA-Cas12a system was optimized to produce maximal fluorescence to increase the amplification signal. The reaction droplets in the platform were all at the microliter level and the detection could be accomplished within 30 min due to the effective mixing of droplets by digital microfluidic technology. The whole process from amplification to recognition is completed in the chip, which reduces the risk of aerosol contamination. One chip can contain multiple detection reaction areas, offering the potential for customized detection. The RCD platform demonstrated a high level of sensitivity, specificity (no false positives or negatives), speed (≤30 min), automation and multiplexing. We also used the RCD platform to detect nucleic acids from influenza patients and COVID-19 patients. The results were consistent with the findings of qPCR. The RCD platform is a one-step, rapid, highly sensitive and specific method with the advantages of digital microfluidic technology, which circumvents the shortcomings of manual operation. The development of the RCD platform provides potential for the isothermal automatic detection of nucleic acids during epidemics. Electronic Supplementary MaterialSupplementary material is available in the online version of this article at 10.1007/s11426-021-1169-1.

Solution for Mass Production of High-Throughput Digital Microfluidic Chip Based on a-Si TFT with In-Pixel Boost Circuit.

As one of the most popular research hotspot of lab-on-chip, digital microfluidic (DMF) technology based on the principle of electrowetting has unique advantages of high-precision, low cost and programmable control. However, due to the limitation of electrodes number, the throughput is hard to further upgrade. Therefore, active matrix electrowetting-on-dielectric (AM-EWOD) technology is a solution to acquire larger scale of driving electrodes. However, the process of manufacturing of AM-EWOD based on thin-film-transistor (TFT) is complex and expensive. Besides, the driving voltage of DMF chip is usually much higher than that of common display products.In this paper, a solution for mass production of AM-EWOD based on amorphous silicon (a-Si) is provided. Samples of 32 × 32 matrix AM-EWOD chips was designed and manufactured. A boost circuit was integrated into the pixel, which can raise the pixel voltage up by about 50%. Customized designed Printed Circuit Board (PCB) was used to supply the timing signals and driving voltage to make the motion of droplets programmable. The process of moving, mixing and generation of droplets was demonstrated.The minimum voltage in need was about 20 V and a velocity of up to 96 mm/s was achieved. Such an DMF device with large-scale matrix and low driving voltage will be very suitable for POCT applications.

A digital microfluidic platform based on a near-infrared light-responsive shape-memory micropillar array.

In this work, we developed a digital microfluidic platform based on a shape memory micropillar array responsive to near-infrared light, and the droplets were programmatically manipulated through light-induced micropillar deformation. The micropillar array was constructed on the surface of a poly(ethylene-vinyl acetate) copolymer, a shape memory polymer sensitive to near-infrared light. Before droplet manipulation, the micropillar array was kept temporarily tilted by heating and pressing. Under the irradiation of a near-infrared laser, the micropillar array achieved the transition from the temporary shape to the original shape. Temperature gradient and micropillar deformation caused by near-infrared light irradiation produce the driving force for droplet movement. The movement of the laser mounted on an electronically controlled displacement platform was controlled by a computer to achieve the programmed control of the droplets. Moreover, we demonstrated light-manipulated droplet movement and fusion, and achieved ascorbic acid detection using this digital microfluidic platform. In particular, the micropillar array chip is able to manipulate droplets in a wide range of 0.1 μL to 10 μL. The proposed digital microfluidic platform will broaden the application of digital microfluidic technology in analytical chemistry and biomedicine.

Designing Splicing Digital Microfluidics Chips Based on Polytetrafluoroethylene Membrane.

As a laboratory-on-a-chip application tool, digital microfluidics (DMF) technology is widely used in DNA-based applications, clinical diagnosis, chemical synthesis, and other fields. Additional components (such as heaters, centrifuges, mixers, etc.) are required in practical applications on DMF devices. In this paper, a DMF chip interconnection method based on electrowetting-on-dielectric (EWOD) was proposed. An open modified slippery liquid-infused porous surface (SLIPS) membrane was used as the dielectric-hydrophobic layer material, which consisted of polytetrafluoroethylene (PTFE) membrane and silicone oil. Indium tin oxide (ITO) glass was used to manufacture the DMF chip. In order to test the relationship between the splicing gap and droplet moving, the effect of the different electrodes on/off time on the minimum driving voltage when the droplet crossed a splicing gap was investigated. Then, the effects of splicing gaps of different widths, splicing heights, and electrode misalignments were investigated, respectively. The experimental results showed that a driving voltage of 119 V was required for a droplet to cross a splicing gap width of 300 μm when the droplet volume was 10 μL and the electrode on/off time was 600 ms. At the same time, the droplet could climb a height difference of 150 μm with 145 V, and 141 V was required when the electrode misalignment was 1000 μm. Finally, the minimum voltage was not obviously changed, when the same volume droplet with different aqueous solutions crossed the splicing gap, and the droplet could cross different chip types. These splicing solutions show high potential for simultaneous detection of multiple components in human body fluids.

Emerging Approaches for Fluorescence-Based Newborn Screening of Mucopolysaccharidoses.

Interest in newborn screening for mucopolysaccharidoses (MPS) is growing, due in part to ongoing efforts to develop new therapies for these disorders and new screening assays to identify increased risk for the individual MPSs on the basis of deficiency in the cognate enzyme. Existing tests for MPSs utilize either fluorescence or mass spectrometry detection methods to measure biomarkers of disease (e.g., enzyme function or glycosaminoglycans) using either urine or dried blood spot (DBS) samples. There are currently two approaches to fluorescence-based enzyme function assays from DBS: (1) manual reaction mixing, incubation, and termination followed by detection on a microtiter plate reader; and (2) miniaturized automation of these same assay steps using digital microfluidics technology. This article describes the origins of laboratory assays for enzyme activity measurement, the maturation and clinical application of fluorescent enzyme assays for MPS newborn screening, and considerations for future expansion of the technology.

Digital Microfluidics for the Detection of Selected Inorganic Ions in Aerosols.

A prototype aerosol detection system is presented that is designed to accurately and quickly measure the concentration of selected inorganic ions in the atmosphere. The aerosol detection system combines digital microfluidics technology, aerosol impaction and chemical detection integrated on the same chip. Target compounds are the major inorganic aerosol constituents: sulfate, nitrate and ammonium. The digital microfluidic system consists of top and bottom plates that sandwich a fluid layer. Nozzles for an inertial impactor are built into the top plate according to known, scaling principles. The deposited air particles are densely concentrated in well-defined deposits on the bottom plate containing droplet actuation electrodes of the chip in fixed areas. The aerosol collection efficiency for particles larger than 100 nm in diameter was higher than 95%. After a collection phase, deposits are dissolved into a scanning droplet. Due to a sub-microliter droplet size, the obtained extract is highly concentrated. Droplets then pass through an air/oil interface on chip for colorimetric analysis by spectrophotometry using optical fibers placed between the two plates of the chip. To create a standard curve for each analyte, six different concentrations of liquid standards were chosen for each assay and dispensed from on-chip reservoirs. The droplet mixing was completed in a few seconds and the final droplet was transported to the detection position as soon as the mixing was finished. Limits of detection (LOD) in the final droplet were determined to be 11 ppm for sulfate and 0.26 ppm for ammonium. For nitrate, it was impossible to get stable measurements. The LOD of the on-chip measurements for sulfate was close to that obtained by an off-chip method using a Tecan spectrometer. LOD of the on-chip method for ammonium was about five times larger than what was obtained with the off-chip method. For the current impactor collection air flow (1 L/min) and 1 h collection time, the converted LODs in air were: 0.275 μg/m3 for sulfate, 6.5 ng/m3 for ammonium, sufficient for most ambient air monitoring applications.

Deformation, speed, and stability of droplet motion in closed electrowetting-based digital microfluidics

Electrowetting-based microdrop manipulation has received considerable attention owing to its wide applications in numerous scientific areas based on the digital microfluidics (DMF) technology. However, the techniques for highly precise droplet handling in such microscopic systems are still unclear. In this work, the deformation, speed, and stability of droplet transporting in closed electrowetting-based DMF systems are comprehensively investigated with both theoretical and numerical analyses. First, a theoretical model is derived which governs the droplet motion and includes the influences of the key electrowetting system parameters. After that, a finite volume formulation with a two-step projection method is used for solving the microfluidic flow on a fixed numerical domain. The liquid-gas interface of the droplet is tracked by a coupled level-set and volume-of-fluid method, and the surface tension at the interface is computed by the continuum surface force scheme. A parametric study has been carried out to examine the effects of the static contact angles (θs,ON and θs,OFF), hysteresis effect (Δθ), channel height (H), and electrode size (LE) on droplet shape, speed, and deformation during transport, which unanimously shows that droplet length, neck width, and transport stability are directly related to a dimensionless parameter κ* that only comprises θs,ON, θs,OFF, H, LE, and the hysteresis angle Δθ. Based on the results, the scaling laws for estimating droplet shape and stability of the transport process have been developed, which can be used for promoting the accuracy and efficiency of droplet manipulation in a large variety of droplet-based DMF applications.

Mechanism of droplets on electrowetting-on-dielectric chips transition from stillness to motion

Digital microfluidics technology based on the electrowetting-on-dielectric effect is a popular emerging technology whose objects of control are individual droplets on the microliter or even nano-liter scales. It has unique advantages such as rapid response, low reagent consumption, and high integration, so it has drawn widespread attention and use in the biological, medical, and chemical fields. However, to date, there has been relatively little research conducted on the mechanism of droplets transition from stillness to motion by electrowetting-on-dielectric actuation. Here, we studied the polarization mechanism underlying solid–liquid contact surface, thus building upon the previous research. The electric field in chip, internal pressure, and flow field of droplet were modeled and simulated numerically. Then, the process and mechanism of droplet transition from stillness to motion was comprehensively analyzed, and the results obtained from the simulation and discussion were in close agreement with experimental results. It is here shown that the process of droplet from stillness to motion involves four successive steps, which helps to better understand electrowetting-on-dielectric-induced droplet motions and physics of digital microfluidics systems. The aim of this work was to research basic physical mechanisms of electrowetting-on-dielectric droplet motion on a common ground so that the researchers may form a clear picture of the fundamentals.

Transport mechanism by which droplets on electrowetting-on-dielectric devices

Digital microfluidics technology based on the electrowetting-on-dielectric (EWOD) effect is a popular emerging technology whose objects of manipulation are individual droplets on the micro-liter or even nano-liter scales. It has the unique advantages such as rapid reaction, low reagent consumption, and high integration, which have drawn extensive attention and application in the fields of biology, medicine, and chemistry. However, there has been no a complete mechanism by which the droplets transition from stillness to motion in EWOD chip, leaving the proper mechanism for droplet movement uncertain. At this point in time, we studied the EWOD theory and polarization mechanism underlying solid-liquid interface, thus building upon previous research. The theoretical derivation and numerical analyses of EWOD driving force were presented. Then, the process and mechanism of droplet movement from stillness to motion was comprehensively analyzed, the results obtained from the simulation and discussion were in agreement with experimental results. It is our hope that this work will inspire new research into less understood EWOD regimes and help design useful EWOD devices.

Energy-efficient generation of controlled vortices on low-voltage digital microfluidic platform

Generating controlled vortices in a sessile surface droplet configuration in an energy efficient manner is an outstanding research problem of interdisciplinary relevance, having implications in widely varying areas ranging from biomedical diagnostics, thermal management to digital microfluidic technology. Here, we experimentally and theoretically demonstrate a simple yet energy efficient strategy for generating controlled vortices inside a surface droplet, by deploying interacting electrical and thermal fields over inter-digitated electrodes on an electrically wetted platform. Unlike the traditional electrically driven mechanisms, this strategy involves significantly low voltage (≤10 V) to induce rotational structures inside the droplet, by exploiting the strong spatial gradient of electrical properties on account of the prevailing thermal field as attributable to intrinsically induced Joule heating effects. Our experiments demonstrate that fluid velocities typically of the order of mm/s can be generated inside the droplet within the standard regimes of operating parameters, bearing far-reaching consequences towards enhancing internal mixing in multifarious droplet based microfluidic applications. An inherent integrability with the existing electrowetting on dielectric platforms renders the process ideal to be used in conjunction with digital microfluidic technology.

Facile profiling of molecular heterogeneity by microfluidic digital melt

This work presents a digital microfluidic platform called HYPER-Melt (high-density profiling and enumeration by melt) for highly parallelized copy-by-copy DNA molecular profiling. HYPER-Melt provides a facile means of detecting and assessing sequence variations of thousands of individual DNA molecules through digitization in a nanowell microchip array, allowing amplification and interrogation of individual template molecules by detecting HRM fluorescence changes due to sequence-dependent denaturation. As a model application, HYPER-Melt is used here for the detection and assessment of intermolecular heterogeneity of DNA methylation within the promoters of classical tumor suppressor genes. The capabilities of this platform are validated through serial dilutions of mixed epialleles, with demonstrated detection limits as low as 1 methylated variant in 2 million unmethylated templates (0.00005%) of a classic tumor suppressor gene, CDKN2A (p14ARF). The clinical potential of the platform is demonstrated using a digital assay for NDRG4, a tumor suppressor gene that is commonly methylated in colorectal cancer, in liquid biopsies of healthy and colorectal cancer patients. Overall, the platform provides the depth of information, simplicity of use, and single-molecule sensitivity necessary for rapid assessment of intermolecular variation contributing to genetic and epigenetic heterogeneity for challenging applications in embryogenesis, carcinogenesis, and rare biomarker detection.

Neonatal screening for four lysosomal storage diseases with a digital microfluidics platform: Initial results in Brazil.

We describe the initial results of a neonatal screening program for four lysosomal storage diseases (MPS I, Pompe, Gaucher and Fabry) using the digital microfluidics methodology. The method successfully identified patients previously diagnosed with these diseases and was used to test dried blood spot samples obtained from 10,527 newborns aged 2 to 14 days. The digital microfluidic technology shows potential for a simple, rapid and high-throughput screening for these four diseases in a standard neonatal screening laboratory.