Heterogeneous immunoassays using magnetic beads on a digital microfluidic platform

Open and closed microfluidics for biosensing

Biosensing is vital for many areas like disease diagnosis, infectious disease prevention, and point-of-care monitoring. Microfluidics has been evidenced to be a powerful tool for biosensing via integrating biological detection processes into a palm-size chip. Based on the chip structure, microfluidics has two subdivision types: open microfluidics and closed microfluidics, whose operation methods would be diverse. In this review, we summarize fundamentals, liquid control methods, and applications of open and closed microfluidics separately, point out the bottlenecks, and propose potential directions of microfluidics-based biosensing.

An All-in-One Platform for On-Site Multiplex Foodborne Pathogen Detection Based on Channel-Digital Hybrid Microfluidics

Recently, significant progress has been made in the development of microdevices for point-of-care infectious disease detection. However, most microdevices only allow limited steps, such as DNA amplification on the chip, while sample preparation, such as lysis and DNA extraction, is conducted off the chip using the traditional method. In this study, an all-in-one platform was developed, which incorporated all necessary procedures for nucleic acid detection. Our on-chip DNA extraction method utilized the magnetic bead-based technology on a hybrid channel-digital microfluidics (C-DMF) microdevice. It yielded high recovery rates, varying from 88.43% to 95.83%, with pathogen concentrations of 103-106 CFU/mL. In particular, the on-chip method exhibited significantly higher efficacy compared to the traditional off-chip manual method, for the DNA extraction of E. coli and S. aureus, representing Gram-negative and Gram-positive bacteria, respectively, at a sample concentration of 103 CFU/mL. To address the need for rapid and accessible diagnostics, colorimetric LAMP amplification was integrated into the proposed microdevice. The results were visually detectable with the naked eye, making it user-friendly for non-specialists. In addition, this platform demonstrated impressive sensitivity in simultaneously detecting common foodborne pathogens in spiked meat samples, achieving the LOD of 102-103 CFU/mL. The entire process, from sampling to result, was fully automated and only required approximately 60 min, offering promising applicability in resource-limited and on-site testing scenarios.

Advanced design and applications of digital microfluidics in biomedical fields: An update of recent progress

Significant breakthroughs have been made in digital microfluidic (DMF)-based technologies over the past decades. DMF technology has attracted great interest in bioassays depending on automatic microscale liquid manipulations and complicated multi-step processing. In this review, the recent advances of DMF platforms in the biomedical field were summarized, focusing on the integrated design and applications of the DMF system. Firstly, the electrowetting-on-dielectric principle, fabrication of DMF chips, and commercialization of the DMF system were elaborated. Then, the updated droplets and magnetic beads manipulation strategies with DMF were explored. DMF-based biomedical applications were comprehensively discussed, including automated sample preparation strategies, immunoassays, molecular diagnosis, blood processing/testing, and microbe analysis. Emerging applications such as enzyme activity assessment and DNA storage were also explored. The performance of each bioassay was compared and discussed, providing insight into the novel design and applications of the DMF technology. Finally, the advantages, challenges, and future trends of DMF systems were systematically summarized, demonstrating new perspectives on the extensive applications of DMF in basic research and commercialization.

Smart enzyme catalysts capable of self-separation by sensing the reaction extent

A smart biocatalyst should dissolve homogeneously for catalysis and recover spontaneously at the end of the reaction. In this study, we present a strategy for preparing self-precipitating enzyme catalysts by exploiting reaction-induced pH decreases, which connect the reaction extent to the catalyst aggregation state. Using poly(methacrylic acid)-functionalized gold nanoparticles as carriers, we construct smart catalysts with three model systems, including the glucose oxidase (GOx)-catalase (CAT) cascade, the alcohol dehydrogenase (ADH)-glucose dehydrogenase (GDH) cascade, and a combination of two lipases. All smart catalysts can self-separate with a nearly 100% recovery efficiency when a certain conversion threshold is reached. The threshold can be adjusted depending on the reaction demand and buffer capacity. By monitoring the optical signals caused by the dissolution/precipitation of smart catalysts, we propose a prototypic automation system that may enable unsupervised batch/fed-batch bioprocessing.

Loading characteristics of streptavidin on polypropylene capillary channeled polymer fibers and capture performance towards biotinylated proteins

The development of higher-throughput, potentially lower-cost means to isolate proteins, for a variety of end uses, is of continuing emphasis. Polypropylene (PP) capillary-channeled polymer (C-CP) fiber columns are modified with the biotin-binding protein streptavidin (SAV) to capture biotinylated proteins. The loading characteristics of SAV on fiber supports were determined using breakthrough curves and frontal analysis. Based on adsorption data, a 3-min on-column loading at a flow rate of 0.5 mL min-1 (295.2 cm h-1) with a SAV feed concentration of 0.5 mg mL-1 produces a SAV loading capacity of 1.4 mg g-1 fiber. SAV has an incredibly high affinity for the small-molecule biotin (10-14 M), such that this binding relationship can be exploited by labeling a target protein with biotin via an Avi-tag. To evaluate the capture of the biotinylated proteins on the modified PP surface, the biotinylated versions of bovine serum albumin (b-BSA) and green fluorescent protein (b-GFP) were utilized as probe species. The loading buffer composition and flow rate were optimized towards protein capture. The non-ionic detergent Tween-20 was added to the deposition solutions to minimize non-specific binding. Values of 0.25-0.50% (v/v) Tween-20 in PBS exhibited better capture efficiency, while minimizing the non-specific binding for b-BSA and b-GFP, respectively. The C-CP fiber platform has the potential to provide a fast and low-cost method to capture targeted proteins for applications including protein purification or pull-down assays.

An Analysis of the Biotin-(Strept)avidin System in Immunoassays: Interference and Mitigation Strategies

An immunoassay is an analytical test method in which analyte quantitation is based on signal responses generated as a consequence of an antibody-antigen interaction. They are the method of choice for the measurement of a large panel of diagnostic markers. Not only are they fully automated, allowing for a short turnaround time and high throughput, but offer high sensitivity and specificity with low limits of detection for a wide range of analytes. Many immunoassay manufacturers exploit the extremely high affinity of biotin for streptavidin in their assay design architectures as a means to immobilize and detect analytes of interest. The biotin-(strept)avidin system is, however, vulnerable to interference with high levels of supplemental biotin that may cause elevated or suppressed test results. Since this system is heavily applied in clinical diagnostics, biotin interference has become a serious concern, prompting the FDA to issue a safety report alerting healthcare workers and the public about the potential harm of ingesting high levels of supplemental biotin contributing toward erroneous diagnostic test results. This review includes a general background and historical prospective of immunoassays with a focus on the biotin-streptavidin system, interferences within the system, and what mitigations are applied to minimize false diagnostic results.

Digital Microfluidic Multiplex RT-qPCR for SARS-CoV-2 Detection and Variants Discrimination

Continuous mutations have occurred in the genome of the SARS-CoV-2 virus since the onset of the COVID-19 pandemic. The increased transmissibility of the mutated viruses has not only imposed medical burdens but also prolonged the duration of the pandemic. A point-of-care (POC) platform that provides multitarget detection will help to track and reduce disease transmissions. Here we detected and discriminated three genotypes of SARS-CoV-2, including the wildtype and two variants of concern (VOCs), the Delta variant and Omicron variant, through reverse transcription quantitative polymerase chain reaction (RT-qPCR) on a digital microfluidics (DMF)-based cartridge. Upon evaluating with the RNA samples of Omicron variant, the DMF RT-qPCR presented a sensitivity of 10 copies/μL and an amplification efficiency of 96.1%, capable for clinical diagnosis. When spiking with SARS-CoV-2 RNA (wildtype, Delta variant, or Omicron variant) and 18S rDNA, the clinical analog samples demonstrated accurate detection and discrimination of different SARS-CoV-2 strains in 49 min.

Magnetic Beads inside Droplets for Agitation and Splitting Manipulation by Utilizing a Magnetically Actuated Platform

We successfully developed a platform for the magnetic manipulation of droplets containing magnetic beads and examined the washing behaviors of the droplets, including droplet transportation, magnetic bead agitation inside droplets, and separation from parent droplets. Magnetic field gradients were produced with two layers of 6 × 1 planar coils fabricated by using printed circuit board technology. We performed theoretical analyses to understand the characteristics of the coils and successfully predicted the magnetic field and thermal temperature of a single coil. We then investigated experimentally the agitation and splitting kinetics of the magnetic beads inside droplets and experimentally observed the washing performance in different neck-shaped gaps. The performance of the washing process was evaluated by measuring both the particle loss ratio and the optical density. The findings of this work will be used to design a magnetic-actuated droplet platform, which will separate magnetic beads from their parent droplets and enhance washing performance. We hope that this study will provide digital microfluidics for application in point-of-care testing. The developed microchip will be of great benefit for genetic analysis and infectious disease detection in the future.

Shaking Device for Homogeneous Dispersion of Magnetic Beads in Droplet Microfluidics

Magnetic beads (or particles) having a size between 1 and 5 µm are largely used in many biochemical assays devoted to both purification and quantification of cells, nucleic acids, or proteins. Unfortunately, the use of these beads within microfluidic devices suffers from natural precipitation because of their size and density. The strategies applied thus far to cells or polymeric particles cannot be extended to magnetic beads, mainly due to their magnetization and their higher densities. We report an effective shaking device capable of preventing the sedimentation of beads that are stored in a custom PCR tube. After the characterization of the operating principle, the device is validated for magnetic beads in droplets, leading to an equal distribution between the droplets, barely affecting their generation.

Digital detection of proteins

This paper reviews methods for detecting proteins based on molecular digitization, i.e., the isolation and detection of single protein molecules or singulated ensembles of protein molecules. The single molecule resolution of these methods has resulted in significant improvements in the sensitivity of immunoassays beyond what was possible using traditional "analog" methods: the sensitivity of some digital immunoassays approach those of methods for measuring nucleic acids, such as the polymerase chain reaction (PCR). The greater sensitivity of digital protein detection has resulted in immuno-diagnostics with high potential societal impact, e.g., the early diagnosis and therapeutic intervention of Alzheimer's Disease. In this review, we will first provide the motivation for developing digital protein detection methods given the limitations in the sensitivity of analog methods. We will describe the paradigm shift catalyzed by single molecule detection, and will describe in detail one digital approach - which we call digital bead assays (DBA) - based on the capture and labeling of proteins on beads, identifying "on" and "off" beads, and quantification using Poisson statistics. DBA based on the single molecule array (Simoa) technology have sensitivities down to attomolar concentrations, equating to ∼10 proteins in a 200 μL sample. We will describe the concept behind DBA, the different single molecule labels used, the ways of analyzing beads (imaging of arrays and flow), the binding reagents and substrates used, and integration of these technologies into fully automated and miniaturized systems. We provide an overview of emerging approaches to digital protein detection, including those based on digital detection of nucleic acids labels, single nanoparticle detection, measurements using nanopores, and methods that exploit the kinetics of single molecule binding. We outline the initial impact of digital protein detection on clinical measurements, highlighting the importance of customized assay development and translational clinical research. We highlight the use of DBA in the measurement of neurological protein biomarkers in blood, and how these higher sensitivity methods are changing the diagnosis and treatment of neurological diseases. We conclude by summarizing the status of digital protein detection and suggest how the lab-on-a-chip community might drive future innovations in this field.

DNA-Encoded Immunoassay in Picoliter Drops: A Minimal Cell-Free Approach

Immunoassays have emerged as indispensable bioanalytical tools but necessitate long preliminary steps for the selection, production, and purification of the antibody(ies) to be used. Here is explored the paradigm shift of creating a rapid and purification-free assay in picoliter drops where the antibody is expressed from coding DNA and its binding to antigens concomitantly characterized in situ. Efficient synthesis in bulk of various functional variable domains of heavy-chain only antibodies (VHH) using reconstituted cell-free expression media, including an anti-green fluorescent protein VHH, is shown first. A microfluidic device is then used to generate monodisperse drops (30 pL) containing all the assay components, including a capture scaffold, onto which the accumulation of VHH:antigen produces a specific fluorescent signal. This allows to assess, in parallel or sequentially at high throughput (500 Hz), the VHH-antigen binding and its specificity in less than 3 h, directly from a VHH-coding DNA, for multiple VHH sequences, various antigens and down to DNA concentrations as low as 12 plasmids per drop. It is anticipated that the ultraminiaturized format, robustness, and programmability of this novel cell-free immunoassay concept will constitute valuable assets in fields as diverse as antibody discovery, point-of-care diagnostics, synthetic biology, and/or bioanalytical assays.

Digital microfluidics for biological analysis and applications

Digital microfluidics (DMF) is an emerging liquid-handling technology based on arrays of microelectrodes for the precise manipulation of discrete droplets. DMF offers the benefits of automation, addressability, integration and dynamic configuration ability, and provides enclosed picoliter-to-microliter reaction space, making it suitable for lab-on-a-chip biological analysis and applications that require high integration and intricate processes. A review of DMF bioassays with a special emphasis on those actuated by electrowetting on dielectric (EWOD) force is presented here. Firstly, a brief introduction is presented on both the theory of EWOD actuation and the types of droplet motion. Subsequently, a comprehensive overview of DMF-based biological analysis and applications, including nucleic acid, protein, immunoreaction and cell assays, is provided. Finally, a discussion on the strengths, challenges, and potential applications and perspectives in this field is presented.

Combining sensors and actuators with electrowetting-on-dielectric (EWOD): advanced digital microfluidic systems for biomedical applications

The concept of digital microfluidics (DMF) enables highly flexible and precise droplet manipulation at a picoliter scale, making DMF a promising approach to realize integrated, miniaturized "lab-on-a-chip" (LOC) systems for research and clinical purposes. Owing to its simplicity and effectiveness, electrowetting-on-dielectric (EWOD) is one of the most commonly studied and applied effects to implement DMF. However, complex biomedical assays usually require more sophisticated sample handling and detection capabilities than basic EWOD manipulation. Alternatively, combined systems integrating EWOD actuators and other fluidic handling techniques are essential for bringing DMF into practical use. In this paper, we briefly review the main approaches for the integration/combination of EWOD with other microfluidic manipulation methods or additional external fields for specified biomedical applications. The form of integration ranges from independently operating sub-systems to fully coupled hybrid actuators. The corresponding biomedical applications of these works are also summarized to illustrate the significance of these innovative combination attempts.

Recent progress of microfluidic chips in immunoassay

Microfluidic chip technology is a technology platform that integrates basic operation units such as processing, separation, reaction and detection into microchannel chip to realize low consumption, fast and efficient analysis of samples. It has the characteristics of small volume need of samples and reagents, fast analysis, low cost, automation, portability, high throughout, and good compatibility with other techniques. In this review, the concept, preparation materials and fabrication technology of microfluidic chip are described. The applications of microfluidic chip in immunoassay, including fluorescent, chemiluminescent, surface-enhanced Raman spectroscopy (SERS), and electrochemical immunoassay are reviewed. Look into the future, the development of microfluidic chips lies in point-of-care testing and high throughput equipment, and there are still some challenges in the design and the integration of microfluidic chips, as well as the analysis of actual sample by microfluidic chips.

Spatial barcoding-enabled highly multiplexed immunoassay with digital microfluidics

Digital microfluidics (DMF), facilitating independent manipulation of microliter samples, provides an ideal platform for immunoassay detection; however, suffering limited multiplexity. To address the need, herein we described a digital microfluidics (DMF) platform that realizes spatial barcoding on the Teflon-coated indium tin oxide (ITO) glass side to fulfill highly multiplexed immunoassay (10+) with low-volume samples (∼4 μL) in parallel, representing the highest multiplexing recorded to date for DMF-actuated immunoassay. Planar-based spatial immobilization of multiple capture antibodies was realized on a Teflon-coated ITO glass side, which was then used as the top plate of the DMF device. Droplets containing analytes, secondary antibodies, and fluorescent signaling reporters with low volume, which were electrically manipulated by our DMF control system, were shuttled sequentially along the working electrodes to complete the immuno-reaction. Evaluation of platform performance with recombinant proteins showed excellent sensitivity and reproducibility. To test the feasibility of our platform in analyzing multiplex biomarkers of the immune response, we used lipopolysaccharide-stimulated macrophages as a model system for protein secretion dynamics studies. As a result, temporal profiling of pro-inflammatory cytokine secretion dynamics was obtained. The spatial barcoding strategy presented here is easy-to-operate to enable a more comprehensive evaluation of protein abundance from biological samples, paving the way for new opportunities to realize multiplexity-associated applications with the DMF platform.

Bead Number Effect in a Magnetic-Beads-Based Digital Microfluidic Immunoassay

In a biomedical diagnosis with a limited sample volume and low concentration, droplet-based microfluidics, also called digital microfluidics, becomes a very attractive approach. Previously, our group developed a magnetic-beads-based digital microfluidic immunoassay with a bead number of around 100, requiring less than 1 μL of sample volume to achieve a pg/mL level limit of detection (LOD). However, the bead number in each measurement was not the same, causing an unstable coefficient of variation (CV) in the calibration curve. Here, we investigated whether a fixed number of beads in this bead-based digital microfluidic immunoassay could provide more stable results. First, the bead screening chips were developed to extract exactly 100, 49, and 25 magnetic beads with diameters of less than 6 μm. Then, four calibration curves were established. One calibration curve was constructed by using varying bead numbers (50–160) in the process. The other three calibration curves used a fixed number of beads, (100, 49, and 25). The results indicated that the CVs for a fixed number of beads were evidently smaller than the CVs for varying bead numbers, especially in the range of 1 pg/mL to 100 pg/mL, where the CVs for 100 beads were less than 10%. Furthermore, the calculated LOD, based on the composite calibration curves, could be reduced by three orders, from 3.0 pg/mL (for the unfixed bead number) to 0.0287 pg/mL (for 100 beads). However, when the bead numbers were too high (more than 500) or too low (25 or fewer), the bead manipulation for aggregation became more difficult in the magnetic-beads-based digital microfluidic immunoassay chip.

Digital Microfluidic qPCR Cartridge for SARS-CoV-2 Detection

Point-of-care (POC) tests capable of individual health monitoring, transmission reduction, and contact tracing are especially important in a pandemic such as the coronavirus disease 2019 (COVID-19). We develop a disposable POC cartridge that can be mass produced to detect the SARS-CoV-2 N gene through real-time quantitative polymerase chain reaction (qPCR) based on digital microfluidics (DMF). Several critical parameters are studied and improved, including droplet volume consistency, temperature uniformity, and fluorescence intensity linearity on the designed DMF cartridge. The qPCR results showed high accuracy and efficiency for two primer-probe sets of N1 and N2 target regions of the SARS-CoV-2 N gene on the DMF cartridge. Having multiple droplet tracks for qPCR, the presented DMF cartridge can perform multiple tests and controls at once.

Magnetic Nanorobots as Maneuverable Immunoassay Probes for Automated and Efficient Enzyme Linked Immunosorbent Assay

As a typical, classical, but powerful biochemical sensing technology in analytical chemistry, enzyme-linked immunosorbent assay (ELISA) shows excellence and wide practicability for quantifying analytes of ultralow concentration. However, long incubation time and burdensome laborious multistep washing processes make it inefficient and labor-intensive for conventional ELISA. Here, we propose rod-like magnetically driven nanorobots (MNRs) for use as maneuverable immunoassay probes that facilitate a strategy for an automated and highly efficient ELISA analysis, termed nanorobots enabled ELISA (nR-ELISA). To prepare the MNRs, the self-assembled chains of Fe₃O₄ magnetic particles are chemically coated with a thin layer of rigid silica oxide (SiO₂), onto which capture antibody (Ab1) is grafted to further achieve magnetically maneuverable immunoassay probes (MNR-Ab1s). We investigate the fluid velocity distribution around the MNRs at microscale using numerical simulation and empirically identify the mixing efficiency of the actively rotating MNRs. To automate the analysis process, we design and fabricate by 3-D printing a detection unit consisting of three function wells. The MNR-Ab1s can be steered into different function wells for required reaction or wishing process. The actively rotating MNR-Ab1s can enhance the binding efficacy with target analytes at microscale and greatly decrease incubation time. The integrated nR-ELISA system can significantly reduce the assay time, more importantly during which process manpower input is greatly minimized. Our simulation of the magnetic field distribution generated by Helmholtz coils demonstrates that our approach can be scaled up, which proves the feasibility of using current strategy to construct high throughput nR-ELISA detection instrument. This work of taking magnetic micro/nanobots as active immunoassay probes for automatic and efficient ELISA not only holds great potential for point-of-care testing (POCT) in future but also extends the practical applications of self-propelled micro/nanorobots into the field of analytical chemistry.

Handheld Microfluidic Filtration Platform Enables Rapid, Low-Cost, and Robust Self-Testing of SARS-CoV-2 Virus

Here, a novel microfluidic test kit combining ultrahigh throughput hydrodynamic filtration and sandwich immunoassay is reported. Specifically, nano and microbeads coated with two different, noncompetitive antibodies, are used to capture the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) nucleocapsid (N) proteins simultaneously, forming larger complexes. Microfluidic filtration discards free nanobeads but retains antigen-bridged complexes in the observation zone, where a display of red color indicates the presence of antigen in the sample. This testing platform exhibits high throughput separation (<30 s) and enrichment of antigen that exceeds the traditional lateral flow assays or microfluidic assays, with a low limit of detection (LoD) < 100 copies mL-1 . In two rounds of clinical trials conducted in December 2020 and August 2021, the assays demonstrate high sensitivities of 95.4% and 100%, respectively, which proves this microfluidic test kit is capable of detecting SARS-CoV-2 virus variants evolved over significant periods of time. Furthermore, the mass-produced chip can be fabricated at a cost of $0.98/test and the robust design allows the chip to be reused for over 50 times. All of these features make the microfluidic test kit particularly suitable for areas with inadequate medical infrastructure and a shortage of laboratory resources.

Single B cell technologies for monoclonal antibody discovery

Monoclonal antibodies (mAbs) are often selected from antigen-specific single B cells derived from different hosts, which are notably short-lived in ex vivo culture conditions and hence, arduous to interrogate. The development of several new techniques and protocols has facilitated the isolation and retrieval of antibody-coding sequences of antigen-specific B cells by also leveraging miniaturization of reaction volumes. Alternatively, mAbs can be generated independently of antigen-specific B cells, comprising display technologies and, more recently, artificial intelligence-driven algorithms. Consequently, a considerable variety of techniques are used, raising the demand for better consolidation. In this review, we present and discuss the major techniques available to interrogate antigen-specific single B cells to isolate antigen-specific mAbs, including their main advantages and disadvantages.

Emerging Biosensors to Detect Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2): A Review

Coronavirus disease (COVID-19) is a global health crisis caused by the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). Real-time reverse transcriptase-polymerase chain reaction (RT-PCR) is the gold standard test for diagnosing COVID-19. Although it is highly accurate, this lab test requires highly-trained personnel and the turn-around time is long. Rapid and inexpensive immuno-diagnostic tests (antigen or antibody test) are available, but these point of care (POC) tests are not as accurate as the RT-PCR test. Biosensors are promising alternatives to these rapid POC tests. Here we review three types of recently developed biosensors for SARS-CoV-2 detection: surface plasmon resonance (SPR)-based, electrochemical and field-effect transistor (FET)-based biosensors. We explain the sensing principles and discuss the advantages and limitations of these sensors. The accuracies of these sensors need to be improved before they could be translated into POC devices for commercial use. We suggest potential biorecognition elements with highly selective target-analyte binding that could be explored to increase the true negative detection rate. To increase the true positive detection rate, we suggest two-dimensional materials and nanomaterials that could be used to modify the sensor surface to increase the sensitivity of the sensor.

Digital immunoassay for biomarker concentration quantification using solid-state nanopores

Single-molecule counting is the most accurate and precise method for determining the concentration of a biomarker in solution and is leading to the emergence of digital diagnostic platforms enabling precision medicine. In principle, solid-state nanopores—fully electronic sensors with single-molecule sensitivity—are well suited to the task. Here we present a digital immunoassay scheme capable of reliably quantifying the concentration of a target protein in complex biofluids that overcomes specificity, sensitivity, and consistency challenges associated with the use of solid-state nanopores for protein sensing. This is achieved by employing easily-identifiable DNA nanostructures as proxies for the presence (“1”) or absence (“0”) of the target protein captured via a magnetic bead-based sandwich immunoassay. As a proof-of-concept, we demonstrate quantification of the concentration of thyroid-stimulating hormone from human serum samples down to the high femtomolar range. Further optimization to the method will push sensitivity and dynamic range, allowing for development of precision diagnostic tools compatible with point-of-care format.

The concentration of a biomarker in solution can be determined by counting single molecules. Here the authors report a digital immunoassay scheme with solid-state nanopore readout to quantify a target protein and use this to measure thyroid-stimulating hormone from human serum.

High Resolution Patterning of an Organic-Inorganic Photoresin for the Fabrication of Platinum Microstructures

Platinum (Pt) is an interesting material for many applications due to its high chemical resilience, outstanding catalytic activity, high electrical conductivity, and high melting point. However, microstructuring and especially 3D microstructuring of platinum is a complex process, based on expensive and specialized equipment often suffering from very slow processing speeds. In this work, organic-inorganic photoresins, which can be structured using direct optical lithography as well as two-photon lithography (TPL) with submicrometer resolution and high-throughput is presented. The printed structures are subsequently converted to high-purity platinum using thermal debinding of the binder and reduction of the salt. With this technique, complex 3D structures with a 3D resolution of 300 nm were fabricated. At a layer thickness of 35 nm, the patterns reach a high conductivity of 67% compared to bulk platinum. Microheaters, thermocouple sensors as well as a Lab-on-a-Chip system are presented as exemplary applications. This technology will enable a broad range of application from electronics, sensing and heating elements to 3D photonics and metamaterials.

"One-to-three" droplet generation in digital microfluidics for parallel chemiluminescence immunoassays

In digital microfluidics, droplet generation is a fundamental operation for quantitative liquid manipulation. The generation of well-defined micro-droplets on a chip with restricted device geometries has become a real obstacle for digital microfluidics platforms to be used in parallel for in vitro diagnostic applications. Here, we propose a "one-to-three" droplet splitting technique that is able to generate sub-microlitre droplets beyond the "well-known" geometry limit in electrowetting-on-dielectric digital microfluidics. Accordingly, we realized an on-chip magnetic bead chemiluminescence immunoassay for parallel detection with the "one-to-three" technique. With the help of the generated micro droplets, we were able to retain the magnetic beads by a significantly reduced magnetic force. We have shown the detection of five B-type natriuretic peptide analyte samples on a single chip for around 10 minutes. The correlation coefficient of the calibration curve was 0.9942, and the detection limit was lower than 5 pg mL^-1.

Application of Microfluidics in Immunoassay: Recent Advancements

In recent years, point-of-care testing has played an important role in immunoassay, biochemical analysis, and molecular diagnosis, especially in low-resource settings. Among various point-of-care-testing platforms, microfluidic chips have many outstanding advantages. Microfluidic chip applies the technology of miniaturizing conventional laboratory which enables the whole biochemical process including reagent loading, reaction, separation, and detection on the microchip. As a result, microfluidic platform has become a hotspot of research in the fields of food safety, health care, and environmental monitoring in the past few decades. Here, the state-of-the-art application of microfluidics in immunoassay in the past decade will be reviewed. According to different driving forces of fluid, microfluidic platform is divided into two parts: passive manipulation and active manipulation. In passive manipulation, we focus on the capillary-driven microfluidics, while in active manipulation, we introduce pressure microfluidics, centrifugal microfluidics, electric microfluidics, optofluidics, magnetic microfluidics, and digital microfluidics. Additionally, within the introduction of each platform, innovation of the methods used and their corresponding performance improvement will be discussed. Ultimately, the shortcomings of different platforms and approaches for improvement will be proposed.

Tumor-Induced Inflammatory Cytokines and the Emerging Diagnostic Devices for Cancer Detection and Prognosis

Chronic inflammation generated by the tumor microenvironment is known to drive cancer initiation, proliferation, progression, metastasis, and therapeutic resistance. The tumor microenvironment promotes the secretion of diverse cytokines, in different types and stages of cancers. These cytokines may inhibit tumor development but alternatively may contribute to chronic inflammation that supports tumor growth in both autocrine and paracrine manners and have been linked to poor cancer outcomes. Such distinct sets of cytokines from the tumor microenvironment can be detected in the circulation and are thus potentially useful as biomarkers to detect cancers, predict disease outcomes and manage therapeutic choices. Indeed, analyses of circulating cytokines in combination with cancer-specific biomarkers have been proposed to simplify and improve cancer detection and prognosis, especially from minimally-invasive liquid biopsies, such as blood. Additionally, the cytokine signaling signatures of the peripheral immune cells, even from patients with localized tumors, are recently found altered in cancer, and may also prove applicable as cancer biomarkers. Here we review cytokines induced by the tumor microenvironment, their roles in various stages of cancer development, and their potential use in diagnostics and prognostics. We further discuss the established and emerging diagnostic approaches that can be used to detect cancers from liquid biopsies, and additionally the technological advancement required for their use in clinical settings.

An electrochemical method for a rapid and sensitive immunoassay on digital microfluidics with integrated indium tin oxide electrodes coated on a PET film

Electrochemical detection is the simplest analytical tool to be integrated into digital microfluidics (DMF). It offers the advantages of small size, with detector electrodes incorporated into the device by patterning, and high compatibility with portable analytical instruments. Indium tin oxide (ITO) coated on glass has been commonly used for the top plate of DMF due to its good conductivity and transparency. However, instability and the low current response of ITO electrodes patterned on glass hindered their application for immunoassays. It has been reported that ITO coated on polyethylene terephthalate (PET) has better conductivity, owing to its higher carrier concentration, faster mobility and lower resistivity. Herein, we investigated the use of ITO electrodes patterned on PET film as the top plate of DMF for a simple and stable electrochemical immunoassay using square wave voltammetry (SWV), with an excellent peak resolution and high sensitivity. A magnetic bead-based immunoassay for H5N1 antigen was performed on a DMF platform with a limit of detection of 0.6 ng mL^-1 in buffer and 18 ng mL^-1 in human serum. These results showed the good electrochemical performance of ITO coated on a PET film, a lightweight, shock resistant and cost-effective material, which is promising for DMF fabrication and transparent electrodes for various electroanalytical methods.

Magnetic nanoparticles in microfluidics-based diagnostics: an appraisal

The use of magnetic nanoparticles (MNPs) in microfluidics based diagnostics is a classic case of micro-, nano- and bio-technology coming together to design extremely controllable, reproducible, and scalable nano and micro 'on-chip bio sensing systems.' In this review, applications of MNPs in microfluidics ranging from molecular diagnostics and immunodiagnostics to clinical uses have been examined. In addition, microfluidic mixing and capture of analytes using MNPs, and MNPs as carriers in microfluidic devices has been investigated. Finally, the challenges and future directions of this upcoming field have been summarized. The use of MNP-based microfluidic devices, will help in developing decentralized or 'point of care' testing globally, contributing to affordable healthcare, particularly, for middle- and low-income developing countries.

Composable Microfluidic Plates (cPlate): A Simple and Scalable Fluid Manipulation System for Multiplexed Enzyme-Linked Immunosorbent Assay (ELISA)

Enzyme-linked immunosorbent assay (ELISA) is the gold standard method for protein biomarkers. However, scaling up ELISA for multiplexed biomarker analysis is not a trivial task due to the lengthy procedures for fluid manipulation and high reagent/sample consumption. Herein, we present a highly scalable multiplexed ELISA that achieves a similar level of performance to commercial single-target ELISA kits as well as shorter assay time, less consumption, and simpler procedures. This ELISA is enabled by a novel microscale fluid manipulation method, composable microfluidic plates (cPlate), which are comprised of miniaturized 96-well plates and their corresponding channel plates. By assembling and disassembling the plates, all of the fluid manipulations for 96 independent ELISA reactions can be achieved simultaneously without any external fluid manipulation equipment. Simultaneous quantification of four protein biomarkers in serum samples is demonstrated with the cPlate system, achieving high sensitivity and specificity (∼ pg/mL), short assay time (∼1 h), low consumption (∼5 μL/well), high scalability, and ease of use. This platform is further applied to probe the levels of three protein biomarkers related to vascular dysfunction under pulmonary nanoparticle exposure in rat's plasma. Because of the low cost, portability, and instrument-free nature of the cPlate system, it will have great potential for multiplexed point-of-care testing in resource-limited regions.

Ultrasensitive Detection of Attomolar Protein Concentrations by Dropcast Single Molecule Assays

Measurements of very low levels of biomolecules, including proteins and nucleic acids, remain a critical challenge in many clinical diagnostic applications due to insufficient sensitivity. While digital measurement methods such as Single Molecule Arrays (Simoa), or digital ELISA, have made significant advances in sensitivity, there are still many potential disease biomarkers that exist in accessible biofluids at levels below the detection limits of these techniques. To overcome this barrier, we have developed a simple strategy for single molecule counting, dropcast single molecule assays (dSimoa), that enables more target molecules to be counted through increased sampling efficiency and with a simpler workflow. In this approach, beads are simply dropcast onto a microscope slide and dried into a monolayer film for digital signal readout. The dSimoa platform achieves attomolar limits of detection, with an up to 25-fold improvement in sensitivity over Simoa, the current state of the art for ultrasensitive protein detection. Furthermore, due to its simple readout process and improved cost-effectiveness compared to existing digital bioassays, dSimoa increases amenability to integration into point-of-care platforms. As an illustration of the potential utility of dSimoa, we demonstrate its ability to measure previously undetectable levels of Brachyury, a tissue biomarker for chordoma, in plasma samples. With its significantly enhanced sensitivity and simplicity, dSimoa can pave the way toward the discovery of new biomarkers for early disease diagnosis and improved health outcomes.

Auto-affitech: an automated ligand binding affinity evaluation platform using digital microfluidics with a bidirectional magnetic separation method

The dissociation constant (Kd) is a crucial parameter for characterizing binding affinity in molecular recognition, including antigen-antibody, DNA-protein, and receptor-ligand interactions. However, conventional methods for Kd characterization usually involve a multi-step process and time-consuming operations for incubation, washing, and detection, thus causing problems, such as time delays, microbead loss, degradation of sensitive molecules, and personal errors. Here we demonstrate an automated ligand binding affinity evaluation platform (Auto-affitech) using digital microfluidics (DMF), with individual droplets at the microliter level, programmed to rapidly perform the incubation and separation of target-beads and binding ligands. Because the loss of the beads influences the detection results, we propose a new strategy for magnetic bead separation using DMF, termed the bidirectional separation method. By splitting one droplet into two asymmetric droplets, high bead retention efficiency (89.57% ± 0.05%) and high washing efficiency (99.59% ± 0.17%, with four washings) were obtained. We demonstrate the determination of Kd of an aptamer-protein system (EpCAM and its corresponding aptamer SYL3C) and an antigen-antibody system (H5N1 antigen and antibody), proving the capability and universality of Auto-affitech in various receptor-ligand systems. Integrating all the sample processing procedures, the Auto-affitech not only saves manual labor and minimizes personal errors, but also conserves samples and shortens analysis time. Overall, this platform successfully demonstrates to be an automated approach for dissociation constant evaluation and exhibits great potential for highly efficient screening of ligands.

Engineering magnetic nanoparticles and their integration with microfluidics for cell isolation

Isolation of cancer cells, bacteria, and viruses from peripheral blood has important applications in cancer diagnosis, therapy monitoring, and drug development. Magnetic particles functionalized with antibodies that target receptors of cancer cells have been shown to isolate such entities using magnetic field gradients. Here, we report enhancement in capture efficiency and specificity by engineering magnetic nanoparticles and integrating them with microfluidics for the enumeration of tumor cells. Nanoparticles were made from iron oxide, coated with poly(ethylene glycol), and conjugated through avidin-biotin chemistry with antibody specifically against epithelial cell adhesion molecule (EpCAM). On exposure of targeted nanoparticles to tumor cells, specific uptake by EpCAM-expressing tumor cells (e.g., BxPC3, a pancreatic cancer cell) was observed, whereas there was negligible uptake by cells with low EpCAM expression (e.g., CCRF-CEM, a leukemia cell). Using an arrangement of magnets called a Halbach array, capture efficiency and specificity towards BxPC3 cells tagged with magnetic nanoparticles were enhanced, compared to conditions without the magnetic field gradient and/or without magnetic nanoparticles, either in buffer or in whole blood. These results illustrate that engineered magnetic nanoparticles and their integration with microfluidics have great potential for tumor cell enumeration and cancer prognosis.

Single-Cell Sequencing of T cell Receptors: A Perspective on the Technological Development and Translational Application

T cells recognize peptides bound to major histocompatibility complex (MHC) class I and class II molecules at the cell surface. This recognition is accomplished by the expression of T cell receptors (TCR) which are required to be diverse and adaptable in order to accommodate the various and vast number of antigens presented on the MHCs. Thus, determining TCR repertoires of effector T cells is necessary to understand the immunological process in responding to cancer progression, infection, and autoimmune development. Furthermore, understanding the TCR repertoires will provide a solid framework to predict and test the antigen which is more critical in autoimmunity. However, it has been a technical challenge to sequence the TCRs and provide a conceptual context in correlation to the vast number of TCR repertoires in the immunological system. The exploding field of single-cell sequencing has changed how the repertoires are being investigated and analyzed. In this review, we focus on the biology of TCRs, TCR signaling and its implication in autoimmunity. We discuss important methods in bulk sequencing of many cells. Lastly, we explore the most pertinent platforms in single-cell sequencing and its application in autoimmunity.

Digital Microfluidic Platform to Maximize Diagnostic Tests with Low Sample Volumes from Newborns and Pediatric Patients

"Children are not tiny adults" is an adage commonly used in pediatrics to emphasize the fact that children often have different physiological responses to sickness and trauma compared to adults. However, despite widespread acceptance of this concept, diagnostic blood testing is an excellent example of clinical care that is not yet customized to the needs of children, especially newborns. Cumulative blood loss resulting from clinical testing does not typically impact critically ill adult patients, but can quickly escalate in children, leading to iatrogenic anemia and related comorbidities. Moreover, the tests prioritized for rapid, near-patient testing in adults are not always the most clinically relevant tests for children or newborns. This report describes the development of a digital microfluidic testing platform and associated clinical assays purposely curated to address current shortcomings in pediatric laboratory testing by using microliter volumes (<50 µL) of samples. The automated platform consists of a small instrument and single-use cartridges, which contain all reagents necessary to prepare the sample and perform the assay. Electrowetting technology is used to precisely manipulate nanoliter-sized droplets of samples and reagents inside the cartridge. To date, we have automated three disparate types of assays (biochemical assays, immunoassays, and molecular assays) on the platform and have developed over two dozen unique tests, each with important clinical application to newborns and pediatric patients. Cell lysis, plasma preparation, magnetic bead washing, thermocycling, incubation, and many other essential functions were all performed on the cartridge without any user intervention. The resulting assays demonstrate performance comparable to standard clinical laboratory assays and are economical due to the reduced hands-on effort required for each assay and lower overall reagent consumption. These capabilities allow a wide range of assays to be run simultaneously on the same cartridge using significantly reduced sample volumes with results in minutes.

Microbioreactors for Process Development and Cell-Based Screening Studies

Microbioreactors (MBRs) have emerged as potent cultivation devices enabling automated small-scale experiments in parallel while enhancing their cost efficiency. The widespread use of MBRs has contributed to recent advances in industrial and pharmaceutical biotechnology, and they have proved to be indispensable tools in the development of many modern bioprocesses. Being predominantly applied in early stage process development, they open up new fields of research and enhance the efficacy of biotechnological product development. Their reduced reaction volume is associated with numerous inherent advantages - particularly the possibility for enabling parallel screening operations that facilitate high-throughput cultivations with reduced sample consumption (or the use of rare and expensive educts). As a result, multiple variables can be examined in a shorter time and with a lower expense. This leads to a simultaneous acceleration of research and process development along with decreased costs.MBRs range from simple miniaturized cultivations vessels (i.e., in the milliliter scale with limited possibilities for process control) to highly complex and automated small-scale microreactors with integrated sensors that allow for comprehensive screenings in very short time or a precise reflection of large-scale cultivation conditions. Progressive developments and improvements in manufacturing and automation techniques are already helping researchers to make use of the advantages that MBRs offer. This overview of current MBR systems surveys the diverse application for microbial and mammalian cell cultivations that have been developed in recent years.

Simultaneous detection of two growth factors from human single-embryo culture medium by a bead-based digital microfluidic chip

The measurement of growth factors released in a culture medium is considered to be an attractive non-invasive approach, apart from the embryo morphology, to identify the condition of an embryo development after fertilization in vitro (IVF), but the available embryo culture medium in the current method is only a few microlitres. This small sample volume, also of small concentration, makes difficult the application of a conventional detection method, such as an enzyme-linked immunosorbent assay (ELISA). A reliable detection of the growth factor from each embryo culture medium of such a small concentration hence remains a challenge. Here for the first time we report the results of measurement of not just one, but two, growth factors, human IL-1β and human TNF-α, from an individual droplet of embryo culture medium with a bead-based digital microfluidic chip. The required sample volume for a single measurement is only 520 nL; the total duration of the on-chip process is less than 40 min. Using the culture media of human embryos with normal morphologic features, we found that the concentrations of TNF-α change little from day 3 to day 5-6, but the concentrations of IL-1β for some embryos might double from day 3 to day 5-6. For other embryos even with similar normal morphologic features, some growth factors, such as IL-1β, might exhibit different expressions during the culture period. Those growth factors could serve to distinguish the development conditions of each embryo, not merely from an observation of embryo morphology.

Liquid-Infused Surfaces: A Review of Theory, Design, and Applications

Due to inspiration from the Nepenthes pitcher plant, a frontier of devices has emerged with unmatched capabilities. Liquid-infused surfaces (LISs), particularly known for their liquid-repelling behavior under low tilting angles (<5°), have demonstrated a plethora of applications in medical, marine, energy, industrial, and environmental materials. This review presents recent developments of LIS technology and its prospective to define the future direction of this technology in solving tomorrow's real-life challenges. First, an introduction to the different models explaining the physical phenomena of these surfaces, their wettability, and viscous-dependent frictional forces is discussed. Then, an outline of different emerging strategies required to fabricate a stable liquid-infused interface is presented, including different substrates, lubricants, surface chemistries, and design parameters which can be tuned depending on the application. Furthermore, applications of LIS coatings in the areas of anticorrosion, antifouling, anti-icing, self-healing, droplet manipulation, and biomedical devices will be presented followed by the limitations and future direction of this technology.

Monitoring biomolecule concentrations in tissue using a wearable droplet microfluidic-based sensor

Knowing how biomarker levels vary within biological fluids over time can produce valuable insight into tissue physiology and pathology, and could inform personalised clinical treatment. We describe here a wearable sensor for monitoring biomolecule levels that combines continuous fluid sampling with in situ analysis using wet-chemical assays (with the specific assay interchangeable depending on the target biomolecule). The microfluidic device employs a droplet flow regime to maximise the temporal response of the device, using a screw-driven push-pull peristaltic micropump to robustly produce nanolitre-sized droplets. The fully integrated sensor is contained within a small (palm-sized) footprint, is fully autonomous, and features high measurement frequency (a measurement every few seconds) meaning deviations from steady-state levels are quickly detected. We demonstrate how the sensor can track perturbed glucose and lactate levels in dermal tissue with results in close agreement with standard off-line analysis and consistent with changes in peripheral blood levels.

Magnetic Nanoparticles Enhance Pore Blockage-Based Electrochemical Detection of a Wound Biomarker

A novel pore blockage-based electrochemical immunosensor based on the combination of 100 nm-magnetic nanoparticles (MNPs), as signal enhancers, and 200 nm-pore diameter nanoporous anodic alumina (NAA) membranes, as sensing platform, is reported. A peptide conjugate mimicking flightless I (Flii), a wound healing biomarker, was chosen as target analyte. The sensing platform consists of an anti-Flii antibody (Ab1)-modified NAA membrane attached onto a gold electrode. Anti-KLH antibody (Ab2)-modified MNPs (MNP-Ab2) were used to selectively capture the Flii peptide conjugate in solution. Sensing was based on pore blockage of the Ab1-modified NAA membrane caused upon specific binding of the MNP-Ab2-analyte complex. The degree of pore blockage, and thus the concentration of the Flii peptide conjugate in the sample, was measured as a reduction in the oxidation current of a redox species ([Fe(CN)6]4-) added in solution. We demonstrated that pore blockage is drastically enhanced by applying an external magnetic field at the membrane backside to facilitate access of the MNP-Ab2-analyte complex into the pores, and thus ensure its availability to bind to the Ab1-modified NAA membrane. Combining the pore blockage-based electrochemical magnetoimmunosensor with an externally applied magnetic field, a limit of detection (LOD) of 0.5 ng/ml of Flii peptide conjugate was achieved, while sensing in the absence of magnetic field could only attain a LOD of 1.2 μg/ml. The developed sensing strategy is envisaged as a powerful solution for the ultra-sensitive detection of an analyte of interest present in a complex matrix.

High density DNA data storage library via dehydration with digital microfluidic retrieval

DNA promises to be a high density data storage medium, but physical storage poses a challenge. To store large amounts of data, pools must be physically isolated so they can share the same addressing scheme. We propose the storage of dehydrated DNA spots on glass as an approach for scalable DNA data storage. The dried spots can then be retrieved by a water droplet using a digital microfluidic device. Here we show that this storage schema works with varying spot organization, spotted masses of DNA, and droplet retrieval dwell times. In all cases, the majority of the DNA was retrieved and successfully sequenced. We demonstrate that the spots can be densely arranged on a microfluidic device without significant contamination of the retrieval. We also demonstrate that 1 TB of data could be stored in a single spot of DNA and successfully retrieved using this method.

Micro-Electrode-Dot-Array Digital Microfluidic Biochips: Technology, Design Automation, and Test Techniques

Digital microfluidic biochips (DMFBs) are being increasingly used for DNA sequencing, point-of-care clinical diagnostics, and immunoassays. DMFBs based on a micro-electrode-dot-array (MEDA) architecture have recently been proposed, and fundamental droplet manipulations, e.g., droplet mixing and splitting, have also been experimentally demonstrated on MEDA biochips. There can be thousands of microelectrodes on a single MEDA biochip, and the fine-grained control of nanoliter volumes of biochemical samples and reagents is also enabled by this technology. MEDA biochips offer the benefits of real-time sensitivity, lower cost, easy system integration with CMOS modules, and full automation. This review paper first describes recent design tools for high-level synthesis and optimization of map bioassay protocols on a MEDA biochip. It then presents recent advances in scheduling of fluidic operations, placement of fluidic modules, droplet-size-aware routing, adaptive error recovery, sample preparation, and various testing techniques. With the help of these tools, biochip users can concentrate on the development of nanoscale bioassays, leaving details of chip optimization and implementation to software tools.

Heterogeneous Immunoassay Using Channels and Droplets in a Digital Microfluidic Platform

This work presents a heterogeneous immunoassay using the integrated functionalities of a channel and droplets in a digital microfluidic (DMF) platform. Droplet functionality in DMF allows for the programmable manipulation of discrete sample and reagent droplets in the range of nanoliters. Pressure-driven channels become advantageous over droplets when sample must be washed, as the supernatant can be thoroughly removed in a convenient and rapid manner while the sample is immobilized. Herein, we demonstrate a magnetic bead-based, enzyme-linked immunosorbent assay (ELISA) using ~60 nL of human interleukin-6 (IL-6) sample. The wash buffer was introduced in the form of a wall-less virtual electrowetting channel by a syringe pump at the flow rate of 10 μL/min with ~100% bead retention rate. Critical parameters such as sample wash flow rate and bead retention rate were optimized for reliable assay results. A colorimetric readout was analyzed in the International Commission on Illumination (CIE) color space without the need for costly equipment. The concepts presented in this work are potentially applicable in rapid neonatal disease screening using a finger prick blood sample in a DMF platform.