Standardized assays to monitor drug sensitivity in hematologic cancers

The principle of drug sensitivity testing is to expose cancer cells to a library of different drugs and measure its effects on cell viability. Recent technological advances, continuous approval of targeted therapies, and improved cell culture protocols have enhanced the precision and clinical relevance of such screens. Indeed, drug sensitivity testing has proven diagnostically valuable for patients with advanced hematologic cancers. However, different cell types behave differently in culture and therefore require optimized drug screening protocols to ensure that their ex vivo drug sensitivity accurately reflects in vivo drug responses. For example, primary chronic lymphocytic leukemia (CLL) and multiple myeloma (MM) cells require unique microenvironmental stimuli to survive in culture, while this is less the case for acute myeloid leukemia (AML) cells. Here, we present our optimized and validated protocols for culturing and drug screening of primary cells from AML, CLL, and MM patients, and a generic protocol for cell line models. We also discuss drug library designs, reproducibility, and quality controls. We envision that these protocols may serve as community guidelines for the use and interpretation of assays to monitor drug sensitivity in hematologic cancers and thus contribute to standardization. The read-outs may provide insight into tumor biology, identify or confirm treatment resistance and sensitivity in real time, and ultimately guide clinical decision-making.

Curvature-mediated programming of liquid crystal microflows

Despite the recognized role of liquid crystal microfluidics in generating programmable, self-organized and guided flow properties, to date, the flow behavior of LCs within curved channels remains unexplored. Using experiments and numerical simulations, we demonstrate that the curvature of microscale conduits allow programming of liquid crystal (LC) flows. Focusing on a nematic LC flowing through U- and L-shaped channels - two simple yet fundamental curved flow paths - with rectangular cross-section, our results reveal that the curvature of flow path can trigger transverse flow-induced director gradients. The emergent director field feeds back into the flow field, ultimately leading to LC flows controlled by the channel curvature. This curvature-mediated flow control, identified by polarizing optical microscopy and supported by the nematofluidic solutions, offers concepts in LC microfluidic valves, wherein the throughput distribution is determined by the Ericksen number and variations in local curvature. Finally, this work leverages curvature to amplify (suppress) LC transport through flow-aligned (homeotropic) regions emerging within channels with bends, in a programmable manner. Our results demonstrating the dependence of the dynamic flow-director coupling on the local curvature will have far-reaching ramifications in advancing the understanding of LC-based passive and active biological systems under real life geometrical constraints.

Droplet-Based Microfluidics: Applications in Pharmaceuticals

Droplet-based microfluidics offer great opportunities for applications in various fields, such as diagnostics, food sciences, and drug discovery. A droplet provides an isolated environment for performing a single reaction within a microscale-volume sample, allowing for a fast reaction with a high sensitivity, high throughput, and low risk of cross-contamination. Owing to several remarkable features, droplet-based microfluidic techniques have been intensively studied. In this review, we discuss the impact of droplet microfluidics, particularly focusing on drug screening and development. In addition, we surveyed various methods of device fabrication and droplet generation/manipulation. We further highlight some promising studies covering drug synthesis and delivery that were updated within the last 5 years. This review provides researchers with a quick guide that includes the most up-to-date and relevant information on the latest scientific findings on the development of droplet-based microfluidics in the pharmaceutical field.

Using Hadamard Transform Multiplexed IR Spectroscopy Together with a Segmented Ion Trap for the Identification of Mobility-Selected Isomers

The high isomeric complexity of glycans makes them particularly difficult to analyze. While ultra-high-resolution ion mobility spectrometry (IMS) can offer rapid baseline separation of many glycan isomers, their unambiguous identification remains a challenging task. One approach to solving this problem is to identify mobility-separated isomers by measuring their highly resolved cryogenic vibrational spectra. To be able to apply this approach to complex mixtures at high throughput, we have recently developed a Hadamard transform multiplexed spectroscopic technique that allows measuring vibrational spectra of all species separated in both IMS and mass spectrometry dimensions in a single laser scan. In the current work, we further develop the multiplexing technique using ion traps incorporated directly into the IMS device based on structures for lossless ion manipulations (SLIM). We also show that multiplexed spectroscopy using perfect sequence matrices can outperform standard multiplexing using Simplex matrices. Lastly, we show that we can increase the measurement speed and throughput further by running multiple multiplexing schemes using several SLIM ion traps in combination with simultaneous spectroscopic measurements in the segmented cryogenic ion trap.

A microfluidic Braille valve platform for on-demand production, combinatorial screening and sorting of chemically distinct droplets

Droplet microfluidics is a powerful tool for a variety of biological applications including single-cell genetics, antibody discovery and directed evolution. All these applications make use of genetic libraries, illustrating the difficulty of generating chemically distinct droplets for screening applications. This protocol describes our Braille Display valving platform for on-demand generation of droplets with different chemical contents (16 different reagents and combinations thereof), as well as sorting droplets with different chemical properties, on the basis of fluorescence signals. The Braille Display platform is compact, versatile and cost efficient (only ~US$1,000 on top of a standard droplet microfluidics setup). The procedure includes manufacturing of microfluidic chips, assembly of custom hardware, co-encapsulation of cells and drugs into droplets, fluorescence detection of readout signals and data analysis using shared, freely available LabVIEW and Python packages. As a first application, we demonstrate the complete workflow for screening cancer cell drug sensitivities toward 74 conditions. Furthermore, we describe here an assay enabling the normalization of the observed drug sensitivity to the number of cancer cells per droplet, which additionally increases the robustness of the system. As a second application, we also demonstrate the sorting of droplets according to enzymatic activity. The drug screening application can be completed within 2 d; droplet sorting takes ~1 d; and all preparatory steps for manufacturing molds, chips and setting up the Braille controller can be accomplished within 1 week.

A droplet-based microfluidic platform enables high-throughput combinatorial optimization of cyanobacterial cultivation

Cyanobacteria are fast-growing, genetically accessible, photoautotrophs. Therefore, they have attracted interest as sustainable production platforms. However, the lack of techniques to systematically optimize cultivation parameters in a high-throughput manner is holding back progress towards industrialization. To overcome this bottleneck, here we introduce a droplet-based microfluidic platform capable of one- (1D) and two-dimension (2D) screening of key parameters in cyanobacterial cultivation. We successfully grew three different unicellular, biotechnologically relevant, cyanobacteria: Synechocystis sp. PCC 6803, Synechococcus elongatus UTEX 2973 and Synechococcus sp. UTEX 3154. This was followed by a highly-resolved 1D screening of nitrate, phosphate, carbonate, and salt concentrations. The 1D screening results suggested that nitrate and/or phosphate may be limiting nutrients in standard cultivation media. Finally, we use 2D screening to determine the optimal N:P ratio of BG-11. Application of the improved medium composition in a high-density cultivation setup led to an increase in biomass yield of up to 15.7%. This study demonstrates that droplet-based microfluidics can decrease the volume required for cyanobacterial cultivation and screening up to a thousand times while significantly increasing the multiplexing capacity. Going forward, microfluidics have the potential to play a significant role in the industrial exploitation of cyanobacteria.

High-throughput single-cell antibody secretion quantification and enrichment using droplet microfluidics-based FRET assay

High-throughput screening and enrichment of antibody-producing cells have many important applications. Herein, we present a droplet microfluidic approach for high-throughput screening and sorting of antibody-secreting cells using a Förster resonance electron transfer (FRET)-based assay. The FRET signal is mediated by the specific binding of the secreted antibody to two fluorescently labeled probes supplied within a droplet. Functional hybridoma cells expressing either membrane-bound or secreted monoclonal antibodies (mAbs), or both, were efficiently differentiated in less than 30 min. The antibody secretion rate by individual hybridoma cells was recorded in the range of 14,000 Abs/min, while the density of membrane-bound fraction was approximately 100 Abs/μm². Combining the FRET assay with droplet-based single-cell sorting, an 800-fold enrichment of antigen-specific cells was achieved after one round of sorting. The presented system overcomes several key limitations observed in conventional FACS-based screening methods and should be applicable to assaying various other secreted proteins.

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FRET-based screening assay of antibody-secreting cells in microfluidic droplets

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Membrane-bound and secreted antibodies of the same cell are efficiently differentiated

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Using mouse hybridoma cells antibody secretion assay is completed in 30 min

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FRET-based droplet sorting enables over 800-fold enrichment in one round of sorting

Micro and nanofluidics for high throughput drug screening

In this book chapter, we elaborate on the state-of-the-art technology developments in high throughput screening, microfluidics and nanofluidics. This book chapter further elaborated on the application of microfluidics and nanofluidics for high throughput drug screening with respect to communicable diseases and non-communicable diseases such as cancer. As a future perspective, there is tremendous potential for microfluidics and nanofluidics to be applied in high throughput drug screening which could be applied for various biotechnology applications such as in cancer precision medicine, point-of-care diagnostics and imaging. With the integration of Fourth industrial revolution (4IR) technologies with micro and nanofluidics technologies, it envisioned that such integration along with digital health would enable next generation technology development in medical field.

Droplet-based microfluidics in biomedical applications

Droplet-based microfluidic systems have been employed to manipulate discrete fluid volumes with immiscible phases. Creating the fluid droplets at microscale has led to a paradigm shift in mixing, sorting, encapsulation, sensing, and designing high throughput devices for biomedical applications. Droplet microfluidics has opened many opportunities in microparticle synthesis, molecular detection, diagnostics, drug delivery, and cell biology. In the present review, we first introduce standard methods for droplet generation (i.e., passive and active methods) and discuss the latest examples of emulsification and particle synthesis approaches enabled by microfluidic platforms. Then, the applications of droplet-based microfluidics in different biomedical applications are detailed. Finally, a general overview of the latest trends along with the perspectives and future potentials in the field are provided.

Multi-Organs-on-Chips for Testing Small-Molecule Drugs: Challenges and Perspectives

Organ-on-a-chip technology has been used in testing small-molecule drugs for screening potential therapeutics and regulatory protocols. The technology is expected to boost the development of novel therapies and accelerate the discovery of drug combinations in the coming years. This has led to the development of multi-organ-on-a-chip (MOC) for recapitulating various organs involved in the drug-body interactions. In this review, we discuss the current MOCs used in screening small-molecule drugs and then focus on the dynamic process of drug absorption, distribution, metabolism, and excretion. We also address appropriate materials used for MOCs at low cost and scale-up capacity suitable for high-performance analysis of drugs and commercial high-throughput screening platforms.

Analysis of deoxyribonuclease activity by conjugation-free fluorescence polarisation in sub-nanolitre droplets

We report the analysis of deoxyribonuclease (DNase) activity by conjugation-free fluorescence polarisation in a droplet-based microfluidic chip. DNase is a DNA cleaving enzyme and its activity is important in the maintenance of normal cellular functions. Alterations in DNase activity have been implicated as the cause of various cancers and autoimmune diseases. To date, various methods for the analysis of DNase activity have been reported. However, they are not cost effective due to the requirement of large sample volumes and the need for the conjugation of fluorescent dyes. In this study, we have used ethidium bromide (EtBr), a DNA intercalating reagent, as a fluorescent reporter without any prior conjugation or modification of DNA. Degradation of DNA by DNase 1 was monitored at a steady state by making changes in the fluorescence polarisation of EtBr in droplets with a volume of 330 picolitre at a 40 hertz frequency under visible light. Using this technique, we successfully determined the half-maximal inhibitory concentration (IC₅₀) of ethylenediaminetetraacetic acid (EDTA) for the inhibition of DNase 1 activity to be 1.56 ± 0.91 mM.

Droplet-based optofluidic systems for measuring enzyme kinetics

The study of enzyme kinetics is of high significance in understanding metabolic networks in living cells and using enzymes in industrial applications. To gain insight into the catalytic mechanisms of enzymes, it is necessary to screen an enormous number of reaction conditions, a process that is typically laborious, time-consuming, and costly when using conventional measurement techniques. In recent times, droplet-based microfluidic systems have proved themselves to be of great utility in large-scale biological experimentation, since they consume a minimal sample, operate at high analytical throughput, are characterized by efficient mass and heat transfer, and offer high levels of integration and automation. The primary goal of this review is the introduction of novel microfluidic tools and detection methods for use in high-throughput and sensitive analysis of enzyme kinetics. The first part of this review focuses on introducing basic concepts of enzyme kinetics and describing most common microfluidic approaches, with a particular focus on segmented flow. Herein, the key advantages include accurate control over the flow behavior, efficient mass and heat transfer, multiplexing, and high-level integration with detection modalities. The second part describes the current state-of-the-art platforms for high-throughput and sensitive analysis of enzyme kinetics. In addition to our categorization of recent advances in measuring enzyme kinetics, we have endeavored to critically assess the limitations of each of these detection approaches and propose strategies to improve measurements in droplet-based microfluidics. Graphical abstract.

New opportunities for creating man-made bioarchitectures utilizing microfluidics

Cells are the basic units of life, and can be mimicked to create artificial analogs enabling the investigation of cellular mechanisms under controlled conditions. Building biomimetic systems ranging from proto-cells to cell-like objects such as compartment membranes can be achieved by collecting biobricks that self-assemble to build simplified models performing specific functions. Hence, scientists can develop and optimize new synthetic cells with biological functions by taking inspiration from nature and exploiting the advantages of synthetic biology. However, the bottom-down approach is not restricted to the basic principles of biological cells, and new mimicry systems can be designed starting with a combination of living and non-living simple molecules to focus on a cellular machinery function. In recent years, microfluidic devices have been well established to engineer bioarchitecture models resembling cell-like structures involving vesicles, compartmentalization, synthetic membranes, and the chip itself as a synthetic cell. This review aims to highlight the role of biological cells and their impact on inspiring the development of biomimetic models. The combination of the principles of synthetic biology with microfluidic technology represents the newly-introduced field of synthetic cells and synthetic membranes that can be further exploited in diagnostic and therapeutic applications.

Passive droplet generation in aqueous two-phase systems with a variable-width microchannel

Passive droplet generation for an aqueous two-phase system (ATPS) was performed with a fracture-based variable microchannel. A jet of dextran-rich phase (DEX) in a polyethylene-glycol (PEG)-rich phase was created by focused flow. The width of the inlet channel could be varied over the range 1-10 μm via mechanical strain, which extended the range of operational back pressure. This enabled the spontaneous formation of DEX droplets with an ultralow surface tension of 12 μN m-1. The production of DEX droplets were examined with regard to driving pressure, flow rate, DEX/PEG concentration. The droplet properties are analyzed in terms of production rate (2-20 droplets per s), droplet diameter (10-100 μm), and diameter variance (5-20%). Controlling the inlet-channel width with other operating conditions widened the range of droplet properties. This simple and robust method significantly strengthened droplet-generation in microfluidics, especially for ATPS of low solute concentrations relevant to live cells.

Multiple splitting of droplets using multi-furcating microfluidic channels

Removing volumes from droplets is a challenging but critical step in many droplet-based applications. Geometry-mediated droplet splitting has the potential to reliably divide droplets and thus facilitate the implementation of this step. In this paper, we report the design of multi-furcating microfluidic channels for efficient droplet splitting. We studied the splitting regimes as the size of the mother droplets varied and investigated the dependence of the transition between splitting regimes on the capillary number and the dimensionless droplet length. We found that the results obtained with our device agreed with the reported dimensionless analysis law in T-junctions. We further investigated the effect of channel lengths on the volume allocation in branch channels and achieved droplet splitting with various splitting ratios. This study proposed an efficient on-demand droplet splitting method and the findings could potentially be applied in washing steps in droplet-based biological assays or assays that require aliquot.

High throughput analysis enables high throughput experimentation in pharmaceutical process research

High throughput experimentation has become widely used in the discovery and development of new medicines.

Microfluidic diamagnetic water-in-water droplets: a biocompatible cell encapsulation and manipulation platform

Droplet microfluidics enables cellular encapsulation for biomedical applications such as single-cell analysis, which is an important tool used by biologists to study cells on a single-cell level, and understand cellular heterogeneity in cell populations. However, most cell encapsulation strategies in microfluidics rely on random encapsulation processes, resulting in large numbers of empty droplets. Therefore, post-sorting of droplets is necessary to obtain samples of purely cell-encapsulating droplets. With the recent advent of aqueous two-phase systems (ATPS) as a biocompatible alternative of the conventional water-in-oil droplet systems for cellular encapsulation, there has also been a focus on integrating ATPS with droplet microfluidics. In this paper, we describe a new technique that combines ATPS-based water-in-water droplets with diamagnetic manipulation to isolate single-cell encapsulating water-in-water droplets, and achieve a purity of 100% in a single pass. We exploit the selective partitioning of ferrofluid in an ATPS of polyethylene glycol-polypropylene glycol-polyethylene glycol triblock copolymer (PEG-PPG-PEG) and dextran (DEX), to achieve diamagnetic manipulation of water-in-water droplets. A cell-triggered Rayleigh-Plateau instability in the dispersed phase thread results in a size distinction between the cell-encapsulating and empty droplets, enabling diamagnetic separation and sorting of the cell-encapsulating droplets from empty droplets. This is a simple and biocompatible all-aqueous platform for single-cell encapsulation and droplet manipulation, with applications in single-cell analysis.

Structural Formation of Oil-in-Water (O/W) and Water-in-Oil-in-Water (W/O/W) Droplets in PDMS Device Using Protrusion Channel without Hydrophilic Surface Treatment

This paper presents a simple method of droplet formation using liquids that easily wet polydimethylsiloxane (PDMS) surfaces without any surface treatment. Using only structural features and uniform flow focusing, Oil-in-Water (O/W) and Water-in-Oil-in-Water (W/O/W) droplets were formed in the full PDMS structure. Extrusion channel and three-dimensional flow focusing resulted in effective fluidic conditions for droplet formation and the droplet size could be precisely controlled by controlling the flow rate of each phase. The proposed structure can be utilized as an important element for droplet based research, as well as a droplet generator.

A droplet microfluidic platform for efficient enzymatic chromatin digestion enables robust determination of nucleosome positioning

The first step in chromatin-based epigenetic assays involves the fragmentation of chromatin to facilitate precise genomic localization of the associated DNA. Here, we report the development of a droplet microfluidic device that can rapidly and efficiently digest chromatin into single nucleosomes starting from whole-cell input material offering simplified and automated processing compared to conventional manual preparation. We demonstrate the digestion of chromatin from 2500-125 000 Jurkat cells using micrococcal nuclease for enzymatic processing. We show that the yield of mononucleosomal DNA can be optimized by controlling enzyme concentration and incubation time, with resulting mononucleosome yields exceeding 80%. Bioinformatic analysis of sequenced mononucleosomal DNA (MNase-seq) indicated a high degree of reproducibility and concordance (97-99%) compared with conventionally processed preparations. Our results demonstrate the feasibility of robust and automated nucleosome preparation using a droplet microfluidic platform for nucleosome positioning and downstream epigenomic assays.

Label-Free Sensing in Microdroplet-Based Microfluidic Systems

Droplet microfluidic systems have evolved as fluidic platforms that use much less sample volume and provide high throughput for biochemical analysis compared to conventional microfluidic devices. The variety of droplet fluidic applications triggered several detection techniques to be applied for analysis of droplets. In this review, we focus on label-free droplet detection techniques that were adapted to various droplet microfluidic platforms. We provide a classification of most commonly used droplet platform technologies. Then we discuss the examples of various label-free droplet detection schemes implemented for these platforms. While providing the research landscape for label-free droplet detection methods, we aim to highlight the strengths and shortcomings of each droplet platform so that a more targeted approach can be taken by researchers when selecting a droplet platform and a detection scheme for any given application.

Microfluidic Devices for Drug Delivery Systems and Drug Screening

Microfluidic devices present unique advantages for the development of efficient drug carrier particles, cell-free protein synthesis systems, and rapid techniques for direct drug screening. Compared to bulk methods, by efficiently controlling the geometries of the fabricated chip and the flow rates of multiphase fluids, microfluidic technology enables the generation of highly stable, uniform, monodispersed particles with higher encapsulation efficiency. Since the existing preclinical models are inefficient drug screens for predicting clinical outcomes, microfluidic platforms might offer a more rapid and cost-effective alternative. Compared to 2D cell culture systems and in vivo animal models, microfluidic 3D platforms mimic the in vivo cell systems in a simple, inexpensive manner, which allows high throughput and multiplexed drug screening at the cell, organ, and whole-body levels. In this review, the generation of appropriate drug or gene carriers including different particle types using different configurations of microfluidic devices is highlighted. Additionally, this paper discusses the emergence of fabricated microfluidic cell-free protein synthesis systems for potential use at point of care as well as cell-, organ-, and human-on-a-chip models as smart, sensitive, and reproducible platforms, allowing the investigation of the effects of drugs under conditions imitating the biological system.

Automating drug discovery

Small-molecule drug discovery can be viewed as a challenging multidimensional problem in which various characteristics of compounds - including efficacy, pharmacokinetics and safety - need to be optimized in parallel to provide drug candidates. Recent advances in areas such as microfluidics-assisted chemical synthesis and biological testing, as well as artificial intelligence systems that improve a design hypothesis through feedback analysis, are now providing a basis for the introduction of greater automation into aspects of this process. This could potentially accelerate time frames for compound discovery and optimization and enable more effective searches of chemical space. However, such approaches also raise considerable conceptual, technical and organizational challenges, as well as scepticism about the current hype around them. This article aims to identify the approaches and technologies that could be implemented robustly by medicinal chemists in the near future and to critically analyse the opportunities and challenges for their more widespread application.

Biocompatibility of fluids for multiphase drops-in-drops microfluidics

This paper addresses the biocompatibility of fluids and surfactants in the context of microfluidics and more specifically in a drops-in-drops system for mammalian cell based drug screening. In the drops-in-drops approach, three immiscible fluids are used to manipulate the flow of aqueous microliter-sized drops; it enables merging of drops containing cells with drops containing drugs within a Teflon tube. Preliminary tests showed that a commonly-used fluid and surfactant combination resulted in significant variability in gene expression levels in Jurkat cells after exposure to a drug for four hours. This result led to further investigations of potential fluid and surfactant combinations that can be used in microfluidic systems for medium to long-term drug screening. Results herein identify a fluid combination, HFE-7500 and 5-cSt silicone oil + 0.25% Abil EM180, which enabled the drops-in-drops approach; this combination also allowed gene expression at normal levels comparable with the conventional drug screening in both magnitude and variability.

Sessile droplets for chemical and biological assays

Sessile droplets are non-movable droplets spanning volumes in the nL-to-μL range. The sessile-droplet-based platform provides a paradigm shift from the conventional, flow-based lab-on-a-chip philosophy, yet offering similar benefits: low reagent/sample consumption, high throughput, automation, and most importantly flexibility and versatility. Moreover, the platform relies less heavily on sophisticated fabrication techniques, often sufficient with a hydrophobic substrate, and no pump is required for operation. In addition, exploiting the physical phenomena that naturally arise when a droplet evaporates, such as the coffee-ring effect or Marangoni flow, can lead to fascinating applications. In this review, we introduce the physics of droplets, and then focus on the different types of chemical and biological assays that have been implemented in sessile droplets, including analyte concentration, particle separation and sorting, cell-based assays, and nucleic acid amplification. Finally, we provide our perspectives on this unique micro-scale platform.

OpenDrop: An Integrated Do-It-Yourself Platform for Personal Use of Biochips

Biochips, or digital labs-on-chip, are developed with the purpose of being used by laboratory technicians or biologists in laboratories or clinics. In this article, we expand this vision with the goal of enabling everyone, regardless of their expertise, to use biochips for their own personal purposes. We developed OpenDrop, an integrated electromicrofluidic platform that allows users to develop and program their own bio-applications. We address the main challenges that users may encounter: accessibility, bio-protocol design and interaction with microfluidics. OpenDrop consists of a do-it-yourself biochip, an automated software tool with visual interface and a detailed technique for at-home operations of microfluidics. We report on two years of use of OpenDrop, released as an open-source platform. Our platform attracted a highly diverse user base with participants originating from maker communities, academia and industry. Our findings show that 47% of attempts to replicate OpenDrop were successful, the main challenge remaining the assembly of the device. In terms of usability, the users managed to operate their platforms at home and are working on designing their own bio-applications. Our work provides a step towards a future in which everyone will be able to create microfluidic devices for their personal applications, thereby democratizing parts of health care.

A microfluidic chip integrated with droplet generation, pairing, trapping, merging, mixing and releasing

Developing a microfluidic chip with multiple functions is highly demanded for practical applications, such as chemical analysis, diagnostics, particles synthesis and drug screening.

Microfabricated tools for quantitative plant biology

The development of microfabricated devices that will provide high-throughput quantitative data and high resolution in a fast, repeatable and reproducible manner is essential for plant biology research.

Droplet-Based Digital PCR: Application in Cancer Research

The efficient characterization of genetic and epigenetic alterations in oncology, virology, or prenatal diagnostics requires highly sensitive and specific high-throughput approaches. Nevertheless, with the use of conventional methods, sensitivity and specificity were largely limited. By partitioning individual target molecules within distinct compartments, digital PCR (dPCR) could overcome these limitations and detect very rare sequences with unprecedented precision and sensitivity. In dPCR, the sample is diluted such that each individual partition will contain no more than one target sequence. Following the assay reaction, the dPCR process provides an absolute value and analyzable quantitative data. The recent coupling of dPCR with microfluidic systems in commercial platforms should lead to an essential tool for the management of patients with cancer, especially adapted to the analysis of precious samples. Applications in cancer research range from the analysis of tumor heterogeneity to that of a range of body fluids. Droplet-based dPCR is indeed particularly appropriate for the emerging field of liquid biopsy analysis. In this review, following an overview of the development in dPCR technology and different strategies based on the use of microcompartments, we will focus particularly on the applications and latest development of microfluidic droplet-based dPCR in oncology.

Suzuki-Miyaura cross-coupling optimization enabled by automated feedback

An automated, droplet-flow microfluidic system explores and optimizes Pd-catalyzed Suzuki-Miyaura cross-coupling reactions. A smart optimal DoE-based algorithm is implemented to increase the turnover number and yield of the catalytic system considering both discrete variables-palladacycle and ligand-and continuous variables-temperature, time, and loading-simultaneously. The use of feedback allows for experiments to be run with catalysts and under conditions more likely to produce an optimum; consequently complex reaction optimizations are completed within 96 experiments. Response surfaces predicting reaction performance near the optima are generated and validated. From the screening results, shared attributes of successful precatalysts are identified, leading to improved understanding of the influence of ligand selection upon transmetalation and oxidative addition in the reaction mechanism. Dialkylbiarylphosphine, trialkylphosphine, and bidentate ligands are assessed.

Droplet Merging on a Lab-on-a-Chip Platform by Uniform Magnetic Fields

Droplet microfluidics offers a range of Lab-on-a-chip (LoC) applications. However, wireless and programmable manipulation of such droplets is a challenge. We address this challenge by experimental and modelling studies of uniform magnetic field induced merging of ferrofluid based droplets. Control of droplet velocity and merging was achieved through uniform magnetic field and flow rate ratio. Conditions for droplet merging with respect to droplet velocity were studied. Merging and mixing of colour dye + magnetite composite droplets was demonstrated. Our experimental and numerical results are in good agreement. These studies are useful for wireless and programmable droplet merging as well as mixing relevant to biosensing, bioassay, microfluidic-based synthesis, reaction kinetics, and magnetochemistry.

Feedback in Flow for Accelerated Reaction Development

The pharmaceutical industry is investing in continuous flow and high-throughput experimentation as tools for rapid process development accelerated scale-up. Coupled with automation, these technologies offer the potential for comprehensive reaction characterization and optimization, but with the cost of conducting exhaustive multifactor screens. Automated feedback in flow offers researchers an alternative strategy for efficient characterization of reactions based on the use of continuous technology to control chemical reaction conditions and optimize in lieu of screening. Optimization with feedback allows experiments to be conducted where the most information can be gained from the chemistry, enabling product yields to be maximized and kinetic models to be generated while the total number of experiments is minimized. This Account opens by reviewing select examples of feedback optimization in flow and applications to chemical research. Systems in the literature are classified into (i) deterministic "black box" optimization systems that do not model the reaction system and are therefore limited in the utility of results for scale-up, (ii) deterministic model-based optimization systems from which reaction kinetics and/or mechanisms can be automatically evaluated, and (iii) stochastic systems. Though diverse in application, flow feedback systems have predominantly focused upon the optimization of continuous variables, i.e., variables such as time, temperature, and concentration that can be ramped from one experiment to the next. Unfortunately, this implies that the screening of discrete variables such as catalyst, ligand, or solvent generally does not factor into automated flow optimization, resulting in incomplete process knowledge. Herein, we present a system and strategy developed for optimizing discrete and continuous variables of a chemical reaction simultaneously. The approach couples automated feedback with high-throughput reaction screening in droplet flow microfluidics. This Account details the system configuration for on-demand creation of sub-20 μL droplets with interchangeable reagents and catalysts. These droplets are reacted in a fully automated microfluidic system and analyzed online by LC/MS. Feeding back from the online analytical results, a design of experiments (DoE)-based adaptive response surface algorithm is employed that deductively removes candidate reagents from the optimization as optimal reaction conditions are refined, leading to rapid convergence. Using the automated optimization platform, case studies are presented for solvent selection in a competitive alkylation chemistry and for catalyst-ligand selection in heteroaromatic Suzuki-Miyaura cross-coupling chemistries. For the monoalkylation of trans-1,2-diaminocyclohexane, polar aprotic solvents at moderate temperatures are shown to be favorable, with optimality accurately identified with dimethyl sulfoxide as the solvent in 67 experiments. For Suzuki-Miyaura cross-couplings, the optimality of precatalysts and continuous variable conditions are observed to change in accordance with the coupling reagents, providing insights into catalyst behavior in the context of the reaction mechanism. Future opportunities in automated reaction development include the incorporation of chemoinformatics for faster analysis and machine-learning algorithms to guide and optimize the synthesis. Adoption of this technology stands to reduce graduate student and postdoc time on routine tasks in the laboratory, while feeding back knowledge used to guide new research directions. Moreover, the application of this technology in industry promises to lessen the cost and time associated with advancing pharmaceutical molecules through development and scale-up.

Water-in-Water Droplets by Passive Microfluidic Flow Focusing

We present a simple microfluidic system that generates water-in-water, aqueous two phase system (ATPS) droplets, by passive flow focusing. ATPS droplet formation is achieved by applying weak hydrostatic pressures, with liquid-filled pipette tips as fluid columns at the inlets, to introduce low speed flows to the flow focusing junction. To control the size of the droplets, we systematically vary the interfacial tension and viscosity of the ATPS fluids and adjust the fluid column height at the fluid inlets. The size of the droplets scales with a power law of the ratio of viscous stresses in the two ATPS phases. Overall, we find a drop size coefficient of variation (CV; i.e., polydispersity) of about 10%. We also find that when drops form very close to the flow focusing junction, the drops have a CV of less than 1%. Our droplet generation method is easily scalable: we demonstrate a parallel system that generates droplets simultaneously and improves the droplet production rate by up to one order of magnitude. Finally, we show the potential application of our system for encapsulating cells in water-in-water emulsions by encapsulating microparticles and cells. To the best of our knowledge, our microfluidic technique is the first that forms low interfacial tension ATPS droplets without applying external perturbations. We anticipate that this simple approach will find utility in drug and cell delivery applications because of the all-biocompatible nature of the water-in-water ATPS environment.

Microfluidics for cell-based high throughput screening platforms - A review

In the last decades, the basic techniques of microfluidics for the study of cells such as cell culture, cell separation, and cell lysis, have been well developed. Based on cell handling techniques, microfluidics has been widely applied in the field of PCR (Polymerase Chain Reaction), immunoassays, organ-on-chip, stem cell research, and analysis and identification of circulating tumor cells. As a major step in drug discovery, high-throughput screening allows rapid analysis of thousands of chemical, biochemical, genetic or pharmacological tests in parallel. In this review, we summarize the application of microfluidics in cell-based high throughput screening. The screening methods mentioned in this paper include approaches using the perfusion flow mode, the droplet mode, and the microarray mode. We also discuss the future development of microfluidic based high throughput screening platform for drug discovery.

Floating Droplet Array: An Ultrahigh-Throughput Device for Droplet Trapping, Real-time Analysis and Recovery

We describe the design, fabrication and use of a dual-layered microfluidic device for ultrahigh-throughput droplet trapping, analysis, and recovery using droplet buoyancy. To demonstrate the utility of this device for digital quantification of analytes, we quantify the number of droplets, which contain a β-galactosidase-conjugated bead among more than 100,000 immobilized droplets. In addition, we demonstrate that this device can be used for droplet clustering and real-time analysis by clustering several droplets together into microwells and monitoring diffusion of fluorescein, a product of the enzymatic reaction of β-galactosidase and its fluorogenic substrate FDG, between droplets.

The Poisson distribution and beyond: methods for microfluidic droplet production and single cell encapsulation

There is a recognized and growing need for rapid and efficient cell assays, where the size of microfluidic devices lend themselves to the manipulation of cellular populations down to the single cell level. An exceptional way to analyze cells independently is to encapsulate them within aqueous droplets surrounded by an immiscible fluid, so that reagents and reaction products are contained within a controlled microenvironment. Most cell encapsulation work has focused on the development and use of passive methods, where droplets are produced continuously at high rates by pumping fluids from external pressure-driven reservoirs through defined microfluidic geometries. With limited exceptions, the number of cells encapsulated per droplet in these systems is dictated by Poisson statistics, reducing the proportion of droplets that contain the desired number of cells and thus the effective rate at which single cells can be encapsulated. Nevertheless, a number of recently developed actively-controlled droplet production methods present an alternative route to the production of droplets at similar rates and with the potential to improve the efficiency of single-cell encapsulation. In this critical review, we examine both passive and active methods for droplet production and explore how these can be used to deterministically and non-deterministically encapsulate cells.

A High Throughput Micro-Chamber Array Device for Single Cell Clonal Cultivation and Tumor Heterogeneity Analysis

Recently, single cell cloning techniques have been gradually developed benefited from their important roles in monoclonal antibody screening, tumor heterogeneity research fields, etc. In this study, we developed a high throughput device containing 1400 lateral chambers to efficiently isolate single cells and carry out long-term single cell clonal cultivation as well as tumor heterogeneity studies. Most of the isolated single cells could proliferate normally nearly as long as three weeks and hundreds of clones could be formed once with one device, which made it possible to study tumor heterogeneity at single cell level. The device was further used to examine tumor heterogeneity such as morphology, growth rate, anti-cancer drug tolerance as well as adenosine triphosphate-binding cassette (ABC) transporter ABCG2 protein expression level. Except for the single cell isolation and tumor heterogeneity studies, the device is expected to be used as an excellent platform for drug screening, tumor biomarker discovering and tumor metastasis assay.