Droplet microfluidics for high-throughput biological assays

Droplet Microfluidics for High-Resolution Virology

Microfluidics has enabled a new era of cellular and molecular assays due to the small length scales, parallelization, and the modularity of various analysis and actuation functions. Droplet microfluidics, in particular, has been instrumental in providing new tools for biology with its ability to quickly and reproducibly generate drops that act as individual reactors. A notable beneficiary of this technology has been single-cell RNA sequencing, which has revealed new heterogeneities and interactions for the fundamental unit of life. However, viruses far surpass the diversity of cellular life, affect the dynamics of all ecosystems, and are a chronic source of global health crises. Despite their impact on the world, high-throughput and high-resolution viral profiling has been difficult, with conventional methods being limited to population-level averaging, large sample volumes, and few cultivable hosts. Consequently, most viruses have not been identified and studied. Droplet microfluidics holds the potential to address many of these limitations and offers new levels of sensitivity and throughput for virology. This Feature highlights recent efforts that have applied droplet microfluidics to the detection and study of viruses, including for diagnostics, virus-host interactions, and cell-independent virus assays. In combination with traditional virology methods, droplet microfluidics should prove a potent tool toward achieving a better understanding of the most abundant biological species on Earth.

Identification of Lipolytic Enzymes Using High-Throughput Single-cell Screening and Sorting of a Metagenomic Library

The demand for novel, robust microbial biocatalysts for use in industrial and pharmaceutical applications continues to increase rapidly. As a result, there is a need to develop advanced tools and technologies to exploit the vast metabolic potential of unculturable microorganisms found in various environments. Single-cell and functional metagenomics studies can explore the enzymatic potential of entire microbial communities in a given environment without the need to culture the microorganisms. This approach has contributed substantially to the discovery of unique microbial genes for industrial and medical applications. Functional metagenomics involves the extraction of microbial DNA directly from environmental samples, constructing expression libraries comprising the entire microbial genome, and screening of the libraries for the presence of desired phenotypes. In this study, lipolytic enzymes from the Red Sea were targeted. A high-throughput single-cell microfluidic platform combined with a laser-based fluorescent screening bioassay was employed to discover new genes encoding lipolytic enzymes. Analysis of the metagenomic library led to the identification of three microbial genes encoding lipases based on their functional similarity and sequence homology to known lipases. The results demonstrated that microfluidics is a robust technology that can be used for screening in functional metagenomics. The results also indicate that the Red Sea is a promising, under-investigated source of new genes and gene products.

Trends in Droplet Microfluidics: From Droplet Generation to Biomedical Applications

Over the past decade, droplet microfluidics has attracted growing interest in biology, medicine, and engineering. In this feature article, we review the advances in droplet microfluidics, primarily focusing on the research conducted by our group. Starting from the introduction to the mechanisms of microfluidic droplet formation and the strategies for cell encapsulation in droplets, we then focus on droplet transformation into microgels. Furthermore, we review three biomedical applications of droplet microfluidics, that is, 3D cell culture, single-cell analysis, and in vitro organ and disease modeling. We conclude with our perspective on future directions in the development of droplet microfluidics for biomedical applications.

Oil-Triggered and Template-Confined Dewetting for Facile and Low-Loss Sample Digitization

This paper proposes a simple and robust method for spontaneously digitizing aqueous samples into a high-density microwell array. The method is based on an oil-triggered template-confined dewetting phenomenon. To realize the dewetting-induced sample digitization, an aqueous sample is first infused into a networked microwell array (NMA) through a pre-degassing-based self-pumping mechanism, and an immiscible oil phase is then applied over the surface of NMA chip to induce the templated dewetting. Due to periodic interfacial tension heterogeneity, such dewetting ruptures the sample at the thinnest parts (i.e., connection channels) and spontaneously splits the sample into droplets in individual microwells. Without requiring any complex pumping or valving systems, this method can discretize a sample into tens of thousands of addressable droplets in a matter of minutes with nearly 98% usage. To demonstrate the utility and universality of this self-digitization method, we exploited it to discretize samples into 40 233 wells for a digital PCR assay, the digital quantification of bacteria, the self-assembly of spherical colloidal photonic crystals, and the spherical crystallization of drugs. We believe this facile technique will provide a substantial benefit to many compartmentalized assays or syntheses where it is necessary to partition samples into a large number of small individual volumes.

Spatial confinement toward creating artificial living systems

Lifeforms are regulated by many physicochemical factors, and these factors could be controlled to play a role in the construction of artificial living systems. Among these factors, spatial confinement is an important one, which mediates biological behaviors at multiscale levels and participates in the biomanufacturing processes accordingly. This review describes how spatial confinement, as a fundamental biological phenomenon, provides cues for the construction of artificial living systems. Current knowledge about the role of spatial confinement in mediating individual cell behavior, collective cellular behavior, and tissue-level behavior are categorized. Endeavors on the synthesis of biomacromolecules, artificial cells, engineered tissues, and organoids in spatially confined bioreactors are then emphasized. After that, we discuss the cutting-edge applications of spatially confined artificial living systems in biomedical fields. Finally, we conclude by assessing the remaining challenges and future trends in the context of fundamental science, technical improvement, and practical applications.

Application of Microfluidic Technology in Antibody Screening

Specific antibodies are widely used in the biomedical field. Current screening methods for specific antibodies mainly involve hybridoma technology and antibody engineering techniques. However, these technologies suffer from tedious screening processes, long preparation periods, high costs, low efficiency, and a degree of automation, which have become a bottleneck for the screening of specific antibodies. To overcome these difficulties, microfluidics has been developed as a promising technology for high-throughput screening and high purity of antibody. In this review, we provide an overview of the recent advances in microfluidic applications for specific antibody screening. In particular, hybridoma technology and four antibody engineering techniques (including phage display, single B cell antibody screening, antibody expression, and cell-free protein synthesis) based on microfluidics have been introduced, challenges, and the future outlook of these technologies are also discussed. This article is protected by copyright. All rights reserved.

Breakup Dynamics of Droplets in Symmetric Y-Junction Microchannels

The experimental method is used to study the droplet breaking characteristics of an immiscible liquid–liquid t8wo-phase fluid in symmetric Y-junction microchannels. Silicone oil is used as the dispersed phase and distilled water containing 0.5% SDS is used as the continuous phase. Three breakup behaviors were observed: breakup with permanent obstruction, breakup with gaps, and no breakup. Two stages of the change of the neck width of the sub-droplet during the breakup process were discovered: a rapid breakup stage and a thread breakup stage. The effect of the breakup behavior on the flow pattern was investigated and it was found that the breakup behavior of the droplets made the slug flow area smaller; further, a new flow pattern was observed, being droplet flow. The length of the sub-droplet increases with an increase of the volume flow rate of the dispersed phase and the ratio of the volume flow rate of the dispersed phase to the continuous phase, while decreasing with an increase of the volume flow rate and the capillary number of the continuous phase. Based on the influence of the two-phase flow parameters on the length of the sub-droplet, a correlation formula for the length of the sub-droplet with good predictive performance is proposed.

Responsive Liquid Metal Droplets: From Bulk to Nano

Droplets exist widely in nature and play an extremely important role in a broad variety of industrial processes. Typical droplets, including water and oil droplets, have received extensive attention and research, however their single properties still cannot meet diverse needs. Fortunately, liquid metal droplets emerging in recent years possess outstanding properties, including large surface tension, excellent electrical and thermal conductivity, convenient chemical processing, easy transition between liquid and solid phase state, and large-scale deformability, etc. More interestingly, liquid metal droplets with unique features can respond to external factors, including the electronic field, magnetic field, acoustic field, chemical field, temperature, and light, exhibiting extraordinary intelligent response characteristics. Their development over the past decade has brought substantial breakthroughs and progress. To better promote the advancement of this field, the present article is devoted to systematically summarizing and analyzing the recent fundamental progress of responsive liquid metal droplets, not only involving droplet characteristics and preparation methods, but also focusing on their diverse response behaviors and mechanisms. On this basis, the challenges and prospects related to the following development of liquid metal droplets are also proposed. In the future, responsive liquid metal droplets with a rapid development trend are expected to play a key role in soft robots, biomedicine, smart matter, and a variety of other fields.

Large-Scale Dried Reagent Reconstitution and Diffusion Control Using Microfluidic Self-Coalescence Modules

The positioning and manipulation of large numbers of reagents in small aliquots are paramount to many fields in chemistry and the life sciences, such as combinatorial screening, enzyme activity assays, and point-of-care testing. Here, a capillary microfluidic architecture based on self-coalescence modules capable of storing thousands of dried reagent spots per square centimeter is reported, which can all be reconstituted independently without dispersion using a single pipetting step and ≤5 μL of a solution. A simple diffusion-based mathematical model is also provided to guide the spotting of reagents in this microfluidic architecture at the experimental design stage to enable either compartmentalization, mixing, or the generation of complex multi-reagent chemical patterns. Results demonstrate the formation of chemical patterns with high accuracy and versatility, and simple methods for integrating reagents and imaging the resulting chemical patterns.

Microfluidics-based strategies for molecular diagnostics of infectious diseases

Traditional diagnostic strategies for infectious disease detection require benchtop instruments that are inappropriate for point-of-care testing (POCT). Emerging microfluidics, a highly miniaturized, automatic, and integrated technology, are a potential substitute for traditional methods in performing rapid, low-cost, accurate, and on-site diagnoses. Molecular diagnostics are widely used in microfluidic devices as the most effective approaches for pathogen detection. This review summarizes the latest advances in microfluidics-based molecular diagnostics for infectious diseases from academic perspectives and industrial outlooks. First, we introduce the typical on-chip nucleic acid processes, including sample preprocessing, amplification, and signal read-out. Then, four categories of microfluidic platforms are compared with respect to features, merits, and demerits. We further discuss application of the digital assay in absolute nucleic acid quantification. Both the classic and recent microfluidics-based commercial molecular diagnostic devices are summarized as proof of the current market status. Finally, we propose future directions for microfluidics-based infectious disease diagnosis.

Microfluidic Microalgae System: A Review

Microalgae that have recently captivated interest worldwide are a great source of renewable, sustainable and economical biofuels. The extensive potential application in the renewable energy, biopharmaceutical and nutraceutical industries have made them necessary resources for green energy. Microalgae can substitute liquid fossil fuels based on cost, renewability and environmental concern. Microfluidic-based systems outperform their competitors by executing many functions, such as sorting and analysing small volumes of samples (nanolitre to picolitre) with better sensitivities. In this review, we consider the developing uses of microfluidic technology on microalgal processes such as cell sorting, cultivation, harvesting and applications in biofuels and biosensing.

Droplet-based valveless microfluidic system for phage-display screening against spheroids

In this study, we proposed a droplet-based valveless microfluidic system that has the necessary functions to perform the binding, washing, eluting, and collecting processes of phage-display screening against spheroids, which can be expected to present a similar repertoire and number of membrane proteins as in vivo. Although spheroids have much larger sizes than single cells, spheroids are difficult to manipulate through manual operation. The proposed microfluidic system actively controls the position and velocity of droplets using a camera, three air pumps, and three liquid pumps to perform the processes for phage-display screening. The cross section of the microchannel is large in width and height for the passage of spheroids. Valves that can close such a large cross-sectional microchannel are not readily available. Thus, we proposed valveless flow control using liquid pumps. In addition, the proposed microfluidic system involves complex flow channels with airflow subchannels to perform phage-display screening. For washing, nonspecific-binding phages remaining in the flow channels must be minimized. The proposed microfluidic system can perform selective blocking and flush washing. Selective blocking can prevent the airflow channels from becoming hydrophilic with blocking liquid, and flush washing can flush phages remaining in the flow channel. We experimentally verified the functions of the developed microfluidic device based on the proposed system.

Tailored Double Emulsions Made Simple

Double emulsions, such as water-in-oil-in-water droplets, are important material platforms for conducting fundamental research and for technological applications. To date, well-defined double-emulsion droplets consisting of a single water core and a thin oil shell can be exclusively formed with sophisticated microfluidic devices. The fabrication, preparation, and operation of such devices is challenging, which reduces the availability of tailored double emulsions to a limited community of experts. Here, a simple method is introduced to produce single-core double emulsions with high yield in large quantities, using a vortex mixer. Utilizing the density difference between the dispersed droplet and the continuous phase, this two-step emulsification method can achieve very small core droplet diameters below 10 μm and ultrathin shells with thicknesses below 1 μm. A detailed picture of the formation mechanism is provided and it is demonstrated that the process can be extended to produce multishell and multicore emulsions. Finally, its application is demonstrated to produce structurally colored colloidal supraparticles with unprecedented uniformity and yield. The method allows the creation of tailored double emulsions with minimal time, cost, effort, and expertise, and may widen its application to nonspecialized scientific communities.

Microfluidics device for drug discovery, screening and delivery

Microfluidics and lab-on-chip are two progressive technologies widely used for drug discovery, screening and delivery. It has been designed in a way to act as a platform for sample preparations, culturing, incubation and screening through multi-channels. These devices require a small amount of reagent in about micro- to nanolitre volume. Microfluidics has the capacity to perform operations in a programmable manner and is easy to fine tune the size, shape and composition of drugs by changing flow rate and precise manipulations. Microfluidics platform comes with the advantage of mixing fluid in droplet reactors. Microfluidics is used in the field of chemistry, biomedical, biology and nanotechnology due to its high-throughput performance in various assays. It is potent enough to be used in microreactors for synthesis of particles and encapsulation of many biological entities for biological and drug delivery applications. Microfluidics therefore has the scope to be uplifted from basic to advanced diagnostic and therapeutic applications.

An outlook on microfluidics: the promise and the challenge

This perspective considers ways in which the field of microfluidics can increase its impact by improving existing technologies and enabling new functionalities. We highlight applications where microfluidics has made or can make important contributions, including diagnostics, food safety, and the production of materials. The success of microfluidics assumes several forms, including fundamental innovations in fluid mechanics that enable the precise manipulation of fluids at small scales and the development of portable microfluidic chips for commercial purposes. We identify outstanding technical challenges whose resolution could increase the accessibility of microfluidics to users with both scientific and non-technical backgrounds. They include the simplification of procedures for sample preparation, the identification of materials for the production of microfluidic devices in both laboratory and commercial settings, and the replacement of auxiliary equipment with automated components for the operation of microfluidic devices.

Microfluidic Compartmentalization Platforms for Single Cell Analysis

Many cellular analytical technologies measure only the average response from a cell population with an assumption that a clonal population is homogenous. The ensemble measurement often masks the difference among individual cells that can lead to misinterpretation. The advent of microfluidic technology has revolutionized single-cell analysis through precise manipulation of liquid and compartmentalizing single cells in small volumes (pico- to nano-liter). Due to its advantages from miniaturization, microfluidic systems offer an array of capabilities to study genomics, transcriptomics, and proteomics of a large number of individual cells. In this regard, microfluidic systems have emerged as a powerful technology to uncover cellular heterogeneity and expand the depth and breadth of single-cell analysis. This review will focus on recent developments of three microfluidic compartmentalization platforms (microvalve, microwell, and microdroplets) that target single-cell analysis spanning from proteomics to genomics. We also compare and contrast these three microfluidic platforms and discuss their respective advantages and disadvantages in single-cell analysis.

Double T-junction microfluidic and conventional dripping systems for Bacillus subtilis immobilization in calcium alginate microparticles for lipase production

Bacillus subtilis immobilization in calcium alginate microparticles was investigated using two techniques: droplet microfluidics-based in T-junction geometry composed with a double droplet generation system and conventional dripping system. Alginate microparticles produced by microfluidic technology presented an average size of 68.35 µm with low polydispersity and immobilization efficiency around 86%. The cell response was evaluated in batch cultivation for 24 h, viewing lipase production compared to free cells. In this study, the batch cultivation with immobilized cells in alginate microparticles presented lipase production about 2.4 and 1.7 times higher than cultivation with cells immobilized cells by conventional technique and free cells cultivations. According to the results, this main novelty of the double T junction technique is an innovative contribution as a tool for cell immobilization on a laboratory scale, since the cultivation of immobilized cells in microparticles of small size and low polydispersity favors cell growth and increases the productivity of important metabolites of industrial biotechnology.

Microfluidics-assisted synthesis and functionalization of monodisperse colloidal hydrogel particles for optomechanical biosensors

The soft colloidal probe (SCP) assay is a highly versatile sensing principle employing micrometer-sized hydrogel particles as optomechanical transducer elements. We report the synthesis, optimization, and conjugation of SCPs with...

Extreme Compressed Sensing of Poisson Rates from Multiple Measurements

Compressed sensing (CS) is a signal processing technique that enables the efficient recovery of a sparse high-dimensional signal from low-dimensional measurements. In the multiple measurement vector (MMV) framework, a set of signals with the same support must be recovered from their corresponding measurements. Here, we present the first exploration of the MMV problem where signals are independently drawn from a sparse, multivariate Poisson distribution. We are primarily motivated by a suite of biosensing applications of microfluidics where analytes (such as whole cells or biomarkers) are captured in small volume partitions according to a Poisson distribution. We recover the sparse parameter vector of Poisson rates through maximum likelihood estimation with our novel Sparse Poisson Recovery (SPoRe) algorithm. SPoRe uses batch stochastic gradient ascent enabled by Monte Carlo approximations of otherwise intractable gradients. By uniquely leveraging the Poisson structure, SPoRe substantially outperforms a comprehensive set of existing and custom baseline CS algorithms. Notably, SPoRe can exhibit high performance even with one-dimensional measurements and high noise levels. This resource efficiency is not only unprecedented in the field of CS but is also particularly potent for applications in microfluidics in which the number of resolvable measurements per partition is often severely limited. We prove the identifiability property of the Poisson model under such lax conditions, analytically develop insights into system performance, and confirm these insights in simulated experiments. Our findings encourage a new approach to biosensing and are generalizable to other applications featuring spatial and temporal Poisson signals.

Robotic automation of droplet microfluidics

Droplet microfluidics enables powerful analytic capabilities but often requires workflows involving macro- and microfluidic processing steps that are cumbersome to perform manually. Here, we demonstrate the automation of droplet microfluidics with commercial fluid-handling robotics. The workflows incorporate common microfluidic devices including droplet generators, mergers, and sorters and utilize the robot's native capabilities for thermal control, incubation, and plate scanning. The ability to automate microfluidic devices using commercial fluid handling will speed up the integration of these methods into biological workflows.

Microfluidic-based functional materials: new prospects for wound healing and beyond

Microfluidics has been applied to fabricate high-performance functional materials contributing to all physiological stages of wound healing. The advances of microfluidic-based functional materials for wound healing have been summarized.

Microfluidic-Based Droplets for Advanced Regenerative Medicine: Current Challenges and Future Trends

Microfluidics is a promising approach for the facile and large-scale fabrication of monodispersed droplets for various applications in biomedicine. This technology has demonstrated great potential to address the limitations of regenerative medicine. Microfluidics provides safe, accurate, reliable, and cost-effective methods for encapsulating different stem cells, gametes, biomaterials, biomolecules, reagents, genes, and nanoparticles inside picoliter-sized droplets or droplet-derived microgels for different applications. Moreover, microenvironments made using such droplets can mimic niches of stem cells for cell therapy purposes, simulate native extracellular matrix (ECM) for tissue engineering applications, and remove challenges in cell encapsulation and three-dimensional (3D) culture methods. The fabrication of droplets using microfluidics also provides controllable microenvironments for manipulating gametes, fertilization, and embryo cultures for reproductive medicine. This review focuses on the relevant studies, and the latest progress in applying droplets in stem cell therapy, tissue engineering, reproductive biology, and gene therapy are separately evaluated. In the end, we discuss the challenges ahead in the field of microfluidics-based droplets for advanced regenerative medicine.

7-Aminocoumarin-4-acetic Acid as a Fluorescent Probe for Detecting Bacterial Dipeptidyl Peptidase Activities in Water-in-Oil Droplets and in Bulk

Droplet-based microfluidic systems are a powerful tool for biological assays with high throughput. Water-in-oil droplets (WODLs) are typically used in droplet-based microfluidic systems to culture microorganisms and perform enzyme assays. However, because of the oil surrounding the nanoliter and picoliter volumes of WODLs, availability of suitable substrates is limited. For instance, although 7-amino-4-methylcoumarin (AMC) is commonly used as a fluorescent probe of the substrate to detect peptidase activity, AMC leaks from WODLs to the oil phase due to its high hydrophobicity. Thus, AMC substrates cannot be used in droplet-based microfluidic systems with WODLs. In this study, we developed a peptidase substrate consisting of a dipeptide and 7-aminocoumarin-4-acetic acid (ACA), an AMC-derived fluorogenic compound. ACA was retained in the WODL for more than 7 days, and the dipeptidyl ACA substrate detected dipeptidyl peptidase (DPP) activity in the WODL. Compared to AMC substrates, the substrate specificity constants of DPPs for ACA substrates increased up to 4.7-fold. Fluorescence-activated droplet sorting made high-throughput screening of microorganisms based on DPP activity using the dipeptidyl ACA substrate possible. Since ACA could be applied to various substrates as a fluorescent probe, detectable microbial enzyme activities for droplet-based microfluidic systems can be largely expanded.

Active single cell encapsulation using SAW overcoming the limitations of Poisson distribution

We demonstrate the use of an acoustic device to actively encapsulate single red blood cells into individual droplets in a T-junction. We compare the active encapsulation with the passive encapsulation depending on the number of loaded cells as well as the created droplet volumes. This method overcomes the Poisson limitation statistical loading of cells for the passive encapsulation. In our experiments we reach a single cell encapsulation efficiency of 97.9 ± 2.1% at droplet formation rates exceeding 15 Hz.

Forty Years of Directed Evolution and its Continuously Evolving Technology Toolbox - A Review of the Patent Landscape

Generating functional protein variants with novel or improved characteristics has been a goal of the biotechnology industry and life sciences, for decades. Rational design and directed evolution are two major pathways to achieve the desired ends. Whilst rational protein design approach has made substantial progress, the idea of using a method based on cycles of mutagenesis and natural selection to develop novel binding proteins, enzymes and structures has attracted great attention. Laboratory evolution of proteins/enzymes requires new tools and analytical approaches to create genetic diversity and identifying variants with desired traits. In this pursuit, construction of sufficiently large libraries of target molecules to search for improved variants and the need for new protocols to alter the properties of target molecules has been a continuing challenge in the directed evolution experiments. This review will discuss the in vivo and in vitro gene diversification tools, library screening or selection approaches, and artificial intelligence/machine-learning-based strategies to mutagenesis developed in the last forty years to accelerate the natural process of evolution in creating new functional protein variants, optimization of microbial strains and transformation of enzymes into industrial machines. Analyzing patent position over these techniques and mechanisms also constitutes an integral and distinctive part of this review. The aim is to provide an up-to-date resource/technology toolbox for research-based and pharmaceutical companies to discover the boundaries of competitor's intellectual property (IP) portfolio, their freedom-to-operate in the relevant IP landscape, and the need for patent due diligence analysis to rule out whether use of a particular patented mutagenesis method, library screening/selection technique falls outside the safe harbor of experimental use exemption. While so doing, we have referred to some recent cases that emphasize the significance of selecting a suitable gene diversification strategy in directed evolution experiments. This article is protected by copyright. All rights reserved.

A plasmonic gold nanofilm-based microfluidic chip for rapid and inexpensive droplet-based photonic PCR

Polymerase chain reaction (PCR) is a powerful tool for nucleic acid amplification and quantification. However, long thermocycling time is a major limitation of the commercial PCR devices in the point-of-care (POC). Herein, we have developed a rapid droplet-based photonic PCR (dpPCR) system, including a gold (Au) nanofilm-based microfluidic chip and a plasmonic photothermal cycler. The chip is fabricated by adding mineral oil to uncured polydimethylsiloxane (PDMS) to suppress droplet evaporation in PDMS microfluidic chips during PCR thermocycling. A PDMS to gold bonding technique using a double-sided adhesive tape is applied to enhance the bonding strength between the oil-added PDMS and the gold nanofilm. Moreover, the gold nanofilm excited by two light-emitting diodes (LEDs) from the top and bottom sides of the chip provides fast heating of the PCR sample to 230 °C within 100 s. Such a design enables 30 thermal cycles from 60 to 95 °C within 13 min with the average heating and cooling rates of 7.37 ± 0.27 °C/s and 1.91 ± 0.03 °C/s, respectively. The experimental results demonstrate successful PCR amplification of the alcohol oxidase (AOX) gene using the rapid plasmonic photothermal cycler and exhibit the great performance of the microfluidic chip for droplet-based PCR.

Pipette-Tip-Enabled Digital Nucleic Acid Analyzer for COVID-19 Testing with Isothermal Amplification

Herein, a pipette-tip-enabled digital nucleic acid analyzer for high-performance COVID-19 testing is demonstrated. This is achieved by digital loop-mediated isothermal amplification (digital LAMP or dLAMP) using common laboratory equipment and materials. It is shown that simply fixing a glass capillary inside conventional pipette tips enables the generation of monodisperse, water-in-oil microdroplets with benchtop centrifugation. It is shown that using LAMP, the ORF1a/b gene, a standard test region for COVID-19 screening, can be amplified without a thermal cycler. The amplification allows counting of fluorescent microdroplets so that Poisson analysis can be performed to allow quantification with a limit of detection that is 1 order of magnitude better than those of nondigital techniques and comparable to those of commercial dLAMP platforms. It is envisioned that this work will inspire studies on ultrasensitive digital nucleic acid analyzers demanding both sensitivity and accessibility, which is pivotal to their large-scale applications.

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.

Microfluidics as a Novel Technique for Tuberculosis: From Diagnostics to Drug Discovery

Tuberculosis (TB) remains a global healthcare crisis, with an estimated 5.8 million new cases and 1.5 million deaths in 2020. TB is caused by infection with the major human pathogen Mycobacterium tuberculosis, which is difficult to rapidly diagnose and treat. There is an urgent need for new methods of diagnosis, sufficient in vitro models that capably mimic all physiological conditions of the infection, and high-throughput drug screening platforms. Microfluidic-based techniques provide single-cell analysis which reduces experimental time and the cost of reagents, and have been extremely useful for gaining insight into monitoring microorganisms. This review outlines the field of microfluidics and discusses the use of this novel technique so far in M. tuberculosis diagnostics, research methods, and drug discovery platforms. The practices of microfluidics have promising future applications for diagnosing and treating TB.

Design and Manufacture of a Low-Cost Microfluidic System for the Synthesis of Giant Liposomes for the Encapsulation of Yeast Homologues: Applications in the Screening of Membrane-Active Peptide Libraries

The discovery of new membrane-active peptides (MAPs) is an area of considerable interest in modern biotechnology considering their ample applicability in several fields ranging from the development of novel delivery vehicles (via cell-penetrating peptides) to responding to the latent threat of antibiotic resistance (via antimicrobial peptides). Different strategies have been devised for such discovery process, however, most of them involve costly, tedious, and low-efficiency methods. We have recently proposed an alternative route based on constructing a non-rationally designed library recombinantly expressed on the yeasts’ surfaces. However, a major challenge is to conduct a robust and high-throughput screening of possible candidates with membrane activity. Here, we addressed this issue by putting forward low-cost microfluidic platforms for both the synthesis of Giant Unilamellar Vesicles (GUVs) as mimicking entities of cell membranes and for providing intimate contact between GUVs and homologues of yeasts expressing MAPs. The homologues were chitosan microparticles functionalized with the membrane translocating peptide Buforin II, while intimate contact was through passive micromixers with different channel geometries. Both microfluidic platforms were evaluated both in silico (via Multiphysics simulations) and in vitro with a high agreement between the two approaches. Large and stable GUVs (5–100 µm) were synthesized effectively, and the mixing processes were comprehensively studied leading to finding the best operating parameters. A serpentine micromixer equipped with circular features showed the highest average encapsulation efficiencies, which was explained by the unique mixing patterns achieved within the device. The microfluidic devices developed here demonstrate high potential as platforms for the discovery of novel MAPs as well as for other applications in the biomedical field such as the encapsulation and controlled delivery of bioactive compounds.

Measurement methods of single cell drug response

In the last decades, a wide multitude of research activity has been focused on the development of new drugs, and devoted to overcome the challenges of high cost and low efficiency in drug evaluation. The measurement of drug response at the single cell level is a quicker, more direct and more accurate way to reflect drug efficacy, which can shorten the drug development period and reduce research costs. Therefore, the single cell drug response (SCDR) measurement technology has aroused extensive attention from researchers, and has become a hot topic in the fields of drug research and cell biology. Recent years have seen the emergence of various SCDR measurement technologies that feature different working principles and different levels of measurement performance. To better examine, compare and summarize the characteristics and functions of these technologies, we select signal-to-noise ratio, throughput, content, invasion, and device complexity as the criteria to evaluate them from the drug efficacy perspective. This review aims to highlight sixteen kinds of SCDR measurement technologies, including patch-clamp technique, live-cell interferometry, capillary electrophoresis, secondary ion mass spectrometry, and more, and report widespread representative examples of SCDR measurement the recent approaches for over the past forty years. Based on their reaction principles, these technologies are classified into four categories: electrical, optical, electrochemical, and mass spectrometry, and a detailed comparison is made between them. After in-depth understanding of these technologies, it is expected to improve or integrate these technologies to propose better SCDR measurement strategies, and explore methods in new drug development and screening, as well as disease diagnosis and treatment.

Microfluidic encapsulation for controlled release and its potential for nanofertilisers

Nanotechnology is increasingly being utilized to create advanced materials with improved or new functional attributes. Converting fertilizers into a nanoparticle-form has been shown to improve their efficacy but the current procedures used to fabricate nanofertilisers often have poor reproducibility and flexibility. Microfluidic systems, on the other hand, have advantages over traditional nanoparticle fabrication methods in terms of energy and materials consumption, versatility, and controllability. The increased controllability can result in the formation of nanoparticles with precise and complex morphologies (e.g., tuneable sizes, low polydispersity, and multi-core structures). As a result, their functional performance can be tailored to specific applications. This paper reviews the principles, formation, and applications of nano-enabled delivery systems fabricated using microfluidic approaches for the encapsulation, protection, and release of fertilizers. Controlled release can be achieved using two main routes: (i) nutrients adsorbed on nanosupports and (ii) nutrients encapsulated inside nanostructures. We aim to highlight the opportunities for preparing a new generation of highly versatile nanofertilisers using microfluidic systems. We will explore several main characteristics of microfluidically prepared nanofertilisers, including droplet formation, shell fine-tuning, adsorbate fine-tuning, and sustained/triggered release behavior.

Recent Advances in Microfluidic Platforms for Programming Cell-Based Living Materials

Cell-based living materials, including single cells, cell-laden fibers, cell sheets, organoids, and organs, have attracted intensive interests owing to their widespread applications in cancer therapy, regenerative medicine, drug development, and so on. Significant progress in materials, microfabrication, and cell biology have promoted the development of numerous promising microfluidic platforms for programming these cell-based living materials with a high-throughput, scalable, and efficient manner. In this review, the recent progress of novel microfluidic platforms for programming cell-based living materials is presented. First, the unique features, categories, and materials and related fabrication methods of microfluidic platforms are briefly introduced. From the viewpoint of the design principles of the microfluidic platforms, the recent significant advances of programming single cells, cell-laden fibers, cell sheets, organoids, and organs in turns are then highlighted. Last, by providing personal perspectives on challenges and future trends, this review aims to motivate researchers from the fields of materials and engineering to work together with biologists and physicians to promote the development of cell-based living materials for human healthcare-related applications.

Sorting droplets into many outlets

Droplet microfluidics is a commercially successful technology, widely used in single cell sequencing and droplet PCR. Combining droplet making with droplet sorting has also been demonstrated, but so far found limited use, partly due to difficulties in scaling manufacture with injection molded plastics. We introduce a droplet sorting system with several new elements, including: 1) an electrode design combining metallic and ionic liquid parts, 2) a modular, multi-sorting fluidic design with features for keeping inter-droplet distances constant, 3) using timing parameters calculated from fluorescence or scatter signal triggers to precisely actuate dozens of sorting electrodes, 4) droplet collection techniques, including ability to collect a single droplet, and 5) a new emulsion breaking method to collect aqueous samples for downstream analysis. We use these technologies to build a fluorescence based cell sorter that can sort with high (>90%) purity. We also show that these microfluidic designs can be translated into injection molded thermoplastic, suitable for industrial production. Finally, we tally the advantages and limitations of these devices.

Direct Measurement of Initial Rate of Enzyme Reaction by Electrokinetic Filtration Using a Hydrogel-plugged Capillary Device

A novel electrokinetic filtration device using a plugged hydrogel was developed to directly measure the initial rate of enzyme reactions. In the proposed method, the enzyme reaction proceeded only for a short time when the substrate was passed through a thin layer of enzyme trapped by the hydrogel without any lag times for mixing and detection. In experimental conditions, alkaline phosphatase (enzyme) was filtrated at a cathodic-side interface of the plugged hydrogel by molecular sieving effect, providing the thin enzyme zone whose thickness was approximately 100 μm. When 4-methylumberiferyl phosphate (substrate) was electrokinetically introduced into the device after trapping the enzyme, 4-methylumberiferone (product) was generated by the enzyme reaction for only 1.26 s as the substrate passed through the trapped enzyme zone. As a result, the initial rate of the enzyme reaction could be directly calculated to 31.0 μM/s by simply dividing the concentration of the product by the tunable reaction time. Compared to the initial rate obtained by mixing the enzyme and substrate solutions, the value of the maximum velocity of the enzyme reaction was 30-fold larger than that in the mixing method due to the preconcentration of the enzyme by trapping. The Michaelis-Menten constant in the proposed method was 2.7-fold larger than that in the mixing method, suggesting the variation of changes in the equilibrium of complex formation under the experimental conditions.

Faster, better, and cheaper: harnessing microfluidics and mass spectrometry for biotechnology

High-throughput screening technologies are widely used for elucidating biological activities. These typically require trade-offs in assay specificity and sensitivity to achieve higher throughput. Microfluidic approaches enable rapid manipulation of small volumes and have found a wide range of applications in biotechnology providing improved control of reaction conditions, faster assays, and reduced reagent consumption. The integration of mass spectrometry with microfluidics has the potential to create high-throughput, sensitivity, and specificity assays. This review introduces the widely-used mass spectrometry ionization techniques that have been successfully integrated with microfluidics approaches such as continuous-flow system, microchip electrophoresis, droplet microfluidics, digital microfluidics, centrifugal microfluidics, and paper microfluidics. In addition, we discuss recent applications of microfluidics integrated with mass spectrometry in single-cell analysis, compound screening, and the study of microorganisms. Lastly, we provide future outlooks towards online coupling, improving the sensitivity and integration of multi-omics into a single platform.

Tunable and Contamination-Free Injection with Microfluidics by Stepinjection

Droplet microfluidics with picoinjection provides significant advantages to multistep reactions and screenings. The T-junction design for picoinjection is convenient in adding picoliter reagents into passing droplets to initiate reactions. However, conventional picoinjectors face difficulties in eliminating cross-contamination between droplets, preventing them from widespread use in sensitive biological and molecular assays. Here, we introduce stepinjection, which uses a T-junction with a stepped channel design to elevate the diffusional buffer zone into the main channel and consequently increases the pressure difference between droplets and the inlet of the injection channel. To demonstrate the stepinjector's ability to perform contamination-sensitive enzymatic assays, we inject casein fluorescein isothiocyanate (FITC-casein) into a mixture of savinase and savinase-free (labeled with a red fluorescent dye) droplets. We observe no cross-contamination using stepinjection but find a severe cross-talk using an optimal picoinjection design. We envision that the simple, tunable, and reliable stepinjector can be easily integrated in various droplet processing devices, and facilitate various biomedical and biochemical applications including multiplex digital PCR, single-cell sequencing, and enzymatic screening.

Optoelectronic manipulation of bio-droplets containing cells or macromolecules by active ferroelectric platforms

Photovoltaic optoelectronic tweezers are a useful platform with many applications in optical manipulation and nanotechnology. They are based on electrical forces associated with the bulk photovoltaic effect presented by certain ferroelectric crystals, such as Fe doped lithium niobate. This manipulation technique has experienced huge developments in recent years, although its use in biology and biomedicine is still scarce. Recently, a novel strategy has been reported that extends the platform capabilities to the manipulation of polar droplets, such as water and aqueous bio-droplets, promising great potential for biological applications. In this work, we are taking this challenge, addressing the manipulation of cells and macromolecules contained inside the droplets by optoelectronic ferroelectric platforms. On the one hand, experiments of photoelectric induced migration of DNA and sperm droplets have been successfully developed and the corresponding droplet dynamics have been analyzed in depth. From this analysis, parameters of the biomaterial such as its concentration and its electrical charge have been evaluated, showing the sensing capabilities of the platform. In fact, the charge of sperm cells has been demonstrated to be negative, and the relative sperm concentration of the samples determined. On the other hand, experiments on the light-induced merging of two droplets have been carried out. Specifically, sperm droplets are mixed with droplets containing acridine orange, a convenient dye for visualization purposes. The spermatozoa become clearly visible in the final droplet through fluorescence imaging. The results point out the multiple possibilities of application of the optoelectronic ferroelectric platform in biology and biomedicine including the development of "lab on a chip" devices. Hence, these capabilities introduce these platforms as an efficient tool in biotechnology.

Portable all-in-one automated microfluidic system (PAMICON) with 3D-printed chip using novel fluid control mechanism

State-of-the-art microfluidic systems rely on relatively expensive and bulky off-chip infrastructures. The core of a system—the microfluidic chip—requires a clean room and dedicated skills to be fabricated. Thus, state-of-the-art microfluidic systems are barely accessible, especially for the do-it-yourself (DIY) community or enthusiasts. Recent emerging technology—3D-printing—has shown promise to fabricate microfluidic chips more simply, but the resulting chip is mainly hardened and single-layered and can hardly replace the state-of-the-art Polydimethylsiloxane (PDMS) chip. There exists no convenient fluidic control mechanism yet suitable for the hardened single-layered chip, and particularly, the hardened single-layered chip cannot replicate the pneumatic valve—an essential actuator for automatically controlled microfluidics. Instead, 3D-printable non-pneumatic or manually actuated valve designs are reported, but their application is limited. Here, we present a low-cost accessible all-in-one portable microfluidic system, which uses an easy-to-print single-layered 3D-printed microfluidic chip along with a novel active control mechanism for fluids to enable more applications. This active control mechanism is based on air or gas interception and can, e.g., block, direct, and transport fluid. As a demonstration, we show the system can automatically control the fluid in microfluidic chips, which we designed and printed with a consumer-grade 3D-printer. The system is comparably compact and can automatically perform user-programmed experiments. All operations can be done directly on the system with no additional host device required. This work could support the spread of low budget accessible microfluidic systems as portable, usable on-the-go devices and increase the application field of 3D-printed microfluidic devices.

Deep-learning on-chip light-sheet microscopy enabling video-rate volumetric imaging of dynamic biological specimens

Volumetric imaging of dynamic signals in a large, moving, and light-scattering specimen is extremely challenging, owing to the requirement on high spatiotemporal resolution and difficulty in obtaining high-contrast signals. Here we report that through combining a microfluidic chip-enabled digital scanning light-sheet illumination strategy with deep-learning based image restoration, we can realize isotropic 3D imaging of a whole crawling Drosophila larva on an ordinary inverted microscope at a single-cell resolution and a high volumetric imaging rate up to 20 Hz. Enabled with high performances even unmet by current standard light-sheet fluorescence microscopes, we in toto record the neural activities during the forward and backward crawling of a 1st instar larva, and successfully correlate the calcium spiking of motor neurons with the locomotion patterns.

Real-time precise microfluidic droplets label-sequencing combined in a velocity detection sensor

Droplets microfluidics is broadening the range of Lab on a Chip solutions that, however, still suffer from the lack of an adequate level of integration of optical detection and sensors. In fact, droplets are currently monitored by imaging techniques, mostly limited by a time-consuming data post-processing and big data storage. This work aims to overcome this weakness, presenting a fully integrated opto-microfluidic platform able to detect, label and characterize droplets without the need for imaging techniques. It consists of optical waveguides arranged in a Mach Zehnder's configuration and a microfluidic circuit both coupled in the same substrate. As a proof of concept, the work demonstrates the performances of this opto-microfluidic platform in performing a complete and simultaneous sequence labelling and identification of each single droplet, in terms of its optical properties, as well as velocity and lengths. Since the sensor is realized in lithium niobate crystals, which is also highly resistant to chemical attack and biocompatible, the future addition of multifunctional stages into the same substrate can be easily envisioned, extending the range of applicability of the final device.

Fused filament fabrication 3D printed polylactic acid electroosmotic pumps

Additive manufacturing (3D printing) offers a flexible approach for the production of bespoke microfluidic structures such as the electroosmotic pump. Here a readily accessible fused filament fabrication (FFF) 3D printing technique has been employed for the first time to produce microcapillary structures using low cost thermoplastics in a scalable electroosmotic pump application. Capillary structures were formed using a negative space 3D printing approach to deposit longitudinal filament arrangements with polylactic acid (PLA) in either "face-centre cubic" or "body-centre cubic" arrangements, where the voids deliberately formed within the deposited structure act as functional micro-capillaries. These 3D printed capillary structures were shown to be capable of functioning as a simple electroosmotic pump (EOP), where the maximum flow rate of a single capillary EOP was up to 1.0 μl min^-1 at electric fields of up to 750 V cm^-1. Importantly, higher flow rates were readily achieved by printing parallel multiplexed capillary arrays.

Development of aldolase-based catalysts for the synthesis of organic chemicals

Aldol chemicals are synthesized by condensation reactions between the carbon units of ketones and aldehydes using aldolases. The efficient synthesis of diverse organic chemicals requires intrinsic modification of aldolases via engineering and design, as well as extrinsic modification through immobilization or combination with other catalysts. This review describes the development of aldolases, including their engineering and design, and the selection of desired aldolases using high-throughput screening, to enhance their catalytic properties and perform novel reactions. Aldolase-containing catalysts, which catalyze the aldol reaction combined with other enzymatic and/or chemical reactions, can efficiently synthesize diverse complex organic chemicals using inexpensive and simple materials as substrates. We also discuss the current challenges and emerging solutions for aldolase-based catalysts.

Digital Hybridization Human Papillomavirus Assay with Attomolar Sensitivity without Amplification

Detection of nucleic acid without amplification can avoid problems associated with thermal cycling such as labor-intensiveness and aerosol pollution. Here we develop a droplet-based digital microfluidic hybridization assay for nucleic acid detection with attomolar sensitivity. This assay provides a clinically useful sensitivity for detecting human papillomavirus (HPV) without amplification. The sensitivity is accomplished using femtoliter-sized droplet microfluidics for concentrating enzyme-catalyzed fluorescent products into a detectable signal and magnetic beads for accelerating reaction time. Meanwhile, using magnetic beads and droplet microfluidic chips, we can improve the sampling efficiency over conventional methods. We characterized the sensitivity, selectivity, detection range, stability, and accuracy of our assay. Our assay is 50-fold more sensitive than the traditional hybrid capture assay. The assay without amplification avoids problems of complex handling procedures and aerosol pollution. The direct and sensitive detection of nucleic acid using a droplet microfluidic system provides an early disease diagnosis tool.

Therapeutic oxygen delivery by perfluorocarbon-based colloids

After the protocol-related indecisive clinical trial of Oxygent, a perfluorooctylbromide/phospholipid nanoemulsion, in cardiac surgery, that often unduly assigned the observed untoward effects to the product, the development of perfluorocarbon (PFC)-based O₂ nanoemulsions ("blood substitutes") has come to a low. Yet, significant further demonstrations of PFC O₂-delivery efficacy have continuously been reported, such as relief of hypoxia after myocardial infarction or stroke; protection of vital organs during surgery; potentiation of O₂-dependent cancer therapies, including radio-, photodynamic-, chemo- and immunotherapies; regeneration of damaged nerve, bone or cartilage; preservation of organ grafts destined for transplantation; and control of gas supply in tissue engineering and biotechnological productions. PFC colloids capable of augmenting O₂ delivery include primarily injectable PFC nanoemulsions, microbubbles and phase-shift nanoemulsions. Careful selection of PFC and other colloid components is critical. The basics of O₂ delivery by PFC nanoemulsions will be briefly reminded. Improved knowledge of O₂ delivery mechanisms has been acquired. Advanced, size-adjustable O₂-delivering nanoemulsions have been designed that have extended room-temperature shelf-stability. Alternate O₂ delivery options are being investigated that rely on injectable PFC-stabilized microbubbles or phase-shift PFC nanoemulsions. The latter combine prolonged circulation in the vasculature, capacity for penetrating tumor tissues, and acute responsiveness to ultrasound and other external stimuli. Progress in microbubble and phase-shift emulsion engineering, control of phase-shift activation (vaporization), understanding and control of bubble/ultrasound/tissue interactions is discussed. Control of the phase-shift event and of microbubble size require utmost attention. Further PFC-based colloidal systems, including polymeric micelles, PFC-loaded organic or inorganic nanoparticles and scaffolds, have been devised that also carry substantial amounts of O₂. Local, on-demand O₂ delivery can be triggered by external stimuli, including focused ultrasound irradiation or tumor microenvironment. PFC colloid functionalization and targeting can help adjust their properties for specific indications, augment their efficacy, improve safety profiles, and expand the range of their indications. Many new medical and biotechnological applications involving fluorinated colloids are being assessed, including in the clinic. Further uses of PFC-based colloidal nanotherapeutics will be briefly mentioned that concern contrast diagnostic imaging, including molecular imaging and immune cell tracking; controlled delivery of therapeutic energy, as for noninvasive surgical ablation and sonothrombolysis; and delivery of drugs and genes, including across the blood-brain barrier. Even when the fluorinated colloids investigated are designed for other purposes than O₂ supply, they will inevitably also carry and deliver a certain amount of O₂, and may thus be considered for O₂ delivery or co-delivery applications. Conversely, O₂-carrying PFC nanoemulsions possess by nature a unique aptitude for ¹⁹F MR imaging, and hence, cell tracking, while PFC-stabilized microbubbles are ideal resonators for ultrasound contrast imaging and can undergo precise manipulation and on-demand destruction by ultrasound waves, thereby opening multiple theranostic opportunities.