Microfluidic large-scale integration

Microfluidic hydraulic oscillators: A comprehensive review of emerging biochemical and biomedical applications

Microfluidics provides microenvironments for drug delivery, transport, mixing, chemical reaction, cell culture, and tissue engineering. Developing microfluidic circuit networks comparable to electronic circuits is beneficial because they can significantly reduce the need for dynamic off-chip controllers and reagent volume. A microfluidic hydraulic oscillator (MHO) is a fluidic circuit network analogous to a hydraulic-electric system that converts constant input into a pulsatile output. The challenge lies in integrating the MHO with an on-chip controller for biochemical applications and precise fluid control. Herein, we present fundamental working principles and components of multiple types of MHO to produce pulsatile pressure and review the current biochemical applications of MHO. First, we present fundamental working principles of multiple types of MHO and components to build the MHO to produce pulsatile pressure. The coupling of the MHO with various on-chip controllers, such as a diode pump, reset valve, droplet generator, filter, etc., will then be explored. Next, current applications are discussed, including their employment in chemistry for mixing, crystallization, coating, biomedical for cellular biology, filtration, staining, and amplification of targets. Finally, we explore the potential future application of MHO to show its versatility. The adaptive nature of MHO highlights their potential to transform biochemical and biomedical applications, from precise fluid control in point-of-care (POC) and lab-on-chip devices to innovative diagnostic and therapeutic solutions.

Controlled Nanoconfinement in a Microfluidic Modular Bead Array Device via Elastomeric Diaphragm Collapse for Enhancing Biomolecular Binding Kinetics

Nanoscale confinement strategies alleviate diffusional transport limitations to enhance target binding kinetics with receptors, motivating their utilization for screening and selecting receptors based on binding affinities with target molecules. Herein, a modular and multiplexed device for creating nanoconfinement is presented through the collapse of an elastomeric diaphragm onto microbead arrays immobilized with biomolecules, followed by repeated diaphragm withdrawal to promote bulk transport, thereby enhancing receptor binding kinetics. To repeatedly create controlled nanoconfinement over large spatial extents on the bead, the diaphragm is integrated on its top side with a strain sensor for modulating vertical displacement, while microfabricated nanoposts (≈500 nm depth) on its bottom side control the lateral extent. The modular platform enables facile assembly of beads, each immobilized with different targets into eight microwells for multiplexed screening of receptors, and facile disassembly for quantifying DNA-binding on each bead by downstream q-PCR. Nanoconfinement enhances biomolecular binding at 1 Hz diaphragm pressurization, as validated by rapid DNA immobilization (time constant of ≈6 min vs >60 min under no confinement) and through saturating the binding of target molecules with optimal aptamer candidates (88% site occupancy vs 5% under no confinement at 10 nm levels), thereby screening candidate receptors based on binding affinity parameters.

Advances in Microfluidics-Enabled Dimensional Design of Micro-/Nanomaterials for Biomedical Applications: A Review

Biomedical materials are of great significance for preventing and treating major diseases and protecting human health. At present, more stringent requirements have been put forward for the preparation methods and dimension control of biomedical materials based on the urgent demand for high-performance biomedical materials, especially the existence of various physiological size thresholds in vitro/in vivo. Microfluidic platforms break the limitations of traditional micro-/nanomaterial synthesis, which provide a miniaturized and highly controlled environment for size-dependent biomaterials. In this review, the basic conceptions and technical characteristics of microfluidics are first described. Then the syntheses of biomedical materials with different dimensions (0D, 1D, 2D, 3D) driven by microfluidics have been systematically summarized. Meanwhile, the applications of microfluidics-driven biomedical materials, including diagnosis, anti-inflammatory, drug delivery, antibacterial, and disease therapy, are discussed. Furthermore, the challenges and developments in the research field are further proposed. This work is expected to facilitate the convergence between the bioscience and engineering communities and continue to contribute to this emerging field.

Rapid High-Throughput Discovery of Molecules With Antimicrobial Activity From Natural Products Enabled by a Nanoliter Matrix SlipChip

Improper use of antibiotics has led to the development of antimicrobial resistance, or "superbugs," outpacing the discovery of new antibiotics. The lack of rapid, high-throughput screening methods is a major bottleneck in discovery novel antibiotics. Traditional methods consume significant amounts of samples, making it challenging to discover new antibiotics from limited natural product extracts. Here, a rapid, high-throughput screening method is reported for natural products with antimicrobial activity enabled by a nanoliter matrix SlipChip (nm-SlipChip). This nm-SlipChip creates a screening matrix with nanoliter droplets for 100 drug candidate-bacterium combinations. The effectiveness of candidate antibiotics is assessed by analyzing microbial phenotypic changes. This nm-SlipChip reduces sample consumption by over 5000-fold and shortens the detection time to three hours. Twenty compounds isolated from Callicarpa integerrima were tested against 10 pathogenic bacteria and identified two previously unreported clerodane diterpenes with activity against methicillin-resistant Staphylococcus aureus (MRSA). Molecular docking and fluorescence probe experiments reveals that their antimicrobial effect results from disruption of bacterial cell membranes and biofilms. The nm-SlipChip provides an effective method for discovering new antimicrobial drugs from natural sources, vital in combating antibiotic resistance.

Bioinspired Liquid Pockets with Externally Induced Internal Microscale Flow

The gastric mucosal barrier, through its gastric pits, serves as a pathway for secretions, ensuring that mucus produced by the gastric glands is transferred to the gastric lumen, providing stable protection. Here a bioinspired liquid pockets material is shown, composed of a thermo-driven hydrogel that acts as an external activation unit to release interflowing liquid responsively, and porous matrices that serve as interconnected pockets to transfer it, enabling controlled internal flow and adaptive barrier functionality. Experiments and theoretical analysis demonstrate the stability and regulatory mechanisms of these liquid pockets, based on the interconnected pockets between the external activation unit and internal fluid flow. It exhibits a new pathway for regulating microscale flow at responsive material interfaces, enabling applications from sequential drug release, self-cleaning, and antifouling to anti-swelling. These unique capabilities address long-standing challenges in microscale flow control, with potential impacts in diverse fields including microfluidics, drug delivery, medical devices, 3D printing, and beyond.

SlipChip Enables the Integration of CRISPR-Cas12a and RPA for Fast and Stand-Alone HPV Detection

Human papillomavirus (HPV) screening is vital for the early detection and prevention of cervical cancer. However, existing methods often face challenges related to speed, simplicity, and multiplexing, especially in resource-limited settings. Here we developed a portable SlipChip-based multiplexed and rapid nucleic acid testing platform, named SMART, designed to simultaneously detect HPV16 and HPV18. SMART allows seamless integration of the RPA and Cas12a assays on the SlipChip and includes a heating membrane to regulate the on-chip assay temperatures. This allows SMART to operate as a stand-alone platform without additional control instruments. The platform also features an All-in-One imaging mode for rapid on-chip data acquisition, enhancing its performance. SMART enables sensitive detection of HPV16 and HPV18 DNA across multiple samples in just 36 min with a detection limit of approximately 6 copies per reaction. Testing of 56 clinical samples at risk of HPV infection validated SMART's performance, showing 97.7% sensitivity and 100% specificity. In summary, SMART offers a stand-alone system capable of rapidly distinguishing between the two most harmful HPV subtypes, showcasing the significant potential for rapid, multiplexed nucleic acid testing in various applications.

A Robust Normally Closed Pneumatic Valve for Integrated Microfluidic Flow Control

Accurate fluid management in microfluidic-based point-of-care testing (POCT) devices is critical. Fluids must be gated and directed in precise sequences to facilitate desired biochemical reactions and signal detection. Pneumatic valves are widely utilized for fluid gating due to their flexibility and simplicity. However, the development of reliable normally closed pneumatic valves remains challenging, despite their increasing demand in advanced POCT applications to prevent uncontrolled fluid flow. Existing normally closed valves often suffer from poor reliability and lack precise control over fluid opening pressure, due to the uncontrolled stretching of the elastomer during assembly. In this study, we propose and develop a robust method for normally closed valves. By precisely controlling the pre-stretching of the elastomer, we achieve reliable valve closure and accurate control of the opening pressure. A robust normally closed valve was designed and fabricated, and its pneumatic opening pressure was systematically studied. Experimental validations were conducted to demonstrate the reliability and effectiveness of the proposed design.

Transformation gap from research findings to large-scale commercialized products in microfluidic field

The field of microfluidics has experienced rapid growth in the last several decades, yet it isn't considered to be a large industry comparable to semiconductor and consumer electronics. In this review, we analyzed the entire process of the transformation from research findings to commercialized products in microfluidics, as well as the significant gap during the whole developing process between microchip fabrication in R&D and large-scale production in the industry. We elaborated in detail on various materials in the microfluidics industry, including silicon, glass, PDMS, and thermoplastics, discussing their characteristics, production processes, and existing products. Despite challenges hindering the large-scale commercialization of microfluidic chips, ongoing advancements and applications are expected to integrate microfluidic technology into everyday life, transforming it into a commercially viable field with substantial potential and promising prospects.

Integrated SERS-Microfluidic Sensor Based on Nano-Micro Hierarchical Cactus-like Array Substrates for the Early Diagnosis of Prostate Cancer

The detection and analysis of cancer cell exosomes with high sensitivity and precision are pivotal for the early diagnosis and treatment strategies of prostate cancer. To this end, a microfluidic chip, equipped with a cactus-like array substrate (CAS) based on surface-enhanced Raman spectroscopy (SERS) was designed and fabricated for the detection of exosome concentrations in Lymph Node Carcinoma of the Prostate (LNCaP). Double layers of polystyrene (PS) microspheres were self-assembled onto a polyethylene terephthalate (PET) film to form an ordered cactus-like nanoarray for detection and analysis. By combining EpCAM aptamer-labeled SERS nanoprobes and a CD63 aptamer-labeled CAS, a 'sandwich' structure was formed and applied to the microfluidic chips, further enhancing the Raman scattering signal of Raman reporter molecules. The results indicate that the integrated microfluidic sensor exhibits a good linear response within the detection concentration range of 105 particles μL-1 to 1 particle μL-1. The detection limit of exosomes in cancer cells can reach 1 particle μL-1. Therefore, we believed that the CAS integrated microfluidic sensor offers a superior solution for the early diagnosis and therapeutic intervention of prostate cancer.

Large-scale-integration and collective oscillations of 2D artificial cells

The on-chip large-scale-integration of genetically programmed artificial cells capable of exhibiting collective expression patterns is important for fundamental research and biotechnology. Here, we report a 3D biochip with a 2D layout of 1024 DNA compartments as artificial cells on a 5 × 5 mm2 area. Homeostatic cell-free protein synthesis reactions driven by genetic circuits occur inside the compartments. We create a reaction-diffusion system with a 30 × 30 square lattice of artificial cells interconnected by thin capillaries for diffusion of products. We program the connected lattice with a synthetic genetic oscillator and observe collective oscillations. The microscopic dimensions of the unit cell and capillaries set the effective diffusion and coupling strength in the lattice, which in turn affects the macroscopic synchronization dynamics. Strongly coupled oscillators exhibit fast and continuous 2D fronts emanating from the boundaries, which generate smooth and large-scale correlated spatial variations of the oscillator phases. This opens a class of 2D genetically programmed nonequilibrium synthetic multicellular systems, where chemical energy dissipated in protein synthesis leads to large-scale spatiotemporal patterns.

Single-Cell Microfluidics: A Primer for Microbiologists

Recent advances in microfluidic technology have made it possible to image live bacterial cells with a high degree of precision and control. In particular, single-cell microfluidic designs have created new opportunities to study phenotypic variation in bacterial populations. However, the development and use of microfluidic devices require specialized resources, and these can be practical barriers to entry for microbiologists. With this review, our intentions are to help demystify the design, construction, and application of microfluidics. Our approach is to present design elements as building blocks from which a multitude of microfluidics applications can be imagined by the microbiologist.

Gas Evolution in Water Electrolysis

Gas bubbles generated by the hydrogen evolution reaction and oxygen evolution reaction during water electrolysis influence the energy conversion efficiency of hydrogen production. Here, we survey what is known about the interaction of gas bubbles and electrode surfaces and the influence of gas evolution on practicable devices used for water electrolysis. We outline the physical processes occurring during the life cycle of a bubble, summarize techniques used to characterize gas evolution phenomena in situ and in practical device environments, and discuss ways that electrodes can be tailored to facilitate gas removal at high current densities. Lastly, we review efforts to model the behavior of individual gas bubbles and multiphase flows produced at gas-evolving electrodes. We conclude our review with a short summary of outstanding questions that could be answered by future efforts to characterize gas evolution in electrochemical device environments or by improved simulations of multiphase flows.

Rapid identification of bacterial isolates using microfluidic adaptive channels and multiplexed fluorescence microscopy

We demonstrate the rapid capture, enrichment, and identification of bacterial pathogens using Adaptive Channel Bacterial Capture (ACBC) devices. Using controlled tuning of device backpressure in polydimethylsiloxane (PDMS) devices, we enable the controlled formation of capture regions capable of trapping bacteria from low cell density samples with near 100% capture efficiency. The technical demands to prepare such devices are much lower compared to conventional methods for bacterial trapping and can be achieved with simple benchtop fabrication methods. We demonstrate the capture and identification of seven species of bacteria with bacterial concentrations lower than 1000 cells per mL, including common Gram-negative and Gram-positive pathogens such as Escherichia coli and Staphylococcus aureus. We further demonstrate that species identification of the trapped bacteria can be undertaken in the order of one-hour using multiplexed 16S rRNA-FISH with identification accuracies of 70-98% with unsupervised classification methods across 7 species of bacteria. Finally, by using the bacterial capture capabilities of the ACBC chip with an ultra-rapid antimicrobial susceptibility testing method employing fluorescence imaging and convolutional neural network (CNN) classification, we demonstrate that we can use the ACBC chip as an imaging flow cytometer that can predict the antibiotic susceptibility of E. coli cells after identification.

Microfluidic programmable strategies for channels and flow

This review summarizes programmable microfluidics, an advanced method for precise fluid control in microfluidic technology through microchannel design or liquid properties, referring to microvalves, micropumps, digital microfluidics, multiplexers, micromixers, slip-, and block-based configurations. Different microvalve types, including electrokinetic, hydraulic/pneumatic, pinch, phase-change and check valves, cater to diverse experimental needs. Programmable micropumps, such as passive and active micropumps, play a crucial role in achieving precise fluid control and automation. Due to their small size and high integration, microvalves and micropumps are widely used in medical devices and biological analysis. In addition, this review provides an in-depth exploration of the applications of digital microfluidics, multiplexed microfluidics, and mixer-based microfluidics in the manipulation of liquid movement, mixing, and splitting. These methodologies leverage the physical properties of liquids, such as capillary forces and dielectric forces, to achieve precise control over fluid dynamics. SlipChip technology, which branches into rotational SlipChip and translational SlipChip, controls fluid through sliding motion of the microchannel. On the other hand, innovative designs in microfluidic systems pursue better modularity, reconfigurability and ease of assembly. Different assembly strategies, from one-dimensional assembly blocks and two-dimensional Lego®-style blocks to three-dimensional reconfigurable modules, aim to enhance flexibility and accessibility. These technologies enhance user-friendliness and accessibility by offering integrated control systems, making them potentially usable outside of specialized technical labs. Microfluidic programmable strategies for channels and flow hold promising applications in biomedical research, chemical analysis and drug screening, providing theoretical and practical guidance for broader utilization in scientific research and practical applications.

SUPERCELLS: a novel microfluidic reactor architecture for ultra-fast sequential delivery of chemical reagents

Applications such as nucleic acid synthesis or next-generation sequencing involve repeated fluidic cycles with the same set of reagents. The large dead volumes present in external valves and pumps with relatively long supply lines mandate the inclusion of extensive rinsing steps in current protocols, resulting in the consumption of significant quantities of reagents. To allow for fast rinsing, to reduce reagent consumption, and to ensure high reagent purity, we propose a fluidic concept based on a hierarchical branching structure. The working principle comprises a 3D fluidic network of supply lines - one line per reagent - that ensures reagents to be provided up to the entrance of every single reaction cavity, called supercells. Because all reagents are always present inside or at the inlet of a supercell, the principle allows for very rapid reagent switching, while a continuous flow avoids cross contamination. Selection of a specific reagent to enter the supercells is controlled by adjusting the pressure over different supply lines. As the pressure is regulated by a single, external controller per reagent, no integrated valves are needed. The very small distances to the reaction cavities also results in the use of minimal reagent volumes and, hence, largely reduces operational costs. We demonstrated the working principle of this concept and show an average switching time of 0.23 ± 0.09 s for the current design at a flow rate of 10 nL s-1. We used a 10 × 10 matrix of supercells to validate the fluidic concept to be scalable towards a large number of reaction sites. In summary, we believe the presented fluidic 3D hierarchical concept allows designing flow cells that enable highly parallel, more cost-efficient, and faster work flows for applications requiring many reagent cycles.

Programmed Internal Reconfigurations in a 3D-Printed Mechanical Metamaterial Enable Fluidic Control for a Vertically Stacked Valve Array

Microfluidic valves play a key role within microfluidic systems by regulating fluid flow through distinct microchannels, enabling many advanced applications in medical diagnostics, lab-on-chips, and laboratory automation. While microfluidic systems are often limited to planar structures, 3D printing enables new capabilities to generate complex designs for fluidic circuits with higher densities and integrated components. However, the control of fluids within 3D structures presents several difficulties, making it challenging to scale effectively and many fluidic devices are still often restricted to quasi-planar structures. Incorporating mechanical metamaterials that exhibit spatially adjustable mechanical properties into microfluidic systems provides an opportunity to address these challenges. Here, we have performed systematic computational and experimental characterization of a modified re-entrant honeycomb structure to generate a modular metamaterial for an active device that allows us to directly regulate flow through integrated, multiplexed fluidic channels "one-at-a-time," in a manner that is highly scalable. We present a design algorithm so that this architecture can be extended to arbitrary geometries, and we expect that by incorporation of mechanical metamaterial designs into 3D printed fluidic systems, which themselves are readily expandable to any complex geometries, will enable new biotechnological and biomedical applications of 3D printed devices.

A programmable and automated optical electrowetting-on-dielectric (oEWOD) driven platform for massively parallel and sequential processing of single cell assay operations

Recently, there has been an increasing emphasis on single cell profiling for high-throughput screening workflows in drug discovery and life sciences research. However, the biology underpinning these screens is often complex and is insufficiently addressed by singleplex assay screens. Traditional single cell screening technologies have created powerful sets of 'omic data that allow users to bioinformatically infer biological function, but have as of yet not empowered direct functional analysis at the level of each individual cell. Consequently, screening campaigns often require multiple secondary screens leading to laborious, time-consuming and expensive workflows in which attrition points may not be queried until late in the process. We describe a platform that harnesses droplet microfluidics and optical electrowetting-on-dielectric (oEWOD) to perform highly-controlled sequential and multiplexed single cell assays in massively parallelised workflows to enable complex cell profiling during screening. Soluble reagents or objects, such as cells or assay beads, are encapsulated into droplets of media in fluorous oil and are actively filtered based on size and optical features ensuring only desirable droplets (e.g. single cell droplets) are retained for analysis, thereby overcoming the Poisson probability distribution. Droplets are stored in an array on a temperature-controlled chip and the history of individual droplets is logged from the point of filter until completion of the workflow. On chip, droplets are subject to an automated and flexible suite of operations including the merging of sample droplets and the fluorescent acquisition of assay readouts to enable complex sequential assay workflows. To demonstrate the broad utility of the platform, we present examples of single-cell functional workflows for various applications such as antibody discovery, infectious disease, and cell and gene therapy.

Functionality integration in stereolithography 3D printed microfluidics using a "print-pause-print" strategy

Stereolithography 3D printing, although an increasingly used fabrication method for microfluidic chips, has the main disadvantage of producing monolithic chips in a single material. We propose to incorporate during printing various objects using a "print-pause-print" strategy. Here, we demonstrate that this novel approach can be used to incorporate glass slides, hydrosoluble films, paper pads, steel balls, elastic or nanoporous membranes and silicon-based microdevices, in order to add microfluidic functionalities as diverse as valves, fluidic diodes, shallow chambers, imaging windows for bacteria tracking, storage of reagents, blue energy harvesting or filters for cell capture and culture.

Experimental Study on SPR Array Sensing Chip Integrated with Microvalves

This paper discusses a microfluidic system designed for surface plasmon resonance (SPR) sensing, incorporating integrated microvalves. This system is built from a layered structure of polydimethylsiloxane (PDMS) and polymethylmethacrylate (PMMA). The functionality of the microvalves is verified through a conductance method involving electrodes positioned at the microfluidic channels' inlets and outlets. These microvalves can fully close at a control pressure of 0.3 MPa, with their operation depending on the duration of the applied pressure. The study further explores the coordinated operation of multiple microvalves to regulate the sequential flow of samples and reagents in the system. In SPR detection experiments, the microfluidic system is integrated with an SPR array sensing system to control the injection of NaCl solutions via the microvalves, and the observation of phase change curves in different chip regions are observed. The findings validate the microvalves' dependability and suitability for use in SPR array sensing.

Single-Cell RNA Sequencing in Organ and Cell Transplantation

Single-cell RNA sequencing is a high-throughput novel method that provides transcriptional profiling of individual cells within biological samples. This method typically uses microfluidics systems to uncover the complex intercellular communication networks and biological pathways buried within highly heterogeneous cell populations in tissues. One important application of this technology sits in the fields of organ and stem cell transplantation, where complications such as graft rejection and other post-transplantation life-threatening issues may occur. In this review, we first focus on research in which single-cell RNA sequencing is used to study the transcriptional profile of transplanted tissues. This technology enables the analysis of the donor and recipient cells and identifies cell types and states associated with transplant complications and pathologies. We also review the use of single-cell RNA sequencing in stem cell implantation. This method enables studying the heterogeneity of normal and pathological stem cells and the heterogeneity in cell populations. With their remarkably rapid pace, the single-cell RNA sequencing methodologies will potentially result in breakthroughs in clinical transplantation in the coming years.

Soft Nanofluidic Machinery

Soft devices integrating flexible structures and versatile material functionalities offer platform technologies for the healthcare, information, and communication industries. The flexibility can be achieved by constructing devices from low-dimensional nanostructures or nanoporous soft materials. By pushing the limits of fabrication and structuring down to the nanometer and Ångstrom scales, nanofluidics with extreme spatial confinement has recently been actively explored for energy-, environment-, and human-friendly device applications as alternative solutions to electronics and mechanotronics. Soft nanofluidic machinery enables ultrafast and selective fluidic transport, efficient energy conversion, and information processing, offering unconventional dimensions of design. The physics behind the design is introduced, followed by discussions on their implementations and performance and an outlook on the opportunities and challenges.

High-speed fluidic processing circuits for dynamic control of haptic and robotic systems

Fluidic logic circuits simplify system design for soft robotics by eliminating bulky components while enabling operation in a range of hostile environments that are incompatible with electronics but at the expense of limited computational capabilities and response times on the order of seconds. This paper presents a four-terminal fluidic transistor optimized for fast switching times, reduced component count, low unit cost, and high reproducibility to achieve complex fluidic control circuits while maintaining flow rates of liters per minute. A ring oscillator using three fluidic transistors achieves oscillation frequencies up to a kilohertz with full signal propagation, tolerating billions of cycles without failure. Fundamental processor circuits like a full adder and a 3-bit analog-to-digital converter require just seven transistors each. A decode circuit drives a high-resolution soft haptic display with refresh times below the human perception threshold for latency, and an electronics-free control circuit performs closed-loop position control of a pneumatic actuator with disturbance rejection, demonstrating the value across domains.

Microfluidic systems for infectious disease diagnostics

Microorganisms, encompassing both uni- and multicellular entities, exhibit remarkable diversity as omnipresent life forms in nature. They play a pivotal role by supplying essential components for sustaining biological processes across diverse ecosystems, including higher host organisms. The complex interactions within the human gut microbiota are crucial for metabolic functions, immune responses, and biochemical signalling, particularly through the gut-brain axis. Viruses also play important roles in biological processes, for example by increasing genetic diversity through horizontal gene transfer when replicating inside living cells. On the other hand, infection of the human body by microbiological agents may lead to severe physiological disorders and diseases. Infectious diseases pose a significant burden on global healthcare systems, characterized by substantial variations in the epidemiological landscape. Fast spreading antibiotic resistance or uncontrolled outbreaks of communicable diseases are major challenges at present. Furthermore, delivering field-proven point-of-care diagnostic tools to the most severely affected populations in low-resource settings is particularly important and challenging. New paradigms and technological approaches enabling rapid and informed disease management need to be implemented. In this respect, infectious disease diagnostics taking advantage of microfluidic systems combined with integrated biosensor-based pathogen detection offers a host of innovative and promising solutions. In this review, we aim to outline recent activities and progress in the development of microfluidic diagnostic tools. Our literature research mainly covers the last 5 years. We will follow a classification scheme based on the human body systems primarily involved at the clinical level or on specific pathogen transmission modes. Important diseases, such as tuberculosis and malaria, will be addressed more extensively.

Development and future of droplet microfluidics

Over the past two decades, advances in droplet-based microfluidics have facilitated new approaches to process and analyze samples with unprecedented levels of precision and throughput. A wide variety of applications has been inspired across multiple disciplines ranging from materials science to biology. Understanding the dynamics of droplets enables optimization of microfluidic operations and design of new techniques tailored to emerging demands. In this review, we discuss the underlying physics behind high-throughput generation and manipulation of droplets. We also summarize the applications in droplet-derived materials and droplet-based lab-on-a-chip biotechnology. In addition, we offer perspectives on future directions to realize wider use of droplet microfluidics in industrial production and biomedical analyses.

Multiplexed immunosensing of cancer biomarkers on a split-float-gate graphene transistor microfluidic biochip

This work reports the development of a novel microfluidic biosensor using a graphene field-effect transistor (GFET) design for the parallel label-free analysis of multiple biomarkers. Overcoming the persistent challenge of constructing μm2-sized FET sensitive interfaces that incorporate multiple receptors, we implement a split-float-gate structure that enables the manipulation of multiplexed biochemical functionalization using microfluidic channels. Immunoaffinity biosensing experiments are conducted using the mixture samples containing three liver cancer biomarkers, carcinoembryonic antigen (CEA), α-fetoprotein (AFP), and parathyroid hormone (PTH). The results demonstrate the capability of our label-free biochip to quantitatively detect multiple target biomarkers simultaneously by observing the kinetics in 10 minutes, with the detection limit levels in the nanomolar range. This microfluidic biosensor provides a valuable analytical tool for rapid multi-target biosensing, which can be potentially utilized for domiciliary tests of cancer screening and prognosis, obviating the need for sophisticated instruments and professional operations in hospitals.

Elastocapillarity-driven 2D nano-switches enable zeptoliter-scale liquid encapsulation

Biological nanostructures change their shape and function in response to external stimuli, and significant efforts have been made to design artificial biomimicking devices operating on similar principles. In this work we demonstrate a programmable nanofluidic switch, driven by elastocapillarity, and based on nanochannels built from layered two-dimensional nanomaterials possessing atomically smooth surfaces and exceptional mechanical properties. We explore operational modes of the nanoswitch and develop a theoretical framework to explain the phenomenon. By predicting the switching-reversibility phase diagram-based on material, interfacial and wetting properties, as well as the geometry of the nanofluidic circuit-we rationally design switchable nano-capsules capable of enclosing zeptoliter volumes of liquid, as small as the volumes enclosed in viruses. The nanoswitch will find useful application as an active element in integrated nanofluidic circuitry and could be used to explore nanoconfined chemistry and biochemistry, or be incorporated into shape-programmable materials.

Optimizing Microfluidic Channel Design with High-Performance Materials for Safe Neonatal Drug Delivery

INTRODUCTION

Designing the microfluidic channel for neonatal drug delivery requires proper considerations to enhance the efficiency and safety of drug substances when used in neonates. Thus, this research aims to evaluate high-performance materials and optimize the channel design by modeling and simulation using COMSOL multiphysics in order to deliver an optimum flow rate between 0. 3 and 1 mL/hr.

METHOD

Some of the materials used in the study included PDMS, glass, COC, PMMA, PC, TPE, and hydrogels, and the evaluation criterion involved biocompatibility, mechanical properties, chemical resistance, and ease of fabrication. The simulation was carried out in the COMSOL multiphysics platform and demonstrated the fog fluid behavior in different channel geometries, including laminar flow and turbulence. The study then used systematic changes in design parameters with the aim of establishing the best implementation models that can improve the efficiency and reliability of the drug delivery system. The comparison was based mostly on each material and its appropriateness in microfluidic usage, primarily in neonatal drug delivery. The biocompatibility of the developed materials was verified using the literature analysis and adherence to the ISO 10993 standard, thus providing safety for the use of neonatal devices. Tensile strength was included to check the strength of each material to withstand its operation conditions. Chemical resistance was also tested in order to determine the compatibility of the materials with various drugs, and the possibility of fabrication was also taken into consideration to identify appropriate materials that could be used in the rapid manufacturing of the product.

RESULTS

The results we obtained show that PDMS, due to its flexibility and simplicity in simulation coupled with more efficient channel designs which have been extracted from COMSOL, present a feasible solution to neonatal drug delivery.

CONCLUSION

The present comparative study serves as a guide on the choice of materials and design of microfluidic devices to help achieve safer and enhanced drug delivery systems suitable for the delicate reception of fragile neonates.

Tuna-step: tunable parallelized step emulsification for the generation of droplets with dynamic volume control to 3D print functionally graded porous materials

We present tuna-step, a novel microfluidic module based on step emulsification that allows for reliable generation of droplets of different sizes. Until now, sizes of droplets generated with step emulsification were hard-wired into the geometry of the step emulsification nozzle. To overcome this, we incorporate a thin membrane underneath the step nozzle that can be actuated by pressure, enabling the tuning of the nozzle size on-demand. By controllably reducing the height of the nozzle, we successfully achieved a three-order-of-magnitude variation in droplet volume without adjusting the flow rates of the two phases. We developed and applied a new hydrophilic surface modification, that ensured long-term stability and prevented swelling of the device when generating oil-in-water droplets. Our system produced functionally graded soft materials with adjustable porosity and material content. By combining our microfluidic device with a custom 3D printer, we generated and extruded oil-in-water emulsions in an agarose gel bath, creating unique self-standing 3D hydrogel structures with porosity decoupled from flow rate and with composition gradients of external phases. We upscaled tuna-step by setting 14 actuatable nozzles in parallel, offering a step-emulsification-based single chip solution that can accommodate various requirements in terms of throughput, droplet volumes, flow rates, and surface chemistry.

Programmable Control of Nanoliter Droplet Arrays using Membrane Displacement Traps

A unique droplet microfluidic technology enabling programmable deterministic control over complex droplet operations is presented. The platform provides software control over user-defined combinations of droplet generation, capture, ejection, sorting, splitting, and merging sequences to enable the design of flexible assays employing nanoliter-scale fluid volumes. The system integrates a computer vision system with an array of membrane displacement traps capable of performing selected unit operations with automated feedback control. Sequences of individual droplet handling steps are defined through a robust Python-based scripting language. Bidirectional flow control within the microfluidic chips is provided using an H-bridge channel topology, allowing droplets to be transported to arbitrary trap locations within the array for increased operational flexibility. By enabling automated software control over all droplet operations, the system significantly expands the potential of droplet microfluidics for diverse biological and biochemical applications by combining the functionality of robotic liquid handling with the advantages of droplet-based fluid manipulation.

Fabrication of 3D-printed molds for polydimethylsiloxane-based microfluidic devices using a liquid crystal display-based vat photopolymerization process: printing quality, drug response and 3D invasion cell culture assays

Microfluidic platforms enable more precise control of biological stimuli and environment dimensionality than conventional macroscale cell-based assays; however, long fabrication times and high-cost specialized equipment limit the widespread adoption of microfluidic technologies. Recent improvements in vat photopolymerization three-dimensional (3D) printing technologies such as liquid crystal display (LCD) printing offer rapid prototyping and a cost-effective solution to microfluidic fabrication. Limited information is available about how 3D printing parameters and resin cytocompatibility impact the performance of 3D-printed molds for the fabrication of polydimethylsiloxane (PDMS)-based microfluidic platforms for cellular studies. Using a low-cost, commercially available LCD-based 3D printer, we assessed the cytocompatibility of several resins, optimized fabrication parameters, and characterized the minimum feature size. We evaluated the response to both cytotoxic chemotherapy and targeted kinase therapies in microfluidic devices fabricated using our 3D-printed molds and demonstrated the establishment of flow-based concentration gradients. Furthermore, we monitored real-time cancer cell and fibroblast migration in a 3D matrix environment that was dependent on environmental signals. These results demonstrate how vat photopolymerization LCD-based fabrication can accelerate the prototyping of microfluidic platforms with increased accessibility and resolution for PDMS-based cell culture assays.

A Deep Reinforcement Learning Approach to Droplet Routing for Erroneous Digital Microfluidic Biochips

Digital microfluidic biochips (DMFBs), which are used in various fields like DNA analysis, clinical diagnosis, and PCR testing, have made biochemical experiments more compact, efficient, and user-friendly than the previous methods. However, their reliability is often compromised by their inability to adapt to all kinds of errors. Errors in biochips can be categorized into two types: known errors, and unknown errors. Known errors are detectable before the start of the routing process using sensors or cameras. Unknown errors, in contrast, only become apparent during the routing process and remain undetected by sensors or cameras, which can unexpectedly stop the routing process and diminish the reliability of biochips. This paper introduces a deep reinforcement learning-based routing algorithm, designed to manage not only known errors but also unknown errors. Our experiments demonstrated that our algorithm outperformed the previous ones in terms of the success rate of the routing, in the scenarios including both known errors and unknown errors. Additionally, our algorithm contributed to detecting unknown errors during the routing process, identifying the most efficient routing path with a high probability.

Millifluidic valves and pumps made of tape and plastic

There is growing interest in producing micro- and milli-fluidic technologies made of thermoplastic with integrated fluidic control elements that are easy to assemble and suitable for mass production. Here, we developed millifluidic valves and pumps made of acrylic layers bonded with double-sided tape that are simple and fast to assemble. We demonstrate that a layer of pressure-sensitive adhesive (PSA) is flexible enough to be deformed at relatively low pressures. A chemical treatment deposited on specific regions of the PSA prevents it from sticking to the thermoplastic, which enabled us to create three different types of valves in normally open or closed configurations. We characterized different aspects of their performance, their operating pressures, the cut-off pressure values to open or close the valves (for different configurations and sizes), and the flow rate and volume pumped by seven different micropumps. As an application, we implemented a glucose assay with integrated pumps and valves, automatically generating glucose dilutions and reagent mixing. The ability to create polymeric microfluidic control units made with tape paves the way for their mass manufacturing.

A microfluidic transistor for automatic control of liquids

Microfluidics have enabled notable advances in molecular biology1,2, synthetic chemistry3,4, diagnostics5,6 and tissue engineering7. However, there has long been a critical need in the field to manipulate fluids and suspended matter with the precision, modularity and scalability of electronic circuits8-10. Just as the electronic transistor enabled unprecedented advances in the automatic control of electricity on an electronic chip, a microfluidic analogue to the transistor could enable improvements in the automatic control of reagents, droplets and single cells on a microfluidic chip. Previous works on creating a microfluidic analogue to the electronic transistor11-13 did not replicate the transistor's saturation behaviour, and could not achieve proportional amplification14, which is fundamental to modern circuit design15. Here we exploit the fluidic phenomenon of flow limitation16 to develop a microfluidic element capable of proportional amplification with flow-pressure characteristics completely analogous to the current-voltage characteristics of the electronic transistor. We then use this microfluidic transistor to directly translate fundamental electronic circuits into the fluidic domain, including the amplifier, regulator, level shifter, logic gate and latch. We also combine these building blocks to create more complex fluidic controllers, such as timers and clocks. Finally, we demonstrate a particle dispenser circuit that senses single suspended particles, performs signal processing and accordingly controls the movement of each particle in a deterministic fashion without electronics. By leveraging the vast repertoire of electronic circuit design, microfluidic-transistor-based circuits enable fluidic automatic controllers to manipulate liquids and single suspended particles for lab-on-a-chip platforms.

Engineering global and local signal generators for probing temporal and spatial cellular signaling dynamics

Cells constantly encounter a wide range of environmental signals and rely on their signaling pathways to initiate reliable responses. Understanding the underlying signaling mechanisms and cellular behaviors requires signal generators capable of providing diverse input signals to deliver to cell systems. Current research efforts are primarily focused on exploring cellular responses to global or local signals, which enable us to understand cellular signaling and behavior in distinct dimensions. This review presents recent advancements in global and local signal generators, highlighting their applications in studying temporal and spatial signaling activity. Global signals can be generated using microfluidic or photochemical approaches. Local signal sources can be created using living or artificial cells in combination with different control methods. We also address the strengths and limitations of each signal generator type, discussing challenges and potential extensions for future research. These approaches are expected to continue to facilitate on-going research to discover novel and intriguing cellular signaling mechanisms.

Tunable resins with PDMS-like elastic modulus for stereolithographic 3D-printing of multimaterial microfluidic actuators

Stereolithographic 3D-printing (SLA) permits facile fabrication of high-precision microfluidic and lab-on-a-chip devices. SLA photopolymers often yield parts with low mechanical compliancy in sharp contrast to elastomers such as poly(dimethyl siloxane) (PDMS). On the other hand, SLA-printable elastomers with soft mechanical properties do not fulfill the distinct requirements for a highly manufacturable resin in microfluidics (e.g., high-resolution printability, transparency, low-viscosity). These limitations restrict our ability to print microfluidic actuators containing dynamic, movable elements. Here we introduce low-viscous photopolymers based on a tunable blend of the monomers poly(ethylene glycol) diacrylate (PEGDA, Mw ∼ 258) and the monoacrylate poly(ethylene glycol methyl ether) methacrylate (PEGMEMA, Mw ∼ 300). In these blends, which we term PEGDA-co-PEGMEMA, tuning the PEGMEMA content from 0% to 40% (v/v) alters the elastic modulus of the printed plastics by ∼400-fold, reaching that of PDMS. Through the addition of PEGMEMA, moreover, PEGDA-co-PEGMEMA retains desirable properties of highly manufacturable PEGDA such as low viscosity, solvent compatibility, cytocompatibility and low drug absorptivity. With PEGDA-co-PEGMEMA, we SLA-printed drastically enhanced fluidic actuators including microvalves, micropumps, and microregulators with a hybrid structure containing a flexible PEGDA-co-PEGMEMA membrane within a rigid PEGDA housing. These components were built using a custom "Print-Pause-Print" protocol, referred to as "3P-printing", that allows for fabricating high-resolution multimaterial parts with a desktop SLA printer without the need for post-assembly. SLA-printing of multimaterial microfluidic actuators addresses the unmet need of high-performance on-chip controls in 3D-printed microfluidic and lab-on-a-chip devices.

Tuning the hydraulic resistance by swelling-induced buckling of membranes in high-aspect-ratio microfluidic devices

Controlling fluid flow in microfluidic devices and adapting it to varying conditions by selectively regulating hydrodynamic properties is of critical importance, as the field of microfluidics faces increasingly complex challenges in its wide range of applications. One way to manipulate flows in microfluidic devices is to introduce elastic elements that can be actively or passively deformed. In this work, we developed a membrane-based microfluidic device that allows us to study the deformation of swollen thin membranes as a function of the volume fractions in binary mixtures - here isopropanol and water. Furthermore, the membrane deformation can be used to control pressure-driven flows within the device. The device consists of two microfluidic channels separated by a thin membrane that deforms by a buckling-based mechanism, when the isopropanol volume fraction of the solvent flowing through it exceeds a certain volume fraction. The buckling membrane causes a sinusoidal height variation in both adjacent channels, resulting in a large increase in hydraulic resistance. We show that buckling-based deflections of elastic membranes can be used to amplify small changes in the degree of swelling to produce large changes in the microchannel geometry of the device, sufficient to manipulate the flow rate of pressure-driven flows in the microdevice.

Recent Advances on Cell Culture Platforms for In Vitro Drug Screening and Cell Therapies: From Conventional to Microfluidic Strategies

The clinical translations of drugs and nanomedicines depend on coherent pharmaceutical research based on biologically accurate screening approaches. Since establishing the 2D in vitro cell culture method, the scientific community has improved cell-based drug screening assays and models. Those advances result in more informative biochemical assays and the development of 3D multicellular models to describe the biological complexity better and enhance the simulation of the in vivo microenvironment. Despite the overall dominance of conventional 2D and 3D cell macroscopic culture methods, they present physicochemical and operational challenges that impair the scale-up of drug screening by not allowing a high parallelization, multidrug combination, and high-throughput screening. Their combination and complementarity with microfluidic platforms enable the development of microfluidics-based cell culture platforms with unequivocal advantages in drug screening and cell therapies. Thus, this review presents an updated and consolidated view of cell culture miniaturization's physical, chemical, and operational considerations in the pharmaceutical research scenario. It clarifies advances in the field using gradient-based microfluidics, droplet-based microfluidics, printed-based microfluidics, digital-based microfluidics, SlipChip, and paper-based microfluidics. Finally, it presents a comparative analysis of the performance of cell-based methods in life research and development to achieve increased precision in the drug screening process.

A microfluidics vapor-membrane-valve generated by laser irradiation on carbon nanocoils

We have investigated a micro vapor membrane valve (MVMV) for closing the microfluidic channels by laser irradiation on carbon nanocoils (CNCs) attached to the inner wall of the microchannels. The microchannel with MVMVs was found to exhibit a "closed" state without the supply of laser energy, which is explained on the basis of the theory of heat and mass transfer. Multiple MVMVs for sealing the channels can be generated in sequence and exist simultaneously at different irradiation sites, independently. The significant advantages of the MVMV generated by the laser irradiation on CNCs are the elimination of extrinsic energy required to maintain the microfluidic channel "closed" state and the simplification of the structure integrated into the microfluidic channels and fluid control circuitries. The CNC-based MVMV is a powerful tool for the investigations of the functions of microchannel switching and sealing on microfluidic chips in biomedicine, chemical analysis and other fields. The study of MVMVs will have great significance for biochemical and cytological analysis.

Synthesis of nanoparticles via microfluidic devices and integrated applications

In recent years, nanomaterials have attracted the research intervention of experts in the fields of catalysis, energy, biomedical testing, and biomedicine with their unrivaled optical, chemical, and biological properties. From basic metal and oxide nanoparticles to complex quantum dots and MOFs, the stable preparation of various nanomaterials has always been a struggle for researchers. Microfluidics, as a paradigm of microscale control, is a remarkable platform for online stable synthesis of nanomaterials with efficient mass and heat transfer in microreactors, flexible blending of reactants, and precise control of reaction conditions. We describe the process of microfluidic preparation of nanoparticles in the last 5 years in terms of microfluidic techniques and the methods of microfluidic manipulation of fluids. Then, the ability of microfluidics to prepare different nanomaterials, such as metals, oxides, quantum dots, and biopolymer nanoparticles, is presented. The effective synthesis of some nanomaterials with complex structures and the cases of nanomaterials prepared by microfluidics under extreme conditions (high temperature and pressure), the compatibility of microfluidics as a superior platform for the preparation of nanoparticles is demonstrated. Microfluidics has a potent integration capability to combine nanoparticle synthesis with real-time monitoring and online detection, which significantly improves the quality and production efficiency of nanoparticles, and also provides a high-quality ultra-clean platform for some bioassays.

A Microfluidic Transistor for Liquid Signal Processing

Microfluidics have enabled significant advances in molecular biology ^1-3 , synthetic chemistry ^4,5 , diagnostics ^6,7 , and tissue engineering ⁸ . However, there has long been a critical need in the field to manipulate fluids and suspended matter with the precision, modularity, and scalability of electronic circuits ^9-11 . Just as the electronic transistor enabled unprecedented advances in the control of electricity on an electronic chip, a microfluidic analogue to the transistor could enable improvements in the complex, scalable control of reagents, droplets, and single cells on an autonomous microfluidic chip. Prior works on creating a microfluidic analogue to the electronic transistor ^12-14 could not replicate the transistor's saturation behavior, which is crucial to perform analog amplification ¹⁵ and is fundamental to modern circuit design ¹⁶ . Here we exploit the fluidic phenomenon of flow-limitation ¹⁷ to develop a microfluidic element with flow-pressure characteristics completely analogous to the current-voltage characteristics of the electronic transistor. As this microfluidic transistor successfully replicates all of the key operating regimes of the electronic transistor (linear, cut-off and saturation), we are able to directly translate a variety of fundamental electronic circuit designs into the fluidic domain, including the amplifier, regulator, level shifter, logic gate, and latch. Finally, we demonstrate a "smart" particle dispenser that senses single suspended particles, performs liquid signal processing, and accordingly controls the movement of said particles in a purely fluidic system without electronics. By leveraging the vast repertoire of electronic circuit design, microfluidic transistor-based circuits are easy to integrate at scale, eliminate the need for external flow control, and enable uniquely complex liquid signal processing and single-particle manipulation for the next generation of chemical, biological, and clinical platforms.

Microfluidic Investigation of the Ion-Specific Effect on Bubble Coalescence in Salt Solutions

A microfluidic method was developed to study the ion-specific effect on bubble coalescence in salt solutions. Compared with other reported methods, microfluidics provides a more direct and accurate means of measuring bubble coalescence in salt solutions. We analyzed the coalescence time and approach velocity between bubbles and used simulation to investigate the pressure evolution during the coalescence process. The coalescence time of the three salt solutions decreased initially and then increased as the concentration of the salt solution was increased. The concentration with the shortest coalescence time is considered as the transition concentration (TC) and exhibits ion-specific. At the TC, the change in coalescence time indicates a shift in the effect of salt on bubble coalescence from facilitation to initial inhibition. Meanwhile, it can be seen that the sodium halide solutions significantly inhibit the bubble coalescence and the inhibition capability follows the order NaCl > NaBr > NaI. The results of the approach velocity show that the coalescence time decreases with increasing approach velocity, as well as the approach velocity was strongly influenced by concentration. The approach velocity undergoes a significant change at the TC. Furthermore, simulations of bubble coalescence in the microchannel indicate that the vertical pressure gradient at the center point of the bubble pairs increases as bubbles approach, driving liquid film drainage until bubble coalescence. The pressure at the center of the bubble pair reaches the maximum when the bubbles have first coalesced. It was further revealed that the concentration of the salt solution has a significant impact on the maximum pressure, as evidenced by the observed trend of decreasing pressure values with increasing concentrations.

A handheld plug-and-play microfluidic liquid handling automation platform for immunoassays

Lab-on-a-chip technologies and microfluidics have pushed miniaturized liquid handling to unprecedented precision, integration, and automation, which improved the reaction efficiency of immunoassays. However, most microfluidic immunoassay systems still require bulky infrastructures, such as external pressure sources, pneumatic systems, and complex manual tubing and interface connections. Such requirements prevent plug-and-play operation at the point-of-care (POC) settings. Here we present a fully automated handheld general microfluidic liquid handling automation platform with a plug-and-play 'clamshell-style' cartridge socket, a miniature electro-pneumatic controller, and injection-moldable plastic cartridges. The system achieved multi-reagent switching, metering, and timing control on the valveless cartridge using electro-pneumatic pressure control. As a demonstration, a SARS-CoV-2 spike antibody sandwich fluorescent immunoassay (FIA) liquid handling was performed on an acrylic cartridge without human intervention after sample introduction. A fluorescence microscope was used to analyze the result. The assay showed a limit of detection at 31.1 ng/mL, comparable to some previously reported enzyme-linked immunosorbent assays (ELISA). In addition to automated liquid handling on the cartridge, the system can operate as a 6-port pressure source for external microfluidic chips. A rechargeable battery with a 12 V 3000 mAh capacity can power the system for 42 h. The footprint of the system is 16.5 × 10.5 × 7 cm, and the weight is 801 g, including the battery. The system can find many other POC and research applications requiring complex liquid manipulation, such as molecular diagnostics, cell analysis, and on-demand biomanufacturing.

Fluorescence coupling strategies in fluorescence-activated droplet sorting (FADS) for ultrahigh-throughput screening of enzymes, metabolites, and antibodies

Fluorescence-activated droplet sorting (FADS) has emerged as a powerful tool for ultrahigh-throughput screening of enzymes, metabolites, and antibodies. Fluorescence coupling strategies (FCSs) are key to the development of new FADS methods through their coupling of analyte properties such as concentration, activities, and affinity with fluorescence signals. Over the last decade, a series of FCSs have been developed, greatly expanding applications of FADS. Here, we review recent advances in FCS for different analyte types, providing a critical comparison of the available FCSs and further classification into four categories according to their principles. We also summarize successful FADS applications employing FCSs in enzymes, metabolites, and antibodies. Further, we outline possible future developments in this area.

In-depth analysis of biocatalysts by microfluidics: An emerging source of data for machine learning

Nowadays, the vastly increasing demand for novel biotechnological products is supported by the continuous development of biocatalytic applications which provide sustainable green alternatives to chemical processes. The success of a biocatalytic application is critically dependent on how quickly we can identify and characterize enzyme variants fitting the conditions of industrial processes. While miniaturization and parallelization have dramatically increased the throughput of next-generation sequencing systems, the subsequent characterization of the obtained candidates is still a limiting process in identifying the desired biocatalysts. Only a few commercial microfluidic systems for enzyme analysis are currently available, and the transformation of numerous published prototypes into commercial platforms is still to be streamlined. This review presents the state-of-the-art, recent trends, and perspectives in applying microfluidic tools in the functional and structural analysis of biocatalysts. We discuss the advantages and disadvantages of available technologies, their reproducibility and robustness, and readiness for routine laboratory use. We also highlight the unexplored potential of microfluidics to leverage the power of machine learning for biocatalyst development.

Paper-based multiplex biosensors for inexpensive healthcare diagnostics: a comprehensive review

Multiplex detection is a smart and an emerging approach in point-of-care testing as it reduces analysis time and testing cost by detecting multiple analytes or biomarkers simultaneously which are crucial for disease detection at an early stage. Application of inexpensive substrate such as paper has immense potential and matter of research interest in the area of point of care testing for multiplexed analysis as it possesses several unique advantages. This study presents the use of paper, strategies adopted to refine the design created on paper and lateral flow strips to enhance the signal, increase the sensitivity and specificity of multiplexed biosensors. An overview of different multiplexed detection studies performed using biological samples has also been reviewed along with the challenges and advantages offered by multiplexed analysis.

AC-electric-field-controlled multi-component droplet coalescence at microscale

Droplet coalescence with fast response, high controllability and monodispersity has been widely investigated in industrial production and bioengineering. Especially for droplets with multiple components, programmable manipulation of such droplets is crucial for practical applications. However, precise control of the dynamics can be challenging, owing to the complex boundaries and the interfacial and fluidic properties. AC electric fields, with their fast response and high flexibility, have attracted our interest. We design and fabricate an improved flow-focusing microchannel configuration together with a non-contact type of electrode featuring asymmetric geometries, based on which we conduct systematic investigations of the AC-electric-field-controlled coalescence of multi-component droplets at the microscale. Parameters such as flow rates, component ratio, surface tension, electric permittivity and conductivity were given our attention. The results show that droplet coalescence in different flow parameters can be achieved in milliseconds by adjusting the electrical conditions, which shows high controllability. Specifically, both the coalescence region and reaction time can be adjusted by a combination of applied voltage and frequency, and unique merging phenomena have appeared. One is contact coalescence with the approach of paired droplets, while the other is squeezing coalescence, which occurs in the start position and promotes the merging process. The fluid properties, such as the electric permittivity, conductivity and surface tension, present a significant influence on merging behavior. The increasing relative dielectric constant leads to a dramatic reduction of the start merging voltage from the original 250 V to 30 V. The range of effective voltage for coalescence decreases with the addition of surfactant, offering a stricter and yet higher selectivity on electrical conditions, about 1500 V. The conductivity presents a negative correlation with the start merging voltage due to the reduction of the dielectric stress, from 400 V to 1500 V. Finally, we achieve the precise fabrication process of the Janus droplet via implementation of the proposed method, where the components of the droplets and the coalescence conditions are well controlled. Our results can serve as a potent methodology to decipher the physics of multi-component droplet electro-coalescence and contribute to applications in chemical synthesis, bioassay and material synthesis.

Microfluidics: Insights into Intestinal Microorganisms

Microfluidics is a system involving the treatment or manipulation of microscale (10^-9 to 10^-18 L) fluids using microchannels (10 to 100 μm) contained on a microfluidic chip. Among the different methodologies used to study intestinal microorganisms, new methods based on microfluidic technology have been receiving increasing attention in recent years. The intestinal tracts of animals are populated by a vast array of microorganisms that have been established to play diverse functional roles beneficial to host physiology. This review is the first comprehensive coverage of the application of microfluidics technology in intestinal microbial research. In this review, we present a brief history of microfluidics technology and describe its applications in gut microbiome research, with a specific emphasis on the microfluidic technology-based intestine-on-a-chip, and also discuss the advantages and application prospects of microfluidic drug delivery systems in intestinal microbial research.

Towards an active droplet-based microfluidic platform for programmable fluid handling

Droplet-based microfluidic systems have emerged as powerful alternatives to conventional high throughput screening platforms, due to their operational flexibility, high-throughput nature and ability to efficiently process small fluid volumes. However, the challenges associated with performing bespoke operations on user-defined droplets often limit their utility in screening applications that involve complex workflows. To this end, the marriage of droplet- and valve-based microfluidic technologies offers the prospect of balancing the controllability of droplet manipulations and analytical throughput. In this spirit, we present a microfluidic platform that combines the capabilities of integrated microvalve technology with droplet-based sample compartmentalization to realize a highly adaptable programmable fluid handling functionality. The microfluidic device consists of a programmable formulator linked to an automated droplet generation device and storage array. The formulator leverages multiple inputs coupled to a mixing ring to produce combinatorial solution mixtures, with a peristaltic pump enabling titration of reagents into the ring with picoliter resolution. The platform allows for the execution of user-defined reaction protocols within an array of storage chambers by consecutively merging programmable sequences of pL-volume droplets containing specified reagents. The precision in formulating solutions with small differences in concentration is perfectly suited for the accurate estimation of kinetic parameters. The utility of our platform is showcased through the performance of enzymatic kinetic measurements of beta-galactosidase and horseradish peroxidase with fluorogenic substrates. The presented platform provides for a range of automated manipulations and paves the way for a more diverse range of droplet-based biological experiments.

The 2D microfluidics cookbook - modeling convection and diffusion in plane flow devices

A growing number of microfluidic systems operate not through networks of microchannels but instead through using 2D flow fields. While the design rules for channel networks are already well-known and exposed in microfluidics textbooks, the knowledge underlying transport in 2D microfluidics remains scattered piecemeal and is not easily accessible to experimentalists and engineers. In this tutorial review, we formulate a unified framework for understanding, analyzing and designing 2D microfluidic technologies. We first show how a large number of seemingly different devices can all be modelled using the same concepts, namely flow and diffusion in a Hele-Shaw cell. We then expose a handful of mathematical tools, accessible to any engineer with undergraduate level mathematics knowledge, namely potential flow, superposition of charges, conformal transforms and basic convection-diffusion. We show how these tools can be combined to obtain a simple "recipe" that models almost any imaginable 2D microfluidic system. We end by pointing to more advanced topics beyond 2D microfluidics, namely interface problems and flow and diffusion in the third dimension. This forms the basis of a complete theory allowing for the design and operation of new microfluidic systems.