Performance tuning of microfluidic flow-focusing droplet generators

3D printing of monolithic gravity-assisted step-emulsification device for scalable production of high viscosity emulsion droplets

Microfluidic technology widely used in generating monodisperse emulsion droplets often suffers from complexity, scalability, applicability to practical fluids, as well as operation instability due to its susceptibility to flow perturbations, low clearance, and depletion of surfactants. Herein, we present a monolithic 3D-printed step-emulsification device (3D-PSD) for scalable and robust production of high viscosity emulsion droplets up to 208.16 mPa s, which cannot be fully addressed using conventional step-emulsification devices. By utilizing stereo-lithography (SLA), 24 triangular nozzles with a pair of 3D void flow distributors are integrated within the 3D-PSD to ensure uniform flow distribution followed by monodisperse droplet formation. The outlets positioned vertically downward enables gravity-assisted clearing to prevent droplet accumulation and thereby maintain size monodispersity. Deposition of silica nanoparticles (SiNP) within the device was also shown to alter the surface wettability from hydrophobic to hydrophilic, enabling the production of both water-in-oil (W/O) as well as oil-in-water (O/W) emulsion droplets, operated at a maximum production rate of up to 50 mL h-1. The utility of the device is further verified through continuous production of biodegradable polycaprolactone (PCL) microparticles using O/W emulsion as templates. We envision that the 3D-PSD presented in this work marks a significant leap in high-throughput production of high viscosity emulsion droplets as well as the particle analogs.

A Magnetically Driven Biodegradable Microsphere with Mass Production Capability for Subunit Vaccine Delivery and Enhanced Immunotherapy

Subunit vaccines have emerged as a promising strategy in immunotherapy for combating viral infections and cancer. Nevertheless, the clinical application of subunit vaccines is hindered by limitations in antigen delivery efficiency, characterized by rapid clearance and inadequate cellular uptake. Here, a novel subunit vaccine delivery system utilizing ovalbumin@magnetic nanoparticles (OVA@MNPs) encapsulated within biodegradable gelatin methacryloyl (GelMA) microspheres was proposed to enhance the efficacy of antigen delivery. OVA@MNPs-loaded GelMA microspheres, denoted as OMGMs, can be navigated through magnetic fields to deliver subunit vaccines into the lymphatic system efficiently. Moreover, the biodegradable OMGMs enabled the sustained release of subunit vaccines, concentrating OVA around lymph nodes and enhancing the efficacy of induced immune response. OMGMs were produced through a microfluidic droplet generation technique, enabling mass production. In murine models, OMGMs successfully accumulated antigens in lymph nodes abundant in antigen-presenting cells, leading to enhanced cellular and humoral immunity and pronounced antitumor effects with a single booster immunization. In conclusion, these findings highlight the promise of OMGMs as a practical subunit vaccination approach, thus addressing the limitations associated with antigen delivery efficiency and paving the way for advanced immunotherapeutic strategies.

Monolithic 3D nanoelectrospray emitters based on a continuous fluid-assisted etching strategy for glass droplet microfluidic chip-mass spectrometry

Glass microfluidic chips are suitable for coupling with mass spectrometry (MS) due to their flexible design, optical transparency and resistance to organic reagents. However, due to the high hardness and brittleness of glass, there is a lack of simple and feasible technology to manufacture a monolithic nanospray ionization (nESI) emitter on a glass microchip, which hinders its coupling with mass spectrometry. Here, a continuous fluid-assisted etching strategy is proposed to fabricate monolithic three-dimensional (3D) nESI emitters integrated into glass microchips. A continuous fluid of methanol is adopted to protect the inner wall of the channels and the bonding interface of the glass microfluidic chip from being wet-etched, forming sharp 3D nESI emitters. The fabricated 3D nESI emitter can form a stable electrospray plume, resulting in consistent nESI detection of acetylcholine with an RSD of 4.5% within 10 min. The fabricated 3D emitter is integrated on a glass microfluidic chip designed with a T-junction droplet generator, which can realize efficient analysis of acetylcholine in picoliter-volume droplets by nESI-MS. Stability testing of over 20 000 droplets detected by the established system resulted in an RSD of 9.1% over approximately 180 min. The detection of ten neurochemicals in rat cerebrospinal fluid droplets is achieved. The established glass droplet microfluidic chip-MS system exhibits potential for broad applications such as in vivo neurochemical monitoring and single-cell analysis in the future.

Pushing the limits of microfluidic droplet production efficiency: engineering microchannels with seamlessly implemented 3D inverse colloidal crystals

Although droplet microfluidics has been studied for the past two decades, its applications are still limited due to the low productivity of microdroplets resulting from the low integration of planar microchannel structures. In this study, a microfluidic system implementing inverse colloidal crystals (ICCs), a spongious matrix with regularly and densely formed three-dimensional (3D) interconnected micropores, was developed to significantly increase the throughput of microdroplet generation. A new bottom-up microfabrication technique was developed to seamlessly integrate the ICCs into planar microchannels by accumulating non-crosslinked spherical PMMA microparticles as sacrificial porogens in a selective area of a mold and later dissolving them. We have demonstrated that the densely arranged micropores on the spongious ICC of the microchannel function as massively parallel micronozzles, enabling droplet formation on the order of >10 kHz. Droplet size could be adjusted by flow conditions, fluid properties, and micropore size, and biopolymer particles composed of polysaccharides and proteins were produced. By further parallelization of the unit structures, droplet formation on the order of >100 kHz was achieved. The presented approach is an upgrade of the existing droplet microfluidics concept, not only in terms of its high throughput, but also in terms of ease of fabrication and operation.

Design automation of microfluidic single and double emulsion droplets with machine learning

Droplet microfluidics enables kHz screening of picoliter samples at a fraction of the cost of other high-throughput approaches. However, generating stable droplets with desired characteristics typically requires labor-intensive empirical optimization of device designs and flow conditions that limit adoption to specialist labs. Here, we compile a comprehensive droplet dataset and use it to train machine learning models capable of accurately predicting device geometries and flow conditions required to generate stable aqueous-in-oil and oil-in-aqueous single and double emulsions from 15 to 250 μm at rates up to 12000 Hz for different fluids commonly used in life sciences. Blind predictions by our models for as-yet-unseen fluids, geometries, and device materials yield accurate results, establishing their generalizability. Finally, we generate an easy-to-use design automation tool that yield droplets within 3 μm (<8%) of the desired diameter, facilitating tailored droplet-based platforms and accelerating their utility in life sciences.

Versatility and stability optimization of flow-focusing droplet generators via quality metric-driven design automation

Droplet generation is a fundamental component of droplet microfluidics, compartmentalizing biological or chemical systems within a water-in-oil emulsion. As adoption of droplet microfluidics expands beyond expert labs or integrated devices, quality metrics are needed to contextualize the performance capabilities, improving the reproducibility and efficiency of operation. Here, we present two quality metrics for droplet generation: performance versatility, the operating range of a single device, and stability, the distance of a single operating point from a regime change. Both metrics were characterized in silico and validated experimentally using machine learning and rapid prototyping. These metrics were integrated into a design automation workflow, DAFD 2.0, which provides users with droplet generators of a desired performance that are versatile or flow stable. Versatile droplet generators with stable operating points accelerate the development of sophisticated devices by facilitating integration of other microfluidic components and improving the accuracy of design automation tools.

Optimizing the Production of Hydrogel Microspheres Using Microfluidic Chips: The Influence of Surface Treatment on Droplet Formation Mechanism

Microfluidic chips have been widely applied in biology and medical research for stably generating uniform droplets that can be solidified into hydrogel microspheres. However, issues such as low microsphere yield, lengthy experimental processes, and susceptibility to environmental interference need to be addressed. In this work, a simple and effective method was developed to modify microfluidic chips at room temperature to improve the production performance of hydrogel microspheres. Numerical simulation-assisted experiments were conducted to comprehensively understand the effect of solution viscosity, hydrophilicity, and flow rate ratio on droplet formation during microsphere production. Chitosan was selected as the main component and combined with poly(ethylene glycol) diacrylate to prepare photocurable hydrogel microspheres as a demonstration. As a result, grafting fluoro-silane (FOTS) increased the contact angle of the channel from 90 to approximately 110°, which led to a 12.2% increase in droplet yield. Additionally, FOTS-modification attenuated the impact of the flow rate ratio on droplet yield by 19.1%. Alternatively, depositing dopamine decreased the channel's contact angle from 90 to 60°, resulting in a 21.4% increase in particle size and enabling the chip to adjust droplet size over a wider range. Further study demonstrates that the obtained hydrogel microspheres can be modified with layers of aldehyde, which can potentially be used for controlled drug release. Overall, this study proposed a facile method for adjusting the yield and droplet size through surface treatment of microfluidic chips while also enhancing the understanding of the synergistic effects of multiple factors in microfluidics-based microsphere production.

Silicon-Based 3D Microfluidics for Parallelization of Droplet Generation

Both the diversity and complexity of microfluidic systems have experienced a tremendous progress over the last decades, enabled by new materials, novel device concepts and innovative fabrication routes. In particular the subfield of high-throughput screening, used for biochemical, genetic and pharmacological samples, has extensively emerged from developments in droplet microfluidics. More recently, new 3D device architectures enabled either by stacking layers of PDMS or by direct 3D-printing have gained enormous attention for applications in chemical synthesis or biomedical assays. While the first microfluidic devices were based on silicon and glass structures, those materials have not yet been significantly expanded towards 3D despite their high chemical compatibility, mechanical strength or mass-production potential. In our work, we present a generic fabrication route based on the implementation of vertical vias and a redistribution layer to create glass-silicon-glass 3D microfluidic structures. It is used to build different droplet-generating devices with several flow-focusing junctions in parallel, all fed from a single source. We study the effect of having several of these junctions in parallel by varying the flow conditions of both the continuous and the dispersed phases. We demonstrate that the generic concept enables an upscaling in the production rate by increasing the number of droplet generators per device without sacrificing the monodispersity of the droplets.

Deep learning with microfluidics for on-chip droplet generation, control, and analysis

Droplet microfluidics has gained widespread attention in recent years due to its advantages of high throughput, high integration, high sensitivity and low power consumption in droplet-based micro-reaction. Meanwhile, with the rapid development of computer technology over the past decade, deep learning architectures have been able to process vast amounts of data from various research fields. Nowadays, interdisciplinarity plays an increasingly important role in modern research, and deep learning has contributed greatly to the advancement of many professions. Consequently, intelligent microfluidics has emerged as the times require, and possesses broad prospects in the development of automated and intelligent devices for integrating the merits of microfluidic technology and artificial intelligence. In this article, we provide a general review of the evolution of intelligent microfluidics and some applications related to deep learning, mainly in droplet generation, control, and analysis. We also present the challenges and emerging opportunities in this field.

High-throughput microfluidic droplets in biomolecular analytical system: A review

Droplet microfluidic technology has revolutionized biomolecular analytical research, as it has the capability to reserve the genotype-to-phenotype linkage and assist for revealing the heterogeneity. Massive and uniform picolitre droplets feature dividing solution to the level that single cell and single molecule in each droplet can be visualized, barcoded, and analyzed. Then, the droplet assays can unfold intensive genomic data, offer high sensitivity, and screen and sort from a large number of combinations or phenotypes. Based on these unique advantages, this review focuses on up-to-date research concerning diverse screening applications utilizing droplet microfluidic technology. The emerging progress of droplet microfluidic technology is first introduced, including efficient and scaling-up in droplets encapsulation, and prevalent batch operations. Then the new implementations of droplet-based digital detection assays and single-cell muti-omics sequencing are briefly examined, along with related applications such as drug susceptibility testing, multiplexing for cancer subtype identification, interactions of virus-to-host, and multimodal and spatiotemporal analysis. Meanwhile, we specialize in droplet-based large-scale combinational screening regarding desired phenotypes, with an emphasis on sorting for immune cells, antibodies, enzymatic properties, and proteins produced by directed evolution methods. Finally, some challenges, deployment and future perspective of droplet microfluidics technology in practice are also discussed.

Ultrahigh-Throughput Enzyme Engineering and Discovery in In Vitro Compartments

Novel and improved biocatalysts are increasingly sourced from libraries via experimental screening. The success of such campaigns is crucially dependent on the number of candidates tested. Water-in-oil emulsion droplets can replace the classical test tube, to provide in vitro compartments as an alternative screening format, containing genotype and phenotype and enabling a readout of function. The scale-down to micrometer droplet diameters and picoliter volumes brings about a >107-fold volume reduction compared to 96-well-plate screening. Droplets made in automated microfluidic devices can be integrated into modular workflows to set up multistep screening protocols involving various detection modes to sort >107 variants a day with kHz frequencies. The repertoire of assays available for droplet screening covers all seven enzyme commission (EC) number classes, setting the stage for widespread use of droplet microfluidics in everyday biochemical experiments. We review the practicalities of adapting droplet screening for enzyme discovery and for detailed kinetic characterization. These new ways of working will not just accelerate discovery experiments currently limited by screening capacity but profoundly change the paradigms we can probe. By interfacing the results of ultrahigh-throughput droplet screening with next-generation sequencing and deep learning, strategies for directed evolution can be implemented, examined, and evaluated.

Numerical study of a double-stair-shaped dielectrophoresis channel for continuous on-chip cell separation and lysis using finite element method

Prognostication of numerous chronic diseases are in need of identifying circulating tumor cells (CTCs), afterwards, separating and reviving contaminated samples are required. Conventional methods of separating blood cells, namely cytometry or magnetically activated cell sorting, in many cases lose their functionality, or efficiency under different conditions. Hence microfluidic methods of separation have been implemented. Herein, an innovative integrated double stair-shaped microchannel is designed and optimized, capable of 'separation', and 'chemical lysis' simultaneously in which the lysis reagent concentration can be controlled to tune the lysis intensity. The method of insulator-based dielectrophoresis (iDEP), which is the main physics in this device, is utilized yielding maximum separation. Pivotal features of the applied voltage, the voltage difference, the angles and the number of stairs, and the width of the throat in the microchannel have been numerically explored in order to optimize the channel in terms of separation and the lysis buffer concentration. The overall state of optimum case for the voltage difference (ΔV) of 10 owns the following features: the number of stairs is 2, the angle of stairs is 110°, the width of throat is 140 μm, and the inlet voltages are 30 V and 40 V. Also, the overall state of optimum cases for delta possess the following features: the number of stairs is 2, the angle of stairs is 110°, the width of throat is 140 μm, and the inlet voltages are 30 V and 35 V.

dDrop-Chip: disposable film-chip microfluidic device for real-time droplet feedback control

A cost-effective, simple to use, and automated technique that can provide real-time feedback control for droplet generation is required to obtain droplets with high-throughput, stability, and uniformity. This study introduces a disposable droplet generation microfluidic device (dDrop-Chip) that can simultaneously control both droplet size and production rate in real time. The dDrop-Chip consists of a reusable sensing substrate and a disposable microchannel that can be assembled using vacuum pressure. It also integrates a droplet detector and a flow sensor on-chip, enabling real-time measurement and feedback control of droplet size and sample flow rate. The dDrop-Chip has the additional advantage of being disposable, which can prevent chemical and biological contamination, due to low manufacturing cost by the film-chip technique. We demonstrate benefits of the dDrop-Chip by controlling droplet size at a fixed sample flow rate and the production rate at a fixed droplet size using real-time feedback control. The experimental results show that the dDrop-Chip consistently generates monodisperse droplets with a length of 219.36 ± 0.08 μm (CV 0.036%) at a production rate of 32.38 ± 0.48 Hz using the feedback control, while without feedback control, there is a significant deviation in droplet length (224.18 ± 6.69 μm, CV 2.98%) and production rate (33.94 ± 1.72 Hz) despite the use of identical devices. Therefore, the dDrop-Chip is a reliable, cost-effective, and automated technique for generating droplets of controlled size and production rate in real time, making it suitable for various droplet-based applications.

A home-made pipette droplet microfluidics rapid prototyping and training kit for digital PCR, microorganism/cell encapsulation and controlled microgel synthesis

Droplet microfluidics offers a platform from which new digital molecular assay, disease screening, wound healing and material synthesis technologies have been proposed. However, the current commercial droplet generation, assembly and imaging technologies are too expensive and rigid to permit rapid and broad-range tuning of droplet features/cargoes. This rapid prototyping bottleneck has limited further expansion of its application. Herein, an inexpensive home-made pipette droplet microfluidics kit is introduced. This kit includes elliptical pipette tips that can be fabricated with a simple DIY (Do-It-Yourself) tool, a unique tape-based or 3D printed shallow-center imaging chip that allows rapid monolayer droplet assembly/immobilization and imaging with a smart-phone camera or miniature microscope. The droplets are generated by manual or automatic pipetting without expensive and lab-bound microfluidic pumps. The droplet size and fluid viscosity/surface tension can be varied significantly because of our particular droplet generation, assembly and imaging designs. The versatility of this rapid prototyping kit is demonstrated with three representative applications that can benefit from a droplet microfluidic platform: (1) Droplets as microreactors for PCR reaction with reverse transcription to detect and quantify target RNAs. (2) Droplets as microcompartments for spirulina culturing and the optical color/turbidity changes in droplets with spirulina confirm successful photosynthetic culturing. (3) Droplets as templates/molds for controlled synthesis of gold-capped polyacrylamide/gold composite Janus microgels. The easily fabricated and user-friendly portable kit is hence ideally suited for design, training and educational labs.

Fabrication and characterization of core-shell microparticles containing an aqueous core

Core-shell microparticles containing an aqueous core have demonstrated their value for microencapsulation and drug delivery systems. The most important step in generating these uniquely structured microparticles is the formation of droplets and double emulsion. The droplet generator must meet the performance and reliability requirements, including accurate size control with tunability and monodispersity. Herein, we present a facile technique to generate surfactant-free core-shell droplets with an aqueous core in a microfluidic device. We demonstrate that the geometry of the core-shell droplets can be precisely adjusted by the flow rates of the droplet components. As the shell is polymerized after the formation of the core-shell droplets, the resulting solid microparticles ensure the encapsulation of the aqueous core and prevent undesired release. We then study experimentally and theoretically the behaviour of resultant microparticles under heating and compression. The microparticles demonstrate excellent stability under both thermal and mechanical loads. We show that the rupture force can be quantitatively predicted from the shell thickness relative to the outer shell radius. Experimental results and theoretical predictions confirm that the rupture force scales directly with the shell thickness.

Hardware, Software, and Wetware Codesign Environment for Synthetic Biology

Synthetic biology is the process of forward engineering living systems. These systems can be used to produce biobased materials, agriculture, medicine, and energy. One approach to designing these systems is to employ techniques from the design of embedded electronics. These techniques include abstraction, standards, modularity, automated design, and formal semantic models of computation. Together, these elements form the foundation of "biodesign automation," where software, robotics, and microfluidic devices combine to create exciting biological systems of the future. This paper describes a "hardware, software, wetware" codesign vision where software tools can be made to act as "genetic compilers" that transform high-level specifications into engineered "genetic circuits" (wetware). This is followed by a process where automation equipment, well-defined experimental workflows, and microfluidic devices are explicitly designed to house, execute, and test these circuits (hardware). These systems can be used as either massively parallel experimental platforms or distributed bioremediation and biosensing devices. Next, scheduling and control algorithms (software) manage these systems' actual execution and data analysis tasks. A distinguishing feature of this approach is how all three of these aspects (hardware, software, and wetware) may be derived from the same basic specification in parallel and generated to fulfill specific cost, performance, and structural requirements.

Machine learning for microfluidic design and control

Microfluidics has developed into a mature field with applications across science and engineering, having particular commercial success in molecular diagnostics, next-generation sequencing, and bench-top analysis. Despite its ubiquity, the complexity of designing and controlling custom microfluidic devices present major barriers to adoption, requiring intuitive knowledge gained from years of experience. If these barriers were overcome, microfluidics could miniaturize biological and chemical research for non-experts through fully-automated platform development and operation. The intuition of microfluidic experts can be captured through machine learning, where complex statistical models are trained for pattern recognition and subsequently used for event prediction. Integration of machine learning with microfluidics could significantly expand its adoption and impact. Here, we present the current state of machine learning for the design and control of microfluidic devices, its possible applications, and current limitations.

A Biodegradable Magnetic Microrobot Based on Gelatin Methacrylate for Precise Delivery of Stem Cells with Mass Production Capability

A great deal of research has focused on small-scale robots for biomedical applications and minimally invasive delivery of therapeutics (e.g., cells, drugs, and genes) to a target area. Conventional fabrication methods, such as two-photon polymerization, can be used to build sophisticated micro- and nanorobots, but the long fabrication cycle for a single microrobot has limited its practical use. This study proposes a biodegradable spherical gelatin methacrylate (GelMA) microrobot for mass production in a microfluidic channel. The proposed microrobot is fabricated in a flow-focusing droplet generator by shearing a mixture of GelMA, photoinitiator, and superparamagnetic iron oxide nanoparticles (SPIONs) with a mixture of oil and surfactant. Human nasal turbinate stem cells (hNTSCs) are loaded on the GelMA microrobot, and the hNTSC-loaded microrobot shows precise rolling motion in response to an external rotating magnetic field. The microrobot is enzymatically degraded by collagenase, and released hNTSCs are proliferated and differentiated into neuronal cells. In addition, the feasibility of the GelMA microrobot as a cell therapeutic delivery system is investigated by measuring electrophysiological activity on a multielectrode array. Such a versatile and fully biodegradable microrobot has the potential for targeted stem cell delivery, proliferation, and differentiation for stem cell-based therapy.

Microfluidic devices for studying bacterial taxis, drug testing and biofilm formation

Some bacteria have coevolved to establish symbiotic or pathogenic relationships with plants, animals or humans. With human association, the bacteria can cause a variety of diseases. Thus, understanding bacterial phenotypes at the single-cell level is essential to develop beneficial applications. Traditional microbiological techniques have provided great knowledge about these organisms; however, they have also shown limitations, such as difficulties in culturing some bacteria, the heterogeneity of bacterial populations or difficulties in recreating some physical or biological conditions. Microfluidics is an emerging technique that complements current biological assays. Since microfluidics works with micrometric volumes, it allows fine-tuning control of the test conditions. Moreover, it allows the recruitment of three-dimensional (3D) conditions, in which several processes can be integrated and gradients can be generated, thus imitating physiological 3D environments. Here, we review some key microfluidic-based studies describing the effects of different microenvironmental conditions on bacterial response, biofilm formation and antimicrobial resistance. For this aim, we present different studies classified into six groups according to the design of the microfluidic device: (i) linear channels, (ii) mixing channels, (iii) multiple floors, (iv) porous devices, (v) topographic devices and (vi) droplet microfluidics. Hence, we highlight the potential and possibilities of using microfluidic-based technology to study bacterial phenotypes in comparison with traditional methodologies.

Single-Cell Microgels for Diagnostics and Therapeutics

Cell encapsulation within hydrogel droplets is transforming what is feasible in multiple fields of biomedical science such as tissue engineering and regenerative medicine, in vitro modeling, and cell-based therapies. Recent advances have allowed researchers to miniaturize material encapsulation complexes down to single-cell scales, where each complex, termed a single-cell microgel, contains only one cell surrounded by a hydrogel matrix while remaining <100 μm in size. With this achievement, studies requiring single-cell resolution are now possible, similar to those done using liquid droplet encapsulation. Of particular note, applications involving long-term in vitro cultures, modular bioinks, high-throughput screenings, and formation of 3D cellular microenvironments can be tuned independently to suit the needs of individual cells and experimental goals. In this progress report, an overview of established materials and techniques used to fabricate single-cell microgels, as well as insight into potential alternatives is provided. This focused review is concluded by discussing applications that have already benefited from single-cell microgel technologies, as well as prospective applications on the cusp of achieving important new capabilities.

Applications of Microfluidics in Liquid Crystal-Based Biosensors

Liquid crystals (LCs) with stimuli-responsive configuration transition and optical anisotropic properties have attracted enormous interest in the development of simple and label-free biosensors. The combination of microfluidics and the LCs offers great advantages over traditional LC-based biosensors including small sample consumption, fast analysis and low cost. Moreover, microfluidic techniques provide a promising tool to fabricate uniform and reproducible LC-based sensing platforms. In this review, we emphasize the recent development of microfluidics in the fabrication and integration of LC-based biosensors, including LC planar sensing platforms and LC droplets. Fabrication and integration of LC-based planar platforms with microfluidics for biosensing applications are first introduced. The generation and entrapment of monodisperse LC droplets with different microfluidic structures, as well as their applications in the detection of chemical and biological species, are then summarized. Finally, the challenges and future perspectives of the development of LC-based microfluidic biosensors are proposed. This review will promote the understanding of microfluidic techniques in LC-based biosensors and facilitate the development of LC-based microfluidic biosensing devices with high performance.

Simultaneous Droplet Generation with In-Series Droplet T-Junctions Induced by Gravity-Induced Flow

Droplet microfluidics offers a wide range of applications, including high-throughput drug screening and single-cell DNA amplification. However, these platforms are often limited to single-input conditions that prevent them from analyzing multiple input parameters (e.g., combined cellular treatments) in a single experiment. Droplet multiplexing will result in higher overall throughput, lowering cost of fabrication, and cutting down the hands-on time in number of applications such as single-cell analysis. Additionally, while lab-on-a-chip fabrication costs have decreased in recent years, the syringe pumps required for generating droplets of uniform shape and size remain cost-prohibitive for researchers interested in utilizing droplet microfluidics. This work investigates the potential of simultaneously generating droplets from a series of three in-line T-junctions utilizing gravity-driven flow to produce consistent, well-defined droplets. Implementing reservoirs with equal heights produced inconsistent flow rates that increased as a function of the distance between the aqueous inlets and the oil inlet. Optimizing the three reservoir heights identified that taller reservoirs were needed for aqueous inlets closer to the oil inlet. Studying the relationship between the ratio of oil-to-water flow rates (Φ) found that increasing Φ resulted in smaller droplets and an enhanced droplet generation rate. An ANOVA was performed on droplet diameter to confirm no significant difference in droplet size from the three different aqueous inlets. The work described here offers an alternative approach to multiplexed droplet microfluidic devices allowing for the high-throughput interrogation of three sample conditions in a single device. It also has provided an alternative method to induce droplet formation that does not require multiple syringe pumps.

Advances in Magnetic Nanoparticles Engineering for Biomedical Applications-A Review

Magnetic iron oxide nanoparticles (MNPs) have been developed and applied for a broad range of biomedical applications, such as diagnostic imaging, magnetic fluid hyperthermia, targeted drug delivery, gene therapy and tissue repair. As one key element, reproducible synthesis routes of MNPs are capable of controlling and adjusting structure, size, shape and magnetic properties are mandatory. In this review, we discuss advanced methods for engineering and utilizing MNPs, such as continuous synthesis approaches using microtechnologies and the biosynthesis of magnetosomes, biotechnological synthesized iron oxide nanoparticles from bacteria. We compare the technologies and resulting MNPs with conventional synthetic routes. Prominent biomedical applications of the MNPs such as diagnostic imaging, magnetic fluid hyperthermia, targeted drug delivery and magnetic actuation in micro/nanorobots will be presented.

Scaling up the throughput of microfluidic droplet-based materials synthesis: A review of recent progress and outlook

The last two decades have witnessed tremendous progress in the development of microfluidic chips that generate micrometer- and nanometer-scale materials. These chips allow precise control over composition, structure, and particle uniformity not achievable using conventional methods. These microfluidic-generated materials have demonstrated enormous potential for applications in medicine, agriculture, food processing, acoustic, and optical meta-materials, and more. However, because the basis of these chips' performance is their precise control of fluid flows at the micrometer scale, their operation is limited to the inherently low throughputs dictated by the physics of multiphasic flows in micro-channels. This limitation on throughput results in material production rates that are too low for most practical applications. In recent years, however, significant progress has been made to tackle this challenge by designing microchip architectures that incorporate multiple microfluidic devices onto single chips. These devices can be operated in parallel to increase throughput while retaining the benefits of microfluidic particle generation. In this review, we will highlight recent work in this area and share our perspective on the key unsolved challenges and opportunities in this field.

Experimental studies on droplet characteristics in a microfluidic flow focusing droplet generator: effect of continuous phase on droplet encapsulation

The efficacy of droplet-based microfluidic assays depends on droplet size, pattern, generation rate, etc. The size of the droplet is affected by numerous variables as flow rate ratio, viscosity ratio, microchannel geometry, surfactants, nature of fluids and other dimensionless numbers. This work reports rigorous analysis and optimization of the behavior of droplets with change in flow rate ratio and viscosity ratio in a flow-focusing device. Droplets were produced for different flow rate ratios maintaining a constant aqueous phase and varying the continuous phase, to have capillary numbers ranging from 0.01 to 0.1. It was observed that the droplet size decreased with the increase in flow rate ratio, and vice versa. It was noted that as the viscosity ratio was increased, the dispersed phase elongated before the complete breakup and long droplets were formed in the microchannel. Smaller droplets were formed for lower viscosity ratios with a combination of higher flow rate ratios. An empirical relation has been developed to predict the droplet length in terms of capillary number and flow rate ratio for different viscosity ratios. In addition, microparticle encapsulation in individual droplets was attempted to realize the effect of flow rate of the continuous phase for various flow rate ratios on encapsulation efficiency.

A portable droplet generation system for ultra-wide dynamic range digital PCR based on a vibrating sharp-tip capillary

Monodisperse droplet has been widely used as a versatile tool in different disciplines including biosensing. Existing methods still struggle to balance the droplet generation performance with system simplicity. Here we introduce a novel droplet generation scheme based on the acoustic streaming generated from a vibrating sharp-tip capillary. The unique fluid pattern enables efficient droplet generation without any external pressure sources. This method achieved real-time modulation of droplet size over an ultra-wide range (6.77-661 μm), high throughput (up to 5000 droplets/s), and good monodispersity (<4%) with a power consumption below 60 mW. This method has enabled a multi-volume digital PCR with a dynamic range of ~6 orders of magnitude and multiplexing capability. It has also enabled a simple protocol to produce cell-laden alginate microcapsules in variable sizes with excellent biocompatibility. Overall, the present method combines high performance with small footprint and portability, which will be especially valuable for droplet applications requiring variable droplet size and performed in resource-limited settings.

Recent advances in the design of microfluidic technologies for the manufacture of drug releasing particles

Drug releasing particles are valued for their ability to deliver therapeutics to targeted locations and for their controllable release patterns. The development of microfluidic technologies, which are designed specifically to manipulate small amounts of fluids, to manufacture particles for drug delivery applications reflects a recent trend due to the advantages they confer in terms of control over particle size and material composition. This review takes a comprehensive look at the different types of microfluidic devices used to fabricate such particles from different types of biomaterials, and at how the on-chip features enable the production of particles with different types of properties. The review concludes by suggesting avenues for future work that will enable these technologies to fulfill their potential and be used in industrial settings for the manufacture of drug releasing particles with unique capabilities.

3D-Printed Microfluidic Droplet Generator with Hydrophilic and Hydrophobic Polymers

Droplet generation has been widely used in conventional two-dimensional (2D) microfluidic devices, and has recently begun to be explored for 3D-printed droplet generators. A major challenge for 3D-printed devices is preventing water-in-oil droplets from sticking to the interior surfaces of the droplet generator when the device is not made from hydrophobic materials. In this study, two approaches were investigated and shown to successfully form droplets in 3D-printed microfluidic devices. First, several printing resin candidates were tested to evaluate their suitability for droplet formation and material properties. We determined that a hexanediol diacrylate/lauryl acrylate (HDDA/LA) resin forms a solid polymer that is sufficiently hydrophobic to prevent aqueous droplets (in a continuous oil flow) from attaching to the device walls. The second approach uses a fully 3D annular channel-in-channel geometry to form microfluidic droplets that do not contact channel walls, and thus, this geometry can be used with hydrophilic resins. Stable droplets were shown to form using the channel-in-channel geometry, and the droplet size and generation frequency for this geometry were explored for various flow rates for the continuous and dispersed phases.

Setting Up an Automated Biomanufacturing Laboratory

Laboratory automation is a key enabling technology for genetic engineering that can lead to higher throughput, more efficient and accurate experiments, better data management and analysis, decrease in the DBT (Design, Build, and Test) cycle turnaround, increase of reproducibility, and savings in lab resources. Choosing the correct framework among so many options available in terms of software, hardware, and skills needed to operate them is crucial for the success of any automation project. This chapter explores the multiple aspects to be considered for the solid development of a biofoundry project including available software and hardware tools, resources, strategies, partnerships, and collaborations in the field needed to speed up the translation of research results to solve important society problems.

Machine learning enables design automation of microfluidic flow-focusing droplet generation

Droplet-based microfluidic devices hold immense potential in becoming inexpensive alternatives to existing screening platforms across life science applications, such as enzyme discovery and early cancer detection. However, the lack of a predictive understanding of droplet generation makes engineering a droplet-based platform an iterative and resource-intensive process. We present a web-based tool, DAFD, that predicts the performance and enables design automation of flow-focusing droplet generators. We capitalize on machine learning algorithms to predict the droplet diameter and rate with a mean absolute error of less than 10 μm and 20 Hz. This tool delivers a user-specified performance within 4.2% and 11.5% of the desired diameter and rate. We demonstrate that DAFD can be extended by the community to support additional fluid combinations, without requiring extensive machine learning knowledge or large-scale data-sets. This tool will reduce the need for microfluidic expertise and design iterations and facilitate adoption of microfluidics in life sciences.

Development of Microdroplet Generation Method for Organic Solvents Used in Chemical Synthesis

Recently, chemical operations with microfluidic devices, especially droplet-based operations, have attracted considerable attention because they can provide an isolated small-volume reaction field. However, analysis of these operations has been limited mostly to aqueous-phase reactions in water droplets due to device material restrictions. In this study, we have successfully demonstrated droplet formation of five common organic solvents frequently used in chemical synthesis by using a simple silicon/glass-based microfluidic device. When an immiscible liquid with surfactant was used as the continuous phase, the organic solvent formed droplets similar to water-in-oil droplets in the device. In contrast to conventional microfluidic devices composed of resins, which are susceptible to swelling in organic solvents, the developed microfluidic device did not undergo swelling owing to the high chemical resistance of the constituent materials. Therefore, the device has potential applications for various chemical reactions involving organic solvents. Furthermore, this droplet generation device enabled control of droplet size by adjusting the liquid flow rate. The droplet generation method proposed in this work will contribute to the study of organic reactions in microdroplets and will be useful for evaluating scaling effects in various chemical reactions.

Rapid and inexpensive microfluidic electrode integration with conductive ink

Electrode integration significantly increases the versatility of droplet microfluidics, enabling label-free sensing and manipulation at a single-droplet (single-cell) resolution. However, common fabrication techniques for integrating electronics into microfluidics are expensive, time-consuming, and can require cleanroom facilities. Here, we present a simple and cost-effective method for integrating electrodes into thermoplastic microfluidic chips using an off-the-shelf conductive ink. The developed conductive ink electrodes cost less than $10 for an entire chip, have been shown here in channel geometries as small as 75 μm by 50 μm, and can go from fabrication to testing within a day without a cleanroom. The geometric fabrication limits of this technique were explored over time, and proof-of-concept microfluidic devices for capacitance sensing, droplet merging, and droplet sorting were developed. This novel method complements existing rapid prototyping systems for microfluidics such as micromilling, laser cutting, and 3D printing, enabling their wider use and application.

Effect of elastic modulus on inertial displacement of cell-like particles in microchannels

Label-free microfluidic-based cell sorters leverage innate differences among cells (e.g., size and stiffness), to separate one cell type from another. This sorting step is crucial for many cell-based applications. Polystyrene-based microparticles (MPs) are the current gold standard for calibrating flow-based cell sorters and analyzers; however, the deformation behavior of these rigid materials is drastically different from that of living cells. Given this discrepancy in stiffness, an alternative calibration particle that better reflects cell elasticity is needed for the optimization of new and existing microfluidic devices. Here, we describe the fabrication of cell-like, mechanically tunable MPs and demonstrate their utility in quantifying differences in inertial displacement within a microfluidic constriction device as a function of particle elastic modulus, for the first time. Monodisperse, fluorescent, cell-like microparticles that replicate the size and modulus of living cells were fabricated from polyacrylamide within a microfluidic droplet generator and characterized via optical and atomic force microscopy. Trajectories of our cell-like MPs were mapped within the constriction device to predict where living cells of similar size/modulus would move. Calibration of the device with our MPs showed that inertial displacement depends on both particle size and modulus, with large/soft MPs migrating further toward the channel centerline than small/stiff MPs. The mapped trajectories also indicated that MP modulus contributed proportionally more to particle displacement than size, for the physiologically relevant ranges tested. The large shift in focusing position quantified here emphasizes the need for physiologically relevant, deformable MPs for calibrating and optimizing microfluidic separation platforms.

A micrometer head integrated microfluidic device for facile droplet size control and automatic measurement of a droplet size

A novel microfluidic droplet generator is proposed, which can control the droplet size through turning an integrated micrometer head with ease, and the size of the produced micro-droplet can be automatically and real-time monitored by an open-sourced software and off-the-shelf hardware.

An ultra-rapid acoustic micromixer for synthesis of organic nanoparticles

Mixing is a crucial step in many chemical analyses and synthesis processes, particularly in nanoparticle formation, where it determines the nucleation rate, homogeneity, and physicochemical characteristics of the products. In this study, we propose an energy-efficient acoustic platform based on boundary-driven acoustic streaming, which provides the rapid mixing required to control nanoprecipitation. The device encompasses oscillatory bubbles and sharp edges in the microchannel to transform the acoustic energy into vigorous vortical fluid motions. The combination of bubbles and sharp edges at their immediate proximity induced substantially stronger acoustic microstreams than the simple superposition of their effects. The device could effectively homogenize DI water and fluorescein within a mixing length of 25.2 μm up to a flow rate of 116 μL min^-1 at a driving voltage of 40 V_pp, corresponding to a mixing time of 0.8 ms. This rapid mixing was employed to mitigate some complexities in nanoparticle synthesis, namely controlling nanoprecipitation and size, batch to batch variation, synthesis throughput, and clogging. Both polymeric nanoparticles and liposomes were synthesized in this platform and showed a smaller effective size and narrower size distribution in comparison to those obtained by a hydrodynamic flow focusing method. Through changing the mixing time, the effective size of the nanoparticles could be fine-tuned for both polymeric nanoparticles and liposomes. The rapid mixing and strong vortices prevent aggregation of nanoparticles, leading to a substantially higher throughput of liposomes in comparison with that by the hydrodynamic flow focusing method. The straightforward fabrication process of the system coupled with low power consumption, high-controllability, and rapid mixing time renders this mixer a practical platform for a myriad of nano and biotechnological applications.

3DμF - Interactive Design Environment for Continuous Flow Microfluidic Devices

The design of microfluidic Lab on a Chip (LoC) systems is an onerous task requiring specialized skills in fluid dynamics, mechanical design drafting, and manufacturing. Engineers face significant challenges during the labor-intensive process of designing microfluidic devices, with very few specialized tools that help automate the process. Typical design iterations require the engineer to research the architecture, manually draft the device layout, optimize for manufacturing processes, and manually calculate and program the valve sequences that operate the microfluidic device. The problem compounds when engineers not only have to test the functionality of the chip but are also expected to optimize them for the robust execution of biological assays. In this paper, we present an interactive tool for designing continuous flow microfluidic devices. 3DμF is the first completely open source interactive microfluidic system designer that readily supports state of the art design automation algorithms. Through various case studies, we show 3DμF can be used to reproduce designs from literature, provide metrics for evaluating microfluidic design complexity and showcase how 3DμF is a platform for integrating a wide assortment of engineering techniques used in the design of microfluidic devices as a part of the standard design workflow.