Droplet morphometry and velocimetry (DMV): a video processing software for time-resolved, label-free tracking of droplet parameters

Gelatin maleimide microgels for hematopoietic progenitor cell encapsulation

Hematopoietic stem cells (HSCs) are the apical cells of the hematopoietic system, giving rise to cells of the blood and lymph lineages. HSCs reside primarily within bone marrow niches that contain matrix and cell-derived signals that help inform stem cell fate. Aspects of the bone marrow microenvironment have been captured in vitro by encapsulating cells within hydrogel matrices that mimic native mechanical and biochemical properties. Hydrogel microparticles, or microgels, are increasingly being used to assemble granular biomaterials for cell culture and noninvasive delivery applications. Here, we report the optimization of a gelatin maleimide hydrogel system to create monodisperse gelatin microgels via a flow-focusing microfluidic process. We report characteristic hydrogel stiffness, stability, and swelling characteristics as well as encapsulation of murine hematopoietic stem and progenitor cells, and mesenchymal stem cells within microgels. Microgels support cell viability, confirming compatibility of the microfluidic encapsulation process with these sensitive bone marrow cell populations. Overall, this work presents a microgel-based gelatin maleimide hydrogel as a foundation for future development of a multicellular artificial bone marrow culture system.

Automated dynamic inlet microfluidics system: 3D printer adaptation for cost-effective, low volume, on-demand multi-analyte droplet generator

The paper demonstrates an adaptation of a 3D printer (Prusa Mini+) with novel modules to develop a droplet generation system that generates combinatorial droplets from a standard 96 well plate. The calibration methodology developed would allow any fused deposition modeling (FDM) printer to generate monodisperse droplets (coefficient of variance (CV%) < 5%) from well plates or vials of any geometry. The system maintains precision across various volumes while maintaining a C.V. range of 0.81% to 3.61%, with an increased precision for larger volumes. The cost of the system developed is 70% less than commercially available droplet generation packages. Successful droplet library storage is accomplished via 3D printed cartridge connectors. The implemented system has been calibrated for Tygon® and PTFE at different velocities and volumetric configurations.

Experimental investigation on the bouncing dynamics of a liquid marble during the impact on a hydrophilic surface

Liquid marbles are droplets coated by hydrophobic particles. At low Weber numbers (We), when impacting a hydrophilic surface, the marble may bounce on the substrate repeatedly without any rupturing until the quiescence condition is achieved. The marble bouncing has gained far less attention, although its rich underlying physics is due to the interaction between liquid core, hydrophobic grain, and surrounding air. Accordingly, this research experimentally scrutinizes the marble impact and subsequent bouncing on a hydrophilic surface for the first time. Additionally, the conversion of kinetic, gravitational potential, inertial, and surface energies occurring regularly during the impact is exhaustively surveyed. Moreover, the effect of Weber and gravitational Bond numbers (Bo) on the bouncing time, maximum spreading time, maximum spreading ratio, maximum elongation ratio, and maximum restitution are investigated, which characterize the marble impact and bouncing dynamics. This study is one of the limited investigations exploring the effects of the gravitational Bond number on the results. Dimensionless correlations are proposed for the mentioned parameters based on the experimental data. Furthermore, utilizing the simplifying theoretical presumptions, correlations are suggested based on the scale analysis for the spreading time and maximum spreading ratio. The results imply that the mentioned parameters behave differently at low and moderate Weber numbers, though the distinction is more pronounced in the case of the bouncing time, maximum spreading time and maximum spreading ratio. Although increasing with the Weber number when WeWecr. In addition, the maximum elongation ratio linearly grows with the Weber number.

Dynamic behavior of floating magnetic liquid marbles under steady and pulse-width-modulated magnetic fields

Liquid marbles show promising potential for digital microfluidic devices due to their lower friction with the platform surface than non-covered droplets. In this study, the manipulation of a biocompatible magnetic liquid marble with a magnetic shell (LMMS) is experimentally studied. The movement of the floating LMMS on the water surface, which is actuated by DC and pulse width modulation (PWM) magnetic fields, is investigated under the influence of various parameters, including the LMMS volume, the initial distance of the LMMS from the magnetic coil tip, the magnetic coil current, the PWM frequency and its duty cycle. The LMMS has a shorter travel time to the magnetic coil tip under a DC magnetic field by increasing the magnetic coil current, decreasing the initial distance and its volume. In the PWM mode, these parameters show similar behavior; moreover, increasing the PWM duty cycle and decreasing the PWM frequency shorten the travel time. It is demonstrated that actuation by a PWM magnetic field with step-by-step movement provides better control over manipulation of the floating magnetic marble. The dynamic behavior of an LMMS is compared to a ferrofluid marble (FM), which is formed using a ferrofluid instead of water as its core. It is observed that the LMMS has a lower velocity than the FM.

Dynamic video recognition for cell-encapsulating microfluidic droplets

Droplet microfluidics is a highly sensitive and high-throughput technology extensively utilized in biomedical applications, such as single-cell sequencing and cell screening. However, its performance is highly influenced by the droplet size and single-cell encapsulation rate (following random distribution), thereby creating an urgent need for quality control. Machine learning has the potential to revolutionize droplet microfluidics, but it requires tedious pixel-level annotation for network training. This paper investigates the application software of the weakly supervised cell-counting network (WSCApp) for video recognition of microdroplets. We demonstrated its real-time performance in video processing of microfluidic droplets and further identified the locations of droplets and encapsulated cells. We verified our methods on droplets encapsulating six types of cells/beads, which were collected from various microfluidic structures. Quantitative experimental results showed that our approach can not only accurately distinguish droplet encapsulations (micro-F1 score > 0.94), but also locate each cell without any supervised location information. Furthermore, fine-tuning transfer learning on the pre-trained model also significantly reduced (>80%) annotation. This software provides a user-friendly and assistive annotation platform for the quantitative assessment of cell-encapsulating microfluidic droplets.

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

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

The Effect of Gelatin Source on the Synthesis of Gelatin-Methacryloyl and the Production of Hydrogel Microparticles

Gelatin methacryloyl (GelMA) is widely used for the formulation of hydrogels in diverse biotechnological applications. After the derivatization of raw gelatin, the degree of functionalization (DoF) is an attribute of particular interest as the functional residues are necessary for crosslinking. Despite progress in the optimization of the process found in the literature, a comparison of the effect of raw gelatin on the functionalization is challenging as various approaches are employed. In this work, the modification of gelatin was performed at room temperature (RT), and eight different gelatin products were employed. The DoF proved to be affected by the bloom strength and by the species of gelatin at an equal reactant ratio. Furthermore, batch-to-batch variability of the same gelatin source had an effect on the produced GelMA. Moreover, the elasticity of GelMA hydrogels depended on the DoF of the protein as well as on bloom strength and source of the raw material. Additionally, GelMA solutions were used for the microfluidic production of droplets and subsequent crosslinking to hydrogel. This process was developed as a single pipeline at RT using protein concentrations up to 20% (w/v). Droplet size was controlled by the ratio of the continuous to dispersed phase. The swelling behavior of hydrogel particles depended on the GelMA concentration.

DUPLETS: Deformability-Assisted Dual-Particle Encapsulation Via Electrically Activated Sorting

Co-encapsulation of bead carriers and biological cells in microfluidics has become a powerful technique for various biological assays in single-cell genomics and drug screening because of its distinct capability of single-cell confinement. However, current co-encapsulation approaches exist a trade-off between cell/bead pairing rate and probability of multiple cells in individual droplets, significantly limiting the effective throughput of single-paired cell-bead droplets production. Deformability-assisted dUal-Particle encapsuLation via Electrically acTivated Sorting (DUPLETS) system is reported to overcome this problem. The DUPLETS can differentiate the encapsulated content in individual droplets and sort out targeted droplets via a combined screening of mechanical and electrical characteristics of single droplets in label-free manners and with the highest effective throughput in comparison to current commercial platforms. The DUPLETS has been demonstrated to enrich single-paired cell-bead droplets to over 80% (above eightfold higher than current co-encapsulation techniques). It eliminates multicell droplets to 0.1% whereas up to ≈24% in 10× Chromium. It is believed that merging DUPLETS into the current co-encapsulation platforms can meaningfully elevate sample quality in terms of high purity of single-paired cell-bead droplets, low fraction of multicell droplets, and high cell viability, which can benefit a multitude of biological assay applications.

WSCNet: Biomedical Image Recognition for Cell Encapsulated Microfluidic Droplets

Microfluidic droplets accommodating a single cell as independent microreactors are frequently demanded for single-cell analysis of phenotype and genotype. However, challenges exist in identifying and reducing the covalence probability (following Poisson's distribution) of more than two cells encapsulated in one droplet. It is of great significance to monitor and control the quantity of encapsulated content inside each droplet. We demonstrated a microfluidic system embedded with a weakly supervised cell counting network (WSCNet) to generate microfluidic droplets, evaluate their quality, and further recognize the locations of encapsulated cells. Here, we systematically verified our approach using encapsulated droplets from three different microfluidic structures. Quantitative experimental results showed that our approach can not only distinguish droplet encapsulations (F1 score > 0.88) but also locate each cell without any supervised location information (accuracy > 89%). The probability of a "single cell in one droplet" encapsulation is systematically verified under different parameters, which shows good agreement with the distribution of the passive method (Residual Sum of Squares, RSS < 0.5). This study offers a comprehensive platform for the quantitative assessment of encapsulated microfluidic droplets.

Multi-objective optimization of multiple droplet impacts on a molten PCM using NSGA-II optimizer and artificial neural network

Embracing an interaction between the phase change material (PCM) and the droplets of a heat transfer fluid, the direct contact (DC) method suggests a cutting-edge solution for expediting the phase change rates of PCMs in thermal energy storage (TES) units. In the direct contact TES configuration, when impacting the molten PCM pool, droplets evaporate, provoking the formation of a solidified PCM area (A). Then, they reduce the created solid temperature, leading to a minimum temperature value (Tmin). As a novelty, this research intends to maximize A and minimize Tmin since augmenting A expedites the discharge rate, and by lowering Tmin, the generated solid is preserved longer, resulting in a higher storage efficacy. To take the influences of interaction between droplets into account, the simultaneous impingement of two ethanol droplets on a molten paraffin wax is surveyed. Impact parameters (Weber number, impact spacing, and the pool temperature) govern the objective functions (A and Tmin). Initially, through high-speed and IR thermal imaging, the experimental values of objective functions are achieved for a wide range of impact parameters. Afterward, exploiting an artificial neural network (ANN), two models are fitted to A and Tmin, respectively. Subsequently, the models are provided for the NSGA-II algorithm to implement multi-objective optimization (MOO). Eventually, utilizing two different final decision-making (FDM) approaches (LINMAP and TOPSIS), optimized impact parameters are attained from the Pareto front. Regarding the results, the optimum amount of Weber number, impact spacing, and pool temperature accomplished by LINMAP and TOPSIS procedures are 309.44, 2.84 mm, 66.89 °C, and 294.98, 2.78 mm, 66.89 °C, respectively. This is the first investigation delving into the optimization of multiple droplet impacts for TES applications.

Multiplex single-cell droplet PCR with machine learning for detection of high-risk human papillomaviruses

High-risk human papillomavirus (HPV) testing can significantly decline the incidence and mortality of cervical cancer. Microfluidic technology provides an effective method for accurate detection of high-risk HPV by utilizing multiplex single-cell droplet polymerase chain reaction (PCR). However, current strategies are limited by low-integration microfluidic chip, complex reagent system, expensive detection equipment and time-consuming droplet identification. Here, we developed a novel multiplex droplet PCR method that directly detected high-risk HPV sequences in single cells. A multiplex microfluidic chip integrating four flow-focusing structures was designed for one-step and parallel droplet preparation. Using single-cell droplet PCR, multi-target sequences were detected simultaneously based on a monochromatic fluorescence signal. We applied machine learning to automatically identify the large populations of single-cell droplets with 97% accuracy. HPV16, 18 and 45 sequences were sensitively detected without cross-contamination in mixed CaSki and Hela cells. The approach enables rapid and reliable detection of multi-target sequences in single cells, making it powerful for investigating cellular heterogeneity related to cancer diagnosis and treatment.

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.

Assessment of microplastics using microfluidic approach

Microplastics are plastic particles smaller than 5 mm, and microplastics have gradually become a severe environmental pollution source that exists in the atmosphere. The identification and quantification of microplastic particles are challenging, current approaches require expensive instruments and are usually time-consuming. In this study, a microfluidic method was introduced to detect and count microplastics using a polymer-based microfluidic chip. Microplastic particles were stained with Nile red, dispersed in the carrier fluid and passed through the microchannel. A fluorescence microscope filmed the whole process as microplastic particles passed through the microchannel. Finally, the software automatically analyzed the video footage for the microplastic particle counting and size analysis. The entire process is fully automated for microplastic particle counting and is much more efficient than the current manual counting method. The proposed study may have broad application potentials in the environmental field.

Experimental and Theoretical Investigation on the Dynamic Response of Ferrofluid Liquid Marbles to Steady and Pulsating Magnetic Fields

Liquid marbles are droplets enwrapped by a layer of hydrophobic micro/nanoparticles. Due to the isolation of fluid from its environment, reduction in evaporation rate, low friction with the surfaces, and capability of manipulation even on hydrophilic surfaces, liquid marbles have attracted the attention of researchers in digital microfluidics. This study investigates the manipulation of ferrofluid liquid marbles (FLMs) under DC and pulse width-modulated (PWM) magnetic fields generated by an electromagnet for the first time. At first, the threshold of the magnetic field for manipulating these FLMs is studied. Afterward, the dynamic response of the FLMs to the DC magnetic field for different FLM volumes, coil currents, and initial distances of FLM from the coil is studied, and a theoretical model is proposed. By applying the PWM magnetic field, it is possible to gain more control over the manipulation of the FLMs on the surface and adjust their position more accurately. Results indicate that with a decrease in FLM volume, coil current, and duty cycle, the FLM step length decreases; hence, FLM manipulation is more precise. Under the PWM magnetic field, it is observed that FLM movement is not synchronous with the generated pulse, and even after the coil is turned off, FLMs keep their motion. In the end, with proper adjustment of the electromagnet pulse width, launching of FLMs at a distance farther than the coil is observed.

Determination of the Dominating Coalescence Pathways in Double Emulsion Formulations by Use of Microfluidic Emulsions

In water-in-oil-in-water (W1/O/W2) double emulsions several irreversible instability phenomena lead to changes. Besides diffusive processes, coalescence of droplets is the main cause of structural changes. In double emulsions, inner droplets can coalesce with each other (W1–W1 coalescence), inner droplets can be released via coalescence (W1–W2 coalescence) and oil droplets can coalesce with each other (O–O coalescence). Which of the coalescence pathways contributes most to the failure of the double emulsion structure cannot be determined by common measurement techniques. With monodisperse double emulsions produced with microfluidic techniques, each coalescence path can be observed and quantified simultaneously. By comparing the occurrence of all possible coalescence events, different hydrophilic surfactants in combination with PGPR are evaluated and discussed with regard to their applicability in double emulsion formulations. When variating the hydrophilic surfactant, the stability against all three coalescence mechanisms changes. This shows that measuring only one of the coalescence mechanisms is not sufficient to describe the stability of a double emulsion. While some surfactants are able to stabilize against all three possible coalescence mechanisms, some display mainly one of the coalescence mechanisms or in some cases all three mechanisms are observed simultaneously.

Magnetically Driven Manipulation of Nonmagnetic Liquid Marbles: Billiards with Liquid Marbles

Liquid marbles are droplets encapsulated by a layer of hydrophobic nanoparticles and have been extensively employed in digital microfluidics and lab-on-a-chip systems in recent years. In this study, magnetic liquid marbles were used to manipulate nonmagnetic liquid marbles. To achieve this purpose, a ferrofluid liquid marble (FLM) was employed and attracted toward an electromagnet, resulting in an impulse to a water liquid marble (WLM) on its way to the electromagnet. It was observed that the manipulation of the WLM by the FLM was similar to the collision of billiard balls except that the liquid marbles exhibited an inelastic collision. Taking the FLM as the projectile ball and the WLM as the other target balls, one can adjust the displacement and direction of the WLM precisely, similar to an expert billiard player. Firstly, the WLM displacement can be adjusted by altering the liquid marble volumes, the initial distances from the electromagnet, and the coil current. Secondly, the WLM direction can be adjusted by changing the position of the WLM relative to the connecting line between the FLM center and the electromagnet. Results show that when the FLM or WLM volume increases by five times, the WLM shooting distance approximately increases by 200% and decreases by 75%, respectively.

Recent Advances of Utilizing Artificial Intelligence in Lab on a Chip for Diagnosis and Treatment

Nowadays, artificial intelligence (AI) creates numerous promising opportunities in the life sciences. AI methods can be significantly advantageous for analyzing the massive datasets provided by biotechnology systems for biological and biomedical applications. Microfluidics, with the developments in controlled reaction chambers, high-throughput arrays, and positioning systems, generate big data that is not necessarily analyzed successfully. Integrating AI and microfluidics can pave the way for both experimental and analytical throughputs in biotechnology research. Microfluidics enhances the experimental methods and reduces the cost and scale, while AI methods significantly improve the analysis of huge datasets obtained from high-throughput and multiplexed microfluidics. This review briefly presents a survey of the role of AI and microfluidics in biotechnology. Also, the incorporation of AI with microfluidics is comprehensively investigated. Specifically, recent studies that perform flow cytometry cell classification, cell isolation, and a combination of them by gaining from both AI methods and microfluidic techniques are covered. Despite all current challenges, various fields of biotechnology can be remarkably affected by the combination of AI and microfluidic technologies. Some of these fields include point-of-care systems, precision, personalized medicine, regenerative medicine, prognostics, diagnostics, and treatment of oncology and non-oncology-related diseases.

Nonequilibrium interfacial diffusion across microdroplet interface

Increases in complexity attainable in molecular self-assembly necessitates both advanced molecular design as well as microenvironmental control. Such control is offered by microfluidics, where precise chemical compositions and gradients can be readily established. A droplet microfluidic platform combining upstream step emulsification with downstream hydrodynamic microtraps has been designed to facilitate molecular self-assembly. The step emulsification rapidly generates uniform droplets which act as reaction chambers. The hydrodynamic microtraps hold droplets against the flow ensuring they are exposed to a continuous supply of fresh fluid for constant reagent extraction and/or delivery. Additionally, the droplet immobilization permits real-time droplet characterization and reaction monitoring. Subsequently, droplets can be released from the traps through flow reversal, allowing post-process characterization. The microfluidic system was demonstrated by the phase separation of lyotropic droplets. Ethanol/water droplets were created in a continuous ambient squalene/monoolein microflow, causing the continuous extraction of ethanol from the droplets and delivery of monoolein from the ambient microflow. Unlike conventional bulk techniques and continuous microfluidics, where finite microchannel lengths necessarily impose limits to the extent to which slow processes can proceed, this approach allows extended duration reactions whilst enabling real time process monitoring.

Machine Learning-Aided Microdroplets Breakup Characteristic Prediction in Flow-Focusing Microdevices by Incorporating Variations of Cross-Flow Tilt Angles

Controlling droplet breakup characteristics such as size, frequency, regime, and droplet quality within flow-focusing microfluidic devices is critical for different biomedical applications of droplet microfluidics such as drug delivery, biosensing, and nanomaterial preparation. The development of a prediction platform capable of forecasting droplet breakup characteristics can significantly improve the iterative design and fabrication processes required for achieving desired performance. The present study aims to develop a multipurpose platform capable of predicting the working conditions of user-specific droplet size and frequency and reporting the quality of the generated droplets, regime, and hydrodynamical breakup characteristics in flow-focusing microdevices with different cross-junction tilt angles. Four different neural network-based prediction platforms were compared to accurately estimate capsule size, generation rate, uniformity, and circle metric. The trained capsule size and frequency networks were optimized using the heuristic optimization approach for establishing the Pareto optimal solution plot. To investigate the transition of the droplet generation regime (i.e., squeezing, dripping, and jetting), two different classification models (LDA and MLP) were developed and compared in terms of their prediction accuracy. The MLP model outperformed the LDA model with a cross-validation measure evaluated as 97.85%, demonstrating that the droplet quality and regime prediction models can provide an engineering judgment for the decision maker to choose between the suggested solutions on the Pareto front. The study followed a comprehensive hydrodynamical analysis of the junction angle effect on the dispersed thread formation, pressure, and velocity domains in the orifice.

On-demand ferrofluid droplet formation with non-linear magnetic permeability in the presence of high non-uniform magnetic fields

The magnetic actuation of ferrofluid droplets offers an inspiring tool in widespread engineering and biological applications. In this study, the dynamics of ferrofluid droplet generation with a Drop-on-Demand feature under a non-uniform magnetic field is investigated by multiscale numerical modeling. Langevin equation is assumed for ferrofluid magnetic susceptibility due to the strong applied magnetic field. Large and small computational domains are considered. In the larger domain, the magnetic field is obtained by solving Maxwell equations. In the smaller domain, a coupling of continuity, Navier Stokes, two-phase flow, and Maxwell equations are solved by utilizing the magnetic field achieved by the larger domain for the boundary condition. The Finite volume method and coupling of level-set and Volume of Fluid methods are used for solving equations. The droplet formation is simulated in a two-dimensional axisymmetric domain. The method of solving fluid and magnetic equations is validated using a benchmark. Then, ferrofluid droplet formation is investigated experimentally, and the numerical results showed good agreement with the experimental data. The effect of 12 dimensionless parameters, including the ratio of magnetic, gravitational, and surface tension forces, the ratio of the nozzle and magnetic coil dimensions, and ferrofluid to continuous-phase properties ratios are studied. The results showed that by increasing the magnetic Bond number, gravitational Bond number, Ohnesorge number, dimensionless saturation magnetization, initial magnetic susceptibility of ferrofluid, the generated droplet diameter reduces, whereas the formation frequency increases. The same results were observed when decreasing the ferrite core diameter to outer nozzle diameter, density, and viscosity ratios.

Point-of-care blood coagulation assay enabled by printed circuit board-based digital microfluidics

The monitoring of coagulation function has great implications in many clinical settings. However, existing coagulation assays are simplex, sample-consuming, and slow in turnaround, making them less suitable for point-of-care testing. In this work, we developed a novel blood coagulation assay that simultaneously assesses both the tendency of clotting and the stiffness of the resultant clot using printed circuit board (PCB)-based digital microfluidics. A drop of blood was actuated to move back and forth on the PCB electrode array, until the motion winded down as the blood coagulated and became thicker. The velocity tracing and the deformation of the clot were calculated via image analysis to reflect the coagulation progression and the clot stiffness, respectively. We investigated the effect of different hardware and biochemical settings on the assay results. To validate the assay, we performed assays on blood samples with hypo- and hyper-coagulability, and the results confirmed the assay's capability in distinguishing different blood samples. We then examined the correlation between the measured metrics in our assays and standard coagulation assays, namely prothrombin time and fibrinogen level, and the high correlation supported the clinical relevance of our assay. We envision that this method would serve as a powerful point-of-care coagulation testing method.

A Lotus shaped acoustofluidic mixer: High throughput homogenisation of liquids in 2 ms using hydrodynamically coupled resonators

This paper presents an acoustically actuated microfluidic mixer that uses an array of hydrodynamically coupled resonators to rapidly homogenise liquid solutions and synthesise nanoparticles. The system relies on 8 identical oscillating cantilevers that are equally spaced on the perimeter of a circular well, through which the liquid solutions are introduced. When an oscillatory electrical signal is applied to a piezoelectric transducer attached to the device, the cantilevers start resonating. Due to the close proximity between the cantilevers, their circular arrangement and the liquid medium in which they are immersed, the vibration of each cantilever affects the response of its neighbours. The streaming fields and shearing rates resulting from the oscillating structures were characterised. It was shown that the system can be used to effectively mix fluids at flow rates up to 1400 µl.min^-1 in time scales as low as 2 ms. The rapid mixing time is especially advantageous for nanoparticle synthesis, which is demonstrated by synthesising Poly lactide-co-glycolic acid (PLGA) nanoparticles with 52.2 nm size and PDI of 0.44.

Continuous size-based DEP separation of particles using a bi-gap electrode pair

The design, fabrication, and characterization of an advanced microfluidic device containing a bi-gap electrode pair for the continuous separation of three different populations of particles based on their size using DEP are presented.

Novel glass capillary microfluidic devices for the flexible and simple production of multi-cored double emulsions

HYPOTHESIS

Double emulsions with many monodispersed internal droplets are required for the fabrication of multicompartment microcapsules and tissue-like synthetic materials. These double emulsions can also help to optically resolve different coalescence mechanisms contributing to double emulsion destabilization. Up to date microfluidic double emulsions are limited to either core-shell droplets or droplets with eight or less inner droplets. By applying a two-step jet break-up within one setup, double emulsion droplets filled with up to several hundred monodispersed inner droplets can be achieved.

EXPERIMENTS

Modular interconnected CNC-milled Lego®-inspired blocks were used to create two separated droplet break-up points within coaxial glass capillaries. Inner droplets were formed by countercurrent flow focusing within a small inner capillary, while outer droplets were formed by co-flow in an outer capillary. The size of inner and outer droplets was independently controlled since the two droplet break-up processes were decoupled.

FINDINGS

With the developed setup W/O/W and O/W/O double emulsions were produced with different surfactants, oils, and viscosity modifiers to encapsulate 25-400 inner droplets in each outer drop with a volume percentage of inner phase between 7% and 50%. From these emulsions monodispersed multicompartment microcapsules were obtained. The report offers insights on the relationship between the coalescence of internal droplets and their release.

Microfluidic On-Chip Production of Alginate Hydrogels Using Double Coflow Geometry

Microfluidic on-chip production of microgels employing external gelation has numerous biological and pharmaceutical applications, particularly for the encapsulation of delicate cargos; however, the on-chip production of microgels in microfluidic devices can be challenging due to problems such as clogging caused by accelerated progress in precursor solution viscosity. Here, we introduce a novel microfluidic design incorporating two consecutive coflow geometries for microfluidic droplet generation. A shielding oil phase is employed to avoid emulsification and gelation stages from occurring simultaneously, thereby preventing clogging. The results revealed that the microfluidic device could generate highly monodispersed spherical droplets (coefficient of variation < 3%) with an average diameter in the range of 60-200 μm. Additionally, it was demonstrated that the device could appropriately create a shelter of the oil phase around the inner aqueous phase regardless of the droplet formation regime and flow conditions. The ability of the proposed microfluidic device in the generation of microgels was validated by producing alginate microgels utilizing an aqueous solution of calcium chloride as the continuous phase.

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

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

A portable droplet microfluidic device for cortisol measurements using a competitive heterogeneous assay

Point-of-care monitoring of chemical biomarkers in real-time holds great potential in rapid disease diagnostics and precision medicine. However, monitoring is still rare in practice, as the measurement of biomarkers often requires time consuming and labour intensive assay procedures such as enzyme linked immunosorbent assay (ELISA), which pose a challenge to an autonomous point-of-care device. This paper describes a prototype device capable of performing ELISA autonomously and repeatedly in a high frequency using droplet microfluidics. Driven by a specially designed peristaltic pump, the device can collect liquid samples from a reservoir, produce trains of droplets, complete magnetic bead based ELISA protocols and provide readouts with colourimetric measurement. Here, cortisol was chosen as a target analyte as its concentration in the human body varies on a circadian rhythm which may be perturbed by disease. The prototype device draws in and analyses 350 nL of the sample containing free bioactive cortisol every 10 seconds, with a sample-to-signal time of 10 minutes, and measures favourably in the analytical range of 3.175-100 ng ml-1, with reliably lower variability compared with the well plate based assay. As most ELISA type assays share similar procedures, we envisage that this approach could form a platform technology for measurement or even continuous monitoring of biomarkers in biological fluids at the point-of-care.

Sheath-assisted versus sheathless dielectrophoretic particle separation

Lab-on-chip devices are widely being used for binary and ternary cell/particle separation applications. Among the lab-on-chip methods, dielectrophoresis (DEP) is a cost-effective and label-free method, with great capabilities for size-based separation of cells and particles, which is mostly performed in sheath-assisted forms. However, the elimination of the sheath flows offers advantages such as ease of operation and higher sample throughput. In this work, we present a comparison of sheath-assisted and sheathless DEP separation of three sizes of microparticles using tilted electrodes. The sheath-assisted design was capable of separating the 5, 10, and 15 μm particles with a separation efficiency as high as 98.0% for 15 μm particles. By adding a DEP focusing region, a sheathless DEP separator was proposed, which offered higher throughputs (up to 10 times) at the cost of lowering the separation efficiency (a reduction up to 10.3% for 15 μm) compared to the sheath-assisted design. To enhance the separation efficiency, a combination of the DEP focusing accompanied by weak sheath flows from both sides was proposed. This design achieved the highest sample separation yield in the outlets (as high as 98.7% for 15 μm) with a sample throughput of more than 4.2 μL/min. This study provides insights into the choice of an appropriate platform for any application in which the yield, purity, throughput, and portability must be considered.

Integration of Droplet Microfluidic Tools for Single-Cell Functional Metagenomics: An Engineering Head Start

Droplet microfluidic techniques have shown promising outcome to study single cells at high throughput. However, their adoption in laboratories studying “-omics” sciences is still irrelevant due to the complex and multidisciplinary nature of the field. To facilitate their use, here we provide engineering details and organized protocols for integrating three droplet-based microfluidic technologies into the metagenomic pipeline to enable functional screening of bioproducts at high throughput. First, a device encapsulating single cells in droplets at a rate of ∼250 Hz is described considering droplet size and cell growth. Then, we expand on previously reported fluorescence-activated droplet sorting systems to integrate the use of 4 independent fluorescence-exciting lasers (i.e., 405, 488, 561, and 637 nm) in a single platform to make it compatible with different fluorescence-emitting biosensors. For this sorter, both hardware and software are provided and optimized for effortlessly sorting droplets at 60 Hz. Then, a passive droplet merger is also integrated into our pipeline to enable adding new reagents to already-made droplets at a rate of 200 Hz. Finally, we provide an optimized recipe for manufacturing these chips using silicon dry-etching tools. Because of the overall integration and the technical details presented here, our approach allows biologists to quickly use microfluidic technologies and achieve both single-cell resolution and high-throughput capability (>50,000 cells/day) for mining and bioprospecting metagenomic data

Liquid-Liquid Flows with Non-Newtonian Dispersed Phase in a T-Junction Microchannel

Immiscible liquid–liquid flows in microchannels are used extensively in various chemical and biological lab-on-a-chip systems when it is very important to predict the expected flow pattern for a variety of fluids and channel geometries. Commonly, biological and other complex liquids express non-Newtonian properties in a dispersed phase. Features and behavior of such systems are not clear to date. In this paper, immiscible liquid–liquid flow in a T-shaped microchannel was studied by means of high-speed visualization, with an aim to reveal the shear-thinning effect on the flow patterns and slug-flow features. Three shear-thinning and three Newtonian fluids were used as dispersed phases, while Newtonian castor oil was a continuous phase. For the first time, the influence of the non-Newtonian dispersed phase on the transition from segmented to continuous flow is shown and quantitatively described. Flow-pattern maps were constructed using nondimensional complex We^0.4·Oh^0.6 depicting similarity in the continuous-to-segmented flow transition line. Using available experimental data, the proposed nondimensional complex is shown to be effectively applied for flow-pattern map construction when the continuous phase exhibits non-Newtonian properties as well. The models to evaluate an effective dynamic viscosity of a shear-thinning fluid are discussed. The most appropriate model of average-shear-rate estimation based on bulk velocity was chosen and applied to evaluate an effective dynamic viscosity of a shear-thinning fluid. For a slug flow, it was found that in the case of shear-thinning dispersed phase at low flow rates of both phases, a jetting regime of slug formation was established, leading to a dramatic increase in slug length.

Sheath-assisted focusing of microparticles on lab-on-a-chip platforms

Lab-on-a-chip (LOC) technologies can take advantage of sheath flows for particle/cell focusing before sensing or sorting. The integration of focusing with other microscale manipulation techniques (e.g., sorting) creates a trade-off between the throughput of the device and its performance. Therefore, exploring the effective parameters for cells/particles focusing enables us to improve the desired output of LOC devices. A common configuration for sheath-assisted focusing is Y junctions, which are parametrically studied in this paper. First, a computational model was developed and validated by comparing it with our experimental results. Using COMSOL Multiphysics modeling, the effects of multiple parameters were studied. These parameters include the sheath flow ratio (sheath flow over total flow), width ratio (width of the sheath inlet over the total width), junction angles, and particle size on the focusing width and the distribution of the particles within the focusing region. Then, the numerical data were used to develop two generalized linear models to predict the focusing width of the particles and the standard deviation of the position of the particles. The results showed that the focusing width is greatly impacted by the sheath flow rate ratio. Further, the standard deviation of the position of the particles, which represents the concentration of the particles, is mostly dependent on the flow rate ratio, width ratio, and particle size. Our results provide a better understanding of how the device geometrical and operational factors affect the position of the particles in the development of high-performance on-chip sensing and sorting of both cells and particles.

Rotating-liquid-based hydrogel bead generator

Hydrogel beads are widely used in various applications, but producing such beads often requires complicated devices. Instead, we propose an easy-to-adopt, cost effective, open source hydrogel bead generator. This generator consists of two modules. The first module rotates two immiscible liquids in rigid body motion: mineral oil as the continuous phase (CP) liquid on top, and a hydrogel cross-linking (CL) liquid at bottom. The second module injects a hydrogel pre-polymer solution as the dispersed phase (DP) liquid into the rotating CP liquid. As the DP liquid flows out of a syringe needle, its drops are pinched off by the shear force from the CP liquid, and move with the CP liquid while settling down. When the drops enter the CL liquid, they become hydrogel beads. Experiments using water and mineral oil showed that the size of produced drops could be controlled by adjusting the flow speed of the CP and DP liquids. A demonstration using alginate showed that the proposed generator could successfully create alginate gel beads of uniform size and shape.

Experimental Investigation on the Dynamics of On-Demand Ferrofluid Drop Formation under a Pulse-Width-Modulated Nonuniform Magnetic Field

Drop formation has been the focus of many studies because of its vast application in biomedicine and engineering, as well as its rich underlying physics. Applying a magnetic force on ferrofluids can provide more control over the formation process of the droplet. In this study, a time-dependent, nonuniform magnetic field was used for the formation of ferrofluid droplets using a nozzle. A pulse-width-modulation signal (PWM) was utilized to induce the time-dependent magnetic field, and a drop-on-demand system was designed using the capability of the PWM magnetic field. Three kinds of drop formation regimes under the PWM magnetic field were seen. Also, a new droplet generation regime was observed in which the drop is formed while it bounces back to the nozzle during the off-time period of the magnetic excitation. As compared to other techniques, the main advantage of droplet formation in this regime is that there will be no satellite droplet during the pinch-off. The regime map of drop formation based on the magnetic Bond number and the dimensionless induced frequency was obtained. Also, the effect of the duty cycle, the induced frequency, the magnetic induction, and the vertical interval between the coil's top surface and the nozzle on the drop formation evolution, the equivalent diameter of the droplets, the frequency of droplet formation, and the pulses that are necessary to form a drop was studied. Additionally, it was illustrated that by prolonging the duty cycle, the magnetic induction, or by decreasing the induced frequency, the equivalent diameter of the drop and the pulses that are necessary to form a drop reduce, while the frequency of drop formation increases. Eventually, a correlation for predicting the nondimensionalized diameter of the droplet, based on dimensionless variables, was presented with a maximum relative error of 8.1% and an average relative error equal to 2.2%.

Droplet applicator module for reproducible and controlled endoscopic laryngeal adductor reflex stimulation

This work presents a droplet applicator module to generate stable droplets with different muzzle energies for the reproducible endoscopic stimulation of the laryngeal adductor reflex (LAR). The LAR is a protective reflex of the human larynx; an abnormal LAR performance may cause aspiration pneumonia. A pathological LAR can be detected by evaluating its onset latency. The reflex can be triggered by shooting a droplet onto the laryngeal mucosa, which is referred to as Microdroplet Impulse Testing of the LAR (MIT-LAR). Stimulation intensity variation is desired as the reflex threshold may vary inter-individually. The kinetic energy of a droplet after detachment from the nozzle, i.e., its muzzle energy, is considered an appropriate metric for the LAR stimulation intensity. In this work, a suitable nozzle channel geometry is identified based on the experimental evaluation of droplet formation using three different nozzle channel geometries. Two nontoxic additives are evaluated regarding their effect on fluid properties and droplet formation. The range of achievable droplet muzzle energies is determined by high-speed cinematography in association with a physically motivated model of the macroscopic droplet motion. The experimental results show that sodium chloride is a suitable additive to enhance droplet stability in the studied parameter range with the proposed system. Droplet muzzle energy variation from 0.02  μ J to 1.37  μ J was achieved while preserving the formation of a single stimulation droplet. These results are an important prerequisite for a safe and reproducible LAR stimulation by MIT-LAR, which could also help to further elucidate the physiological mechanisms underlying this laryngeal reflex.

Microfluidic Droplet-Storage Array

A microfluidic droplet-storage array that is capable of the continuous operation of droplet formation, storing, repositioning, retrieving, injecting and restoring is demonstrated. The microfluidic chip comprised four valve-assisted droplet generators and a 3 × 16 droplet-storage array. The integrated pneumatically actuated microvalves enable the precise control of aqueous phase dispensing, as well as carrier fluid flow path and direction for flexible manipulating water-in-oil droplets in the chip. The size of droplets formed by the valve-assisted droplet generators was validated under various operating conditions such as pressures for introducing solutions and dispensing time. In addition, flexible droplet addressing in the storage array was demonstrated by storing droplets with various numbers and compositions in different storage units as well as rearranging their stored positions. Moreover, serial injections of new droplets into a retrieved droplet from a storage unit was performed to show the potential of the platform in sequential dosing on incubated droplet-based reactors at the desired timeline. The droplet-storage array with great freedom and flexibility in droplet handling could be applied for performing complex chemical and biologic reactions, especially in which incubation and dosing steps are necessary.

Droplet encapsulation of electrokinetically-focused analytes without loss of resolution

Lab-on-chip electrokinetic focusing and separation techniques are widely used in several scientific fields. In a number of cases, these techniques have been combined with a selective analyte extraction for off-chip analysis. Nevertheless, the usability of the extracts is limited by diffusion which reduces the separation resolution. In this paper we propose the integration of a droplet generator capable of continuous or on-demand generation and extraction of electrokinetically separated and focused analytes. We demonstrate the selective droplet extraction of model analytes separated and concentrated via ion concentration polarization focusing (ICPF). We report extracted droplets with 1000-fold increased concentration. Importantly, the droplet generator does not interrupt the ICPF process making it suitable for integration with the majority of electrokinetic separation techniques.

Off-Chip-Controlled Droplet-on-Demand Method for Precise Sample Handling

We present a simple, stable, and highly reproducible off-chip-controlled method for generating droplets-on-demand. To induce the droplet generation, externally pre-programmed positive pressure pulses are applied to the dispersed phase input while the continuous phase channel remains at constant input pressure. By controlling solely one fluid phase, the method allows for connecting multiple independent dispersed-phase channels to a single continuous channel. Experimental results show that the method allows for a droplet generation frequency of 33 Hz and a high reproducibility of droplets with standard deviations less than 5% of the mean value. Moreover, utilization of the off-chip-controlled method results in the simplicity in chip design and allows rapid (∼5 min) and cost-efficient (0.5 USD) prototyping of the device.

Aperture Ratio Improvement by Optimizing the Voltage Slope and Reverse Pulse in the Driving Waveform for Electrowetting Displays

Electrowetting display (EWD) performance is severely affected by ink distribution and charge trapping in pixel cells. Therefore, a multi structural driving waveform is proposed for improving the aperture ratio of EWDs. In this paper, the hysteresis characteristic (capacitance–voltage, C-V) curve of the EWD pixel is tested and analyzed for obtaining the driving voltage value at the inflection point of the driving waveform. In the composition of driving waveform, a voltage slope is designed for preventing ink dispersion and a reverse pulse is designed for releasing the trapped charge which is caused by hysteresis characteristic. Finally, the frequency and the duty cycle of the driving waveform are optimized for the max aperture ratio by a series of testing. The experimental results show that the proposed driving waveform can improve the ink dispersion behavior, and the aperture ratio of the EWD is about 8% higher than the conventional driving waveform. At the same time, the response speed of the driving waveform can satisfy the dynamic display in EWDs, which provides a new idea for the design of the EWD driving scheme.

Real-time impedimetric droplet measurement (iDM)

Droplet-based microfluidic systems require a precise control of droplet physical properties; hence, measuring the morphological properties of droplets is critical to obtain high sensitivity analysis. The ability to perform such measurements in real-time is another demand which has not been addressed yet. In this study, we used coplanar electrodes configured in the differential measurement mode for impedimetric measurement of size and velocity. To obtain the size of the droplets, detailed 3D finite element simulations of the system were performed. The interaction of the non-uniform electric field and the droplet was investigated. Electrode geometry optimization steps were described and design guideline rules were laid out. User-friendly software was developed for real-time observation of droplet length and velocity together with in situ statistical analysis results. A comparison between impedimetric and optical measurement tools is given. Finally, to illustrate the benefit of having real-time analysis, iDM was used to synthesize particles with a predefined monodispersity limit and to study the response times of syringe pump and pressure pump driven droplet generation devices. This analysis allows one to evaluate the 'warm-up' time for a droplet generator system, after which droplets reach the desired steady-state size required by the application of interest.

Microfluidic impedance cytometry device with N-shaped electrodes for lateral position measurement of single cells/particles

Tracking the lateral position of single cells and particles plays an important role in evaluating the efficiency of microfluidic cell focusing, separation and sorting. In this work, we present an N-shaped electrode-based microfluidic impedance cytometry device for the measurement of the lateral position of single cells and particles in continuous flows. Specifically, a simple analytical expression for determining the particle lateral position is derived from the measured electrical signal and geometry relationship among the positions of the flowing particles, electrodes and microchannel. This microfluidic system is experimentally validated by measuring the lateral positions of 5, 7 and 10 μm diameter beads and human red blood cells (RBCs) flowing in a 200 μm wide channel at varying flow rates up to 59.3 μl min^-1. Statistical analyses show a good correlation (R² = 0.99) and agreement (Bland-Altman analysis) between our results and those obtained by a microscopy imaging method. The resolution of our system reflected by the root-mean-square deviation (RMSD) is 10.3 μm (5.15% of the channel width) for 5 and 10 μm beads, and 11.4 μm (5.7% of the channel width) for RBCs at a flow rate of 42.4 μl min^-1. Compared to the existing impedance-based methods for measuring the particle lateral position, we achieve the highest resolution, highest flow rate and smallest measured particle size (3.6 μm beads). The experimental results of the mixture with 5 and 10 μm beads demonstrate that our device does not merely measure the lateral position of single particles or cells, but also can characterize their physical properties (e.g., size) simultaneously. Furthermore, we demonstrate the position monitoring of sheath flow-induced particle focusing, which is in quantitative agreement with the results by imaging quantification. With the advantages of rapid and accurate processing of electrical signal and high throughput of the impedance flow cytometry, this novel N-shaped electrode-based system can be easily integrated with other microfluidic platforms as a downstream approach for the real-time measurement of the lateral position and physical properties of single cells and particles.