Rails and anchors: guiding and trapping droplet microreactors in two dimensions

Microrail-assisted liposome trapping and aligning in microfluidic channels

Liposome assemblies with a specific shape are potential cell tissue models for studying intercellular communication. Microfluidic channels that can trap liposomes have been constructed to achieve efficient and high-throughput manipulation and observation of liposomes. However, the trapping and alignment of multiple liposomes in a specific space are still challenging because the liposomes are soft and easily ruptured. In this study, we focused on a microrail-assisted technique for manipulating water-in-oil (w/o) emulsions. In this technique, w/o emulsions are trapped under the microrails through a surface energy gradient. First, we investigated whether the microrail channel can be applied for liposome trapping and alignment and found that the numerical simulations showed that drag forces in the direction of the microrail acted on the liposomes, thereby moving the liposomes from the main channel to the microrail. Next, we designed a microrail device based on the simulation results and trapped liposomes using the device. Resultantly, 24.7 ± 8.5 liposomes were aligned under the microrail within an hour, and the microrail was filled with liposomes for 3 hours. Finally, we prepared the microrail devices with y-shaped and ring-shaped microrails and demonstrated the construction of liposome assemblies with specific shapes, not only the straight shape. Our results indicate that the microrail-assisted technique is a valuable method for manipulating liposomes because it has the potential to provide various-shaped liposome assemblies. We believe the microrail channel will be a powerful tool for constructing liposome-based cell-cell interaction models.

Cancer-on-a-chip model shows that the adenomatous polyposis coli mutation impairs T cell engagement and killing of cancer spheroids

Evaluating the ability of cytotoxic T lymphocytes (CTLs) to eliminate tumor cells is crucial, for instance, to predict the efficiency of cell therapy in personalized medicine. However, the destruction of a tumor by CTLs involves CTL migration in the extra-tumoral environment, accumulation on the tumor, antigen recognition, and cooperation in killing the cancer cells. Therefore, identifying the limiting steps in this complex process requires spatio-temporal measurements of different cellular events over long periods. Here, we use a cancer-on-a-chip platform to evaluate the impact of adenomatous polyposis coli (APC) mutation on CTL migration and cytotoxicity against 3D tumor spheroids. The APC mutated CTLs are found to have a reduced ability to destroy tumor spheroids compared with control cells, even though APC mutants migrate in the extra-tumoral space and accumulate on the spheroids as efficiently as control cells. Once in contact with the tumor however, mutated CTLs display reduced engagement with the cancer cells, as measured by a metric that distinguishes different modes of CTL migration. Realigning the CTL trajectories around localized killing cascades reveals that all CTLs transition to high engagement in the 2 h preceding the cascades, which confirms that the low engagement is the cause of reduced cytotoxicity. Beyond the study of APC mutations, this platform offers a robust way to compare cytotoxic cell efficiency of even closely related cell types, by relying on a multiscale cytometry approach to disentangle complex interactions and to identify the steps that limit the tumor destruction.

Droplet Microfluidics for Current Cancer Research: From Single-Cell Analysis to 3D Cell Culture

Cancer is the second leading cause of death worldwide. Differences in drug resistance and treatment response caused by the heterogeneity of cancer cells are the primary reasons for poor cancer therapy outcomes in patients. In addition, current in vitro anticancer drug-screening methods rely on two-dimensional monolayer-cultured cancer cells, which cannot accurately predict drug behavior in vivo. Therefore, a powerful tool to study the heterogeneity of cancer cells and produce effective in vitro tumor models is warranted to leverage cancer research. Droplet microfluidics has become a powerful platform for the single-cell analysis of cancer cells and three-dimensional cell culture of in vitro tumor spheroids. In this review, we discuss the use of droplet microfluidics in cancer research. Droplet microfluidic technologies, including single- or double-emulsion droplet generation and passive- or active-droplet manipulation, are concisely discussed. Recent advances in droplet microfluidics for single-cell analysis of cancer cells, circulating tumor cells, and scaffold-free/based 3D cell culture of tumor spheroids have been systematically introduced. Finally, the challenges that must be overcome for the further application of droplet microfluidics in cancer research are discussed.

Deformability-Based Microfluidic Microdroplet Screening to Obtain Agarolytic Bacterial Cells

Environmental microorganisms possess enzymes that can digest macromolecules such as agarose into smaller molecules that can be utilized for growth. These enzymes could be valuable for the effective utilization of global resources. However, since most of the microorganisms on Earth remain uncultured, there is significant untapped enzymatic potential in nature. Therefore, it is necessary to develop innovative tools and strategies for exploring these enzymatic resources. To address this, we developed a method for screening microbial cells that secrete hydrogel-degrading enzymes using deformability-based microfluidic microdroplet sorting. In this method, microbial cells are encapsulated as single cells in water-in-oil (W/O) microdroplets with a hydrogel whose shape becomes deformable as the hydrogel is progressively degraded into smaller molecules. Screening is achieved using a microfluidic device that passively sorts the deformed W/O microdroplets. Using this method, we successfully sorted agarose-containing microdroplets, encapsulating single bacterial cells that hydrolyzed agarose. This method can be used to screen various hydrogel-degrading microbial cells.

Microfluidic integrated gas sensors for smart analyte detection: a comprehensive review

The utilization of gas sensors has the potential to enhance worker safety, mitigate environmental issues, and enable early diagnosis of chronic diseases. However, traditional sensors designed for such applications are often bulky, expensive, difficult to operate, and require large sample volumes. By employing microfluidic technology to miniaturize gas sensors, we can address these challenges and usher in a new era of gas sensors suitable for point-of-care and point-of-use applications. In this review paper, we systematically categorize microfluidic gas sensors according to their applications in safety, biomedical, and environmental contexts. Furthermore, we delve into the integration of various types of gas sensors, such as optical, chemical, and physical sensors, within microfluidic platforms, highlighting the resultant enhancements in performance within these domains.

Combinatorial drug screening on 3D Ewing sarcoma spheroids using droplet-based microfluidics

Culturing and screening cells in microfluidics, particularly in three-dimensional formats, has the potential to impact diverse areas from fundamental biology to cancer precision medicine. Here, we use a platform based on anchored droplets for drug screening. The response of spheroids of Ewing sarcoma (EwS) A673 cells to simultaneous or sequential combinations of etoposide and cisplatin was evaluated. This was done by culturing spheroids of EwS cells inside 500 nL droplets then merging them with secondary droplets containing fluorescent-barcoded drugs at different concentrations. Differences in EwS spheroid growth and viability were measured by microscopy. After drug exposure such measurements enabled estimation of their IC50 values, which were in agreement with values obtained in standard multiwell plates. Then, synergistic drug combination was evaluated. Sequential combination treatment of EwS with etoposide applied 24 h before cisplatin resulted in amplified synergistic effect. As such, droplet-based microfluidics offers the modularity required for evaluation of drug combinations.

Droplet microfluidics for CTC-based liquid biopsy: a review

Circulating tumor cells (CTCs) are important biomarkers of liquid biopsy. The number and heterogeneity of CTCs play an important role in cancer diagnosis and personalized medicine. However, owing to the low-abundance biomarkers of CTCs, conventional assays are only able to detect CTCs at the population level. Therefore, there is a pressing need for a highly sensitive method to analyze CTCs at the single-cell level. As an important branch of microfluidics, droplet microfluidics is a high-throughput and sensitive single-cell analysis platform for the quantitative detection and heterogeneity analysis of CTCs. In this review, we focus on the quantitative detection and heterogeneity analysis of CTCs using droplet microfluidics. Technologies that enable droplet microfluidics, particularly high-throughput droplet generation and high-efficiency droplet manipulation, are first discussed. Then, recent advances in detecting and analyzing CTCs using droplet microfluidics from the different aspects of nucleic acids, proteins, and metabolites are introduced. The purpose of this review is to provide guidance for the continued study of droplet microfluidics for CTC-based liquid biopsy.

Rail induced lateral migration of particles across intact co-flowing liquids

This paper presents a rail guided method to apply a Layer-by-Layer (LbL) coating on particles in a microfluidic device. The passive microfluidic approach allows handling suspensions of particles to be coated in the system. The trajectory of the particles is controlled using engraved rails, inducing lateral movement of particles while keeping the axially oriented liquid flow (and the interface of different liquids) undisturbed. The depth and angle of the rails together with the liquid velocity were studied to determine a workable geometry of the device. A discontinuous LbL coating procedure was converted into one continuous process, demonstrating that the chip can perform seven consecutive steps normally conducted in batch operation, further easily extendable to larger cycle numbers. Coating of the particles with two bilayers was confirmed by fluorescence microscopy.

Droplet Microfluidic Technology for the Early and Label-Free Isolation of Highly-Glycolytic, Activated T-Cells

A label-free, fixation-free and passive sorting method is presented to isolate activated T-cells shortly after activation and prior to the display of activation surface markers. It uses a recently developed sorting platform dubbed "Sorting by Interfacial Tension" (SIFT) that sorts droplets based on pH. After polyclonal (anti-CD3/CD28 bead) activation and a brief incubation on chip, droplets containing activated T-cells display a lower pH than those containing naive cells due to increased glycolysis. Under specific surfactant conditions, a change in pH can lead to a concurrent increase in droplet interfacial tension. The isolation of activated T-cells on chip is hence achieved as flattened droplets are displaced as they encounter a micro-fabricated trench oriented diagonally with respect to the direction of flow. This technique leads to an enrichment of activated primary CD4+ T-cells to over 95% from an initial mixed population of naive cells and cells activated for as little as 15 min. Moreover, since the pH change is correlated to successful activation, the technique allows the isolation of T-cells with the earliest activation and highest glycolysis, an important feature for the testing of T-cell activation modulators and to determine regulators and predictors of differentiation outcomes.

High resolution microfluidic assay and probabilistic modeling reveal cooperation between T cells in tumor killing

Cytotoxic T cells are important components of natural anti-tumor immunity and are harnessed in tumor immunotherapies. Immune responses to tumors and immune therapy outcomes largely vary among individuals, but very few studies examine the contribution of intrinsic behavior of the T cells to this heterogeneity. Here we show the development of a microfluidic-based in vitro method to track the outcome of antigen-specific T cell activity on many individual cancer spheroids simultaneously at high spatiotemporal resolution, which we call Multiscale Immuno-Oncology on-Chip System (MIOCS). By combining parallel measurements of T cell behaviors and tumor fates with probabilistic modeling, we establish that the first recruited T cells initiate a positive feedback loop to accelerate further recruitment to the spheroid. We also provide evidence that cooperation between T cells on the spheroid during the killing phase facilitates tumor destruction. Thus, we propose that both T cell accumulation and killing function rely on collective behaviors rather than simply reflecting the sum of individual T cell activities, and the possibility to track many replicates of immune cell-tumor interactions with the level of detail our system provides may contribute to our understanding of immune response heterogeneity.

Anti-cancer cytotoxic T cell responses largely vary among individuals. Here authors show, by stochastic modeling on high throughput T cell behavior and matched tumor spheroid fate data generated by a microfluidics system, that tumor killing is dependent on T cell cooperativity, which might contribute to the heterogeneity of T cell responses.

Electrochemical pH regulation in droplet microfluidics

We report a method for electrochemical pH regulation in microdroplets generated in a microfluidic device. The key finding is that controlled quantities of reagents can be generated electrochemically in moving microdroplets confined within a microfluidic channel. Additionally, products generated at the anode and cathode can be isolated within descendant microdroplets. Specifically, ∼5 nL water-in-oil microdroplets are produced at a T-junction and then later split into two descendant droplets. During splitting, floor-patterned microelectrodes drive water electrolysis within the aqueous microdroplets to produce H⁺ and OH^-. This results in a change in the pHs of the descendant droplets. The droplet pH can be regulated over a range of 5.9 to 7.7 by injecting controlled amounts of charge into the droplets. When the injected charge is between -6.3 and 54.5 nC nL^-1, the measured pH of the resulting droplets is within ±0.1 pH units of that predicted based on the magnitude of the injected charge. This technique can likely be adapted to electrogeneration of other reagents within microdroplets.

3D-printed capillary force trap reactors (CFTRs) for multiphase catalytic flow chemistry

Figure of 3D illustration of a capillary trap force reactor (CFTR) with transiently trapped liquid nanoparticle catalysts in dimple-shaped capillary traps in the presence of a gas–liquid segmented flow, for the hydrogenation of 1-hexene to n-hexane.

The role of temperature in the formation of human-mimetic artificial cell membranes using droplet interface bilayers (DIBs)

Droplet interface bilayers (DIBs) have recently started to be used as human-mimetic artificial cell membranes. DIBs are bilayer sections created at the interface of two aqueous droplets, such that one droplet can be used as a donor compartment and the other as an acceptor compartment for the quantification of molecular transport across the artificial cell membrane. However, synthetic phospholipids are overwhelmingly used to create DIBs instead of naturally derived phospholipids, even though the diverse distribution of phospholipids in the latter is more biomimetic. We present the first systematic study of the role of temperature in DIB formation, which shows that the temperature at which DIBs are formed is a key parameter for the formation of DIBs using naturally derived phospholipids in a microfluidic platform. The phospholipids that are most abundant in mammalian cell membranes (phosphatidylcholine (PC), phosphatidylethanolamine (PE), phosphatidylserine (PS), and phosphatidylinositol (PI)) only form DIBs when the temperature is above the phase transition temperature (T_m). Similarly, DIB formation usually only occurs above the highest T_m of a single phospholipid in a bespoke formulation. In addition, we show a new phenomenon wherein the DIB "melts" without disintegrating for bilayers formed predominantly of phospholipids that occupy cylindrical spaces. We also demonstrate differences in DIB formation rates as well as permeability of these biomimetic membranes. Given the difficulties associated with making DIBs using naturally derived phospholipids, we anticipate this work will illuminate the role of phospholipid phase transition in mono- and bilayer formation and lay the foundation for DIBs to be used as human-mimetic artificial cell membranes.

Sequential storage and release of microdroplets

Droplet microfluidic methods have opened up the possibility of studying a plethora of phenomena ranging from biological to physical or chemical processes at ultra low volumes and high throughput. A key component of such approaches is the ability to trap droplets for observation, and many device architectures for achieving this objective have been developed. A challenge with such approaches is, however, recovering the droplets following their confinement for applications involving further analysis. Here, we present a device capable of generating, confining and releasing microdroplets in a sequential manner. Through a combination of experimental and computational simulations, we shed light on the key features required for successful droplet storage and retrieval. Moreover, we explore the effect of the flow rate of the continuous phase on droplet release, determining that a critical rate is needed to ensure complete droplet deformation through constrictions holding the droplets in place prior to release. Finally, we find that once released, droplets can be retrieved and collected off chip. The ability to generate, store and sequentially release droplets renders such a device particularly promising for future applications where reactions may not only be monitored on-chip, but droplets can also be retrieved for further analysis, facilitating new exploratory avenues in the fields of analytical chemistry and biology.

Microfluidic Device for Droplet Pairing by Combining Droplet Railing and Floating Trap Arrays

Droplet microfluidics are characterized by the generation and manipulation of discrete volumes of solutions, generated with the use of immiscible phases. Those droplets can then be controlled, transported, analyzed or their content modified. In this wide droplet microfluidic toolbox, no means are available to generate, in a controlled manner, droplets co-encapsulating to aqueous phases. Indeed, current methods rely on random co-encapsulation of two aqueous phases during droplet generation or the merging of two random droplets containing different aqueous phases. In this study, we present a novel droplet microfluidic device to reliably and efficiently co-encapsulate two different aqueous phases in micro-droplets. In order to achieve this, we combined existing droplet microfluidic modules in a novel way. The different aqueous phases are individually encapsulated in droplets of different sizes. Those droplet populations are then filtered in order to position each droplet type towards its adequate trapping compartment in traps of a floating trap array. Single droplets, each containing a different aqueous phase, are thus paired and then merged. This pairing at high efficiency is achieved thanks to a unique combination of floating trap arrays, a droplet railing system and a droplet size-based filtering mechanism. The microfluidic chip design presented here provides a filtering threshold with droplets larger than 35 μm (big droplets) being deviated to the lower rail while droplets smaller than 20 μm (small droplets) remain on the upper rail. The effects of the rail height and the distance between the two (upper and lower) rails were investigated. The optimal trap dimensions provide a trapping efficiency of 100% for small and big droplets with a limited double trapping (both compartments of the traps filled with the same droplet type) of 5%. The use of electrocoalescence enables the generation of a droplet while co-encapsulating two aqueous phases. Using the presented microfluidic device libraries of 300 droplets, dual aqueous content can be generated in less than 30 min.

Chiral lipid bilayers are enantioselectively permeable

Homochiral membrane bilayers organize biological functions in all domains of life. The membrane’s permeability–its key property–correlates with a molecule’s lipophilicity, but the role of the membrane’s rich and uniform stereochemistry as a permeability determinant is largely ignored in empirical and computational measurements. Here, we describe a new approach to measuring permeation using continuously generated microfluidic droplet interface bilayers (DIBs, 480/min) and benchmark this system by monitoring fluorescent dye DIB permeation over time. Permeation of non-fluorescent, alkyne-labeled molecules was measured using a fluorogenic click reaction. DIB transport measurements revealed enantioselective permeation of alkyne-labeled amino acids (Ala, Val, Phe, Pro) and dipeptides through a chiral phospholipid bilayer; the biological L enantiomers permeated faster than D (1.2–6-fold; Ala–Pro). Enantioselective permeation both poses a potentially unanticipated criterion for drug design and offers a kinetic mechanism for the abiotic emergence of homochirality via chiral transfer between sugars, amino acids, and lipids.

Individual Control and Quantification of 3D Spheroids in a High-Density Microfluidic Droplet Array

As three-dimensional cell culture formats gain in popularity, there emerges a need for tools that produce vast amounts of data on individual cells within the spheroids or organoids. Here, we present a microfluidic platform that provides access to such data by parallelizing the manipulation of individual spheroids within anchored droplets. Different conditions can be applied in a single device by triggering the merging of new droplets with the spheroid-containing drops. This allows cell-cell interactions to be initiated for building microtissues, studying stem cells’ self-organization, or observing antagonistic interactions. It also allows the spheroids’ physical or chemical environment to be modulated, as we show by applying a drug over a large range of concentrations in a single parallelized experiment. This convergence of microfluidics and image acquisition leads to a data-driven approach that allows the heterogeneity of 3D culture behavior to be addressed across the scales, bridging single-cell measurements with population measurements.

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Microfluidic droplet pairs sequentially trapped in capillary anchors before merging

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1 spheroid/droplet, with microenvironment modulations driven by droplet merging

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A wide range of drug concentrations tested on hepatic-like spheroids in a single chip

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Data-driven approach unravels 3D tissue-level dynamic drug response

Integration of capillary-hydrodynamic logic circuitries for built-in control over multiple droplets in microfluidic networks

Here, we show the successful implementation of advanced sequential logic in droplet microfluidics, whose principles rely on capillary wells establishing stationary states, where droplets can communicate remotely via pressure impulses, influencing each other and switching the device states. All logic operations perform spontaneously due to the utilization of nothing more than capillary-hydrodynamic interactions, inherent for the confined biphasic flow. Our approach offers integration feasibility allowing to encode unprecedentedly long algorithms, e.g., 1000-droplet counting. This work has the potential for the advancement of liquid computers and thereby could participate in the development of the next generation of portable microfluidic systems with embedded control, enabling applications from single-cell analysis and biochemical assays to materials science.

Internal circulation and mixing within tight-squeezing deformable droplets

The internal flow and mixing properties inside deformable droplets, after reaching the steady state within two types of passive droplet traps, are visualized and analyzed as dynamical systems. The first droplet trap (constriction) is formed by three spheres arranged in an equilateral triangle, while the second consists of two parallel spherocylinders (capsules). The systems are assumed to be embedded in a uniform far-field flow at low Reynolds number, and the steady shapes and interfacial velocities on the drops are generated using the boundary-integral method. The internal velocity field is recovered by solving the internal Dirichlet problem, also via a desingularized boundary-integral method. Calculation of 2D streamlines within planes of symmetry reveals the internal equilibria of the flow. The type of each equilibrium is classified in 3D and their interactions probed using passive tracers and their Poincaré maps. For the two-capsule droplet, saddle points located on orthogonal symmetry planes influence the regular flow within the drop. For the three-sphere droplet, large regions of chaos are observed, embedded with simple periodic orbits. Flow is visualized via passive dyes, using material lines and surfaces. In 2D, solely the interface between two passive interior fluids is advected using an adaptive number of linked tracer particles. The reduction in dimension decreases the number of required tracer points, and also resolves arbitrarily thin filaments, in contrast to backward cell-mapping methods. In 3D, the advection of a material surface, bounded by the droplet interface, is enabled using an adaptive mesh scheme. Off-lattice 3D contour advection allows for highly resolved visualizations of the internal flow and quantification of the associated degree of mixing. Analysis of the time-dependent growth of material surfaces and 3D mixing numbers suggests the three-sphere droplet exhibits superior mixing properties compared to the two-capsule droplet.

Evaluation of 3D-printed molds for fabrication of non-planar microchannels

Replica obtained from micromolds patterned by simple photolithography has features with uniform heights, and attainable microchannels are thus quasi-two-dimensional. Recent progress in three-dimensional (3D) printing has enabled facile desktop fabrication of molds to replicate microchannels with varying heights. We investigated the replica obtained from four common techniques of 3D printing-fused deposition modeling, selective laser sintering, photo-polymer inkjet printing (PJ), and stereolithography (SL)-for the suitability to form microchannels in terms of the surface roughness inherent to the mechanism of 3D printing. There have been limited quantitative studies that focused on the surface roughness of a 3D-printed mold with different methods of 3D printing. We discussed that the surface roughness of the molds affected (1) transparency of the replica and (2) delamination pressure of poly(dimethylsiloxane) replica bonded to flat glass substrates. Thereafter, we quantified the accuracy of replication from 3D-printed molds by comparing the dimensions of the replicated parts to the designed dimensions and tested the ability to fabricate closely spaced microchannels. This study suggested that molds printed by PJ and SL printers were suitable for replica molding to fabricate microchannels with varying heights. The insight from this study shall be useful to fabricate 3D microchannels with controlled 3D patterns of flows guided by the geometry of the microchannels.

Dynamic pneumatic rails enabled microdroplet manipulation

This study presented a convenient method of gathering, splitting, merging, and sorting microdroplets by dynamic pneumatic rails in double-layered microfluidic devices. In these devices, the pneumatic rails were placed below the droplet channel, with a thin elastic polydimethylsiloxane (PDMS) film between them. The PDMS film would sag down to the rail channel, forming a groove pattern at the bottom of the droplet channel, when the fluid pressure in the droplet channel was higher than the air pressure in the rail channel. The groove could capture the flattened droplets and guide the flow path of them due to the lowered surface energy when they extended into the groove. We have designed different components consisting of pneumatic rails to split, merge and sort droplets, and demonstrated that the components maintained good performance in manipulating droplets only by controlling the air pressure. Furthermore, a pneumatic rail-based sorter has been successfully used to sort out single-cell droplets. The pneumatic rail can be integrated into pneumatic valve-based microfluidic devices to be a flexible tool for droplet-based biological and chemical analysis.

Everything in its right place: controlling the local composition of hydrogels using microfluidic traps

Many natural materials display locally varying compositions that impart unique mechanical properties to them which are still unmatched by manmade counterparts. Synthetic materials often possess structures that are well-defined on the molecular level, but poorly defined on the microscale. A fundamental difference that leads to this dissimilarity between natural and synthetic materials is their processing. Many natural materials are assembled from compartmentalized reagents that are released in well-defined and spatially confined regions, resulting in locally varying compositions. By contrast, synthetic materials are typically processed in bulk. Inspired by nature, we introduce a drop-based technique that enables the design of microstructured hydrogel sheets possessing tuneable locally varying compositions. This control in the spatial composition and microstructure is achieved with a microfluidic Hele-Shaw cell that possesses traps with varying trapping strengths to selectively immobilize different types of drops. This modular platform is not limited to the fabrication of hydrogels but can be employed for any material that can be processed into drops and solidified within them. It likely opens up new possibilities for the design of structured, load-bearing hydrogels, as well as for the next generation of soft actuators and sensors.

Method for Passive Droplet Sorting after Photo-Tagging

We present a method to photo-tag individual microfluidic droplets for latter selection by passive sorting. The use of a specific surfactant leads to the interfacial tension to be very sensitive to droplet pH. The photoexcitation of droplets containing a photoacid, pyranine, leads to a decrease in droplet pH. The concurrent increase in droplet interfacial tension enables the passive selection of irradiated droplets. The technique is used to select individual droplets within a droplet array as illuminated droplets remain in the wells while other droplets are eluted by the flow of the external oil. This method was used to select droplets in an array containing cells at a specific stage of apoptosis. The technique is also adaptable to continuous-flow sorting. By passing confined droplets over a microfabricated trench positioned diagonally in relation to the direction of flow, photo-tagged droplets were directed toward a different chip exit based on their lateral movement. The technique can be performed on a conventional fluorescence microscope and uncouples the observation and selection of droplets, thus enabling the selection on a large variety of signals, or based on qualitative user-defined features.

A bespoke microfluidic pharmacokinetic compartment model for drug absorption using artificial cell membranes

Early prediction of the rate and extent of intestinal absorption is vital for the efficient development of orally administered drugs. Here we show a new type of pharmacokinetic compartment model that shows a threefold improvement in the prediction of molecular absorption in the jejunum than the current state-of-the-art in vitro technique, parallel artificial membrane permeability assays (PAMPA). Our three-stage pharmacokinetic compartment model uses microfluidic droplets and bespoke, biomimetic artificial cells to model the path of a drug proxy from the intestinal space into the blood via an enterocyte. Each droplet models the buffer and salt composition of each pharmacokinetic compartment. The artificial cell membranes are made from the major components of human intestinal cell membranes (l-α-phosphatidylcholine, PC and l-α-phosphatidylethanolamine, PE) and sizes are comparable to human cells (∼0.5 nL). We demonstrate the use of the microfluidic platform to quantify common pharmacokinetic parameters such as half-life, flux and the apparent permeability coefficient (P_app). Our determined P_app more closely resembles that of actual intestinal tissue than PAMPA, which overestimates it by a factor of 20.

Microfluidic Platform for the Isolation of Cancer-Cell Subpopulations Based on Single-Cell Glycolysis

High rates of glycolysis in tumors have been associated with cancer metastasis, tumor recurrence, and poor outcomes. In this light, single cells that exhibit high glycolysis are specific targets for therapy. However, the study of these cells requires efficient tools for their isolation. We use a droplet microfluidic technique developed in our lab, Sorting by Interfacial Tension (SIFT), to isolate cancer cell subpopulations based on glycolysis without the use of labels or active sorting components. By controlling the flow conditions on chip, the threshold of selection can be modified, enabling the isolation of cells with different levels of glycolysis. Hypoxia in tumors, that can be simulated with treatment with CoCl₂, leads to an increase in glycolysis, and more dangerous tumors. The device was used to enrich CoCl₂ treated MDA-MB 231 breast cancer cells from an untreated population. It is also used to sort K562 human chronic myelogenous leukemia cells that have either been treated or untreated with 2-deoxy-d-glucose (2DG), a pharmaceutical that targets cell metabolism. The technique provides a facile and robust way of separating cells based on elevated glycolytic activity; a biomarker associated with cancer cell malignancy.

Scalable microfluidic droplet on-demand generator for non-steady operation of droplet-based assays

We developed a microfluidic droplet on-demand (DoD) generator that enables the production of droplets with a volume solely governed by the geometry of the generator for a range of operating conditions. The prime reason to develop this novel type of DoD generator is that its robustness in operation enables scale out and operation under non-steady conditions, which are both essential features for the further advancement of droplet-based assays. We first detail the working principle of the DoD generator and study the sensitivity of the volume of the generated droplets with respect to the used fluids and control parameters. We next compare the performance of our DoD generator when scaled out to 8 parallel generators to the performance of a conventional DoD generator in which the droplet volume is not geometry-controlled, showing its superior performance. Further scale out to 64 parallel DoD generators shows that all generators produce droplets with a volume between 91% and 105% of the predesigned volume. We conclude the paper by presenting a simple droplet-based assay in which the DoD generator enables sequential supply of reagent droplets to a droplet stored in the device, illustrating its potential to be used in droplet-based assays for biochemical studies under non-steady operation conditions.

Microfluidic Chamber Design for Controlled Droplet Expansion and Coalescence

The defined formation and expansion of droplets are essential operations for droplet-based screening assays. The volumetric expansion of droplets causes a dilution of the ingredients. Dilution is required for the generation of concentration graduation which is mandatory for many different assay protocols. Here, we describe the design of a microfluidic operation unit based on a bypassed chamber and its operation modes. The different operation modes enable the defined formation of sub-µL droplets on the one hand and the expansion of low nL to sub-µL droplets by controlled coalescence on the other. In this way the chamber acts as fluidic interface between two fluidic network parts dimensioned for different droplet volumes. Hence, channel confined droplets of about 30–40 nL from the first network part were expanded to cannel confined droplets of about 500 to about 2500 nL in the second network part. Four different operation modes were realized: (a) flow rate independent droplet formation in a self-controlled way caused by the bypassed chamber design, (b) single droplet expansion mode, (c) multiple droplet expansion mode, and (d) multiple droplet coalescence mode. The last mode was used for the automated coalescence of 12 droplets of about 40 nL volume to produce a highly ordered output sequence with individual droplet volumes of about 500 nL volume. The experimental investigation confirmed a high tolerance of the developed chamber against the variation of key parameters of the dispersed-phase like salt content, pH value and fluid viscosity. The presented fluidic chamber provides a solution for the problem of bridging different droplet volumes in a fluidic network.

Microfluidic-based linear-optics label-free imager

Linear optics based nanoscopy previously reached resolution beyond the diffraction limit, illuminating samples in the visible light regime while allowing light to interact with freely moving metallic nanoparticles. However, the hydrodynamics governing the nanoparticle motion used to scan the sample is very complex and has low probability of achieving appropriate and fast mapping in practice. Hence, an implementation of the technique on real biological samples has not been demonstrated so far. Moreover, a suitable way to perform controlled nanoparticle scanning of biological samples is required. Here we show a solution where a microfluidic channel is used to flow and trap biological samples inside a water droplet along with suspended nanoparticles surrounded by silicone oil. The evanescent light scattered from the sample and is rescattered by the nanoparticles in the vicinity. This encodes the sub-wavelength features of the sample which can later on be decoded and reconstructed from measurements in the far field. The microfluidic system-controlled flow allows better nanoparticle scanning of the sample and maintains an isolated system for each sample in each droplet. A more localized scan at the droplet water/oil interface is also conducted using amphiphilic nanoparticles where their hydrophilic side is constrained to the droplet and their hydrophobic side is constrained to the oil. This allows higher probability of capturing evanescent fields closer to their origin, yielding better resolution and a higher signal to noise ratio. Using this system, we obtained images of an E. coli sample and demonstrated how the method yield fine resolution of the sample contours. To the best of our knowledge, this is the first time that a linear and label free optics imaging process was performed using a micro-fluidic device.

Microfluidics as an Enabling Technology for Personalized Cancer Therapy

Tailoring patient-specific treatments for cancer is necessary in order to achieve optimal results but requires new diagnostic approaches at affordable prices. Microfluidics has immense potential to provide solutions for this, as it enables the processing of samples that are not available in large quantities (e.g., cells from patient biopsies), is cost efficient, provides a high level of automation, and allows the set-up of complex models for cancer studies. In this review, individual solutions in the fields of genetics, circulating tumor cell monitoring, biomarker analysis, phenotypic drug sensitivity tests, and systems providing controlled environments for disease modeling are discussed. An overview on how these early stage achievements can be combined or developed further is showcased, and the required translational steps before microfluidics becomes a routine tool for clinical applications are critically discussed.

Quantifying the sol-gel process and detecting toxic gas in an array of anchored microfluidic droplets

The detection of toxic gases is becoming an important element in tackling increased air pollution. This has led to the development of gas sensors based on porous solid materials, which are produced using sol-gel chemistry and functionalized to change their optical qualities when in contact with the gas. In this context it is interesting to explore how microfluidics can be used to miniaturize these sensors, to improve their sensitivity and dynamic range, or to multiplex many gas measurements on a single chip. In this article we show how the sol-gel process can be implemented using anchored droplet microfluidics. The sensor material is partitioned into droplets while in the sol phase and maintained using capillary anchors. The ability to hold the droplets in place first allows us to study the sol-gel process. We use an original rheology method, which consists of observing the flows within stationary droplets that are submitted to an external flow, to measure the gelation time of the droplets. These measurements show a gelation time that decreases from 50 minutes to below 10 minutes as the temperature increases from 20 to 50 °C. We also measure the shrinkage of individual gel beads after gelation and find that this syneresis process is nearly finished after about 12 hours, leading to a final bead size that is 50% smaller than the initial droplet. Finally, we show that the beads can be functionalized and used to detect the presence of formaldehyde. These results first provide a new way to observe the physics of the sol-gel process in a well-controlled and quantitative fashion. Moreover they highlight how the coupling of microfluidics and sol-gel chemistry can be used to detect toxic gases, in view of answering the challenges surrounding gas detection in real-world settings.

Microfluidic SlipChip device for multistep multiplexed biochemistry on a nanoliter scale

We have developed a multistep microfluidic device that expands the current SlipChip capabilities by enabling multiple steps of droplet merging and multiplexing. Harnessing the interfacial energy between carrier and sample phases, this manually operated device accurately meters nanoliter volumes of reagents and transfers them into on-device reaction wells. Judiciously shaped microfeatures and surface-energy traps merge droplets in a parallel fashion. Wells can be tuned for different volumetric capacities and reagent types, including for pre-spotted reagents that allow for unique identification of original well contents even after their contents are pooled. We demonstrate the functionality of the multistep SlipChip by performing RNA transcript barcoding on-device for synthetic spiked-in standards and for biologically derived samples. This technology is a good candidate for a wide range of biological applications that require multiplexing of multistep reactions in nanoliter volumes, including single-cell analyses.

DropSOAC: Stabilizing Microfluidic Drops for Time-Lapse Quantification of Single-Cell Bacterial Physiology

The physiological heterogeneity of cells within a microbial population imparts resilience to stresses such as antimicrobial treatments and nutrient limitation. This resilience is partially due to a subpopulation of cells that can survive such stresses and regenerate the community. Microfluidic approaches now provide a means to study microbial physiology and bacterial heterogeneity at the single cell level, improving our ability to isolate and examine these subpopulations. Drop-based microfluidics provides a high-throughput approach to study individual cell physiology within bacterial populations. Using this approach, single cells are isolated from the population and encapsulated in growth medium dispersed in oil using a 15 μm diameter drop making microfluidic device. The drops are arranged as a packed monolayer inside a polydimethylsiloxane (PDMS) microfluidic device. Growth of thousands of individual cells in identical microenvironments can then be imaged using confocal laser scanning microscopy (CLSM). A challenge for this approach has been the maintenance of drop stability during extended time-lapse imaging. In particular, the drops do not maintain their volume over time during incubation in PDMS devices, due to fluid transport into the porous PDMS surroundings. Here, we present a strategy for PDMS device preparation that stabilizes drop position and volume within a drop array on a microfluidic chip for over 20 h. The stability of water-in-oil drops is maintained by soaking the device in a reservoir containing both water and oil in thermodynamic equilibrium. This ensures that phase equilibrium of the drop emulsion fluids within the porous PDMS material is maintained during drop incubation and imaging. We demonstrate the utility of this approach, which we label DropSOAC (Drop Stabilization On A Chip), for time-lapse studies of bacterial growth. We characterize growth of Pseudomonas aeruginosa and its Δhpf mutant derivative during resuscitation and growth following starvation. We demonstrate that growth rate and lag time heterogeneity of hundreds of individual bacterial cells can be determined starting from single isolated cells. The results show that the DropSOAC capsule provides a high-throughput approach toward studies of microbial physiology at the single cell level, and can be used to characterize physiological differences of cells from within a larger population.

Sorting by interfacial tension (SIFT): Label-free enzyme sorting using droplet microfluidics

Droplet microfluidics has the ability to greatly increase the throughput of screening and sorting of enzymes by carrying reagents in picoliter droplets flowing in inert oils. It was found with the use of a specific surfactant, the interfacial tension of droplets can be very sensitive to droplet pH. This enables the sorting of droplets of different pH when confined droplets encounter a microfabricated trench. The device can be extended to sort enzymes, as a large number of enzymatic reactions lead to the production of an acidic or basic product and a concurrent change in solution pH. The progress of an enzymatic reaction is tracked from the position of a flowing train of droplets. We demonstrate the sorting of esterase isoenzymes based on their enzymatic activity. This label-free technology, that we dub droplet sorting by interfacial tension (SIFT), requires no active components and would have applications for enzyme sorting in high-throughput applications that include enzyme screening and directed evolution of enzymes.

Multiplexed Single-Cell Measurements of FDG Uptake and Lactate Release Using Droplet Microfluidics

INTRODUCTION

Glucose utilization and lactate release are 2 important indicators of cancer metabolism. Most tumors consume glucose and release lactate at a higher rate than normal tissues due to enhanced aerobic glycolysis. However, these 2 indicators of metabolism have not previously been studied on a single-cell level, in the same cell.

OBJECTIVE

To develop and characterize a novel droplet microfluidic device for multiplexed measurements of glucose uptake (via its analog ¹⁸F-fluorodeoxyglucose) and lactate release, in single live cells encapsulated in an array of water-in-oil droplets.

RESULTS

Surprisingly, ¹⁸F-fluorodeoxyglucose uptake and lactate release were only marginally correlated at the single-cell level, even when assayed in a standard cell line (MDA-MB-231). While ¹⁸F-fluorodeoxyglucose-avid cells released substantial amounts of lactate, the reverse was not true, and many cells released high amounts of lactate without taking up ¹⁸F-fluorodeoxyglucose.

DISCUSSION

These results confirm that cancer cells rely on multiple metabolic pathways in addition to aerobic glycolysis and that the use of these pathways is highly heterogeneous, even under controlled culture conditions. Clinically, the large cell-to-cell variability suggests that positron emission tomography measurements of ¹⁸F-fluorodeoxyglucose uptake represent metabolic flux only in an aggregate sense, not for individual cancer cells within the tumor.

Microfluidic Handling and Analysis of Giant Vesicles for Use as Artificial Cells: A Review

One of the goals of synthetic biology is the bottom-up construction of an artificial cell, the successful realization of which could shed light on how cellular life emerged and could also be a useful tool for studying the function of modern cells. Using liposomes as biomimetic containers is particularly promising because lipid membranes are biocompatible and much of the required machinery can be reconstituted within them. Giant lipid vesicles have been used extensively in other fields such as biophysics and drug discovery, but their use as artificial cells has only recently seen an increase. Despite the prevalence of giant vesicles, many experiments remain challenging or impossible due to their delicate nature compared to biological cells. This review aims to highlight the effectiveness of microfluidic technologies in handling and analyzing giant vesicles. The advantages and disadvantages of different microfluidic approaches and what new insights can be gained from various applications are introduced. Finally, future directions are discussed in which the unique combination of microfluidics and giant lipid vesicles can push forward the bottom-up construction of artificial cells.

Growing from a few cells: combined effects of initial stochasticity and cell-to-cell variability

The growth of a cell population from a large inoculum appears deterministic, although the division process is stochastic at the single-cell level. Microfluidic observations, however, display wide variations in the growth of small populations. Here we combine theory, simulations and experiments to explore the link between single-cell stochasticity and the growth of a population starting from a small number of individuals. The study yields descriptors of the probability distribution function (PDF) of the population size under three sources of stochasticity: cell-to-cell variability, uncertainty in the number of initial cells and generation-dependent division times. The PDF, rescaled to account for the exponential growth of the population, is found to converge to a stationary distribution. All moments of the PDF grow exponentially with the same growth rate, which depends solely on cell-to-cell variability. The shape of the PDF, however, contains the signature of all sources of stochasticity, and is dominated by the early stages of growth, and not by the cell-to-cell variability. Thus, probabilistic predictions of the growth of bacterial populations can be obtained with implications for both naturally occurring conditions and technological applications of single-cell microfluidics.

Sorting by interfacial tension (SIFT): label-free selection of live cells based on single-cell metabolism

Selection of live cells from a population is critical in many biological studies and biotechnologies. We present here a novel droplet microfluidic approach that allows for label-free and passive selection of live cells using the glycolytic activity of individual cells. It was observed that with the use of a specific surfactant utilized to stabilize droplet formation, the interfacial tension of droplets was very sensitive to pH. After incubation, cellular lactate release results in droplets containing a live cell to attain a lower pH than other droplets. This enables the sorting of droplets containing live cells when confined droplets flow over a microfabricated trench oriented diagonally with respect to the direction of flow. The technique is demonstrated with human U87 glioblastoma cells for the selection of only droplets containing a live cell while excluding either empty droplets or droplets containing a dead cell. This label-free sorting method, dubbed sorting by interfacial tension (SIFT) presents a new strategy to sort diverse cell types based on metabolic activity.

Microfluidic on-demand droplet generation, storage, retrieval, and merging for single-cell pairing

A multifunctional microfluidic platform combining on-demand aqueous-phase droplet generation, multi-droplet storage, and controlled merging of droplets selected from a storage library in a single integrated microfluidic device is described. A unique aspect of the technology is a microfluidic trap design comprising a droplet trap chamber and lateral bypass channels integrated with a microvalve that supports the capture and merger of multiple droplets over a wide range of individual droplet sizes. A storage unit comprising an array of microfluidic traps operates in a first-in first-out manner, allowing droplets stored within the library to be analyzed before sequentially delivering selected droplets to a downstream merging zone, while shunting other droplets to waste. Performance of the microfluidic trap is investigated for variations in bypass/chamber hydrodynamic resistance ratio, micro-chamber geometry, trapped droplet volume, and overall flow rate. The integrated microfluidic platform is then utilized to demonstrate the operational steps necessary for cell-based assays requiring the isolation of defined cell populations with single cell resolution, including encapsulation of individual cells within an aqueous-phase droplet carrier, screening or incubation of the immobilized cell-encapsulated droplets, and generation of controlled combinations of individual cells through the sequential droplet merging process. Beyond its utility for cell analysis, the presented platform represents a versatile approach to robust droplet generation, storage, and merging for use in a wide range of droplet-based microfluidics applications.

Observation of molecular self-assembly events in massively parallel microdroplet arrays

The self-assembly of peptide and protein molecules into nanoscale filaments is a process associated with both biological function and malfunction. Microfluidic techniques can provide powerful tools in the study of such aggregation phenomena while providing access to exploring the role of molecular interactions in disease development. Yet, a common challenge encountered in the study of protein aggregation is the difficulty in achieving spatial and temporal control of the underlying processes. Here, we present a planar (2-D) device allowing for both the generation and confinement of 10 000 monodisperse water-in-oil droplets in an array of chambers with a trapping efficiency of 99%. Due to the specific geometry of the device, droplets can be formed and immediately trapped on the same chip, without the need for continuous flow of the oil phase. Furthermore, we demonstrate the capability of this device as a platform to study the aggregation kinetics and determine stochastic molecular nanoscale self-assembly events in a highly parallel manner for the aggregation of the dipeptide, diphenylalanine, the core recognition motif of the Aβ-42 peptide associated with Alzheimer's disease. The ability to reproducibly generate and confine monodisperse water-in-oil droplets with an extremely high trapping efficiency while maintaining entrapment under zero-flow conditions, on timescales compatible with observing molecular self-assembly events, renders it promising for numerous potential further applications in the biological and biophysical fields.

Size-Dependent and Property-Independent Passive Microdroplet Sorting by Droplet Transfer on Dot Rails

A fully passive microdroplet sorting method is presented in this paper. On the rails with dot patterns, the droplets were sorted in different ways depending on their size. However, the effect of droplet properties on the threshold size of the sorting was eliminated. The droplet positions on two railways and the Laplace pressure of the droplets on the dot patterns allowed selective droplet transfer according to size. Different gaps between the rails altered the threshold size of the transfer. However, the threshold size was independent of the droplet's surface tension and viscosity because the droplet transfer utilized only the droplet position and Laplace pressure without lateral flow to sort targets. This feature has a high potential for bio/chemical applications requiring categorization of droplet targets consisting of various mixtures as pre- or post-elements.

Fast selective trapping and release of picoliter droplets in a 3D microfluidic PDMS multi-trap system with bubbles

The selective manipulation and incubation of individual picoliter drops in high-throughput droplet based microfluidic devices still remains challenging. We used a surface acoustic wave (SAW) to induce a bubble in a 3D designed multi-trap polydimethylsiloxane (PDMS) device to manipulate multiple droplets and demonstrate the selection, incubation and on-demand release of aqueous droplets from a continuous oil flow. By controlling the position of the acoustic actuation, individual droplets are addressed and selectively released from a droplet stream of 460 drops per s. A complete trapping and releasing cycle can be as short as 70 ms and has no upper limit for incubation time. We characterize the fluidic function of the hybrid device in terms of electric power, pulse duration and acoustic path.

Universal anchored-droplet device for cellular bioassays

The ability to encapsulate cells individually in droplets has many potential applications, for example for observing the heterogeneity of behaviors within a population. However, implementing operations on moving droplets require feedback control and instruments that provide precise timing. These technical difficulties impede the adoption of droplet microfluidic protocols in nonspecialist labs. In this chapter we describe an approach to produce and manipulate droplets that remain stationary within a microfluidic chamber, by fabricating a microfluidic device having three-dimensional topography. The method uses microchannels that confine the fluids everywhere except in predefined regions where the channels have a large height, a technique known as "rails and anchors." By relying on the natural tendency of droplets to minimize their surface area, the approach provides a wide range of droplet manipulation tools. This chapter shows how this can be used to produce droplets, and several biological applications are demonstrated.