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.

Programmable Control of Nanoliter Droplet Arrays using Membrane Displacement Traps

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

Self-synchronization of reinjected droplets for high-efficiency droplet pairing and merging

Droplet merging serves as a powerful tool to add reagents to moving droplets for biological and chemical reactions. However, unsynchronized droplet pairing impedes high-efficiency merging. Here, we develop a microfluidic design for the self-synchronization of reinjected droplets. A periodic increase in the hydrodynamic resistance caused by droplet blocking a T-junction enables automatic pairing of droplets. After inducing spacing, the paired droplets merge downstream under an electric field. The blockage-based design can achieve a 100% synchronization efficiency even when the mismatch rate of droplet frequencies reaches 10%. Over 98% of the droplets can still be synchronized at nonuniform droplet sizes and fluctuating reinjection flow rates. Moreover, the droplet pairing ratio can be adjusted flexibly for on-demand sample addition. Using this system, we merge two groups of droplets encapsulating enzyme/substrate, demonstrating its capacity to conduct multi-step reactions. We also combine droplet sorting and merging to coencapsulate single cells and single beads, providing a basis for high-efficiency single-cell sequencing. We expect that this system can be integrated with other droplet manipulation systems for a broad range of chemical and biological applications.

Analysis of double-emulsion droplets with ESI mass spectrometry for monitoring lipase-catalyzed ester hydrolysis at nanoliter scale

Microfluidic double-emulsion droplets allow the realization and study of biphasic chemical processes such as chemical reactions or extractions on the nanoliter scale. Double emulsions of the rare type (o₁/w/o₂) are used here to realize a lipase-catalyzed reaction in the non-polar phase. The surrounding aqueous phase induces the transfer of the hydrophilic product from the core oil phase, allowing on-the-fly MS analysis in single double droplets. A microfluidic two-step emulsification process is developed to generate the (o₁/w/o₂) double-emulsion droplets. In this first example of microfluidic double-emulsion MS coupling, we show in proof-of-concept experiments that the chemical composition of the water layer can be read online using ESI–MS. Double-emulsion droplets were further employed as two-phase micro-reactors for the hydrolysis of the lipophilic ester p-nitrophenyl palmitate catalyzed by the Candida antarctica lipase B (CalB). Finally, the formation of the hydrophilic reaction product p-nitrophenol within the double-emulsion droplet micro-reactors is verified by subjecting the double-emulsion droplets to online ESI–MS analysis.

Microfluidic Printing-Based Method for the Multifactorial Study of Cell-Free Protein Networks

Protein networks can be assembled in vitro for basic biochemistry research, drug screening, and the creation of artificial cells. Two standard methodologies are used: manual pipetting and pipetting robots. Manual pipetting has limited throughput in the number of input reagents and the combination of reagents in a single sample. While pipetting robots are evident in improving pipetting efficiency and saving hands-on time, their liquid handling volume usually ranges from a few to hundreds of microliters. Microfluidic methods have been developed to minimize the reagent consumption and speed up screening but are challenging in multifactorial protein studies due to their reliance on complex structures and labeling dyes. Here, we engineered a new impact-printing-based methodology to generate printed microdroplet arrays containing water-in-oil droplets. The printed droplet volume was linearly proportional (R2 = 0.9999) to the single droplet number, and each single droplet volume was around 59.2 nL (coefficient of variation = 93.8%). Our new methodology enables the study of protein networks in both membrane-unbound and -bound states, without and with anchor lipids DGS-NTA(Ni), respectively. The methodology is demonstrated using a subnetwork of mitogen-activated protein kinase (MAPK). It takes less than 10 min to prepare 100 different droplet-based reactions, using <1 μL reaction volume at each reaction site. We validate the kinase (ATPase) activity of MEK1 (R4F)* and ERK2 WT individually and together under different concentrations, without and with the selective membrane attachment. Our new methodology provides a reagent-saving, efficient, and flexible way for protein network research and related applications.

Facile and scalable tubing-free sample loading for droplet microfluidics

Droplet microfluidics has in recent years found a wide range of analytical and bioanalytical applications. In droplet microfluidics, the samples that are discretized into droplets within the devices are predominantly loaded through tubings, but such tubing-based sample loading has drawbacks such as limited scalability for processing many samples, difficulty for automation, and sample wastage. While advances in autosamplers have alleviated some of these drawbacks, sample loading that can instead obviate tubings offers a potentially promising alternative but has been underexplored. To fill the gap, we introduce herein a droplet device that features a new Tubing Eliminated Sample Loading Interface (TESLI). TESLI integrates a network of programmable pneumatic microvalves that regulate vacuum and pressure sources so that successive sub-microliter samples can be directly spotted onto the open-to-atmosphere TESLI inlet, vacuumed into the device, and pressurized into nanoliter droplets within the device with minimal wastage. The same vacuum and pressure regulation also endows TESLI with cleaning and sample switching capabilities, thus enabling scalable processing of many samples in succession. Moreover, we implement a pair of TESLIs in our device to parallelize and alternate their operation as means to minimizing idle time. For demonstration, we use our device to successively process 44 samples into droplets—a number that can further scale. Our results demonstrate the feasibility of tubing-free sample loading and a promising approach for advancing droplet microfluidics.

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.

Active Flow Control and Dynamic Analysis in Droplet Microfluidics

Droplet-based microfluidics has emerged as an important subfield within the microfluidic and general analytical communities. Indeed, several unique applications such as digital assay readout and single-cell sequencing now have commercial systems based on droplet microfluidics. Yet there remains room for this research area to grow. To date, most analytical readouts are optical in nature, relatively few studies have integrated sample preparation, and passive means for droplet formation and manipulation have dominated the field. Analytical scientists continue to expand capabilities by developing droplet-compatible method adaptations, for example, by interfacing to mass spectrometers or automating droplet sampling for temporally resolved analysis. In this review, we highlight recently developed fluidic control techniques and unique integrations of analytical methodology with droplet microfluidics-focusing on automation and the connections to analog/digital domains-and we conclude by offering a perspective on current challenges and future applications.

Deterministic assembly of chromosome ensembles in a programmable membrane trap array

Selective spatial isolation and manipulation of single chromosomes and the controlled formation of defined chromosome ensembles in a droplet-based microfluidic system is presented. The multifunctional microfluidic technology employs elastomer valves and membrane displacement traps to support deterministic manipulation of individual droplets. Picoliter droplets are formed in the 2D array of microscale traps by self-discretization of a nanoliter sample plug, with membranes positioned over each trap allowing controllable metering or full release of selected droplets. By combining discretization, optical interrogation, and selective droplet release for sequential delivery to a downstream merging zone, the system enables efficient manipulation of multiple chromosomes into a defined ensemble with single macromolecule resolution. Key design and operational parameters are explored, and co-compartmentalization of three chromosome pairs is demonstrated as a first step toward formation of precisely defined chromosome ensembles for applications in genetic engineering and synthetic biology.

Droplet Microfluidics for Tumor Drug-Related Studies and Programmable Artificial Cells

Anticancer drug development is a crucial step toward cancer treatment, that requires realistic predictions of malignant tissue development and sophisticated drug delivery. Tumors often acquire drug resistance and drug efficacy, hence cannot be accurately predicted in 2D tumor cell cultures. On the other hand, 3D cultures, including multicellular tumor spheroids (MCTSs), mimic the in vivo cellular arrangement and provide robust platforms for drug testing when grown in hydrogels with characteristics similar to the living body. Microparticles and liposomes are considered smart drug delivery vehicles, are able to target cancerous tissue, and can release entrapped drugs on demand. Microfluidics serve as a high-throughput tool for reproducible, flexible, and automated production of droplet-based microscale constructs, tailored to the desired final application. In this review, it is described how natural hydrogels in combination with droplet microfluidics can generate MCTSs, and the use of microfluidics to produce tumor targeting microparticles and liposomes. One of the highlights of the review documents the use of the bottom-up construction methodologies of synthetic biology for the formation of artificial cellular assemblies, which may additionally incorporate both target cancer cells and prospective drug candidates, as an integrated "droplet incubator" drug assay platform.

Trapping and Coalescence of Diamagnetic Aqueous Droplets Using Negative Magnetophoresis

The manipulation of aqueous droplets has a profound significance in biochemical assays. Magnetic field-driven droplet manipulation, offering unique advantages, is consequently gaining attention. However, the phenomenon relating to diamagnetic droplets is not well understood. Here, we report the understanding of trapping and coalescence of flowing diamagnetic aqueous droplets in a paramagnetic (oil-based ferrofluid) medium using negative magnetophoresis. Our study revealed that the trapping phenomenon is underpinned by the interplay of magnetic energy (E_m) and frictional (viscous) energy (E_f), in terms of magnetophoretic stability number, S_m = (E_m/E_f). The trapping and nontrapping regimes are characterized based on the peak value of magnetophoretic stability number, S_mp, and droplet size, D*. The study of coalescence of a trapped droplet with a follower droplet (and a train of droplets) revealed that the film-drainage Reynolds number (Re_fd) representing the coalescence time depends on the magnetic Bond number, Bo_m. The coalesced droplet continues to remain trapped or gets self-released obeying the S_mp and D* criterion. Our study offers an understanding of the magnetic manipulation of diamagnetic aqueous droplets that can potentially be used for biochemical assays in microfluidics.

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.

Multifunctional Droplet Microfluidic Platform for Rapid Immobilization of Oligonucleotides on Semiconductor Quantum Dots

Quantum dot-DNA oligonucleotide (QD-DNA) conjugates have been used in many fields such as nucleic acid bioassays, intracellular probes, and drug delivery systems. A typical solid-phase method that achieves rapid loading of oligonucleotides on surfaces of QDs involves a two-step reaction and is performed in a batch-based approach. In contrast, droplet microfluidics offers many advantages that are unavailable when using batch processing, providing rapid and dense immobilized DNA oligonucleotides on QDs. The presented droplet microfluidic approach allows high-quality QD-DNA conjugates to be produced using one single device, which is designed to have two droplet generators, one droplet merger, and one mixer. One of the droplet generators coencapsulates QDs and magnetic beads (MBs) into nanoliter-sized droplets for the production of QD-MB conjugates and the other encapsulates oligonucleotides in nanoliter-sized droplets. These two streams of droplets then merge at a one-to-one ratio in a chamber. The merged droplets travel along the mixer, which is a serpentine microchannel with 30 turns, resulting in QD-DNA conjugation structures of high quality. This multifunctional microfluidic device provides advantages such as higher degree of control over the reaction conditions, minimized cross-contamination and impurities, and reduction of reagent consumption while eliminating any need for external vortexing and pipetting. To evaluate the quality of the QD-DNA conjugates, they were used as Forster resonance energy transfer (FRET) probes to quantify oligonucleic targets.

A programmable microfluidic platform for multisample injection, discretization, and droplet manipulation

A programmable microfluidic platform enabling on-demand sampling, compartmentalization, and manipulation of multiple aqueous volumes is presented. The system provides random-access actuation of a microtrap array supporting selective discretization of picoliter volumes from multiple sample inputs. The platform comprises two interconnected chips, with parallel T-junctions and multiplexed microvalves within one chip enabling programmable injection of aqueous sample plugs, and nanoliter volumes transferred to a second microtrap array chip in which the plugs are actively discretized into picoliter droplets within a static array of membrane displacement actuators. The system employs two different multiplexer designs that reduce the number of input signals required for both sample injection and discretization. This versatile droplet-based technology offers flexible sample workflows and functionalities for the formation and manipulation of heterogeneous picoliter droplets, with particular utility for applications in biochemical synthesis and cell-based assays requiring flexible and programmable operation of parallel and multistep droplet processes. The platform is used here for the selective encapsulation of differentially labeled cells within a discrete droplet array.

A Scalable Random Access Micro-traps Array for Formation, Selective Retrieval and Capturing of Individual Droplets

Formation, selective retrieval and capturing of individual droplets are key operational capabilities needed for a broad range of droplet microfluidic applications. The membrane displacement trap (MDT) element gives a robust method for uniform discretization and controllable manipulation of aqueous droplets using an enclosed micro-well covered by an elastomer membrane. This capability can be scaled up by combining the modular elements with a system design that requires a minimal number of signal inputs. Incorporation of MDT elements with a pneumatically-controllable multiplexer system can lead to a scalable random access MDT array platform for liquid discretization and selective manipulation. Herein, we report the design and development of a programmable droplet microfluidic platform for liquid sampling and selectively handling up to 32 individual droplets using 10 pneumatic signal inputs. The multiplexer system can logarithmically scale up capacity of the MDT array platform, making it possible to manipulate hundreds droplets.

Release of a trapped droplet in a single micro pore throat

The critical condition (i.e., critical capillary number Ca_c) for the release of trapped droplets is of practical importance in enhanced oil recovery and droplet microfluidics. In a recent study, Ca_c was obtained for a long droplet with a size much larger than the channel size. However, in real applications, trapped droplets are often finite with a size comparable to the channel, in which capillary and hydrostatic pressures alternate significantly. We hypothesize that Ca_c of finite droplets has a discrepancy from that of long droplets. Microfluidic experiments were performed to obtain the critical condition for the release of a finite droplet trapped in a single pore throat. A theoretical prediction via analyzing capillary and hydrostatic pressures was derived for Ca_c of both finite and long droplets. We find that Ca_c strongly depends on the droplet-to-channel size ratio (i.e., the droplet-to-convergent channel length ratio L/L_c). In particular, Ca_c increases with L/L_c for finite droplets (i.e., L/L_c < 1) but shows an opposite tendency for long droplets (i.e., L/L_c > 1), as demonstrated in previous studies. Via theoretical analysis, we established a predictive criterion for Ca_c versus L/L_c, and this criterion quantitatively agrees well with experimental data for both finite and long droplets.

Semi-automated on-demand control of individual droplets with a sample application to a drug screening assay

Automated control of individual droplets in microfluidic channels offers tremendous potential for applications requiring high accuracy and minimal user involvement. The feasibility of active droplet control has been previously demonstrated with pressure-driven flow control and visual feedback, but the manual operation required to perform droplet manipulations limited the accuracy, repeatability, and throughput. The present study improves upon the aforementioned challenges with a higher-level algorithm capturing the dynamics of droplet motion for a semi-automated control system. With a simple T junction geometry, droplets can now be automatically and precisely controlled on-demand. Specifically, there is ±10% accuracy for droplet generation, ±1.3% monodispersity for 500 μm long droplets and ±4% accuracy for splitting ratios. On-demand merging, mixing, and sorting are also demonstrated as well as the application of a drug screening assay related to neurodegenerative disorders. Overall, this system serves as a foundation for a fully automated system that does not require valves, embedded electrodes, or complex multi-layer fabrication.

Integrating Immunology and Microfluidics for Single Immune Cell Analysis

The field of immunoengineering aims to develop novel therapies and modern vaccines to manipulate and modulate the immune system and applies innovative technologies toward improved understanding of the immune system in health and disease. Microfluidics has proven to be an excellent technology for analytics in biology and chemistry. From simple microsystem chips to complex microfluidic designs, these platforms have witnessed an immense growth over the last decades with frequent emergence of new designs. Microfluidics provides a highly robust and precise tool which led to its widespread application in single-cell analysis of immune cells. Single-cell analysis allows scientists to account for the heterogeneous behavior of immune cells which often gets overshadowed when conventional bulk study methods are used. Application of single-cell analysis using microfluidics has facilitated the identification of several novel functional immune cell subsets, quantification of signaling molecules, and understanding of cellular communication and signaling pathways. Single-cell analysis research in combination with microfluidics has paved the way for the development of novel therapies, point-of-care diagnostics, and even more complex microfluidic platforms that aid in creating in vitro cellular microenvironments for applications in drug and toxicity screening. In this review, we provide a comprehensive overview on the integration of microsystems and microfluidics with immunology and focus on different designs developed to decode single immune cell behavior and cellular communication. We have categorized the microfluidic designs in three specific categories: microfluidic chips with cell traps, valve-based microfluidics, and droplet microfluidics that have facilitated the ongoing research in the field of immunology at single-cell level.

Trapping a moving droplet train by bubble guidance in microfluidic networks

Trapping a train of moving droplets into preset positions within a microfluidic device facilitates the long-term observation of biochemical reactions inside the droplets. In this paper, a new bubble-guided trapping method, which can remarkably improve the limited narrow two-phase flow rate range of uniform trapping, was proposed by taking advantage of the unique physical property that bubbles do not coalescence with two-phase fluids and the hydrodynamic characteristic of large flow resistance of bubbles. The flow behaviors of bubble-free and bubble-guided droplet trains were compared and analyzed under the same two-phase flow rates. The experimental results show that the droplets trapped by bubble-free guided trapping exhibit the four trapping modes of sequentially uniform trapping, non-uniform trapping induced by break-up and collision, and failed trapping due to squeezing through, and the droplets exhibit the desired uniform trapping in a relatively small two-phase flow rate range. Compared with bubble-free guided droplets, bubble-guided droplets also show four trapping modes. However, the two-phase flow rate range in which uniform trapping occurs is increased significantly and the uniformity of the trapped droplet array is improved. This investigation is beneficial to enhance the applicability of microfluidic chips for storing droplets in a passive way.

Simulation before fabrication: a case study on the utilization of simulators for the design of droplet microfluidic networks

The functional performance of passively operated droplet microfluidics is sensitive with respect to the dimensions of the channel network, the fabrication precision as well as the applied pressure because the entire network is coupled together. Especially, the local and global hydrodynamic resistance changes caused by droplets make the task to develop a robust microfluidic design challenging as plenty of interdependencies which all affect the intended behavior have to be considered by the designer. After the design, its functionality is usually validated by fabricating a prototype and testing it with physical experiments. In case that the functionality is not implemented as desired, the designer has to go back, revise the design, and repeat the fabrication as well as experiments. This current design process based on multiple iterations of refining and testing the design produces high costs (financially as well as in terms of time). In this work, we show how a significant amount of those costs can be avoided when applying simulation before fabrication. To this end, we demonstrate how simulations on the 1D circuit analysis model can help in the design process by means of a case study. Therefore, we compare the design process with and without using simulation. As a case study, we use a microfluidic network which is capable of trapping and merging droplets with different content on demand. The case study demonstrates how simulation can help to validate the derived design by considering all local and global hydrodynamic resistance changes. Moreover, the simulations even allow further exploration of different designs which have not been considered before due to the high costs.

Simulating microfluidic networks allows to check a design even before first prototypes are realized.

Numerical simulation of high inertial liquid-in-gas droplet in a T-junction microchannel

Two new flow regimes named unstable dripping and unstable jetting are identified in aqueous droplet generation within high inertial air flow inside a T-Junction microchannel.