Mass Activated Droplet Sorting (MADS) Enables High‐Throughput Screening of Enzymatic Reactions at Nanoliter Scale

Microfluidic droplet sorting enables the high‐throughput screening and selection of water‐in‐oil microreactors at speeds and volumes unparalleled by traditional well‐plate approaches. Most such systems sort using fluorescent reporters on modified substrates or reactions that are rarely industrially relevant. We describe a microfluidic system for high‐throughput sorting of nanoliter droplets based on direct detection using electrospray ionization mass spectrometry (ESI‐MS). Droplets are split, one portion is analyzed by ESI‐MS, and the second portion is sorted based on the MS result. Throughput of 0.7 samples s^?1 is achieved with 98 % accuracy using a self‐correcting and adaptive sorting algorithm. We use the system to screen ≈15 000 samples in 6 h and demonstrate its utility by sorting 25 nL droplets containing transaminase expressed in vitro. Label‐free ESI‐MS droplet screening expands the toolbox for droplet detection and recovery, improving the applicability of droplet sorting to protein engineering, drug discovery, and diagnostic workflows.     

High-throughput sorting of nanoliter droplets enabled by a sequentially addressable dielectrophoretic array

Droplet microfluidics has emerged as a powerful tool for a diverse range of biomedical and industrial applications such as single-cell analysis, directed evolution, and metabolic engineering. In these applications, droplet sorting has been effective for isolating small droplets encapsulating molecules, cells, or crystals of interest. Recently, there is an increased interest in extending the applicability of droplet sorting to larger droplets to utilize their size advantage. However, sorting throughputs of large droplets have been limited, hampering their wide adoption. Here, we report our demonstration of high-throughput fluorescence-activated droplet sorting of 1 nL droplets using an upgraded version of the sequentially addressable dielectrophoretic array (SADA), which we reported previously. The SADA is an array of electrodes that are individually and sequentially activated/deactivated according to the speed and position of a droplet passing nearby the array. We upgraded the SADA by increasing the number of driving electrodes constituting the SADA and incorporating a slanted microchannel. By using a ten-electrode SADA with the slanted microchannel, we achieved fluorescence-activated droplet sorting of 1 nL droplets at a record high throughput of 1752 droplets/s, twice as high as the previously reported maximum sorting throughput of 1 nL droplets.     

微流控芯片液滴生成与检测技术研究进展

微流控芯片液滴技术是一种操控微小体积液体的新技术,既可实现高通量微观样本的生成及控制,也可进行独立液滴的操作.分散的微液滴单元可作为理想的微反应器,在生物医药中的药物筛选、材料筛选和高附加值微颗粒材料合成领域展现出巨大的应用潜力.液滴微流控芯片是利用流体剪切力的改变,使互不相溶的两相流体在其界面处生成稳定、有序的液滴,...     

Fusion and sorting of two parallel trains of droplets using a railroad-like channel network and guiding tracks

We propose a robust droplet fusion and sorting method for two parallel trains of droplets that is relatively insensitive to frequency and phase mismatch. Conventional methods of droplet fusion require an extremely precise control of aqueous/oil flows for perfect frequency matching between two trains of droplets. In this work, by combining our previous two methods (i.e., droplet synchronization using railroad-like channels and manipulation of shape-dependent droplets using guiding tracks), we realized an error-free droplet fusion/sorting device for the two parallel trains of droplets. If droplet pairs are synchronized through a railroad-like channel, they are electrically fused and the fused droplets transit to a middle guiding track to flow in a middle channel; otherwise non-synchronized non-fused droplets will be discarded into the side waste channels by flowing through their own guiding tracks. The simple droplet synchronization, fusion, and sorting technology will have widespread application in droplet-based chemical or biological experiments, where two trains of the chemically or biologically treated or pre-formed droplets yield a train of 100% one-to-one fused droplets at the desired outlet channel by sorting all the non-synchronized non-fused droplets into waste outlets.     

Compressed Air-Driven Continuous-Flow Thermocycled Digital PCR for HBV Diagnosis in Clinical-Level Serum Sample Based on Single Hot Plate

We report a novel compressed air-driven continuous-flow digital PCR (dPCR) system based on a 3D microfluidic chip and self-developed software system to realize real-time monitoring. The system can ensure the steady transmission of droplets in long tubing without an external power source and generate stable droplets of suitable size for dPCR by two needles and a narrowed Teflon tube. The stable thermal cycle required by dPCR can be achieved by using only one constant temperature heater. In addition, our system has realized the real-time detection of droplet fluorescence in each thermal cycle, which makes up for the drawbacks of the end-point detection method used in traditional continuous-flow dPCR. This continuous-flow digital PCR by the compressed air-driven method can meet the requirements of droplet thermal cycle and diagnosis in a clinical-level serum sample. Comparing the detection results of clinical samples (hepatitis B virus serum) with commercial instruments (CFX Connect; Bio Rad, Hercules, CA, USA), the linear correlation reached 0.9995. Because the system greatly simplified the traditional dPCR process, this system is stable and user-friendly.     

Design of microfluidic channel geometries for the control of droplet volume, chemical concentration, and sorting

Passive microfluidic channel geometries for control of droplet fission, fusion and sorting are designed, fabricated, and tested. In droplet fission, the inlet width of the bifurcating junction is used to control the range of breakable droplet sizes and the relative resistances of the daughter channels were used to control the volume of the daughter droplets. Droplet fission is shown to produce concentration differences in the daughter droplets generated from a primary drop with an incompletely mixed chemical gradient, and for droplets in each of the bifurcated channels, droplets were found to be monodispersed with a less than 2% variation in size. Droplet fusion is demonstrated using a flow rectifying design that can fuse multiple droplets of same or different sizes generated at various frequencies. Droplet sorting is achieved using a bifurcating flow design that allows droplets to be separated base on their sizes by controlling the widths of the daughter channels. Using this sorting design, submicron satellite droplets are separated from the larger droplets.     

Rapid and label-free screening of enzyme inhibitors using segmented flow electrospray ionization mass spectrometry

Electrospray ionization mass spectrometry (ESI-MS) is an attractive analytical tool for high-throughput screening because of its rapid scan time and ability to detect compounds without need for labels. Impediments to the use of ESI-MS for screening have been the relatively large sample consumed and slow sample introduction rates associated with commonly used flow injection analysis. We have previously shown that by segmenting nanoliter plugs of sample with air, an array of discrete samples can be delivered to a platinum-coated emitter tip for ESI-MS analysis with throughput as high as 0.8 Hz and carry-over between samples less than 0.1%. This method was applied to screening for inhibitors of acetylcholinesterase as a demonstration of the potential of segmented flow ESI-MS for such applications. Each enzyme assay consumed 10 nL of sample. At 1 μL/min infusion rate, 102 samples were analyzed, corresponding to a 0.65 Hz sample analysis rate. Linear quantification of choline was achieved from 200 μM to 10 mM using this method and Z′ values were over 0.8 for the assay. Detailed pharmacologic dose-response curves of selected inhibitors were also measured in high-throughput fashion to validate the method.     

1-Million droplet array with wide-field fluorescence imaging for digital PCR

Digital droplet reactors are useful as chemical and biological containers to discretize reagents into picolitre or nanolitre volumes for analysis of single cells, organisms, or molecules. However, most DNA based assays require processing of samples on the order of tens of microlitres and contain as few as one to as many as millions of fragments to be detected. Presented in this work is a droplet microfluidic platform and fluorescence imaging setup designed to better meet the needs of the high-throughput and high-dynamic-range by integrating multiple high-throughput droplet processing schemes on the chip. The design is capable of generating over 1-million, monodisperse, 50 picolitre droplets in 2-7 minutes that then self-assemble into high density 3-dimensional sphere packing configurations in a large viewing chamber for visualization and analysis. This device then undergoes on-chip polymerase chain reaction (PCR) amplification and fluorescence detection to digitally quantify the sample's nucleic acid contents. Wide-field fluorescence images are captured using a low cost 21-megapixel digital camera and macro-lens with an 8-12 cm(2) field-of-view at 1× to 0.85× magnification, respectively. We demonstrate both end-point and real-time imaging ability to perform on-chip quantitative digital PCR analysis of the entire droplet array. Compared to previous work, this highly integrated design yields a 100-fold increase in the number of on-chip digitized reactors with simultaneous fluorescence imaging for digital PCR based assays.     

A critical analysis of electrospray techniques for the determination of accelerated rates and mechanisms of chemical reactions in droplets

Electrospray and Electrosonic Spray Ionization Mass Spectrometry (ESI-MS and ESSI-MS) have been widely used to report evidence that many chemical reactions in micro- and nano-droplets are dramatically accelerated by factors of ∼10² to 10⁶ relative to macroscale bulk solutions. Despite electrospray''s relative simplicity to both generate and detect reaction products in charged droplets using mass spectrometry, substantial complexity exists in how the electrospray process itself impacts the interpretation of the mechanism of these observed accelerated rates. ESI and ESSI are both coupled multi-phase processes, in which analytes in small charged droplets are transferred and detected as gas-phase ions with a mass spectrometer. As such, quantitative examination is needed to evaluate the impact of multiple experimental factors on the magnitude and mechanisms of reaction acceleration. These include: (1) evaporative concentration of reactants as a function of droplet size and initial concentration, (2) competition from gas-phase chemistry and reactions on experimental surfaces, (3) differences in ionization efficiency and ion transmission and (4) droplet charge. We examine (1–4) using numerical models, new ESI/ESSI-MS experimental data, and prior literature to assess the limitations of these approaches and the experimental best practices required to robustly interpret acceleration factors in micro- and nano-droplets produced by ESI and ESSI.

The application of Electrospray and Electrosonic Spray Ionization Mass Spectrometry (ESI-MS and ESSI-MS) to study accelerated reaction kinetics in droplets is examined using numerical models, new experimental data, and prior literature.     

A homogeneous assay for protein analysis in droplets by fluorescence polarization

We present a novel homogeneous (“mix‐incubate‐read”) droplet microfluidic assay for specific protein detection in picoliter volumes by fluorescence polarization (FP), for the first time demonstrating the use of FP in a droplet microfluidic assay. Using an FP‐based assay we detect streptavidin concentrations as low as 500 nM and demonstrate that an FP assay allows us to distinguish droplets containing 5 μM rabbit IgG from droplets without IgG with an accuracy of 95%, levels relevant for hybridoma screening. This adds to the repertoire of droplet assay techniques a direct protein detection method which can be performed entirely inside droplets without the need for labeling of the analyte molecules.     

Microfluidic droplet sorting with a high frequency ultrasound beam

This paper presents experimental results demonstrating the feasibility of high frequency ultrasonic sensing and sorting for screening single oleic acid (lipid or oil) droplets under continuous flow in a microfluidic channel. In these experiments, hydrodynamically focused lipid droplets of two different diameters (50 μm and 100 μm) are centered along the middle of the channel, which is filled with deionized (DI) water. A 30 MHz lithium niobate (LiNbO(3)) transducer, placed outside the channel, first transmits short sensing pulses to non-invasively determine the acoustic scattering properties of the individual droplets passing through the beam's focus. Integrated backscatter (IB) coefficients, utilized as a sorting criterion, are measured by analyzing the received echo signals from each droplet. When the IB values corresponding to 100 μm droplets are obtained, a custom-built LabVIEW panel commands the transducer to emit sinusoidal burst signals to commence the sorting operation. The number of droplets tested for the sorting is 139 for 50 μm droplets and 95 for 100 μm droplets. The sensing efficiencies are estimated to be 98.6% and 99.0%, respectively. The sorting is carried out by applying acoustic radiation forces to 100 μm droplets to direct them towards the upper sheath flow, thus separating them from the centered droplet flow. The sorting efficiencies are 99.3% for 50 μm droplets and 85.3% for 100 μm droplets. The results suggest that this proposed technique has the potential to be further developed into a cost-effective and efficient cell/microparticle sorting instrument.     

Microfluidic platform for on-demand generation of spatially indexed combinatorial droplets

We propose a highly versatile and programmable nanolitre droplet-based platform that accepts an unlimited number of sample plugs from a multi-well plate, performs digitization of these sample plugs into smaller daughter droplets and subsequent synchronization-free, robust injection of multiple reagents into the sample daughter droplets on-demand. This platform combines excellent control of valve-based microfluidics with the high-throughput capability of droplet microfluidics. We demonstrate the functioning of a proof-of-concept device which generates combinatorial mixture droplets from a linear array of sample plugs and four different reagents, using food dyes to mimic samples and reagents. Generation of a one dimensional array of the combinatorial mixture droplets on the device leads to automatic spatial indexing of these droplets, precluding the need to include a barcode in each droplet to identify its contents. We expect this platform to further expand the range of applications of droplet microfluidics to include applications requiring a high degree of multiplexing as well as high throughput analysis of multiple samples.     

Digital droplet immunoassay based on a microfluidic chip with magnetic beads for the detection of prostate-specific antigen

Sensitive biomarker detection techniques are beneficial for both disease diagnosis and postoperative examinations. In this study, we report an integrated microfluidic chip designed for the immunodetection of prostate-specific antigens (PSAs). The microfluidic chip is based on the three-dimensional structure of quartz capillaries. The outlet channel extends to 1.8 cm, effectively facilitating the generation of uniform droplets ranging in size from 3 to 50 μm. Furthermore, we successfully immobilized the captured antibodies onto the surface of magnetic beads using an activator, and we constructed an immunosandwich complex by employing biotinylated antibodies. A key feature of this microfluidic chip is its integration of microfluidic droplet technology advantages, such as high-throughput parallelism, enzymatic signal amplification, and small droplet size. This integration results in an exceptionally sensitive PSA detection capability, with the detection limit reduced to 7.00 ± 0.62 pg/mL.     

Single-molecule emulsion PCR in microfluidic droplets

The application of microfluidic droplet PCR for single-molecule amplification and analysis has recently been extensively studied. Microfluidic droplet technology has the advantages of compartmentalizing reactions into discrete volumes, performing highly parallel reactions in monodisperse droplets, reducing cross-contamination between droplets, eliminating PCR bias and nonspecific amplification, as well as enabling fast amplification with rapid thermocycling. Here, we have reviewed the important technical breakthroughs of microfluidic droplet PCR in the past five years and their applications to single-molecule amplification and analysis, such as high-throughput screening, next generation DNA sequencing, and quantitative detection of rare mutations. Although the utilization of microfluidic droplet single-molecule PCR is still in the early stages, its great potential has already been demonstrated and will provide novel solutions to today's biomedical engineering challenges in single-molecule amplification and analysis.     

Applying microdroplets as sensors for label-free detection of chemical reactions

Despite its tremendous high-throughput screening capabilities, widespread applications of droplet-based microfluidics are still limited by the poor availability of appropriate analytical assays. Here we report on a novel sensor method that exploits the osmosis-driven change in droplet size as a quantitative and label-free marker for reactions inside the droplets. We present an analysis of the underlying mechanism and apply the method for monitoring metabolic activity at a single-cell level.     

Coalescence-assisted generation of single nanoliter droplets with predefined composition

We demonstrate the generation of highly accurate nanoliter droplets with a predefined composition. This composition control over a single droplet is achieved by merging two droplets with known concentrations and defined volumes. A forced coalescence is accomplished by synchronizing two piezoelectric-based active droplet generators. A microscope-mounted CCD camera is used to record, quantify and monitor the process to assure its high fidelity. The device is disposable, surfactant free, simple to operate and does not require microelectrode fabrication. It delivers a single on-demand droplet with adjustable high resolution mixing ratios up to 9 at a volume range of 1-10 nanoliters. The presented platform offers, for the first time, a means to perform droplet-based high-throughput screening in the nanoliter range.     

Graphene-polyaniline modified electrochemical droplet-based microfluidic sensor for high-throughput determination of 4-aminophenol

We report herein the first development of graphene-polyaniline modified carbon paste electrode (G-PANI/CPE) coupled with droplet-based microfluidic sensor for high-throughput detection of 4-aminophenol (4-AP) in pharmaceutical paracetamol (PA) formulations. A simple T-junction microfluidic platform using an oil flow rate of 1.8 μL/min and an aqueous flow rate of 0.8 μL/min was used to produce aqueous testing microdroplets continuously. The microchannel was designed to extend the aqueous droplet to cover all 3 electrodes, allowing for electrochemical measurements in a single droplet. Parameters including flow rate, water fraction, and applied detection potential (E_det) were investigated to obtain optimal conditions. Using G-PANI/CPE significantly increased the current response for both cyclic voltammetric detections of ferri/ferrocyanide [Fe(CN)₆]^3−/4− (10 times) and 4-AP (2 times), compared to an unmodified electrode. Using the optimized conditions in the droplet system, 4-AP in the presence of PA was selectively determined. The linear range of 4-AP was 50–500 μM (R² = 0.99), limit of detection (LOD, S/N = 3) was 15.68 μM, and limit of quantification (LOQ, S/N = 10) was 52.28 μM. Finally, the system was used to determine 4-AP spiked in commercial PA liquid samples and the amounts of 4-AP were found in good agreement with those obtained from the conventional capillary zone electrophoresis/UV–Visible spectrophotometry (CZE/UV–Vis). The proposed microfluidic device could be employed for a high-throughput screening (at least 60 samples h⁻¹) of pharmaceutical purity requiring low sample and reagent consumption.     

Crosstalk-free colloidosomes for high throughput single-molecule protein analysis

Droplet microfluidics is a powerful platform for high-throughput single-molecule protein analysis. However, the issues of coalescence and crosstalk of droplets compromise the accuracy of detection and hinder its wide application. To address these limitations, a novel colloidosome-based method was presented by combining a Pickering emulsion with droplet microfluidics for single-molecule protein analysis. Utilizing the self-assembly of easily synthesized colloidal surfactant F-SiO_2 NPs at the water/oil interface, the colloidosomes are rigidly stabilized and can effectively avoid the leakage of fluorescent molecules. The crosstalk-free colloidosomes enable high-throughput single-molecule protein analysis, including heterogenous dynamic studies and digital detection. As a robust and accurate method, colloidosome-based microfluidics is promising as a powerful tool for a wide variety of applications, such as directed enzyme evolution, digital enzyme-linked immunosorbent assay(ELISA), and screening of antibiotics.     

Droplet size based separation by deterministic lateral displacement-separating droplets by cell--induced shrinking

We present a novel method for passive separation of microfluidic droplets by size at high throughput using deterministic lateral displacement (DLD). We also show that droplets containing Saccharomyces cerevisiae shrink significantly during incubation while droplets containing only yeast media retain or slightly increase their size. We demonstrate the DLD device by sorting out shrunken yeast-cell containing droplets from 31% larger diameter droplets which were generated at the same time containing only media, present at a >40-fold excess. This demonstrates the resolving power of droplet separation by DLD and establishes that droplets can be separated for a biological property of the droplet contents discriminated by a change of the physical properties of the droplet. Thus suggesting that this technique may be used for e.g. clonal selection. The same device also separates 11 μm from 30 μm droplets at a rate of 12,000 droplets per second, more than twofold faster than previously demonstrated passive hydrodynamic separation devices.