Electric Control of Droplets in Microfluidic Devices

Miniaturization and parallelization of biological and chemical assays in microfluidic devices

Microfluidic systems are an attractive solution for the miniaturization of biological and chemical assays. The typical sample volume can be reduced up to 1 million-fold, and a superb level of spatiotemporal control is possible, facilitating highly parallelized assays with drastically increased throughput and reduced cost. In this review, we focus on systems in which multiple reactions are spatially separated by immobilization of reagents on two-dimensional arrays, or by compartmentalization in microfabricated reaction chambers or droplets. These systems have manifold applications, and some, such as next-generation sequencing are already starting to transform biology. This is likely the first step in a biotechnological transformation comparable to that already brought about by the microprocessor in electronics. We discuss both current applications and likely future impacts in areas such as the study of single cells/single organisms and high-throughput screening.

Droplet microfluidics for high-throughput analysis of cells and particles

Droplet microfluidics (DM) is an area of research which combines lab-on-a-chip (LOC) techniques with emulsion compartmentalization to perform high-throughput, chemical and biological assays. The key issue of this approach lies in the generation, over tens of milliseconds, of thousands of liquid vessels which can be used either as a carrier, to transport encapsulated particles and cells, or as microreactors, to perform parallel analysis of a vast number of samples. Each compartment comprises a liquid droplet containing the sample, surrounded by an immiscible fluid. This microfluidic technique is capable of generating subnanoliter and highly monodispersed liquid droplets, which offer many opportunities for developing novel single-cell and single-molecule studies, as well as high-throughput methodologies for the detection and sorting of encapsulated species in droplets. The aim of this chapter is to give an overview of the features of DM in a broad microfluidic context, as well as to show the advantages and limitations of the technology in the field of LOC analytical research. Examples are reported and discussed to show how DM can provide novel systems with applications in high-throughput, quantitative cell and particle analysis.

Interfacial tension controlled fusion of individual femtolitre droplets and triggering of confined chemical reactions on demand

This paper describes stepwise on-demand generation and fusion of femtolitre aqueous droplets based on interfacial tension. Sub-millisecond reaction times from droplet fusion were demonstrated, as well as a reversible chemical toggle switch based on alternating fusion of droplets containing acidic or basic solution, monitored with the pH-dependent emission of fluorescein.

Passive self-synchronized two-droplet generation

We describe the use of two passive components to achieve controllable and alternating droplet generation in a microfluidic device. The approach overcomes the problems associated with irregularities in channel dimensions and fluid flow rates, and allows precise pairing of alternating droplets in a high-throughput manner. We study droplet generation and self-synchronization in a quantitative fashion by using high-speed image analysis.

Predictive model for the size of bubbles and droplets created in microfluidic T-junctions

We present a closed-form expression that allows the reader to predict the size of bubbles and droplets created in T-junctions without fitting. Despite the wide use of microfluidic devices to create bubbles and droplets, a physically sound expression for the size of bubbles and droplets, key in many applications, did not yet exist. The theoretical foundation of our expression comprises three main ingredients: continuity, geometrics and recently gained understanding of the mechanism which leads to pinch-off. Our simple theoretical model explains why the size of bubbles and droplets strongly depends on the shape of a T-junction, and teaches how the shape can be tuned to obtain the desired size. We successfully validated our model experimentally by analyzing the formation of gas bubbles, as well as liquid droplets, in T-junctions with a wide variety of shapes under conditions typical to multiphase microfluidics.

Microfluidic droplets: new integrated workflows for biological experiments

Miniaturization of the classical test tube to picoliter dimensions is possible in monodisperse water-in-oil droplets that are generated in microfluidic devices. The establishment of standard unit operations for droplet handling and the ability to carry out experiments with DNA, proteins, cells and organisms provides the basis for the design of more complex workflows to address biological challenges. The emerging experimental format makes possible a quantitative readout for large numbers of experiments with a precision comparable to the macroscopic scale. Directed evolution, diagnostics and compound screening are areas in which the first steps are being taken toward the long-term goal of transforming the way we design and carry out experiments.

Dynamics of microfluidic droplets

This critical review discusses the current understanding of the formation, transport, and merging of drops in microfluidics. We focus on the physical ingredients which determine the flow of drops in microchannels and recall classical results of fluid dynamics which help explain the observed behaviour. We begin by introducing the main physical ingredients that differentiate droplet microfluidics from single-phase microfluidics, namely the modifications to the flow and pressure fields that are introduced by the presence of interfacial tension. Then three practical aspects are studied in detail: (i) The formation of drops and the dominant interactions depending on the geometry in which they are formed. (ii) The transport of drops, namely the evaluation of drop velocity, the pressure-velocity relationships, and the flow field induced by the presence of the drop. (iii) The fusion of two drops, including different methods of bridging the liquid film between them which enables their merging.

Continuous two-phase flow miniaturised bioreactor for monitoring anaerobic biocatalysis by pentaerythritol tetranitrate reductase

A novel continuous recirculating two-phase flow miniaturised bioreactor was developed for biocatalytic transformations with the enzyme pentaerythritol tetranitrate reductase using on-chip spectroscopic detection of the organic and aqueous phases. A phase separation technique is described that uses electrostatic attraction to force charged droplets to merge back into the aqueous phase and thus allow the monitoring of both reaction phases during enzymatic turnover. We report an increased rate of enzyme catalysed reduction of trans-2-(2-nitrovinyl)thiophene, which was used as a model system to demonstrate the principles of the bioreactor design, compared to conventional macroscale experiments. Additional data obtained with ketoisophorone, trans-cinnamaldehyde and 2-methylmaleimide support our findings and provide a basis for improving the chemistry of biocatalysis.

Protein crystallization using microfluidic technologies based on valves, droplets, and SlipChip

To obtain protein crystals, researchers must search for conditions in multidimensional chemical space. Empirically, thousands of crystallization experiments are carried out to screen various precipitants at multiple concentrations. Microfluidics can manipulate fluids on a nanoliter scale, and it affects crystallization twofold. First, it miniaturizes the experiments that can currently be done on a larger scale and enables crystallization of proteins that are available only in small amounts. Second, it offers unique experimental approaches that are difficult or impossible to implement on a larger scale. Ongoing development of microfluidic techniques and their integration with protein production, characterization, and in situ diffraction promises to accelerate the progress of structural biology.

Optimized droplet-based microfluidics scheme for sol-gel reactions

A droplet-based microfluidic reaction scheme is developed where the chemical reactants are dispensed with precise volume control into pairs of droplets. The reaction is activated by coalescing droplet pairs and fast mixing inside the coalesced droplets. Furthermore, the pre-processing of the chemical products is included in the microfluidic device. This reaction scheme allows the performing of precisely volume controlled reactions and long operation times without any clogging even if precipitates or sticky gels are formed during the reaction. Using this approach and optimizing the reaction parameters, we generate mesoporous silica microspheres from a rapid gelation optimized sol-gel synthesis route. The produced silica particles have a superior surface area of 820 m(2) g(-1) and a narrow pore radius distribution of around 2.4 nm.

Multiphase bioreaction microsystem with automated on-chip droplet operation

A droplet-based bioreaction microsystem has been developed with automated droplet generation and confinement. On-chip electronic sensing is employed to track the position of the droplets by sensing the oil/aqueous interface in real time. The sensing signal is also used to control the pneumatic supply for moving as well as automatically generating four different nanolitre-sized droplets. The actual size of droplets is very close to the designed droplet size with a standard deviation less than 3% of the droplet size. The automated droplet generation can be completed in less than 2 s, which is 5 times faster than using manual operation that takes at least 10 s. Droplets can also be automatically confined in the reaction region with feedback pneumatic control and digital or analog sensing. As an example bioreaction, PCR has been successfully performed in the automated generated droplets. Although the amplification yield was slightly reduced with the droplet confinement, especially while using the analog sensing method, adding additional reagents effectively alleviated this inhibition.

Electric charge-mediated coalescence of water droplets for biochemical microreactors

This work proposes the use of charged droplets driven by the Coulombic force as solution-phase reaction chambers for biological microreactions. A droplet can be charged near an electrode under dc voltage by direct contact to the electrode. This process is called electrical charging of droplet (ECOD). This charged droplet can then be transported rapidly between electrodes following the arc of an electric field line by exploiting electrostatic force. As on-demand electrocoalescence, both alkalization of phenolphthalein and bioluminescence reaction of luciferase in the presence of adenosine triphosphate are studied to test the feasibility of the biochemical microreactors using ECOD. Two oppositely charged droplets are merged to have a color change immediately after microchemical reaction. The applicability of an ECOD-driven droplet to measurement of glucose concentration is also tested. The glucose concentration is measured using a colorimetric enzyme-kinetic method based on Trinder's reaction [J. Clin. Pathol. 22, 158 (1969)]. The color change in the merged droplet is detected with an absorbance measurement system consisting of a photodiode and a light emitting diode.

Microfluidic encapsulation of cells in alginate capsules for high throughput screening

Microdroplet systems can drastically reduce costs and increase throughput in high throughput screening (HTS) assays. While droplets are well suited for biomolecular screening, cell-based screens are more problematic because eukaryotes typically require attachment to solid supports to maintain viability and function. This paper describes an economical, off-the-shelf microfluidic system which encapsulates eukaryotic cells in gelatinous alginate capsules for the purpose of HTS. The flow-through system consists of i) a cross junction, which forms monodisperse droplets of alginate and cell suspension in an immiscible carrier fluid, followed by ii) a T junction which delivers BaCl(2) to crosslink and solidify each droplet. With an appropriate carrier fluid, the system is self-synchronized and can produce cell-alginate-BaCl(2) capsules with virtually 100% reliability. Droplet volumes and frequency are determined by flow rates and the diameter of the cross junction. The present implementation, which utilizes 1.5 mm Teflon tubing and plastic junctions, can generate 0.4-1.4 microL droplets at frequencies >10 droplets/sec. Cell viability is >80% at 4 hours post-encapsulation. With low recurring cost (<USD 2) and no need for automation robots, this can be an initial step towards economical cell-based HTS.

High-throughput automated droplet microfluidic system for screening of reaction conditions

We demonstrate a new droplet on demand (DOD) technique and an integrated system for scanning of arbitrary combinations of 3 miscible solutions in approximately 1.5 microL droplets at 3 Hz. The DOD system uses standard electromagnetic valves that are external to the microfluidic chip. This feature makes up for modularity, simplicity of assembly and compatibility with virtually any microfluidic chip and yields an on-chip footprint of less than 1 mm(2). A novel protocol for formation of DOD enables generation of an arbitrarily large range of volumes of droplets at a maximum operational frequency of approximately 30 Hz. The integrated system that we demonstrate can be used to scan up to 10,000 conditions of chemical and biochemical reactions per hour using approximately 10 mL of solutions in total.

On-demand droplet release for droplet-based microfluidic system

On-demand droplet release from microwell was successfully implemented and well combined with droplet trapping/fusion functions to make an ideal and integrated droplet based microfluidic system.

Droplet on demand system utilizing a computer controlled microvalve integrated into a stiff polymeric microfluidic device

An integrated microvalve fabricated in a stiff polymeric (polycarbonate) device allows for formation of droplets and bubbles of arbitrary volumes and at arbitrary times of emission. Predesigned protocols of formation of sequences of volumes, intervals between the droplets and their chemical compositions can be practically and reproducibly realised and controlled from a personal computer. The design of the microvalve enables its utilization in chips made in a range of other materials, including plastics and glass and in devices operated both with positive and negative pressure providing a useful module for a range of applications of microfluidic droplet systems.

Dynamic memory in a microfluidic system of droplets traveling through a simple network of microchannels

The flow of droplets through the simplest microfluidic network--a set of two parallel channels with a common inlet and a common outlet--exhibits a rich variety of dynamic behaviors parametrized by the frequency of feeding of droplets into the system and by the asymmetry of the arms of the microfluidic loop. Finite ranges of these two parameters form islands of regular (cyclic) behaviors of a well defined period that can be estimated via simple theoretical arguments. These islands are separated by regions of behaviors that are either irregular or cyclic with a very long periodicity. Interestingly, theoretical arguments and numerical simulations show that within the islands of regular behaviors the state of the system can be degenerate: there can exist a number of distinct sequences of trajectories of droplets, each stable and--in the absence of disturbances--continuing ad infinitum. The system can be switched between these cyclic trajectories with a single stimulus.

Microfluidic Droplet Technique for In Vitro Directed Evolution

Increasingly over the past two decades, biotechnologists have been exploiting various molecular technologies for high-throughput screening of genes and their protein products to isolate novel functionalities with a wide range of industrial applications. One particular technology now widely used for these purposes involves directed evolution, an artificial form of evolution in which genes and proteins are evolved towards new or improved functions by imposing intense selection pressures on libraries of mutant genes generated by molecular biology techniques and expressed in heterologous systems such as Escherichia coli. Most recently, the rapid development of droplet-based microfluidics has created the potential to dramatically increase the power of directed evolution by increasing the size of the libraries and the throughput of the screening by several orders of magnitude. Here, we review the methods for generating and controlling droplets in microfluidic systems, and their applications in directed evolution. We focus on the methodologies for cell-based assays, in vitro protein expression and DNA amplification, and the prospects for using such platforms for directed evolution in next-generation biotechnologies.

Generation and mixing of subfemtoliter aqueous droplets on demand

We describe a novel method of generating monodisperse subfemtoliter aqueous droplets on demand by means of piezoelectric injection. Droplets with volumes down to 200 aL are generated by this technique. The droplets are injected into a low refractive index perfluorocarbon so that they can be optically trapped. We demonstrate the use of optical tweezers to manipulate and mix droplets. For example, using optical tweezers we bring two droplets, one containing a calcium sensitive dye and the other calcium chloride, into contact. The droplets coalesce with a resulting reaction time of about 1 ms. The monodispersity, manipulability, repeatability, small size, and fast mixing afforded by this system offer many opportunities for nanochemistry and observation of chemical reactions on a molecule-by-molecule basis.

Multi-step microfluidic droplet processing: kinetic analysis of an in vitro translated enzyme

Microdroplets in water-in-oil emulsions can be used as microreactors with volumes 10(3) to 10(9) times smaller than the smallest working volumes in a microtitre plate well (1-2 microL). However, many reactions and assays require multiple steps where new reagents are added at defined times, to start, modify or terminate a reaction. The most flexible way to add new reagents to pre-formed droplets is by controlled, pairwise droplet fusion. We describe a droplet-based microfluidic system capable of performing multiple operations, including pairwise droplet fusion, to analyze complex and sequential multi-step reactions. It is exemplified by performing a series of six on-chip and two off-chip operations which enable the coupled in vitro transcription and translation of cotA laccase genes in droplets and, after performing a controlled fusion with droplets containing laccase assay reagents, the end-point and kinetic analysis of the catalytic activity of the translated protein. In vitro translation and the laccase assay must be performed sequentially as the conditions for the laccase assay are not compatible with in vitro translation. Droplet fusion was performed by electrocoalescence at a rate of approximately 3000 fusion events per second and nearly 90% of droplets were fused one-to-one (one droplet containing in vitro translated laccase fused to one droplet containing the reagents for the laccase assay). The ability to uncouple the enzymatic assay from in vitro translation greatly extends the range of activities of in vitro translated proteins that can potentially be screened in droplet-based microfluidic systems. Furthermore, the system also opens up the possibility of performing a wide range of other new (bio)chemical reactions in droplets.

Non-coalescence of oppositely charged drops

Electric fields induce motion in many fluid systems, including polymer melts, surfactant micelles and colloidal suspensions. Likewise, electric fields can be used to move liquid drops. Electrically induced droplet motion manifests itself in processes as diverse as storm cloud formation, commercial ink-jet printing, petroleum and vegetable oil dehydration, electrospray ionization for use in mass spectrometry, electrowetting and lab-on-a-chip manipulations. An important issue in practical applications is the tendency for adjacent drops to coalesce, and oppositely charged drops have long been assumed to experience an attractive force that favours their coalescence. Here we report the existence of a critical field strength above which oppositely charged drops do not coalesce. We observe that appropriately positioned and oppositely charged drops migrate towards one another in an applied electric field; but whereas the drops coalesce as expected at low field strengths, they are repelled from one another after contact at higher field strengths. Qualitatively, the drops appear to 'bounce' off one another. We directly image the transient formation of a meniscus bridge between the bouncing drops, and propose that this temporary bridge is unstable with respect to capillary pressure when it forms in an electric field exceeding a critical strength. The observation of oppositely charged drops bouncing rather than coalescing in strong electric fields should affect our understanding of any process involving charged liquid drops, including de-emulsification, electrospray ionization and atmospheric conduction.

Concurrent droplet charging and sorting by electrostatic actuation

This paper presents a droplet-based microfluidic device for concurrent droplet charging and sorting by electrostatic actuation. Water-in-oil droplets can be charged on generation by synchronized electrostatic actuation. Then, simultaneously, the precharged droplets can be electrostatically steered into any designated laminar streamline, thus they can be sorted into one of multiple sorting channels one by one in a controlled fashion. In this paper, we studied the size dependence of the water droplets under various relative flow rates of water and oil. We demonstrated the concurrent charging and sorting of up to 600 dropletss by synchronized electrostatic actuation. Finally, we investigated optimized voltages for stable droplet charging and sorting. This is an essential enabling technology for fast, robust, and multiplexed sorting of microdroplets, and for the droplet-based microfluidic systems.

High-throughput confinement and detection of single DNA molecules in aqueous microdroplets

A droplet-based microfluidic system combined with high-sensitivity optical detection is used as a tool for high-throughput confinement and detection of single DNA molecules.

Electrically initiated upstream coalescence cascade of droplets in a microfluidic flow

Two phase microfluidic systems creating size-controlled microdroplets have recently emerged as powerful tools to achieve liquid compartmentalization for high throughput chemical and biological assays. Emulsion electrocoalescence is a destabilization process that can be used in droplet-based platforms for water phase separation to enable lab-on-a-chip applications in biotechnology, including particle or cell recovery. In this paper, we report upon a series of phenomena associated with electrocoalescence of water microdroplet-in-oil populations in microfluidics. In our experiments, we formed microdroplets whose size and dispersion in the channel were varied according to the ratio of the flow rates of the two phases. Different types of electrocoalescence between droplets were obtained. For low applied voltages, drops merged in pairs over the electrode region; for higher values of the applied voltage, a cascade of droplet coalescence was produced against the flow direction, for a range of droplet sizes, lateral distributions of droplets in the channel and localized electric fields.

On-chip electrocoalescence of microdroplets as a function of voltage, frequency and droplet size

Electric fields have previously been used in microfluidic devices for the manipulation, sorting and mixing of microemulsions. Here, an active system for on-demand electrocoalescence of water droplets in oil is presented. The platform does not require precise electrode alignment nor droplet-droplet or droplet-electric field synchronisation. Droplets can be reliably merged in pairs at a rate up to 50 fusion events per second. The fusion mechanism is based on the balance between viscous, electric and interfacial stresses at the droplet interface and depends upon the flow behaviour in the microchannel. Experimental results show that, under different conditions of frequency, applied potential and size of the droplets with respect to the channel geometry, diverse types of droplet coalescence occur. The fusion mechanism and general trends which enabled different merging results are proposed. This system has potential for being applied and multiplexed for high throughput, emulsion-based applications in the field of combinatorial reactions and screening bioassays.

A fast and efficient microfluidic system for highly selective one-to-one droplet fusion

Microdroplets in microfluidic systems can be used as independent microreactors to perform a range of chemical and biological reactions. However, in order to add new reagents to pre-formed droplets at defined times, to start, modify, or terminate a reaction, it is necessary to perform a controlled fusion with a second droplet. We describe and characterize a simple and extremely reliable technique for the one-to-one fusion of droplet pairs in a microfluidic system at kHz frequencies. The technique does not require special channel treatment, electrical fields or lasers to induce droplet fusion. Instead, we make use of transient states in the stabilization of the droplet interface by surfactant, coupled to a proper geometrical design of a coalescence module, to induce the selective fusion of a droplet stabilized by surfactant (re-injected) with a droplet which is not fully stabilized (generated on-chip). Using a 1.2-fold excess of the surfactant-stabilized droplets approximately 99% of the partially stabilized droplets were fused one-to-one with surfactant-stabilized droplets. Even when the surfactant-stablized droplets were in 5-fold excess, over 96% of the partially stabilized droplets were fused one-to-one. The fused droplet contains enough surfactant to inhibit further fusion events. After fusion, the droplets were fully stabilized by additional surfactant provided in the carrier oil, which allowed the fused droplets to be collected, incubated off-chip and re-injected onto a microfluidic device without any undesired coalescence.

High-throughput quantitative polymerase chain reaction in picoliter droplets

Limiting dilution PCR has become an increasingly useful technique for the detection and quantification of rare species in a population, but the limit of detection and accuracy of quantification are largely determined by the number of reactions that can be analyzed. Increased throughput may be achieved by reducing the reaction volume and increasing processivity. We have designed a high-throughput microfluidic chip that encapsulates PCR reagents in millions of picoliter droplets in a continuous oil flow. The oil stream conducts the droplets through alternating denaturation and annealing zones, resulting in rapid (55-s cycles) and efficient PCR amplification. Inclusion of fluorescent probes in the PCR reaction mix permits the amplification process to be monitored within individual droplets at specific locations within the microfluidic chip. We show that amplification of a 245-bp adenovirus product can be detected and quantified in 35 min at starting template concentrations as low as 1 template molecule/167 droplets (0.003 pg/microL). The frequencies of positive reactions over a range of template concentrations agree closely with the frequencies predicted by Poisson statistics, demonstrating both the accuracy and sensitivity of this platform for limiting dilution and digital PCR applications.

Droplet-based microfluidic systems for high-throughput single DNA molecule isothermal amplification and analysis

We have developed a method for high-throughput isothermal amplification of single DNA molecules in a droplet-based microfluidic system. DNA amplification in droplets was analyzed using an intercalating fluorochrome, allowing fast and accurate "digital" quantification of the template DNA based on the Poisson distribution of DNA molecules in droplets. The clonal amplified DNA in each 2 pL droplet was further analyzed by measuring the enzymatic activity of the encoded proteins after fusion with a 15 pL droplet containing an in vitro translation system.

Droplet microfluidic technology for single-cell high-throughput screening

We present a droplet-based microfluidic technology that enables high-throughput screening of single mammalian cells. This integrated platform allows for the encapsulation of single cells and reagents in independent aqueous microdroplets (1 pL to 10 nL volumes) dispersed in an immiscible carrier oil and enables the digital manipulation of these reactors at a very high-throughput. Here, we validate a full droplet screening workflow by conducting a droplet-based cytotoxicity screen. To perform this screen, we first developed a droplet viability assay that permits the quantitative scoring of cell viability and growth within intact droplets. Next, we demonstrated the high viability of encapsulated human monocytic U937 cells over a period of 4 days. Finally, we developed an optically-coded droplet library enabling the identification of the droplets composition during the assay read-out. Using the integrated droplet technology, we screened a drug library for its cytotoxic effect against U937 cells. Taken together our droplet microfluidic platform is modular, robust, uses no moving parts, and has a wide range of potential applications including high-throughput single-cell analyses, combinatorial screening, and facilitating small sample analyses.

Fluorescence-activated droplet sorting (FADS): efficient microfluidic cell sorting based on enzymatic activity

We describe a highly efficient microfluidic fluorescence-activated droplet sorter (FADS) combining many of the advantages of microtitre-plate screening and traditional fluorescence-activated cell sorting (FACS). Single cells are compartmentalized in emulsion droplets, which can be sorted using dielectrophoresis in a fluorescence-activated manner (as in FACS) at rates up to 2000 droplets s(-1). To validate the system, mixtures of E. coli cells, expressing either the reporter enzyme beta-galactosidase or an inactive variant, were compartmentalized with a fluorogenic substrate and sorted at rates of approximately 300 droplets s(-1). The false positive error rate of the sorter at this throughput was <1 in 10(4) droplets. Analysis of the sorted cells revealed that the primary limit to enrichment was the co-encapsulation of E. coli cells, not sorting errors: a theoretical model based on the Poisson distribution accurately predicted the observed enrichment values using the starting cell density (cells per droplet) and the ratio of active to inactive cells. When the cells were encapsulated at low density ( approximately 1 cell for every 50 droplets), sorting was very efficient and all of the recovered cells were the active strain. In addition, single active droplets were sorted and cells were successfully recovered.

Droplets for ultrasmall-volume analysis

By using methods that permit the generation and manipulation of ultrasmall-volume droplets, researchers are pushing the boundaries of ultrasensitive chemical analyses. (To listen to a podcast about this feature, please go to the Analytical Chemistry Web site at pubs.acs.org/ancham.).

On-demand microfluidic droplet trapping and fusion for on-chip static droplet assays

On-demand droplet trapping and droplet fusion through novel approaches were successfully demonstrated to form a static droplet assay on-chip for timelapse studies of droplet based microreactions.

Performance of nanoliter-sized droplet-based microfluidic PCR

A microfluidic device was used to characterize PCR in aqueous-in-oil droplets for potential point-of-care applications. Droplets with a volume range of 5-250 nL can be formed on-chip reproducibly, and PCR in the droplets shows amplification efficiencies comparable to benchtop reactions with no evaporation loss. A higher polymerase concentration is required in the reaction droplet while the optimal Magnesium ion concentration is the same for both on-chip and benchtop systems. The optimal hold time is 9 s and 30 s for denaturation and annealing/extension in thermal cycling, respectively. With the optimized cycling parameters, the total reaction time is reduced to half of that required for benchtop PCR. For the droplets containing the same quantity of template DNA, the PCR yield is approximately the same with either fixed droplet size or fixed template DNA concentration. The droplet-based PCR can be monitored in real time with FRET probes, and provide amplification with a cycle threshold of ~10 cycles earlier than the benchtop instruments.