Droplet microfluidics

Hybrid microfluidics: a digital-to-channel interface for in-line sample processing and chemical separations

Microchannels can separate analytes faster with higher resolution, higher efficiency and with lower reagent consumption than typical column techniques. Unfortunately, an impediment in the path toward fully integrated microchannel-based labs-on-a-chip is the integration of pre-separation sample processing. Although possible in microchannels, such steps are challenging because of the difficulty in maintaining spatial control over many reagents simultaneously. In contrast, the alternative format of digital microfluidics (DMF), in which discrete droplets are manipulated on an array of electrodes, is well-suited for carrying out sequential chemical reactions. Here, we report the development of the first digital-channel hybrid microfluidic device for integrated pre-processing reactions and chemical separations. The device was demonstrated to be useful for on-chip labeling of amino acids and primary amines in cell lysate, as well as enzymatic digestion of peptide standards, followed by separation in microchannels. Given the myriad applications requiring pre-processing and chemical separations, the hybrid digital-channel format has the potential to become a powerful new tool for micro total analysis systems.

Microfluidic means of achieving attomolar detection limits with molecular beacon probes

We used inline, micro-evaporators to concentrate and transport DNA targets to a nanoliter single molecule fluorescence detection chamber for subsequent molecular beacon probe hybridization and analysis. This use of solvent removal as a unique means of target transport in a microanalytical platform led to a greater than 5000-fold concentration enhancement and detection limits that pushed below the femtomolar barrier commonly reported using confocal fluorescence detection. This simple microliter-to-nanoliter interconnect for single molecule counting analysis resolved several common limitations, including the need for excessive fluorescent probe concentrations at low target levels and inefficiencies in direct handling of highly dilute biological samples. In this report, the hundreds of bacteria-specific DNA molecules contained in approximately 25 microliters of a 50 aM sample were shuttled to a four nanoliter detection chamber through micro-evaporation. Here, the previously undetectable targets were enhanced to the pM regime and underwent probe hybridization and highly-efficient fluorescent event analysis via microfluidic recirculation through the confocal detection volume. This use of microfluidics in a single molecule detection (SMD) platform delivered unmatched sensitivity and introduced compliment technologies that may serve to bring SMD to more widespread use in replacing conventional methodologies for detecting rare target biomolecules in both research and clinical labs.

Coalescence and splitting of confined droplets at microfluidic junctions

The ability to merge two droplets is an important component of droplet-based lab-on-a-chip devices, yet flow-induced coalescence is difficult to achieve due to long film drainage times compared with relatively short residence times. We examine droplet collisions at a simple microfluidic T-junction and characterize the response for a wide range of droplet sizes and speeds. We find that three primary responses occur, where coalescence occurs easily at low collision speeds, smaller droplets traveling faster slip past one another without coalescing, and larger and faster droplets can break one another into multiple segments. The critical capillary number for coalescence agrees well with previously reported scaling for isolated droplet pairs when local curvature and speed are taken into account. The critical capillary number for splitting of droplets agrees well with a previously reported stability condition for individual droplets stretching in an extensional flow. Quantifying the necessary conditions for coalescence and non-coalescence behavior should enable the informed design of lab on chip devices based on discrete liquid segments.

On-Chip Droplet Dispensing by a Magnetically Driven Microtool

We have investigated an active size controlled droplet generation system by using magnetically driven microtool (MMT). With a lateral motion of the MMT in microchannels, the continuous phase can be pinched off by the movement of MMT to obtain size-controlled droplets actively. With this method particle-enclosed droplet can be produced on demand to fit the size of each enclosed particle, and which is difficult to carry out by fluid dynamic force. For the current study, the system has been evaluated in terms of the frequency of MMT actuation and the size of droplets produced and which contribute to the effective transportation of cells in microchannel.

Hydrodynamic resistance of single confined moving drops in rectangular microchannels

We integrate a sensitive microfluidic comparator into a T-junction device and report measurements of the excess pressure drop due to a single moving droplet confined in a rectangular microchannel. We specifically focus on drops that are not coated with surfactants and study the effects of drop size, droplet viscosity and capillary number on their hydrodynamic resistance. In the capillary number range of approximately 10(-3)-10(-2), we find two distinct regimes for hydrodynamic resistance behavior based on drop size. In regime I associated with small drops (drop length/channel width approximately <4), we find that the pressure drop is independent of the drop size and the capillary number, and depends weakly on the ratio of the viscosities of the two immiscible phases. In regime II, associated with large drops (drop length/channel width > approximately 4), depending on the viscosity ratio of the two phases, the hydrodynamic resistance could increase, decrease or remain unchanged with drop size. We present a simple model that qualitatively captures these experimental trends. This model reveals that the pressure drop in regime I is dominated by the dissipation due to the end caps, and in regime II by both the end caps and the central body of the droplet. Such fundamental understanding will enable the design of large-scale energy-efficient fluidic circuits by minimizing the overall pressure drop in a network and may also provide insights into controlling droplet traffic to build functional passively-driven two-phase microfluidic technologies.

Microfluidic assisted synthesis of multi-functional polycaprolactone microcapsules: incorporation of CdTe quantum dots, Fe3O4 superparamagnetic nanoparticles and tamoxifen anticancer drugs

This paper demonstrates a proof-of-concept approach for encapsulating the anticancer drug tamoxifen, Fe3O4 nanoparticles (NPs) and CdTe quantum dots (QDs) into size-controlled polycaprolactone (PCL) microcapsules utilizing microfluidic emulsification, which combined magnetic targeting, fluorescence imaging and drug controlled release properties into one drug delivery system. Cross-linking the composite PCL microcapsules with poly(vinyl alcohol) (PVA) tailored their size, morphology, optical and magnetic properties and drug release behaviors. The flow conditions of the two immiscible solutions were adjusted in order to successfully generate various sizes of polymer droplets. The result showed superparamagnetic and fluorescent properties, and was used as a controlled drug release vehicle. The composite magnetic and fluorescent PCL microcapsules are potential candidates for a smart drug delivery system.

Microfluidic DNA amplification--a review

The application of microfluidic devices for DNA amplification has recently been extensively studied. Here, we review the important development of microfluidic polymerase chain reaction (PCR) devices and discuss the underlying physical principles for the optimal design and operation of the device. In particular, we focus on continuous-flow microfluidic PCR on-chip, which can be readily implemented as an integrated function of a micro-total-analysis system. To overcome sample carryover contamination and surface adsorption associated with microfluidic PCR, microdroplet technology has recently been utilized to perform PCR in droplets, which can eliminate the synthesis of short chimeric products, shorten thermal-cycling time, and offers great potential for single DNA molecule and single-cell amplification. The work on chip-based PCR in droplets is highlighted.

Microfluidic controlling monodisperse microdroplet for 5-fluorouracil loaded genipin-gelatin microcapsules

This paper demonstrates a proof-of-concept approach for producing genipin-gelatin microcapsules of precisely controlled and monodisperse size distributions by the microfluidic channels. We have demonstrated that one could control the size of emulsions from 130 microm to 580 microm in diameter (with a variation of less than 5%) by altering the relative sheath/sample flow rate ratio. In addition, Results show that the encapsulation and in vitro release of a model drug, 5-fluorouracil, to enhance the effect of controlled release. We demonstrated that the appropriate particle size for different release patterns is predictable, enabling better application of genipin-gelatin microcapsules as a drug carrier. The proposed microfluidic chip is capable of generating relatively uniform micro-droplets with well controllable diameter, and it has the added advantages of being a simple, low cost, and high throughput process.

Spectroscopic characterisation and manipulation of arrays of sub-picolitre aerosol droplets

Arrays of optically tweezed aerosol droplets, each of sub-picolitre volume, are manipulated by holographic optical tweezers and characterised by cavity enhanced Raman spectroscopy. A spatial light modulator is employed to generate arrays of optical traps from a single laser beam and to control the array dimensions and relative trap positions. Comparative hygroscopicity measurements are performed concurrently on five trapped droplets by monitoring the evolving size of each droplet. This is extended to the controlled coalescence of an array of droplets accompanied by spectroscopic measurements. These data represent the first ever simultaneous measurements of the evolving composition and size of an array of aerosol droplets. We consider the possibility of using aerosol arrays as a platform for studying chemical reactions in sub-picolitre volumes, exploiting the versatility of aerosol arrays for performing optical digital microfluidic operations accompanied by micro-total analysis.

Liquid-liquid phase separation: characterisation of a novel device capable of separating particle carrying multiphase flows

Capillary forces on the microscale are exploited to create a continuous flow liquid-liquid phase separator. Segmented flow regimes of immiscible fluids are generated and subsequently separated into their component phases through an array of high aspect ratio, laser machined, separation ducts (36 microm wide, 130 microm deep) in a planar, integrated, polytetrafluoroethylene (PTFE) microdevice. A controlled pressure differential across the phase separator architecture facilitates the selective passage of the wetting, organic, phase through the separator ducts, enabling separation of microfluidic multiphase flow streams. The reported device is demonstrated to separate water and chloroform segmented flow regimes at flow rates up to 0.4 ml min(-1). Separation efficiency is quantified over a range of flow rates and applied pressure differentials, characterising device behaviour and limits of operation. Experimental measurements and observations are supported by theoretical hydrodynamic and capillary pressure modelling. The influence of material properties and geometric design parameters on phase separation is quantified and optimisation strategies proposed. The novel ability of the membrane free device to separate an organic phase containing suspended microparticulates, from an aqueous phase, is also demonstrated.

Electrowetting of complex fluids: perspectives for rheometry on chip

We explore the possibilities of electrowetting (EW) as a tool to assess the elastic properties of aqueous jellifying materials present in the form of a small droplet on a hydrophobic substrate. We monitored the EW response of aqueous solutions of gelatin (2-10 wt %) in ambient oil for various temperatures (8-40 degrees C) below and above the gel point. Whereas the drops remained approximately spherical cap-shaped under all conditions, the voltage-induced reduction of the contact angle became progressively less pronounced upon entering the gel state at lower temperatures. We modeled the decrease in contact angle by minimizing the total energy of the drops consisting of interfacial energies, electrostatic energy, and the elastic energy due to the deformation of the drop, which was taken into account in a modified Hertz model. This allowed fitting the data and extracting the elastic modulus G, which were found to agree well with macroscopic storage moduli G' obtained with oscillatory shear rheometry. These results show that EW can be used as a tool for characterizing soft materials with the elastic moduli ranging (at least) from 10 to 1000 Pa. Our observations also create interesting perspectives for performing in situ rheological measurement inside microfluidic chips.

Microfluidic emulsification and sorting assisted preparation of monodisperse chitosan microparticles

A microfluidic device for generating monodisperse chitosan microparticles and separating the desired particle from smaller particles created as a byproduct of this process was described. The purpose of this study is to separate the satellite droplets from their parent droplets to enhance the size uniformity of the desired microparticles. A double T-junction design was first employed to control the emulsification and the separation, respectively. The results show that the size and gap of the parent droplets are tunable by adjusting the water and oil flow rates. A separation ratio of the satellite droplets of more than 99% was observed. The proposed microfluidic chip is capable of generating relatively uniform micro-droplets with well controllable diameter, and it has the added advantages of being a simple, low cost, and high throughput process. In the future this apparatus can be used to fabricate size-controlled monodisperse microparticles to act as drug carriers for biotechnology and biomedicine applications.

Microfluidic crystallization

Microfluidics offers a wide range of new tools that permit one to revisit the formation of crystals in solution and yield insights into crystallization processes. We review such recent microfluidic devices and particularly emphasize lab-on-chips dedicated to the high-throughput screening of crystallization conditions of proteins with nanolitre consumption. We also thoroughly discuss the possibilities offered by the microfluidic tools to acquire thermodynamic and kinetic data that may improve industrial processes and shed a new light on nucleation and growth mechanisms.

Nanodroplet real-time PCR system with laser assisted heating

We report the successful application of low-power (approximately 30 mW) laser radiation as an optical heating source for high-speed real-time polymerase chain reaction (PCR) amplification of DNA in nanoliter droplets dispersed in an oil phase. Light provides the heating, temperature measurement, and Taqman real-time readout in nanoliter droplets on a disposable plastic substrate. A selective heating scheme using an infrared laser appears ideal for driving PCR because it heats only the droplet, not the oil or plastic substrate, providing fast heating and completing the 40 cycles of PCR in 370 seconds. No microheaters or microfluidic circuitry were deposited on the substrate, and PCR was performed in one droplet without affecting neighboring droplets. The assay performance was quantitative and its amplification efficiency was comparable to that of a commercial instrument.

Microfluidics as a functional tool for cell mechanics

Living cells are a fascinating demonstration of nature's most intricate and well-coordinated micromechanical objects. They crawl, spread, contract, and relax-thus performing a multitude of complex mechanical functions. Alternatively, they also respond to physical and chemical cues that lead to remodeling of the cytoskeleton. To understand this intricate coupling between mechanical properties, mechanical function and force-induced biochemical signaling requires tools that are capable of both controlling and manipulating the cell microenvironment and measuring the resulting mechanical response. In this review, the power of microfluidics as a functional tool for research in cell mechanics is highlighted. In particular, current literature is discussed to show that microfluidics powered by soft lithographic techniques offers the following capabilities that are of significance for understanding the mechanical behavior of cells: (i) Microfluidics enables the creation of in vitro models of physiological environments in which cell mechanics can be probed. (ii) Microfluidics is an excellent means to deliver physical cues that affect cell mechanics, such as cell shape, fluid flow, substrate topography, and stiffness. (iii) Microfluidics can also expose cells to chemical cues, such as growth factors and drugs, which alter their mechanical behavior. Moreover, these chemical cues can be delivered either at the whole cell or subcellular level. (iv) Microfluidic devices offer the possibility of measuring the intrinsic mechanical properties of cells in a high throughput fashion. (v) Finally, microfluidic methods provide exquisite control over drop size, generation, and manipulation. As a result, droplets are being increasingly used to control the physicochemical environment of cells and as biomimetic analogs of living cells. These powerful attributes of microfluidics should further stimulate novel means of investigating the link between physicochemical cues and the biomechanical response of cells. Insights from such studies will have implications in areas such as drug delivery, medicine, tissue engineering, and biomedical diagnostics.

Micro- and nanofluidic systems for high-throughput biological screening

High-throughput screening (HTS) is a method of scientific experimentation widely used in drug discovery and relevant to the fields of biology. The development of micro- and nanofluidic systems for use in the biological sciences has been driven by a range of fundamental attributes that accompany miniaturization and massively parallel experimentation. We review recent advances in both arraying strategies based on nano/microfluidics and novel nano/microfluidic devices with high analytical throughput rates.

Development of a digital microfluidic platform for point of care testing

Point of care testing is playing an increasingly important role in improving the clinical outcome in health care management. The salient features of a point of care device are rapid results, integrated sample preparation and processing, small sample volumes, portability, multifunctionality and low cost. In this paper, we demonstrate some of these salient features utilizing an electrowetting-based Digital Microfluidic platform. We demonstrate the performance of magnetic bead-based immunoassays (cardiac troponin I) on a digital microfluidic cartridge in less than 8 minutes using whole blood samples. Using the same microfluidic cartridge, a 40-cycle real-time polymerase chain reaction was performed within 12 minutes by shuttling a droplet between two thermal zones. We further demonstrate, on the same cartridge, the capability to perform sample preparation for bacterial infectious disease pathogen, methicillin-resistant Staphylococcus aureus and for human genomic DNA using magnetic beads. In addition to rapid results and integrated sample preparation, electrowetting-based digital microfluidic instruments are highly portable because fluid pumping is performed electronically. All the digital microfluidic chips presented here were fabricated on printed circuit boards utilizing mass production techniques that keep the cost of the chip low. Due to the modularity and scalability afforded by digital microfluidics, multifunctional testing capability, such as combinations within and between immunoassays, DNA amplification, and enzymatic assays, can be brought to the point of care at a relatively low cost because a single chip can be configured in software for different assays required along the path of care.

Microfluidic polymer multilayer adsorption on liquid crystal droplets for microcapsule synthesis

Exploiting microfluidic principles, the potential for chip-based multilayer assembly for the synthesis of polymer microcapsules was investigated. We demonstrate that continuous flow microfluidic multilayer synthesis is a fast, efficient, automated alternative to conventional batch synthesis. In this work, we dispersed liquid crystal (LC) molecules (organic phase) as monodisperse droplets in an aqueous continuous phase containing the primary polymer and a suitable surfactant. The primary polymer was coadsorbed with the surfactant at the organic/aqueous interface, stabilizing the LC droplets against coalescence and providing a template for subsequent polymer adsorption. As the droplet templates are transported through the microfluidic channel, the polymer-containing aqueous continuous phase is selectively withdrawn and replaced with rinse solution, and then with an alternative polymer solution. This selective withdrawal and infusion cycle was repeated to assemble polymer multilayers onto the emulsion droplets. The process was followed using fluorescence microscopy of the fluorescently-labelled polymers at the LC interface and of the flowing polymer solutions during the sequential rinse stages. Cross-linking of the multilayers and removal of the dispersed LC phase resulted in polymer capsules retaining the high monodispersity of the droplet templates. This microfluidic approach significantly reduces the multilayer formation time (to <2 min for 3-layer capsules) of well-defined capsules that are envisaged to have benefits in biomedical applications, including drug delivery and encapsulated biochemical reactions.

Identification of mitochondrial disease genes through integrative analysis of multiple datasets

Determining the genetic factors in a disease is crucial to elucidating its molecular basis. This task is challenging due to a lack of information on gene function. The integration of large-scale functional genomics data has proven to be an effective strategy to prioritize candidate disease genes. Mitochondrial disorders are a prevalent and heterogeneous class of diseases that are particularly amenable to this approach. Here we explain the application of integrative approaches to the identification of mitochondrial disease genes. We first examine various datasets that can be used to evaluate the involvement of each gene in mitochondrial function. The data integration methodology is then described, accompanied by examples of common implementations. Finally, we discuss how gene networks are constructed using integrative techniques and applied to candidate gene prioritization. Relevant public data resources are indicated. This report highlights the success and potential of data integration as well as its applicability to the search for mitochondrial disease genes.

Heterogeneous immunoassays using magnetic beads on a digital microfluidic platform

A digital microfluidic platform for performing heterogeneous sandwich immunoassays based on efficient handling of magnetic beads is presented in this paper. This approach is based on manipulation of discrete droplets of samples and reagents using electrowetting without the need for channels where the droplets are free to move laterally. Droplet-based manipulation of magnetic beads therefore does not suffer from clogging of channels. Immunoassays on a digital microfluidic platform require the following basic operations: bead attraction, bead washing, bead retention, and bead resuspension. Several parameters such as magnetic field strength, pull force, position, and buffer composition were studied for effective bead operations. Dilution-based washing of magnetic beads was demonstrated by immobilizing the magnetic beads using a permanent magnet and splitting the excess supernatant using electrowetting. Almost 100% bead retention was achieved after 7776-fold dilution-based washing of the supernatant. Efficient resuspension of magnetic beads was achieved by transporting a droplet with magnetic beads across five electrodes on the platform and exploiting the flow patterns within the droplet to resuspend the beads. All the magnetic-bead droplet operations were integrated together to generate standard curves for sandwich heterogeneous immunoassays on human insulin and interleukin-6 (IL-6) with a total time to result of 7 min for each assay.

Manipulations of microfluidic droplets using electrorheological carrier fluid

Electrorheological (ER) fluids are a type of ''smart'' colloid capable of reversible viscosity variations, or even solidification, in response to an applied electric field. The response time can be as short as a few milliseconds. By using the ER fluid as the carrier fluid in microfluidic chips, we report the generation and manipulation of microdroplets and bubbles via integrated, digitally controlled micro-electrodes equipped with a feedback system. By utilizing the strong electric response of the ER fluid, the flow rate can be easily controlled digitally, thereby making tunable the size of the droplets generated and their separations. In particular, ordering change in a chain of droplets is demonstrated. The maneuverability presented in this paper may have potential applications in a variety of lab chips for chemical reactions, bioassays, as well as microfluidic logic computation.

Screening the cellular microenvironment: a role for microfluidics

The cellular microenvironment is an increasingly discussed topic in cell biology as it has been implicated in the progression of cancer and the maintenance of stem cells. The microenvironment of a cell is an organized combination of extracellular matrix (ECM), cells, and interstitial fluid that influence cellular phenotype through physical, mechanical, and biochemical mechanisms. Screening can be used to map combinations of cells and microenvironments to phenotypic outcomes in a way that can help develop more predictive in vitro models and to better understand phenotypic mechanisms from a systems biology perspective. This paper examines microenvironmental screening in terms of outcomes and benefits, key elements of the screening process, challenges for implementation, and a possible role for microfluidics as the screening platform. To assess microfluidics for use in microenvironmental screening, examples and categories of micro-scale and microfluidic technology are highlighted. Microfluidic technology shows promise for simultaneous control of multiple parameters of the microenvironment and can provide a base for scaling advanced cell-based experiments into automated high-throughput formats.

Formation of miscible fluid microstructures by hydrodynamic focusing in plane geometries

We experimentally investigate the flow structures formed when two miscible fluids that have large viscosity contrasts are injected and hydrodynamically focused in plane microchannels. Parallel viscous flows composed of a central stream surrounded by symmetric sheath streams are examined as a function of the flow rates, fluid viscosities, and rates of molecular diffusion. We study miscible interfacial morphologies and show a route for manipulating viscous flow-segregation processes in plane microsystems. The diffusion layer at the boundary of an ensheathed fluid grows as function of the distance downstream and depends on the Péclet number. In particular, we observe diffusion-enhanced viscous ensheathing processes. In the presence of a constriction, we investigate the formation of a lubricated viscous thread in the converging flow and also the buckling morphologies of the thread in the diverging flow. This study, relevant to multifluid flow between a "thick" material and a "thin" solvent, demonstrates the possibility to further control steady and oscillatory miscible fluid microstructures.

The electrochemical detection of droplets in microfluidic devices

This paper presents a new electrochemical method for the detection and characterisation of aqueous droplets in an organic carrier fluid (1,2-dichloroethane) formed in flow-focusing microfluidic devices. The devices consist of a conventional flow-focusing channel 250 microm wide and 250 microm deep cast out of poly(dimethylsiloxane) (PDMS) which is sealed onto a glass substrate containing a set of microelectrodes 100 microm long. Chronoamperometric analysis of a suitable electrolyte contained in the organic phase is presented for characterising the droplet frequency and size. This chronoamperometric method is then extended to a dual working electrode approach in order to determine the velocity of the droplet. Good agreement between experimental measurements and theory was observed.

Microscale LC-MS-NMR platform applied to the identification of active cyanobacterial metabolites

An LC-MS-NMR platform is demonstrated, which combines two innovations in microscale analysis, nanoSplitter LC-MS and microdroplet NMR, for the identification of unknown compounds found at low concentrations in complex sample matrixes as frequently encountered in metabolomics or natural products discovery. The nanoSplitter provides the high sensitivity of nanoelectrospray MS while allowing 98% of the HPLC effluent from a large-bore LC column to be collected and concentrated for NMR. Microdroplet NMR is a droplet microfluidic NMR loading method providing severalfold higher sample efficiency than conventional flow injection methods. Performing NMR offline from LC-UV-MS accommodates the disparity between MS and NMR in their sample mass and time requirements, as well as allowing NMR spectra to be requested retrospectively, after review of the LC-MS data. Interpretable 1D NMR spectra were obtained from analytes at the 200-ng level, in 1 h/well automated NMR data acquisitions. The system also showed excellent intra- and interdetector reproducibility with retention time RSD values less than 2% and sample recovery on the order of 93%. When applied to a cyanobacterial extract showing antibacterial activity, the platform recognized several previously known metabolites, down to the 1% level, in a single 30-mug injection, and prioritized one unknown for further study.

Droplet motion on designed microtextured superhydrophobic surfaces with tunable wettability

Superhydrophobic surfaces have shown promising applications in microfluidic systems as a result of their water-repellent and low-friction properties over the past decade. Recently, designed microstructures have been experimentally applied to construct wettability gradients and direct the droplet motion. However, thermodynamic mechanisms responsible for the droplet motion on such regular rough surfaces have not been well understood such that at present specific guidelines for the design of tunable superhydrophobic surfaces are not available. In this study, we propose a simple but robust thermodynamic methodology to gain thorough insight into the physical nature for the controllable motion of droplets. On the basis of the thermodynamic calculations of free energy (FE) and the free-energy barrier (FEB), the effects of surface geometry of a pillar microtexture are systematically investigated. It is found that decreasing the pillar width and spacing simultaneously is required to lower the advancing and receding FEBs to effectively direct droplets on the roughness gradient surface. Furthermore, the external energy plays a role in the actuation of spontaneous droplet motion with the cooperation of the roughness gradient. In addition, it is suggested that the so-called "virtual wall" used to confine the liquid flow along the undesired directions could be achieved by constructing highly advancing FEB areas around the microchannels, which is promising for the design of microfluidic systems.

Microfluidic production of droplet pairs

We study a microfluidic dual nozzle for the production of water-in-oil droplet pairs. Droplets are paired by the hydrodynamic coupling of two nozzles over a wide range of aqueous and oil flow rates provided that they are larger than the channel dimensions. The droplet production frequencies and volumes are related to the flow rates through a single, experimentally determined power-law. The data are in good agreement with a model based on a geometrical decomposition of the dual nozzle leading to a general equation of droplet frequencies as a function of the various flow rates.

Electrowetting-based microdrop tensiometer

We performed electrowetting (EW) contact angle measurements to determine the interfacial tension between aqueous drops laden with various inorganic and organic solutes and various ambient oils. Using low frequency AC voltage, we obtained interfacial tensions from 5 to 72 mJ/m 2, in close agreement with macroscopic tensiometry for drop volumes between 20 and 2000 nL. In addition to the conventional EW geometry, we demonstrate the possibility of performing "contact-less" measurements without any loss of accuracy using interdigitated coplanar electrodes.

Sampling and electrophoretic analysis of segmented flow streams using virtual walls in a microfluidic device

A method for sampling and electrophoretic analysis of aqueous plugs segmented in a stream of immiscible oil is described. In the method, an aqueous buffer and oil stream flow parallel to each other to form a stable virtual wall in a microfabricated K-shaped fluidic element. As aqueous sample plugs in the oil stream make contact with the virtual wall, coalescence occurs and sample is electrokinetically transferred to the aqueous stream. Using this virtual wall, two methods of injection for channel electrophoresis were developed. In the first, discrete sample zones flow past the inlet of an electrophoresis channel and a portion is injected by electroosmotic flow, termed the "discrete injector". With this approach at least 800 plugs could be injected without interruption from a continuous segmented stream with 5.1% RSD in peak area. This method generated up to 1,050 theoretical plates, although analysis of the injector suggested that improvements may be possible. In a second method, aqueous plugs are sampled in a way that allows them to form a continuous stream that is directed to a microfluidic cross-style injector, termed the "desegmenting injector". This method does not analyze each individual plug but instead allows periodic sampling of a high-frequency stream of plugs. Using this system at least 1000 injections could be performed sequentially with 5.8% RSD in peak area and 53,500 theoretical plates. This method was demonstrated to be useful for monitoring concentration changes from a sampling device with 10 s temporal resolution. Aqueous plugs in segmented flows have been applied to many different chemical manipulations including synthesis, assays, sampling processing and sampling. Nearly all such studies have used optical methods to analyze plug contents. This method offers a new way to analyze such samples and should enable new applications of segmented flow systems.

Digital bioanalysis

Digital microfluidics has recently emerged as a new paradigm in the world of lab-on-a-chip technology. A wide variety of bioanalyses have been successfully implemented in this format. This paper reviews the various techniques that have been adapted to digital microfluidic systems, and the current state of the field.

Droplet-based microfluidic system for individual Caenorhabditis elegans assay

A droplet-based microfluidic system integrating a droplet generator and a droplet trap array is described for encapsulating individual Caenorhabditis elegans into a parallel series of droplets, enabling characterization of the worm behavior in response to neurotoxin at single-animal resolution.

Novel one-pot route to monodisperse thermosensitive hollow microcapsules in a microfluidic system

We present a simple one-pot synthetic approach for the preparation of monodisperse thermo-sensitive poly(N-isopropylacrylamide) (PNIPAM) microcapsules in a microfluidic system. Based on the mechanism of shear force-driven break-off, aqueous droplets of monomer solution are continuously generated in an immiscible continuous phase containing photoinitiators. Under UV irradiation, activated initiators are diffused into the interface between the continuous phase and the aqueous droplets, which trigger polymerization of NIPAM monomers. The PNIPAM microcapsules produced are hollow microcapsules with a thin shell membrane, high monodispersity, and fast response to environmental temperature. In addition, the size of microcapsules produced can be manipulated by the flow rate of the continuous phase or aqueous phase and different concentrations of surfactant to control interfacial tension between continuous phase and aqueous phase. Furthermore, the versatility of this approach enables the preparation of monodisperse microcapsules having the capability to encapsulate various materials such as proteins and nanoparticles under mild conditions. The in situ microfluidic synthetic method provides a novel approach for the preparation of monodisperse hollow microcapsules via a one-pot route.

Controlled encapsulation of single-cells into monodisperse picolitre drops

Encapsulation of cells within picolitre-size monodisperse drops provides new means to perform quantitative biological studies on a single-cell basis for large cell populations. Variability in the number of cells per drop due to stochastic cell loading is a major barrier to these techniques. We overcome this limitation by evenly spacing cells as they travel within a high aspect-ratio microchannel; cells enter the drop generator with the frequency of drop formation.

Rapid microfabrication of solvent-resistant biocompatible microfluidic devices

This paper presents a rapid, simple, and low-cost fabrication method to prepare solvent resistant and biocompatible microfluidic devices with three-dimensional geometries. The devices were fabricated in thiolene and replicated from PDMS master with high molding fidelity. Good chemical compatibility for organic solvents allows volatile chemicals in synthesis and analysis applications. The surface can be processed to be hydrophobic or hydrophilic for water-in-oil and oil-in-water emulsions. Monodisperse organic solvent droplet generation is demonstrated to be reproducible in thiolene microchannels without swelling. The thiolene surface prevents cell adhesion but normal cell growth and adhesion on glass substrates is not affected by the adjacent thiolene patterns.