Droplet microfluidics

A microchip fabricated with a vapor-diffusion self-assembled-monolayer method to transport droplets across superhydrophobic to hydrophilic surfaces

A wettability gradient to transport a droplet across superhydrophobic to hydrophilic surfaces is fabricated on combining a structure gradient and a self-assembled-monolayer (SAM) gradient. The combination of these two gradients is realized with a simple but versatile SAM technique, in which the textured silicon wafer strip is placed vertically in a bottle that contains a decyltrichlorosilane solution to form concurrently a saturated SAM below the liquid surface and a wettability gradient above. The platform fabricated in this way has a water-contact angle from 151.2 degrees to 39.7 degrees; the self-transport distance is hence increased significantly to about 9 mm. A theoretical model that approximates the shape of a moving drop to a spheroidal cap is developed to predict the self-transport behavior. Satisfactory agreement is shown for most regions except where the hysteresis effect is unmeasurable and an unsymmetrical deformation occurs. A double-directional gradient surface to alter the direction of movement of a droplet is also realized. The platforms we developed serve not only to transport a fluid over a long distance but also for a broad spectrum of biomedical applications such as protein adsorption, cell adhesion and DNA-based biosensors.

Microfluidic tools for cell biological research

Microfluidic technology is creating powerful tools for cell biologists to control the complete cellular microenvironment, leading to new questions and new discoveries. We review here the basic concepts and methodologies in designing microfluidic devices, and their diverse cell biological applications.

Transverse acoustic trapping using a gaussian focused ultrasound

The optical tweezer has become a popular device to manipulate particles in nanometer scales and to study the underlying principles of many cellular or molecular interactions. Theoretical analysis was previously carried out at the authors' laboratory, to show that similar acoustic trapping of microparticles may be possible with a single beam ultrasound. This article experimentally presents the transverse trapping of 125 microm lipid droplets under an acoustically transparent mylar film, which is an intermediate step toward achieving acoustic tweezers in three-dimension. Despite the lack of axial trapping capability in the current experimental arrangement, it was found that a 30 MHz focused beam could be used to laterally direct the droplets toward the focus. The spatial range within which acoustic traps may guide droplet motion was in the range of hundreds of micrometers, much greater than that of optical traps. This suggests that this acoustic device may offer an alternative for manipulating microparticles in a wider spatial range.

Counting single molecules in sub-nanolitre droplets

We demonstrate single biomolecule detection and quantification within sub-nanolitre droplets through application of Cylindrical Illumination Confocal Spectroscopy (CICS) and droplet confinement within a retractable microfluidic constriction.

User-loaded SlipChip for equipment-free multiplexed nanoliter-scale experiments

This paper describes a microfluidic approach to perform multiplexed nanoliter-scale experiments by combining a sample with multiple different reagents, each at multiple mixing ratios. This approach employs a user-loaded, equipment-free SlipChip. The mixing ratios, characterized by diluting a fluorescent dye, could be controlled by the volume of each of the combined wells. The SlipChip design was validated on an approximately 12 nL scale by screening the conditions for crystallization of glutaryl-CoA dehydrogenase from Burkholderia pseudomallei against 48 different reagents; each reagent was tested at 11 different mixing ratios, for a total of 528 crystallization trials. The total consumption of the protein sample was approximately 10 microL. Conditions for crystallization were successfully identified. The crystallization experiments were successfully scaled up in well plates using the conditions identified in the SlipChip. Crystals were characterized by X-ray diffraction and provided a protein structure in a different space group and at a higher resolution than the structure obtained by conventional methods. In this work, this user-loaded SlipChip has been shown to reliably handle fluids of diverse physicochemical properties, such as viscosities and surface tensions. Quantitative measurements of fluorescent intensities and high-resolution imaging were straighforward to perform in these glass SlipChips. Surface chemistry was controlled using fluorinated lubricating fluid, analogous to the fluorinated carrier fluid used in plug-based crystallization. Thus, we expect this approach to be valuable in a number of areas beyond protein crystallization, especially those areas where droplet-based microfluidic systems have demonstrated successes, including measurements of enzyme kinetics and blood coagulation, cell-based assays, and chemical reactions.

A PMMA microfluidic droplet platform for in vitro protein expression using crude E. coli S30 extract

Droplet based microfluidics are promising new tools for biological and chemical assays. In this paper, a high throughput and high sensitivity microfluidic droplet platform is described for in vitro protein expression using crude Escherichia coli S30 extract. A flow-focusing polymethylmethacrylate (PMMA) microchip was designed and integrated with different functions involving droplet generation, storage, separation and detection. The material used for the chip is superior to the previously tested polydimethylsiloxane (PDMS) due to its mechanical and chemical properties. Droplet formation characteristics such as size and generation rate are investigated systematically. The effect of surfactants Abil EM90 and Span80 in the oil phase on droplet formation and optical detection is also studied. The performance of the system is demonstrated by the high throughput and stable droplet generation and ultralow detection limit. The robustness of the system is also demonstrated by the successful synthesis of a green fluorescent protein (GFP) using E. coli S30 extract as a source of RNA translation reagents.

Simultaneous determination of gene expression and enzymatic activity in individual bacterial cells in microdroplet compartments

A microfluidic device capable of storing picoliter droplets containing single bacteria at constant volumes has been fabricated in PDMS. Once captured in droplets that remain static in the device, bacteria express both a red fluorescent protein (mRFP1) and the enzyme, alkaline phosphatase (AP), from a biscistronic construct. By measuring the fluorescence intensity of both the mRFP1 inside the cells and a fluorescent product formed as a result of the enzymatic activity outside the cells, gene expression and enzymatic activity can be simultaneously and continuously monitored. By collecting data from many individual cells, the distribution of activities in a cell is quantified and the difference in activity between two AP mutants is measured.

On-chip manipulation of continuous picoliter-volume superparamagnetic droplets using a magnetic force

A microfluidic device for generating monodisperse superparamagnetic droplets and rapidly manipulating desired droplets into designated sub-microchannels by an external magnetic force is described. Superparamagnetic magnetite (Fe3O4) nanoparticles are synthesized by a chemical co-precipitation method. They are well dispersed in the water-phase to form a superparamagnetic fluid that is sheared into picoliter-volume monodisperse superparamagnetic droplets by the oil-phase in a T-junction PDMS microchannel. Superparamagnetic droplets always flow into sub-microchannel 1 due only to laminar flow without a magnetic field. But they are deflected from the direction of laminar flow by a perpendicular magnetic field. The results show that the deflection is proportional to the magnetic field gradient and magnetic nanoparticle concentration, and it is closely related to the magnet position. The observed experimental results make a good match with theoretical analysis. Single or bulk superparamagnetic droplets are successfully manipulated into the designated sub-microchannels 2 and 3 respectively, only by changing the positions of the magnet. Relatively high efficiency is obtained with more than 10 superparamagnetic droplets precisely manipulated per second. This simple and robust apparatus has wide applications in high throughput drug delivery/screening, immunoassay, cell research and synthesis of magnetic microparticles due to good biological compatibility and monodispersity of superparamagnetic droplets.

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.

Nonviral gene vector formation in monodispersed picolitre incubator for consistent gene delivery

A novel picolitre incubator based microfluidic system for consistent nonviral gene carrier formulation is presented. A cationic lipid-based carrier is the most attractive nonviral solution for delivering plasmid DNA, shRNA, or drugs for pharmaceutical research and RNAi applications. The size of the cationic lipid and DNA complex (CL-DNA), or the lipoplex, is one of the important variations for consistency of gene transfection. CL-DNA size, in turn, may be controlled by factors such as the cationic lipid and DNA mixing order, mixing rate, and mixture incubation time. The Picolitre Microfluidic Reactor and Incubator (PMRI) system described here is able to control these parameters in order to create homogeneous CL-DNA. Compared with conventional CL-DNA preparation techniques involving hand-shaking or vortexing, the PMRI system demonstrates a greater ability to constantly and uniformly mix cationic lipids and DNA simultaneously. After mixing in the picolitre droplet reactors, the cationic lipid and DNA is incubated within the picolitre incubator to form CL-DNA. The PMRI generates a narrower size distribution band, while also turning the sample loading, mixing and incubation steps into an integrated process enabling the consistent formation of CL-DNA. The coefficient of variation (CV) of transfection efficiency is 0.05 and 0.30 for PMRI-based and conventional methods, respectively. In addition, this paper demonstrates that the gene transfection efficiency of lipoplex created in the PMRI is more reproducible.

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.

Beating Poisson encapsulation statistics using close-packed ordering

Loading drops with discrete objects, such as particles and cells, is often necessary when performing chemical and biological assays in microfluidic devices. However, random loading techniques are inefficient, yielding a majority of empty and unusable drops. We use deformable particles that are close packed to insert a controllable number of particles into every drop. This provides a simple, flexible means of efficiently encapsulating a controllable number of particles per drop.

Surface molecular property modifications for poly(dimethylsiloxane) (PDMS) based microfluidic devices

Fast advancements of microfabrication processes in past two decades have reached to a fairly matured stage that we can manufacture a wide range of microfluidic devices. At present, the main challenge is the control of nanoscale properties on the surface of lab-on-a-chip to satisfy the need for biomedical applications. For example, poly(dimethylsiloxane) (PDMS) is a commonly used material for microfluidic circuitry, yet the hydrophobic nature of PDMS surface suffers serious nonspecific protein adsorption. Thus the current major efforts are focused on surface molecular property treatments for satisfying specific needs in handling macro functional molecules. Reviewing surface modifications of all types of materials used in microfluidics will be too broad. This review will only summarize recent advances in nonbiofouling PDMS surface modification strategies applicable to microfluidic technology and classify them into two main categories: (1) physical approach including physisorption of charged or amphiphilic polymers and copolymers, as well as (2) chemical approach including self assembled monolayer and thick polymer coating. Pros and cons of a collection of available yet fully exploited surface modification methods are briefly compared among subcategories.

Eletrowetting effect in a nanoporous silica

In the past, electrowetting was usually analyzed on large solid surfaces. In the current study, the effective solid-liquid interfacial tension in a nanoporous silica, which is measured by the ion transport pressure, is investigated experimentally. The interfacial tension decreases as the applied potential difference increases, while the magnitude of variation is much smaller than its bulk counterpart. The effect of the external electric field is saturated at a relatively low voltage. These unique phenomena can be attributed to the confinement effect of nanopore walls.

Single beam acoustic trapping

A single beam acoustic device, with its relatively simple scheme and low intensity, can trap a single lipid droplet in a manner similar to optical tweezers. Forces in the order of hundreds of nanonewtons direct the droplet toward the beam focus, within the range of hundreds of micrometers. This trapping method, therefore, can be a useful tool for particle manipulation in areas where larger particles or forces are involved.

Single beam acoustic trapping

A single beam acoustic device, with its relatively simple scheme and low intensity, can trap a single lipid droplet in a manner similar to optical tweezers. Forces in the order of hundreds of nanonewtons direct the droplet toward the beam focus, within the range of hundreds of micrometers. This trapping method, therefore, can be a useful tool for particle manipulation in areas where larger particles or forces are involved.

Deformation and breakup of high-viscosity droplets with symmetric microfluidic cross flows

The dynamic response of highly viscous droplets to a sharp increase in the surrounding liquid velocity is experimentally investigated in a square microchannel junction. The local injection of the continuous phase from symmetric side channels onto a train of droplets produces a large velocity contrast between the front and the rear of droplets, yielding a broad range of time-dependent deformation and breakup. In particular, due to microscale confinement, the system displays a nonlinear behavior with the initial droplet size. Deformations, relaxation times, and fragmentation processes are examined as a function of flow parameters and fluids properties with emphasis on the formation of slender viscous structures such as spoon-shaped droplets, i.e., asymmetrical droplets.

Microfluidic system for controlled gelation of a thermally reversible hydrogel

The integration of cell culture and characterization onto a miniaturized platform promises to benefit many applications such as tissue engineering, drug screening, and those involving small, precious cell populations. This paper presents the controlled on-chip gelation of a thermally-reversible hydrogel. Channel design and flowrate control are crucial in determining hydrogel geometry, while integrated temperature control triggers reversible gel formation. Formation of hydrogel droplets through shearing of immiscible flows is demonstrated with subsequent on-chip gelation. The temperature of phase transition occurs between 32degC-34degC, well within the range for mammalian cell encapsulation and culture.

Impact of inlet channel geometry on microfluidic drop formation

We study the impact of inlet channel geometry on microfluidic drop formation. We show that drop makers with T-junction style inlets form monodisperse emulsions at low and moderate capillary numbers and those with Flow-Focus style inlets do so at moderate and high capillary numbers. At low and moderate capillary number, drop formation is dominated by interfacial forces and mediated by the confinement of the microchannels; drop size as a function of flow-rate ratio follows a simple functional form based on a blocking-squeezing mechanism. We summarize the stability of the drop makers with different inlet channel geometry in the form of a phase diagram as a function of capillary number and flow-rate ratio.

Preparation of evaporation-resistant aqueous microdroplet arrays as a model system for the study of molecular order at the liquid/air interface

Aqueous arrays of microdroplets typically sized between 2 and 10 microm were generated by microfluid contact printing and stabilized with respect to evaporation by incorporation of poly(ethylene oxide). The arrays are used as a model system for the study of structure formation at liquid/air or liquid/liquid interfaces. In particular, we demonstrated the self-organization of fatty acids with photopolymerizable diacetylene units (10,12-pentacosadiynoic acid) at the liquid/air interface of the microdroplets. Topochemical polymerization behavior of this compound and the autofluorescence property of the resulting polyconjugated polymer are appropriate features to prove the molecular order of the amphiphilic molecules at the interface.

Inkjet formation of unilamellar lipid vesicles for cell-like encapsulation

Encapsulation of macromolecules within lipid vesicles has the potential to drive biological discovery and enable development of novel, cell-like therapeutics and sensors. However, rapid and reliable production of large numbers of unilamellar vesicles loaded with unrestricted and precisely-controlled contents requires new technologies that overcome size, uniformity, and throughput limitations of existing approaches. Here we present a high-throughput microfluidic method for vesicle formation and encapsulation using an inkjet printer at rates up to 200 Hz. We show how multiple high-frequency pulses of the inkjet's piezoelectric actuator create a microfluidic jet that deforms a bilayer lipid membrane, controlling formation of individual vesicles. Variations in pulse number, pulse voltage, and solution viscosity are used to control the vesicle size. As a first step toward cell-like reconstitution using this method, we encapsulate the cytoskeletal protein actin and use co-encapsulated microspheres to track its polymerization into a densely entangled cytoskeletal network upon vesicle formation.

Acoustically driven programmable liquid motion using resonance cavities

Performance and utility of microfluidic systems are often overshadowed by the difficulties and costs associated with operation and control. As a step toward the development of a more efficient platform for microfluidic control, we present a distributed pressure generation scheme whereby independently tunable pressure sources can be simultaneously controlled by using a single acoustic source. We demonstrate how this scheme can be used to perform precise droplet positioning as well as merging, splitting, and sorting within open microfluidic networks. We further show how this scheme can be implemented for control of continuous-flow systems, specifically for generation of acoustically tunable liquid gradients. Device operation hinges on a resonance-decoding and rectification mechanism by which the frequency content in a composite acoustic input is decomposed into multiple independently buffered output pressures. The device consists of a bank of 4 uniquely tuned resonance cavities (404, 484, 532, and 654 Hz), each being responsible for the actuation of a single droplet, 4 identical flow-rectification structures, and a single acoustic source. Cavities selectively amplify resonant tones in the input signal, resulting in highly elevated local cavity pressures. Fluidic-rectification structures then serve to convert the elevated oscillating cavity pressures into unidirectional flows. The resulting pressure gradients, which are used to manipulate fluids in a microdevice, are tunable over a range of approximately 0-200 Pa with a control resolution of 10 Pa.

Microfluidic emulsions with dynamic compound drops

We demonstrate a new class of microfluidic emulsion where the 'drops' of the emulsion are dynamic reversible bubble-drop pairs, with potential applications in microfluidic technology for chemical synthesis, molecular separations and screening.

In situ formation, manipulation, and imaging of droplet-encapsulated fibrin networks

The protein fibrin plays a principal role in blood clotting and forms robust three dimensional networks. Here, microfluidic devices have been tailored to strategically generate and study these bionetworks by confinement in nanoliter volumes. The required protein components are initially encapsulated in separate droplets, which are subsequently merged by electrocoalescence. Next, distinct droplet microenvironments are created as the merged droplets experience one of two conditions: either they traverse a microfluidic pathway continuously, or they "park" to fully evolve an isotropic network before experiencing controlled deformations. High resolution fluorescence microscopy is used to image the fibrin networks in the microchannels. Aggregation (i.e."clotting") is significantly affected by the complicated flow fields in moving droplets. In stopped-flow conditions, an isotropic droplet-spanning network forms after a suitable ripening time. Subsequent network deformation, induced by the geometric structure of the microfluidic channel, is found to be elastic at low rates of deformation. A shape transition is identified for droplets experiencing rates of deformation higher than an identified threshold value. In this condition, significant densification of protein within the droplet due to hydrodynamic forces is observed. These results demonstrate that flow fields considerably affect fibrin in different circumstances exquisitely controlled using microfluidic tools.

Oil droplet generation in PDMS microchannel using an amphiphilic continuous phase

This paper reports an amphiphilic solution can be used as a new continuous phase to generate double droplet emulsions (water/oil/IPA) with neither surface treatment nor surfactant in PDMS microfluidic chip. The affinity of various amphiphilic solutions in the microchannel was influenced by the polarity ratio and the size of molecules. The polarity ratio of isopropyl alcohol (IPA) was closest to that of the recovered PDMS surface and the chain length of IPA was also suitable for high affinity. IPA showed the highest affinity for the recovered PDMS and was selected as the continuous phase to form oil droplets in a PDMS microchannel. With this new continuous phase solution, IPA, we could successfully generate not only oil droplets but also double emulsions in the PDMS microfluidic chips.

Controllable microfluidic synthesis of multiphase drug-carrying lipospheres for site-targeted therapy

We report the production of micrometer-sized gas-filled lipospheres using digital (droplet-based) microfluidics technology for chemotherapeutic drug delivery. Advantages of on-chip synthesis include a monodisperse size distribution (polydispersity index (sigma) values of <5%) with consistent stability and uniform drug loading. Photolithography techniques are applied to fabricate novel PDMS-based microfluidic devices that feature a combined dual hydrodynamic flow-focusing region and expanding nozzle geometry with a narrow orifice. Spherical vehicles are formed through flow-focusing by the self-assembly of phospholipids to a lipid layer around the gas core, followed by a shear-induced break off at the orifice. The encapsulation of an extra oil layer between the outer lipid shell and inner bubble gaseous core allows the transport of highly hydrophobic and toxic drugs at high concentrations. Doxorubicin (Dox) entrapment is estimated at 15 mg mL(-1) of particles packed in a single ordered layer. In addition, the attachment of targeting ligands to the lipid shell allows for direct vehicle binding to cancer cells. Preliminary acoustic studies of these monodisperse gas lipospheres reveal a highly uniform echo correlation of greater than 95%. The potential exists for localized drug concentration and release with ultrasound energy.

Catalytic microtubular jet engines self-propelled by accumulated gas bubbles

Strain-engineered microtubes with an inner catalytic surface serve as self-propelled microjet engines with speeds of up to approximately 2 mm s(-1) (approximately 50 body lengths per second). The motion of the microjets is caused by gas bubbles ejecting from one opening of the tube, and the velocity can be well approximated by the product of the bubble radius and the bubble ejection frequency. Trajectories of various different geometries are well visualized by long microbubble tails. If a magnetic layer is integrated into the wall of the microjet engine, we can control and localize the trajectories by applying external rotating magnetic fields. Fluid (i.e., fuel) pumping through the microtubes is revealed and directly clarifies the working principle of the catalytic microjet engines.

Simultaneous measurement of reactions in microdroplets filled by concentration gradients

This work describes a technology for performing and monitoring simultaneously several reactions confined in strings of microdroplets having identical volumes but different composition, and travelling with the same speed in parallel channels of a microfluidic device. This technology, called parallel microdroplets technology (PmicroD), uses an inverted optical microscope and a charge-coupled device (CCD) camera to collect images and analyze them so as to report on the reactions occurring in these microdroplets. A concentration gradient of one reactant is created in the microfluidic device. In each channel, a different concentration of this reactant is mixed with a fixed amount of a second reactant. Using planar flow-focusing methodology, these mixtures are confined in microdroplets of pL size which travel in oil as continuous medium, avoiding laminar dispersion. By analyzing the images of parallel strings of microdroplets, the time courses of several reactions with different reagent compositions are investigated simultaneously. In order to design the microfluidic device that consists in a complex network of channels having well-defined geometries and restricted positions, the theoretical concept of equivalent channels (i.e. channels having identical hydraulic resistance) is exploited and developed. As a demonstration of the PmicroD technology, an enzyme activity assay was carried out and the steady-state kinetic constants were determined.

EWOD-driven droplet microfluidic device integrated with optoelectronic tweezers as an automated platform for cellular isolation and analysis

We report the integration of two technologies: droplet microfluidics using electrowetting-on-dielectric (EWOD) and individual particle manipulation using optoelectronic tweezers (OET)-in one microfluidic device. The integrated device successfully demonstrates a sequence involving both EWOD and OET operations. We encountered various challenges during integration of the two different technologies and present how they are addressed. To show the applicability of the device in cellular biology, live HeLa cells are used in the experiments. The unique advantages of EWOD and OET make their integration a significant step towards a powerful tool for many applications, such as single cell studies involving multiplexed environmental stimuli.