Droplet-based microfluidic device for multiple-droplet clustering

We present a multiple-droplet clustering device that can perform sequential droplet trapping and storing. Shape-dependent droplet manipulation in forward and backward flows has been incorporated to achieve high trapping and storing efficiency in a 10 × 12 array of clustering structures (e.g., storing well, storing chamber, trapping well, and guiding track). In the forward flow, flattened droplets are trapped in each trapping well. In the backward flow, the trapped droplets are released from the trapping well and follow the guiding tracks to their corresponding storing wells. The guided droplets float up out of the confining channel to the super stratum of the storing chamber due to interfacial energy and buoyancy effects. This forward/backward flow-based trapping/storing process can be repeated several times to cluster droplets with different contents and samples in the storing chambers. We expect that the proposed platform will be a valuable tool to study complex droplet-based reactions in clustered droplets.     

Droplet-based microfluidic lab-on-a-chip for glucose detection

A microfluidic lab-on-a-chip (LoC) platform for in vitro measurement of glucose for clinical diagnostic applications is presented in this paper. The LoC uses a discrete droplet format in contrast to conventional continuous flow microfluidic systems. The droplets act as solution-phase reaction chambers and are manipulated using the electrowetting effect. Glucose is measured using a colorimetric enzyme-kinetic method based on Trinder’s reaction. The color change is detected using an absorbance measurement system consisting of a light emitting diode and a photodiode. The linear range of the assay is 9-100 mg/dl using a sample dilution factor of 2 and 15-300 mg/dl using a sample dilution factor of 3. The results obtained on the electrowetting system compare favorably with conventional measurements done on a spectrophotometer, indicating that there is no change in enzyme activity under electrowetting conditions.     

Drop-based microfluidic devices for encapsulation of single cells

We use microfluidic devices to encapsulate, incubate, and manipulate individual cells in picoliter aqueous drops in a carrier fluid at rates of up to several hundred Hz. We use a modular approach with individual devices for each function, thereby significantly increasing the robustness of our system and making it highly flexible and adaptable to a variety of cell-based assays. The small volumes of the drops enables the concentrations of secreted molecules to rapidly attain detectable levels. We show that single hybridoma cells in 33 pL drops secrete detectable concentrations of antibodies in only 6 h and remain fully viable. These devices hold the promise of developing microfluidic cell cytometers and cell sorters with much greater functionality, allowing assays to be performed on individual cells in their own microenvironment prior to analysis and sorting.     

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.     

A microfluidic photolithography for controlled encapsulation of single cells inside hydrogel microstructures

A microfluidic approach to generate hydrogel microstructures inside microchannels for controlled encapsulation of single cells was developed. The method was based on a modified microscope projection photolithography which allowed for the photopolymerization of poly(ethylene glycol) diacrylate (PEG-DA) inside microchannels. Uniform-sized hydrogel microstructures (~50 μm in diameter) were generated one by one with determined positions to encapsulate single cells without losing the viability. Cells of interest could be identified by any kinds of visible labels to be selectively encapsulated inside the formed hydrogel microstructures. Large-scale encapsulation of single cells was achieved with a relatively high efficiency of 80% and the viability of encapsulated cells could be guaranteed by removing the dead cells identified with Trypan blue. This method is simple, fast and convenient to pattern the microchannels with single cells for a wide range of cell-based applications. For demonstration, two intracellular enzyme assays of carboxylesterase were performed to investigate the distribution of enzyme concentrations and the kinetic information within the encapsulated single HepG2 cells.     

Integration of microcolumns and microfluidic fractionators on multitasking centrifugal microfluidic platforms for the analysis of biomolecules

This work demonstrates the development of microfluidic compact discs (CDs) for protein purification and fractionation integrating a series of microfluidic features, such as microreservoirs, microchannels, and microfluidic fractionators. The CDs were fabricated with polydimethylsiloxane (PDMS), and each device contained multiple identical microfluidic patterns. Each pattern employed a microfluidic fractionation feature with operation that was based on the redirection of fluid into an isolation chamber as a result of an overflow. This feature offers the advantage of automated operation without the need for any external manipulation, which is independent of the size and the charge of the fractionated molecules. The performance of the microfluidic fractionator was evaluated by its integration into a protein purification microfluidic architecture. The microfluidic architecture employed a microchamber that accommodated a monolithic microcolumn, the fractionator, and an isolation chamber, which was also utilized for the optical detection of the purified protein. The monolithic microcolumn was polymerized “in situ” on the CD from a monolith precursor solution by microwave-initiated polymerization. This technique enabled the fast, efficient, and simultaneous polymerization of monoliths on disposable CD microfluidic platforms. The design of the CD employed allows the integration of various processes on a single microfluidic device, including protein purification, fractionation, isolation, and detection.      

Controlled microfluidic encapsulation of cells, proteins, and microbeads in lipid vesicles

Cells have been encapsulated inside lipid vesicles by using a new microfluidic lipid vesicle formulation technique. Lipid vesicles are formulated within minutes without using toxic lipid solvents. The encapsulation efficiency inside the vesicles is controlled by the microfluidic flows. Green fluorescent proteins (GFP), carcinoma cells, and bead encapsulated vesicles have mean diameters of 27.2 mum, 62.4 mum, and 55.9 mum, respectively. The variations of vesicle sizes are approximately 20% for the GFP and cell encapsulated vesicles and approximately 10% for the bead encapsulated vesicles.     

Reorientation of microfluidic channel enables versatile dielectrophoretic platforms for cell manipulations

Dielectrophoresis is a versatile tool for the sorting, immobilization, and characterization of cells in microfluidic systems. The performance of dielectrophoretic systems strongly relies on the configuration of microelectrodes, which produce a nonuniform electric field. However, once fabricated, the microelectrodes cannot be reconfigured to change the characteristics of the system. Here, we show that the reorientation of the microfluidic channel with respect to the microelectrodes can be readily utilized to alter the characteristics of the system. This enables us to change the location and density of immobilized viable cells across the channel, release viable cells along customized numbers of streams within the channel, change the deflection pattern of nonviable cells along the channel, and improve the sorting of viable and nonviable cells in terms of flow throughput and efficiency of the system. We demonstrate that the reorientation of the microfluidic channel is an effective tool to create versatile dielectrophoretic platforms using the same microelectrode design.     

Emerging trends in miniaturized and microfluidic electrochemical sensing platforms

Electrochemical sensing has established a strong presence in diverse areas. The conventional electrochemical sensing approach consumes large sample volumes and reagents and requires bulky potentiostat, macro-electrodes, and other equipment. The synergistic integration of electrochemical sensing systems with miniaturized or microfluidic electrochemical devices and microelectrodes in a single platform provides rapid analysis with a disposable, reusable, and cost-effective platform for multiplexed point-of-care detections. Such microdevices have created scope for using several materials as electrodes and sensing platforms by using appropriate fabrication techniques. One of the most recent advancements in miniaturized devices includes the integration of automation and Internet of Things to realize fully automated and robust electrochemical microdevices. The review summarizes the emerging trends in fabrication methods of miniaturized and microfluidic devices, their multiple applications in real-time, integration of Internet of Things, automation, identifying research gaps with strategies for bridging these gaps, future outlook, and recent approaches to intelligent electrochemical sensing.     

A microfluidic platform for pharmaceutical salt screening

We describe a microfluidic platform comprised of 48 wells to screen for pharmaceutical salts. Solutions of pharmaceutical parent compounds (PCs) and salt formers (SFs) are mixed on-chip in a combinatorial fashion in arrays of 87.5-nanolitre wells, which constitutes a drastic reduction of the volume of PC solution needed per condition screened compared to typical high throughput pharmaceutical screening approaches. Nucleation and growth of salt crystals is induced by diffusive and/or convective mixing of solutions containing, respectively, PCs and SFs in a variety of solvents. To enable long term experiments, solvent loss was minimized by reducing the thickness of the absorptive polymeric material, polydimethylsiloxane (PDMS), and by using solvent impermeable top and bottom layers. Additionally, well isolation was enhanced via the incorporation of pneumatic valves that are closed at rest. Brightfield and polarized light microscopy and Raman spectroscopy were used for on-chip analysis and crystal identification. Using a gold-coated glass substrate and minimizing the thickness of the PDMS control layer drastically improved the signal-to-noise ratio for Raman spectra. Two drugs, naproxen (acid) and ephedrine (base), were used for validation of the platform's ability to screen for salts. Each PC was mixed combinatorially with potential SFs in a variety of solvents. Crystals were visualized using brightfield polarized light microscopy. Subsequent on-chip analyses of the crystals with Raman spectroscopy identified four different naproxen salts and five different ephedrine salts.     

Artificial intelligence-based microfluidic platforms for the sensitive detection of environmental pollutants: Recent advances and prospects

Environmental pollution and its drastic effects on human and animal health have urged governments to implement strict policies to minimize damage. The first step in applying such policies is to find reliable methods to detect pollution in various media, including water, food, soil, and air. In this regard, various approaches such as spectrophotometric, chromatographic, and electrochemical techniques have been proposed. To overcome the limitations associated with conventional analytical methods, microfluidic devices have emerged as sensitive technologies capable of generating high content information during the past few years. The passage of contaminant samples through the microfluidic channels provides essential details about the whole environment after detection by the detector. In the meantime, artificial intelligence is an ideal means to identify, classify, characterize, and even predict the data obtained from microfluidic systems. The development of microfluidic devices with integrated machine learning and artificial intelligence is promising for the development of next-generation monitoring systems. Combination of the two systems ensures time efficient setups with easy operation. This review article is dedicated to the recent developments in microfluidic chips coupled with artificial intelligence technology for the evolution of more convenient pollution monitoring systems.     

Hydrogels for encapsulation of mammalian cells

In our attempt to prepare new hydrogels for cell encapsulation, we decided to use synthetic copolymers presuming that chemical bonds should bring better mechanical stability of capsules than electrostatic forces between polyanions and polycations in the so far predominantly used materials based on polyelectrolyte complexes. Two series of copolymers of 2‐hydroxyethyl methacrylate (HEMA) with alkyl acrylates (methacrylates) or N‐alkylacrylamides (methacrylamides) were prepared. The copolymers, prepared by solution copolymerization in ethanol, were characterized by swelling in water and PBS solution, viscometric molecular weight, and NMR analysis, and their encapsulation processability was tested. Their characteristics and the first biological tests are the subject of this paper.     

Continuous cytometric bead processing within a microfluidic device for bead based sensing platforms

A microfluidic device for continuous biosensing based on analyte binding with cytometric beads is introduced. The operating principle of the continuous biosensing is based on a novel concept named the "particle cross over" mechanism in microfluidic channels. By carefully designing the microfluidic network the beads are able to "cross-over" from a carrier fluid stream into a recipient fluid stream without mixing of the two streams and analyte dilution. After crossing over into the recipient stream, bead processing such as analyte-bead binding may occur. The microfluidic device is composed of a bead solution inlet, an analyte solution inlet, two washing solution inlets, and a fluorescence detection window. To achieve continuous particle cross over in microfluidic channels, each microfluidic channel is precisely designed to allow the particle cross over to occur by conducting a series of studies including an analogous electrical circuit study to find optimal fluidic resistances, an analytical determination of device dimensions, and a numerical simulation to verify microflow structures within the microfluidic channels. The functionality of the device was experimentally demonstrated using a commercially available fluorescent biotinylated fluorescein isothiocyanate (FITC) dye and streptavidin coated 8 microm-diameter beads. After, demonstrating particle cross over and biotin-streptavidin binding, the fluorescence intensity of the 8 microm-diameter beads was measured at the detection window and linearly depends on the concentration of the analyte (biotinylated FITC) at the inlet. The detection limit of the device was a concentration of 50 ng ml(-1) of biotinylated FITC.     

Interfacing multicellular organisms with polyelectrolyte shells and nanoparticles: a Caenorhabtidis elegans study

We report the surface modification of microscopic live multicellular nematodes Caenorhabtidis elegans with polyelectrolyte multilayers (pure and doped with 20 nm gold nanoparticles) and the direct magnetic functionalization of nematodes with biocompatible magnetic nanoparticles. Magnetically functionalized "ironoxideclad" nematodes can be effectively separated and moved using an external magnetic field. The surface-functionalized nematodes preserve their viability and reproduction.     

Droplet-based microreactor for the production of micro/nano-materials

Droplet-based microreactors are of great interest to researchers due to their incredible ability in the synthesis of micro/nano-materials with multi-function and complex geometry. In recent years, a broad range of micro/nano-materials has been synthesized in droplet-based microreactors, which provide apparent advantages, such as better reproducibility, reliable automation, and accurate manipulation. In this review, we give a comprehensive and in-depth insight into droplet-based microreactors, covering fundamental research from droplet generation and manipulation to the applications of droplet-based microreactors in micro/nano-material generation. We also explore the outlook for droplet-based microreactors and challenges that lie ahead and give a possible effort direction. We hope this review will promote communications among researchers and entrepreneurs.     

A programmable microfluidic cell array for combinatorial drug screening

We describe the development of a fully automatic and programmable microfluidic cell culture array that integrates on-chip generation of drug concentrations and pair-wise combinations with parallel culture of cells for drug candidate screening applications. The device has 64 individually addressable cell culture chambers in which cells can be cultured and exposed either sequentially or simultaneously to 64 pair-wise concentration combinations of two drugs. For sequential exposure, a simple microfluidic diffusive mixer is used to generate different concentrations of drugs from two inputs. For generation of 64 pair-wise combinations from two drug inputs, a novel time dependent variable concentration scheme is used in conjunction with the simple diffusive mixer to generate the desired combinations without the need for complex multi-layer structures or continuous medium perfusion. The generation of drug combinations and exposure to specific cell culture chambers are controlled using a LabVIEW interface capable of automatically running a multi-day drug screening experiment. Our cell array does not require continuous perfusion for keeping cells exposed to concentration gradients, minimizing the amount of drug used per experiment, and cells cultured in the chamber are not exposed to significant shear stress continuously. The utility of this platform is demonstrated for inducing loss of viability of PC3 prostate cancer cells using combinations of either doxorubicin or mitoxantrone with TRAIL (TNF-alpha Related Apoptosis Inducing Ligand) either in a sequential or simultaneous format. Our results demonstrate that the device can capture the synergy between different sensitizer drugs and TRAIL and demonstrate the potential of the microfluidic cell array for screening and optimizing combinatorial drug treatments for cancer therapy.     

Spherical phospholipid polymer hydrogels for cell encapsulation prepared with a flow-focusing microfluidic channel device

To prepare spherical polymer hydrogels, we used a flow-focusing microfluidic channel device for mixing aqueous solutions of two water-soluble polymers. Continuous encapsulation of cells in the hydrogels was also examined. The polymers were bioinspired 2-methacryloyloxyethyl phosphorylcholine polymer bearing phenyl boronic acid groups (PMBV) and poly(vinyl alcohol) (PVA), which spontaneously form a hydrogel in aqueous medium via specific molecular complexation upon mixing, even when they were in cell culture medium. The microfluidic device was prepared with polydimethylsiloxan, and the surface of the channel was treated with fluoroalkyl compound to prevent sticking of the polymers on the surface. The microfluidic channel process could control the diameter of the spherical hydrogels in the range of 30-90 μm and generated highly monodispersed diameter spherical hydrogels. We found that the polymer distribution in the hydrogel was influenced by the PVA concentration and that the hydrogel could be dissociated by the addition of d-sorbitol to the suspension. The single cells could be encapsulated and remain viable in the hydrogels. The localized distribution of polymers in the hydrogel may provide an environment for modulating cell function. It is concluded that the spontaneous hydrogel formation between PMBV and PVA in the flow-focusing microfluidic channel device is applicable for continuous preparation of a spherical hydrogel-encapsulating living cell.     

Truly nonionic polymer shells for the encapsulation of living cells

Engineering surfaces of living cells with natural or synthetic compounds can mediate intercellular communication and provide a protective barrier from hostile agents. We report on truly nonionic hydrogen-bonded LbL coatings for cell surface engineering. These ultrathin, highly permeable polymer membranes are constructed on living cells without the cationic component typically employed to increase the stability of LbL coatings. Without the cytotoxic cationic PEI pre-layer, the viability of encapsulated cells drastically increases to 94%, in contrast to 20% viability in electrostatically-bonded LbL shells. Moreover, the long-term growth of encapsulated cells is not affected, thus facilitating efficient function of protected cells in hostile environment.