On-chip polyelectrolyte coating onto magnetic droplets - towards continuous flow assembly of drug delivery capsules

One-step formation of polymorphous sperm-like microswimmers by vortex turbulence-assisted microfluidics

Microswimmers are considered promising candidates for active cargo delivery to benefit a wide spectrum of biomedical applications. Yet, big challenges still remain in designing the microswimmers with effective propelling, desirable loading and adaptive releasing abilities all in one. Inspired by the morphology and biofunction of spermatozoa, we report a one-step formation strategy of polymorphous sperm-like magnetic microswimmers (PSMs) by developing a vortex turbulence-assisted microfluidics (VTAM) platform. The fabricated PSM is biodegradable with a core-shell head and flexible tail, and their morphology can be adjusted by vortex flow rotation speed and calcium chloride solution concentration. Benefiting from the sperm-like design, our PSM exhibits both effective motion ability under remote mag/netic actuation and protective encapsulation ability for material loading. Further, it can also realize the stable sustain release after alginate-chitosan-alginate (ACA) layer coating modification. This research proposes and verifies a new strategy for the sperm-like microswimmer construction, offering an alternative solution for the target delivery of diverse drugs and biologics for future biomedical treatment. Moreover, the proposed VTAM could also be a general method for other sophisticated polymorphous structures fabrication that isn't achievable by conventional laminar flow.

Acoustofluidics-Assisted Coating of Microparticles

Microparticles have been applied in many areas, ranging from drug delivery, diagnostics, cosmetics, personal care, and the food industry to chemical and catalytic reactions, sensing, and environmental remediation. Coating further provides additional functionality to the microparticles, such as controlled release, surface modification, bio-fouling resistance, stability, protection, etc. In this study, the conformal coating of microparticles with a positively charged polyelectrolyte (polyallylamine hydrochloride, PAH) by utilizing an acoustofluidic microchip was proposed and demonstrated. The multiple laminar streams, including the PAH solution, were formed inside the microchannel, and, under the traveling surface acoustic wave, the microparticles traversed through the streams, where they were coated with PAH. The results showed that the coating of microparticles can be achieved in a rapid fashion via a microfluidic approach compared to that obtained by the batch method. Moreover, the zeta potentials of the microparticles coated via the microfluidic approach were more uniform. For the unfunctionalized microparticles, the charge reversal occurred after coating, and the zeta potential increased as the width of the microchannel or the concentration of the PAH solution increased. As for the carboxylate-conjugated microparticles, the charge reversal again occurred after coating; however, the magnitudes of the zeta potentials were similar when using the microchannels with different widths or different concentrations of PAH solution.

Applications of magnetic and electromagnetic forces in micro-analytical systems

Novel applications of magnetic fields in analytical chemistry have become a remarkable trend in the last two decades. Various magnetic forces have been employed for the migration, orientation, manipulation, and trapping of microparticles, and new analytical platforms for separating and detecting molecules have been proposed. Magnetic materials such as functional magnetic nanoparticles, magnetic nanocomposites, and specially designed magnetic solids and liquids have also been developed for analytical purposes. Numerous attractive applications of magnetic and electromagnetic forces on magnetic and non-magnetic materials have been studied, but fundamental studies to understand the working principles of magnetic forces have been challenging. These studies will form a new field of magneto-analytical science, which should be developed as an interdisciplinary field. In this review, essential pioneering works and recent attractive developments are presented.

Droplet Manipulation under a Magnetic Field: A Review

The magnetic manipulation of droplets is one of the emerging magnetofluidic technologies that integrate multiple disciplines, such as electromagnetics, fluid mechanics and so on. The directly driven droplets are mainly composed of ferrofluid or liquid metal. This kind of magnetically induced droplet manipulation provides a remote, wireless and programmable approach beneficial for research and engineering applications, such as drug synthesis, biochemistry, sample preparation in life sciences, biomedicine, tissue engineering, etc. Based on the significant growth in the study of magneto droplet handling achieved over the past decades, further and more profound explorations in this field gained impetus, raising concentrations on the construction of a comprehensive working mechanism and the commercialization of this technology. Current challenges faced are not limited to the design and fabrication of the magnetic field, the material, the acquisition of precise and stable droplet performance, other constraints in processing speed and so on. The rotational devices or systems could give rise to additional issues on bulky appearance, high cost, low reliability, etc. Various magnetically introduced droplet behaviors, such as deformation, displacement, rotation, levitation, splitting and fusion, are mainly introduced in this work, involving the basic theory, functions and working principles.

Fabrication of microcapsules with core-shell structure for oral delivery of dual drugs and real-time computed tomography imaging

Although multidrug combinations are an effective therapeutic strategy for serious disease in clinical practice, their therapeutic effect may be reduced because they conflict with each other medicinally in certain cases. Hence, there is an urgent need to develop a single drug carrier for precise multidrug delivery to avoid this interference. A reverse coordination method is reported that fabricates a double‐layer barium sulphate microcapsule (DL@BS MS) for two drugs separately loading simultaneously. In addition, BS nanoclusters were synthesised in situ inside the DL@BS MSs for real‐time computed tomography (CT) imaging. The results showed that the DL@BS MSs with a particle size of approximately 2 mm exhibited a uniform sphere. Because BS nanoclusters have a high X‐ray attenuation coefficient, the retention of DL@BS MSs in the digestive tract could be monitored through CT imaging in real time. More important, the core‐shell structure of DL@BS MSs encapsulating two different drugs could be released in spatiotemporal order in an acidic stomach environment. The as‐synthesis DL@BS MSs with a core‐shell structure and real‐time imaging performance provide an ideal carrier for the oral administration of multiple drugs simultaneously loaded but sequentially released.

Development of an in-line magnetometer for flow chemistry and its demonstration for magnetic nanoparticle synthesis

Despite the wide usage of magnetic nanoparticles, it remains challenging to synthesise particles with properties that exploit each application's full potential. Time consuming experimental procedures and particle analysis hinder process development, which is commonly constrained to a handful of experiments without considering particle formation kinetics, reproducibility and scalability. Flow reactors are known for their potential of large-scale production and high-throughput screening of process parameters. These advantages, however, have not been utilised for magnetic nanoparticle synthesis where particle characterisation is performed, with a few exceptions, post-synthesis. To overcome this bottleneck, we developed a highly sensitive magnetometer for flow reactors to characterise magnetic nanoparticles in solution in-line and in real-time using alternating current susceptometry. This flow magnetometer enriches the flow-chemistry toolbox by facilitating continuous quality control and high-throughput screening of magnetic nanoparticle syntheses. The sensitivity required to monitor magnetic nanoparticle syntheses at the typically low concentrations (<100 mM of Fe) was achieved by comparing the signals induced in the sample and reference cell, each of which contained near-identical pairs of induction and pick-up coils. The reference cell was filled only with air, whereas the sample cell was a flow cell allowing sample solution to pass through. Balancing the flow and reference cell impedance with a newly developed electronic circuit was pivotal for the magnetometer's sensitivity. To showcase its potential, the flow magnetometer was used to monitor two iron oxide nanoparticle syntheses with well-known particle formation kinetics, i.e., co-precipitation syntheses with sodium carbonate and sodium hydroxide as base, which have been previously studied via synchrotron X-ray diffraction. The flow magnetometer facilitated batch (on-line) and flow (in-line) synthesis monitoring, providing new insights into the particle formation kinetics as well as, effect of temperature and pH. The compact lab-scale flow device presented here, opens up new possibilities for magnetic nanoparticle synthesis and manufacturing, including 1) early stage reaction characterisation 2) process monitoring and control and 3) high-throughput screening in combination with flow reactors.

Dielectrophoretic medium exchange around droplets for on-chip fabrication of layer-by-layer microcapsules

Continuous medium exchange within a microchannel represents a highly sought-after technique in functionalizing micro-objects with coating layers, enabling a myriad of applications ranging from biomedical engineering to materials science. Herein, we introduce a unique medium exchange approach, namely, tilted-angle dielectrophoresis, to accomplish layer-by-layer (LbL) coating on droplets in a wide microchannel. Pairs of adjacent tilted parallel electrodes arranged in a zigzag fashion are exploited to consecutively and repeatedly guide particles/droplets travelling through three parallel laminar streams comprising two reagents and a washing buffer. The performance of medium exchange is demonstrated using PS microparticles and oil droplets. We show that multi-cycle medium exchange, droplet transfer accompanied with purification, and multi-mode medium exchange around different micro-objects are achieved by conveniently regulating the applied voltage and the inlet flow rate, indicating a flexible, versatile and label-free alternative for characterizing and handling colloidal particles. Furthermore, LbL coating on droplets utilizing the presented strategy is implemented in the parallel coating-chemical and washing streams to obtain multiple layers of microcapsules. The linearly increasing fluorescence intensity of the coated droplets with each subsequent fluorescent coating demonstrates the capability of the tilted-angle dielectrophoretic medium exchanger for on-chip generation of LbL microcapsules on demand. The presented medium exchange strategy, together with its unique features of simple geometric configuration, facile control and multifunctionality, can provide a refined alternative for further expanding the utility scope in functional particles and cells.

Droplet Microfluidics for Tumor Drug-Related Studies and Programmable Artificial Cells

Anticancer drug development is a crucial step toward cancer treatment, that requires realistic predictions of malignant tissue development and sophisticated drug delivery. Tumors often acquire drug resistance and drug efficacy, hence cannot be accurately predicted in 2D tumor cell cultures. On the other hand, 3D cultures, including multicellular tumor spheroids (MCTSs), mimic the in vivo cellular arrangement and provide robust platforms for drug testing when grown in hydrogels with characteristics similar to the living body. Microparticles and liposomes are considered smart drug delivery vehicles, are able to target cancerous tissue, and can release entrapped drugs on demand. Microfluidics serve as a high-throughput tool for reproducible, flexible, and automated production of droplet-based microscale constructs, tailored to the desired final application. In this review, it is described how natural hydrogels in combination with droplet microfluidics can generate MCTSs, and the use of microfluidics to produce tumor targeting microparticles and liposomes. One of the highlights of the review documents the use of the bottom-up construction methodologies of synthetic biology for the formation of artificial cellular assemblies, which may additionally incorporate both target cancer cells and prospective drug candidates, as an integrated "droplet incubator" drug assay platform.

Manipulating Dispersions of Magnetic Nanoparticles

Dispersions of magnetic nanoparticles (MNPs) can exhibit paramagnetic ferrofluid or ferromagnetic liquid behavior. By modifying the surface functionality of MNPs, ferrofluids have been used to fabricate novel magnetically actuated devices. If the surface-functionalized MNPs interact with complementary ligands at a fluid-fluid interface, MNP surfactants form and in situ assemble at the interface. When jammed interfacially, MNP surfactants give rise to ferromagnetic behavior of the liquid (droplet), which is endowed with permanent magnetic dipoles while maintaining all of the characteristics of a fluid system. Here, we give a brief overview of the developments in the dispersion of MNPs in liquids from ferrofluids to ferromagnetic liquid droplets, their responses to external fields, and the manipulation of these responses for end uses. The reversible room-temperature para-to-ferro transformation of magnetic liquids is highlighted. We discuss challenges in the synthesis and characterization of these unusual liquids along with potential technological applications.

Synthesis and applications of surface-modified magnetic nanoparticles: progress and future prospects

The growing use of magnetic nanoparticles (MNPs) demands cost-effective methods for their synthesis that allow proper control of particle size and size distribution. The unique properties of MNPs include high specific surface area, ease of functionalization, chemical stability and superparamagnetic behavior, with applications in catalysis, data and energy storage, environmental remediation and biomedicine. This review highlights breakthroughs in the use of MNPs since their initial introduction in biomedicine to the latest challenging applications; special attention is paid to the importance of proper coating and functionalization of the particle surface, which dictates the specific properties for each application. Starting from the first report following LaMer’s theory in 1950, this review discusses and analyzes methods of synthesizing MNPs, with an emphasis on functionality and applications. However, several hurdles, such as the design of reactors with suitable geometries, appropriate control of operating conditions and, in particular, reproducibility and scalability, continue to prevent many applications from reaching the market. The most recent strategy, the use of microfluidics to achieve continuous and controlled synthesis of MNPs, is therefore thoroughly analyzed. This review is the first to survey continuous microfluidic coating or functionalization of particles, including challenging properties and applications.

Formation and manipulation of ferrofluid droplets with magnetic fields in a microdevice: a numerical parametric study

We present a numerical model that describes the microfluidic generation and manipulation of ferrofluid droplets under an external magnetic field. We developed a numerical Computational Fluid Dynamics (CFD) analysis for predicting and optimizing continuous flow generation and processing of ferrofluid droplets with and without the presence of a permanent magnet. More specifically, we explore the dynamics of oil-based ferrofluid droplets within an aqueous continuous phase under an external inhomogeneous magnetic field. The developed model determines the effect of the magnetic field on the droplet generation, which is carried out in a flow-focusing geometry, and its sorting in T-junction channels. Three-channel depths (25 μm, 30 μm, and 40 μm) were investigated to study droplet deformation under magnetic forces. Among the three, the 30 μm channel depth showed the most consistent droplet production for the studied range of flow rates. Ferrofluids with different loadings of magnetic nanoparticles were used to observe the behavior for different ratios of magnetic and hydrodynamic forces. Our results show that the effect of these factors on droplet size and generation rate can be tuned and optimized to produce consistent droplet generation and sorting. This approach involves fully coupled magnetic-fluid mechanics models and can predict critical details of the process including droplet size, shape, trajectory, dispensing rate, and the perturbation of the fluid co-flow for different flow rates. The model enables better understanding of the physical phenomena involved in continuous droplet processing and allows efficient parametric analysis and optimization.

Tuning the Splitting Behavior of Droplet in a Bifurcating Channel through Wettability-Capillarity Interaction

We present a comprehensive computational physics-based study of the influence of surface wettability on the displacement behavior of a droplet in a three-dimensional bifurcating channel. Various surface wettability configurations for the daughter branches are considered to gain insight into the wettability-capillarity interaction. Also, the influence of initial droplet size on the splitting dynamics for different wettability configurations is investigated. Time evolution of the droplet displacement behavior in the bifurcating channel is discussed for different physicochemical parameters including capillary number and wettability. Three distinct flow regimes are identified as the droplet interacts with the bifurcating tip of the channel, namely, splitting, nonsplitting, and oscillating regimes. Furthermore, the occurrence of Rayleigh-Plateau instability in different wettability scenarios is discussed. Additionally, the intricacies associated with the droplet dynamics are elucidated through the temporal evolution of the droplet surface area and mass outflow of the continuous phase. A flow regime map based on the capillary number and wettability contrast of the daughter branches is proposed for a comprehensive description of the droplet dynamics.

Magnetic polyelectrolyte microcapsules via water-in-water droplet microfluidics

Polyelectrolyte microcapsules (PEMCs) have biocompatible microcompartments. Therefore, PEMCs are useful for applications in cosmetics, food, pharmaceutics, and other industries. The fabrication of PEMCs often involves the use of harsh chemicals or cytotoxic organic phases that make biomedical applications of the microcapsules challenging. In this report, we present an all-aqueous droplet microfluidics platform for the generation of magnetic PEMCs. In the platform, we use an aqueous-two-phase system (ATPS) of polyethylene glycol (PEG) and dextran (Dex), to generate water-in-water droplets, which are magnetically functionalized with ferrofluid. Strong polyelectrolytes (PEs) with opposite charges are used in each ATPS phase. We make emulsion templates of magnetic Dex, containing the polycations, in a continuous phase of PEG. We then apply a magnetic field to move the magnetic droplets to a second PEG phase, which contains the polyanions. By careful tuning of the fluxes of the two PEs in their respective phases, we trigger the formation of a shell at the droplet interface. Owing to the presence of the ferrofluid, the resulting microcapsules are magnetically responsive. We show that the magnetic PEMCs are capable of passive release of large pseudo-drugs as well as triggered release using external stimuli such as osmotic shock and pH change. We expect that magnetic PEMCs from this biocompatible all-aqueous platform will find utility in the fabrication of functionalized drug carriers for targeted drug delivery.

Continuous-Flow Separation of Magnetic Particles from Biofluids: How Does the Microdevice Geometry Determine the Separation Performance?

The use of functionalized magnetic particles for the detection or separation of multiple chemicals and biomolecules from biofluids continues to attract significant attention. After their incubation with the targeted substances, the beads can be magnetically recovered to perform analysis or diagnostic tests. Particle recovery with permanent magnets in continuous-flow microdevices has gathered great attention in the last decade due to the multiple advantages of microfluidics. As such, great efforts have been made to determine the magnetic and fluidic conditions for achieving complete particle capture; however, less attention has been paid to the effect of the channel geometry on the system performance, although it is key for designing systems that simultaneously provide high particle recovery and flow rates. Herein, we address the optimization of Y-Y-shaped microchannels, where magnetic beads are separated from blood and collected into a buffer stream by applying an external magnetic field. The influence of several geometrical features (namely cross section shape, thickness, length, and volume) on both bead recovery and system throughput is studied. For that purpose, we employ an experimentally validated Computational Fluid Dynamics (CFD) numerical model that considers the dominant forces acting on the beads during separation. Our results indicate that rectangular, long devices display the best performance as they deliver high particle recovery and high throughput. Thus, this methodology could be applied to the rational design of lab-on-a-chip devices for any magnetically driven purification, enrichment or isolation.

Layer-by-Layer Self-Assembly of Polyelectrolytes on Superparamagnetic Nanoparticle Surfaces

Designing and manufacturing multifunctional nanoparticles (NPs) are of considerable interest for both academic and industrial research. Among NPs used in this field, iron oxide NPs show low toxicity compared to metallic ones and are thus of high interest for biomedical applications. In this work, superparamagnetic Fe3-δO4-based core/shell NPs were successfully prepared and characterized by the combination of different techniques, and their physical properties were investigated. We demonstrate the efficiency of the layer-by-layer process to graft polyelectrolytes on the surface of iron oxide NPs. The influence of the polyelectrolyte chain configuration on the magnetic properties of the Fe3-δO4/polymer core/shell NPs was enlightened. The simple and fast process described in this work is efficient for the grafting of polyelectrolytes from surfaces, and thus, derived Fe3-δO4 NPs display both the physical properties of the core and of the macromolecular shell. Finally, the cytotoxicity toward the human THP-1 monocytic cell line of the core/shell NPs was assessed. The results showed that the polymer-capped Fe3-δO4 NPs exhibited almost no toxicity after 24 h of exposure at concentrations up to 25 μg mL-1. Our results show that these smart superparamagnetic nanocarriers with stealth properties are promising for applications in multimodal cancer therapy, including drug delivery.

Magnetic water-in-water droplet microfluidics: Systematic experiments and scaling mathematical analysis

A major barrier to the clinical utilization of microfluidically generated water-in-oil droplets is the cumbersome washing steps required to remove the non-biocompatible organic oil phase from the droplets. In this paper, we report an on-chip magnetic water-in-water droplet generation and manipulation platform using a biocompatible aqueous two-phase system of a polyethylene glycol-polypropylene glycol-polyethylene glycol triblock copolymer (PEG-PPG-PEG) and dextran (DEX), eliminating the need for subsequent washing steps. By careful selection of a ferrofluid that shows an affinity toward the DEX phase (the dispersed phase in our microfluidic device), we generate magnetic DEX droplets in a non-magnetic continuous phase of PEG-PPG-PEG. We apply an external magnetic field to manipulate the droplets and sort them into different outlets. We also perform scaling analysis to model the droplet deflection and find that the experimental data show good agreement with the model. We expect that this type of all-biocompatible magnetic droplet microfluidic system will find utility in biomedical applications, such as long-term single cell analysis. In addition, the model can be used for designing experimental parameters to achieve a desired droplet trajectory.

Laborless, Automated Microfluidic Tandem Cell Processor for Visualizing Intracellular Molecules of Mammalian Cells

Visualization and quantification of intracellular molecules of mammalian cells are crucial steps in clinical diagnosis, drug development, and basic biological research. However, conventional methods rely mostly on labor-intensive, centrifugation-based manual operations for exchanging the cell carrier medium and have limited reproducibility and recovery efficiency. Here we present a microfluidic cell processor that can perform four-step exchange of carrier medium, simply by introducing a cell suspension and fluid reagents into the device. The reaction time period for each reaction step, including fixation, membrane permeabilization, and staining, was tunable in the range of 2 to 15 min by adjusting the volume of the reaction tube connecting the neighboring exchanger modules. We double-stained the cell nucleus and cytoskeleton (F-actin) using the presented device with an overall reaction period of ∼30 min, achieving a high recovery ratio and high staining efficiency. Additionally, intracellular cytokine (IL-2) was visualized for T cells to demonstrate the feasibility of the device as a pretreatment system for downstream flow-cytometric analysis. The presented approach would facilitate the development of laborless, automated microfluidic systems that integrate cell processing and analysis operations and would pave a new path to high-throughput biological experiments.

Flexible Ferrofluids: Design and Applications

Ferrofluids, also known as ferromagnetic particle suspensions, are materials with an excellent magnetic response, which have attracted increasing interest in both industrial production and scientific research areas. Because of their outstanding features, such as rapid magnetic reaction, flexible flowability, as well as tunable optical and thermal properties, ferrofluids have found applications in various fields, including material science, physics, chemistry, biology, medicine, and engineering. Here, a comprehensive, in-depth insight into the diverse applications of ferrofluids from material fabrication, droplet manipulation, and biomedicine to energy and machinery is provided. Design of ferrofluid-related devices, recent developments, as well as present challenges and future prospects are also outlined.

Numerical Analysis of Bead Magnetophoresis from Flowing Blood in a Continuous-Flow Microchannel: Implications to the Bead-Fluid Interactions

In this work, we report a numerical flow-focused study of bead magnetophoresis inside a continuous-flow microchannel in order to provide a detailed analysis of bead motion and its effect on fluid flow. The numerical model involves a Lagrangian approach and predicts the bead separation from blood and their collection into a flowing buffer by the application of a magnetic field generated by a permanent magnet. The following scenarios are modelled: (i) one-way coupling wherein momentum is transferred from the fluid to beads, which are treated as point particles, (ii) two-way coupling wherein the beads are treated as point particles and momentum is transferred from the bead to the fluid and vice versa, and (iii) two-way coupling taking into account the effects of bead volume in fluid displacement. The results indicate that although there is little difference in the bead trajectories for the three scenarios, there is significant variation in the flow fields, especially when high magnetic forces are applied on the beads. Therefore, an accurate full flow-focused model that takes into account the effects of the bead motion and volume on the flow field should be solved when high magnetic forces are employed. Nonetheless, when the beads are subjected to medium or low magnetic forces, computationally inexpensive models can be safely employed to model magnetophoresis.

Droplet CAR-Wash: continuous picoliter-scale immunocapture and washing

To address current limitations in adapting solid phase sample capture and washing techniques to continuously flowing droplet microfluidics, we have developed the "Coalesce-Attract-Resegment Wash" (CAR-Wash) approach. This module provides efficient, high-throughput magnetic washing by electrocoalescing magnetic bead-laden input droplets with a washing buffer flow and magnetophoretically transporting beads through the buffer into a secondary droplet formation streamline. In this work, we first characterized the technology in terms of throughput, sample retention, and flow-based exclusion of waste volume, demonstrating >500 Hz droplet processing with >98% bead retention and >100-fold dilution in final droplets. Next, we showed that the technique can be adapted to alternative commercially available magnetic beads with lower magnetite content per particle. Then, we demonstrated the CAR-Wash module's effectiveness in washing away a small molecule competitive inhibitor to restore the activity of magnetic bead-immobilized β-galactosidase. Finally, we applied the system to immunomagnetically enrich a green fluorescent protein-histone H2B fusion protein from cell lysate while washing away mCherry and other lysate components. We believe this approach will bridge the gap between powerful biochemical and bioanalytical techniques and current droplet microfluidic capabilities, and we envision future application in droplet-based immunoassays, solid phase extraction, and other complex, multi-step operations.

Droplet Mechanical Hand Based on Anisotropic Water Adhesion of Hydrophobic-Superhydrophobic Patterned Surfaces

Superhydrophobic copper surfaces patterned with non-round hydrophobic areas were fabricated by a combination of through-mask chemical oxidation and fluorocarbon film deposition techniques. The anisotropic sliding resistance of droplets on typical non-round hydrophobic patterns such as semicircle, V-shape, and line segment hydrophobic patterns was observed. The dependence of sliding anisotropy on the pattern shape and dimensions was investigated. Results showed that the experimental sliding resistance was in good agreement with the calculated data using a classical drag-resistance model (Furmidge equation). By taking advantage of the anisotropic sliding resistance, these patterned surfaces can be used as droplet mechanical hands to capture, transfer, mix, and release in situ micro droplets by simply moving the surfaces in different directions. A droplet pinned on a non-round hydrophobic pattern can be captured by lifting a surface with another non-round hydrophobic pattern in a large-sliding-resistance direction after touching it, while the captured droplet can be released in situ with nearly no mass loss by horizontally moving the surface in the low-sliding-resistance direction. The lossless droplet manipulations using hydrophobic/superhydrophobic patterned surfaces have advantages of being low in cost and easy to operate and may have great promising applications to high throughput drug screening, molecular detection, and other lab-on-chip devices.

Microfluidic curved-channel centrifuge for solution exchange of target microparticles and their simultaneous separation from bacteria

One of the common operations in sample preparation is to separate specific particles (e.g. target cells, embryos or microparticles) from non-target substances (e.g. bacteria) in a fluid and to wash them into clean buffers for further processing like detection (called solution exchange in this paper). For instance, solution exchange is widely needed in preparing fluidic samples for biosensing at the point-of-care and point-of-use, but still conducted via the use of cumbersome and time-consuming off-chip analyte washing and purification techniques. Existing small-scale and handheld active and passive devices for washing particles are often limited to very low throughputs or require external sources of energy. Here, we integrated Dean flow recirculation of two fluids in curved microchannels with selective inertial focusing of target particles to develop a microfluidic centrifuge device that can isolate specific particles (as surrogates for target analytes) from bacteria and wash them into a clean buffer at high throughput and efficiency. We could process micron-size particles at a flow rate of 1 mL min-1 and achieve throughputs higher than 104 particles per second. Our results reveal that the device is capable of singleplex solution exchange of 11 μm and 19 μm particles with efficiencies of 86 ± 2% and 93 ± 0.7%, respectively. A purity of 96 ± 2% was achieved in the duplex experiments where 11 μm particles were isolated from 4 μm particles. Application of our device in biological assays was shown by performing duplex experiments where 11 μm or 19 μm particles were isolated from an Escherichia coli bacterial suspension with purities of 91-98%. We envision that our technique will have applications in point-of-care devices for simultaneous purification and solution exchange of cells and embryos from smaller substances in high-volume suspensions at high throughput and efficiency.