Microfluidic Flow-focusing Device Research Articles

Microfluidic-based preparation of PLGA microspheres facilitating peptide sustained-release

It is challenging for the manufacturing of microspheres loaded with peptide due to the complicated preparation process and the issue of burst release. In this construct, a microfluidic flow-focusing device is developed to prepare and optimize poly (lactic-co-glycolic acid) (PLGA) Exenatide −loaded microspheres. Surfactants was added to greatly improve the stability of the emulsion, leading to increase drug loading capacity. By increasing the particle size of the drug-loaded microspheres to 30 μm, the burst release was significantly reduced. Additionally, multi-channel droplet microfluidic devices are prepared with the capability to achieve a 100-fold increase in throughput, which could achieve a production rate of 2 g per hour.

Effect of Mold Materials Used During Hot Embossing on Feature Fidelity for Microfabrication in Cyclic Olefin Polymer (COP) Substrate

During the transition from research to market, the fabrication of microfluidic devices in thermoplastic substrates is inevitable. For short production runs of several hundred products, hot embossing is the typical method before moving on to a typically more expensive injection molding process for higher production volumes. In this work, we investigated the effect of mold material used during hot embossing on feature fidelity for microfabrication in cyclic olefin polymer (COP) substrate. Specifically, we designed a simple flow-focusing microfluidic device and fabricated three different molds using silicon wafer by deep reactive ion etching (DRIE), aluminum filled high temperature epoxy by soft lithography and aluminum by CNC milling. We performed hot embossing experiments with 2mm thick COP substrates and these three different molds using automatic bench top Carver hot press. Finally, we characterized the hot embossed substrates by optical and scanning electron microscopy. Fabrication results demonstrate that the mold material plays a big role in feature fidelity. Among the mold materials used, silicon substrate performed the worst based on defects after demolding. Epoxy and aluminum molds were similar in terms of microfabricated feature defects in the substrate which could be mostly attributed to their coefficient of thermal expansion (CTE). A mold material with a CTE closer to the thermoplastic will result in much better feature fidelity.

One-Step Microfluidic Fabrication of Bioinspired Microfibers with a Spindle-Knot Structure for Fog Harvest.

Many biomimetic microfibers have been designed from spider silk to collect water efficiently from humid air as a result of its periodic spindle-knot structure, which enhances the direct movement and convergence of captured fog droplets. Here, a hydrodynamic flow-focusing microfluidic device with a theta-shaped tube is designed for the one-step fabrication of bioinspired microfibers with a spindle-knot structure for fog harvest. The morphology of the structured microfibers, including height, width, and spacing of spindle knots, can be adjusted readily by regulating the flow rate of specific phases. The production rate of these structured microfibers can reach 1100 cm/min. Moreover, the capture, transportation, and collection performance of fog droplets on various microfibers are investigated in a fog collection platform. It is demonstrated that our one-step microfluidic device presents a ready method for the fabrication of structured microfibers with spindle knots, which provide a significant facilitation on efficient fog capture and water collection.

Performance optimization of droplet formation and break up within a microfluidic device – Numerical and experimental evaluation

Droplet-based microfluidics, including the controlled manipulation of materials in the form of monodisperse droplets on the micrometer scale in microfluidic chips, has gained much attention due to their ability in biological assays. The capillary number of the continuous phase, as the ratio of viscous force and interfacial tension, is so important in the mechanism of droplet formation. This study aims to survey the type of continuous-phase fluid, the change in the parameters of the continuous phase, and the effect of the different flow rates ratios on the droplet formation dynamics, droplet size, distance between droplets and frequency of droplet formation. Three types of oil, including mineral, olive, and paraffin in a flow-focusing microfluidic device have been utilized numerically and experimentally. To characterize the parameters of the continuous phase affecting the droplet size, a microfluidic device has been designed to estimate of the droplet size for three different types of oil used in the assay. The results show that for a constant continuous phase velocity, the diameter of the droplets formed in olive oil has the lowest amount, followed by paraffin and mineral oil, respectively. Regarding the effects of oil type on the frequency of droplet formation and the distance between droplets, the rate of droplet formation in mineral oil, in the ratios of the variable flow rate and the constant continuous phase velocity, is the lowest and, the distance between the droplets has the highest level.

Large-Scale Preparation of Hair Follicle Germs Using a Microfluidic Device.

Hair follicle morphogenesis during embryonic development is driven by the formation of hair follicle germs (HFGs) via interactions between epithelial and mesenchymal cells. Bioengineered HFGs are potential tissue grafts for hair regenerative medicine because they can replicate interactions and hair follicle morphogenesis after transplantation. However, a mass preparation approach for HFGs is necessary for clinical applications, given that thousands of de novo hair follicles are required to improve the appearance of a single patient with alopecia. In this study, we developed a microfluidics-based approach for the large-scale preparation of HFGs. A simple flow-focusing microfluidic device allowed collagen solutions containing epithelial and mesenchymal cells to flow and generate collagen microbeads with distinct Janus structures. During the 3 days of culture, the collagen beads contracted owing to cellular traction forces, resulting in collagen- and cell-dense HFGs. The transplantation of HFGs into nude mice resulted in highly efficient de novo hair follicle regeneration. This method provides a scalable and robust tissue graft preparation approach for hair regeneration.

Effective colloidal emulsion droplet regulation in flow-focusing glass capillary microfluidic device via collection tube variation.

Colloidal emulsion droplets, created using glass capillary microfluidic devices, have been found in a myriad of applications, serving as subtle microcarriers, delicate templates, etc. To meet the objective requirements under varying circumstances, it is crucial to efficiently control the morphology and dimensions of the droplets on demand. The glass capillary collection tube is a crucial component of the flow-focusing microfluidic system due to its close association with the geometrical confinement of the multiphasic flow. However, there are currently no guidelines for the design of the morphology and dimensions of the glass capillary collection tube, which shall result in a delay in assessing serviceability until after the microfluidic device is prepared, thereby causing a loss of time and effort. Herein, an experimental study was conducted to investigate the effect of the geometrical characteristics of glass capillary collection tubes on the production of colloidal emulsion droplets. After characterizing the generated colloidal emulsion droplets, it was found that the geometrical variations of the glass capillary collection tube resulted in numerical disparities of droplets due to different degrees of flow-focusing effects. The stronger flow-focusing effect produced smaller droplets at a higher frequency, and the dimensional variation of colloidal emulsion droplets was more responsive to varying flow rates. Furthermore, the transformation from colloidal single-core double-emulsion droplets to multi-core double-emulsion droplets also changed with the flow rate due to the glass capillary collection tube morphology-determined varying flow-focusing effect. These experimental findings can offer qualitative guidance for the design of glass capillary microfluidic devices in the preliminary stage, thus facilitating the smooth production of desired colloidal emulsion droplets.

A Versatile Photocrosslinkable Silicone Composite for 3D Printing Applications

AbstractEmbedded printing has emerged as a valuable tool for fabricating complex structures and microfluidic devices. Currently, an ample of amount of research is going on to develop new materials to advance its capabilities and increase its potential applications. Here, a novel, transparent, printable, photocrosslinkable, and tuneable silicone composite is demonstrated that can be utilized as a support bath or an extrudable ink for embedded printing. Its properties can be tuned to achieve ideal rheological properties, such as optimal self‐recovery and yield stress, for use in 3D printing. When used as a support bath, it facilitated the generation microfluidic devices with circular channels of diameter up to 30 µm. To demonstrate its utility, flow focusing microfluidic devices are fabricated for generation of Janus microrods, which can be easily modified for multitude of applications. When used as an extrudable ink, 3D printing of complex‐shaped constructs are achieved with integrated electronics, which greatly extends its potential applications toward soft robotics. Further, its biocompatibility is tested with multiple cell types to validate its applicability for tissue engineering. Altogether, this material offers a myriad of potential applications (i.e., soft robotics, microfluidics, bioprinting) by providing a facile approach to develop complicated 3D structures and interconnected channels.

Parameter control, characterization and stability of soy protein emulsion prepared by microfluidic technology.

A flow-focusing microfluidic device driven by pressure was employed in soy protein emulsions with uniform droplet size and good morphology. The results suggested that pressure was an essential factor for droplet formation. The optimum parameter was at a continuous phase pressure of 140mbar and dispersed phase pressure of 80mbar. Under this condition, the droplet formation time was shortened to 0.20s, with average sizes of 39-43μm and coefficient of variation of about 2%. Emulsion stability was improved with increasing soy protein isolate (SPI) concentrations. At SPI concentrations higher than 20mg/mL, the emulsions exhibited improved stability against changes in temperature, pH and salt concentration. Emulsions prepared in this manner exhibited superior oxidative stability than those prepared by conventional methods utilizing homogenizers. This study showed that microfluidic technology can be applied to soy protein emulsions as an effective tool for preparing droplets with uniform size and enhanced stability.

Simplified 3D hydrodynamic flow focusing for lab-on-chip single particle study

Accurately control of the position of a fluid and particle within lab-on-a-chip platform is a critical prerequisite for many downstream analysis processes, such as detection, trapping and separation, moving the sensing at the single-particle level. With the development of microfluidic fabrication technology, particle/cell focusing has shifted from two to three dimensions. 3D hydrodynamic focusing, which sorts and aligns the incoming cloud of particles so that they pass through the interrogation area one by one, enables new possibilities and breakthroughs in the single-cell analysis system. Despite the excellent results shown in literature, there is still a lack of a device that can simultaneously fulfilling the requirements of high throughput, compactness, high integrability, and ease of use operation to become a widely accepted work center for biomedical research and clinical applications. Here, we proposed a unique 3D flow focusing microfluidic device buried in fused silica substrate that potentially combines all this advantages. By designing a sample channel suspended inside a larger buffer channel, manufactured by exploiting the laser-assisted micromachine technique, a not size-dependent focusing capability is shown. A spatially and temporally stable central flow of a mixture of 15 μm and 6 μm PS particles to a 1 μm PS microsphere solution has been obtained with high accuracy. Finally, to test the achievable focusing resolution, the chip was tested for the detection of Escherichia Coli bacteria in water solution as proof of concept of biological application.

Machine learning enhanced droplet microfluidics

Machine learning has recently been introduced in the context of droplet microfluidics to simplify the process of droplet formation, which is usually controlled by a variety of parameters. However, the studies introduced so far have mainly focused on droplet size control using water and mineral oil in microfluidic devices fabricated using soft lithography or rapid prototyping. This approach negated the applicability of machine learning results to other types of fluids more relevant to biomedical applications, while also preventing users that do not have access to microfluidic fabrication facilities to take advantage of previous findings. There are a number of different algorithms that could be used as part of a data driven approach, and no clear comparison has been previously offered among multiple machine learning architectures with respect to the predictions of flow rate values and generation rate. We here employed machine learning to predict the experimental parameters required for droplet generation in three commercialized microfluidic flow-focusing devices using phosphate buffer saline and biocompatible fluorinated oil as dispersed and continuous liquid phases, respectively. We compared three different machine learning architectures and established the one leading to more accurate predictions. We also compared the predictions with a new set of experiments performed at a different day to account for experimental variability. Finally, we provided a proof of concept related to algae encapsulation and designed a simple app that can be used to generate accurate predictions for a given droplet size and generation rate across the three commercial devices.

Deep reinforcement learning-based digital twin for droplet microfluidics control

This study applied deep reinforcement learning (DRL) with the Proximal Policy Optimization (PPO) algorithm within a two-dimensional computational fluid dynamics (CFD) model to achieve closed-loop control in microfluidics. The objective was to achieve the desired droplet size with minimal variability in a microfluidic capillary flow-focusing device. An artificial neural network was utilized to map sensing signals (flow pressure and droplet size) to control actions (continuous phase inlet pressure). To validate the numerical model, simulation results were compared with experimental data, which demonstrated a good agreement with errors below 11%. The PPO algorithm effectively controlled droplet size across various targets (50, 60, 70, and 80 μm) with different levels of precision. The optimized DRL + CFD framework successfully achieved droplet size control within a coefficient of variation (CV%) below 5% for all targets, outperforming the case without control. Furthermore, the adaptability of the PPO agent to external disturbances was extensively evaluated. By subjecting the system to sinusoidal mechanical vibrations with frequencies ranging from 10 Hz to 10 KHz and amplitudes between 50 and 500 Pa, the PPO algorithm demonstrated efficacy in handling disturbances within limits, highlighting its robustness. Overall, this study showcased the implementation of the DRL+CFD framework for designing and investigating novel control algorithms, advancing the field of droplet microfluidics control research.

Modular assembly of microswimmers with liquid compartments.

Artificial microswimmers, i.e. colloidal scale objects capable of self-propulsion, have garnered significant attention due to their central role as models for out of equilibrium systems. Moreover, their potential applications in diverse fields such as biomedicine, environmental remediation, and materials science have long been hypothesized, often in conjunction with their ability to deliver cargoes to overcome mass transport limitations. A very efficient way to load molecular cargoes is to disperse them in a liquid compartment, however, fabricating microswimmers with multiple liquid compartments remains a significant challenge. To address this challenge, we present a modular fabrication platform that combines microfluidic synthesis and sequential capillarity-assisted particle assembly (sCAPA) for microswimmers with various liquid compartments. We demonstrate the synthesis of monodisperse, small polymer-based microcapsules (Ø = 3-6μm) with different liquid cargoes using a flow-focusing microfluidic device. By employing the sCAPA technique, we assemble multiple microcapsules into microswimmers with high precision, resulting in versatile microswimmers with multiple liquid compartments and programmable functionalities. Our work provides a flexible approach for the fabrication of modular microswimmers, which could potentially actively transport cargoes and release them on demand in the future.

Optical detection of bovine serum albumin using charged cholesteric liquid crystal droplets functionalized with surfactant

Monodispersed micrometer-sized Cholesteric liquid crystal (CLC) droplets were prepared by using the flow-focusing microfluidic device, which was used for the detection of protein bovine serum albumin (BSA). The PVA/SC12S stabilized micro-sized CLC droplets (CLCPVA/SC12S) exhibited flashing spots with red reflection. These CLC droplets were effective in the detection of BSA in an aqueous solution through a configurational change from red reflection flashing spots (homeotropic configuration) to a central spot of red reflection (planar configuration) with a detection limit of 0.15 µM. We found that the electrostatic interaction caused by the adsorption of negatively charged BSA below isoelectric point on the positively charged CLC droplets can cause the arrangement of the CLC molecules in droplets to change from flashing spot to central spot reflection. Our results also highlight that the limit of detection and response times of CLCPVA/SC12S droplets are tunable as a function of the concentration and pH of protein. These CLCPVA/SC12S droplets are highly sensitive and are inexpensive and simple to make. With this novel approach, we can detect protein by observing optical reflection patterns with/without crossed polarizers in CLCPVA/SC12S droplets, which, on the contrary, are required in the case of the nematic biosensors.

Synthesis and Characterization of Hydrogel Droplets Containing Magnetic Nano Particles, in a Microfluidic Flow-Focusing Chip.

Magnetic hybrid hydrogels have exhibited remarkable efficacy in various areas, particularly in the biomedical sciences, where these inventive substances exhibit intriguing prospects for controlled drug delivery, tissue engineering, magnetic separation, MRI contrast agents, hyperthermia, and thermal ablation. Additionally, droplet-based microfluidic technology enables the fabrication of microgels possessing monodisperse characteristics and controlled morphological shapes. Here, alginate microgels containing citrated magnetic nanoparticles (MNPs) were produced by a microfluidic flow-focusing system. Superparamagnetic magnetite nanoparticles with an average size of 29.1 ± 2.5 nm and saturation magnetization of 66.92 emu/g were synthesized via the co-precipitation method. The hydrodynamic size of MNPs was changed from 142 nm to 826.7 nm after the citrate group's attachment led to an increase in dispersion and the stability of the aqueous phase. A microfluidic flow-focusing chip was designed, and the mold was 3D printed by stereo lithographic technology. Depending on inlet fluid rates, monodisperse and polydisperse microgels in the range of 20-120 μm were produced. Different conditions of droplet generation in the microfluidic device (break-up) were discussed considering the model of rate-of-flow-controlled-breakup (squeezing). Practically, this study indicates guidelines for generating droplets with a predetermined size and polydispersity from liquids with well-defined macroscopic properties, utilizing a microfluidic flow-focusing device (MFFD). Fourier transform infrared spectrometer (FT-IR) results indicated a chemical attachment of citrate groups on MNPs and the existence of MNPs in the hydrogels. Magnetic hydrogel proliferation assay after 72 h showed a better rate of cell growth in comparison to the control group (p = 0.042).

Mass transfer and size control of partially miscible fluid drops in a flow focusing microfluidic device

We conducted laboratory experiments and numerical simulations to investigate the formation and evolution of drops formed by partially miscible two-phase fluid, n-butanol (the continuous phase) and water (the dispersed phase), in a flow focusing microfluidic system. We carefully calibrated the numerical model to obtain good agreement with experimental data in drop velocity and mass transfer, demonstrating the model's capability to capture realistic drop dynamics. Our detailed investigation of the numerical results allowed us to determine the mechanism of drop formation and obtain a relevant criterion in terms of the disperse-to-continuous flow ratio beyond which the tubing patterns would occur. Additionally, we found that the mass transfer between the two phases, specifically at the drop interface, strongly depends on the local distribution of dissolved concentration of the dispersed phase. To enhance mass transfer, we conducted numerical simulations on alternating curved channels, which allows for the lateral advection of the dispersed phase concentration in the continuous phase at the curved section. We found that this lateral movement enhances mass transfer at the drop interface. Through detailed investigation of numerical results, we addressed mechanisms of mass transfer enhancement in the curved channel. Overall, our findings provide insight into the mechanisms of drop formation and mass transfer in partially miscible two-phase fluids in microfluidic systems, which could be useful in designing and optimizing such systems for various applications.

Enhancing Structural Stability of Oil-Shell Microbubbles via Incorporation of a Gold Nanoparticle Protective Shell for Theranostic Applications

Phospholipid-stabilized microbubbles are utilized as contrast agents in medical ultrasound imaging, and researchers are currently investigating their potential as theranostic agents. Due to the inadequate water solubility and poor stability of numerous new therapeutics, the development of stable microbubbles with the capacity to encapsulate hydrophobic therapeutics is necessary. Herein, we proposed a flow-focusing microfluidic device to generate highly monodispersed, phospholipid-stabilized dual-layer microbubbles for theranostic applications. The stability and microstructural evolution of these microbubbles were investigated by microscopy and machine-learning-assisted segmentation techniques at different phospholipid and gold nanoparticle concentrations. The double-emulsion microbubbles, formed with the combination of phospholipids and gold nanoparticles, developed a protective gold nanoparticle shell that not only acted as a steric barrier against gas diffusion and microbubble coalescence but also alleviated the progressive dewetting instability and the subsequent cascade of coalescence events.

Sunset Yellow Confined in Curved Geometry: A Microfluidic Approach.

The behavior of lyotropic chromonic liquid crystals (LCLCs) in confined environments is an interesting research field that still awaits exploration, with multiple key variables to be uncovered and understood. Microfluidics is a highly versatile technique that allows us to confine LCLCs in micrometric spheres. As microscale networks offer distinct interplays between the surface effects, geometric confinement, and viscosity parameters, rich and unique interactions emerging at the LCLC-microfluidic channel interfaces are expected. Here, we report on the behavior of pure and chiral doped nematic Sunset Yellow (SSY) chromonic microdroplets produced through a microfluidic flow-focusing device. The continuous production of SSY microdroplets with controllable size gives the possibility to systematically study their topological textures as the function of their diameters. Indeed, doped SSY microdroplets produced via microfluidics, show topologies that are typical of common chiral thermotropic liquid crystals. Furthermore, few droplets exhibit a peculiar texture never observed for chiral chromonic liquid crystals. Finally, the achieved precise control of the produced LCLC microdroplets is a crucial step for technological applications in biosensing and anticounterfeiting.

Numerical investigation of electrohydrodynamic effect for size-tunable droplet formation in a flow-focusing microfluidic device

ABSTRACT Droplet-based microfluidics has received much attention in biofabrication due to its compatibility with 3D printers that use cell-laden bioinks. In order to tailor the printing resolution, droplet generation under a DC electric field in a flow-focusing device is explored numerically. The major purpose of simulations is to investigate how geometry affects droplet production and electric field intensity. The effects of the orifice’s length, injection angle, and shape are discussed regarding the electric capillary number . Based on the retardation effect, the necking stage of droplet formation changes to expansion, and the frequency of droplet formation declines when the capillary number rises. For a particular value of the applied electric potential, orifice elongation reduces the induced electric field within the orifice. By adjusting the angle of side flow, smaller droplets can be formed, and the linear decrease in droplet size is provided across a wider range. The electric field in the orifice was amplified by 64% by creating a wedge-shaped orifice. In other words, when the notch angle was sharpened, the electric force increased, and the droplet diameter reduced. Since the frequency of droplet production varied only slightly between different notch angles, it could be preferable to generate smaller droplets without significantly altering the frequency of droplet formation.

Parallel Photothermal Coalescence of Biocompatible Photocurable PEGDA Droplets

Droplet microfluidics is a powerful tool for high-throughput experimentation, and droplet coalescence is necessary for mixing and chemical reactions. Droplet merging and polymerization will release the limits on particle synthesis and widen the technical potential of droplet microfluidics. Previously, a focused laser beam has been used to induce the coalescence of droplets. This paper reports the parallel photothermal coalescence of biocompatible photocurable polyethylene glycol diacrylate (PEGDA) droplets using a 2D image for developing a more efficient coalescence process. PEGDA droplets with diameters of 18 µm to 50 µm were generated in a microfluidic flow-focusing device and stored in a microchannel. A 2D image of violet light induced the parallel coalescence of PEGDA droplets with diameters of around 30 µm. When continuous-phase oil was replaced with nitrogen, PEGDA droplets were photopolymerized.

Biomimetic Extracellular Scaffolds by Microfluidic Superstructuring of Nanofibers.

The extracellular matrix is a dynamic framework bearing chemical and morphological cues that support many cellular functions, and artificial analogs with well-defined chemistry are of great interest for biomedical applications. Herein, we describe hierarchical, extracellular-matrix-mimetic microgels, termed "superbundles" (SBs) composed of peptide amphiphile (PA) supramolecular nanofiber networks created using flow-focusing microfluidic devices. We explore the effects of altered flow rate ratio and PA concentration on the ability to create SBs and develop design rules for producing SBs with both cationic and anionic PA nanofibers and gelators. We demonstrate the morphological similarities of SBs to decellularized extracellular matrices and showcase their ability to encapsulate and retain proteinaceous cargos with a wide variety of isoelectric points. Finally, we demonstrate that the novel SB morphology does not affect the well-established biocompatibility of PA gels.