Novel glass capillary microfluidic devices for the flexible and simple production of multi-cored double emulsions

Glass Microdroplet Generator for Lipid-Based Double Emulsion Production

Microfluidics offers a highly controlled and reproducible route to synthesize lipid vesicles. In recent years, several microfluidic approaches have been introduced for this purpose, but double emulsions, such as Water-in-Oil-in-Water (W/O/W) droplets, are preferable to produce giant vesicles that are able to maximize material encapsulation. Flow focusing (FF) is a technique used to generate double emulsion droplets with high monodispersity, a controllable size, and good robustness. Many researchers use polydimethylsiloxane as a substrate material to fabricate microdroplet generators, but it has some limitations due to its hydrophobicity, incompatibility with organic solvents, and the molecular adsorption on the microchannel walls. Thus, specific surface modification and functionalization steps, which are uncomfortable to perform in closed microchannels, are required to overcome these shortcomings. Here, we propose glass as a material to produce a chip with a six-inlet junction geometry. The peculiar geometry and the glass physicochemical properties allow for W/O/W droplet formation without introducing microchannel wall functionalization and using a variety of reagents and organic solvents. The robust glass chip can be easily cleaned and used repeatedly, bringing advantages in terms of cost and reproducibility in emulsion preparation.

Fabrication of polymeric microspheres for biomedical applications

Polymeric microspheres (PMs) have attracted great attention in the field of biomedicine in the last several decades due to their small particle size, special functionalities shown on the surface and high surface-to-volume ratio. However, how to fabricate PMs which can meet the clinical needs and transform laboratory achievements to industrial scale-up still remains a challenge. Therefore, advanced fabrication technologies are pursued. In this review, we summarize the technologies used to fabricate PMs, including emulsion-based methods, microfluidics, spray drying, coacervation, supercritical fluid and superhydrophobic surface-mediated method and their advantages and disadvantages. We also review the different structures, properties and functions of the PMs and their applications in the fields of drug delivery, cell encapsulation and expansion, scaffolds in tissue engineering, transcatheter arterial embolization and artificial cells. Moreover, we discuss existing challenges and future perspectives for advancing fabrication technologies and biomedical applications of PMs.

Revolutionizing targeting precision: microfluidics-enabled smart microcapsules for tailored delivery and controlled release

As promising delivery systems, smart microcapsules have garnered significant attention owing to their targeted delivery loaded with diverse active materials. By precisely manipulating fluids on the micrometer scale, microfluidic has emerged as a powerful tool for tailoring delivery systems based on potential applications. The desirable characteristics of smart microcapsules are associated with encapsulation capacity, targeted delivery capability, and controlled release of encapsulants. In this review, we briefly describe the principles of droplet-based microfluidics for smart microcapsules. Subsequently, we summarize smart microcapsules as delivery systems for efficient encapsulation and focus on target delivery patterns, including passive targets, active targets, and microfluidics-assisted targets. Additionally, based on release mechanisms, we review controlled release modes adjusted by smart membranes and on/off gates. Finally, we discuss existing challenges and potential implications associated with smart microcapsules.

Production and Reconfiguration of Double Emulsions by Temperature Control

Double emulsions are of great importance for both science and engineering. However, the production of multicore double-emulsion droplets is challenging and normally requires sophisticated microfluidic devices, which limits their availability to broader communities. Here, we propose a simple, precise, and scalable batch method for producing double emulsions with monodispersed multicores at milliliter per minute rates, using the most common means in laboratory, temperature. By rapidly cooling liquid crystal emulsions, the introduced temperature gradient around the emulsion droplets leads to the injection of monodispersed guest droplets to form double-emulsion droplets. The number of injected water droplets can be precisely controlled by adjusting the thermally induced mechanical force through the temperature difference and the cooling rate. In contrast to conventional microfluidic fabrication, this method processes all emulsion droplets simultaneously in a noncontact and in situ manner. Therefore, it has great flexibility, allows multiple processing of double emulsions of arbitrary shape, has good capacity for mass production, and offers excellent compatibility with technologies such as microfluidics. Finally, we demonstrate that temperature changes can also be used to release the inner droplets from the double emulsion. The proposed method offers a reversible tool for processing double emulsions with minimal cost and expertise and is applicable to droplet-based microsystems in materials science, photonics, sensors, pharmaceuticals, and biotechnology.

Droplet-Based Microfluidics as a Platform to Design Food-Grade Delivery Systems Based on the Entrapped Compound Type

Microfluidic technology has emerged as a powerful tool for several applications, including chemistry, physics, biology, and engineering. Due to the laminar regime, droplet-based microfluidics enable the development of diverse delivery systems based on food-grade emulsions, such as multiple emulsions, microgels, microcapsules, solid lipid microparticles, and giant liposomes. Additionally, by precisely manipulating fluids on the low-energy-demand micrometer scale, it becomes possible to control the size, shape, and dispersity of generated droplets, which makes microfluidic emulsification an excellent approach for tailoring delivery system properties based on the nature of the entrapped compounds. Thus, this review points out the most current advances in droplet-based microfluidic processes, which successfully use food-grade emulsions to develop simple and complex delivery systems. In this context, we summarized the principles of droplet-based microfluidics, introducing the most common microdevice geometries, the materials used in the manufacture, and the forces involved in the different droplet-generation processes into the microchannels. Subsequently, the encapsulated compound type, classified as lipophilic or hydrophilic functional compounds, was used as a starting point to present current advances in delivery systems using food-grade emulsions and their assembly using microfluidic technologies. Finally, we discuss the limitations and perspectives of scale-up in droplet-based microfluidic approaches, including the challenges that have limited the transition of microfluidic processes from the lab-scale to the industrial-scale.

Research on Integrated 3D Printing of Microfluidic Chips

Microfluidic chips have the advantages of miniaturization, integration, and portability, and are widely used in the early diagnosis of major diseases, personalized medical treatment, environmental detection, health quarantine, and other fields. The existing microfluidic chip manufacturing process is difficult to operate because of complex three-dimensional channels, complicated manufacturing steps, limited printing materials, the difficulty of operating the bonding process, and the need to purchase expensive new equipment. In this paper, an integrated molding method for microfluidic chips that integrates 3D printing and polymer dissolution technology is proposed. First, the channel mold of poly(vinyl alcohol) (PVA) or high impact polystyrene (HIPS) is dissolved to complete the manufacturing of the microfluidic chip channel. The integrated 3D-forming method of microfluidic chips proposed in this paper can manufacture microchannels inside the microfluidic chip, avoid the bonding process, and eliminate the need for rapid alignment of microchannels, material modification, and other operations, thus improving the stability of the process. Finally, by comparing the microchannels made by PVA and HIPS, it is concluded that the quality of the microchannels made by HIPS is obviously better than that made by PVA. This paper provides a new idea for the fabrication of microfluidic chips and the application of HIPS.

Lego-Inspired Glass Capillary Microfluidic Device: A Technique for Bespoke Microencapsulation of Phase Change Materials

We report a Lego-inspired glass capillary microfluidic device capable of encapsulating both organic and aqueous phase change materials (PCMs) with high reproducibility and 100% PCM yield. Oil-in-oil-in-water (O/O/W) and water-in-oil-in-water (W/O/W) core-shell double emulsion droplets were formed to encapsulate hexadecane (HD, an organic PCM) and salt hydrate SP21EK (an aqueous PCM) in a UV-curable polymeric shell, Norland Optical Adhesive (NOA). The double emulsions were consolidated through on-the-fly polymerization, which followed thiol-ene click chemistry for photoinitiation. The particle diameters and shell thicknesses of the microcapsules were controlled by manipulating the geometry of glass capillaries and fluid flow rates. The microcapsules were monodispersed and exhibited the highest encapsulation efficiencies of 65.4 and 44.3% for HD and SP21EK-based materials, respectively, as determined using differential scanning calorimetry (DSC). The thermogravimetric (TGA) analysis confirmed much higher thermal stability of both encapsulated PCMs compared to pure PCMs. Polarization microscopy revealed that microcapsules could sustain over 100 melting-crystallization cycles without any structural changes. Bifunctional microcapsules with remarkable photocatalytic activity along with thermal energy storage performance were produced after the addition of 1 wt % titanium dioxide (TiO₂) nanoparticles (NPs) into the polymeric shell. The presence of TiO₂ NPs in the shell was confirmed by higher opacity and whiteness of these microcapsules and was quantified by energy dispersive X-ray (EDX) spectroscopy. Young's modulus of HD-based microcapsules estimated using micromanipulation analysis increased from 58.5 to 224 MPa after TiO₂ incorporation in the shell.

Determination of the Dominating Coalescence Pathways in Double Emulsion Formulations by Use of Microfluidic Emulsions

In water-in-oil-in-water (W1/O/W2) double emulsions several irreversible instability phenomena lead to changes. Besides diffusive processes, coalescence of droplets is the main cause of structural changes. In double emulsions, inner droplets can coalesce with each other (W1–W1 coalescence), inner droplets can be released via coalescence (W1–W2 coalescence) and oil droplets can coalesce with each other (O–O coalescence). Which of the coalescence pathways contributes most to the failure of the double emulsion structure cannot be determined by common measurement techniques. With monodisperse double emulsions produced with microfluidic techniques, each coalescence path can be observed and quantified simultaneously. By comparing the occurrence of all possible coalescence events, different hydrophilic surfactants in combination with PGPR are evaluated and discussed with regard to their applicability in double emulsion formulations. When variating the hydrophilic surfactant, the stability against all three coalescence mechanisms changes. This shows that measuring only one of the coalescence mechanisms is not sufficient to describe the stability of a double emulsion. While some surfactants are able to stabilize against all three possible coalescence mechanisms, some display mainly one of the coalescence mechanisms or in some cases all three mechanisms are observed simultaneously.

Recent Advances in Drug Delivery System Fabricated by Microfluidics for Disease Therapy

Traditional drug therapy faces challenges such as drug distribution throughout the body, rapid degradation and excretion, and extensive adverse reactions. In contrast, micro/nanoparticles can controllably deliver drugs to target sites to improve drug efficacy. Unlike traditional large-scale synthetic systems, microfluidics allows manipulation of fluids at the microscale and shows great potential in drug delivery and precision medicine. Well-designed microfluidic devices have been used to fabricate multifunctional drug carriers using stimuli-responsive materials. In this review, we first introduce the selection of materials and processing techniques for microfluidic devices. Then, various well-designed microfluidic chips are shown for the fabrication of multifunctional micro/nanoparticles as drug delivery vehicles. Finally, we describe the interaction of drugs with lymphatic vessels that are neglected in organs-on-chips. Overall, the accelerated development of microfluidics holds great potential for the clinical translation of micro/nanoparticle drug delivery systems for disease treatment.

Facile Microfluidic Fabrication of Biocompatible Hydrogel Microspheres in a Novel Microfluidic Device

Poly(ethylene glycol) diacrylate (PEGDA) microgels with tuneable size and porosity find applications as extracellular matrix mimics for tissue-engineering scaffolds, biosensors, and drug carriers. Monodispersed PEGDA microgels were produced by modular droplet microfluidics using the dispersed phase with 49–99 wt% PEGDA, 1 wt% Darocur 2959, and 0–50 wt% water, while the continuous phase was 3.5 wt% silicone-based surfactant dissolved in silicone oil. Pure PEGDA droplets were fully cured within 60 s at the UV light intensity of 75 mW/cm2. The droplets with higher water content required more time for curing. Due to oxygen inhibition, the polymerisation started in the droplet centre and advanced towards the edge, leading to a temporary solid core/liquid shell morphology, confirmed by tracking the Brownian motion of fluorescent latex nanoparticles within a droplet. A volumetric shrinkage during polymerisation was 1–4% for pure PEGDA droplets and 20–32% for the droplets containing 10–40 wt% water. The particle volume increased by 36–50% after swelling in deionised water. The surface smoothness and sphericity of the particles decreased with increasing water content in the dispersed phase. The porosity of swollen particles was controlled from 29.7% to 41.6% by changing the water content in the dispersed phase from 10 wt% to 40 wt%.

Simplified, Shear Induced Generation of Double Emulsions for Robust Compartmentalization during Single Genome Analysis

Drop microfluidics has driven innovations for high throughput, low input analysis techniques such as single-cell RNA-seq. However, the instability of single emulsion (SE) drops occasionally causes significant merging during drop processing, limiting most applications to single-step reactions in drops. Here, we show that double emulsion (DE) drops address this critical limitation and completely prevent drop contents from mixing. DEs show excellent stability during thermal cycling. More importantly, DEs undergo rupture into the continuous phase instead of merging, preventing content mixing and eliminating unstable drops from the downstream analysis. Due to the lack of drop merging, the monodispersity of drops is maintained throughout a workflow, enabling the deterministic manipulation of drops downstream. We also developed a simple, one-layer DE drop maker compatible with simple surface treatment using a plasma cleaner. The device allows for the robust production of single-core DEs at a wide range of flow rates and better control over the shell thickness, both of which have been significant limitations of conventional two-layer devices. This approach makes the fabrication of DE devices much more accessible, facilitating its broader adoption. Finally, we show that DE droplets eliminate content mixing and maintain compartmentalization of single virus genomes during PCR-based amplification and barcoding, while SEs mixed contents due to merging. With their resistance to content mixing, DE drops have key advantages for multistep reactions in drops, which is limited in SEs due to merging and content mixing.

Oil Droplet Coalescence in W/O/W Double Emulsions Examined in Models from Micrometer- to Millimeter-Sized Droplets

Water-in-oil-in-water (W1/O/W2) double emulsions must resist W1–W1, O–O and W1–W2 coalescence to be suitable for applications. This work isolates the stability of the oil droplets in a double emulsion, focusing on the impact of the concentration of the hydrophilic surfactant. The stability against coalescence was measured on droplets ranging in size from millimeters to micrometers, evaluating three different measurement methods. The time between the contact and coalescence of millimeter-sized droplets at a planar interface was compared to the number of coalescence events in a microfluidic emulsion and to the change in the droplet size distributions of micrometer-sized single and double emulsions. For the examined formulations, the same stability trends were found in all three droplet sizes. When the concentration of the hydrophilic surfactant is reduced drastically, lipophilic surfactants can help to increase the oil droplets’ stability against coalescence. This article also provides recommendations as to which purpose each of the model experiments is suited and discusses advantages and limitations compared to previous research carried out directly on double emulsions.