One-step generation of cell-laden microgels using double emulsion drops with a sacrificial ultra-thin oil shell

Droplet Microfluidics Powered Hydrogel Microparticles for Stem Cell-Mediated Biomedical Applications

Stem cell-related therapeutic technologies have garnered significant attention of the research community for their multi-faceted applications. To promote the therapeutic effects of stem cells, the strategies for cell microencapsulation in hydrogel microparticles have been widely explored, as the hydrogel microparticles have the potential to facilitate oxygen diffusion and nutrient transport alongside their ability to promote crucial cell-cell and cell-matrix interactions. Despite their significant promise, there is an acute shortage of automated, standardized, and reproducible platforms to further stem cell-related research. Microfluidics offers an intriguing platform to produce stem cell-laden hydrogel microparticles (SCHMs) owing to its ability to manipulate the fluids at the micrometer scale as well as precisely control the structure and composition of microparticles. In this review, the typical biomaterials and crosslinking methods for microfluidic encapsulation of stem cells as well as the progress in droplet-based microfluidics for the fabrication of SCHMs are outlined. Moreover, the important biomedical applications of SCHMs are highlighted, including regenerative medicine, tissue engineering, scale-up production of stem cells, and microenvironmental simulation for fundamental cell studies. Overall, microfluidics holds tremendous potential for enabling the production of diverse hydrogel microparticles and is worthy for various stem cell-related biomedical applications.

Gravity-Oriented Microfluidic Device for Biocompatible End-to-End Fabrication of Cell-Laden Microgels

Droplet microfluidics are extensively utilized to generate monodisperse cell-laden microgels in biomedical applications. However, maintaining cell viability is still challenging due to overexposure to harsh conditions in subsequent procedures that recover the microgels from the oil phase. Here, a gravity-oriented microfluidic device for end-to-end fabrication of cell-laden microgels is reported, which integrates dispersion, gelation, and extraction into a continuous workflow. This innovative on-chip extraction, driven by native buoyancy and kinetically facilitated by pseudosurfactant, exhibits 100% retrieval efficiency for microgels with a wide range of sizes and stiffnesses. The viability of encapsulated cells is perfectly maintained at ≈98% with minimal variations within and between batches. The end-to-end fabrication remarkably enhances the biocompatibility and practicality of microfluidics-based cell encapsulation and is promising to be compatible with various applications ranging from single-cell analysis to clinical therapy.

Single-Step Biofabrication of In Situ Spheroid-Forming Compartmentalized Hydrogel for Clinical-Sized Cartilage Tissue Formation

3D cellular spheroids offer more biomimetic microenvironments than conventional 2D cell culture technologies, which has proven value for many tissue engineering applications. Despite beneficiary effects of 3D cell culture, clinical translation of spheroid tissue engineering is challenged by limited scalability of current spheroid formation methods. Although recent adoption of droplet microfluidics can provide a continuous production process, use of oils and surfactants, generally low throughput, and requirement of additional biofabrication steps hinder clinical translation of spheroid culture. Here, the use of clean (e.g., oil-free and surfactant-free), ultra-high throughput (e.g., 8.5 mL min-1 , 10 000 spheroids s-1 ), single-step, in-air microfluidic biofabrication of spheroid forming compartmentalized hydrogels is reported. This novel technique can reliably produce 1D fibers, 2D planes, and 3D volumes compartmentalized hydrogel constructs, which each allows for distinct (an)isotropic orientation of hollow spheroid-forming compartments. Spheroids produced within ink-jet bioprinted compartmentalized hydrogels outperform 2D cell cultures in terms of chondrogenic behavior. Moreover, the cellular spheroids can be harvested from compartmentalized hydrogels and used to build shape-stable centimeter-sized biomaterial-free living tissues in a bottom-up manner. Consequently, it is anticipated that in-air microfluidic production of spheroid-forming compartmentalized hydrogels can advance production and use of cellular spheroids for various biomedical applications.

Sacrificial biomaterials in 3D fabrication of scaffolds for tissue engineering applications

Three-dimensional (3D) printing technology has progressed exceedingly in the area of tissue engineering. Despite the tremendous potential of 3D printing, building scaffolds with complex 3D structure, especially with soft materials, still exist as a challenge due to the low mechanical strength of the materials. Recently, sacrificial materials have emerged as a possible solution to address this issue, as they could serve as temporary support or templates to fabricate scaffolds with intricate geometries, porous structures, and interconnected channels without deformation or collapse. Here, we outline the various types of scaffold biomaterials with sacrificial materials, their pros and cons, and mechanisms behind the sacrificial material removal, compare the manufacturing methods such as salt leaching, electrospinning, injection-molding, bioprinting with advantages and disadvantages, and discuss how sacrificial materials could be applied in tissue-specific applications to achieve desired structures. We finally conclude with future challenges and potential research directions.

Formation Mechanism of Starch-Based Double Emulsions from the Interfacial Perspective

Double emulsions are of significant practical value in protecting the core material owing to their multicomponent structure and have thus been applied in various fields, such as food, cosmetics, and drugs. However, the mechanism of double emulsion formation by native starch is not well established. Herein, we demonstrate a facile route to develop type-A, type-B, and type-C double emulsions using native starch and develop an innovative design for a carrier. Interfacial interaction, enthalpy changes of starch, and interfacial properties are key factors governing the formation of double emulsions and controlling the type of double emulsions formed. Therefore, the results of this study provide a better understanding of how and what type of starch-based double emulsions are formed.

Microfluidic Droplet-Assisted Fabrication of Vessel-Supported Tumors for Preclinical Drug Discovery

High-fidelity in vitro tumor models are important for preclinical drug discovery processes. Currently, the most commonly used model for in vitro drug testing remains the two-dimensional (2D) cell monolayer. However, the natural in vivo tumor microenvironment (TME) consists of extracellular matrix (ECM), supporting stromal cells and vasculature. They not only participate in the progression of tumors but also hinder drug delivery and effectiveness on tumor cells. Here, we report an integrated engineering system to generate vessel-supported tumors for preclinical drug screening. First, gelatin-methacryloyl (GelMA) hydrogel was selected to mimic tumor extracellular matrix (ECM). HCT-116 tumor cells were encapsulated into individual micro-GelMA beads with microfluidic droplet technique to mimic tumor-ECM interactions in vitro. Then, normal human lung fibroblasts were mingled with tumor cells to imitate the tumor-stromal interaction. The tumor cells and fibroblasts reconstituted in the individual GelMA microbead and formed a biomimetic heterotypic tumor model with a core-shell structure. Next, the cell-laden beads were consociated into a functional on-chip vessel network platform to restore the tumor-tumor microenvironment (TME) interaction. Afterward, the anticancer drug paclitaxel was tested on the individual and vessel-supported tumor models. It was demonstrated that the blood vessel-associated TME conferred significant additional drug resistance in the drug screening experiment. The reported system is expected to enable the large-scale fabrication of vessel-supported heterotypic tumor models of various cellular compositions. It is believed to be promising for the large-scale fabrication of biomimetic in vitro tumor models and may be valuable for improving the efficiency of preclinical drug discovery processes.

Hybrid cell constructs consisting of bioprinted cell-spheroids

Bioprinted cell constructs have been investigated for regeneration of various tissues. However, poor cell-cell interactions have limited their utility. Although cell-spheroids offer an alternative for efficient cell-cell interactions, they complicate bioprinting. Here, we introduce a new cell-printing process, fabricating cell-spheroids and cell-loaded constructs together without preparation of cell-spheroids in advance. Cells in mineral oil droplets self-assembled to form cell-spheroids due to the oil-aqueous interaction, exhibiting similar biological functions to the conventionally prepared cell-spheroids. By controlling printing parameters, spheroid diameter and location could be manipulated. To demonstrate the feasibility of this process, we fabricated hybrid cell constructs, consisting of endothelial cell-spheroids and stem cells loaded decellularized extracellular matrix/β-tricalcium phosphate struts for regenerating vascularized bone. The hybrid cell constructs exhibited strong angiogenic/osteogenic activities as a result of increased secretion of signaling molecules and synergistic crosstalk between the cells.

Microfluidic Fabrication of Natural Polymer-Based Scaffolds for Tissue Engineering Applications: A Review

Natural polymers, thanks to their intrinsic biocompatibility and biomimicry, have been largely investigated as scaffold materials for tissue engineering applications. Traditional scaffold fabrication methods present several limitations, such as the use of organic solvents, the obtainment of a non-homogeneous structure, the variability in pore size and the lack of pore interconnectivity. These drawbacks can be overcome using innovative and more advanced production techniques based on the use of microfluidic platforms. Droplet microfluidics and microfluidic spinning techniques have recently found applications in the field of tissue engineering to produce microparticles and microfibers that can be used as scaffolds or as building blocks for three-dimensional structures. Compared to standard fabrication technologies, microfluidics-based ones offer several advantages, such as the possibility of obtaining particles and fibers with uniform dimensions. Thus, scaffolds with extremely precise geometry, pore distribution, pore interconnectivity and a uniform pores size can be obtained. Microfluidics can also represent a cheaper manufacturing technique. In this review, the microfluidic fabrication of microparticles, microfibers and three-dimensional scaffolds based on natural polymers will be illustrated. An overview of their applications in different tissue engineering fields will also be provided.

Droplets microfluidics platform-A tool for single cell research

Cells are the most basic structural and functional units of living organisms. Studies of cell growth, differentiation, apoptosis, and cell-cell interactions can help scientists understand the mysteries of living systems. However, there is considerable heterogeneity among cells. Great differences between individuals can be found even within the same cell cluster. Cell heterogeneity can only be clearly expressed and distinguished at the level of single cells. The development of droplet microfluidics technology opens up a new chapter for single-cell analysis. Microfluidic chips can produce many nanoscale monodisperse droplets, which can be used as small isolated micro-laboratories for various high-throughput, precise single-cell analyses. Moreover, gel droplets with good biocompatibility can be used in single-cell cultures and coupled with biomolecules for various downstream analyses of cellular metabolites. The droplets are also maneuverable; through physical and chemical forces, droplets can be divided, fused, and sorted to realize single-cell screening and other related studies. This review describes the channel design, droplet generation, and control technology of droplet microfluidics and gives a detailed overview of the application of droplet microfluidics in single-cell culture, single-cell screening, single-cell detection, and other aspects. Moreover, we provide a recent review of the application of droplet microfluidics in tumor single-cell immunoassays, describe in detail the advantages of microfluidics in tumor research, and predict the development of droplet microfluidics at the single-cell level.

Biohybrid materials: Structure design and biomedical applications

Biohybrid materials are proceeded by integrating living cells and non-living materials to endow materials with biomimetic properties and functionalities by supporting cell proliferation and even enhancing cell functions. Due to the outstanding biocompatibility and programmability, biohybrid materials provide some promising strategies to overcome current problems in the biomedical field. Here, we review the concept and unique features of biohybrid materials by comparing them with conventional materials. We emphasize the structure design of biohybrid materials and discuss the structure-function relationships. We also enumerate the application aspects of biohybrid materials in biomedical frontiers. We believe this review will bring various opportunities to promote the communication between cell biology, material sciences, and medical engineering.

Microfluidic Formulation of Topological Hydrogels for Microtissue Engineering

Microfluidics has recently emerged as a powerful tool in generation of submillimeter-sized cell aggregates capable of performing tissue-specific functions, so-called microtissues, for applications in drug testing, regenerative medicine, and cell therapies. In this work, we review the most recent advances in the field, with particular focus on the formulation of cell-encapsulating microgels of small "dimensionalities": "0D" (particles), "1D" (fibers), "2D" (sheets), etc., and with nontrivial internal topologies, typically consisting of multiple compartments loaded with different types of cells and/or biopolymers. Such structures, which we refer to as topological hydrogels or topological microgels (examples including core-shell or Janus microbeads and microfibers, hollow or porous microstructures, or granular hydrogels) can be precisely tailored with high reproducibility and throughput by using microfluidics and used to provide controlled "initial conditions" for cell proliferation and maturation into functional tissue-like microstructures. Microfluidic methods of formulation of topological biomaterials have enabled significant progress in engineering of miniature tissues and organs, such as pancreas, liver, muscle, bone, heart, neural tissue, or vasculature, as well as in fabrication of tailored microenvironments for stem-cell expansion and differentiation, or in cancer modeling, including generation of vascularized tumors for personalized drug testing. We review the available microfluidic fabrication methods by exploiting various cross-linking mechanisms and various routes toward compartmentalization and critically discuss the available tissue-specific applications. Finally, we list the remaining challenges such as simplification of the microfluidic workflow for its widespread use in biomedical research, bench-to-bedside transition including production upscaling, further in vivo validation, generation of more precise organ-like models, as well as incorporation of induced pluripotent stem cells as a step toward clinical applications.

A double-step emulsification device for direct generation of double emulsions

In microfluidic step emulsification, the size of droplets generated in the dripping regime is predominantly determined by the nozzle's height and only weakly depends on the applied flow rates or liquid properties. While the generation of monodisperse emulsions at high throughput using step emulsifiers has been well established, the generation of double emulsions, i.e., liquid core-shell structures, is still challenging. Here, we demonstrate a novel double-step emulsification method for the direct generation of multi-core double-emulsions and provide a predictive model for the number of cores. While the mechanism of the formation of the core droplets or empty shell droplets follows the well-established scenario of simple step emulsification, the formation of double-emulsion droplets is strongly affected by the presence of the cores. Passing of the cores through the narrowing neck of the shell postpones shell pinch-off. In particular, we demonstrate that our system can be used for the generation of arbitrary large, tightly packed droplet clusters consisting of a controllable number of droplets. Finally, we discuss the options of upscaling the system for high-throughput generation of tailored double emulsions.

A Novel Step-T-Junction Microchannel for the Cell Encapsulation in Monodisperse Alginate-Gelatin Microspheres of Varying Mechanical Properties at High Throughput

Cell encapsulation has been widely employed in cell therapy, characterization, and analysis, as well as many other biomedical applications. While droplet-based microfluidic technology is advantageous in cell microencapsulation because of its modularity, controllability, mild conditions, and easy operation when compared to other state-of-art methods, it faces the dilemma between high throughput and monodispersity of generated cell-laden microdroplets. In addition, the lack of a biocompatible method of de-emulsification transferring cell-laden hydrogel from cytotoxic oil phase into cell culture medium also hurtles the practical application of microfluidic technology. Here, a novel step-T-junction microchannel was employed to encapsulate cells into monodisperse microspheres at the high-throughput jetting regime. An alginate–gelatin co-polymer system was employed to enable the microfluidic-based fabrication of cell-laden microgels with mild cross-linking conditions and great biocompatibility, notably for the process of de-emulsification. The mechanical properties of alginate-gelatin hydrogel, e.g., stiffness, stress–relaxation, and viscoelasticity, are fully adjustable in offering a 3D biomechanical microenvironment that is optimal for the specific encapsulated cell type. Finally, the encapsulation of HepG2 cells into monodisperse alginate–gelatin microgels with the novel microfluidic system and the subsequent cultivation proved the maintenance of the long-term viability, proliferation, and functionalities of encapsulated cells, indicating the promising potential of the as-designed system in tissue engineering and regenerative medicine.

Recent Advances in Microgels: From Biomolecules to Functionality

The emerging applications of hydrogel materials at different length scales, in areas ranging from sustainability to health, have driven the progress in the design and manufacturing of microgels. Microgels can provide miniaturized, monodisperse, and regulatable compartments, which can be spatially separated or interconnected. These microscopic materials provide novel opportunities for generating biomimetic cell culture environments and are thus key to the advances of modern biomedical research. The evolution of the physical and chemical properties has, furthermore, highlighted the potentials of microgels in the context of materials science and bioengineering. This review describes the recent research progress in the fabrication, characterization, and applications of microgels generated from biomolecular building blocks. A key enabling technology allowing the tailoring of the properties of microgels is their synthesis through microfluidic technologies, and this paper highlights recent advances in these areas and their impact on expanding the physicochemical parameter space accessible using microgels. This review finally discusses the emerging roles that microgels play in liquid-liquid phase separation, micromechanics, biosensors, and regenerative medicine.

Hydrogels as functional components in artificial cell systems

Recent years have seen substantial efforts aimed at constructing artificial cells from various molecular components with the aim of mimicking the processes, behaviours and architectures found in biological systems. Artificial cell development ultimately aims to produce model constructs that progress our understanding of biology, as well as forming the basis for functional bio-inspired devices that can be used in fields such as therapeutic delivery, biosensing, cell therapy and bioremediation. Typically, artificial cells rely on a bilayer membrane chassis and have fluid aqueous interiors to mimic biological cells. However, a desire to more accurately replicate the gel-like properties of intracellular and extracellular biological environments has driven increasing efforts to build cell mimics based on hydrogels. This has enabled researchers to exploit some of the unique functional properties of hydrogels that have seen them deployed in fields such as tissue engineering, biomaterials and drug delivery. In this Review, we explore how hydrogels can be leveraged in the context of artificial cell development. We also discuss how hydrogels can potentially be incorporated within the next generation of artificial cells to engineer improved biological mimics and functional microsystems.

Hydrogels for Single-Cell Microgel Production: Recent Advances and Applications

Single-cell techniques have become more and more incorporated in cell biological research over the past decades. Various approaches have been proposed to isolate, culture, sort, and analyze individual cells to understand cellular heterogeneity, which is at the foundation of every systematic cellular response in the human body. Microfluidics is undoubtedly the most suitable method of manipulating cells, due to its small scale, high degree of control, and gentle nature toward vulnerable cells. More specifically, the technique of microfluidic droplet production has proven to provide reproducible single-cell encapsulation with high throughput. Various in-droplet applications have been explored, ranging from immunoassays, cytotoxicity assays, and single-cell sequencing. All rely on the theoretically unlimited throughput that can be achieved and the monodispersity of each individual droplet. To make these platforms more suitable for adherent cells or to maintain spatial control after de-emulsification, hydrogels can be included during droplet production to obtain “microgels.” Over the past years, a multitude of research has focused on the possibilities these can provide. Also, as the technique matures, it is becoming clear that it will result in advantages over conventional droplet approaches. In this review, we provide a comprehensive overview on how various types of hydrogels can be incorporated into different droplet-based approaches and provide novel and more robust analytic and screening applications. We will further focus on a wide range of recently published applications for microgels and how these can be applied in cell biological research at the single- to multicell scale.

Microfluidic Tissue Engineering and Bio-Actuation

Bio-hybrid technologies aim to replicate the unique capabilities of biological systems that could surpass advanced artificial technologies. Soft bio-hybrid robots consist of synthetic and living materials and have the potential to self-assemble, regenerate, work autonomously, and interact safely with other species and the environment. Cells require a sufficient exchange of nutrients and gases, which is guaranteed by convection and diffusive transport through liquid media. The functional development and long-term survival of biological tissues in vitro can be improved by dynamic flow culture, but only microfluidic flow control can develop tissue with fine structuring and regulation at the microscale. Full control of tissue growth at the microscale will eventually lead to functional macroscale constructs, which are needed as the biological component of soft bio-hybrid technologies. This review summarizes recent progress in microfluidic techniques to engineer biological tissues, focusing on the use of muscle cells for robotic bio-actuation. Moreover, the instances in which bio-actuation technologies greatly benefit from fusion with microfluidics are highlighted, which include: the microfabrication of matrices, biomimicry of cell microenvironments, tissue maturation, perfusion, and vascularization.

Large-scale single-cell encapsulation in microgels through metastable droplet-templating combined with microfluidic-integration

Current techniques for the generation of cell-laden microgels are limited by numerous challenges, including poorly uncontrolled batch-to-batch variations, processes that are both labor- and time-consuming, the high expense of devices and reagents, and low production rates; this hampers the translation of laboratory findings to clinical applications. To address these challenges, we develop a droplet-based microfluidic strategy based on metastable droplet-templating and microchannel integration for the substantial large-scale production of single cell-laden alginate microgels. Specifically, we present a continuous processing method for microgel generation by introducing amphiphilic perfluoronated alcohols to obtain metastable emulsion droplets as sacrificial templates. In addition, to adapt to the metastable emulsion system, integrated microfluidic chips containing 80 drop-maker units are designed and optimized based on the computational fluid dynamics simulation. This strategy allows single cell encapsulation in microgels at a maximum production rate of 10 ml hr-1 of cell suspension while retaining cell viability and functionality. These results represent a significant advance toward using cell-laden microgels for clinical-relevant applications, including cell therapy, tissue regeneration and 3D bioprinting.

Oral delivery of stem-cell-loaded hydrogel microcapsules restores gut inflammation and microbiota

Mesenchymal stem cells (MSCs) are an attractive candidate for the treatment of inflammatory bowel disease (IBD), but their poor delivery rate to an inflamed colon is a major factor hampering the clinical potential of stem cell therapies. Moreover, there remains a formidable hurdle to overcome with regard to survival and homing in to injured sites. Here, we develop a strategy utilizing monodisperse hydrogel microcapsules with a thin intermediate oil layer prepared by a triple-emulsion drop-based microfluidic approach as an in-situ oral delivering carrier. The oral delivery of stem-cell-loaded hydrogel microcapsules (SC-HM) enhances MSC survival and retention in the hostile stomach environment due to the intermediate oil layer and low value of the overall stiffness, facilitating programmable cell release during gastrointestinal peristalsis. SC-HM is shown to induce tissue repair, reduce the colonic macrophage infiltration responsible for the secretion of the pro-inflammatory factors, and significantly mitigate the severity of IBD in a mouse model, where MSCs released by SC-HM successfully accumulate at the colonic crypt. Moreover, a metagenomics analysis reveals that SC-HM ameliorates the dysbiosis of specific bacterial genera, including Bacteroides acidifaciens, Lactobacillus (L.) gasseri, Lactobacillus reuteri, and L. intestinalis, implying optimization of the microorganism's composition and abundance. These findings demonstrate that SC-HM is a potential IBD treatment candidate.

Gradient Printing Alginate Herero Gel Microspheres for Three-Dimensional Cell Culture

Hydrogel microspheres are widely used in tissue engineering, such as 3D cell culture and injection therapy, and among which, heterogeneous microspheres are drawing much attention as a promising tool to carry multiple cell types in separated phases. However, it is still a big challenge to fabricate heterogeneous gel microspheres with excellent resolution and different material components in limited sizes. Here, we developed a multi-channel dynamic micromixer, which can use active mechanical mixing to achieve rapid mixing with multi-component materials and extrude the homogenized material. By changing the flow rate ratio of the solutions of the two components and by rapidly mixing in the micromixer, real-time concentration change of the mixed material at the outlet could be monitored in a process so-called “gradient printing”. By studying the mixing efficiency of the micromixer, its size and process parameters were optimized. Using the novel dynamic gradient printing method, the composition of the hydrogel microspheres can be distributed in any proportion and alginate heterogeneous gel microspheres with adjustable cell concentration were fabricated. The effects of cell concentration on cell viability and proliferation ability under three-dimensional culture conditions were also studied. The results showed that cells have very low death rate and can exchange substances within the microspheres. Due to the micromixing ability of the micromixers, the demand for biological reagents and materials such as cells, proteins, cytokines and other materials could be greatly reduced, which helps reduce the experimental cost and improve the feasibility of the method in practical use. The heterogeneous gel microsphere can be greatly valuable for research in various fields such as analytical chemistry, microarray, drug screening, and tissue culture.

Cell-based biocomposite engineering directed by polymers

Biological cells such as bacterial, fungal, and mammalian cells always exploit sophisticated chemistries and exquisite micro- and nano-structures to execute life activities, providing numerous templates for engineering bioactive and biomorphic materials, devices, and systems. To transform biological cells into functional biocomposites, polymer-directed cell surface engineering and intracellular functionalization have been developed over the past two decades. Polymeric materials can be easily adopted by various cells through polymer grafting or in situ hydrogelation and can successfully bridge cells with other functional materials as interfacial layers, thus achieving the manufacture of advanced biocomposites through bioaugmentation of living cells and transformation of cells into templated materials. This review article summarizes the recent progress in the design and construction of cell-based biocomposites by polymer-directed strategies. Furthermore, the applications of cell-based biocomposites in broad fields such as cell research, biomedicine, and bioenergy are discussed. Last, we provide personal perspectives on challenges and future trends in this interdisciplinary area.

Tailored Double Emulsions Made Simple

Double emulsions, such as water-in-oil-in-water droplets, are important material platforms for conducting fundamental research and for technological applications. To date, well-defined double-emulsion droplets consisting of a single water core and a thin oil shell can be exclusively formed with sophisticated microfluidic devices. The fabrication, preparation, and operation of such devices is challenging, which reduces the availability of tailored double emulsions to a limited community of experts. Here, a simple method is introduced to produce single-core double emulsions with high yield in large quantities, using a vortex mixer. Utilizing the density difference between the dispersed droplet and the continuous phase, this two-step emulsification method can achieve very small core droplet diameters below 10 μm and ultrathin shells with thicknesses below 1 μm. A detailed picture of the formation mechanism is provided and it is demonstrated that the process can be extended to produce multishell and multicore emulsions. Finally, its application is demonstrated to produce structurally colored colloidal supraparticles with unprecedented uniformity and yield. The method allows the creation of tailored double emulsions with minimal time, cost, effort, and expertise, and may widen its application to nonspecialized scientific communities.

One-Step Generation and Purification of Cell-Encapsulated Hydrogel Microsphere With an Easily Assembled Microfluidic Device

Cell-laden hydrogel microspheres with uniform size show great potential for tissue repair and drug screening applications. Droplet microfluidic systems have been widely used for the generation of cell-laden hydrogel microspheres. However, existing droplet microfluidic systems are mostly based on complex chips and are not compatible with well culture plates. Moreover, microspheres produced by droplet microfluidics need demulsification and purification from oil, which requires time and effort and may compromise cell viability. Herein, we present a simple one-step approach for producing and purifying hydrogel microspheres with an easily assembled microfluidic device. Droplets were generated and solidified in the device tubing. The obtained hydrogel microspheres were then transferred to a tissue culture plate filled with cell culture media and demulsified through evaporation of the oil at 37°C. The removal of oil caused the gelled microspheres to be released into the cell culture media. The encapsulated cells demonstrated good viability and grew into tumor spheroids in 12–14 days. Single cell-laden hydrogel microspheres were also obtained and grown into spheroid in 14 days. This one-step microsphere generation method shows good potential for applications in automated spheroid and organoid cultures as well as drug screening, and could potentially offer benefits for translation of cell/microgel technologies.

Cell-Inspired Hydrogel Microcapsules with a Thin Oil Layer for Enhanced Retention of Highly Reactive Antioxidants

In nature, individual cells are compartmentalized by a membrane that protects the cellular elements from the surrounding environment while simultaneously equipped with an antioxidant defense system to alleviate the oxidative stress resulting from light, oxygen, moisture, and temperature. However, this mechanism has not been realized in cellular mimics to effectively encapsulate and retain highly reactive antioxidants. Here, we report cell-inspired hydrogel microcapsules with an interstitial oil layer prepared by utilizing triple emulsion drops as templates to achieve enhanced retention of antioxidants. We employ ionic gelation for the hydrogel shell to prevent exposure of the encapsulated antioxidants to free radicals typically generated during photopolymerization. The interstitial oil layer in the microcapsule serves as an stimulus-responsive diffusion barrier, enabling efficient encapsulation and retention of antioxidants by providing an adequate pH microenvironment until osmotic pressure is applied to release the cargo on-demand. Moreover, addition of a lipophilic reducing agent in the oil layer induces a complementary reaction with the antioxidant, similar to the nonenzymatic antioxidant defense system in cells, leading to enhanced retention of the antioxidant activity. Furthermore, we show the complete recovery and even further enhancement in antioxidant activity by lowering the storage temperature, which decreases the oxidation rate while retaining the complementary reaction with the lipophilic reducing agent.

Droplet microfluidics-based biomedical microcarriers

Droplet microfluidic technology provides a new platform for controllable generation of microdroplets and droplet-derived materials. In particular, because of the ability in high-throughput production and accurate control of the size, structure, and function of these materials, droplet microfluidics presents unique advantages in the preparation of functional microcarriers, i.e., microsized liquid containers or solid particles that serve as substrates of biomolecules or cells. These microcarriers could be extensively applied in the areas of cell culture, tissue engineering, and drug delivery. In this review, we focus on the fabrication of microcarriers from droplet microfluidics, and discuss their applications in the biomedical field. We start with the basic principle of droplet microfluidics, including droplet generation regimes and its control methods. We then introduce the fabrication of biomedical microcarriers based on single, double, and multiple emulsion droplets, and emphasize the various applications of microcarriers in biomedical field, especially in 3D cell culture, drug development and biomedical detection. Finally, we conclude this review by discussing the limitations and challenges of droplet microfluidics in preparing microcarriers. STATEMENT OF SIGNIFICANCE: Because of its precise control and high throughput, droplet microfluidics has been employed to generate functional microcarriers, which have been widely used in the areas of drug development, tissue engineering, and regenerative medicine. This review is significant because it emphasizes recent progress in research on droplet microfluidics in the preparation and application of biomedical microcarriers. In addition, this review suggests research directions for the future development of biomedical microcarriers based on droplet microfluidics by presenting existing shortcomings and challenges.

Engineering Biological Tissues from the Bottom-Up: Recent Advances and Future Prospects

Tissue engineering provides a powerful solution for current organ shortages, and researchers have cultured blood vessels, heart tissues, and bone tissues in vitro. However, traditional top-down tissue engineering has suffered two challenges: vascularization and reconfigurability of functional units. With the continuous development of micro-nano technology and biomaterial technology, bottom-up tissue engineering as a promising approach for organ and tissue modular reconstruction has gradually developed. In this article, relevant advances in living blocks fabrication and assembly techniques for creation of higher-order bioarchitectures are described. After a critical overview of this technology, a discussion of practical challenges is provided, and future development prospects are proposed.

Microfluidic-templating alginate microgels crosslinked by different metal ions as engineered microenvironment to regulate stem cell behavior for osteogenesis

Cell microenvironment is a collection of dynamic biochemical and biophysical cues which functions as the key factor in determining cell behavior. Encapsulating single cell into micrometer-scale hydrogels which mimics the cell microenvironment can be used for single cell analysis, cell therapies, and tissue engineering. Here, we developed a microfluidics-based platform to engineer the niche environment at single cell level using alginate microgels crosslinked by different metal ions to regulate stem cell behavior for bone regeneration. Specifically, we revealed that Ca²⁺ in the engineered microenvironment promoted osteogenic differentiation of encapsulated stem cells and substantially accelerated the matrix mineralization compared to Sr²⁺in vitro. However, the superior osteoinductive capacity of Ca²⁺ compared with Sr²⁺ led to comparable bone healing in a rat bone defect model. This attributed to Sr²⁺ in microgels to inhibit the osteoclast activity and bone resorption after implantation. In summary, the present study demonstrates metal ions as a critical factor in the environmental cues to affect cell behavior and influence the efficacy of stem cell-based therapy in tissue regeneration, and provides new insights to engineer an expecting microenvironment for regenerative medicine.

Droplet microfluidic devices for organized stem cell differentiation into germ cells: capabilities and challenges

Demystifying the mechanisms that underlie germline development and gamete production is critical for expanding advanced therapies for infertile couples who cannot benefit from current infertility treatments. However, the low number of germ cells, particularly in the early stages of development, represents a serious challenge in obtaining sufficient materials required for research purposes. In this regard, pluripotent stem cells (PSCs) have provided an opportunity for producing an unlimited source of germ cells in vitro. Achieving this ambition is highly dependent on accurate stem cell niche reconstitution which is achievable through applying advanced cell engineering approaches. Recently, hydrogel microparticles (HMPs), as either microcarriers or microcapsules, have shown promising potential in providing an excellent 3-dimensional (3D) biomimetic microenvironment alongside the systematic bioactive agent delivery. In this review, recent studies of utilizing various HMP-based cell engineering strategies for appropriate niche reconstitution and efficient in vitro differentiation are highlighted with a special focus on the capabilities of droplet-based microfluidic (DBM) technology. We believe that a deep understanding of the current limitations and potentials of the DBM systems in integration with stem cell biology provides a bright future for germ cell research.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s12551-021-00907-5.

Dynamics of polydisperse multiple emulsions in microfluidic channels

Multiple emulsions are a class of soft fluid in which small drops are immersed within a larger one and stabilized over long periods of time by a surfactant. We recently showed that, if a monodisperse multiple emulsion is subject to a pressure-driven flow, a wide variety of nonequilibrium steady states emerges at late times, whose dynamics relies on a complex interplay between hydrodynamic interactions and multibody collisions among internal drops. In this work, we use lattice Boltzmann simulations to study the dynamics of polydisperse double emulsions driven by a Poiseuille flow within a microfluidic channel. Our results show that their behavior is critically affected by multiple factors, such as initial position, polydispersity index, and area fraction occupied within the emulsion. While at low area fraction inner drops may exhibit either a periodic rotational motion (at low polydispersity) or arrange into nonmotile configurations (at high polydispersity) located far from each other, at larger values of area fraction they remain in tight contact and move unidirectionally. This decisively conditions their close-range dynamics, quantitatively assessed through a time-efficiency-like factor. Simulations also unveil the key role played by the capsule, whose shape changes can favor the formation of a selected number of nonequilibrium states in which both motile and nonmotile configurations are found.

Polysaccharide-Based Hydrogels for Microencapsulation of Stem Cells in Regenerative Medicine

Stem cell-based therapy appears as a promising strategy to induce regeneration of damaged and diseased tissues. However, low survival, poor engraftment and a lack of site-specificity are major drawbacks. Polysaccharide hydrogels can address these issues and offer several advantages as cell delivery vehicles. They have become very popular due to their unique properties such as high-water content, biocompatibility, biodegradability and flexibility. Polysaccharide polymers can be physically or chemically crosslinked to construct biomimetic hydrogels. Their resemblance to living tissues mimics the native three-dimensional extracellular matrix and supports stem cell survival, proliferation and differentiation. Given the intricate nature of communication between hydrogels and stem cells, understanding their interaction is crucial. Cells are incorporated with polysaccharide hydrogels using various microencapsulation techniques, allowing generation of more relevant models and further enhancement of stem cell therapies. This paper provides a comprehensive review of human stem cells and polysaccharide hydrogels most used in regenerative medicine. The recent and advanced stem cell microencapsulation techniques, which include extrusion, emulsion, lithography, microfluidics, superhydrophobic surfaces and bioprinting, are described. This review also discusses current progress in clinical translation of stem-cell encapsulated polysaccharide hydrogels for cell delivery and disease modeling (drug testing and discovery) with focuses on musculoskeletal, nervous, cardiac and cancerous tissues.

Recent Advances in Microfluidic Platforms for Programming Cell-Based Living Materials

Cell-based living materials, including single cells, cell-laden fibers, cell sheets, organoids, and organs, have attracted intensive interests owing to their widespread applications in cancer therapy, regenerative medicine, drug development, and so on. Significant progress in materials, microfabrication, and cell biology have promoted the development of numerous promising microfluidic platforms for programming these cell-based living materials with a high-throughput, scalable, and efficient manner. In this review, the recent progress of novel microfluidic platforms for programming cell-based living materials is presented. First, the unique features, categories, and materials and related fabrication methods of microfluidic platforms are briefly introduced. From the viewpoint of the design principles of the microfluidic platforms, the recent significant advances of programming single cells, cell-laden fibers, cell sheets, organoids, and organs in turns are then highlighted. Last, by providing personal perspectives on challenges and future trends, this review aims to motivate researchers from the fields of materials and engineering to work together with biologists and physicians to promote the development of cell-based living materials for human healthcare-related applications.

Thermo-responsive fluorinated surfactant for on-demand demulsification of microfluidic droplets

Droplet microfluidics has recently emerged as a powerful platform for a variety of biomedical applications including microreactors, bioactive compound encapsulation, and single cell culture and analysis; all these applications require long-term droplet stability, which, however, makes breaking the emulsion and retrieving the loaded samples difficult. Herein, we developed a novel class of thermo-responsive fluorosurfactants to control the droplet status simply by temperature. The surfactants were synthesized by coupling perfluorinated polyethers (PFPEs) with a thermo-responsive block of poly(N-isopropylacrylamide) (pNIPAM) or poly(2-ethyl-2-oxazoline) (pEtOx) with lower critical solution temperature (LCST). These diblock surfactants can stabilize the emulsion at temperatures below LCST due to the hydrophilic head, which became hydrophobic upon increasing the ambient temperature above LCST, thereby destabilizing the droplets and realizing demulsification simply via temperature control. The diblock surfactant can be applied for templating cell encapsulation using alginate microgels, which allowed one-step and high-throughput microfluidic generation of cell-laden microgels without compromising cell viability. This non-invasive, on-demand demulsification strategy provides a high degree of freedom for microencapsulation and on-demand recovery of the samples or reaction products within the droplets, which opens a new avenue for a wide range of applications of droplet-templating microfluidics.

Rapid, Simple, and Inexpensive Spatial Patterning of Wettability in Microfluidic Devices for Double Emulsion Generation

Water-in-oil-in-water (w/o/w) double emulsion (DE) encapsulation has been widely used as a promising platform technology for various applications in the fields of food, cosmetics, pharmacy, chemical engineering, materials science, and synthetic biology. Unfortunately, DEs formed by conventional emulsion generation approaches in most cases are highly polydisperse, making them less desirable for quantitative assays, controlled biomaterial synthesis, and entrapped ingredient release. Microfluidic devices can generate monodisperse DEs with controllable size, morphology, and production rate, but these generally require multistep fabrication processes and use of different solvents or bulky external instrumentation to pattern channel wettability. To overcome these limitations, we propose a rapid, simple, and inexpensive method to spatially pattern wettability in microfluidic devices for the continuous generation of monodisperse DEs. This is achieved by applying corona-plasma treatment to a select zone of the microchannel surface aided by a custom-designed corona resistance microchannel to strictly confine the plasma-treatment zone in a single polydimethylsiloxane (PDMS) microfluidic device. The properties of PDMS channel surfaces and key microchannel regions for DE generation are characterized under different levels of treatment. The size, shell thickness, and number of inner cores of generated DEs are shown to be highly controllable by tuning the phase flow rate ratios. Using DEs as templates, we successfully achieve a one-step generation and collection of gelatin microgels. Additionally, we demonstrate the biological capability of generated DEs by flow cytometric screening of the encapsulation and growth of yeast cells within DEs. We expect that the proposed approach will be widely used to create microfluidic devices with more complex wettability patterns.

Alginate microgels as delivery vehicles for cell-based therapies in tissue engineering and regenerative medicine

Conventional stem cell delivery typically utilize administration of directly injection of allogenic cells or domesticated autogenic cells. It may lead to immune clearance of these cells by the host immune systems. Alginate microgels have been demonstrated to improve the survival of encapsulated cells and overcome rapid immune clearance after transplantation. Moreover, alginate microgels can serve as three-dimensional extracellular matrix to support cell growth and protect allogenic cells from rapid immune clearance, with functions as delivery vehicles to achieve sustained release of therapeutic proteins and growth factors from the encapsulated cells. Besides, cell-loaded alginate microgels can potentially be applied in regenerative medicine by serving as injectable engineered scaffolds to support tissue regrowth. In this review, the properties of alginate and different methods to produce alginate microgels are introduced firstly. Then, we focus on diverse applications of alginate microgels for cell delivery in tissue engineering and regenerative medicine.

Double emulsions with ultrathin shell by microfluidic step-emulsification

Double emulsions with ultrathin shells are important in some biomedical applications, such as controlled drug release. However, the existing production techniques require two or more manipulation steps, or more complicated channel geometry, to form thin-shell double emulsions. This work presents a novel microfluidic tri-phasic step-emulsification device, with an easily fabricated double-layer PDMS channel, for production of oil-in-oil-in-water and water-in-water-in-oil double emulsions in a single step. The shell thickness is controlled by the flow rates and can reach 1.4% of the μm-size droplet diameter. Four distinct emulsification regimes are observed depending on the experimental conditions. A theoretical model for the tri-phasic step-emulsification is proposed to predict the boundaries separating the four regimes of emulsification in plane of two dimensionless capillary numbers, Ca. The theory yields two coupled nonlinear differential equations that can be solved numerically to find the approximate shape of the free interfaces in the shallow (Hele-Shaw) microfluidic channel. This approximation is then used as the initial guess for the more accurate finite element method solution, showing very good agreement with the experimental findings. This study demonstrates the feasibility of co-flow step-emulsification as a promising method to production of double (and multiple) emulsions and micro-capsules with ultrathin shells of controllable thickness.

A Versatile Microencapsulation Platform for Hyaluronic Acid and Polyethylene Glycol

Cell microencapsulation is a rapidly expanding field with broad potential for stem cell therapies and tissue engineering research. Traditional alginate microspheres suffer from poor biocompatibility, and microencapsulation of more advanced hydrogels is challenging due to their slower gelation rates. We have developed a novel, noncytotoxic, nonemulsion-based method to produce hydrogel microspheres compatible with a wide variety of materials, called core-shell spherification (CSS). Fabrication of microspheres by CSS derived from two slow-hardening hydrogels, hyaluronic acid (HA) and polyethylene glycol diacrylate (PEGDA), was characterized. HA microspheres were manufactured with two different crosslinking methods: thiolation and methacrylation. Microspheres of methacrylated HA (MeHA) had the greatest swelling ratio, the largest average diameter, and the lowest diffusion barrier. In contrast, PEGDA microspheres had the smallest diameters, the lowest swelling ratio, and the highest diffusion barrier, while microspheres of thiolated HA had characteristics that were in between the other two groups. To test the ability of the hydrogels to protect cells, while promoting function, diabetic NOD mice received intraperitoneal injections of PEGDA or MeHA microencapsulated canine islets. PEGDA microspheres reversed diabetes for the length of the study (up to 16 weeks). In contrast, islets encapsulated in MeHA microspheres at the same dose restored normoglycemia, but only transiently (3-4 weeks). Nonencapsulated canine islet transplanted at the same dose did not restore normoglycemia for any length of time. In conclusion, CSS provides a nontoxic microencapsulation procedure compatible with various hydrogel types.

Surface-tension-induced double emulsion drops via phase separation of polymeric fluid confined in micromolds for capsule templates

We report a simple and rapid route to produce double emulsion drops by utilizing phase separation of the confined fluid in micromolds and surface-tension-induced drop formation. Specifically, we use cross-shaped micromolds containing prepolymer solution that phase-separates into two compartments upon addition of wetting fluid with separation agent (SA). Subsequently, Laplace pressure-driven flow allows it to form double emulsion drops without use of any surfactants and complex formulations of fluids. The size of each compartment in the emulsion drops can be controlled by tuning composition of the prepolymer solution and separation agent, making the double emulsion drops with varying shell thicknesses. The phase separation creates two compartments with different polarity (i.e. water-soluble and water-insoluble), enabling encapsulation of both hydrophilic and/-or hydrophobic cargoes in desired compartments depending on their solubility. In addition, we produce poly(N-isopropylacrylamide) (pNIPAm) hydrogel microcapsules by solidifying middle phase in the double emulsion drops; thus, hydrophilic large cargo loaded priorly in the core can be encapsulated within hydrogel shells. Finally, by taking advantage of hydrophilic-hydrophobic phase transition behavior of pNIPAm, we achieve encapsulation of small cargo via post-loading approach; the encapsulated cargo can be released by tuning temperature.

The vortex-driven dynamics of droplets within droplets

Understanding the fluid-structure interaction is crucial for an optimal design and manufacturing of soft mesoscale materials. Multi-core emulsions are a class of soft fluids assembled from cluster configurations of deformable oil-water double droplets (cores), often employed as building-blocks for the realisation of devices of interest in bio-technology, such as drug-delivery, tissue engineering and regenerative medicine. Here, we study the physics of multi-core emulsions flowing in microfluidic channels and report numerical evidence of a surprisingly rich variety of driven non-equilibrium states (NES), whose formation is caused by a dipolar fluid vortex triggered by the sheared structure of the flow carrier within the microchannel. The observed dynamic regimes range from long-lived NES at low core-area fraction, characterised by a planetary-like motion of the internal drops, to short-lived ones at high core-area fraction, in which a pre-chaotic motion results from multi-body collisions of inner drops, as combined with self-consistent hydrodynamic interactions. The onset of pre-chaotic behavior is marked by transitions of the cores from one vortex to another, a process that we interpret as manifestations of the system to maximize its entropy by filling voids, as they arise dynamically within the capsule.

Assembly of Multi-Spheroid Cellular Architectures by Programmable Droplet Merging

Artificial multicellular systems are gaining importance in the field of tissue engineering and regenerative medicine. Reconstruction of complex tissue architectures in vitro is nevertheless challenging, and methods permitting controllable and high-throughput fabrication of complex multicellular architectures are needed. Here, a facile and high-throughput method is developed based on a tunable droplet-fusion technique, allowing programmed assembly of multiple cell spheroids into complex multicellular architectures. The droplet-fusion technique allows for construction of various multicellular architectures (double-spheroids, multi-spheroids, hetero-spheroids) in a miniaturized high-density array format. As an example of application, the propagation of Wnt signaling is investigated within hetero-spheroids formed from two fused Wnt-releasing and Wnt-reporter cell spheroids. The developed method provides an approach for miniaturized, high-throughput construction of complex 3D multicellular architectures and can be applied for studying various biological processes including cell signaling, cancer invasion, embryogenesis, and neural development.

Tumor localization of oncolytic adenovirus assisted by pH-degradable microgels with JQ1-mediated boosting replication and PD-L1 suppression for enhanced cancer therapy

Oncolytic therapy is a fast-developing cancer treatment field based on the promising clinical performance from the selective tumor cell killing and induction of systemic antitumor immunity. The virotherapy efficacy, however, is strongly hindered by the limited virus propagation and negative immune regulation in the tumor microenvironments. To enhance the antitumor activity, we developed injectable pH-degradable PVA microgels encapsulated with oncolytic adenovirus (OA) by microfluidics for localized OA delivery and cancer treatments. PVA microgels were tailored with an OA encapsulation efficiency of 68% and exhibited a pH-dependent OA release as the microgel degradation at mildly acidic conditions. PVA microgels mediated fast viral release and increased replication in HEK293T and A549 cells at a lower pH, and the replication efficiency could be further reinforced by co-loading with one BET bromodomain inhibitor JQ1, inducing significant cytotoxicity against A549 cells. An in vivo study revealed that OA release was highly located at the tumor tissue assisted by PVA microgels, and the OA infection was also enhanced with the addition of JQ1 treatment, meanwhile greatly inhibiting the PD-L1 expression to overcome the immune suppression. OA/JQ1 co-encapsulated injectable microgels exhibited a superior in vivo antitumor activity on the A549 lung tumor-bearing mice by the combination of inhibited proliferation, amplified oncolysis, and potential immune regulation.

Microfluidic Encapsulation of Single Cells by Alginate Microgels Using a Trigger-Gellified Strategy

Microfluidics-based alginate microgels have shown great potential to encapsulate cells in a high-throughput and controllable manner. However, cell viability and biological functions are substantially compromised due to the harsh conditions for gelation, which remains a major challenge for cell encapsulation. Herein, we presented an efficient and biocompatible method by on-chip triggered gelation to generate microfluidic alginate microgels for single-cell encapsulation. Two calcium complexes of calcium–ethylenediaminetetraacetic acid (Ca-EDTA) and calcium–nitrilotriacetic (Ca-NTA) as crosslinkers for triggered gelation of alginate were compared and investigated for feasible application. By triggered release of Ca²⁺ ions from the calcium complex via adding acetic acid in the oil phase, the alginate precursor in the aqueous droplets can be crosslinked to form alginate microgels. Although using Ca-EDTA and Ca-NTA both achieved on-chip gelation, Ca-NTA led to significantly higher cell viability since the dissociation of Ca²⁺ ions from Ca-NTA can be obtained using less concentration of acid compared to Ca-EDTA. We further demonstrated the functionality of encapsulated mesenchymal stem cells (MSCs) in alginate microgels prepared using Ca-NTA, as evidenced by the osteogenesis of encapsulated MSCs upon inductive culture. In summary, our study provided a biocompatible strategy to prepare alginate microgels for single-cell encapsulation which can be further used for applications in tissue engineering and cell therapies.

Enhanced single-cell encapsulation in microfluidic devices: From droplet generation to single-cell analysis

Single-cell analysis to investigate cellular heterogeneity and cell-to-cell interactions is a crucial compartment to answer key questions in important biological mechanisms. Droplet-based microfluidics appears to be the ideal platform for such a purpose because the compartmentalization of single cells into microdroplets offers unique advantages of enhancing assay sensitivity, protecting cells against external stresses, allowing versatile and precise manipulations over tested samples, and providing a stable microenvironment for long-term cell proliferation and observation. The present Review aims to give a preliminary guidance for researchers from different backgrounds to explore the field of single-cell encapsulation and analysis. A comprehensive and introductory overview of the droplet formation mechanism, fabrication methods of microchips, and a myriad of passive and active encapsulation techniques to enhance single-cell encapsulation efficiency were presented. Meanwhile, common methods for single-cell analysis, especially for long-term cell proliferation, differentiation, and observation inside microcapsules, are briefly introduced. Finally, the major challenges faced in the field are illustrated, and potential prospects for future work are discussed.

Multiphase flow in microfluidics: From droplets and bubbles to the encapsulated structures

Microfluidic technologies have a unique ability to control more precisely and effectively on two-phase flow systems in comparison with macro systems. Controlling the size of the droplets and bubbles has led to an ever-increasing expansion of this technology in two-phase systems. Liquid-liquid and gas-liquid two-phase flows because of their numerous applications in different branches such as reactions, synthesis, emulsions, cosmetic, food, drug delivery, etc. have been the most critical two-phase flows in microfluidic systems. This review highlights recent progress in two-phase flows in microfluidic devices. The fundamentals of two-phase flows, including some essential dimensionless numbers, governing equations, and some most well-known numerical methods are firstly introduced, followed by a review of standard methods for producing segmented flows such as emulsions in microfluidic systems. Then various encapsulated structures, a common two-phase flow structure in microfluidic devices, and different methods of their production are reviewed. Finally, applications of two-phase microfluidic flows in drug-delivery, biotechnology, mixing, and microreactors are briefly discussed.

Continuous microfluidic encapsulation of single mesenchymal stem cells using alginate microgels as injectable fillers for bone regeneration

The encapsulation of cells in microscale hydrogels can provide a mimic of a three-dimensional (3D) microenvironment to support cell viability and functions and to protect cells from the environmental stress, which have been widely used in tissue regeneration and cell therapies. Here, a microfluidics-based approach is developed for continuous encapsulation of mesenchymal stem cells (MSCs) at the single-cell level using alginate microgels. This microfluidic technique integrated on-chip encapsulation, gelation, and de-emulsification into a one-step fabrication process, which enables scalable cell encapsulation while retaining the viability and functionality of loaded cells. Remarkably, we observed MSCs encapsulated in Ca-alginate microgels at the single-cell level showed significantly enhanced osteogenesis and accelerated mineralization of the microgels which occurred only after 7 days of induction. Furthermore, MSCs laden in alginate microgels displayed significantly enhanced bone formation compared to MSCs mixed with microgels and acellular microgels in a rat tibial ablation model. To conclude, the current microfluidic technique represents a significant step toward continuous single cell encapsulation, fabrication, and purification. These microgels can boost bone regeneration by providing a controlled osteogenic microenvironment for encapsulated MSCs and facilitate stem cell therapy in the treatment of bone defects in a minimally invasive delivery way. STATEMENT OF SIGNIFICANCE: The biological functions and therapeutic activities of single cells laden in microgels for tissue engineering remains less investigated. Here, we reported a microfluidic-based method for continuous encapsulation of single MSCs with high viability and functionality by integrating on-chip encapsulation, gelation, and de-emulsification into a one-step fabrication process. More importantly, MSCs encapsulated in alginate microgels at the single-cell level showed significantly enhanced osteogenesis, remarkably accelerated mineralization in vitro and bone formation capacity in vivo. Therefore, this single-cell encapsulation technique can facilitate stem cell therapy for bone regeneration and be potentially used in a variety of tissue engineering applications.

Void-free 3D Bioprinting for In-situ Endothelialization and Microfluidic Perfusion

Two major challenges of 3D bioprinting are the retention of structural fidelity and efficient endothelialization for tissue vascularization. We address both of these issues by introducing a versatile 3D bioprinting strategy, in which a templating bioink is deposited layer-by-layer alongside a matrix bioink to establish void-free multimaterial structures. After crosslinking the matrix phase, the templating phase is sacrificed to create a well-defined 3D network of interconnected tubular channels. This void-free 3D printing (VF-3DP) approach circumvents the traditional concerns of structural collapse, deformation and oxygen inhibition, moreover, it can be readily used to print materials that are widely considered "unprintable". By pre-loading endothelial cells into the templating bioink, the inner surface of the channels can be efficiently cellularized with a confluent endothelial layer. This in-situ endothelialization method can be used to produce endothelium with a far greater uniformity than can be achieved using the conventional post-seeding approach. This VF-3DP approach can also be extended beyond tissue fabrication and towards customized hydrogel-based microfluidics and self-supported perfusable hydrogel constructs.

Development of a tunable method to generate various three-dimensional microstructures by replenishing macromolecules such as extracellular matrix components and polysaccharides

Multicellular spheroids (spheroids) are expected to be a promising approach to mimic in vivo organ functions and cell microenvironments. However, conventional spheroids do not fully consider the existence of extracellular matrices (ECMs). In this study, we developed a tunable method for replenishing macromolecules, including ECM components and polysaccharides, into spheroids without compromising cell viability by injecting a microvolume cell suspension into a high density of methylcellulose dissolved in the culture medium. Adjusting the ECM concentration in the cell suspension enabled the generation of different three-dimensional microstructures, such as "ECM gel capsules", which contained individually separated cells, and "ECM-loaded spheroids", which had thin ECM layers between cells. ECM-loaded spheroids with a 30-fold dilution of Matrigel (0.3 mg/ml) showed significantly higher albumin secretion than control spheroids composed of Hep G2 or HuH-7 cells. Additionally, the expression levels of major CYP genes were decreased in ECM gel capsules with undiluted Matrigel (9 mg/ml) compared to those in control spheroids. However, 0.3 mg/ml Matrigel did not disrupt gene expression. Furthermore, cell polarity associated with tight junction proteins (ZO-1 and Claudin-1) and the transporter protein MRP2 was markedly induced by using 0.3 mg/ml Matrigel. Thus, high-performance three-dimensional tissues fabricated by this method are applicable to increasing the efficiency of drug screening and to regenerative medicine.

Gelatin-Based 3D Microgels for In Vitro T Lineage Cell Generation

T cells are predominantly produced by the thymus and play a significant role in maintaining our adaptive immune system. Physiological involution of the thymus occurs gradually with age, compromising naive T cell output, which can have severe clinical complications. Also, T cells are utilized as therapeutic agents in cancer immunotherapies. Therefore, there is an increasing need for strategies aimed at generating naive T cells. The majority of in vitro T cell generation studies are performed in two-dimensional (2D) cultures, which ignore the physiological thymic microenvironment and are not scalable; therefore, we applied a new three-dimensional (3D) approach. Here, we use a gelatin-based 3D microgel system for T lineage induction by co-culturing OP9-DL4 cells and mouse fetal-liver-derived hematopoietic stem cells (HSCs). Flow cytometric analysis revealed that microgel co-cultures supported T lineage induction similar to 2D cultures while providing a 3D environment. We also encapsulated mouse embryonic thymic epithelial cells (TECs) within the microgels to provide a defined 3D culture platform. The microgel system supported TEC maintenance and retained their phenotype. Together, these data show that our microgel system has the capacity for TEC maintenance and induction of in vitro T lineage differentiation with potential for scalability.

Microfluidics-based fabrication of cell-laden microgels

Microfluidic principles have been extensively utilized as powerful tools to fabricate controlled monodisperse cell-laden hydrogel microdroplets for various biological applications, especially tissue engineering. In this review, we report recent advances in microfluidic-based droplet fabrication and provide our rationale to justify the superiority of microfluidics-based techniques over other microtechnology methods in achieving the encapsulation of cells within hydrogels. The three main components of such a system-hydrogels, cells, and device configurations-are examined thoroughly. First, the characteristics of various types of hydrogels including natural and synthetic types, especially concerning cell encapsulation, are examined. This is followed by the elucidation of the reasoning behind choosing specific cells for encapsulation. Next, in addition to a detailed discussion of their respective droplet formation mechanisms, various device configurations including T-junctions, flow-focusing, and co-flowing that aid in achieving cell encapsulation are critically reviewed. We then present an outlook on the current applications of cell-laden hydrogel droplets in tissue engineering such as 3D cell culturing, rapid generation and repair of tissues, and their usage as platforms for studying cell-cell and cell-microenvironment interactions. Finally, we shed some light upon the prospects of microfluidics-based production of cell-laden microgels and propose some directions for forthcoming research that can aid in overcoming challenges currently impeding the translation of the technology into clinical success.