Cell-enclosing gelatin-based microcapsule production for tissue engineering using a microfluidic flow-focusing system

Phototuning of Hyaluronic-Acid-Based Hydrogel Properties to Control Network Formation in Human Vascular Endothelial Cells

In vitro network formation by endothelial cells serves as a fundamental model for studies aimed at understanding angiogenesis. The morphogenesis of these cells to form a network is intricately regulated by the mechanical and biochemical properties of the extracellular matrix. Here the effects of modulating these properties in hydrogels derived from phenolated hyaluronic acid (HA-Ph) and phenolated gelatin (Gelatin-Ph) are presented. Visible-light irradiation in the presence of tris(2,2'-bipyridyl)ruthenium(II) chloride hexahydrate and sodium persulfate induces the crosslinking of these polymers, thereby forming a hydrogel and degrading HA-Ph. Human vascular endothelial cells form networks on the hydrogel prepared by visible-light irradiation for 45 min (42 W cm-2 at 450 nm) but not on the hydrogels prepared by irradiation for 15, 30, or 60 min. The irradiation time-dependent degradation of HA-Ph and the changes in the mechanical stiffness of the hydrogels, coupled with the expressions of RhoA and β-actin genes and CD44 receptors in the cells, reveal that the network formation is synergistically influenced by the hydrogel stiffness and HA-Ph degradation. These findings highlight the potential of tailoring HA-based hydrogel properties to modulate human vascular endothelial cell responses, which is critical for advancing their application in vascular tissue engineering.

Effect of cell adhesiveness of Cell Dome shell on enclosed HeLa cells

The Cell Dome is a dome-shaped structure (diameter: 1 mm, height: 270 μm) with cells enclosed within a cavity, covered by a hemispherical hydrogel shell, and immobilized on a glass plate. Given that the cells within Cell Dome are in contact with the inner walls of the hydrogel shell, the properties of the shell are anticipated to influence cell behavior. To date, the impact of the hydrogel shell properties on the enclosed cells has not been investigated. In this study, we explored the effects of the cell adhesiveness of hydrogel shell on the behavior of enclosed cancer cells. Hydrogel shells with varying degrees of cell adhesiveness were fabricated using aqueous solutions containing either an alginate derivative with phenolic hydroxyl moieties exclusively or a mixture of alginate and gelatin derivatives with phenolic hydroxyl moieties. Hydrogel formation was mediated by horseradish peroxidase. We used the HeLa human cervical cancer cell line, which expresses fucci2, a cell cycle marker, to observe cell behavior. Cells cultured in hydrogel shells with cell adhesiveness proliferated along the inner wall of the hydrogel shell. Conversely, cells in hydrogel shells without cell adhesiveness grew uniformly at the bottom of the cavities. Furthermore, cells in non-adhesive hydrogel shells had a higher percentage of cells in the G1/G0 phase compared to those in adhesive shells and exhibited increased resistance to mitomycin hydrochloride when the cavities became filled with cells. These results highlight the need to consider the cell adhesiveness of the hydrogel shell when selecting materials for constructing Cell Dome.

Effect of Flow Rate Modulation on Alginate Emulsification in Multistage Microfluidics

The encapsulation of stem cells into alginate microspheres is an important aspect of tissue engineering or bioprinting which ensures cell growth and development. We previously demonstrated the encapsulation of stem cells using the hanging drop method. However, this conventional process takes a relatively long time and only produces a small-volume droplet. Here, an experimental approach for alginate emulsification in multistage microfluidics is reported. By using the microfluidic method, the emulsification of alginate in oil can be manipulated by tuning the flow rate for both phases. Two-step droplet emulsification is conducted in a series of polycarbonate and polydimethylsiloxane microfluidic chips. Multistage emulsification of alginate for stem cell encapsulation has been successfully reported in this study under certain flow rates. Fundamental non-dimensional numbers such as Reynolds and capillary are used to evaluate the effect of flow rate on the emulsification process. Reynolds numbers of around 0.5-2.5 for alginate/water and 0.05-0.2 for oil phases were generated in the current study. The capillary number had a maximum value of 0.018 to ensure the formation of plug flow. By using the multistage emulsification system, the flow rates of each process can be tuned independently, offering a wider range of droplet sizes that can be produced. A final droplet size of 500-1000 µm can be produced using flow rates of 0.1-0.5 mL/h and 0.7-2.4 mL/h for the first stage and second stage, respectively.

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.

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.

Cytoprotection of Human Progenitor and Stem Cells through Encapsulation in Alginate Templated, Dual Crosslinked Silk and Silk-Gelatin Composite Hydrogel Microbeads

Susceptibility of mammalian cells against harsh processing conditions limit their use in cell transplantation and tissue engineering applications. Besides modulation of the cell microenvironment, encapsulation of mammalian cells within hydrogel microbeads attract attention for cytoprotection through physical isolation of the encapsulated cells. The hydrogel formulations used for cell microencapsulation are largely dominated by ionically crosslinked alginate (Alg), which suffer from low structural stability under physiological culture conditions and poor cell-matrix interactions. Here the fabrication of Alg templated silk and silk/gelatin composite hydrogel microspheres with permanent or on-demand cleavable enzymatic crosslinks using simple and cost-effective centrifugation-based droplet processing are demonstrated. The composite microbeads display structural stability under ion exchange conditions with improved mechanical properties compared to ionically crosslinked Alg microspheres. Human mesenchymal stem and neural progenitor cells are successfully encapsulated in the composite beads and protected against environmental factors, including exposure to polycations, extracellular acidosis, apoptotic cytokines, ultraviolet (UV) irradiation, anoikis, immune recognition, and particularly mechanical stress. The microbeads preserve viability, growth, and differentiation of encapsulated stem and progenitor cells after extrusion in viscous polyethylene oxide solution through a 27-gauge fine needle, suggesting potential applications in injection-based delivery and three-dimensional bioprinting of mammalian cells with higher success rates.

Trends in Droplet Microfluidics: From Droplet Generation to Biomedical Applications

Over the past decade, droplet microfluidics has attracted growing interest in biology, medicine, and engineering. In this feature article, we review the advances in droplet microfluidics, primarily focusing on the research conducted by our group. Starting from the introduction to the mechanisms of microfluidic droplet formation and the strategies for cell encapsulation in droplets, we then focus on droplet transformation into microgels. Furthermore, we review three biomedical applications of droplet microfluidics, that is, 3D cell culture, single-cell analysis, and in vitro organ and disease modeling. We conclude with our perspective on future directions in the development of droplet microfluidics for biomedical applications.

Current advances in cell therapeutics: A biomacromolecules application perspective

INTRODUCTION

Many chronic diseases have evolved and to circumvent the limitations of using conventional drug therapies, smart cell encapsulating delivery systems have been explored to customize the treatment with alignment to disease longevity. Cell therapeutics has advanced in tandem with improvements in biomaterials that can suitably deliver therapeutic cells to achieve targeted therapy. Among the promising biomacromolecules for cell delivery are those that share bio-relevant architecture with the extracellular matrix and display extraordinary compatibility in the presence of therapeutic cells. Interestingly, many biomacromolecules that fulfil these tenets occur naturally and can form hydrogels.

AREAS COVERED

This review provides a concise incursion into the paradigm shift to cell therapeutics using biomacromolecules. Advances in the design and use of biomacromolecules to assemble smart therapeutic cell carriers is discussed in light of their pivotal role in enhancing cell encapsulation and delivery. In addition, the principles that govern the application of cell therapeutics in diabetes, neuronal disorders, cancers and cardiovascular disease are outlined.

EXPERT OPINION

Cell therapeutics promises to revolutionize the treatment of various secretory cell dysfunctions. Current and future advances in designing functional biomacromolecules will be critical to ensure that optimal delivery of therapeutic cells is achieved with desired biosafety and potency.

Shape-Programmable Three-Dimensional Microfluidic Structures

Microfluidic devices are gaining extensive interest due to their potential applications in wide-ranging areas, including lab-on-a-chip devices, fluid delivery, and artificial vascular networks. Most current microfluidic devices are in a planar design with fixed configurations once formed, which limits their applications such as in engineered vascular networks in biology and programmable drug delivery systems. Here, shape-programmable three-dimensional (3D) microfluidic structures, which are assembled from a bilayer of channel-embedded polydimethylsiloxane (PDMS) and shape-memory polymers (SMPs) via compressive buckling, are reported. 3D microfluidics in diverse geometries including those in open-mesh configurations are presented. In addition, they can be programmed into temporary shapes and recover their original shape under thermal stimuli due to the shape memory effect of the SMP component, with fluid flow in the microfluidic channels well maintained in both deformed and recovered shapes. Furthermore, the shape-fixing effect of SMPs enables freestanding open-mesh 3D microfluidic structures without the need for a substrate to maintain the 3D shape as used in previous studies. By adding magnetic particles into the PDMS layer, magnetically responsive 3D microfluidic structures are enabled to achieve fast, remote programming of the structures via a portable magnet. A 3D design phase diagram is constructed to show the effects of the magnetic PDMS/SMP thickness ratio and the volume fraction of magnetic particles on the shape programmability of the 3D microfluidic structures. The developed shape-programmable, open-mesh 3D microfluidic structures offer many opportunities for applications including tissue engineering, drug delivery, and many others.

Polymer microcapsules and microbeads as cell carriers for in vivo biomedical applications

Polymer microcarriers are being extensively explored as cell delivery vehicles in cell-based therapies and hybrid tissue and organ engineering. Spherical microcarriers are of particular interest due to easy fabrication and injectability. They include microbeads, composed of a porous matrix, and microcapsules, where matrix core is additionally covered with a semipermeable membrane. Microcarriers provide cell containment at implantation site and protect the cells from host immunoresponse, degradation and shear stress. Immobilized cells may be genetically altered to release a specific therapeutic product directly at the target site, eliminating side effects of systemic therapies. Cell microcarriers need to fulfil a number of extremely high standards regarding their biocompatibility, cytocompatibility, immunoisolating capacity, transport, mechanical and chemical properties. To obtain cell microcarriers of specified parameters, a wide variety of polymers, both natural and synthetic, and immobilization methods can be applied. Yet so far, only a few approaches based on cell-laden microcarriers have reached clinical trials. The main issue that still impedes progress of these systems towards clinical application is limited cell survival in vivo. Herein, we review polymer biomaterials and methods used for fabrication of cell microcarriers for in vivo biomedical applications. We describe their key limitations and modifications aiming at improvement of microcarrier in vivo performance. We also present the main applications of polymer cell microcarriers in regenerative medicine, pancreatic islet and hepatocyte transplantation and in the treatment of cancer. Lastly, we outline the main challenges in cell microimmobilization for biomedical purposes, the strategies to overcome these issues and potential future improvements in this area.

Designing Microgels for Cell Culture and Controlled Assembly of Tissue Microenvironments

Micron-sized hydrogels, termed microgels, are emerging as multifunctional platforms that can recapitulate tissue heterogeneity in engineered cell microenvironments. The microgels can function as either individual cell culture units or can be assembled into larger scaffolds. In this manner, individual microgels can be customized for single or multi-cell co-culture applications, or heterogeneous populations can be used as building blocks to create microporous assembled scaffolds that more closely mimic tissue heterogeneities. The inherent versatility of these materials allows user-defined control of the microenvironments, from the order of singly encapsulated cells to entire three-dimensional cell scaffolds. These hydrogel scaffolds are promising for moving towards personalized medicine approaches and recapitulating the multifaceted microenvironments that exist in vivo.

Slip Molding for Precision Fabrication of Microparts

Microparts with precise sizes, custom shapes, and a wide selection of materials have various applications, including biomedical microelectromechanical systems (MEMS), drug delivery, single-cell studies, and tissue engineering. Janus microparts containing multiple components are also demonstrated for biomolecule analysis, cell-cell interaction studies, and self-assembly. Small-footprint, affordable, and rapid technologies to fabricate microparts with customized morphologies and a wide selection of materials are highly desired. This paper reports on a SlipChip-based microfluidic molding method to control the interface for the synthesis of microparts-on-demand (mPods) with fast and easy loading-slipping-solidification operations that do not require pumps, masks, or other auxiliary fluidic control instruments. This method is based on the relative movement of two microfluidic plates that are in close contact, and the size and shape of the microparts can be accurately controlled by the geometry of the microcavities imprinted on the contacting surfaces of these microfluidic plates. To demonstrate the capability of this method, mPods of different sizes and various shapes are presented with photosensitive resin via a photopolymerization reaction. The synthesis of two-layer Janus microparts is also demonstrated by a slip overmolding method. This SlipChip-based molding method can offer new opportunities for producing customized microparts with great flexibility for a broad spectrum of applications.

Rapid Production of Cell-Laden Microspheres Using a Flexible Microfluidic Encapsulation Platform

This study establishes a novel microfluidic platform for rapid encapsulation of cells at high densities in photocrosslinkable microspherical hydrogels including poly(ethylene glycol)-diacrylate, poly(ethylene glycol)-fibrinogen, and gelatin methacrylate. Cell-laden hydrogel microspheres are advantageous for many applications from drug screening to regenerative medicine. Employing microfluidic systems is considered the most efficient method for scale-up production of uniform microspheres. However, existing platforms have been constrained by traditional microfabrication techniques for device fabrication, restricting microsphere diameter to below 200 µm and making iterative design changes time-consuming and costly. Using a new molding technique, the microfluidic device employs a modified T-junction design with readily adjustable channel sizes, enabling production of highly uniform microspheres with cell densities (10-60 million cells mL^-1 ) and a wide range of diameters (300-1100 µm), which are critical for realizing downstream applications, through rapid photocrosslinking (≈1 s per microsphere). Multiple cell types are encapsulated at rates of up to 1 million cells per min, are evenly distributed throughout the microspheres, and maintain high viability and appropriate cellular activities in long-term culture. This microfluidic encapsulation platform is a valuable and readily adoptable tool for numerous applications, including supporting injectable cell therapy, bioreactor-based cell expansion and differentiation, and high throughput tissue sphere-based drug testing assays.

Microfluidic three-dimensional cell culture of stem cells for high-throughput analysis

Although the recent advances in stem cell engineering have gained a great deal of attention due to their high potential in clinical research, the applicability of stem cells for preclinical screening in the drug discovery process is still challenging due to difficulties in controlling the stem cell microenvironment and the limited availability of high-throughput systems. Recently, researchers have been actively developing and evaluating three-dimensional (3D) cell culture-based platforms using microfluidic technologies, such as organ-on-a-chip and organoid-on-a-chip platforms, and they have achieved promising breakthroughs in stem cell engineering. In this review, we start with a comprehensive discussion on the importance of microfluidic 3D cell culture techniques in stem cell research and their technical strategies in the field of drug discovery. In a subsequent section, we discuss microfluidic 3D cell culture techniques for high-throughput analysis for use in stem cell research. In addition, some potential and practical applications of organ-on-a-chip or organoid-on-a-chip platforms using stem cells as drug screening and disease models are highlighted.

In-air production of 3D co-culture tumor spheroid hydrogels for expedited drug screening

Three-dimensional (3D) in vitro tumor spheroids are becoming popular as pre-clinical platforms for testing the performance of existing drugs or for discovery of innovative anti-cancer therapeutics. This focus is correlated with in vitro 3D tumor models ability to mimic the multicellular compact structure and spatial architecture of human solid tumors. However, these microphysiological systems generally lack the pre-existence of tumor-ECM, a critical aspect that can affect the overall therapeutic performance and the decision of advancing candidate drugs to later stages of the pipeline. Aiming to face this drawback and mimic tumors-ECM, herein we rapidly fabricated in-air hyaluronan-methacrylate (HA-MA) and gelatin-methacrylate (GelMA) photocrosslinkable 3D spheroid microgels by using superhydrophobic surfaces. These platforms were used for establishing heterotypic 3D co-culture models of prostate cancer cells (PC-3) and human osteoblasts (hOB) to mimic prostate cancer-to-bone metastasis cellular heterogeneity and the tumor-ECM microenvironment. 3D microgel microtumors morphology, size and cell number were easily controlled via digital droplet generation on polystyrene superhydrophobic surfaces and under solvent-free conditions when compared to microfluidics or electrospray. Co-culture 3D microgels formed by 2.5%HA-MA-5%GelMA and 5%HA-MA-5%GelMA ratios showed the highest calcium deposition after 14 days of culture, evidencing osteoblasts viability and the establishment of functional mineralization in the 3D hydrogel matrix. Cisplatin cytotoxicity evaluation showed that 3D microgels are more resistant to platin chemotherapeutics than single or co-culture 3D multicellular spheroid counterparts. Overall, our findings indicate that solvent-free, in-air produced 3D microgel microenvironments are cost-effective and robust tumor mimicking platforms for in vitro high-throughput screening of therapeutics targeted to prostate-to-bone metastasis microenvironments. STATEMENT OF SIGNIFICANCE: The generation of robust microphysiological systems that recapitulate the complexity of the metastatic prostate-to-bone tumor microenvironment is crucial for pre-clinical evaluation of new therapeutics that can eradicate these secondary tumors. In this study, we employed superhydrophobic (SH) surfaces to rapidly fabricate photocrosslinkable hyaluronan-methacrylate/gelatin-methacrylate 3D spheroid microgels for prostate cancer cells and human osteoblasts co-culture models that simultaneously mimic the cellular and ECM tumor components. The use of SH platforms overcomes the issues of standard in-liquid microgel production technologies by providing a robust control over 3D microgels size/morphology and cell-cell co-encapsulation numbers, while avoiding the use of oil-based microgel droplets generation. Overall, SH surfaces allowed a solvent-free, cost-effective, reproducible and adaptable fabrication of heterotypic 3D spherical microgels for high throughput drug screening.

Cell encapsulation in liquified compartments: Protocol optimization and challenges

Cell encapsulation is a widely used technique in the field of Tissue Engineering and Regenerative Medicine (TERM). However, for the particular case of liquefied compartmentalised systems, only a limited number of studies have been reported in the literature. We have been exploring a unique cell encapsulation system composed by liquefied and multilayered capsules. This system transfigured the concept of 3D scaffolds for TERM, and was already successfully applied for bone and cartilage regeneration. Due to a number of appealing features, we envisage that it can be applied in many other fields, including in advanced therapies or as disease models for drug discovery. In this review, we intend to highlight the advantages of this new system, while discussing the methodology, and sharing the protocol optimization and results. The different liquefied systems for cell encapsulation reported in the literature will be also discussed, considering the different encapsulation matrixes as core templates, the types of membranes, and the core liquefaction treatments.

Cancer-on-a-chip for Drug Screening

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The oncology pharmaceutical research spent a shocking amount of money on target validation and drug optimization in preclinical models because many oncology drugs fail during clinical trial phase III. One of the most important reasons for oncology drug failures in clinical trials may due to the poor predictive tool of existing preclinical models. Therefore, in cancer research and personalized medicine field, it is critical to improve the effectiveness of preclinical predictions of the drug response of patients to therapies and to reduce costly failures in clinical trials. Three dimensional (3D) tumor models combine micro-manufacturing technologies mimic critical physiologic parameters present in vivo, including complex multicellular architecture with multicellular arrangement and extracellular matrix deposition, packed 3D structures with cell–cell interactions, such as tight junctions, barriers to mass transport of drugs, nutrients and other factors, which are similar to in vivo tumor tissues. These systems provide a solution to mimic the physiological environment for improving predictive accuracy in oncology drug discovery.

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his review gives an overview of the innovations, development and limitations of different types of tumor-like construction techniques such as self-assemble spheroid formation, spheroids formation by micro-manufacturing technologies, micro-dissected tumor tissues and tumor organoid. Combination of 3D tumor-like construction and microfluidic techniques to achieve tumor on a chip for in vitro tumor environment modeling and drug screening were all included. Eventually, developmental directions and technical challenges in the research field are also discussed. We believe tumor on chip models have provided better sufficient clinical predictive power and will bridge the gap between proof-of-concept studies and a wider implementation within the oncology drug development for pathophysiological applications.

Hydrogel Biomaterials for Stem Cell Microencapsulation

Stem cell transplantation has been recognized as a promising strategy to induce the regeneration of injured and diseased tissues and sustain therapeutic molecules for prolonged periods in vivo. However, stem cell-based therapy is often ineffective due to low survival, poor engraftment, and a lack of site-specificity. Hydrogels can offer several advantages as cell delivery vehicles, including cell stabilization and the provision of tissue-like environments with specific cellular signals; however, the administration of bulk hydrogels is still not appropriate to obtain safe and effective outcomes. Hence, stem cell encapsulation in uniform micro-sized hydrogels and their transplantation in vivo have recently garnered great attention for minimally invasive administration and the enhancement of therapeutic activities of the transplanted stem cells. Several important methods for stem cell microencapsulation are described in this review. In addition, various natural and synthetic polymers, which have been employed for the microencapsulation of stem cells, are reviewed in this article.

Toward Customized Extracellular Niche Engineering: Progress in Cell-Entrapment Technologies

The primary aim in tissue engineering is to repair, replace, and regenerate dysfunctional tissues to restore homeostasis. Cell delivery for repair and regeneration is gaining impetus with our understanding of constructing tissue-like environments. However, the perpetual challenge is to identify innovative materials or re-engineer natural materials to model cell-specific tissue-like 3D modules, which can seamlessly integrate and restore functions of the target organ. To devise an optimal functional microenvironment, it is essential to define how simple is complex enough to trigger tissue regeneration or restore cellular function. Here, the purposeful transition of cell immobilization from a cytoprotection point of view to that of a cell-instructive approach is examined, with advances in the understanding of cell-material interactions in a 3D context, and with a view to further application of the knowledge for the development of newer and complex hierarchical tissue assemblies for better examination of cell behavior and offering customized cell-based therapies for tissue engineering.

Controlling Differentiation of Stem Cells for Developing Personalized Organ-on-Chip Platforms

Organ-on-chip (OOC) platforms have attracted attentions of pharmaceutical companies as powerful tools for screening of existing drugs and development of new drug candidates. OOCs have primarily used human cell lines or primary cells to develop biomimetic tissue models. However, the ability of human stem cells in unlimited self-renewal and differentiation into multiple lineages has made them attractive for OOCs. The microfluidic technology has enabled precise control of stem cell differentiation using soluble factors, biophysical cues, and electromagnetic signals. This study discusses different tissue- and organ-on-chip platforms (i.e., skin, brain, blood-brain barrier, bone marrow, heart, liver, lung, tumor, and vascular), with an emphasis on the critical role of stem cells in the synthesis of complex tissues. This study further recaps the design, fabrication, high-throughput performance, and improved functionality of stem-cell-based OOCs, technical challenges, obstacles against implementing their potential applications, and future perspectives related to different experimental platforms.

Microfluidic production of degradable thermoresponsive poly(N-isopropylacrylamide)-based microgels

Highly monodisperse and hydrolytically degradable thermoresponsive microgels on the tens-to-hundreds of micron size scale have been fabricated based on simultaneous on-chip mixing and emulsification of aldehyde and hydrazide-functionalized poly(N-isopropylacrylamide) precursor polymers. The microfluidic chip can run for extended periods without upstream gelation and can produce monodisperse (<10% particle size variability) microgels on the size range of ∼30-90 μm, with size tunable according to the flow rate of the oil continuous phase. Fluorescence analysis indicates a uniform distribution of each reactive pre-polymer inside the microgels while micromechanical testing suggests that smaller microfluidic-produced microgels exhibit significantly higher compressive moduli compared to bulk hydrogels of the same composition, an effect we attribute to improved mixing (and thus crosslinking) of the precursor polymer solutions within the microfluidic device. The microgels retain the reversible volume phase transition behavior of conventional microgels but can be hydrolytically degraded back into their oligomeric precursor polymer fragments, offering potential for microgel clearance following use in vivo.

Assessment of hydrogels for bioprinting of endothelial cells

In tissue engineering applications, vascularization can be accomplished by coimplantation of tissue forming cells and endothelial cells (ECs), whereby the latter are able to form functional blood vessels. The use of three-dimensional (3D) bioprinting technologies has the potential to improve the classical tissue engineering approach because these will allow the generation of scaffolds with high spatial control of endothelial cell allocation. This study focuses on a side by side comparison of popular commercially available bioprinting hydrogels (Matrigel, fibrin, collagen, gelatin, agarose, Pluronic F-127, alginate, and alginate/gelatin) in the context of their physicochemical parameters, their swelling/degradation characteristics, their biological effects on vasculogenesis-related EC parameters and their printability. The aim of this study was to identify the most suitable hydrogel or hydrogel combination for inkjet printing of ECs to build prevascularized tissue constructs. Most tested hydrogels displayed physicochemical characteristics suitable for inkjet printing. However, Pluronic F-127 and the alginate/gelatin blend were rapidly degraded when incubated in cell culture medium. Agarose, Pluronic F-127, alginate and alginate/gelatin hydrogels turned out to be unsuitable for bioprinting of ECs because of their non-adherent properties and/or their incapability to support EC proliferation. Gelatin was able to support EC proliferation and viability but was unable to support endothelial cell sprouting. Our experiments revealed fibrin and collagen to be most suitable for bioprinting of ECs, because these hydrogels showed acceptable swelling/degradation characteristics, supported vasculogenesis-related EC parameters and showed good printability. Moreover, ECs in constructs of preformed spheroids survived the printing process and formed capillary-like cords. © 2017 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 106A: 935-947, 2018.

Growth of hollow cell spheroids in microbead templated chambers

Cells form hollow, spheroidal structures during the development of many tissues, including the ocular lens, inner ear, and many glands. Therefore, techniques for in vitro formation of hollow spheroids are valued for studying developmental and disease processes. Current in vitro methods require cells to self-organize into hollow morphologies; we explored an alternative strategy based on cell growth in predefined, spherical scaffolds. Our method uses sacrificial, gelatin microbeads to simultaneously template spherical chambers within a hydrogel and deliver cells into the chambers. We use mouse lens epithelial cells to demonstrate that cells can populate the internal surfaces of the chambers within a week to create numerous hollow spheroids. The platform supports manipulation of matrix mechanics, curvature, and biochemical composition to mimic in vivo microenvironments. It also provides a starting point for engineering organoids of tissues that develop from hollow spheroids.

Nanoemulsion-induced enzymatic crosslinking of tyramine-functionalized polymer droplets

In situ gelation of water-in-oil polymer emulsions is a key method to produce hydrogel particles. Although this approach is in principle ideal for encapsulating bioactive components such as cells, the oil phase can interfere with straightforward presentation of crosslinker molecules. Several approaches have been developed to induce in-emulsion gelation by exploiting the triggered generation or release of crosslinker molecules. However, these methods typically rely on photo- or acid-based reactions that are detrimental to cell survival and functioning. In this work, we demonstrate the diffusion-based supplementation of small molecules for the in-emulsion gelation of multiple tyramine-functionalized polymers via enzymatic crosslinking using a H₂O₂/oil nanoemulsion. This strategy is compatible with various emulsification techniques, thereby readily supporting the formation of monodisperse hydrogel particles spanning multiple length scales ranging from the nano- to the millimeter. As proof of principle, we leveraged droplet microfluidics in combination with the cytocompatible nature of enzymatic crosslinking to engineer hollow cell-laden hydrogel microcapsules that support the formation of viable and functional 3D microtissues. The straightforward, universal, and cytocompatible nature of nanoemulsion-induced enzymatic crosslinking facilitates its rapid and widespread use in numerous food, pharma, and life science applications.

Advances in multicellular spheroids formation

Three-dimensional multicellular spheroids (MCSs) have a complex architectural structure, dynamic cell-cell/cell-matrix interactions and bio-mimicking in vivo microenvironment. As a fundamental building block for tissue reconstruction, MCSs have emerged as a powerful tool to narrow down the gap between the in vitro and in vivo model. In this review paper, we discussed the structure and biology of MCSs and detailed fabricating methods. Among these methods, the approach in microfluidics with hydrogel support for MCS formation is promising because it allows essential cell-cell/cell-matrix interactions in a confined space.

Impact of immobilizing of low molecular weight hyaluronic acid within gelatin-based hydrogel through enzymatic reaction on behavior of enclosed endothelial cells

The hydrogels having the ability to promote migration and morphogenesis of endothelial cells (ECs) are useful for fabricating vascularized dense tissues in vitro. The present study explores the immobilization of low molecular weight hyaluronic acid (LMWHA) derivative within gelatin-based hydrogel to stimulate migration of ECs. The LMWHA derivative possessing phenolic hydroxyl moieties (LMWHA-Ph) was bound to gelatin-based derivative hydrogel through the horseradish peroxidase-catalyzed reaction. The motility of ECs was analyzed by scratch migration assay and microparticle-based cell migration assay. The incorporated LMWHA-Ph molecules within hydrogel was found to be preserved stably through covalent bonds during incubation. The free and immobilized LMWHA-Ph did not lose an inherent stimulatory effect on human umbilical vein endothelial cells (HUVECs). The immobilized LMWHA-Ph within gelatin-based hydrogel induced the high motility of HUVECs, accompanied by robust cytoskeleton extension, and cell subpopulation expressing CD44 cell receptor. In the presence of immobilized LMWHA-Ph, the migration distance and the number of existing HUVECs were demonstrated to be encouraged in dose-dependent and time-dependent manners. Based on the results obtained in this work, it was concluded that the enzymatic immobilization of LMWHA-Ph within gelatin-based hydrogel represents a promising approach to promote ECs' motility and further exploitation for vascular tissue engineering applications.

Cell-laden microfluidic microgels for tissue regeneration

Regeneration of diseased tissue is one of the foremost concerns for millions of patients who suffer from tissue damage each year. Local delivery of cell-laden hydrogels offers an attractive approach for tissue repair. However, due to the typical macroscopic size of these cell constructs, the encapsulated cells often suffer from poor nutrient exchange. These issues can be mitigated by incorporating cells into microscopic hydrogels, or microgels, whose large surface-to-volume ratio promotes efficient mass transport and enhanced cell-matrix interactions. Using microfluidic technology, monodisperse cell-laden microgels with tunable sizes can be generated in a high-throughput manner, making them useful building blocks that can be assembled into tissue constructs with spatially controlled physicochemical properties. In this review, we examine microfluidics-generated cell-laden microgels for tissue regeneration applications. We provide a brief overview of the common biomaterials, gelation mechanisms, and microfluidic device designs that are used to generate these microgels, and summarize the most recent works on how they are applied to tissue regeneration. Finally, we discuss future applications of microfluidic cell-laden microgels as well as existing challenges that should be resolved to stimulate their clinical application.

Can microfluidics address biomanufacturing challenges in drug/gene/cell therapies?

Translation of any inventions into products requires manufacturing. Development of drug/gene/cell delivery systems will eventually face manufacturing challenges, which require the establishment of standardized processes to produce biologically-relevant products of high quality without incurring prohibitive cost. Microfluidicu technologies present many advantages to improve the quality of drug/gene/cell delivery systems. They also offer the benefits of automation. What remains unclear is whether they can meet the scale-up requirement. In this perspective, we discuss the advantages of microfluidic-assisted synthesis of nanoscale drug/gene delivery systems, formation of microscale drug/cell-encapsulated particles, generation of genetically engineered cells and fabrication of macroscale drug/cell-loaded micro-/nano-fibers. We also highlight the scale-up challenges one would face in adopting microfluidic technologies for the manufacturing of these therapeutic delivery systems.

Emulsion technologies for multicellular tumour spheroid radiation assays

Emulsion technology is used to assess the cytotoxic effect of radiation on spheroids by mimicking tumour growth and tumour quiescence.

Cell-Instructive Microgels with Tailor-Made Physicochemical Properties

A microfluidic in vitro cell encapsulation platform to systematically test the effects of microenvironmental parameters on cell fate in 3D is developed. Multiple cell types including fibroblasts, embryonic stem cells, and cancer cells are incorporated in enzymatically cross-linked poly(ethylene glycol)-based microgels having defined and tunable mechanical and biochemical properties. Furthermore, different approaches to prevent cell "escape" from the microcapsules are explored and shown to substantially enhance the potential of this technology. Finally, coencapsulation of microgels within nondegradable gels allows cell viability, proliferation, and morphology to be studied in different microenvironmental conditions up to two weeks in culture.

Formation of Cucurbit[8]uril-Based Supramolecular Hydrogel Beads Using Droplet-Based Microfluidics

Herein we describe the use of microdroplets as templates for the fabrication of uniform-sized supramolecular hydrogel beads, assembled by supramolecular cross-linking of functional biopolymers with the macrocyclic host molecule, cucurbit[8]uril (CB[8]). The microdroplets were formed containing diluted hydrogel precursors in solution, including the functional polymers and CB[8], in a microfluidic device. Subsequent evaporation of water from collected microdroplets concentrated the contents, driving the formation of the CB[8]-mediated host-guest ternary complex interactions and leading to the assembly of condensed three-dimensional polymeric scaffolds. Rehydration of the dried particles gave monodisperse hydrogel beads. Their equilibrium size was shown to be dependent on both the quantity of material loaded and the dimensions of the microfluidic flow focus. Fluorescein-labeled dextran was used to evaluate the efficacy of the hydrogel beads as a vector for controlled cargo release. Both passive, sustained release (hours) and triggered, fast release (minutes) of the FITC-dextran was observed, with the rate of sustained release dependent on the formulation. The kinetics of release was fitted to the Ritger-Peppas controlled release equation and shown to follow an anomalous (non-Fickian) transport mechanism.

Mixed hydrogel bead-based tumor spheroid formation and anticancer drug testing

Three-dimensional multicellular tumor spheroids have become critical for anticancer study since they may provide a better model than conventional monolayer cultures of cancer cells. Various methods for tumor spheroid formation have been explored. However, only one kind of hydrogel was used in these methods, which has an influence on the size and morphology of the obtained tumor spheroids. Herein, we present a microfluidic droplet-based method for the formation of multicellular tumor spheroids using alginate and matrigel mixed hydrogel beads. By on-chip changing the flow rate of the two hydrogel solutions, mixed hydrogel beads with different volume ratios between alginate and matrigel are obtained. Meanwhile, human cervical carcinoma (HeLa) cells are encapsulated in the mixed hydrogel beads. Acridine orange and propidium iodide double-staining assay shows that the viability of cells encapsulated in the mixed hydrogel beads was more than 90%. After 4 day culture, the multicellular tumor spheroids were successfully formed with spherical shape and uniform size distribution compared with spheroids formed in pure alginate beads. Cytoskeletal analysis by TRITC-phalloidin staining show that HeLa cells in the mixed hydrogel beads closely link to each other. The dose-dependent response assay of HeLa cell spheroids to vincristine show that multicellular spheroids have more powerful resistance to vincristine compared to conventional monolayer culture cells. Taken together, this novel technology may be of importance to facilitate in vitro culture of tumor spheroids for their ever-increasing utilization in modern cell-based medicine.

Natural polymers for the microencapsulation of cells

The encapsulation of living mammalian cells within a semi-permeable hydrogel matrix is an attractive procedure for many biomedical and biotechnological applications, such as xenotransplantation, maintenance of stem cell phenotype and bioprinting of three-dimensional scaffolds for tissue engineering and regenerative medicine. In this review, we focus on naturally derived polymers that can form hydrogels under mild conditions and that are thus capable of entrapping cells within controlled volumes. Our emphasis will be on polysaccharides and proteins, including agarose, alginate, carrageenan, chitosan, gellan gum, hyaluronic acid, collagen, elastin, gelatin, fibrin and silk fibroin. We also discuss the technologies commonly employed to encapsulate cells in these hydrogels, with particular attention on microencapsulation.

Preparation and validation of low cost microfluidic chips using a shrinking approach

The present paper describes the production of microfluidic chips using an approach based on shrinkable biocompatible polymers (i.e. agarose) for the production of size controlled microfluidic channels. In addition, all steps of chip production were carried out using an inexpensive approach that uses low cost chemicals and equipment. The produced chips were then validated by producing monodisperse polymeric microparticles for drug delivery and hydrogel microfibers for cell embedding.

Rapid one-step purification of single-cells encapsulated in alginate microcapsules from oil to aqueous phase using a hydrophobic filter paper: implications for single-cell experiments

By virtue of the biocompatibility and physical properties of hydrogel, picoliter-sized hydrogel microcapsules have been considered to be a biometric signature containing several features similar to that of encapsulated single cells, including phenotype, viability, and intracellular content. To maximize the experimental potential of encapsulating cells in hydrogel microcapsules, a method that enables efficient hydrogel microcapsule purification from oil is necessary. Current methods based on centrifugation for the conventional stepwise rinsing of oil, are slow and laborious and decrease the monodispersity and yield of the recovered hydrogel microcapsules. To remedy these shortcomings we have developed a simple one-step method to purify alginate microcapsules, containing a single live cell, from oil to aqueous phase. This method employs oil impregnation using a commercially available hydrophobic filter paper without multistep centrifugal purification and complicated microchannel networks. The oil-suspended alginate microcapsules encapsulating single cells from mammalian cancer cell lines (MCF-7, HepG2, and U937) and microorganisms (Chlorella vulgaris) were successfully exchanged to cell culture media by quick (~10 min) depletion of the surrounding oil phase without coalescence of neighboring microcapsules. Cell proliferation and high integrity of the microcapsules were also demonstrated by long-term incubation of microcapsules containing a single live cell. We expect that this method for the simple and rapid purification of encapsulated single-cell microcapsules will attain widespread adoption, assisting cell biologists and clinicians in the development of single-cell experiments.

Rapid formation of multicellular spheroids in double-emulsion droplets with controllable microenvironment

An attractive option for tissue engineering is to use of multicellular spheroids as microtissues, particularly with stem cell spheroids. Conventional approaches of fabricating spheroids suffer from low throughput and polydispersity in size, and fail to supplement cues from extracellular matrix (ECM) for enhanced differentiation. In this study, we report the application of microfluidics-generated water-in-oil-in-water (w/o/w) double-emulsion (DE) droplets as pico-liter sized bioreactor for rapid cell assembly and well-controlled microenvironment for spheroid culture. Cells aggregated to form size-controllable (30–80 μm) spheroids in DE droplets within 150 min and could be retrieved via a droplet-releasing agent. Moreover, precursor hydrogel solution can be adopted as the inner phase to produce spheroid-encapsulated microgels after spheroid formation. As an example, the encapsulation of human mesenchymal stem cells (hMSC) spheroids in alginate and alginate-arginine-glycine-aspartic acid (-RGD) microgel was demonstrated, with enhanced osteogenic differentiation further exhibited in the latter case.

Preparation of cell-encapsulation devices in confined microenvironment

The entrapment of cells into hydrogel microdevice in form of microparticles or microfibers is one of the most appealing and useful tools for cell-based therapy and tissue engineering. Cell encapsulation procedures allow the immunoisolation of cells from the surrounding environment, after their transplantation and the maintenance of the normal cellular physiology. Factors affecting the efficacy of microdevices, which include size, size distribution, morphology, and porosity are all highly dependent on the method of preparation. In this respect, microfluidic based methods offer a promising strategy to fabricate highly uniform and morphologically controlled microdevices with tunable chemical and mechanical properties. In the current review, various cell microencapsulation procedures, based on a microfluidics, are critically analyzed with a special focus on the effect of the procedure on the morphology, viability and functions of the embedded cells. Moreover, a brief introduction about the optimal characteristics of microdevice intended for cell encapsulation, together with the currently used materials for the production is reported. A further challenging application of microfluidics for the development of "living microchip" is also presented. Finally, the limitations, challenging and future work on the microfluidic approach are also discussed.

Supramolecular hydrogel capsules based on PEG: a step toward degradable biomaterials with rational design

Supramolecular microgel capsules based on polyethylene glycol (PEG) are a promising class of soft particulate scaffolds with tailored properties. An approach to fabricate such particles with exquisite control by droplet-based microfluidics is presented. Linear PEG precursor polymers that carry bipyridine moieties on both chain termini are gelled by complexation to iron(II) ions. To investigate the biocompatibility of the microgels, living mammalian cells are encapsulated within them. The microgel elasticity is controlled by using PEG precursors of different molecular weights at different concentrations and the influence of these parameters on the cell viabilities, which can be optimized to exceed 90% is studied. Reversion of the supramolecular polymer cross-linking allows the microcapsules to be degraded at mild conditions with no effect on the viability of the encapsulated and released cells.

Microdevices for nanomedicine

This review surveys selected methods of manufacture and applications of microdevices-miniaturized functional devices capable of handling cell and tissue cultures or producing particles-and discusses their potential relevance to nanomedicine. Many characteristics of microdevices such as miniaturization, increased throughput, and the ability to mimic organ-specific microenvironments are promising for the rapid, low-cost evaluation of the efficacy and toxicity of nanomaterials. Their potential to accurately reproduce the physiological environments that occur in vivo could reduce dependence on animal models in pharmacological testing. Technologies in microfabrications and microfluidics are widely applicable for nanomaterial synthesis and for the development of diagnostic devices. Although the use of microdevices in nanomedicine is still in its infancy, these technologies show promise for enhancing fundamental and applied research in nanomedicine.

Dripping and jetting in microfluidic multiphase flows applied to particle and fiber synthesis

Dripping and jetting regimes in microfluidic multiphase flows have been investigated extensively, and this review summarizes the main observations and physical understandings in this field to date for three common device geometries: coaxial, flow-focusing and T-junction. The format of the presentation allows for simple and direct comparison of the different conditions for drop and jet formation, as well as the relative ease and utility of forming either drops or jets among the three geometries. The emphasis is on the use of drops and jets as templates for microparticle and microfiber syntheses, and a description is given of the more common methods of solidification and strategies for achieving complex multicomponent microparticles and microfibers.

Dripping and jetting in microfluidic multiphase flows applied to particle and fiber synthesis

Dripping and jetting regimes in microfluidic multiphase flows have been investigated extensively, and this review summarizes the main observations and physical understandings in this field to date for three common device geometries: coaxial, flow-focusing and T-junction. The format of the presentation allows for simple and direct comparison of the different conditions for drop and jet formation, as well as the relative ease and utility of forming either drops or jets among the three geometries. The emphasis is on the use of drops and jets as templates for microparticle and microfiber syntheses, and a description is given of the more common methods of solidification and strategies for achieving complex multicomponent microparticles and microfibers.