Cell-laden microfluidic microgels for tissue regeneration

Multi-well plate-based versatile platform for online fabricating alginate hydrogel microspheres and in-situ 3D cell culture

BACKGROUND

Hydrogel microspheres with monodisperse and homogeneous dimensions have potential application in the field of three-dimensional (3D) cell culture due to its ability to provide a similar microenvironment. Currently, alginate hydrogel microspheres (AHMs) have received much attention due to the favorable properties of alginate such as biocompatibility, inexpensiveness, nontoxicity, and biodegradability. The fabrication methods of AHMs mainly include extrusion, electrostatic dripping and microfluidic chip techniques. These current methods suffer trade-offs between operational complexity, fabrication cost and practical application.

RESULTS

We proposed a novel and versatile multi-well plate-based platform for online fabricating AHMs and in-situ 3D cell culture. The AHMs could be easily fabricated based on gravity-driven gelation combined with our recently developed bent-capillary-centrifugal-driven (BCCD) system. Ca-EDTA complex was used as Ca2+ source for crosslinking reaction of the alginate chains. The whole preparation process of AHMs included four steps: emulsification, pre-gelation, spontaneous demulsification and further solidification. The gravity-driven hydrogel microsphere gelation could produce the AHMs with good sphericity (Φ = 0.96) and monodispersity (PDI% = 0.94 %). The rapid drug susceptibility testing and single-cell encapsulation in the AHMs were well demonstrated. It also provided a novel in-situ 3D cell culture strategy, which demonstrated more than 85 % cell viability in practice.

SIGNIFICANCE

The proposed platform avoided the complex and laborious microfabrication. Moreover, cell-encapsulated AHMs could be directly produced in the multi-well plate, which could facilitate the subsequent cultivation and observation. It is expected to be a versatile in-situ 3D cell culture tool in the fields of biomedicine and tissue engineering.

Decellularized cartilage tissue bioink formulation for osteochondral graft development

Articular cartilage and osteochondral defect repair and regeneration presents significant challenges to the field of tissue engineering (TE). TE and regenerative medicine strategies utilizing natural and synthetic-based engineered scaffolds have shown potential for repair, however, they face limitations in replicating the intricate native microenvironment and structure to achieve optimal regenerative capacity and functional recovery. Herein, we report the development of a cartilage extracellular matrix (ECM) as a printable biomaterial for tissue regeneration. The biomaterial was prepared through decellularization and solubilization of articular cartilage. The effects of two different viscosity modifiers, xanthan gum and Laponite®, and the introduction of a secondary photo-crosslinkable component on the rheological behavior and stability were studied. dcECM-Laponite® bioink formulations demonstrated storage modulus (G') ranging from 750 to 4000 Pa, which is three orders of magnitude higher than that of the dcECM-XG bioink formulations. The rheological evaluation of the bioinks demonstrated the tunability of the bioinks in terms of their viscosity and degree of shear thinning, allowing the formulations to be readily extruded during 3D printing. Also, a spreadable ink composition was identified to form a uniform cartilage layer post-printing. The choice of viscosity modifier along with UV cross-linking warrants shape fidelity of the structure post-printing, as well as improvements in the storage and loss moduli. The modified ECM-based bioink also significantly improved the stability and allowed for prolonged and sustained release of loaded growth factors through the addition of Laponite®. The ECM-based bioink supported human bone-marrow derived stromal cell and chondrocyte viability and increased chondrogenic differentiationin vitro. By forming decellularized cartilage ECM biomaterials in a printable and stable bioink form, we develop a 'Cartilage Ink' that can support cartilaginous tissue formation by closely resembling the native cartilage ECM in structure and function.

Advances in the Construction and Application of Bone-on-a-Chip Based on Microfluidic Technologies

Bone-on-a-chip (BOC) models that based on microfluidic technology have widely applied to understand bone physiology and the pathogenesis of related diseases. In this review, we provide an overview of bone biology and related diseases, explain the advantages and applications of microfluidic technology in the construction of BOC models, and summarize their progress in physiology, pathology, and drug development. Finally, we discussed the problems to be solved and the future directions of microfluidic technology and BOC platforms, so as to provide a reference for researchers to design better BOC models.

Controlling the size and elastic modulus of in-aqueous alginate micro-beads

The fabrication of microgels, particularly those ranging from tens to hundreds of micrometers in size, represents a thriving area of research, particularly for biologists seeking controlled and isotropic media for cell encapsulation. In this article, we present a novel and robust method for producing structurally homogeneous alginate beads with a reduced environmental footprint, employing a co-flow focusing microfluidic device. These beads can be easily recovered in an oil-free aqueous medium, making the fabrication method highly suitable for diverse applications. We demonstrate precise control over the production of perfectly spherical beads across a wide range of diameters, from about 30 to 300 μm. We then measure Young's moduli of the beads, revealing a wide accessible range from 90 Pa to 11 kPa, contingent upon controlling the type (e.g. chain length) and concentration of alginate.

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.

Advances in the Development of Granular Microporous Injectable Hydrogels with Non-spherical Microgels and Their Applications in Tissue Regeneration

Granular microporous hydrogels are emerging as effective biomaterial scaffolds for tissue engineering due to their improved characteristics compared to traditional nanoporous hydrogels, which better promote cell viability, cell migration, cellular/tissue infiltration, and tissue regeneration. Recent advances have resulted in the development of granular hydrogels made of non-spherical microgels, which compared to those made of spherical microgels have higher macroporosity, more stable mechanical properties, and better ability to guide the alignment and differentiation of cells in anisotropic tissue. The development of these hydrogels as an emerging research area is attracting increasing interest in regenerative medicine. This review first summarizes the fabrication techniques available for non-spherical microgels with different aspect-ratios. Then, it introduces the development of granular microporous hydrogels made of non-spherical microgels, their physicochemical characteristics, and their applications in tissue regeneration. The limitations and future outlook of research on microporous granular hydrogels are also critically discussed.

Hydrogel microspheres for bone regeneration through regulation of the regenerative microenvironment

Bone defects are a prevalent category of skeletal tissue disorders in clinical practice, with a range of pathogenic factors and frequently suboptimal clinical treatment effects. In bone regeneration of bone defects, the bone regeneration microenvironment-composed of physiological, chemical, and physical components-is the core element that dynamically coordinates to promote bone regeneration. In recent years, medical biomaterials with bioactivity and functional tunability have been widely researched upon and applied in the fields of tissue replacement/regeneration, and remodelling of organ structure and function. The biomaterial treatment system based on the comprehensive regulation strategy of bone regeneration microenvironment is expected to solve the clinical problem of bone defect. Hydrogel microspheres (HMS) possess a highly specific surface area and porosity, an easily adjustable physical structure, and high encapsulation efficiency for drugs and stem cells. They can serve as highly efficient carriers for bioactive factors, gene agents, and stem cells, showing potential advantages in the comprehensive regulation of bone regeneration microenvironment to enhance bone regeneration. This review aims to clarify the components of the bone regeneration microenvironment, the application of HMS in bone regeneration, and the associated mechanisms. It also discusses various preparation materials and methods of HMS and their applications in bone tissue engineering. Furthermore, it elaborates on the relevant mechanisms by which HMS regulates the physiological, chemical, and physical microenvironment in bone regeneration to achieve bone regeneration. Finally, we discuss the future prospects of the HMS system application for comprehensive regulation of bone regeneration microenvironment, to provide novel perspectives for the research and application of HMS in the bone tissue engineering field.

Customizable Hydrogel Coating of ECM-Based Microtissues for Improved Cell Retention and Tissue Integrity

Overcoming the oxygen diffusion limit of approximately 200 µm remains one of the most significant and intractable challenges to be overcome in tissue engineering. The fabrication of hydrogel microtissues and their assembly into larger structures may provide a solution, though these constructs are not without their own drawbacks; namely, these hydrogels are rapidly degraded in vivo, and cells delivered via microtissues are quickly expelled from the area of action. Here, we report the development of an easily customized protocol for creating a protective, biocompatible hydrogel barrier around microtissues. We show that calcium carbonate nanoparticles embedded within an ECM-based microtissue diffuse outwards and, when then exposed to a solution of alginate, can be used to generate a coated layer around the tissue. We further show that this technique can be fine-tuned by adjusting numerous parameters, granting us full control over the thickness of the hydrogel coating layer. The microtissues' protective hydrogel functioned as hypothesized in both in vitro and in vivo testing by preventing the cells inside the tissue from escaping and protecting the microdroplets against external degradation. This technology may provide microtissues with customized properties for use as sources of regenerative therapies.

Triple click chemistry for crosslinking, stiffening, and annealing of gelatin-based microgels

Microgels are spherical hydrogels with physicochemical properties ideal for many biomedical applications. For example, microgels can be used as individual carriers for suspension cell culture or jammed/annealed into granular hydrogels with micron-scale pores highly permissive to molecular transport and cell proliferation/migration. Conventionally, laborious optimization processes are often needed to create microgels with different moduli, sizes, and compositions. This work presents a new microgel and granular hydrogel preparation workflow using gelatin-norbornene-carbohydrazide (GelNB-CH). As a gelatin-derived macromer, GelNB-CH presents cell adhesive and degradable motifs while being amenable to three orthogonal click chemistries, namely the thiol-norbornene photo-click reaction, hydrazone bonding, and the inverse electron demand Diels-Alder (iEDDA) click reaction. The thiol-norbornene photo-click reaction (with thiol-bearing crosslinkers) and hydrazone bonding (with aldehyde-bearing crosslinkers) were used to crosslink the microgels and to realize on-demand microgel stiffening, respectively. The tetrazine-norbornene iEDDA click reaction (with tetrazine-bearing crosslinkers) was used to anneal microgels into granular hydrogels. In addition to materials development, we demonstrated the value of the triple-click chemistry granular hydrogels via culturing human mesenchymal stem cells and pancreatic cancer cells.

A Pump-Free Strategy for the Controllable Generation of Alginate Microgels as Cellular Microcarriers

Microgels are advanced scaffolds for tissue engineering due to their proper biodegradability, good biocompatibility, and high specific surface area for effective oxygen and nutrient transfer. However, most of the current monodispersed microgel fabrication systems rely heavily on various precision pumps, which highly increase the cost and complexity of their downstream application. In this work, we developed a simple and facile system for the controllable generation of uniform alginate microgels by integrating a gas-shearing strategy into a glass microfluidic device. Importantly, the cell-laden microgels can be rapidly prepared in a pump-free manner under an all-aqueous environment. The three-dimensional cultured green fluorescent protein-human A549 cells in alginate microgels exhibited enhanced stemness and drug resistance compared to those under two-dimensional conditions. The pancreatic cancer organoids in alginate microgels exhibited some of the key features of pancreatic cancer. The proposed microgels showed decent monodispersity, biocompatibility, and versatility, providing great opportunities in various biomedical applications such as microcarrier fabricating, organoid engineering, and high-throughput drug screening.

The microparticulate inks for bioprinting applications

Three-dimensional (3D) bioprinting has emerged as a groundbreaking technology for fabricating intricate and functional tissue constructs. Central to this technology are the bioinks, which provide structural support and mimic the extracellular environment, which is crucial for cellular executive function. This review summarizes the latest developments in microparticulate inks for 3D bioprinting and presents their inherent challenges. We categorize micro-particulate materials, including polymeric microparticles, tissue-derived microparticles, and bioactive inorganic microparticles, and introduce the microparticle ink formulations, including granular microparticles inks consisting of densely packed microparticles and composite microparticle inks comprising microparticles and interstitial matrix. The formulations of these microparticle inks are also delved into highlighting their capabilities as modular entities in 3D bioprinting. Finally, existing challenges and prospective research trajectories for advancing the design of microparticle inks for bioprinting are discussed.

Hydrogel Microparticles for Bone Regeneration

Hydrogel microparticles (HMPs) stand out as promising entities in the realm of bone tissue regeneration, primarily due to their versatile capabilities in delivering cells and bioactive molecules/drugs. Their significance is underscored by distinct attributes such as injectability, biodegradability, high porosity, and mechanical tunability. These characteristics play a pivotal role in fostering vasculature formation, facilitating mineral deposition, and contributing to the overall regeneration of bone tissue. Fabricated through diverse techniques (batch emulsion, microfluidics, lithography, and electrohydrodynamic spraying), HMPs exhibit multifunctionality, serving as vehicles for drug and cell delivery, providing structural scaffolding, and functioning as bioinks for advanced 3D-printing applications. Distinguishing themselves from other scaffolds like bulk hydrogels, cryogels, foams, meshes, and fibers, HMPs provide a higher surface-area-to-volume ratio, promoting improved interactions with the surrounding tissues and facilitating the efficient delivery of cells and bioactive molecules. Notably, their minimally invasive injectability and modular properties, offering various designs and configurations, contribute to their attractiveness for biomedical applications. This comprehensive review aims to delve into the progressive advancements in HMPs, specifically for bone regeneration. The exploration encompasses synthesis and functionalization techniques, providing an understanding of their diverse applications, as documented in the existing literature. The overarching goal is to shed light on the advantages and potential of HMPs within the field of engineering bone tissue.

From cells to organs: progress and potential in cartilaginous organoids research

While cartilage tissue engineering has significantly improved the speed and quality of cartilage regeneration, the underlying metabolic mechanisms are complex, making research in this area lengthy and challenging. In the past decade, organoids have evolved rapidly as valuable research tools. Methods to create these advanced human cell models range from simple tissue culture techniques to complex bioengineering approaches. Cartilaginous organoids in part mimic the microphysiology of human cartilage and fill a gap in high-fidelity cartilage disease models to a certain extent. They hold great promise to elucidate the pathogenic mechanism of a diversity of cartilage diseases and prove crucial in the development of new drugs. This review will focus on the research progress of cartilaginous organoids and propose strategies for cartilaginous organoid construction, study directions, and future perspectives.

Core-Shell Microfiber Encapsulation Enables Glycerol-Free Cryopreservation of RBCs with High Hematocrit

Cryopreservation of red blood cells (RBCs) provides great potential benefits for providing transfusion timely in emergencies. High concentrations of glycerol (20% or 40%) are used for RBC cryopreservation in current clinical practice, which results in cytotoxicity and osmotic injuries that must be carefully controlled. However, existing studies on the low-glycerol cryopreservation of RBCs still suffer from the bottleneck of low hematocrit levels, which require relatively large storage space and an extra concentration process before transfusion, making it inconvenient (time-consuming, and also may cause injury and sample lose) for clinical applications. To this end, we develop a novel method for the glycerol-free cryopreservation of human RBCs with a high final hematocrit by using trehalose as the sole cryoprotectant to dehydrate RBCs and using core-shell alginate hydrogel microfibers to enhance heat transfer during cryopreservation. Different from previous studies, we achieve the cryopreservation of human RBCs at high hematocrit (> 40%) with high recovery (up to 95%). Additionally, the washed RBCs post-cryopreserved are proved to maintain their morphology, mechanics, and functional properties. This may provide a nontoxic, high-efficiency, and glycerol-free approach for RBC cryopreservation, along with potential clinical transfusion benefits.

Integrating spheroid-on-a-chip with tubeless rocker platform: A high-throughput biological screening platform

Spheroid-on-a-chip platforms are emerging as promising in vitro models that enable screening of the efficacy of biologically active ingredients. Generally, the supply of liquids to spheroids occurs in the steady flow mode with the use of syringe pumps; however, the utilization of tubing and connections, especially for multiplexing and high-throughput screening applications, makes spheroid-on-a-chip platforms labor- and cost-intensive. Gravity-induced flow using rocker platforms overcomes these challenges. Here, a robust gravity-driven technique was developed to culture arrays of cancer cell spheroids and dermal fibroblast spheroids in a high-throughput manner using a rocker platform. The efficiency of the developed rocker-based platform was benchmarked to syringe pumps for generating multicellular spheroids and their use for screening biologically active ingredients. Cell viability, internal spheroid structure as well as the effect of vitamin C on spheroids' protein synthesis was studied. The rocker-based platform not only offers comparable or enhanced performance in terms of cell viability, spheroids formation, and protein production by dermal fibroblast spheroids but also, from a practical perspective, offers a smaller footprint, requires a lower cost, and offers an easier method for handling. These results support the application of rocker-based microfluidic spheroid-on-a-chip platforms for in vitro screening in a high-throughput manner with industrial scaling-up opportunities.

Development of Organs-on-Chips and Their Impact on Precision Medicine and Advanced System Simulation

Drugs may undergo costly preclinical studies but still fail to demonstrate their efficacy in clinical trials, which makes it challenging to discover new drugs. Both in vitro and in vivo models are essential for disease research and therapeutic development. However, these models cannot simulate the physiological and pathological environment in the human body, resulting in limited drug detection and inaccurate disease modelling, failing to provide valid guidance for clinical application. Organs-on-chips (OCs) are devices that serve as a micro-physiological system or a tissue-on-a-chip; they provide accurate insights into certain functions and the pathophysiology of organs to precisely predict the safety and efficiency of drugs in the body. OCs are faster, more economical, and more precise. Thus, they are projected to become a crucial addition to, and a long-term replacement for, traditional preclinical cell cultures, animal studies, and even human clinical trials. This paper first outlines the nature of OCs and their significance, and then details their manufacturing-related materials and methodology. It also discusses applications of OCs in drug screening and disease modelling and treatment, and presents the future perspective of OCs.

Engineered biomimetic micro/nano-materials for tissue regeneration

The incidence of tissue and organ damage caused by various diseases is increasing worldwide. Tissue engineering is a promising strategy of tackling this problem because of its potential to regenerate or replace damaged tissues and organs. The biochemical and biophysical cues of biomaterials can stimulate and induce biological activities such as cell adhesion, proliferation and differentiation, and ultimately achieve tissue repair and regeneration. Micro/nano materials are a special type of biomaterial that can mimic the microstructure of tissues on a microscopic scale due to its precise construction, further providing scaffolds with specific three-dimensional structures to guide the activities of cells. The study and application of biomimetic micro/nano-materials have greatly promoted the development of tissue engineering. This review aims to provide an overview of the different types of micro/nanomaterials, their preparation methods and their application in tissue regeneration.

Integration of immune cells in organs-on-chips: a tutorial

Viral and bacterial infections continue to pose significant challenges for numerous individuals globally. To develop novel therapies to combat infections, more insight into the actions of the human innate and adaptive immune system during infection is necessary. Human in vitro models, such as organs-on-chip (OOC) models, have proven to be a valuable addition to the tissue modeling toolbox. The incorporation of an immune component is needed to bring OOC models to the next level and enable them to mimic complex biological responses. The immune system affects many (patho)physiological processes in the human body, such as those taking place during an infection. This tutorial review introduces the reader to the building blocks of an OOC model of acute infection to investigate recruitment of circulating immune cells into the infected tissue. The multi-step extravasation cascade in vivo is described, followed by an in-depth guide on how to model this process on a chip. Next to chip design, creation of a chemotactic gradient and incorporation of endothelial, epithelial, and immune cells, the review focuses on the hydrogel extracellular matrix (ECM) to accurately model the interstitial space through which extravasated immune cells migrate towards the site of infection. Overall, this tutorial review is a practical guide for developing an OOC model of immune cell migration from the blood into the interstitial space during infection.

Application of microfluidic chips in the simulation of the urinary system microenvironment

The urinary system, comprising the kidneys, ureters, bladder, and urethra, has a unique mechanical and fluid microenvironment, which is essential to the urinary system growth and development. Microfluidic models, based on micromachining and tissue engineering technology, can integrate pathophysiological characteristics, maintain cell-cell and cell-extracellular matrix interactions, and accurately simulate the vital characteristics of human tissue microenvironments. Additionally, these models facilitate improved visualization and integration and meet the requirements of the laminar flow environment of the urinary system. However, several challenges continue to impede the development of a tissue microenvironment with controllable conditions closely resemble physiological conditions. In this review, we describe the biochemical and physical microenvironment of the urinary system and explore the feasibility of microfluidic technology in simulating the urinary microenvironment and pathophysiological characteristics in vitro. Moreover, we summarize the current research progress on adapting microfluidic chips for constructing the urinary microenvironment. Finally, we discuss the current challenges and suggest directions for future development and application of microfluidic technology in constructing the urinary microenvironment in vitro.

Hyaluronic acid-based multifunctional carriers for applications in regenerative medicine: A review

Hyaluronic acid (HA) is an important type of naturally derived carbohydrate polymer with specific polysaccharide macromolecular structures and multifaceted biological functions, including biocompatibility, low immunogenicity, biodegradability, and bioactivity. Specifically, HA hydrogels in a microscopic scale have been widely used for biomedical applications, such as drug delivery, tissue engineering, and medical cosmetology, considering their superior properties outperforming the more conventional monolithic hydrogels in network homogeneity, degradation profile, permeability, and injectability. Herein, we reviewed the recent progress in the preparation and applications of HA microgels in biomedical fields. We first summarized the fabrication of HA microgels by focusing on the different crosslinking/polymerization schemes for HA gelation and the miniaturized fabrication techniques for producing HA-based microparticles. We then highlighted the use of HA-based microgels for different applications in regenerative medicine, including cartilage repair, bioactive delivery, diagnostic imaging, modular tissue engineering. Finally, we discussed the challenges and future perspectives in bridging the translational gap in the utilization of HA-based microgels in regenerative medicine.

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.

A clinical feasible stem cell encapsulation ensures an improved wound healing

Cell encapsulation has proven to be promising in stem cell therapy. However, there are issues needed to be addressed, including unsatisfied yield, unmet clinically friendly formulation, and unacceptable viability of stem cells after cryopreservation and thawing. We developed a novel biosynsphere technology to encapsulate stem cells in clinically-ready biomaterials with controlled microsphere size. We demonstrated that biosynspheres ensure the bioviability and functionality of adipose-derived stromal cells (ADSCs) encapsulated, as delineated by a series of testing procedures. We further demonstrated that biosynspheres protect ADSCs from the hardness of clinically handling such as cryopreservation, thawing, high-speed centrifugation and syringe/nozzle injection. In a swine full skin defect model, we showed that biosynspheres were integrated to the destined tissues and promoted the repair of injured tissues with an accelerating healing process, less scar tissue formation and normalized deposition of collagen type I and type III, the ratio similar to that found in normal skin. These findings underscore the potential of biosynsphere as an improved biofabrication technology for tissue regeneration in clinical setting.

Chitosan and Pectin Hydrogels for Tissue Engineering and In Vitro Modeling

Hydrogels are fascinating biomaterials that can act as a support for cells, i.e., a scaffold, in which they can organize themselves spatially in a similar way to what occurs in vivo. Hydrogel use is therefore essential for the development of 3D systems and allows to recreate the cellular microenvironment in physiological and pathological conditions. This makes them ideal candidates for biological tissue analogues for application in the field of both tissue engineering and 3D in vitro models, as they have the ability to closely mimic the extracellular matrix (ECM) of a specific organ or tissue. Polysaccharide-based hydrogels, because of their remarkable biocompatibility related to their polymeric constituents, have the ability to interact beneficially with the cellular components. Although the growing interest in the use of polysaccharide-based hydrogels in the biomedical field is evidenced by a conspicuous number of reviews on the topic, none of them have focused on the combined use of two important polysaccharides, chitosan and pectin. Therefore, the present review will discuss the biomedical applications of polysaccharide-based hydrogels containing the two aforementioned natural polymers, chitosan and pectin, in the fields of tissue engineering and 3D in vitro modeling.

Microfluidic-templated cell-laden microgels fabricated using phototriggered imine-crosslinking as injectable and adaptable granular gels for bone regeneration

Injectable granular gels consisting of densely packed microgels serving as scaffolding biomaterial have recently shown great potential for applications in tissue regeneration, which allow administration via minimally invasive surgery, on-target cargo delivery, and high efficiency in nutrient/waste exchange. However, limitations such as insufficient mechanical strength, structural integrity, and uncontrollable differentiation of the encapsulated cells in the scaffolds hamper their further applications in the biomedical field. Herein, we developed a new class of granular gels via bottom-up assembly of cell-laden microgels via photo-triggered imine-crosslinking (PIC) chemistry based on the microfluidic technique. The particulate nature of the granular gels rendered them with shear-thinning and self-healing behavior, thereby functioning as an injectable and adaptable cellularized scaffold for bone tissue regeneration. Specifically, single cell-laden, monodisperse microgels composed of methacrylate- and o-nitrobenzene-functionalized hyaluronic acid and gelatin were prepared using a high-throughput microfluidic technique with a production rate up to 3.7 × 10⁸ microgels/hr, wherein the PIC chemistry alleviated the oxygen inhibition on free-radical polymerization and facilitated enhanced fabrication accuracy, accelerated gelation rate, and improved network strength. Further in vitro and in vivo studies demonstrated that the microgels can serve as carriers to support the activity of the encapsulated mesenchymal stem cells; these cell-laden microgels can also be used as cellularized bone fillers to induce the regeneration of bone tissues as evidenced by the in vivo experiment using the rat femoral condyle defect model. In general, these results represent a significant step toward the precise fabrication of engineered tissue mimics with single-cell resolution and high cell-density and can potentially offer a powerful tool for the design and applications of a next generation of tissue engineering strategy. STATEMENT OF SIGNIFICANCE: Using microfluidic droplet-based technology, we hereby developed a new class of injectable and moldable granular gels via bottom-up assembly of cell-laden microgels as a versatile platform for tissue regeneration. Phototriggered imine-crosslinking chemistry was introduced for microgel cross-linkage, which allowed for the fabrication of microgels with improved matrix homogeneity, accelerated gelation process, and enhanced mechanical strength. We demonstrated that the microgel building blocks within the granular gels facilitated the proliferation and differentiation of the encapsulated mesenchymal stem cells, which can further serve as a cellularized scaffold for the treatment of bone defects.

Polymeric microcarriers for minimally-invasive cell delivery

Tissue engineering (TE) aims at restoring tissue defects by applying the three-dimensional (3D) biomimetic pre-formed scaffolds to restore, maintain, and enhance tissue growth. Broadly speaking, this approach has created a potential impact in anticipating organ-building, which could reduce the need for organ replacement therapy. However, the implantation of such cell-laden biomimetic constructs based on substantial open surgeries often results in severe inflammatory reactions at the incision site, leading to the generation of a harsh adverse environment where cell survival is low. To overcome such limitations, micro-sized injectable modularized units based on various biofabrication approaches as ideal delivery vehicles for cells and various growth factors have garnered compelling interest owing to their minimally-invasive nature, ease of packing cells, and improved cell retention efficacy. Several advancements have been made in fabricating various 3D biomimetic microscale carriers for cell delivery applications. In this review, we explicitly discuss the progress of the microscale cell carriers that potentially pushed the borders of TE, highlighting their design, ability to deliver cells and substantial tissue growth in situ and in vivo from different viewpoints of materials chemistry and biology. Finally, we summarize the perspectives highlighting current challenges and expanding opportunities of these innovative carriers.

Progress of Microfluidic Hydrogel-Based Scaffolds and Organ-on-Chips for the Cartilage Tissue Engineering

Cartilage degeneration is among the fundamental reasons behind disability and pain across the globe. Numerous approaches have been employed to treat cartilage diseases. Nevertheless, none have shown acceptable outcomes in the long run. In this regard, the convergence of tissue engineering and microfabrication principles can allow developing more advanced microfluidic technologies, thus offering attractive alternatives to current treatments and traditional constructs used in tissue engineering applications. Herein, the current developments involving microfluidic hydrogel-based scaffolds, promising structures for cartilage regeneration, ranging from hydrogels with microfluidic channels to hydrogels prepared by the microfluidic devices, that enable therapeutic delivery of cells, drugs, and growth factors, as well as cartilage-related organ-on-chips are reviewed. Thereafter, cartilage anatomy and types of damages, and present treatment options are briefly overviewed. Various hydrogels are introduced, and the advantages of microfluidic hydrogel-based scaffolds over traditional hydrogels are thoroughly discussed. Furthermore, available technologies for fabricating microfluidic hydrogel-based scaffolds and microfluidic chips are presented. The preclinical and clinical applications of microfluidic hydrogel-based scaffolds in cartilage regeneration and the development of cartilage-related microfluidic chips over time are further explained. The current developments, recent key challenges, and attractive prospects that should be considered so as to develop microfluidic systems in cartilage repair are highlighted.

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.

Microgels based on 0D-3D carbon materials: Synthetic techniques, properties, applications, and challenges

Microgels are three-dimensional (3D) colloidal hydrogel particles with outstanding features such as biocompatibility, good mechanical properties, tunable sizes from submicrometer to tens of nanometers, and large surface areas. Because of these unique qualities, microgels have been widely used in various applications. Carbon-based materials (CMs) with various dimensions (0-3D) have recently been investigated as promising candidates for the design and fabrication of microgels because of their large surface area, excellent conductivity, unique chemical stability, and low cost. Here, we provide a critical review of the specific characteristics of CMs that are being incorporated into microgels, as well as the state-of-the art applications of CM-microgels in pollutant adsorption and photodegradation, H₂ evoluation, CO₂ capture, soil conditioners, water retention, drug delivery, cell encapsulation, and tissue engineering. Advanced preparation techniques for CM-microgel systems are also summarized and discussed. Finally, challenges related to the low colloidal stability of CM-microgels and development strategies are examined. This review shows that CM-microgels have the potential to be widely used in various practical applications.

Cell Response in Free-Packed Granular Systems

The study of the interactions of living adherent cells with mechanically stable (visco)elastic materials enables understanding and exploitation of physiological phenomena mediated by cell-extracellular communication. Insights into the interaction of cells and surrounding objects with different stability patterns upon cell contact might unveil biological responses to engineer innovative applications. Here, we hypothesize that the efficiency of cell attachment, spreading, and movement across a free-packed granular bed of microparticles depends on the microparticle diameter, raising the possibility of a necessary minimum traction force for the reinforcement of cell-particle bonds and long-term cell adhesion. The results suggest that microparticles with diameters of 14-20 μm are prone to cell-mediated mobility, holding the potential of inducing early cell detachment, while objects with diameters from 38 to 85 μm enable long-lasting cell adhesion and proliferation. An in silico hybrid particle-based model that addresses the time-dependent biological mechanisms of cell adhesion is proposed, providing inspiration for engineering platforms to address healthcare-related challenges.

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.

Aggressive strategies for regenerating intervertebral discs: stimulus-responsive composite hydrogels from single to multiscale delivery systems

As our research on the physiopathology of intervertebral disc degeneration (IVD degeneration, IVDD) has advanced and tissue engineering has rapidly evolved, cell-, biomolecule- and nucleic acid-based hydrogel grafting strategies have been widely investigated for their ability to overcome the harsh microenvironment of IVDD. However, such single delivery systems suffer from excessive external dimensions, difficult performance control, the need for surgical implantation, and difficulty in eliminating degradation products. Stimulus-responsive composite hydrogels have good biocompatibility and controllable mechanical properties and can undergo solution-gel phase transition under certain conditions. Their combination with ready-to-use particles to form a multiscale delivery system may be a breakthrough for regenerative IVD strategies. In this paper, we focus on summarizing the progress of research on the stimulus response mechanisms of regenerative IVD-related biomaterials and their design as macro-, micro- and nanoparticles. Finally, we discuss multi-scale delivery systems as bioinks for bio-3D printing technology for customizing personalized artificial IVDs, which promises to take IVD regenerative strategies to new heights.

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.

Highlights on Advancing Frontiers in Tissue Engineering

The field of tissue engineering continues to advance, sometimes in exponential leaps forward, but also sometimes at a rate that does not fulfill the promise that the field imagined a few decades ago. This review is in part a catalog of success in an effort to inform the process of innovation. Tissue engineering has recruited new technologies and developed new methods for engineering tissue constructs that can be used to mitigate or model disease states for study. Key to this antecedent statement is that the scientific effort must be anchored in the needs of a disease state and be working toward a functional product in regenerative medicine. It is this focus on the wildly important ideas coupled with partnered research efforts within both academia and industry that have shown most translational potential. The field continues to thrive and among the most important recent developments are the use of three-dimensional bioprinting, organ-on-a-chip, and induced pluripotent stem cell technologies that warrant special attention. Developments in the aforementioned areas as well as future directions are highlighted in this article. Although several early efforts have not come to fruition, there are good examples of commercial profitability that merit continued investment in tissue engineering. Impact statement Tissue engineering led to the development of new methods for regenerative medicine and disease models. Among the most important recent developments in tissue engineering are the use of three-dimensional bioprinting, organ-on-a-chip, and induced pluripotent stem cell technologies. These technologies and an understanding of them will have impact on the success of tissue engineering and its translation to regenerative medicine. Continued investment in tissue engineering will yield products and therapeutics, with both commercial importance and simultaneous disease mitigation.

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.

PBPK Modeling on Organs-on-Chips: An Overview of Recent Advancements

Organ-on-a-chip (OoC) is a new and promising technology, which aims to improve the efficiency of drug development and realize personalized medicine by simulating in vivo environment in vitro. Physiologically based pharmacokinetic (PBPK) modeling is believed to have the advantage of better reflecting the absorption, distribution, metabolism and excretion process of drugs in vivo than traditional compartmental or non-compartmental pharmacokinetic models. The combination of PBPK modeling and organ-on-a-chip is believed to provide a strong new tool for new drug development and have the potential to replace animal testing. This article provides the recent development of organ-on-a-chip technology and PBPK modeling including model construction, parameter estimation and validation strategies. Application of PBPK modeling on Organ-on-a-Chip (OoC) has been emphasized, and considerable progress has been made. PBPK modeling on OoC would become an essential part of new drug development, personalized medicine and other fields.

Microgel assembly: Fabrication, characteristics and application in tissue engineering and regenerative medicine

Microgel assembly, a macroscopic aggregate formed by bottom-up assembly of microgels, is now emerging as prospective biomaterials for applications in tissue engineering and regenerative medicine (TERM). This mini-review first summarizes the fabrication strategies available for microgel assembly, including chemical reaction, physical reaction, cell-cell interaction and external driving force, then highlights its unique characteristics, such as microporosity, injectability and heterogeneity, and finally itemizes its applications in the fields of cell culture, tissue regeneration and biofabrication, especially 3D printing. The problems to be addressed for further applications of microgel assembly are also discussed.

Translating Therapeutic Microgels into Clinical Applications

Microgels are crosslinked, water-swollen networks with a 10 nm to 100 µm diameter and can be modified chemically or biologically to render them biocompatible for advanced clinical applications. Depending on their intended use, microgels require different mechanical and structural properties, which can be engineered on demand by altering the biochemical composition, crosslink density of the polymer network, and the fabrication method. Here, the fundamental aspects of microgel research and development, as well as their specific applications for theranostics and therapy in the clinic, are discussed. A detailed overview of microgel fabrication techniques with regards to their intended clinical application is presented, while focusing on how microgels can be employed as local drug delivery materials, scavengers, and contrast agents. Moreover, microgels can act as scaffolds for tissue engineering and regeneration application. Finally, an overview of microgels is given, which already made it into pre-clinical and clinical trials, while future challenges and chances are discussed. This review presents an instructive guideline for chemists, material scientists, and researchers in the biomedical field to introduce them to the fundamental physicochemical properties of microgels and guide them from fabrication methods via characterization techniques and functionalization of microgels toward specific applications in the clinic.

Increased connectivity of hiPSC-derived neural networks in multiphase granular hydrogel scaffolds

To reflect human development, it is critical to create a substrate that can support long-term cell survival, differentiation, and maturation. Hydrogels are promising materials for 3D cultures. However, a bulk structure consisting of dense polymer networks often leads to suboptimal microenvironments that impedes nutrient exchange and cell-to-cell interaction. Herein, granular hydrogel-based scaffolds were used to support 3D human induced pluripotent stem cell (hiPSC)-derived neural networks. A custom designed 3D printed toolset was developed to extrude hyaluronic acid hydrogel through a porous nylon fabric to generate hydrogel granules. Cells and hydrogel granules were combined using a weaker secondary gelation step, forming self-supporting cell laden scaffolds. At three and seven days, granular scaffolds supported higher cell viability compared to bulk hydrogels, whereas granular scaffolds supported more neurite bearing cells and longer neurite extensions (65.52 ± 11.59 μm) after seven days compared to bulk hydrogels (22.90 ± 4.70 μm). Long-term (three-month) cultures of clinically relevant hiPSC-derived neural cells in granular hydrogels supported well established neuronal and astrocytic colonies and a high level of neurite extension both inside and beyond the scaffold. This approach is significant as it provides a simple, rapid and efficient way to achieve a tissue-relevant granular structure within hydrogel cultures.

Engineering Hydrogels for the Development of Three-Dimensional In Vitro Models

The superiority of in vitro 3D cultures over conventional 2D cell cultures is well recognized by the scientific community for its relevance in mimicking the native tissue architecture and functionality. The recent paradigm shift in the field of tissue engineering toward the development of 3D in vitro models can be realized with its myriad of applications, including drug screening, developing alternative diagnostics, and regenerative medicine. Hydrogels are considered the most suitable biomaterial for developing an in vitro model owing to their similarity in features to the extracellular microenvironment of native tissue. In this review article, recent progress in the use of hydrogel-based biomaterial for the development of 3D in vitro biomimetic tissue models is highlighted. Discussions of hydrogel sources and the latest hybrid system with different combinations of biopolymers are also presented. The hydrogel crosslinking mechanism and design consideration are summarized, followed by different types of available hydrogel module systems along with recent microfabrication technologies. We also present the latest developments in engineering hydrogel-based 3D in vitro models targeting specific tissues. Finally, we discuss the challenges surrounding current in vitro platforms and 3D models in the light of future perspectives for an improved biomimetic in vitro organ system.

Cell encapsulation in alginate-based microgels using droplet microfluidics; a review on gelation methods and applications

Cell encapsulation within the microspheres using a semi-permeable polymer allows the two-way transfer of molecules such as oxygen, nutrients, and growth factors. The main advantages of cell encapsulation technology include controlling the problems involved in transplanting rejection in tissue engineering applications and reducing the long-term need for immunosuppressive drugs following organ transplantation to eliminate the side effects. Cell-laden microgels can also be used in 3D cell cultures, wound healing, and cancerous clusters for drug testing. Since cell encapsulation is used for different purposes, several techniques have been developed to encapsulate cells. Droplet-based microfluidics is one of the most valuable techniques in cell encapsulating. This study aimed to review the geometries and the mechanisms proposed in microfluidic systems to precisely control cell-laden microgels production with different biopolymers. We also focused on alginate gelation techniques due to their essential role in cell encapsulation applications. Finally, some applications of these microgels and researches will be explored.

Single-cell droplet microfluidics for biomedical applications

This review focuses on the recent advances in the fundamentals of single-cell droplet microfluidics and its applications in biomedicine, providing insights into design and establishment of single-cell microsystems and their further performance.

Microfluidic-Based Droplets for Advanced Regenerative Medicine: Current Challenges and Future Trends

Microfluidics is a promising approach for the facile and large-scale fabrication of monodispersed droplets for various applications in biomedicine. This technology has demonstrated great potential to address the limitations of regenerative medicine. Microfluidics provides safe, accurate, reliable, and cost-effective methods for encapsulating different stem cells, gametes, biomaterials, biomolecules, reagents, genes, and nanoparticles inside picoliter-sized droplets or droplet-derived microgels for different applications. Moreover, microenvironments made using such droplets can mimic niches of stem cells for cell therapy purposes, simulate native extracellular matrix (ECM) for tissue engineering applications, and remove challenges in cell encapsulation and three-dimensional (3D) culture methods. The fabrication of droplets using microfluidics also provides controllable microenvironments for manipulating gametes, fertilization, and embryo cultures for reproductive medicine. This review focuses on the relevant studies, and the latest progress in applying droplets in stem cell therapy, tissue engineering, reproductive biology, and gene therapy are separately evaluated. In the end, we discuss the challenges ahead in the field of microfluidics-based droplets for advanced regenerative medicine.

Layer-by-Layer Biomimetic Microgels for 3D Cell Culture and Nonviral Gene Delivery

Localized release of nucleic acid therapeutics is essential for many biomedical applications, including gene therapy, tissue engineering, and medical implant coatings. We applied the substrate-mediated transfection and layer-by-layer (LbL) technique to achieve an efficient local gene delivery. In the experiments presented herein, we embeded lipoplexes containing plasmid DNA encoding for enhanced green fluorescent protein (pEGFP) within polyelectrolyte alginate-based microgels composed of poly(allylamine hydrochloride) (PAH), chondroitin sulfate (CS), and poly-l-lysine (PLL) with diameters between 70 and 90 μm. Droplet-based microfluidics was used as the main process to produce the alginate (ALG)-based microgels with discrete size, shape, and low coefficient of variation. The physicochemical and morphological properties of the polyelectrolyte microgels were characterized via optical microscopy, scanning electron microscopy (SEM), and zeta potential analysis. We found that polyelectrolyte microgels provide low cytotoxicity and cell-material interactions (adhesion, spreading, and proliferation). In addition, the microsystem showed the ability to load lipoplexes and a loading efficiency equal to 83%, and it enabled in vitro surface-based transfection of MCF-7 cells. This approach provides a new suitable route for cell adhesion and local gene delivery.

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.

Fullerol-hydrogel microfluidic spheres for in situ redox regulation of stem cell fate and refractory bone healing

The balance of redox homeostasis is key to stem cell maintenance and differentiation. However, this balance is disrupted by the overproduced reactive oxygen species (ROS) in pathological conditions, which seriously impair the therapeutic efficacy of stem cells. In the present study, highly dispersed fullerol nanocrystals with enhanced bioreactivity were incorporated into hydrogel microspheres using one-step innovative microfluidic technology to construct fullerol-hydrogel microfluidic spheres (FMSs) for in situ regulating the redox homeostasis of stem cells and promoting refractory bone healing. It was demonstrated that FMSs exhibited excellent antioxidant activity to quench both intracellular and extracellular ROS, sparing stem cells from oxidative stress damage. Furthermore, these could effectively promote the osteogenic differentiation of stem cells with the activation of FoxO1 signaling, indicating the intrinsically osteogenic property of FMSs. By injecting the stem cells-laden FMSs into rat calvarial defects, the formation of new bone was remarkably reinforced, which is a positive synergic effect from modulating the ROS microenvironment and enhancing the osteogenesis of stem cells. Collectively, the antioxidative FMSs, as injectable stem cell carriers, hold enormous promise for refractory bone healing, which can also be expanded to deliver a variety of other cells, targeting diseases that require in situ redox regulation.

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.