Cell-Instructive Microgels with Tailor-Made Physicochemical Properties

Uniform sized cancer spheroids production using hydrogel-based droplet microfluidics: a review

Three-dimensional (3D) cell culture models have been extensively utilized in various mechanistic studies as well as for drug development studies as superior in vitro platforms than conventional two-dimensional (2D) cell culture models. This is especially the case in cancer biology, where 3D cancer models, such as spheroids or organoids, have been utilized extensively to understand the mechanisms of cancer development. Recently, many sophisticated 3D models such as organ-on-a-chip models are emerging as advanced in vitro models that can more accurately mimic the in vivo tissue functions. Despite such advancements, spheroids are still considered as a powerful 3D cancer model due to the relatively simple structure and compatibility with existing laboratory instruments, and also can provide orders of magnitude higher throughput than complex in vitro models, an extremely important aspects for drug development. However, creating well-defined spheroids remain challenging, both in terms of throughputs in generation as well as reproducibility in size and shape that can make it challenging for drug testing applications. In the past decades, droplet microfluidics utilizing hydrogels have been highlighted due to their potentials. Importantly, core-shell structured gel droplets can avoid spheroid-to-spheroid adhesion that can cause large variations in assays while also enabling long-term cultivation of spheroids with higher uniformity by protecting the core organoid area from external environment while the outer porous gel layer still allows nutrient exchange. Hence, core-shell gel droplet-based spheroid formation can improve the predictivity and reproducibility of drug screening assays. This review paper will focus on droplet microfluidics-based technologies for cancer spheroid production using various gel materials and structures. In addition, we will discuss emerging technologies that have the potential to advance the production of spheroids, prospects of such technologies, and remaining challenges.

Towards single cell encapsulation for precision biology and medicine

The primary impetus of therapeutic cell encapsulation in the past several decades has been to broaden the options for donor cell sources by countering against immune-mediated rejection. However, another significant advantage of encapsulation is to provide donor cells with physiologically relevant cues that become compromised in disease. The advances in biomaterial design have led to the fundamental insight that cells sense and respond to various signals encoded in materials, ranging from biochemical to mechanical cues. The biomaterial design for cell encapsulation is becoming more sophisticated in controlling specific aspects of cellular phenotypes and more precise down to the single cell level. This recent progress offers a paradigm shift by designing single cell-encapsulating materials with predefined cues to precisely control donor cells after transplantation.

Lifelike Transformative Materials for Biohybrid Implants: Inspired by Nature, Driven by Technology

Today's living world is enriched with a myriad of natural biological designs, shaped by billions of years of evolution. Unraveling the construction rules of living organisms offers the potential to create new materials and systems for biomedicine. From the close examination of living organisms, several concepts emerge: hierarchy, pattern repetition, adaptation, and irreducible complexity. All these aspects must be tackled to develop transformative materials with lifelike behavior. This perspective article highlights recent progress in the development of transformative biohybrid systems for applications in the fields of tissue regeneration and biomedicine. Advances in computational simulations and data-driven predictions are also discussed. These tools enable the virtual high-throughput screening of implant design and performance before committing to fabrication, thus reducing the development time and cost of biomimetic and biohybrid constructs. The ongoing progress of imaging methods also constitutes an essential part of this matter in order to validate the computation models and enable longitudinal monitoring. Finally, the current challenges of lifelike biohybrid materials, including reproducibility, ethical considerations, and translation, are discussed. Advances in the development of lifelike materials will open new biomedical horizons, where perhaps what is currently envisioned as science fiction will become a science-driven reality in the future.

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.

Microfluidics Fabrication of Micrometer-Sized Hydrogels with Precisely Controlled Geometries for Biomedical Applications

Micrometer-sized hydrogels are cross-linked three-dimensional network matrices with high-water contents and dimensions ranging from several to hundreds of micrometers. Due to their excellent biocompatibility and capability to mimic physiological microenvironments in vivo, micrometer-sized hydrogels have attracted much attention in the biomedical engineering field. Their biological properties and applications are primarily influenced by their chemical compositions and geometries. However, inhomogeneous morphologies and uncontrollable geometries limit traditional micrometer-sized hydrogels obtained by bulk mixing. In contrast, microfluidic technology holds great potential for the fabrication of micrometer-sized hydrogels since their geometries, sizes, structures, compositions, and physicochemical properties can be precisely manipulated on demand based on the excellent control over fluids. Therefore, micrometer-sized hydrogels fabricated by microfluidic technology have been applied in the biomedical field, including drug encapsulation, cell encapsulation, and tissue engineering. This review introduces micrometer-sized hydrogels with various geometries synthesized by different microfluidic devices, highlighting their advantages in various biomedical applications over those from traditional approaches. Overall, emerging microfluidic technologies enrich the geometries and morphologies of hydrogels and accelerate translation for industrial production and clinical applications.

Recent Advances in Microgels: From Biomolecules to Functionality

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

Advancements in Three Dimensional In-Vitro Cell Culture Models

The scientific field is observing a gradual shift from monolayer cultures to three-dimensional (3D) models, as they give a more relevant data in pre-clinical stages. This review summarizes the major techniques and materials used to develop 3D platforms, especially for cancer. It also discusses the challenges and some unresolved issues of the field and highlights some techniques that have made it to the market.

Cell-based biocomposite engineering directed by polymers

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

Core-shell microparticles: From rational engineering to diverse applications

Core-shell microparticles, composed of solid, liquid, or gas bubbles surrounded by a protective shell, are gaining considerable attention as intelligent and versatile carriers that show great potential in biomedical fields. In this review, an overview is given of recent developments in design and applications of biodegradable core-shell systems. Several emerging methodologies including self-assembly, gas-shearing, and coaxial electrospray are discussed and microfluidics technology is emphasized in detail. Furthermore, the characteristics of core-shell microparticles in artificial cells, drug release and cell culture applications are discussed and the superiority of these advanced multi-core microparticles for the generation of artificial cells is highlighted. Finally, the respective developing orientations and limitations inherent to these systems are addressed. It is hoped that this review can inspire researchers to propel the development of this field with new ideas.

Microfluidic droplet-based functional materials for cell manipulation

Functional materials from the microfluidic-based droplet community are emerging as enabling tools for various applications in tissue engineering and cell biology. The innovative micro- and nano-scale materials with diverse sizes, shapes and components can be fabricated without the use of complicated devices, allowing unprecedented control over the cells that interact with them. Here, we review the current development of microfluidic-based droplet techniques for creation of functional materials (i.e., liquid droplet, microcapsule, and microparticle). We also describe their various applications for manipulating cell fate and function.

Delivery of Endothelial Cell-Laden Microgel Elicits Angiogenesis in Self-Assembling Ultrashort Peptide Hydrogels In Vitro

Blood vessel generation is an essential process for tissue formation, regeneration, and repair. Notwithstanding, vascularized tissue fabrication in vitro remains a challenge, as current fabrication techniques and biomaterials lack translational potential in medicine. Naturally derived biomaterials harbor the risk of immunogenicity and pathogen transmission, while synthetic materials need functionalization or blending to improve their biocompatibility. In addition, the traditional top-down fabrication techniques do not recreate the native tissue microarchitecture. Self-assembling ultrashort peptides (SUPs) are promising chemically synthesized natural materials that self-assemble into three-dimensional nanofibrous hydrogels resembling the extracellular matrix (ECM). Here, we use a modular tissue-engineering approach, embedding SUP microgels loaded with human umbilical vein endothelial cells (HUVECs) into a 3D SUP hydrogel containing human dermal fibroblast neonatal (HDFn) cells to trigger angiogenesis. The SUPs IVFK and IVZK were used to fabricate microgels that gel in a water-in-oil emulsion using a microfluidic droplet generator chip. The fabricated SUP microgels are round structures that are 300-350 μm diameter in size and have ECM-like topography. In addition, they are stable enough to keep their original size and shape under cell culture conditions and long-term storage. When the SUP microgels were used as microcarriers for growing HUVECs and HDFn cells on the microgel surface, cell attachment, stretching, and proliferation could be demonstrated. Finally, we performed an angiogenesis assay in both SUP hydrogels using all SUP combinations between micro- and bulky hydrogels. Endothelial cells were able to migrate from the microgel to the surrounding area, showing angiogenesis features such as sprouting, branching, coalescence, and lumen formation. Although both SUP hydrogels support vascular network formation, IVFK outperformed IVZK in terms of vessel network extension and branching. Overall, these results demonstrated that cell-laden SUP microgels have great potential to be used as a microcarrier cell delivery system, encouraging us to study the angiogenesis process and to develop vascularized tissue-engineering therapies.

Controlling Structure with Injectable Biomaterials to Better Mimic Tissue Heterogeneity and Anisotropy

Tissue regeneration of sensitive tissues calls for injectable scaffolds, which are minimally invasive and offer minimal damage to the native tissues. However, most of these systems are inherently isotropic and do not mimic the complex hierarchically ordered nature of the native extracellular matrices. This review focuses on the different approaches developed in the past decade to bring in some form of anisotropy to the conventional injectable tissue regenerative matrices. These approaches include introduction of macroporosity, in vivo pattering to present biomolecules in a spatially and temporally controlled manner, availability of aligned domains by means of self-assembly or oriented injectable components, and in vivo bioprinting to obtain structures with features of high resolution that resembles native tissues. Toward the end of the review, different techniques to produce building blocks for the fabrication of heterogeneous injectable scaffolds are discussed. The advantages and shortcomings of each approach are discussed in detail with ideas to improve the functionality and versatility of the building blocks.

Acoustic focusing of beads and cells in hydrogel droplets

The generation of hydrogel droplets using droplet microfluidics has emerged as a powerful tool with many applications in biology and medicine. Here, a microfluidic system to control the position of particles (beads or astrocyte cells) in hydrogel droplets using bulk acoustic standing waves is presented. The chip consisted of a droplet generator and a 380 µm wide acoustic focusing channel. Droplets comprising hydrogel precursor solution (polyethylene glycol tetraacrylate or a combination of polyethylene glycol tetraacrylate and gelatine methacrylate), photoinitiator and particles were generated. The droplets passed along the acoustic focusing channel where a half wavelength acoustic standing wave field was generated, and the particles were focused to the centre line of the droplets (i.e. the pressure nodal line) by the acoustic force. The droplets were cross-linked by exposure to UV-light, freezing the particles in their positions. With the acoustics applied, 89 ± 19% of the particles (polystyrene beads, 10 µm diameter) were positioned in an area ± 10% from the centre line. As proof-of-principle for biological particles, astrocytes were focused in hydrogel droplets using the same principle. The viability of the astrocytes after 7 days in culture was 72 ± 22% when exposed to the acoustic focusing compared with 70 ± 19% for samples not exposed to the acoustic focusing. This technology provides a platform to control the spatial position of bioparticles in hydrogel droplets, and opens up for the generation of more complex biological hydrogel structures.

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.

Microdroplet enabled cultivation of single yeast cells correlates with bulk growth and reveals subpopulation phenomena

Yeast has been engineered for cost-effective organic acid production through metabolic engineering and synthetic biology techniques. However, cell growth assays in these processes were performed in bulk at the population level, thus obscuring the dynamics of rare single cells exhibiting beneficial traits. Here, we introduce the use of monodisperse picolitre droplets as bioreactors to cultivate yeast at the single-cell level. We investigated the effect of acid stress on growth and the effect of potassium ions on propionic acid tolerance for single yeast cells of different species, genotypes, and phenotypes. The results showed that the average growth of single yeast cells in microdroplets experiences the same trend to those of yeast populations grown in bulk, and microdroplet compartments do not significantly affect cell viability. This approach offers the prospect of detecting cell-to-cell variations in growth and physiology and is expected to be applied for the engineering of yeast to produce value-added bioproducts.

3D aggregation of cells in packed microgel media

In both natural and applied contexts, investigating cell self-assembly and aggregation within controlled 3D environments leads to improved understanding of how structured cell assemblies emerge, what determines their shapes and sizes, and whether their structural features are stable. However, the inherent limits of using solid scaffolding or liquid spheroid culture for this purpose restrict experimental freedom in studies of cell self-assembly. Here we investigate multi-cellular self-assembly using a 3D culture medium made from packed microgels as a bridge between the extremes of solid scaffolds and liquid culture. We find that cells dispersed at different volume fractions in this microgel-based 3D culture media aggregate into clusters of different sizes and shapes, forming large system-spanning networks at the highest cell densities. We find that the transitions between different states of assembly can be controlled by the level of cell-cell cohesion and by the yield stress of the packed microgel environment. Measurements of aggregate fractal dimension show that those with increased cell-cell cohesion are less sphere-like and more irregularly shaped, indicating that cell stickiness inhibits rearrangements in aggregates, in analogy to the assembly of colloids with strong cohesive bonds. Thus, the effective surface tension often expected to emerge from increased cell cohesion is suppressed in this type of cell self-assembly.

Improving Single-Cell Encapsulation Efficiency and Reliability through Neutral Buoyancy of Suspension

Single-cell analysis is of critical importance in revealing cell-to-cell heterogeneity by characterizing individual cells and identifying minority sub-populations of interest. Droplet-based microfluidics has been widely used in the past decade to achieve high-throughput single-cell analysis. However, to maximize the proportion of single-cell emulsification is challenging due to cell sedimentation and aggregation. The purpose of this study was to investigate the influence of single-cell encapsulation and incubation through the use of neutral buoyancy. As a proof of concept, OptiPrep™ was used to create neutrally buoyant cell suspensions of THP-1, a human monocytic leukemia cell line, for single-cell encapsulation and incubation. We found that using a neutrally buoyant suspension greatly increased the efficiency of single-cell encapsulation in microdroplets and eliminated unnecessary cell loss. Moreover, the presence of OptiPrep™ was shown to not affect cellular viability. This method significantly improved the effectiveness of single-cell study in a non-toxic environment and is expected to broadly facilitate single-cell analysis.

Hydrogel microparticles for biomedical applications

Hydrogel microparticles (HMPs) are promising for biomedical applications, ranging from the therapeutic delivery of cells and drugs to the production of scaffolds for tissue repair and bioinks for 3D printing. Biologics (cells and drugs) can be encapsulated into HMPs of predefined shapes and sizes using a variety of fabrication techniques (batch emulsion, microfluidics, lithography, electrohydrodynamic (EHD) spraying and mechanical fragmentation). HMPs can be formulated in suspensions to deliver therapeutics, as aggregates of particles (granular hydrogels) to form microporous scaffolds that promote cell infiltration or embedded within a bulk hydrogel to obtain multiscale behaviours. HMP suspensions and granular hydrogels can be injected for minimally invasive delivery of biologics, and they exhibit modular properties when comprised of mixtures of distinct HMP populations. In this Review, we discuss the fabrication techniques that are available for fabricating HMPs, as well as the multiscale behaviours of HMP systems and their functional properties, highlighting their advantages over traditional bulk hydrogels. Furthermore, we discuss applications of HMPs in the fields of cell delivery, drug delivery, scaffold design and biofabrication.

A Layer-by-Layer Single-Cell Coating Technique To Produce Injectable Beating Mini Heart Tissues via Microfluidics

Human induced pluripotent stem cells (hiPSCs) are used as an alternative for human embryonic stem cells. Cardiomyocytes derived from hiPSCs are employed in cardiac tissue regeneration constructs due to the heart's low regeneration capacity after infarction. A coculture of hiPSC-CM and primary dermal fibroblasts is encapsulated in injectable poly(ethylene glycol)-based microgels via microfluidics to enhance the efficiency of regenerative cell transplantations. The microgels are prepared via Michael-type addition of multi-arm PEG-based molecules with an enzymatically degradable peptide as a cross-linker and modified with a cell-adhesive peptide. Cell-cell interactions and, consequently, cell viability are improved by a thin extracellular matrix (ECM) coating formed on the cell surfaces via layer-by-layer (LbL) deposition. The beating strength of encapsulated cardiomyocytes (∼60 BPM) increases by 2-fold compared to noncoated cells. The combination of microfluidics with the LbL technique offers a new technology to fabricate functional cardiac mini tissues for cell transplantation therapies.

Cell Encapsulation in Soft, Anisometric Poly(ethylene) Glycol Microgels Using a Novel Radical-Free Microfluidic System

Complex 3D artificial tissue constructs are extensively investigated for tissue regeneration. Frequently, materials and cells are delivered separately without benefitting from the synergistic effect of combined administration. Cell delivery inside a material construct provides the cells with a supportive environment by presenting biochemical, mechanical, and structural signals to direct cell behavior. Conversely, the cell/material interaction is poorly understood at the micron scale and new systems are required to investigate the effect of micron-scale features on cell functionality. Consequently, cells are encapsulated in microgels to avoid diffusion limitations of nutrients and waste and facilitate analysis techniques of single or collective cells. However, up to now, the production of soft cell-loaded microgels by microfluidics is limited to spherical microgels. Here, a novel method is presented to produce monodisperse, anisometric poly(ethylene) glycol microgels to study cells inside an anisometric architecture. These microgels can potentially direct cell growth and can be injected as rod-shaped mini-tissues that further assemble into organized macroscopic and macroporous structures post-injection. Their aspect ratios are adjusted with flow parameters, while mechanical and biochemical properties are altered by modifying the precursors. Encapsulated primary fibroblasts are viable and spread and migrate across the 3D microgel structure.

Microgels: Modular, tunable constructs for tissue regeneration

Biopolymer microgels are emerging as a versatile tool for aiding in the regeneration of damaged tissues due to their biocompatible nature, tunable microporous structure, ability to encapsulate bioactive factors, and tailorable properties such as stiffness and composition. These properties of microgels, along with their injectability, have allowed for their utilization in a multitude of different tissue engineering applications. Controlled release of growth factors, antibodies, and other bioactive factors from microgels have demonstrated their capabilities as transporters for essential bioactive molecules necessary for guiding tissue reconstruction. Additionally, recent in vitro studies of cellular interaction and proliferation within microgel structures have laid the initial groundwork for regenerative tissue engineering using these materials. Microgels have even been crosslinked together in various ways or 3D printed to form three-dimensional scaffolds to support cell growth. In vivo studies of microgels have pioneered the clinical relevance of these novel and innovative materials for regenerative tissue engineering. This review will cover recent developments and research of microgels as they pertain to bioactive factor release, cellular interaction and proliferation in vitro, and tissue regeneration in vivo. STATEMENT OF SIGNIFICANCE: This review is focused on state-of-the-art microgel technology and innovations within the tissue engineering field, focusing on the use of microgels in bioactive factor delivery and as cell-interactive scaffolds, both in vitro and in vivo. Microgels are hydrogel microparticles that can be tuned based on the biopolymer from which they are derived, the crosslinking chemistry used, and the fabrication method. The emergence of microgels for tissue regeneration applications in recent years illuminates their versatility and applicability in clinical settings.

Neural tissue engineering with structured hydrogels in CNS models and therapies

The development of techniques to create and use multiphase microstructured hydrogels (granular hydrogels or microgels) has enabled the generation of cultures with more biologically relevant architecture and use of structured hydrogels is especially pertinent to the development of new types of central nervous system (CNS) culture models and therapies. We review material choice and the customisation of hydrogel structure, as well as the use of hydrogels in developmental models. Combining the use of structured hydrogel techniques with developmentally relevant tissue culture approaches will enable the generation of more relevant models and treatments to repair damaged CNS tissue architecture.

High-throughput stem cell-based phenotypic screening through microniches

Methods for screening combinations of signals for their effects on stem cell behavior are needed in the field of tissue engineering. We introduce a microgel-based screening platform for testing combinations of proteins on stem cell fate.

Perspective: The promise of multi-cellular engineered living systems

Recent technological breakthroughs in our ability to derive and differentiate induced pluripotent stem cells, organoid biology, organ-on-chip assays, and 3-D bioprinting have all contributed to a heightened interest in the design, assembly, and manufacture of living systems with a broad range of potential uses. This white paper summarizes the state of the emerging field of "multi-cellular engineered living systems," which are composed of interacting cell populations. Recent accomplishments are described, focusing on current and potential applications, as well as barriers to future advances, and the outlook for longer term benefits and potential ethical issues that need to be considered.

Injectable degradable PVA microgels prepared by microfluidic technology for controlled osteogenic differentiation of mesenchymal stem cells

The direct injection of bone marrow mesenchymal stem cells (hMSCs) is a promising strategy for bone tissue engineering applications. Herein, we have developed injectable degradable poly(vinyl alcohol) (PVA) microgels loaded with hMSCs and growth factors and prepared by a high-throughput microfluidic technology. The PVA-based microgels with tunable mechanical and degradable properties were composed of vinyl ether acrylate-functionalized PVA (PVA-VEA) and thiolated PVA-VEA (PVA-VEA-SH) through a Michael-type crosslinking reaction under mild conditions. The hMSCs sustain high viability in PVA microgels, and cell proliferation and migration behaviors can easily be adjusted by varying crosslinking densities of PVA microgels. Additionally, bone morphogenetic protein-2 (BMP-2) co-encapsulated into the microgel environments enhanced osteogenic differentiation of hMSCs as indicated by a significant increase in alkaline phosphatase activity, calcium content, and Runx2 and OPN gene expression levels. These results demonstrate the degradable PVA microgels with tailored stem cell microenvironments and controlled release profile of the growth factor to promote and direct differentiation. These PVA-based microgels have promising potential as ideal cell vehicles for applications in regenerative medicine.

STATEMENT OF SIGNIFICANCE

Stem cell transplantation by an injectable, minimally invasive method has great and promising potential for various injuries, diseases, and tissue regeneration. However, its applications are largely limited owing to the low cell retention and engraftment at the lesion location after administration. We have developed an injectable degradable poly(vinyl alcohol) (PVA) microgel prepared by a high-throughput microfluidic technology and co-loaded with bone marrow mesenchymal stem cells (hMSCs) and growth factor to protect the stem cells from harsh environmental stress and realize controlled cell differentiation in well-defined microenvironments for bone regeneration. We demonstrated that these degradable PVA microgels can be used as stem cell scaffolds with tailored cell microenvironments and controlled release profile of growth factor to promote and direct differentiation. We are convinced that these PVA-based microgels have promising potential in the future as cellular scaffolds for applications in regenerative medicine.

Microfluidic fabrication of microparticles for biomedical applications

Droplet microfluidics offers exquisite control over the flows of multiple fluids in microscale, enabling fabrication of advanced microparticles with precisely tunable structures and compositions in a high throughput manner. The combination of these remarkable features with proper materials and fabrication methods has enabled high efficiency, direct encapsulation of actives in microparticles whose features and functionalities can be well controlled. These microparticles have great potential in a wide range of bio-related applications including drug delivery, cell-laden matrices, biosensors and even as artificial cells. In this review, we briefly summarize the materials, fabrication methods, and microparticle structures produced with droplet microfluidics. We also provide a comprehensive overview of their recent uses in biomedical applications. Finally, we discuss the existing challenges and perspectives to promote the future development of these engineered microparticles.

Cell-Surface Engineering for Advanced Cell Therapy

Stem cells opened great opportunity to overcome diseases that conventional therapy had only limited success. Use of scaffolds made from biomaterials not only helps handling of stem cells for delivery or transplantation but also supports enhanced cell survival. Likewise, cell encapsulation can provide stability for living animal cells even in a state of separateness. Although various chemical reactions were tried to encapsulate stolid microbial cells such as yeasts, a culture environment for the growth of animal cells allows only highly biocompatible reactions. Therefore, the animal cells were mostly encapsulated in hydrogels, which resulted in enhanced cell survival. Interestingly, major findings of chemistry on biological interfaces demonstrate that cell encapsulation in hydrogels have a further a competence for modulating cell characteristics that can go beyond just enhancing the cell survival. In this review, we present a comprehensive overview on the chemical reactions applied to hydrogel-based cell encapsulation and their effects on the characteristics and behavior of living animal cells.

Single-Cell Microgels: Technology, Challenges, and Applications

Single-cell-laden microgels effectively act as the engineered counterpart of the smallest living building block of life: a cell within its pericellular matrix. Recent breakthroughs have enabled the encapsulation of single cells in sub-100-μm microgels to provide physiologically relevant microniches with minimal mass transport limitations and favorable pharmacokinetic properties. Single-cell-laden microgels offer additional unprecedented advantages, including facile manipulation, culture, and analysis of individual cell within 3D microenvironments. Therefore, single-cell microgel technology is expected to be instrumental in many life science applications, including pharmacological screenings, regenerative medicine, and fundamental biological research. In this review, we discuss the latest trends, technical challenges, and breakthroughs, and present our vision of the future of single-cell microgel technology and its applications.

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.

Centering Single Cells in Microgels via Delayed Crosslinking Supports Long-Term 3D Culture by Preventing Cell Escape

Single-cell-laden microgels support physiological 3D culture conditions while enabling straightforward handling and high-resolution readouts of individual cells. However, their widespread adoption for long-term cultures is limited by cell escape. In this work, it is demonstrated that cell escape is predisposed to off-center encapsulated cells. High-speed microscopy reveals that cells are positioned at the microgel precursor droplets' oil/water interface within milliseconds after droplet formation. In conventional microencapsulation strategies, the droplets are typically gelled immediately after emulsification, which traps cells in this off-center position. By delaying crosslinking, driving cells toward the centers of microgels is succeeded. The centering of cells in enzymatically crosslinked microgels prevents their escape during at least 28 d. It thereby uniquely enables the long-term culture of individual cells within <5-µm-thick 3D uniform hydrogel coatings. Single cell analysis of mesenchymal stem cells in enzymatically crosslinked microgels reveals unprecedented high cell viability (>90%), maintained metabolic activity (>70%), and multilineage differentiation capacity (>60%) over a period of 28 d. The facile nature of this microfluidic cell-centering method enables its straightforward integration into many microencapsulation strategies and significantly enhances control, reproducibility, and reliability of 3D single cell cultures.

Generation and manipulation of hydrogel microcapsules by droplet-based microfluidics for mammalian cell culture

Hydrogel microcapsules provide miniaturized and biocompatible niches for three-dimensional (3D) in vitro cell culture. They can be easily generated by droplet-based microfluidics with tunable size, morphology, and biochemical properties. Therefore, microfluidic generation and manipulation of cell-laden microcapsules can be used for 3D cell culture to mimic the in vivo environment towards applications in tissue engineering and high throughput drug screening. In this review of recent advances mainly since 2010, we will first introduce general characteristics of droplet-based microfluidic devices for cell encapsulation with an emphasis on the fluid dynamics of droplet breakup and internal mixing as they directly influence microcapsule's size and structure. We will then discuss two on-chip manipulation strategies: sorting and extraction from oil into aqueous phase, which can be integrated into droplet-based microfluidics and significantly improve the qualities of cell-laden hydrogel microcapsules. Finally, we will review various applications of hydrogel microencapsulation for 3D in vitro culture on cell growth and proliferation, stem cell differentiation, tissue development, and co-culture of different types of cells.

Single cell-laden protease-sensitive microniches for long-term culture in 3D

Single cell-laden three-dimensional (3D) microgels that can serve to mimic stem cell niches in vitro, and are therefore termed microniches, can be efficiently fabricated by droplet-based microfluidics. In this technique an aqueous polymer solution along with a highly diluted cell solution is injected into a microfluidic device to create monodisperse pre-microgel droplets that are then solidified by a polymer crosslinking reaction to obtain monodisperse single cell-laden microniches. However, problems limiting this approach studying the fate of single cells include Poisson encapsulation statistics that result in mostly empty microniches, and cells egressing from the microniches during subsequent cell culture. Here, we present a strategy to bypass Poisson encapsulation statistics in synthetic microniches by selective crosslinking of only cell-laden pre-microgel droplets. Furthermore, we show that we can position cells in the center of the microniches, and that even in protease-sensitive microniches this greatly reduces cell egress. Collectively, we present the development of a versatile protocol that allows for unprecedented efficiency in creation of synthetic protease-sensitive microniches for probing single stem cell fate in 3D.

Single Cell Microgel Based Modular Bioinks for Uncoupled Cellular Micro- and Macroenvironments

Modular bioinks based on single cell microgels within distinct injectable prepolymers enable uncoupling of biomaterials' micro- and macroenvironments. These inks allow biofabrication of 3D constructs that recapitulate the multiscale modular design of native tissues with a single cell resolution. This approach represents a major step forward in endowing engineered constructs with the multifunctionality that underlies the behavior of native tissues.

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.

Enzymatic Crosslinking of Polymer Conjugates is Superior over Ionic or UV Crosslinking for the On-Chip Production of Cell-Laden Microgels

Cell-laden micrometer-sized hydrogels (microgels) hold great promise for improving high throughput ex-vivo drug screening and engineering biomimetic tissues. Microfluidics is a powerful tool to produce microgels. However, only a limited amount of biomaterials have been reported to be compatible with on-chip microgel formation. Moreover, these biomaterials are often associated with mechanical instability, cytotoxicity, and cellular senescence. To resolve this challenge, dextran-tyramine has been explored as a novel biomaterial for on-chip microgel formation. In particular, dextran-tyramine is compared with two commonly used biomaterials, namely, polyethylene-glycol diacrylate (PEGDA) and alginate, which crosslink through enzymatic reaction, UV polymerization, and ionic interaction, respectively. Human mesenchymal stem cells (hMSCs) encapsulated in dextran-tyramine microgels demonstrate significantly higher (95%) survival as compared to alginate (81%) and PEGDA (69%). Long-term cell cultures demonstrate that hMSCs in PEGDA microgels become senescent after 7 d. Alginate microgels dissolve within 7 d due to Ca²⁺ loss. In contrast, dextran-tyramine based microgels remain stable, sustain hMSCs metabolic activity, and permit for single-cell level analysis for at least 28 d of culture. In conclusion, enzymatically crosslinking dextran-tyramine conjugates represent a novel biomaterial class for the on-chip production of cell-laden microgels, which possesses unique advantages as compared to the commonly used UV and ionic crosslinking biomaterials.

Cells Cultured on Core-Shell Photonic Crystal Barcodes for Drug Screening

The development of effective drug screening platforms is an important task for biomedical engineering. Here, a novel methacrylated gelatin (GelMA) hydrogel-encapsulated core-shell photonic crystal (PhC) barcode particle was developed for three-dimensional cell aggregation culture and drug screening. The GelMA shells of the barcode particles enable creation of a three-dimensional extracellular matrix (ECM) microenvironment for cell adhesion and growth, while the PhC cores of the barcode particles provide stable diffraction peaks that can encode different cell spheroids during culture and distinguish their biological response during drug testing. The applicability of this cell spheroids-on-barcodes platform was investigated by testing the cytotoxic effect of tegafur (TF), a prodrug of 5-fluorouracil (5-FU), on barcode particle-loaded liver HepG2 and HCT-116 colonic tumor cell spheroids. The cytotoxicity of TF against the HCT-116 tumor cell spheroids was enhanced in systems using cocultures of HepG2 and NIH-3T3 cells, indicating the effectiveness of this multiple cell spheroids-on-barcodes platform for drug screening.

Directed Assembly and Development of Material-Free Tissues with Complex Architectures

Material-free tissues are assembled using solely cells. Microstructured hydrogel templates and high content screening allow the formation of centimeter-scale tissues with precise architectures. Similar to developing tissues, these contract autonomously, controllably shift shape, self-scaffold by secreting extracellular matrix, and undergo morphogenesis.

Methods for Generating Hydrogel Particles for Protein Delivery

Proteins represent a major class of therapeutic molecules with vast potential for the treatment of acute and chronic diseases and regenerative medicine applications. Hydrogels have long been investigated for their potential in carrying and delivering proteins. As compared to bulk hydrogels, hydrogel microparticles (microgels) hold promise in improving aspects of delivery owing to their less traumatic route of entry into the body and improved versatility. This review discusses common methods of fabricating microgels, including emulsion polymerization, microfluidic techniques, and lithographic techniques. Microgels synthesized from both natural and synthetic polymers are discussed, as are a series of microgels fashioned from environment-responsive materials.