Influence of key processing parameters and seeding density effects of microencapsulated chondrocytes fabricated using electrohydrodynamic spraying

Design and preparation of hydrogel microspheres for spinal cord injury repair

A severe disorder known as spinal cord damage causes both motor and sensory impairment in the limbs, significantly reducing the patients' quality of life. After a spinal cord injury, functional recovery and therapy have emerged as critical concerns. Hydrogel microspheres have garnered a lot of interest lately because of their enormous promise in the field of spinal cord injury rehabilitation. The material classification of hydrogel microspheres (natural and synthetic macromolecule polymers) and their synthesis methods are examined in this work. This work also covers the introduction of several kinds of hydrogel microspheres and their use as carriers in the realm of treating spinal cord injuries. Lastly, the study reviews the future prospects for hydrogel microspheres and highlights their limitations and problems. This paper can offer feasible ideas for researchers to advance the application of hydrogel microspheres in the field of spinal cord injury.

An injectable self-healing alginate hydrogel with desirable mechanical and degradation properties for enhancing osteochondral regeneration

Articular cartilage and subchondral bone defects have always been problematic because the osteochondral tissue plays a crucial role in the movement of the body and does not recover spontaneously. Here, an injectable hydrogel composed of oxidized sodium alginate/gelatin/chondroitin sulfate (OSAGC) was designed for the minimally invasive treatment and promotion of osteochondral regeneration. The OSAGC hydrogel had a double network based on dynamic covalent bonds, demonstrating commendable injectability and self-healing properties. Chondroitin sulfate was organically bound to the hydrogel network, retaining its own activity and gradually releasing during the degradation process as well as improving mechanical properties. The compressive strength could be increased up to 3 MPa by regulating the concentration of chondroitin sulphate and the oxidation level, and this mechanical stimulation could help repair injured tissue. The OSAGC hydrogel had a favourable affinity to articular cartilage and was able to release active ingredients in a sustained manner over 3 months. The OSAGC showed no cytotoxic effects. Results from animal studies demonstrated its capacity to regenerate new bone tissue in four weeks and new cartilage tissue in twelve weeks. The OSAGC hydrogel represented a promising approach to simplify bone surgery and repair damaged osteochondral tissue.

Development and characterization of antacid microcapsules to buffer the acidic intervertebral disc microenvironment

During intervertebral disc (IVD) degeneration, microenvironmental challenges such as decreasing levels of glucose, oxygen, and pH play crucial roles in cell survival and matrix turnover. Antacids, such as Mg(OH)2 and CaCO3, entrapped in microcapsules are capable of neutralizing acidic microenvironments in a controlled fashion and therefore may offer the potential to improve the acidic niche of the degenerated IVD and enhance cell-based regeneration strategies. The objectives of this work were, first, to develop and characterize antacid microcapsules and assess their neutralization capacity in an acidic microenvironment and, second, to combine antacid microcapsules with cellular microcapsules in a hybrid gel system to investigate their neutralization effect as a potential therapeutic in a disc explant model. To achieve this, we screened five different pH- neutralizing agents (Al(OH)3, Mg(OH)2, CaCO3, and HEPES) in terms of their pH neutralization capacities, with Mg(OH)2 or CaCO3 being carried forward for further investigation. Antacid-alginate microcapsules were formed at different concentrations using the electrohydrodynamic spraying process and assessed in terms of size, buffering kinetics, cell compatibility, and cytotoxicity. Finally, the combination of cellular microcapsules and antacid capsules was examined in a bovine disc explant model under physiological degenerative conditions. Overall, CaCO3 was found to be superior in terms of neutralization capacities, release kinetics, and cellular response. Specifically, CaCO3 elevated the acidic pH to neutral levels and is estimated to be maintained for several weeks based on Ca2+ release. Using a disc explant model, it was demonstrated that CaCO3 microcapsules were capable of increasing the local pH within the core of a hybrid cellular gel system. This work highlights the potential of antacid microcapsules to positively alter the challenging acidic microenvironment conditions typically observed in degenerative disc disease, which may be used in conjunction with cell therapies to augment regeneration.

Granular Hydrogels in Biofabrication: Recent Advances and Future Perspectives

Granular hydrogels, which are formed by densely packing microgels, are promising materials for bioprinting due to their extrudability, porosity, and modularity. However, the multidimensional parameter space involved in granular hydrogel design makes material optimization challenging. For example, design inputs such as microgel morphology, packing density, or stiffness can influence multiple rheological properties that govern printability and the behavior of encapsulated cells. This review provides an overview of fabrication methods for granular hydrogels, and then examines how important design inputs can influence material properties associated with printability and cellular responses across multiple scales. Recent applications of granular design principles in bioink engineering are described, including the development of granular support hydrogels for embedded printing. Further, the paper provides an overview of how key physical properties of granular hydrogels can influence cellular responses, highlighting the advantages of granular materials for promoting cell and tissue maturation after the printing process. Finally, potential future directions for advancing the design of granular hydrogels for bioprinting are discussed.

Nano-Micron Combined Hydrogel Microspheres: Novel Answer for Minimal Invasive Biomedical Applications

Hydrogels, key in biomedical research for their hydrophilicity and versatility, have evolved with hydrogel microspheres (HMs) of micron-scale dimensions, enhancing their role in minimally invasive therapeutic delivery, tissue repair, and regeneration. The recent emergence of nanomaterials has ushered in a revolutionary transformation in the biomedical field, which demonstrates tremendous potential in targeted therapies, biological imaging, and disease diagnostics. Consequently, the integration of advanced nanotechnology promises to trigger a new revolution in the realm of hydrogels. HMs loaded with nanomaterials combine the advantages of both hydrogels and nanomaterials, which enables multifaceted functionalities such as efficient drug delivery, sustained release, targeted therapy, biological lubrication, biochemical detection, medical imaging, biosensing monitoring, and micro-robotics. Here, this review comprehensively expounds upon commonly used nanomaterials and their classifications. Then, it provides comprehensive insights into the raw materials and preparation methods of HMs. Besides, the common strategies employed to achieve nano-micron combinations are summarized, and the latest applications of these advanced nano-micron combined HMs in the biomedical field are elucidated. Finally, valuable insights into the future design and development of nano-micron combined HMs are provided.

A review of advanced hydrogels for cartilage tissue engineering

With the increase in weight and age of the population, the consumption of tobacco, inappropriate foods, and the reduction of sports activities in recent years, bone and joint diseases such as osteoarthritis (OA) have become more common in the world. From the past until now, various treatment strategies (e.g., microfracture treatment, Autologous Chondrocyte Implantation (ACI), and Mosaicplasty) have been investigated and studied for the prevention and treatment of this disease. However, these methods face problems such as being invasive, not fully repairing the tissue, and damaging the surrounding tissues. Tissue engineering, including cartilage tissue engineering, is one of the minimally invasive, innovative, and effective methods for the treatment and regeneration of damaged cartilage, which has attracted the attention of scientists in the fields of medicine and biomaterials engineering in the past several years. Hydrogels of different types with diverse properties have become desirable candidates for engineering and treating cartilage tissue. They can cover most of the shortcomings of other treatment methods and cause the least secondary damage to the patient. Besides using hydrogels as an ideal strategy, new drug delivery and treatment methods, such as targeted drug delivery and treatment through mechanical signaling, have been studied as interesting strategies. In this study, we review and discuss various types of hydrogels, biomaterials used for hydrogel manufacturing, cartilage-targeting drug delivery, and mechanosignaling as modern strategies for cartilage treatment.

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.

Anti-Inflammatory Effects of Encapsulated Human Mesenchymal Stromal/Stem Cells and a Method to Scale-Up Cell Encapsulation

Mesenchymal stem/stromal cells (MSC) promote recovery in a wide range of animal models of injury and disease. They can act in vivo by differentiating and integrating into tissues, secreting factors that promote cell growth and control inflammation, and interacting directly with host effector cells. We focus here on MSC secreted factors by encapsulating the cells in alginate microspheres, which restrict cells from migrating out while allowing diffusion of factors including cytokines across the capsules. One week after intrathecal lumbar injection of human bone marrow MSC encapsulated in alginate (eMSC), rat IL-10 expression was upregulated in distant rat spinal cord injury sites. Detection of human IL-10 protein in rostrally derived cerebrospinal fluid (CSF) indicated distribution of this human MSC-secreted cytokine throughout rat spinal cord CSF. Intraperitoneal (IP) injection of eMSC in a rat model for endotoxemia reduced serum levels of inflammatory cytokines within 5 h. Detection of human IL-6 in sera after injection of human eMSC indicates rapid systemic distribution of this human MSC-secreted cytokine. Despite proof of concept for eMSC in various disorders using animal models, translation of encapsulation technology has not been feasible primarily because methods for scale-up are not available. To scale-up production of eMSC, we developed a rapid, semi-continuous, capsule collection system coupled to an electrosprayer. This system can produce doses of encapsulated cells sufficient for use in clinical translation.

Magnetic hydrogels with ordered structure for biomedical applications

Magnetic hydrogels composed of hydrogel matrices and magnetic nanomaterials have attracted widespread interests. Thereinto, magnetic hydrogels with ordered structure possessing enhanced functionalities and unique architectures, show tremendous advantages in biomedical fields. The ordered structure brought unique anisotropic properties and excellent physical properties. Furthermore, the anisotropic properties of magnetic ordered hydrogels are more analogous to biological tissues in morphology and mechanical property, showing better biocompatibility and bioinducibility. Thus, we aim to systematically describe the latest advances of magnetic hydrogels with ordered structure. Firstly, this review introduced the synthetic methods of magnetic hydrogels focus on constructing ordered structure. Then, their functionalities and biomedical applications are also summarized. Finally, the current challenges and a compelling perspective outlook of magnetic ordered hydrogel are present.

Genipin-crosslinked fibrin seeded with oxidized alginate microbeads as a novel composite biomaterial strategy for intervertebral disc cell therapy

Discectomy procedures alleviate disability caused by intervertebral disc (IVD) herniation, but do not repair herniation-induced annulus fibrosus (AF) defects. Cell therapy shows promise for IVD repair, yet cell delivery biomaterials capable of sealing AF defects and restoring biomechanical function have poor biological performance. To balance the biomechanical and biological demands of IVD cell delivery biomaterials, we engineered an injectable composite biomaterial using cell-laden, degradable oxidized alginate (OxAlg) microbeads (MBs) to deliver AF cells within high-modulus genipin-crosslinked fibrin (FibGen) hydrogels (FibGen + MB composites). Conceptually, the high-modulus FibGen would immediately stabilize injured IVDs, while OxAlg MBs would protect and release cells required for long-term healing. We first showed that AF cells microencapsulated in OxAlg MBs maintained high viability and, upon release, displayed phenotypic AF cell morphology and gene expression. Next, we created cell-laden FibGen + MB composites and demonstrated that OxAlg MBs functionalized with RGD peptides (MB-RGD) minimized AF cell apoptosis and retained phenotypic gene expression. Further, we showed that cell-laden FibGen + MB composites are biomechanically stable and promote extracellular matrix (ECM) synthesis in long-term in vitro culture. Lastly, we evaluated cell-laden FibGen + MB-RGD composites in a long-term bovine caudal IVD organ culture bioreactor and found that composites had low herniation risk, provided superior biomechanical and biological repair to discectomy controls, and retained anabolic cells within the IVD injury space. This novel injectable composite hydrogel strategy shows promise as an IVD cell delivery sealant with potentially broad applications for its capacity to balance biomechanical and biological performance.

A pH-sensor scaffold for mapping spatiotemporal gradients in three-dimensional in vitro tumour models

The detection of extracellular pH at single cell resolution is challenging and requires advanced sensibility. Sensing pH at high spatial and temporal resolution might provide crucial information in understanding the role of pH and its fluctuations in a wide range of physio-pathological cellular processes, including cancer. Here, a method to embed silica-based fluorescent pH sensors into alginate-based three-dimensional (3D) microgels tumour models, coupled with a computational method for fine data analysis, is presented. By means of confocal laser scanning microscopy, live-cell time-lapse imaging of 3D alginate microgels was performed and the extracellular pH metabolic variations were monitored in both in vitro 3D mono- and 3D co-cultures of tumour and stromal pancreatic cells. The results show that the extracellular pH is cell line-specific and time-dependent. Moreover, differences in pH were also detected between 3D monocultures versus 3D co-cultures, thus suggesting the existence of a metabolic crosstalk between tumour and stromal cells. In conclusion, the system has the potential to image multiple live cell types in a 3D environment and to decipher in real-time their pH metabolic interplay under controlled experimental conditions, thus being also a suitable platform for drug screening and personalized medicine.

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.

Three-Dimensional-Printed Flexible Scaffolds Have Tunable Biomimetic Mechanical Properties for Intervertebral Disc Tissue Engineering

The intervertebral disc (IVD) exhibits complex structure and biomechanical function, which supports the weight of the body and permits motion. Surgical treatments for IVD degeneration (e.g., lumbar fusion, disc replacement) often disrupt the mechanical environment of the spine which lead to adjacent segment disease. Alternatively, disc tissue engineering strategies, where cell-seeded hydrogels or fibrous biomaterials are cultured in vitro to promote matrix deposition, do not recapitulate the complex IVD mechanical properties. In this study, we use 3D printing of flexible polylactic acid (FPLA) to fabricate a viscoelastic scaffold with tunable biomimetic mechanics for whole spine motion segment applications. We optimized the mechanical properties of the scaffolds for equilibrium and dynamic moduli in compression and tension by varying fiber spacing or porosity, generating scaffolds with de novo mechanical properties within the physiological range of spine motion segments. The biodegradation analysis of the 3D printed scaffolds showed that FPLA exhibits lower degradation rate and thus has longer mechanical stability than standard PLA. FPLA scaffolds were biocompatible, supporting viability of nucleus pulposus (NP) cells in 2D and in FPLA+hydrogel composites. Composite scaffolds cultured with NP cells maintained baseline physiological mechanical properties and promoted matrix deposition up to 8 weeks in culture. Mesenchymal stromal cells (MSCs) cultured on FPLA adhered to the scaffold and exhibited fibrocartilaginous differentiation. These results demonstrate for the first time that 3D printed FPLA scaffolds have de novo viscoelastic mechanical properties that match the native IVD motion segment in both tension and compression and have the potential to be used as a mechanically stable and biocompatible biomaterial for engineered disc replacement.

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

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

SUPPLEMENTARY INFORMATION

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

Microencapsulated phages show prolonged stability in gastrointestinal environments and high therapeutic efficiency to treat Escherichia coli O157:H7 infection

Escherichia coli (E. coli) O157:H7 bacterial infection causes severe disease in mammals and results in substantial economic losses worldwide. Due to the development of antibiotic resistance, bacteriophage (phage) therapy has become an alternative to control O157:H7 infection. However, the therapeutic effects of phages are frequently disappointing because of their low resistance to the gastrointestinal environment. In this study, to improve the stability of phages in the gastrointestinal tract, E. coli O157:H7 phages were microencapsulated and their in vitro stability and in vivo therapeutic efficiency were investigated. The results showed that compared to free phages, the resistance of microencapsulated phages to simulated gastric fluid and bile salts significantly increased. The microencapsulated phages were efficiently released into simulated intestinal fluid, leading to a better therapeutic effect in rats infected with E. coli O157:H7 compared to the effects of the free phages. In addition, the microencapsulated phages were more stable during storage than the free phages, showing how phage microencapsulation can play an essential role in phage therapy.

Engineered Microgels-Their Manufacturing and Biomedical Applications

Microgels are hydrogel particles with diameters in the micrometer scale that can be fabricated in different shapes and sizes. Microgels are increasingly used for biomedical applications and for biofabrication due to their interesting features, such as injectability, modularity, porosity and tunability in respect to size, shape and mechanical properties. Fabrication methods of microgels are divided into two categories, following a top-down or bottom-up approach. Each approach has its own advantages and disadvantages and requires certain sets of materials and equipments. In this review, we discuss fabrication methods of both top-down and bottom-up approaches and point to their advantages as well as their limitations, with more focus on the bottom-up approaches. In addition, the use of microgels for a variety of biomedical applications will be discussed, including microgels for the delivery of therapeutic agents and microgels as cell carriers for the fabrication of 3D bioprinted cell-laden constructs. Microgels made from well-defined synthetic materials with a focus on rationally designed ultrashort peptides are also discussed, because they have been demonstrated to serve as an attractive alternative to much less defined naturally derived materials. Here, we will emphasize the potential and properties of ultrashort self-assembling peptides related to microgels.

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

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

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.