The characteristics of mussel‐inspired nHA/OSA injectable hydrogel and repaired bone defect in rabbit

Polydopamine-Based Biomaterials in Orthopedic Therapeutics: Properties, Applications, and Future Perspectives

Polydopamine is a versatile and modifiable polymer, known for its excellent biocompatibility and adhesiveness. It can also be engineered into a variety of nanoparticles and biomaterials for drug delivery, functional modification, making it an excellent choice to enhance the prevention and treatment of orthopedic diseases. Currently, the application of polydopamine biomaterials in orthopedic disease prevention and treatment is in its early stages, despite some initial achievements. This article aims to review these applications to encourage further development of polydopamine for orthopedic therapeutic needs. We detail the properties of polydopamine and its biomaterial types, highlighting its superior performance in functional modification on nanoparticles and materials. Additionally, we also explore the challenges and future prospects in developing optimal polydopamine biomaterials for clinical use in orthopedic disease prevention and treatment.

Mussel-Inspired Injectable Adhesive Hydrogels for Biomedical Applications

The impressive adhesive capacity of marine mussels has inspired various fascinating designs in biomedical fields. Mussel-inspired injectable adhesive hydrogels, as a type of promising mussel-inspired material, have attracted much attention due to their minimally invasive property and desirable functions provided by mussel-inspired components. In recent decades, various mussel-inspired injectable adhesive hydrogels have been designed and widely applied in numerous biomedical fields. The rational incorporation of mussel-inspired catechol groups endows the injectable hydrogels with the potential to exhibit many properties, including tissue adhesiveness and self-healing, antimicrobial, and antioxidant capabilities, broadening the applications of injectable hydrogels in biomedical fields. In this review, we first give a brief introduction to the adhesion mechanism of mussels and the characteristics of injectable hydrogels. Further, the typical design strategies of mussel-inspired injectable adhesive hydrogels are summarized. The methodologies for integrating catechol groups into polymers and the crosslinking methods of mussel-inspired hydrogels are discussed in this section. In addition, we systematically overview recent mussel-inspired injectable adhesive hydrogels for biomedical applications, with a focus on how the unique properties of these hydrogels benefit their applications in these fields. The challenges and perspectives of mussel-inspired injectable hydrogels are discussed in the last section. This review may provide new inspiration for the design of novel bioinspired injectable hydrogels and facilitate their application in various biomedical fields.

Preparation and properties of biomimetic bone repair hydrogel with sandwich structure

One of the critical factors that determines the biological properties of scaffolds is their structure. Due to the mechanical and structural discrepancies between the target bone and implants, the poor internal architecture design and difficulty in degradation of conventional bone implants may cause several adverse outcomes. To date, many scaffolds, such as 3-D printed sandwich structures, have been successfully developed for the repair of bone defects; however, the steps of these methods are complex and costly. Hydrogels have emerged as a unique scaffold material for repairing bone defects because of their good biocompatibility and excellent physicochemical properties. However, studies exploring bioinspired hydrogel scaffolds with hierarchical structures are scarce. More efforts are needed to incorporate bioinspired structures into hydrogel scaffolds to achieve optimal osteogenic properties. In this study, we developed a low-cost and easily available hydrogel matrix that mimicked the natural structure of the bone's porous sandwich to promote new bone growth and tissue integration. A comprehensive evaluation was conducted on the microstructure, swelling rate, and mechanical properties of this hydrogel. Furthermore, a 3D finite element analysis was employed to model the structure-property relationship. The results indicate that the sandwich-structured hydrogel is a promising scaffold material for bone injury repair, exhibiting enhanced compressive stress, elastic modulus, energy storage modulus, and superior force transmission.

A chitosan-coated PCL/nano-hydroxyapatite aerogel integrated with a nanofiber membrane for providing antibacterial activity and guiding bone regeneration

A guided bone regeneration (GBR) membrane can act as a barrier to prevent the invasion and interference from foreign soft tissues, promoting infiltration and proliferation of osteoblasts in the bone defect area. Herein, a composite scaffold with dual functions of osteogenesis and antibacterial effects was prepared for GBR. A polycaprolactone (PCL)/nano-hydroxyapatite (n-HA) aerogel produced by electrospinning and freeze-drying techniques was fabricated as the loose layer of the scaffold, while a PCL nanofiber membrane was used as the dense layer. Chitosan (CS) solution served as a middle layer to provide mechanical support and antibacterial effects between the two layers. Morphological results showed that the loose layer had a porous structure with n-HA successfully dispersed in the aerogels, while the dense layer possessed a sufficiently dense structure. In vitro antibacterial experiments illustrated that the CS solution in the middle layer stabilized the scaffold structure and endowed the scaffold with good antibacterial properties. The cytocompatibility results indicated that both fibroblasts and osteoblasts exhibited superior cell activity on the dense and loose layers, respectively. In particular, the dense layer made of nanofibers could work as a barrier layer to inhibit the infiltration of fibroblasts into the loose layer. In vitro osteogenesis analysis suggested that the PCL/n-HA aerogel could enhance the bone induction ability of bone mesenchymal stem cells, which was confirmed by the increased expression of the alkaline phosphatase activity. The loose structure facilitated the infiltration and migration of bone mesenchymal stem cells for better osteogenesis. In summary, such a composite scaffold exhibited excellent osteogenic and antibacterial properties as well as the barrier effect, thus holding promising potential for use as GBR materials.

Research Progress on Injectable Microspheres as New Strategies for the Treatment of Osteoarthritis Through Promotion of Cartilage Repair

Osteoarthritis (OA) is a degenerative disease caused by a variety of factors with joint pain as the main symptom, including fibrosis, chapping, ulcers, and loss of cartilage. Traditional treatment can only delay the progression of OA, and classical delivery system have many side effects. In recent years, microspheres have shown great application prospects in the field of OA treatment. Microspheres can support cells, reproduce the natural tissue microenvironment in vitro and in vivo, and are an efficient delivery system for the release of drugs or biological agents, which can promote cell proliferation, migration, and differentiation. Thus, they have been widely used in cartilage repair and regeneration. In this review, preparation processes, basic materials, and functional characteristics of various microspheres commonly used in OA treatment are systematically reviewed. Then it is introduced surface modification strategies that can improve the biological properties of microspheres and discussed a series of applications of microsphere functionalized scaffolds in OA treatment. Finally, based on bibliometrics research, the research development, future potential, and possible research hotspots of microspheres in the field of OA therapy is systematically and dynamically evaluated. The comprehensive and systematic review will bring new understanding to the field of microsphere treatment of OA.

Augmented physical, mechanical, and cellular responsiveness of gelatin-aldehyde modified xanthan hydrogel through incorporation of silicon nanoparticles for bone tissue engineering

Bioactive scaffolds fabricated from a combination of organic and inorganic biomaterials are a promising approach for addressing defects in bone tissue engineering. In the present study, a self-crosslinked nanocomposite hydrogel, composed of gelatin/aldehyde-modified xanthan (Gel-AXG) is successfully developed by varying concentrations of porous silicon nanoparticles (PSiNPs). The effect of PSiNPs incorporation on physical, mechanical, and biological performance of the nanocomposite hydrogel is evaluated. Morphological analysis reveals formation of highly porous 3D microstructures with interconnected pores in all nanocomposite hydrogels. Increased content of PSiNPs results in a lower swelling ratio, reduced porosity and pore size, which in turn impeded media penetration and slowed down the degradation process. In addition, remarkable enhancements in dynamic mechanical properties are observed in Gel-AXG-8%Si (compressive strength: 0.6223 MPa at 90 % strain and compressive modulus: 0.054 MPa), along with improved biomineralization ability via hydroxyapatite formation after immersion in simulated body fluid (SBF). This optimized nanocomposite hydrogel provides a sustained release of Si ions at safe dose levels. Furthermore, in-vitro cytocompatibility studies using MG-63 cells exhibited remarkable performance in terms of cell attachment, proliferation, and ALP activity for Gel-AXG-8%Si. These findings suggest that the prepared nanocomposite hydrogel holds promising potential as a scaffold for bone tissue engineering.

Synthesis of Poly-γ-Glutamic Acid and Its Application in Biomedical Materials

Poly-γ-glutamic acid (γ-PGA) is a natural polymer composed of glutamic acid monomer and it has garnered substantial attention in both the fields of material science and biomedicine. Its remarkable cell compatibility, degradability, and other advantageous characteristics have made it a vital component in the medical field. In this comprehensive review, we delve into the production methods, primary application forms, and medical applications of γ-PGA, drawing from numerous prior studies. Among the four production methods for PGA, microbial fermentation currently stands as the most widely employed. This method has seen various optimization strategies, which we summarize here. From drug delivery systems to tissue engineering and wound healing, γ-PGA's versatility and unique properties have facilitated its successful integration into diverse medical applications, underlining its potential to enhance healthcare outcomes. The objective of this review is to establish a foundational knowledge base for further research in this field.

Metal Ion-Doped Hydroxyapatite-Based Materials for Bone Defect Restoration

Hydroxyapatite (HA)-based materials are widely used in the bone defect restoration field due to their stable physical properties, good biocompatibility, and bone induction potential. To further improve their performance with extra functions such as antibacterial activity, various kinds of metal ion-doped HA-based materials have been proposed and synthesized. This paper offered a comprehensive review of metal ion-doped HA-based materials for bone defect restoration based on the introduction of the physicochemical characteristics of HA followed by the synthesis methods, properties, and applications of different kinds of metal ion (Ag+, Zn2+, Mg2+, Sr2+, Sm3+, and Ce3+)-doped HA-based materials. In addition, the underlying challenges for bone defect restoration using these materials and potential solutions were discussed.

Hydrogel Tissue Bioengineered Scaffolds in Bone Repair: A Review

Large bone defects due to trauma, infections, and tumors are difficult to heal spontaneously by the body's repair mechanisms and have become a major hindrance to people's daily lives and economic development. However, autologous and allogeneic bone grafts, with their lack of donors, more invasive surgery, immune rejection, and potential viral transmission, hinder the development of bone repair. Hydrogel tissue bioengineered scaffolds have gained widespread attention in the field of bone repair due to their good biocompatibility and three-dimensional network structure that facilitates cell adhesion and proliferation. In addition, loading natural products with nanoparticles and incorporating them into hydrogel tissue bioengineered scaffolds is one of the most effective strategies to promote bone repair due to the good bioactivity and limitations of natural products. Therefore, this paper presents a brief review of the application of hydrogels with different gel-forming properties, hydrogels with different matrices, and nanoparticle-loaded natural products loaded and incorporated into hydrogels for bone defect repair in recent years.

Bionic ordered structured hydrogels: structure types, design strategies, optimization mechanism of mechanical properties and applications

Natural organisms, such as lobsters, lotus, and humans, exhibit exceptional mechanical properties due to their ordered structures. However, traditional hydrogels have limitations in their mechanical and physical properties due to their disordered molecular structures when compared with natural organisms. Therefore, inspired by nature and the properties of hydrogels similar to those of biological soft tissues, researchers are increasingly focusing on how to investigate bionic ordered structured hydrogels and render them as bioengineering soft materials with unique mechanical properties. In this paper, we systematically introduce the various structure types, design strategies, and optimization mechanisms used to enhance the strength, toughness, and anti-fatigue properties of bionic ordered structured hydrogels in recent years. We further review the potential applications of bionic ordered structured hydrogels in various fields, including sensors, bioremediation materials, actuators, and impact-resistant materials. Finally, we summarize the challenges and future development prospects of bionic ordered structured hydrogels in preparation and applications.

Injectable hydrogels for cartilage and bone tissue regeneration: A review

Annually, millions of patients suffer from irreversible injury owing to the loss or failure of an organ or tissue caused by accident, aging, or disease. The combination of injectable hydrogels and the science of stem cells have emerged to address this persistent issue in society by generating minimally invasive treatments to augment tissue function. Hydrogels are composed of a cross-linked network of polymers that exhibit a high-water retention capacity, thereby mimicking the wet environment of native cells. Due to their inherent mechanical softness, hydrogels can be used as needle-injectable stem cell carrier materials to mend tissue defects. Hydrogels are made of different natural or synthetic polymers, displaying a broad portfolio of eligible properties, which include biocompatibility, low cytotoxicity, shear-thinning properties as well as tunable biological and physicochemical properties. Presently, novel ongoing developments and native-like hydrogels are increasingly being used broadly to improve the quality of life of those with disabling tissue-related diseases. The present review outlines various future and in-vitro applications of injectable hydrogel-based biomaterials, focusing on the newest ongoing developments of in-situ forming injectable hydrogels for bone and cartilage tissue engineering purposes.

Alginate: Microbial production, functionalization, and biomedical applications

Alginates are natural polysaccharides widely participating in food, pharmaceutical, and environmental applications due to their excellent gelling capacity. Their excellent biocompatibility and biodegradability further extend their application to biomedical fields. The low consistency in molecular weight and composition of algae-based alginates may limit their performance in advanced biomedical applications. It makes microbial alginate production more attractive due to its potential for customizing alginate molecules with stable characteristics. Production costs remain the primary factor limiting the commercialization of microbial alginates. However, carbon-rich wastes from sugar, dairy, and biodiesel industries may serve as potential substitutes for pure sugars for microbial alginate production to reduce substrate costs. Fermentation parameter control and genetic engineering strategies may further improve the production efficiency and customize the molecular composition of microbial alginates. To meet the specific needs of biomedical applications, alginates may need functionalization, such as functional group modifications and crosslinking treatments, to achieve enhanced mechanical properties and biochemical activities. The development of alginate-based composites incorporated with other polysaccharides, gelatin, and bioactive factors can integrate the advantages of each component to meet multiple requirements in wound healing, drug delivery, and tissue engineering applications. This review provided a comprehensive insight into the sustainable production of high-value microbial alginates. It also discussed recent advances in alginate modification strategies and alginate-based composites for representative biomedical applications.

Advances of Mussel-Inspired Nanocomposite Hydrogels in Biomedical Applications

Hydrogels, with 3D hydrophilic polymer networks and excellent biocompatibilities, have emerged as promising biomaterial candidates to mimic the structure and properties of biological tissues. The incorporation of nanomaterials into a hydrogel matrix can tailor the functions of the nanocomposite hydrogels to meet the requirements for different biomedical applications. However, most nanomaterials show poor dispersion in water, which limits their integration into the hydrophilic hydrogel network. Mussel-inspired chemistry provides a mild and biocompatible approach in material surface engineering due to the high reactivity and universal adhesive property of catechol groups. In order to attract more attention to mussel-inspired nanocomposite hydrogels, and to promote the research work on mussel-inspired nanocomposite hydrogels, we have reviewed the recent advances in the preparation of mussel-inspired nanocomposite hydrogels using a variety of nanomaterials with different forms (nanoparticles, nanorods, nanofibers, nanosheets). We give an overview of each nanomaterial modified or hybridized by catechol or polyphenol groups based on mussel-inspired chemistry, and the performances of the nanocomposite hydrogel after the nanomaterial's incorporation. We also highlight the use of each nanocomposite hydrogel for various biomedical applications, including drug delivery, bioelectronics, wearable/implantable biosensors, tumor therapy, and tissue repair. Finally, the challenges and future research direction in designing mussel-inspired nanocomposite hydrogels are discussed.

Injectable hydrogel loaded with 4-octyl itaconate enhances cartilage regeneration by regulating macrophage polarization

Macrophages play a distinctive role in the early stage of inflammation after cartilage defects. Previous studies have shown that macrophages can express different phenotypes, among which M2 polarization is important to maintain the balance of the inflammatory microenvironment and promote cartilage regeneration. In this study, 4-octyl itaconic acid (4-OI), a derivative of the endogenous metabolite itaconic acid, was used to regulate the polarization behavior of macrophages and enhance cartilage repair. Oxidized sodium alginate (OSA) and gelatin (GEL) were selected as materials to form injectable hydrogels with the function of sustained release of 4-OI. In vivo and in vitro experiments have verified that the OSA/GEL hydrogel system loaded with 4-OI could promote M2 macrophage polarization and inhibit the inflammatory reaction. A rat knee joint cartilage defect model further confirmed its role in promoting cartilage regeneration in the later stage. In this study, the OSA/GEL hydrogel was successfully fabricated as a vehicle for delivering 4-OI, which could evidently alleviate the inflammatory reaction and thus accelerate tissue regeneration. The results of this study provide a new method for promoting subsequent tissue regeneration by regulating the early immune response.

Research Progress of Polydopamine Hydrogel in the Prevention and Treatment of Oral Diseases

Oral diseases represent one of the most prevalent diseases globally and are associated with serious health and economic burdens, greatly altering the quality of life of affected individuals. Various biomaterials play important roles in the treatment of oral diseases. To some extent, the development of biomaterials has promoted progress in clinically available oral medicines. Hydrogels have unique tunable advantages that make them useful in the next generation of regenerative strategies and have been widely applied in both oral soft and hard tissues repair. However, most hydrogels lack self-adhesive properties, which may result in low repair efficacy. Polydopamine (PDA), the primary adhesive component, has attracted increasing attention in recent years. PDA-modified hydrogels exhibit reliable and suitable adherence to tissues and easily integrate into tissues to promote repair efficiency. This paper reviews the latest research progress on PDA hydrogels and elaborates on the mechanism of the reaction between PDA functional groups and hydrogels, and summarizes the biological properties and the applications of PDA hydrogels in the prevention and treatment of the field of oral diseases. It is also proposed that in future research we should simulate the complex microenvironment of the oral cavity as much as possible, coordinate and plan various biological events rationally, and realize the translation from scientific research to clinical practice.

Application of "Click" Chemistry in Biomedical Hydrogels

Since "click" chemistry was first reported in 2001, it has remained a popular research topic in the field of chemistry due to its high yield without byproducts, fast reaction rate, simple reaction, and biocompatibility. It has achieved good applications in various fields, especially for the preparation of hydrogels. The development of biomedicine presents new challenges and opportunities for hydrogels, and "click" chemistry provides a library of chemical tools for the preparation of various innovative hydrogels, including cell culture, 3D bioprinting, and drug release. This article summarizes several common "click" reactions, including copper-catalyzed azide-alkyne cycloaddition reactions, strain-promoted azide-alkyne cycloaddition (SPAAC) reaction, thiol-ene reaction, the Diels-Alder reaction, and the inverse electron demand Diels-Alder (IEDDA) reaction. We introduce the "click" reaction in the nucleic acid field to expand the concept of "click" chemistry. This article focuses on the application of "click" chemistry for preparing various types of biomedical hydrogels and highlights the advantages of "click" reactions for cross-linking to obtain hydrogels. This review also discusses applications of "click" chemistry outside the field of hydrogels, such as drug synthesis, targeted delivery, and surface modification, hydrogels have great application potential in these fields in the future and hopefully inspire other applications of hydrogels.

Alendronate-functionalized double network hydrogel scaffolds for effective osteogenesis

Development of artificial bone substitutes mimicking the extracellular matrix is a promising strategy for bone repair and regeneration. In views of the actual requirement of biomechanics, biodegradability, and bioactivity, herein, a double-network (DN) hydrogel was constructed by interspersing a methacrylated gelatin (GelMA) network into alendronate (ALN)-modified oxidized alginate (OSA) network via Schiff base reaction and photo-crosslinking process to promote in situ bone regeneration. This GelMA@OSA-ALN DN hydrogel possessed favorable network and pores, good biocompatibility, and enhanced biomechanics. Notably, the introduction of Schiff base furnished the ND hydrogel scaffold with pH-responsive biodegradation and sustained ALN drug release delivery, which could provide effective bioactivity, upregulate osteogenesis-related genes, and promote the cell viability, growth, proliferation, and osteogenesis differentiation for bone regeneration. Therefore, we provide a new insight to develop functional DN hydrogel scaffold toward governing the on-demand drug release and achieving the stem cell therapy, which will be developed into the minimally invasive gelling system to prolong local delivery of bisphosphonates for the bone-related diseases.

Stiff and Tough Hydrogels Prepared Through Integration of Ionic Cross-linking and Enzymatic Mineralization

Enzymatic mineralization has become an effective approach to enhancing the stiffness of hydrogels for bone tissue engineering, but generally with limited toughness. On the other hand, double network cross-linking provides hydrogel with enhanced toughness. In this study, we integrated double cross-linking method with enzymatic mineralization to synthesize stiff and tough hydrogels. We have synthesized three kinds of sodium alginate-polyacrylamide (SA-PAM) double-network hydrogels and systematically compared the composition and structure differences, mechanical properties, and biological properties of the different hydrogels in the absence and presence of mineralization. In particular, we examined the role of specific cross-linking ions, i.e., calcium, zinc and strontium ions, in modulating the mineralization process. Synergistic effect of ionic cross-linking and enzymatic mineralization was clearly observed with dramatic increase in compressive modulus. In particular, mineralized hydrogel cross-linked with Sr2+ showed the highest compressive Young's modulus of 17.28 ± 3.56 MPa, which was 37 times of that of the original hydrogel. In addition, it had the highest tensile Young's modulus at 2.60 ± 0.25 MPa and 84 ± 5.5% elongation at break. Such synergistic effect from Sr2+ was attributed to a more uniformed mineralization process due to the early initiation of a more homogeneous nucleation process and subsequent denser mineralized structure. Cellular study also suggested that cell proliferation, adhesion and osteogenic differentiation were improved as a result of enzymatic mineralization. Our results provided an effective way for the preparation of stiff and tough hydrogels with osteogenesis, and demonstrated potential in bone tissue engineering applications. STATEMENT OF SIGNIFICANCE: : Hydrogels with excellent stiffness, stability and biocompatibility have attracted significant attentions in the bone tissue engineering applications. Our results suggested that the synergistic effect of ionic cross-linking and enzymatic mineralization rendered more enhancement of the compressive and tensile stiffness of SA-PAM DN hydrogels, as well as the toughness, swelling stability and cellular response. In particular, mineralized hydrogel cross-linked with Sr2+ showed the highest compressive Young's modulus of 17283 ± 3556KPa, which was 37 times of that of the original hydrogel. Such synergistic effect from Sr2+ was attributed to a more uniformed mineralization process. The cell proliferation, adhesion and osteogenic differentiation were greatly improved as a result of enzymatic mineralization, where the MSCs cultured on strontium ion cross-linked mineralized hydrogel showed the best performance.

Advances in Hydrogels for Stem Cell Therapy: Regulation Mechanisms and Tissue Engineering Applications

Stem cell therapy has shown unparalleled potential in tissue engineering, but it still faces challenges in the regulation of stem cell fate. Inspired by the native stem cell niche, a...

[Methods of improving the mechanical properties of hydrogels and their research progress in bone tissue engineering]

OBJECTIVE

To review the methods of improving the mechanical properties of hydrogels and the research progress in bone tissue engineering.

METHODS

The recent domestic and foreign literature on hydrogels in bone tissue engineering was reviewed, and the methods of improving the mechanical properties of hydrogels and the effect of bone repair in vivo and in vitro were summarized.

RESULTS

Hydrogels are widely used in bone tissue engineering, but their mechanical properties are poor. Improving the mechanical properties of hydrogels can enhance bone repair. The methods of improving the mechanical properties of hydrogels include the construction of dual network structures, inorganic nanoparticle composites, introduction of conductive materials, and fiber network reinforcement. These methods can improve the mechanical properties of hydrogels to various degrees while also demonstrating a significant bone repair impact.

CONCLUSION

The mechanical properties of hydrogels can be effectively improved by modifying the system, components, and fiber structure, and bone repair can be effectively promoted.

Alginate-chitosan oligosaccharide-ZnO composite hydrogel for accelerating wound healing

Moist, breathable and antibacterial microenvironment can promote cell proliferation and migration, which is beneficial to wound healing. Here, we fabricated a novel sodium alginate-chitosan oligosaccharide‑zinc oxide (SA-COS-ZnO) composite hydrogel by spontaneous Schiff base reaction, using aldehydated sodium alginate (SA), chitosan oligosaccharide (COS), and zinc oxide (ZnO) nanoparticles, which can provide a moist and antibacterial environment for wound healing. The porosity and swelling degree of SA-COS-ZnO hydrogel are 80% and 150%, respectively, and its water vapor permeability is 682 g/m2/24h. The composite hydrogel showed good biocompatibility to blood cells, 3T3 cells, and 293T cells, and significant antibacterial activity against Escherichia coli, Staphylococcus aureus, Candida albicans, and Bacillus subtilis. Moreover, the hydrogel showed a promoting effect on wound healing in a rat scald model. The present study suggests that marine carbohydrates composite hydrogels are promising in wound care management.

Hydrogels for Large-Scale Expansion of Stem Cells

Stem cells demonstrate considerable promise for various preclinical and clinical applications, including drug screening, disease treatments, and regenerative medicine. Producing high-quality and large amounts of stem cells is in demand for these applications. Despite challenges, as hydrogel-based cell culture technology has developed, tremendous progress has been made in stem cell expansion and directed differentiation. Hydrogels are soft materials with abundant water. Many hydrogel properties, including biodegradability, mechanical strength, and porosity, have been shown to play essential roles in regulating stem cell proliferation and differentiation. The biochemical and physical properties of hydrogels can be specifically tailored to mimic the native microenvironment that various stem cells reside in vivo. A few hydrogel-based systems have been developed for successful stem cell cultures and expansion in vitro. In this review, we summarize various types of hydrogels that have been designed to effectively enhance the proliferation of hematopoietic stem cells (HSCs), mesenchymal stem/stromal cells (MSCs), and pluripotent stem cells (PSCs), respectively. According to each stem cell type's preference, we also discuss strategies for fabricating hydrogels with biochemical and mechanical cues and other characteristics representing microenvironments of stem cells in vivo. STATEMENT OF SIGNIFICANCE: In this review article we summarize current progress on the construction of hydrogel systems for the culture and expansion of various stem cells, including hematopoietic stem cells (HSCs), mesenchymal stem/stromal cells (MSCs), and pluripotent stem cells (PSCs). The Significance includes: (1) Provide detailed discussion on the stem cell niches that should be considered for stem cell in vitro expansion. (2) Summarize various strategies to construct hydrogels that can largely recapture the microenvironment of native stem cells. (3) Suggest a few future directions that can be implemented to improve current in vitro stem cell expansion systems.

Enhancing Tissue Adhesion and Osteoblastic Differentiation of MC3T3-E1 Cells on Poly(aryl ether ketone) by Chemically Anchored Hydroxyapatite Nanocomposite Hydrogel Coating

Tissue adhesion to bone implant and osteoblastic differentiation are the key factors to achieve poly(aryl ether ketone) (PAEK) implant osseointegration. However, physical interaction of implant with tissue and hydroxyapatite coating suffers from slow implant tissue integration and lack of long-term stability. In this study, a novel poly(phthalazinone ether sulfone ketone) containing allyl groups (APPBAESK) is coated onto PPBESK sheet for reacting with the allyl groups of the hydrogel coating to enhance its stability. N-Succinimidyl (NHS)-ester activated group and nano-hydroxyapatite (nano-HA) are introduced into the hydrogel synthesized from gelatin methacrylate (GelMA) and acrylic acid to construct a nanocomposite hydrogel coating on PPBESK which is a promising PAEK implant material. The hydrophilicity of the PPBESK sheet is improved by the hydrogel coating. The chemical components of the nanocomposite hydrogel coating are confirmed by X-ray photoelectron spectroscope, Attenuated total reflection infrared, and X-ray powder diffraction. The tissue shear adhesion strength of the hydrogel coating toward pig skin is enhanced due to the synergism of NHS-ester activated group and nano-HA. The osteogenic differentiation of MC3T3-E1 preosteoblasts is promoted by nano-HA in nanocomposite hydrogel coating. Therefore, the bifunctional nanocomposite hydrogel coating provides a great application prospect in the surface modification of PAEK implants in bone tissue engineering.

Applications of oxidized alginate in regenerative medicine

Because of its ideal degradation rate and features, oxidized alginate (OA) is selected as an appropriate substitute and has been introduced into hydrogels, microspheres, 3D-printed/composite scaffolds, membranes, and electrospinning and coating materials. By taking advantage of OA, the OA-based materials can be easily functionalized and deliver drugs or growth factors to promote tissue regeneration. In 1928, it was first found that alginate could be oxidized using periodate, yielding OA. Since then, considerable progress has been made in the research on the modification and application of alginate after oxidation. In this article, we summarize the key properties and existing applications of OA and various OA-based materials and discuss their prospects in regenerative medicine.

Click chemistry-based biopolymeric hydrogels for regenerative medicine

Click chemistry is not a single specific reaction, but describes ways of generating products which emulate examples in nature. Click reactions occur in one pot, are not disturbed by water, generate minimal and inoffensive byproducts, and are characterized by a high thermodynamic driving force, driving the reaction quickly and irreversibly towards a high yield of a single reaction product. As a result, over the past 15 years it has become a very useful bio-orthogonal method for the preparation of chemical cross-linked biopolymer-based hydrogel, in the presence of e.g. growth factors and live cells, or in-vivo. Biopolymers are renewable and non-toxic, providing a myriad of potential backbone toolboxes for hydrogel design. The goal of this review is to summarize recent advances in the development of click chemistry-based biopolymeric hydrogels, and their applications in regenerative medicine. In particular, various click chemistry approaches, including copper-catalyzed azide-alkyne cycloaddition reactions, copper-free click reactions (e.g. the Diels-Alder reactions, the strain-promoted azide-alkyne cycloaddition reactions, the radical mediated thiol-ene reactions, and the oxime-forming reactions), and pseudo-click reactions (e.g. the thiol-Michael addition reactions and the Schiff base reactions) are highlighted in the first section. In addition, numerous biopolymers, including proteins (e.g. collagen, gelatin, silk, and mucin), polysaccharides (e.g. hyaluronic acid, alginate, dextran, and chitosan) and polynucleotides (e.g. deoxyribonucleic acid), are discussed. Finally, we discuss biopolymeric hydrogels, cross-linked by click chemistry, intended for the regeneration of skin, bone, spinal cord, cartilage, and cornea. This article provides new insights for readers in terms of the design of regenerative medicine, and the use of biopolymeric hydrogels based on click chemistry reactions.

A Long-Acting Thermoresponsive Injectable Formulation of Tin Protoporphyrin Sustains Antitubercular Efficacy in a Murine Infection Model

Tuberculosis is the leading cause of death from a single infectious agent, ranking above the human immunodeficiency virus (HIV). Effective treatment using antibiotics is achievable, but poor patient compliance constitutes a major challenge impeding successful pharmacotherapeutic outcomes. This is often due to the prolonged treatment periods required and contributes significantly to the rising incidence of drug resistance, which is a major cause of tuberculosis mortality. Thus, innovative interventions capable of encouraging compliance and decreasing lengthy and frequent dosing are needed. Previously, aqueous tin protoporphyrin IX (SnPPIX), a heme oxygenase-1 inhibitor, administered as multiple daily intraperitoneal (IP) injections, showed considerable antitubercular efficacy and treatment shortening capabilities as a host-directed therapy in infected mice. Since daily IP injection is a clinically impractical administration approach, this proof-of-concept study aims to develop a novel, sustained action injectable formulation of SnPPIX for safe intramuscular (IM) administration. Herein, a SnPPIX-loaded poloxamer-poly(acrylic acid)-based thermoresponsive injectable formulation (SnPPIX-TIF) is designed for effective IM delivery. Results show SnPPIX-TIF is microparticulate, syringeable, injectable, and exhibits complete in vitro/in vivo gelation. Administered once weekly, SnPPIX-TIF significantly prolonged absorption and antimicrobial efficacy in infected mice. In addition, SnPPIX-TIF is well-tolerated in vivo; results from treated animals show no significant histopathologic alterations and were indistinguishable from the untreated control group, thus supporting its biocompatibility and preclinical safety. Overall, the IM delivery of the thermoresponsive injectable formulation safely sustains antitubercular effect in an infected murine model and decreases the number of injections required, signifying a potentially practical approach for future clinical translation.

Synthesis and characterization of multifunctional organic-inorganic composite hydrogel formed with tissue-adhesive property and inhibiting infection

Organic-inorganic composite hydrogel materials have been widely studied. In order to expand the application of organic-inorganic composite hydrogel materials, in this work, we prepared a viscous hydrogel with antibacterial properties (OSA-GelDA@ACP/DA/Ag). First, we used polydopamine coating to deposit elemental silver on the surface of amorphous calcium phosphate (ACP) particles to prepare ACP/DA/Ag particles. Next, dopamine was grafted on the gelatin molecular chain by EDC/NHS to activate the carboxyl group to obtain dopamine-modified gelatin (GelDA). The content of DA in GelDA is about 14.09% by standard curve method. Then, the Schiff base reaction took place between the amino group on the GelDA molecular chain and the aldehyde group on the OSA molecular chain, and an OSA-GelDA viscous hydrogel was prepared. Finally, by changing the content of ACP/DA/Ag3 particles added to the OSA-GelDA gel, the corresponding performance of material was investigated. The results show that the introduction of dopamine provides wet viscosity for the hydrogel, and the ACP/DA/Ag3 particles introduced in the viscous hydrogel provide antibacterial properties. This hydrogel with antibacterial and wet viscosity is expected to become an injectable bone repair material for clinical use.

Femoral bone defect healing using two novel biocompatible degradable materials

This study was conducted as an in vivo experiment in adult miniature pigs with the aim to test two new biomaterials. An iatrogenic defect was made into the central femoral diaphysis in the experimental animals and subsequently fixated by bridging plate osteosynthesis. Into the defect we implanted a cancellous autograft (control group), a pasty injectable scaffold (EXP A), and a porous 3D cylinder (EXP B). Radiological examination was performed in all animals at 0, 10, 20, 30 weeks after surgical procedure and histological assessment was performed. In the newly formed bone the osteoblastic activity was monitored. In terms of radiology, the most effective method was observed in the control group (completely healed 100%) compared to experimental groups EXP A (70.0%) and EXP B (62.5%). Histological assessment showed a higher cell count in the place of bone defect in the control group compared to experimental groups. Between the experimental groups, a higher count of bone marrow cells was found in group EXP B. Both newly developed biomaterials seem to be suitable as replacements for large bone defects, having good workability and applicability. However, compared to the control group treated with a cancellous autograft, the newly formed bone did not reach the same number of cells settling in and in some cases, full radiological healing was not reached. Nevertheless, the material was found to be grown into the original bone in all cases within the experimental groups. The new biomaterials have a great potential as a substitute in the treatment of large bone defects.

Mesoporous Hydroxyapatite/Chitosan Loaded With Recombinant-Human Amelogenin Could Enhance Antibacterial Effect and Promote Periodontal Regeneration

The recovery of impaired periodontium is still a challenge to the treatment of periodontitis. This study was the first to apply the mesoporous hydroxyapatites/chitosan (mHA/CS) composite scaffold to periodontal regeneration. The aim of our study is to evaluate the biological effects of mesoporous hydroxyapatite/chitosan (mHA/CS) loaded with recombinant human amelogenin (rhAm) on periodontal regeneration. The physicochemical properties of mHA/CS scaffolds were examined by Fourier transform infrared spectroscopy (FTIR), transmission electron microscopy (TEM), and Brunauer-Emmett-Teller (BET) analysis. Then, the biological effects of the mHA/CS loaded with rhAm were evaluated, including antibacterial effect, controlled-release capacity, osteogenic and cementogenic effects in vitro and in vivo. The antibacterial effect was tested on 1.5 mg/mL CS; 3 mg/mL mHA; 2.25 mg/mL mHA/CS; 4.5 mg/mL mHA/CS and 20 μg/mL rhAm. Tryptic Soy Broth culture medium was used as a baseline control. Osteogenic effect of rhAm (20 μg/mL rhAm), mHA/CS (4.5 mg/mL mHA/CS), and mHA/CS-rhAm (4.5 mg/mL mHA/CS and 20 μg/mL rhAm) on human periodontal ligament cells (hPDLCs) was evaluated in osteogenic media. The hPDLCs treated either with osteogenic media or Dulbecco's modified Eagle's medium (DMEM) alone were used as the baseline control. In the animal model, 4-week-old nude mice (BALB/c) (n = 6) implanted with root slices subcutaneously were used to observe the cementogenic effect in vivo. The root slices were treated with rhAm (20 μg/mL rhAm), mHA/CS (4.5 mg/mL mHA/CS), and mHA/CS-rhAm (4.5 mg/mL mHA/CS and 20 μg/mL rhAm). The root slices treated with osteogenic medium alone were used as the baseline control. The analyses showed that the mHA/CS particles were 2 μm in diameter and had a uniform pore size. The mesoporous structure was 7 nm in diameter and its surface area was 33.95 m²/g. The scaffold exhibited antibacterial effects against Fusobacterium nucleatum and Porphyromonas gingivalis. The mHA/CS scaffold sustainably released rhAm. The mHA/CS loaded with 20 μg/mL rhAm upregulated ALP activity, the expression levels of osteogenesis-related genes and proteins in vitro. Additionally, it promoted the formation of cementum-like tissue in vivo. Our findings suggest that mHA/CS loaded with 20 μg/mL rhAm could inhibit the growth of periodontal pathogens and promote the formation of bone and cementum-like tissue.