Injectable composite hydrogel promotes osteogenesis and angiogenesis in spinal fusion by optimizing the bone marrow mesenchymal stem cell microenvironment and exosomes secretion

Extracellular vesicles: essential agents in critical bone defect repair and therapeutic enhancement

Bone serves as a fundamental structural component in the body, playing pivotal roles in support, protection, mineral supply, and hormonal regulation. However, critical-sized bone injuries have become increasingly prevalent, necessitating extensive medical interventions due to limitations in the body's capacity for self-repair. Traditional approaches, such as autografts, allografts, and xenografts, have yielded unsatisfactory results. Stem cell therapy emerges as a promising avenue, but challenges like immune rejection and low cell survival rates hinder its widespread clinical implementation. Extracellular vesicles (EVs) derived from mesenchymal stem cells (MSCs) have garnered attention for their regenerative capabilities, which surpass those of MSCs themselves. EVs offer advantages such as reduced immunogenicity, enhanced stability, and simplified storage, positioning them as a promising tool in stem cell-based therapies. This review explores the potential of EV-based therapy in bone tissue regeneration, delving into their biological characteristics, communication mechanisms, and preclinical applications across various physiological and pathological conditions. The mechanisms underlying EV-mediated bone regeneration, including angiogenesis, osteoblast proliferation, mineralization, and immunomodulation, are elucidated. Preclinical studies demonstrate the efficacy of EVs in promoting bone repair and neovascularization, even in pathological conditions like osteoporosis. EVs hold promise as a potential alternative for regenerating bone tissue, particularly in the context of critical-sized bone defects, offering new avenues for effective bone defect repair and management.

Progress in the Application of Multifunctional Composite Hydrogels in Promoting Tissue Repair

Tissue repair is an extremely complex process, and effectively promoting tissue regeneration remains a significant clinical challenge. Hydrogel materials, which exhibit physical properties closely resembling those of living tissues, including high water content, oxygen permeability, and softness, have the potential to revolutionize the field of tissue repair. However, the presence of various complex conditions, such as infection, ischemia, and hypoxia in tissue defects, means that hydrogels with simple structures and functions are often insufficient to meet the diverse needs of tissue repair. Researchers have focused on integrating multiple drugs, nanomaterials, bioactive substances, and stem cells into hydrogel matrices to develop novel multifunctional composite hydrogels for addressing these challenges, which have superior antibacterial properties, hemostatic abilities, self-healing capacities, and excellent mechanical properties. These composite hydrogels are designed to enhance tissue repair and have become an important direction in the current research. This review provides a comprehensive review of the recent advances in the application of multifunctional composite hydrogels in promoting tissue repair, including drug-loaded hydrogels, nanomaterial composite hydrogels, bioactive substance composite hydrogels, and stem cell composite hydrogels.

The Opportunities and Challenges of Mesenchymal Stem Cells-Derived Exosomes in Theranostics and Regenerative Medicine

Cell-secreted nanovesicles of endosomal origin, called exosomes, are vital for mediating intracellular communication. As local or distal transporters of intracellular cargo, they reflect the unique characteristics of secretory cells and establish cell-specific interactions via characteristic surface proteins and receptors. With the advent of rapid isolation, purification, and identification techniques, exosomes have become an attractive choice for disease diagnosis (exosomal content as biomarkers), cell-free therapy, and tissue regeneration. Mesenchymal stem cell (MSC)-derived exosomes (MSC-exosomes) display angiogenic, immune-modulatory, and other therapeutic effects crucial for cytoprotection, ischemic wound repair, myocardial regeneration, etc. The primary focus of this review is to highlight the widespread application of MSC-exosomes in therapeutics, theranostics, and tissue regeneration. After a brief introduction of exosome properties, biogenesis, isolation, and functions, recent studies on therapeutic and regenerative applications of MSC-exosomes are described, focusing on bone, cartilage, periodontal, cardiovascular, skin, and nerve regeneration. Finally, the review highlights the theranostic potential of exosomes followed by challenges, summary, and outlook.

Exosome-Integrated Hydrogels for Bone Tissue Engineering

Exosome-integrated hydrogels represent a promising frontier in bone tissue engineering, leveraging the unique biological properties of exosomes to enhance the regenerative capabilities of hydrogels. Exosomes, as naturally occurring extracellular vesicles, carry a diverse array of bioactive molecules that play critical roles in intercellular communication and tissue regeneration. When combined with hydrogels, these exosomes can be spatiotemporally delivered to target sites, offering a controlled and sustained release of therapeutic agents. This review aims to provide a comprehensive overview of the recent advancements in the development, engineering, and application of exosome-integrated hydrogels for bone tissue engineering, highlighting their potential to overcome current challenges in tissue regeneration. Furthermore, the review explores the mechanistic pathways by which exosomes embedded within hydrogels facilitate bone repair, encompassing the regulation of inflammatory pathways, enhancement of angiogenic processes, and induction of osteogenic differentiation. Finally, the review addresses the existing challenges, such as scalability, reproducibility, and regulatory considerations, while also suggesting future directions for research in this rapidly evolving field. Thus, we hope this review contributes to advancing the development of next-generation biomaterials that synergistically integrate exosome and hydrogel technologies, thereby enhancing the efficacy of bone tissue regeneration.

Bioinspired injectable hydrogels for bone regeneration

The effective regeneration of bone/cartilage defects remains a significant clinical challenge, causing irreversible damage to millions annually.Conventional therapies such as autologous or artificial bone grafting often yield unsatisfactory outcomes, emphasizing the urgent need for innovative treatment methods. Biomaterial-based strategies, including hydrogels and active scaffolds, have shown potential in promoting bone/cartilage regeneration. Among them, injectable hydrogels have garnered substantial attention in recent years on account of their minimal invasiveness, shape adaptation, and controlled spatiotemporal release. This review systematically discusses the synthesis of injectable hydrogels, bioinspired approaches-covering microenvironment, structural, compositional, and bioactive component-inspired strategies-and their applications in various bone/cartilage disease models, highlighting bone/cartilage regeneration from an innovative perspective of bioinspired design. Taken together, bioinspired injectable hydrogels offer promising and feasible solutions for promoting bone/cartilage regeneration, ultimately laying the foundations for clinical applications. Furthermore, insights into further prospective directions for AI in injectable hydrogels screening and organoid construction are provided.

The impact of exosomes on bone health: A focus on osteoporosis

Osteoporosis is a widespread chronic condition. Although standard treatments are generally effective, they are frequently constrained by side effects and the risk of developing drug resistance. A promising area of research is the investigation of extracellular vesicles (EVs), including exosomes, microvesicles, and apoptotic bodies, which play a crucial role in bone metabolism. Exosomes, in particular, have shown significant potential in both the diagnosis and treatment of osteoporosis. EVs derived from osteoclasts, osteoblasts, mesenchymal stem cells, and other sources can influence bone metabolism, while exosomes from inflammatory and tumor cells may exacerbate bone loss, highlighting their dual role in osteoporosis pathology. This review offers a comprehensive overview of EV biogenesis, composition, and function in osteoporosis, focusing on their diagnostic and therapeutic potential. We examine the roles of various types of EVs and their cargo-proteins, RNAs, and lipids-in bone metabolism. Additionally, we explore the emerging applications of EVs as biomarkers and therapeutic agents, emphasizing the need for further research to address current challenges and enhance EV-based strategies for managing osteoporosis.

Engineered extracellular vesicles for tissue repair and regeneration

Extracellular vesicles (EVs) are heterogeneous membrane-like vesicles secreted by living cells that are involved in many physiological and pathological processes and act as intermediaries of intercellular communication and molecular transfer. Recent studies have shown that EVs from specific sources regulate tissue repair and regeneration by delivering proteins, lipids, and nucleic acids to target cells as signaling molecules. Nanotechnology breakthroughs have facilitated the development and exploration of engineered EVs for tissue repair. Enhancements through gene editing, surface modification, and content modification have further improved their therapeutic efficacy. This review summarizes the potential of EVs in tissue repair and regeneration, their mechanisms of action, and their research progress in regenerative medicine. This review highlights their design logic through typical examples and explores the development prospects of EVs in tissue repair. The aim of this review is to provide new insights into the design of EVs for tissue repair and regeneration applications, thereby expanding their use in regenerative medicine.

Hydrogel-exosome system in tissue engineering: A promising therapeutic strategy

Characterized by their pivotal roles in cell-to-cell communication, cell proliferation, and immune regulation during tissue repair, exosomes have emerged as a promising avenue for "cell-free therapy" in clinical applications. Hydrogels, possessing commendable biocompatibility, degradability, adjustability, and physical properties akin to biological tissues, have also found extensive utility in tissue engineering and regenerative repair. The synergistic combination of exosomes and hydrogels holds the potential not only to enhance the efficiency of exosomes but also to collaboratively advance the tissue repair process. This review has summarized the advancements made over the past decade in the research of hydrogel-exosome systems for regenerating various tissues including skin, bone, cartilage, nerves and tendons, with a focus on the methods for encapsulating and releasing exosomes within the hydrogels. It has also critically examined the gaps and limitations in current research, whilst proposed future directions and potential applications of this innovative approach.

Citric Acid: A Nexus Between Cellular Mechanisms and Biomaterial Innovations

Citrate-based biodegradable polymers have emerged as a distinctive biomaterial platform with tremendous potential for diverse medical applications. By harnessing their versatile chemistry, these polymers exhibit a wide range of material and bioactive properties, enabling them to regulate cell metabolism and stem cell differentiation through energy metabolism, metabonegenesis, angiogenesis, and immunomodulation. Moreover, the recent US Food and Drug Administration (FDA) clearance of the biodegradable poly(octamethylene citrate) (POC)/hydroxyapatite-based orthopedic fixation devices represents a translational research milestone for biomaterial science. POC joins a short list of biodegradable synthetic polymers that have ever been authorized by the FDA for use in humans. The clinical success of POC has sparked enthusiasm and accelerated the development of next-generation citrate-based biomaterials. This review presents a comprehensive, forward-thinking discussion on the pivotal role of citrate chemistry and metabolism in various tissue regeneration and on the development of functional citrate-based metabotissugenic biomaterials for regenerative engineering applications.

Exosome-loaded hydrogels for craniofacial bone tissue regeneration

It is common for maladies and trauma to cause significant bone deterioration in the craniofacial bone, which can cause patients to experience complications with their appearance and their ability to function. Regarding grafting procedures' complications and disadvantages, the newly emerging field of tissue regeneration has shown promise. Tissue -engineered technologies and their applications in the craniofacial region are increasingly gaining prominence with limited postoperative risk and cost. MSCs-derived exosomes are widely applied in bone tissue engineering to provide cell-free therapies since they not only do not cause immunological rejection in the same way that cells do, but they can also perform a cell-like role. Additionally, the hydrogel system is a family of multipurpose platforms made of cross-linked polymers with considerable water content, outstanding biocompatibility, and tunable physiochemical properties for the efficient delivery of commodities. Therefore, the promising exosome-loaded hydrogels can be designed for craniofacial bone regeneration. This review lists the packaging techniques for exosomes and hydrogel and discusses the development of a biocompatible hydrogel system and its potential for exosome continuous delivery for craniofacial bone healing.

Fabrication of vascularized tissue-engineered bone models using triaxial bioprinting

Bone tissue is a highly vascularized tissue. When constructing tissue-engineered bone models, both the osteogenic and angiogenic capabilities of the construct should be carefully considered. However, fabricating a vascularized tissue-engineered bone to promote vascular formation and bone generation, while simultaneously establishing nutrition channels to facilitate nutrient exchange within the constructs, remains a significant challenge. Triaxial bioprinting, which not only allows the independent encapsulation of different cell types while simultaneously forming nutrient channels, could potentially emerge as a strategy for fabricating vascularized tissue-engineered bone. Moreover, bioinks should also be applied in combination to promote both osteogenesis and angiogenesis. In this study, employing triaxial bioprinting, we used a blend bioink of gelatin methacryloyl (GelMA), sodium alginate (Alg), and different concentrations of nano beta-tricalcium phosphate (nano β-TCP) encapsulated MC3T3-E1 preosteoblasts as the outer layer, a mixed bioink of GelMA and Alg loaded with human umbilical vein endothelial cells (HUVEC) as the middle layer, and gelatin as a sacrificial material to form nutrient channels in the inner layer to fabricate vascularized bone constructs simulating the microenvironment for bone and vascular tissues. The results showed that the addition of nano β-TCP could adjust the mechanical, swelling, and degradation properties of the constructs. Biological assessments revealed the cell viability of constructs containing different concentrations of nano β-TCP was higher than 90% on day 7, The cell-laden constructs containing 3% (w/v) nano β-TCP exhibited better osteogenic (higher Alkaline phosphatase activity and larger Osteocalcin positive area) and angiogenic (the gradual increased CD31 positive area) potential. Therefore, using triaxial bioprinting technology and employing GelMA, Alg, and nano β-TCP as bioink components could fabricate vascularized bone tissue constructs, offering a novel strategy for vascularized bone tissue engineering.

Exosomes: A New Hope for Angiogenesis-Mediated Bone Regeneration

Bone is a metabolically dynamic structure that is generally remodeled throughout the lifetime of an individual but often causes problems with increasing age. A key player for bone development and homeostasis, but also under pathological conditions, is the bone vasculature. This complex system of arteries, veins, and capillaries forms distinct structures where each subset of endothelial cells has important functions. Starting with the basic process of angiogenesis and bone-specific blood vessel formation, coupled with initial bone formation, the importance of different vascular structures is highlighted with respect to how these structures are maintained or changed during homeostasis, aging, and pathological conditions. After exemplifying the current knowledge on bone vasculature, this review will move on to exosomes, a novel hotspot of scientific research. Exosomes will be introduced starting from their discovery via current isolation procedures and state-of-the-art characterization to their role in bone vascular development, homeostasis, and bone regeneration and repair while summarizing the underlying signal transduction pathways. With respect to their role in these processes, especially mesenchymal stem cell-derived extracellular vesicles are of interest, which leads to a discussion on patented applications and an update on ongoing clinical trials. Taken together, this review provides an overview of bone vasculature and bone regeneration, with a major focus on how exosomes influence this intricate system, as they might be useful for therapeutic purposes in the near future.

Mesenchymal Stem Cell-Derived Extracellular Vesicles in Bone-Related Diseases: Intercellular Communication Messengers and Therapeutic Engineering Protagonists

Extracellular vesicles (EVs) can deliver various bioactive molecules among cells, making them promising diagnostic and therapeutic alternatives in diseases. Mesenchymal stem cell-derived EVs (MSC-EVs) have shown therapeutic potential similar to MSCs but with drawbacks such as lower yield, reduced biological activities, off-target effects, and shorter half-lives. Improving strategies utilizing biotechniques to pretreat MSCs and enhance the properties of released EVs, as well as modifying MSC-EVs to enhance targeting abilities and achieve controlled release, shows potential for overcoming application limitations and enhancing therapeutic effects in treating bone-related diseases. This review focuses on recent advances in functionalizing MSC-EVs to treat bone-related diseases. Firstly, we underscore the significance of MSC-EVs in facilitating crosstalk between cells within the skeletal environment. Secondly, we highlight strategies of functional-modified EVs for treating bone-related diseases. We explore the pretreatment of stem cells using various biotechniques to enhance the properties of resulting EVs, as well as diverse approaches to modify MSC-EVs for targeted delivery and controlled release. Finally, we address the challenges and opportunities for further research on MSC-EVs in bone-related diseases.

The role and applications of extracellular vesicles in osteoporosis

Osteoporosis is a widely observed condition characterized by the systemic deterioration of bone mass and microarchitecture, which increases patient susceptibility to fragile fractures. The intricate mechanisms governing bone homeostasis are substantially impacted by extracellular vesicles (EVs), which play crucial roles in both pathological and physiological contexts. EVs derived from various sources exert distinct effects on osteoporosis. Specifically, EVs released by osteoblasts, endothelial cells, myocytes, and mesenchymal stem cells contribute to bone formation due to their unique cargo of proteins, miRNAs, and cytokines. Conversely, EVs secreted by osteoclasts and immune cells promote bone resorption and inhibit bone formation. Furthermore, the use of EVs as therapeutic modalities or biomaterials for diagnosing and managing osteoporosis is promising. Here, we review the current understanding of the impact of EVs on bone homeostasis, including the classification and biogenesis of EVs and the intricate regulatory mechanisms of EVs in osteoporosis. Furthermore, we present an overview of the latest research progress on diagnosing and treating osteoporosis by using EVs. Finally, we discuss the challenges and prospects of translational research on the use of EVs in osteoporosis.

Current Status and Progress in Stem Cell Therapy for Intracerebral Hemorrhage

Intracerebral hemorrhage is a highly prevalent and prognostically poor disease, imposing immeasurable harm on human life and health. However, the treatment options for intracerebral hemorrhage are severely limited, particularly in terms of improving the microenvironment of the lesion, promoting neuronal cell survival, and enhancing neural function. This review comprehensively discussed the application of stem cell therapy for intracerebral hemorrhage, providing a systematic summary of its developmental history, types of transplants, transplantation routes, and transplantation timing. Moreover, this review presented the latest research progress in enhancing the efficacy of stem cell transplantation, including pretransplantation preconditioning, genetic modification, combined therapy, and other diverse strategies. Furthermore, this review pioneeringly elaborated on the barriers to clinical translation for stem cell therapy. These discussions were of significant importance for promoting stem cell therapy for intracerebral hemorrhage, facilitating its clinical translation, and improving patient prognosis.

Effects of Bioactive Glasses (BGs) on Exosome Production and Secretion: A Critical Review

There is an increasing trend toward the application of bioactive glasses in different areas of biomedicine, including tissue engineering and oncology. The reason for this increase is mostly attributed to the inherent properties of BGs, such as excellent biocompatibility, and the ease of tailoring their properties by changing, for example, the chemical composition. Previous experiments have demonstrated that the interactions between BGs and their ionic dissolution products, and mammalian cells, can affect and change cellular behaviors, and thereby govern the performance of living tissues. However, limited research exists on their critical role in the production and secretion of extracellular vesicles (EVs) such as exosomes. Exosomes are nanosized membrane vesicles that carry various therapeutic cargoes such as DNA, RNA, proteins, and lipids, and thereby can govern cell-cell communication and subsequent tissue responses. The use of exosomes is currently considered a cell-free approach in tissue engineering strategies, due to their positive roles in accelerating wound healing. On the other hand, exosomes are known as key players in cancer biology (e.g., progression and metastasis), due to their capability to carry bioactive molecules between tumor cells and normal cells. Recent studies have demonstrated that the biological performance of BGs, including their proangiogenic activity, is accomplished with the help of exosomes. Indeed, therapeutic cargos (e.g., proteins) produced in BG-treated cells are transferred by a specific subset of exosomes toward target cells and tissues, and lead to a biological phenomenon. On the other hand, BGs are suitable delivery vehicles that can be utilized for the targeted delivery of exosomes to cells and tissues of interest. Therefore, it seems necessary to have a deeper understanding of the potential effects of BGs in the production of exosomes in cells that are involved in tissue repair and regeneration (mostly mesenchymal stem cells), as well as in those that play roles in cancer progression (e.g., cancer stem cells). This review aims to present an updated report on this critical issue, to provide a roadmap for future research in the fields of tissue engineering and regenerative medicine.

Stem cell-derived exosomes in bone healing: focusing on their role in angiogenesis

Fractures in bone, a tissue critical in protecting other organs, affect patients' quality of life and have a heavy economic burden on societies. Based on regenerative medicine and bone tissue engineering approaches, stem cells have become a promising and attractive strategy for repairing bone fractures via differentiation into bone-forming cells and production of favorable mediators. Recent evidence suggests that stem cell-derived exosomes could mediate the therapeutic effects of their counterpart cells and provide a cell-free therapeutic strategy in bone repair. Since bone is a highly vascularized tissue, coupling angiogenesis and osteogenesis is critical in bone fracture healing; thus, developing therapeutic strategies to promote angiogenesis will facilitate bone regeneration and healing. To this end, stem cell-derived exosomes with angiogenic potency have been developed to improve fracture healing. This review summarizes the effects of stem cell-derived exosomes on the repair of bone tissue, focusing on the angiogenesis process.

Engineering exosomes and biomaterial-assisted exosomes as therapeutic carriers for bone regeneration

Mesenchymal stem cell-based therapy has become an effective therapeutic approach for bone regeneration. However, there are still limitations in successful clinical translation. Recently, the secretome of mesenchymal stem cells, especially exosome, plays a critical role in promoting bone repair and regeneration. Exosomes are nanosized, lipid bilayer-enclosed structures carrying proteins, lipids, RNAs, metabolites, growth factors, and cytokines and have attracted great attention for their potential application in bone regenerative medicine. In addition, preconditioning of parental cells and exosome engineering can enhance the regenerative potential of exosomes for treating bone defects. Moreover, with recent advancements in various biomaterials to enhance the therapeutic functions of exosomes, biomaterial-assisted exosomes have become a promising strategy for bone regeneration. This review discusses different insights regarding the roles of exosomes in bone regeneration and summarizes the applications of engineering exosomes and biomaterial-assisted exosomes as safe and versatile bone regeneration agent delivery platforms. The current hurdles of transitioning exosomes from bench to bedside are also discussed.

A novel biologically hierarchical hydrogel with osteoblast precursor-targeting extracellular vesicles ameliorates bone loss in vivo via the sequential action of antagomiR-200b-3p and antagomiR-130b-3p

Osteoporotic fracture is a major health problem plaguing the ageing society, and improving its treatment is an urgent challenge. How to ameliorate bone loss determines the recovery of such fractures. Extracellular vesicle (EV)-loaded hydrogel has the capacity to treat osteoporotic fractures due to its pro-osteogenic property. And balancing proliferation and maturation of osteoblast precursors (OBPs) is of great significance to avoid OBP depletion, which is lacking in current treatment. Based on osteoblastogenic miRNAs, this study aimed to explore the efficacies of the combination of hierarchical hydrogel and EVs altering functional miRNAs level in bone loss. Through bioinformatics analyses, we screened out proliferative gene-targeting miR-200b-3p and osteogenic gene-targeting miR-130b-3p. And antagomiR-200b-3p (ant-200b) enhanced OBP proliferation, and antagomiR-130b-3p (ant-130b) promoted OBP differentiation. After confirming the directional effect of Fibronectin (Fn1) on OBPs, we prepared OBP-targeting EVs. Furthermore, encapsulation of two antagomiRNAs in EVs enhanced the respective effect of ant-200b and ant-130b. Notably, hierarchically injectable hydrogel exerted an effective function in promoting the sequential delivery of EVs-200b and EVs-130b. Importantly, hierarchical hydrogel containing dual EVs effectively ameliorated bone loss. Overall, hierarchical hydrogel based on two antagomiRNAs effectively improves bone loss in vivo due to its role in promoting OBP proliferation and maturation sequentially.

Hydrogel scaffolds in bone regeneration: Their promising roles in angiogenesis

Bone tissue engineering (BTE) has become a hopeful potential treatment strategy for large bone defects, including bone tumors, trauma, and extensive fractures, where the self-healing property of bone cannot repair the defect. Bone tissue engineering is composed of three main elements: progenitor/stem cells, scaffold, and growth factors/biochemical cues. Among the various biomaterial scaffolds, hydrogels are broadly used in bone tissue engineering owing to their biocompatibility, controllable mechanical characteristics, osteoconductive, and osteoinductive properties. During bone tissue engineering, angiogenesis plays a central role in the failure or success of bone reconstruction via discarding wastes and providing oxygen, minerals, nutrients, and growth factors to the injured microenvironment. This review presents an overview of bone tissue engineering and its requirements, hydrogel structure and characterization, the applications of hydrogels in bone regeneration, and the promising roles of hydrogels in bone angiogenesis during bone tissue engineering.

Extracellular vesicle-loaded hydrogels for tissue repair and regeneration

Extracellular vesicles (EVs) are a collective term for nanoscale or microscale vesicles secreted by cells that play important biological roles. Mesenchymal stem cells are a class of cells with the potential for self-healing and multidirectional differentiation. In recent years, numerous studies have shown that EVs, especially those secreted by mesenchymal stem cells, can promote the repair and regeneration of various tissues and, thus, have significant potential in regenerative medicine. However, due to the rapid clearance capacity of the circulatory system, EVs are barely able to act persistently at specific sites for repair of target tissues. Hydrogels have good biocompatibility and loose and porous structural properties that allow them to serve as EV carriers, thereby prolonging the retention in certain specific areas and slowing the release of EVs. When EVs are needed to function at specific sites, the EV-loaded hydrogels can stand as an excellent approach. In this review, we first introduce the sources, roles, and extraction and characterization methods of EVs and describe their current application status. We then review the different types of hydrogels and discuss factors influencing their abilities to carry and release EVs. We summarize several strategies for loading EVs into hydrogels and characterizing EV-loaded hydrogels. Furthermore, we discuss application strategies for EV-loaded hydrogels and review their specific applications in tissue regeneration and repair. This article concludes with a summary of the current state of research on EV-loaded hydrogels and an outlook on future research directions, which we hope will provide promising ideas for researchers.

The potential therapeutic role of extracellular vesicles in critical-size bone defects: Spring of cell-free regenerative medicine is coming

In recent years, the incidence of critical-size bone defects has significantly increased. Critical-size bone defects seriously affect patients' motor functions and quality of life and increase the need for additional clinical treatments. Bone tissue engineering (BTE) has made great progress in repairing critical-size bone defects. As one of the main components of bone tissue engineering, stem cell-based therapy is considered a potential effective strategy to regenerate bone tissues. However, there are some disadvantages including phenotypic changes, immune rejection, potential tumorigenicity, low homing efficiency and cell survival rate that restrict its wider clinical applications. Evidence has shown that the positive biological effects of stem cells on tissue repair are largely mediated through paracrine action by nanostructured extracellular vesicles (EVs), which may overcome the limitations of traditional stem cell-based treatments. In addition to stem cell-derived extracellular vesicles, the potential therapeutic roles of nonstem cell-derived extracellular vesicles in critical-size bone defect repair have also attracted attention from scholars in recent years. Currently, the development of extracellular vesicles-mediated cell-free regenerative medicine is still in the preliminary stage, and the specific mechanisms remain elusive. Herein, the authors first review the research progress and possible mechanisms of extracellular vesicles combined with bone tissue engineering scaffolds to promote bone regeneration via bioactive molecules. Engineering modified extracellular vesicles is an emerging component of bone tissue engineering and its main progression and clinical applications will be discussed. Finally, future perspectives and challenges of developing extracellular vesicle-based regenerative medicine will be given. This review may provide a theoretical basis for the future development of extracellular vesicle-based biomedicine and provide clinical references for promoting the repair of critical-size bone defects.

Concentration-Dependent Efficacy of Recombinant Human Bone Morphogenetic Protein-2 Using a HA/β-TCP Hydrogel Carrier in a Mini-Pig Vertebral Oblique Lateral Interbody Fusion Model

Bone morphogenetic protein-2 (BMP-2) is used in the treatment of degenerative spinal disease and vertebral fractures, spine fusion, dental surgery, and facial surgery. However, high doses are associated with side effects such as inflammation and osteophytes. In this study, we performed spinal fusion surgery on mini-pigs using BMP-2 and a HA/β-TCP hydrogel carrier, and evaluated the degree of fusion and osteophyte growth according to time and dosage. Increasing the dose of BMP-2 led to a significantly higher fusion rate than was observed in the control group, and there was no significant difference between the 8-week and 16-week samples. We also found that the HA + β-TCP hydrogel combination helped maintain the rate of BMP-2 release. In conclusion, the BMP-2-loaded HA/β-TCP hydrogel carrier used in this study overcame the drawback of potentially causing side effects when used at high concentrations by enabling the sustained release of BMP-2. This method is also highly efficient, since it provides mineral matter to accelerate the fusion rate of the spine and improve bone quality.

Engineering exosomes for bone defect repair

Currently, bone defect repair is still an intractable clinical problem. Numerous treatments have been performed, but their clinical results are unsatisfactory. As a key element of cell-free therapy, exosome is becoming a promising tool of bone regeneration in recent decades, because of its promoting osteogenesis and osteogenic differentiation function in vivo and in vitro. However, low yield, weak activity, inefficient targeting ability, and unpredictable side effects of natural exosomes have limited the clinical application. To overcome the weakness, various approaches have been applied to produce engineering exosomes by regulating their production and function at present. In this review, we will focus on the engineering exosomes for bone defect repair. By summarizing the exosomal cargos affecting osteogenesis, the strategies of engineering exosomes and properties of exosome-integrated biomaterials, this work will provide novel insights into exploring advanced engineering exosome-based cell-free therapy for bone defect repair.

Manufacturing functional hydrogels for inducing angiogenic-osteogenic coupled progressions in hard tissue repairs: prospects and challenges

In large bone defects, inadequate vascularization within the engineered constructs has been a major challenge in developing clinically impactful products. It is fairly determined that bone tissues and blood vessels are established concurrently throughout tissue repairs after an injury. Thus, the coupling of angiogenesis-osteogenesis is an essential course of action in bone tissue restoration. The manufacture of biomaterial-based scaffolds plays a decisive role in stimulating angiogenic and osteogenic progressions (instruction of neovascularization and bone mineralization). Bone hydrogels with optimal conditions are more efficient at healing bone defects. There has been a remarkable advancement in producing bone substitutes in the tissue engineering area, but the sufficient and timely vascularization of engineered constructs for optimal tissue integration and regeneration is lacking due to mismatch in the scaffold characteristics and new bone tissue reconstruction. Therefore, various key challenges remain to be overcome. A deep understanding of angiogenesis and osteogenesis progressions is required to manufacture bone hydrogels with satisfactory results. The current review briefly discusses the fundamentals of bone tissues, the significance of angiogenesis-osteogenesis progressions and their inducers, the efficacy of biomaterials and composite hydrogel-promoted neo-vasculogenesis (i.e. angiogenesis) and bone mineralization (i.e. osteogenesis), and related challenges, including future research directions.

Mesenchymal Stem Cell-Derived Extracellular Vesicles for Bone Defect Repair

The repair of critical bone defects is a hotspot of orthopedic research. With the development of bone tissue engineering (BTE), there is increasing evidence showing that the combined application of extracellular vesicles (EVs) derived from mesenchymal stem cells (MSCs) (MSC-EVs), especially exosomes, with hydrogels, scaffolds, and other bioactive materials has made great progress, exhibiting a good potential for bone regeneration. Recent studies have found that miRNAs, proteins, and other cargo loaded in EVs are key factors in promoting osteogenesis and angiogenesis. In BTE, the expression profile of the intrinsic cargo of EVs can be changed by modifying the gene expression of MSCs to obtain EVs with enhanced osteogenic activity and ultimately enhance the osteoinductive ability of bone graft materials. However, the current research on MSC-EVs for repairing bone defects is still in its infancy, and the underlying mechanism remains unclear. Therefore, in this review, the effect of bioactive materials such as hydrogels and scaffolds combined with MSC-EVs in repairing bone defects is summarized, and the mechanism of MSC-EVs promoting bone defect repair by delivering active molecules such as internal miRNAs is further elucidated, which provides a theoretical basis and reference for the clinical application of MSC-EVs in repairing bone defects.

Bone Engineering Scaffolds With Exosomes: A Promising Strategy for Bone Defects Repair

The treatment of bone defects is still an intractable clinical problem, despite the fact that numerous treatments are currently available. In recent decades, bone engineering scaffolds have become a promising tool to fill in the defect sites and remedy the deficiencies of bone grafts. By virtue of bone formation, vascular growth, and inflammation modulation, the combination of bone engineering scaffolds with cell-based and cell-free therapy is widely used in bone defect repair. As a key element of cell-free therapy, exosomes with bioactive molecules overcome the deficiencies of cell-based therapy and promote bone tissue regeneration via the potential of osteogenesis, angiogenesis, and inflammation modulation. Hence, this review aimed at overviewing the bone defect microenvironment and healing mechanism, summarizing current advances in bone engineering scaffolds and exosomes in bone defects to probe for future applications.

Preconditioning and Engineering Strategies for Improving the Efficacy of Mesenchymal Stem Cell-Derived Exosomes in Cell-Free Therapy

Mesenchymal stem cells (MSCs) have been widely applied to regenerative medicine owing to their multiple differentiation, self-renewal, and immunomodulatory abilities. Exosomes are cell-secreted natural nanovesicles and thought to be mediators of intercellular communication and material transport. The therapeutic potential of MSCs can be largely attributed to MSC-derived exosomes (MSC-exosomes). Emerging evidence suggests that the therapeutic efficacy of MSC-exosomes is highly dependent on the status of MSCs, and optimization of the extracellular environment affects the exosomal content. Pretreatment methods including three-dimensional cultures, hypoxia, and other biochemical cues have been shown to potentially enhance the biological activity of MSC-exosomes while maintaining or enhancing their production. On the other hand, engineering means to enhance the desired function of MSC-exosomes has been rapidly gaining attention. In particular, biologically active molecule encapsulation and membrane modification can alter or enhance biological functions and targeting of MSC-exosomes. In this review, we summarize two possible strategies to improve the therapeutic activity of MSC-exosomes: preconditioning approaches and engineering exosomes. We also explore the underlying mechanisms of different strategies and discuss their advantages and limitations of the upcoming clinical applications.

State of the field: cellular and exosomal therapeutic approaches in vascular regeneration

Pathologies of the vasculature including the microvasculature are often complex in nature, leading to loss of physiological homeostatic regulation of patency and adequate perfusion to match tissue metabolic demands. Microvascular dysfunction is a key underlying element in the majority of pathologies of failing organs and tissues. Contributing pathological factors to this dysfunction include oxidative stress, mitochondrial dysfunction, endoplasmic reticular (ER) stress, endothelial dysfunction, loss of angiogenic potential and vascular density, and greater senescence and apoptosis. In many clinical settings, current pharmacologic strategies use a single or narrow targeted approach to address symptoms of pathology rather than a comprehensive and multifaceted approach to address their root cause. To address this, efforts have been heavily focused on cellular therapies and cell-free therapies (e.g., exosomes) that can tackle the multifaceted etiology of vascular and microvascular dysfunction. In this review, we discuss 1) the state of the field in terms of common therapeutic cell population isolation techniques, their unique characteristics, and their advantages and disadvantages, 2) common molecular mechanisms of cell therapies to restore vascularization and/or vascular function, 3) arguments for and against allogeneic versus autologous applications of cell therapies, 4) emerging strategies to optimize and enhance cell therapies through priming and preconditioning, and, finally, 5) emerging strategies to bolster therapeutic effect. Relevant and recent clinical and animal studies using cellular therapies to restore vascular function or pathologic tissue health by way of improved vascularization are highlighted throughout these sections.

Mesenchymal Stem Cell-Derived Extracellular Vesicles in Liver Immunity and Therapy

Mesenchymal stem cells (MSCs), as the most common cell source for stem cell therapy, play an important role in the modulation of innate and adaptive immune responses and have been widely used in clinical trials to treat autoimmune and inflammatory diseases. Recent experimental and clinical studies have shown that MSC-derived extracellular vesicles (MSC-EVs) can inhibit the activation and proliferation of a variety of proinflammatory cells, such as Th1, Th17 and M1 macrophages, reducing the secretion of proinflammatory cytokines, while promoting the proliferation of anti-inflammatory cells, such as M2 macrophages and Tregs, and increasing the secretion of anti-inflammatory cytokines, thus playing a role in immune regulation and exhibiting immunomodulatory functions. Besides MSC-EVs are more convenient and less immunogenic than MSCs. There is growing interest in the role of MSC-EVs in liver diseases owing to the intrinsic liver tropism of MSC-EVs. In this review, we focus on the immunomodulatory effects of MSC-EVs and summarize the pivotal roles of MSC-EVs as a cell-free therapy in liver diseases, including NAFLD, AIH, acute liver failure, liver fibrosis and hepatic ischemia–reperfusion injury. Moreover, we provide a concise overview of the potential use and limits of MSC-EVs in clinical application.

Modern approaches on stem cells and scaffolding technology for osteogenic differentiation and regeneration

The process of bone repair has always been a natural mystery. Although bones do repair themselves, supplemental treatment is required for the initiation of the self-regeneration process. Predominantly, surgical procedures are employed for bone regeneration. Recently, cell-based therapy for bone regeneration has proven to be more effective than traditional methods, as it eliminates the immune risk and painful surgeries. In clinical trials, various stem cells, especially mesenchymal stem cells, have shown to be more efficient for the treatment of several bone-related diseases, such as non-union fracture, osteogenesis imperfecta, osteosarcoma, and osteoporosis. Furthermore, the stem cells grown in a suitable three-dimensional scaffold support were found to be more efficient for osteogenesis. It has been shown that the three-dimensional bioscaffolds support and simulate an in vivo environment, which helps in differentiation of stem cells into bone cells. Bone regeneration in patients with bone disorders can be improved through modification of stem cells with several osteogenic factors or using stem cells as carriers for osteogenic factors. In this review, we focused on the various types of stem cells and scaffolds that are being used for bone regeneration. In addition, the molecular mechanisms of various transcription factors, signaling pathways that support bone regeneration and the senescence of the stem cells, which limits bone regeneration, have been discussed.

Current strategies in tailoring methods for engineered exosomes and future avenues in biomedical applications

Exosomes are naturally occurring nanovesicles of endosomal origin, responsible for cellular communication. Depending on the cell type, exosomes display disparity in the cargo and are involved in up/down regulation of different biological pathways. Naturally secreted exosomes, owing to their inherent delivery potential, non-immunogenic nature and limited structural resemblance to the cells have emerged as ideal candidates for various drug delivery and therapeutic applications. Moreover, the structural versatility of exosomes provides greater flexibility for surface modifications to be made in the native configuration, by different methods, like genetic-engineering, chemical procedures, physical methods and microfluidic-technology, to enhance the cargo quality for expanded biomedical applications. Post isolation and prior to engineering exosomes for various applications, the internal and external structural compositions of exosomes are studied via different techniques. Efficiency and scalability of the exosome modification methods are pivotal in determining the scope of the technique for clinical applications. This review majorly focuses on different methods employed for engineering exosomes, and advantages/disadvantages associated with different tailoring approaches, along with the efficacy of engineered exosomes in biomedical applications. Further, the review highlights the importance of a relatively recent avenue for delivery of exosomes via scaffold-based delivery of naïve/engineered exosomes for regenerative medicine and tissue engineering. This review is based on the recent knowledge generated in this field and our comprehension in this domain.

Cell-free exosome-laden scaffolds for tissue repair

With the development of regenerative medicine, tissue repair at the molecular, cellular, tissue, and organ level has seen continuous improvements over traditional techniques. As the core of tissue repair, seed cells are widely used in various fields of regenerative medicine. However, their use is still associated with problems such as decreased cell survival and regeneration capacity after transplantation, immune rejection, and ethical concerns. Therefore, it is difficult to universally and safely apply stem cell banks for regenerative medicine. The paracrine effects of cells, especially secretion of exosomes, play vital roles in cell communication, immune response, angiogenesis, scar formation, tissue repair, and other biological functions. Exosomes are a type of nanoscale extracellular vesicle that contain biologically active molecules such as RNA and proteins; therefore, exosomes can replicate the functions of their parental cells. Meanwhile, exosomes can be used as nanocarriers to deliver active factors or small molecules to promote tissue repair. Preclinical studies of exosomes in tissue engineering and regenerative medicine have been carried in the fields of bone/cartilage repair, nerve regeneration, liver and kidney regeneration, skin repair, vascular tissue regeneration, etc. This review introduces exosomes from the aspects of biogenesis, composition, identification, and isolation, and focuses on the development status of scaffold materials for exosome delivery. In addition, we highlight examples of exosome-laden scaffolds for preclinical applications in tissue repair. We look forward to the broad application prospects of exosome-laden scaffolds.