Alginate Hydrogels for In Vivo Bone Regeneration: The Immune Competence of the Animal Model Matters

Advances in the application and research of biomaterials in promoting bone repair and regeneration through immune modulation

With the ongoing development of osteoimmunology, increasing evidence indicates that the local immune microenvironment plays a critical role in various stages of bone formation. Consequently, modulating the immune inflammatory response triggered by biomaterials to foster a more favorable immune microenvironment for bone regeneration has emerged as a novel strategy in bone tissue engineering. This review first examines the roles of various immune cells in bone tissue injury and repair. Then, the contributions of different biomaterials, including metals, bioceramics, and polymers, in promoting osteogenesis through immune regulation, as well as their future development directions, are discussed. Finally, various design strategies, such as modifying the physicochemical properties of biomaterials and integrating bioactive substances, to optimize material design and create an immune environment conducive to bone formation, are explored. In summary, this review comprehensively covers strategies and approaches for promoting bone tissue regeneration through immune modulation. It offers a thorough understanding of current research trends in biomaterial-based immune regulation, serving as a theoretical reference for the further development and clinical application of biomaterials in bone tissue engineering.

Copper ions-photo dual-crosslinked alginate hydrogel for angiogenesis and osteogenesis

Early healing of bone defects is still a clinical challenge. Many bone-filling materials have been studied, among which photocrosslinked alginate has received significant attention due to its good biocompatibility and morphological plasticity. Although it has been confirmed that photocrosslinked alginate can be used as an extracellular matrix for 3D cell culture, it lacks osteogenesis-related biological functions. This study constructed a copper ions-photo dual-crosslinked alginate hydrogel scaffold by controlling the copper ion concentration. The scaffolds were shaped by photocrosslinking and then endowed with biological functions by copper ions crosslinking. According to in vitro research, the dual-crosslinked hydrogel increased the compressive strength and favored copper dose-dependent osteoblast differentiation and cell surface adherence of rat bone marrow mesenchymal stem cells and the expression of type I collagen (Col1), runt-related transcription factor 2 (Runx2), osteocalcin (OCN), vascular endothelial growth factor (VEGF). In addition, hydrogel scaffolds were implanted into rat skull defects, and more angiogenesis and osteogenesis could be observed in in vivo studies. The above results show that the copper-photo-crosslinked hydrogel scaffold has excellent osseointegration properties and can potentially promote angiogenesis and early healing of bone defects, providing a reference solution for bone tissue engineering materials.

Exosome-Laden Hydrogels as Promising Carriers for Oral and Bone Tissue Engineering: Insight into Cell-Free Drug Delivery

Mineralization is a key biological process that is required for the development and repair of tissues such as teeth, bone and cartilage. Exosomes (Exo) are a subset of extracellular vesicles (~50-150 nm) that are secreted by cells and contain genetic material, proteins, lipids, nucleic acids, and other biological substances that have been extensively researched for bone and oral tissue regeneration. However, Exo-free biomaterials or exosome treatments exhibit poor bioavailability and lack controlled release mechanisms at the target site during tissue regeneration. By encapsulating the Exos into biomaterials like hydrogels, these disadvantages can be mitigated. Several tissue engineering approaches, such as those for wound healing processes in diabetes mellitus, treatment of osteoarthritis (OA) and cartilage degeneration, repair of intervertebral disc degeneration, and cardiovascular diseases, etc., have been exploited to deliver exosomes containing a variety of therapeutic and diagnostic cargos to target tissues. Despite the significant efficacy of Exo-laden hydrogels, their use in mineralized tissues, such as oral and bone tissue, is very sparse. This review aims to explore and summarize the literature related to the therapeutic potential of hydrogel-encapsulated exosomes for bone and oral tissue engineering and provides insight and practical procedures for the development of future clinical techniques.

Engineering Innervated Musculoskeletal Tissues for Regenerative Orthopedics and Disease Modeling

Musculoskeletal (MSK) disorders significantly burden patients and society, resulting in high healthcare costs and productivity loss. These disorders are the leading cause of physical disability, and their prevalence is expected to increase as sedentary lifestyles become common and the global population of the elderly increases. Proper innervation is critical to maintaining MSK function, and nerve damage or dysfunction underlies various MSK disorders, underscoring the potential of restoring nerve function in MSK disorder treatment. However, most MSK tissue engineering strategies have overlooked the significance of innervation. This review first expounds upon innervation in the MSK system and its importance in maintaining MSK homeostasis and functions. This will be followed by strategies for engineering MSK tissues that induce post-implantation in situ innervation or are pre-innervated. Subsequently, research progress in modeling MSK disorders using innervated MSK organoids and organs-on-chips (OoCs) is analyzed. Finally, the future development of engineering innervated MSK tissues to treat MSK disorders and recapitulate disease mechanisms is discussed. This review provides valuable insights into the underlying principles, engineering methods, and applications of innervated MSK tissues, paving the way for the development of targeted, efficacious therapies for various MSK conditions.

Recent advances in immunomodulatory hydrogels biomaterials for bone tissue regeneration

There is a high incidence of fractures in clinical practice and therapy. The repairment of critical size defects in the skeletal system remains a huge challenge for surgeons and researchers, which can be overcame by the application of bone tissue-engineered biomaterials. An increasing number of investigations have revealed that the immune system plays a vital role in the repair of bone defects, especially macrophages, which can modulate the integration of biomaterials and bone regeneration in multiple ways. Therefore, it has become increasingly important in regenerative medicine to regulate macrophage polarization to prevent inflammation caused by biomaterial implantation. Recent studies have stressed the importance of hydrogel-based modifications and the incorporation of various cellular and molecular signals for regulating immune responses to promote bone tissue regeneration and integrate biomaterials. In this review, we first elaborate briefly on the described the general physiological mechanism and process of bone tissue regeneration. Then, we summarized the immunomodulatory role macrophages play in bone repair. In addition, the role of hydrogel-based immune modification targeting macrophage modulation in accelerating and enhancing bone tissue regeneration was also discussed. Finally, we highlighted future directions and research strategies related to hydrogel optimization for the regulation of the immune response during bone regeneration and healing.

Natural and Synthetic Polymeric Biomaterials for Application in Wound Management

Biomaterials are at the forefront of the future, finding a variety of applications in the biomedical field, especially in wound healing, thanks to their biocompatible and biodegradable properties. Wounds spontaneously try to heal through a series of interconnected processes involving several initiators and mediators such as cytokines, macrophages, and fibroblasts. The combination of biopolymers with wound healing properties may provide opportunities to synthesize matrices that stimulate and trigger target cell responses crucial to the healing process. This review outlines the optimal management and care required for wound treatment with a special focus on biopolymers, drug-delivery systems, and nanotechnologies used for enhanced wound healing applications. Researchers have utilized a range of techniques to produce wound dressings, leading to products with different characteristics. Each method comes with its unique strengths and limitations, which are important to consider. The future trajectory in wound dressing advancement should prioritize economical and eco-friendly methodologies, along with improving the efficacy of constituent materials. The aim of this work is to give researchers the possibility to evaluate the proper materials for wound dressing preparation and to better understand the optimal synthesis conditions as well as the most effective bioactive molecules to load.

Hydration-induced Void-containing Hydrogels for Encapsulation and Sustained Release of Small Hydrophilic Molecules

Efficient encapsulation and sustained release of small hydrophilic molecules from traditional hydrogel systems have been challenging due to the large mesh size of 3D networks and high water content. Furthermore, the encapsulated molecules are prone to early release from the hydrogel prior to use, resulting in a short shelf life of the formulation. Here, we present a hydration-induced void-containing hydrogel (HVH) based on hyperbranched polyglycerol-poly(propylene oxide)-hyperbranched polyglycerol (HPG-PPG-HPG) as a robust and efficient delivery system for small hydrophilic molecules. Specifically, after the HPG-PPG-HPG is incubated overnight at 4 °C in the drug solution, it is hydrated into a hydrogel containing micron-sized voids, which could encapsulate hydrophilic drugs and achieve 100% drug encapsulation efficiency. In addition, the voids are surrounded by a densely packed polymer matrix, which restricts drug transport to achieve sustained drug release. The hydrogel/drug formulation can be stored for several months without changing the drug encapsulation and release properties. HVH hydrogels are injectable due to shear thinning properties. In rats, a single injection of the HPG-PPG-HPG hydrogel containing 8 μg of tetrodotoxin (TTX) produced sciatic nerve block lasting up to 10 hours without any TTX-related systemic toxicity nor local toxicity to nerves and muscles.

Stem Cell Membrane-Coated Microribbon Scaffolds Induce Regenerative Innate and Adaptive Immune Responses in a Critical-Size Cranial Bone Defect Model

Naturally-derived cell membranes have shown great promise in functionalizing nanoparticles to enhance biointerfacing functions for drug delivery applications. However, its potential for functionalizing macroporous scaffolds to enhance tissue regeneration in vivo remains unexplored. Engineering scaffolds with immunomodulatory functions represents an exciting strategy for tissue regeneration but is largely limited to soft tissues. Critical-sized bone defects cannot heal on their own, and the role of adaptive immune cells in scaffold-mediated healing of cranial bone defects remains largely unknown. Here, mensenchymal stem cell membrane (MSCM)-coated microribbon (µRB) scaffolds for treating critical size cranial bone defects via targeting immunomodulation are reported. Confocal imaging and proteomic analyses are used to confirm successful coating and characterize the compositions of cell membrane coating. It is demonstrated that MSCM coating promotes macrophage (Mφ) polarization toward regenerative phenotype, induces CD8+ T cell apoptosis, and enhances regulatory T cell differentiation in vitro and in vivo. When combined with a low dosage of BMP-2, MSCM coating further accelerates bone regeneration and suppresses inflammation. These results establish cell membrane-coated microribbon scaffolds as a promising strategy for treating critical size bone defects via immunomodulation. The platform may be broadly used with different cell membranes and scaffolds to enhance regeneration of multiple tissue types.

Promoting lacunar bone regeneration with an injectable hydrogel adaptive to the microenvironment

Injectable hydrogel is suitable for the repair of lacunar bone deficiency. This study fabricated an injectable, self-adaptive silk fibroin/mesoporous bioglass/sodium alginate (SMS) composite hydrogel system. With controllable and adjustable physical and chemical properties, the SMS hydrogel could be easily optimized adaptively to different clinical applications. The SMS hydrogel effectively showed great injectability and shapeability, allowing defect filling with no gap. Moreover, the SMS hydrogel displayed self-adaptability in mechanical reinforcement and degradation, responsive to the concentration of Ca²⁺ and inflammatory-like pH value in the microenvironment of bone deficiency, respectively. In vitro biological studies indicated that SMS hydrogel could promote osteogenic differentiation of bone marrow mesenchymal stem cells by activation of the MAPK signaling pathway. The SMS hydrogel also could improve migration and tube formation of human umbilical vein endothelial cells. Investigations of the crosstalk between osteoblasts and macrophages confirmed that SMS hydrogel could regulate macrophage polarization from M1 to M2, which could create a specific favorable environment to induce new bone formation and angiogenesis. Meanwhile, SMS hydrogel was proved to be antibacterial, especially for gram-negative bacteria. Furthermore, in vivo study indicated that SMS could be easily applied for maxillary sinus elevation, inducing sufficient new bone formation. Thus, it is convincing that SMS hydrogel could be potent in a simple, minimally invasive and efficient treatment for the repair of lacunar bone deficiency.

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Mesoporous bioglass was used as the crosslinking agent and in-situ porogen to form a porous injectable hydrogel.

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The composite hydrogel had suitable injectability and self-adaptability for lacunar bone regeneration.

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The composite hydrogel can simultaneously regulate macrophage polarization and osteogenic differentiation.

Polarization Behavior of Bone Macrophage as Well as Associated Osteoimmunity in Glucocorticoid-Induced Osteonecrosis of the Femoral Head

Glucocorticoid-induced osteonecrosis of the femoral head (GIONFH) is a disabling disease with high mortality in China but the detailed molecular and cellular mechanisms remain to be investigated. Macrophages are considered the key cells in osteoimmunology, and the cross-talk between bone macrophages and other cells in the microenvironment is involved in maintaining bone homeostasis. M1 polarized macrophages launch a chronic inflammatory response and secrete a broad spectrum of cytokines (eg, TNF-α, IL-6 and IL-1β) and chemokines to initiate a chronic inflammatory state in GIONFH. M2 macrophage is the alternatively activated anti-inflammatory type distributed mainly in the perivascular area of the necrotic femoral head. In the development of GIONFH, injured bone vascular endothelial cells and necrotic bone activate the TLR4/NF-κB signal pathway, promote dimerization of PKM2 and subsequently enhance the production of HIF-1, inducing metabolic transformation of macrophage to the M1 phenotype. Considering these findings, putative interventions by local chemokine regulation to correct the imbalance between M1/M2 polarized macrophages by switching macrophages to an M2 phenotype, or inhibiting the adoption of an M1 phenotype appear to be plausible regimens for preventing or intervening GIONFH in the early stage. However, these results were mainly obtained by in vitro tissue or experimental animal model. Further studies to completely elucidate the alterations of the M1/M2 macrophage polarization and functions of macrophages in glucocorticoid-induced osteonecrosis of the femoral head are imperative.

Polysaccharides for Medical Technology: Properties and Applications

Over the past decade, the use of polysaccharides has gained tremendous attention in the field of medical technology. They have been applied in various sectors such as tissue engineering, drug delivery system, face mask, and bio-sensing. This review article provides an overview and background of polysaccharides for biomedical uses. Different types of polysaccharides, for example, cellulose and its derivatives, chitin and chitosan, hyaluronic acid, alginate, and pectin are presented. They are fabricated in various forms such as hydrogels, nanoparticles, membranes, and as porous mediums. Successful development and improvement of polysaccharide-based materials will effectively help users to enhance their quality of personal health, decrease cost, and eventually increase the quality of life with respect to sustainability.

Standard in vitro evaluations of engineered bone substitutes are not sufficient to predict in vivo preclinical model outcomes

Understanding the optimal conditions required for bone healing can have a substantial impact to target the problem of non-unions and large bone defects. The combination of bioactive factors, regenerative progenitor cells and biomaterials to form a tissue engineered (TE) complex is a promising solution but translation to the clinic has been slow. We hypothesized the typical material testing algorithm used is insufficient and leads to materials being mischaracterized as promising. In the first part of this study, human bone marrow - derived mesenchymal stromal cells (hBM-MSCs) were embedded in three commonly used biomaterials (hyaluronic acid methacrylate, gelatin methacrylate and fibrin) and combined with relevant bioactive osteogenesis factors (dexamethasone microparticles and polyphosphate nanoparticles) to form a TE construct that underwent in vitro osteogenic differentiation for 28 days. Gene expression of relevant transcription factors and osteogenic markers, and von Kossa staining were performed. In the second and third part of this study, the same combination of TE constructs were implanted subcutaneously (cell containing) in T cell-deficient athymic Crl:NIH-Foxn1^rnu rats for 8 weeks or cell free in an immunocompetent New Zealand white rabbit calvarial model for 6 weeks, respectively. Osteogenic performance was investigated via MicroCT imaging and histology staining. The in vitro study showed enhanced upregulation of relevant genes and significant mineral deposition within the three biomaterials, generally considered as a positive result. Subcutaneous implantation indicates none to minor ectopic bone formation. No enhanced calvarial bone healing was detected in implanted biomaterials compared to the empty defect. The reasons for the poor correlation of in vitro and in vivo outcomes are unclear and needs further investigation. This study highlights the discrepancy between in vitro and in vivo outcomes, demonstrating that in vitro data should be interpreted with extreme caution. In vitro models with higher complexity are necessary to increase value for translational studies. STATEMENT OF SIGNIFICANCE: Preclinical testing of newly developed biomaterials is a crucial element of the development cycle. Despite this, there is still significant discrepancy between in vitro and in vivo test results. Within this study we investigate multiple combinations of materials and osteogenic stimulants and demonstrate a poor correlation between the in vitro and in vivo data. We propose rationale for why this may be the case and suggest a modified testing algorithm.

Application of Nano-Inspired Scaffolds-Based Biopolymer Hydrogel for Bone and Periodontal Tissue Regeneration

This review's objectives are to provide an overview of the various kinds of biopolymer hydrogels that are currently used for bone tissue and periodontal tissue regeneration, to list the advantages and disadvantages of using them, to assess how well they might be used for nanoscale fabrication and biofunctionalization, and to describe their production processes and processes for functionalization with active biomolecules. They are applied in conjunction with other materials (such as microparticles (MPs) and nanoparticles (NPs)) and other novel techniques to replicate physiological bone generation more faithfully. Enhancing the biocompatibility of hydrogels created from blends of natural and synthetic biopolymers can result in the creation of the best scaffold match to the extracellular matrix (ECM) for bone and periodontal tissue regeneration. Additionally, adding various nanoparticles can increase the scaffold hydrogel stability and provide a number of biological effects. In this review, the research study of polysaccharide hydrogel as a scaffold will be critical in creating valuable materials for effective bone tissue regeneration, with a future impact predicted in repairing bone defects.

Immunomodulatory strategies for bone regeneration: A review from the perspective of disease types

Tissue engineering strategies for treating bone loss to date have largely focused on targeting stem cells or vascularization. Immune cells, including macrophages and T cells, can also indirectly enhance bone healing via cytokine secretion to interact with other bone niche cells. Bone niche cues and local immune environment vary depending on anatomical location, size of defects and disease types. As such, it is critical to evaluate the role of the immune system in the context of specific bone niche and different disease types. This review focuses on immunomodulation research for bone applications using biomaterials and cell-based strategies, with a unique perspective from different disease types. We first reviewed applications for prolonging orthopaedic implant lifetime and enhancing fracture healing, two clinical challenges where immunomodulatory strategies were initially developed for orthopedic applications. We then reviewed recent research progress in harnessing immunomodulatory strategies for regenerating critical-sized, long bone or cranial bone defects, and treating osteolytic bone diseases. Remaining gaps in knowledge, future directions and opportunities were also discussed.

A Review of Recent Advances in Natural Polymer-Based Scaffolds for Musculoskeletal Tissue Engineering

The musculoskeletal (MS) system consists of bone, cartilage, tendon, ligament, and skeletal muscle, which forms the basic framework of the human body. This system plays a vital role in appropriate body functions, including movement, the protection of internal organs, support, hematopoiesis, and postural stability. Therefore, it is understandable that the damage or loss of MS tissues significantly reduces the quality of life and limits mobility. Tissue engineering and its applications in the healthcare industry have been rapidly growing over the past few decades. Tissue engineering has made significant contributions toward developing new therapeutic strategies for the treatment of MS defects and relevant disease. Among various biomaterials used for tissue engineering, natural polymers offer superior properties that promote optimal cell interaction and desired biological function. Natural polymers have similarity with the native ECM, including enzymatic degradation, bio-resorb and non-toxic degradation products, ability to conjugate with various agents, and high chemical versatility, biocompatibility, and bioactivity that promote optimal cell interaction and desired biological functions. This review summarizes recent advances in applying natural-based scaffolds for musculoskeletal tissue engineering.

Bone Tissue Engineering Using Osteogenic Cells: From the Bench to the Clinical Application

The use of tissue engineering to restore and to build new bone tissue is under active research at present. The following review summarizes the latest studies and clinical trials related to the use of osteogenic cells, biomaterials, and scaffolds to regenerate bone defects in the human jaws. Bone tissue engineering (BTE) combined with scaffolds have provided a range of advantages not only to transport the target cells to their desired destination but also to support the early phases of the mineralization process. The mechanical, chemical, and physical properties of scaffolds have been evaluated as they affect the quantity of bone regeneration, particularly in the oral cavity. This review also highlighted the mechanisms underlying bone homeostasis, including the key transcription factors and signaling pathways responsible for regulating the differentiation of osteoblast lineage. Furthering understanding of the mechanisms of cellular signaling in skeletal remodeling with the use of mesenchymal stem cells and the proper scaffold properties are key-factors to enable the incorporation of new and effective treatment methods into clinical practice for bone tissue regeneration using BTE. Impact Statement The use of mesenchymal stem cells able to differentiate in osteoblast lineage for bone tissue engineering (BTE) remains a major challenge. Viable cells and signaling pathways play an essential role in bone repair and regeneration of critical size defects. Recent advances in scaffolds and biological factors such as growth factors (e.g., cytokines and hormones) controlling the osteogenic signaling cascade are now becoming new players affecting the osteogenic potential of cells. Such techniques will significantly impact the maxillofacial bone tissue replacement, repair, and regeneration for patients without having to rely on donor banks or other surgical sites.

Biohybrid Variable-Stiffness Soft Actuators that Self-Create Bone

Inspired by the dynamic process of initial bone development, in which a soft tissue turns into a solid load-bearing structure, the fabrication, optimization, and characterization of bioinduced variable-stiffness actuators that can morph in various shapes and change their properties from soft to rigid are hereby presented. Bilayer devices are prepared by combining the electromechanically active properties of polypyrrole with the compliant behavior of alginate gels that are uniquely functionalized with cell-derived plasma membrane nanofragments (PMNFs), previously shown to mineralize within 2 days, which promotes the mineralization in the gel layer to achieve the soft to stiff change by growing their own bone. The mineralized actuator shows an evident frozen state compared to the movement before mineralization. Next, patterned devices show programmed directional and fixated morphing. These variable-stiffness devices can wrap around and, after the PMNF-induced mineralization in and on the gel layer, adhere and integrate onto bone tissue. The developed biohybrid variable-stiffness actuators can be used in soft (micro-)robotics and as potential tools for bone repair or bone tissue engineering.

Ectopic Bone Tissue Engineering in Mice Using Human Gingiva or Bone Marrow-Derived Stromal/Progenitor Cells in Scaffold-Hydrogel Constructs

Three-dimensional (3D) spheroid culture can promote the osteogenic differentiation and bone regeneration capacity of mesenchymal stromal cells (MSC). Gingiva-derived progenitor cells (GPC) represent a less invasive alternative to bone marrow MSC (BMSC) for clinical applications. The aim of this study was to test the in vivo bone forming potential of human GPC and BMSC cultured as 3D spheroids or dissociated cells (2D). 2D and 3D cells encapsulated in constructs of human platelet lysate hydrogels (HPLG) and 3D-printed poly (L-lactide-co-trimethylene carbonate) scaffolds (HPLG-PLATMC) were implanted subcutaneously in nude mice; cell-free HPLG-PLATMC constructs served as a control. Mineralization was assessed using micro-computed tomography (µCT), histology, scanning electron microscopy (SEM) and in situ hybridization (ISH). After 4–8 weeks, µCT revealed greater mineralization in 3D-BMSC vs. 2D-BMSC and 3D-GPC (p < 0.05), and a similar trend in 2D-GPC vs. 2D-BMSC (p > 0.05). After 8 weeks, greater mineralization was observed in cell-free constructs vs. all 2D- and 3D-cell groups (p < 0.05). Histology and SEM revealed an irregular but similar mineralization pattern in all groups. ISH revealed similar numbers of 2D and 3D BMSC/GPC within and/or surrounding the mineralized areas. In summary, spheroid culture promoted ectopic mineralization in constructs of BMSC, while constructs of dissociated GPC and BMSC performed similarly. The combination of HPLG and PLATMC represents a promising scaffold for bone tissue engineering applications.

Hydrogel-Assisted 3D Model to Investigate the Osteoinductive Potential of MC3T3-Derived Extracellular Vesicles

Effective and rapid regeneration of bone defects often pose substantial challenges in severe accidental injuries and disabilities occurring due to diseases and/or advanced age, especially in patients having reduced tissue regeneration competence. The success of mesenchymal stromal cell (MSC)-based research strategies in improving bone regeneration was hampered not only due to the limited knowledge of therapeutic actions of MSCs but also due to difficulties as well as expenses related to cell manufacturing and time taken for ethical approvals for clinical use of living cells and engineered tissues. The recent trend indicated that there is a shift from the direct usage of MSCs toward the application of paracrine factors and extracellular vesicles (EVs) isolated from their MSC secretome in bone tissue regeneration. This shift has directed research into the development of "cell-free" therapeutics, which could be a better alternative due to its several advantages over the use of their parental MSCs. Furthermore, accumulating evidence suggested that the 3D microenvironment influences the paracrine effects of MSCs. Although the osteogenic role of EVs has been explored recently, the current study showed, for the first time, that encapsulation of EVs along with MC3T3 cells in a 3D hydrogel-assisted culture with a distinct porous microenvironment having meso and macro (0.05-200 μm) pore size distribution resulted in an improved osteogenic response in vitro. The present work was primarily focused on investigating the influence of EVs isolated under distinct priming conditions to enhance the osteogenic potential. In addition, in the current work, the osteogenic ability of different types of EVs (microvesicles and exosomes) and total EVs isolated at different time points was also examined when encapsulated with MC3T3 cells in an alginate gel. Using various biochemical assays, such as alkaline phosphatase (ALP) production and calcium secretion, it was observed that both microvesicles and exosomes collected from MC3T3 cells independently had osteogenic potential; however, their collective activity was found to be superior. We further showed that EVs induce an early osteogenic response in MC3T3 cells as indicated by ALP and calcium secretion at a much earlier time point, compared to the controls. Our data suggested that this 3D hydrogel-assisted system provides close proximity of cells and EVs, and thus, mimics the in vivo scenario, making it clinically useful for bone tissue engineering.

3D Encapsulation and tethering of functionally engineered extracellular vesicles to hydrogels

Mesenchymal stem cell (MSC) derived extracellular vesicles (EVs) in their naïve and engineered forms have emerged as potential alternatives to stem cell therapy. While they have a defined therapeutic potential, the spatial and temporal control of their activity in vivo remains a challenge. The objective of this study was to devise a methodology to encapsulate EVs in 3D hydrogels for prolonged delivery. To achieve this, we have leveraged the MSC EV interactions with ECM proteins and their derivative peptides. Using osteoinductive functionally engineered EVs (FEEs) derived from MSCs, we show that FEEs bind to mimetic peptides from collagen (DGEA, GFPGER) and fibronectin (RGD). In in vitro experiments, photocrosslinkable alginate hydrogels containing RGD were able to encapsulate, tether and retain the FEEs over a period of 7 days while maintaining the structural integrity and osteoinductive functionality of the EVs. When employed in a calvarial defect model in vivo, alginate-RGD hydrogels containing the FEEs enhanced bone regeneration by a factor of 4 compared to controls lacking FEEs and by a factor of 2 compared to controls lacking the tethering peptide. These results show that EVs can be tethered to biomaterials to promote bone repair and the importance of prolonged delivery in vivo. Results also provide a prelude to the possible use of this technology for controlled delivery of EVs for other regenerative medicine applications. STATEMENT OF SIGNIFICANCE: The beneficial effects of human MSC (HMSC) therapy are attributable to paracrine effects of the HMSC derived EVs. While EV engineering has the potential to impact several fields of regenerative medicine, targeted delivery of the engineered EVs with spatial and temporal control is necessary to prevent off-target effects and enhance tissue specificity. Here, we have leveraged the interactions of MSC EVs with ECM proteins to develop a tethering system that can be utilized to prolong EV delivery in vivo while maintaining the structural and functional integrity of the EVs. Our work has provided a tunable platform for EV delivery that we envision can be formulated as an injectable material or a bulk hydrogel suitable for regenerative medicine applications.

Polysaccharide-Based Biomaterials in Tissue Engineering: A Review

In addition to proteins and nucleic acids, polysaccharides are an important type of biomacromolecule widely distributed in plants, animals, and microorganisms. Polysaccharides are considered as promising biomaterials due to their significant bioactivities, natural abundance, immunoactivity, and chemical modifiability for tissue engineering (TE) applications. Due to the similarities of the biochemical properties of polysaccharides and the extracellular matrix of human bodies, polysaccharides are increasingly recognized and accepted. Furthermore, the degradation behavior of these macromolecules is generally nontoxic. Certain delicate properties, such as remarkable mechanical properties and tunable tissue response, can be obtained by modifying the functional groups on the surface of polysaccharide molecules. The applications of polysaccharide-based biomaterials in the TE field have been growing intensively in recent decades, for example, bone/cartilage regeneration, cardiac regeneration, neural regeneration, and skin regeneration. This review summarizes the main essential properties of polysaccharides, including their chemical properties, crosslinking mechanisms, and biological properties, and focuses on the association between their structures and properties. The recent progress in polysaccharide-based biomaterials in various TE applications is reviewed, and the prospects for future studies are addressed as well. We intend this review to offer a comprehensive understanding of and inspiration for the research and development of polysaccharide-based materials in TE.

A Bibliometric Analysis of the Global Trend of Using Alginate, Gelatine, and Hydroxyapatite for Bone Tissue Regeneration Applications

Collecting information from previous investigations and expressing it in a scientometrics study can be a priceless guide to getting a complete overview of a specific research area. The aim of this study is to explore the interrelated connection between alginate, gelatine, and hydroxyapatite within the scope of bone tissue and scaffold. A review of traditional literature with data mining procedures using bibliometric analyses was considered to identify the evolution of the selected research area between 2009 and 2019. Bibliometric methods and knowledge visualization technologies were implemented to investigate diverse publications based on the following indicators: year of publication, document type, language, country, institution, author, journal, keyword, and number of citations. An analysis using a bibliometric study found that 7446 papers were located with the keywords "bone tissue" and "scaffold", and 1767 (alginate), 185 (gelatine), 5658 (hydroxyapatite) papers with those specific sub keywords. The number of publications that relate to "tissue engineering" and bone more than doubled between 2009 (1352) and 2019 (2839). China, the United States and India are the most productive countries, while Sichuan University and the Chinese Academy of Science from China are the most important institutions related to bone tissue scaffold. Materials Science and Engineering C is the most productive journal, followed by the Journal of Biomedical Materials Research Part A. This paper is a starting point, providing the first bibliometric analysis study of bone tissue and scaffold considering alginate, gelatine and hydroxyapatite. A bibliometric analysis would greatly assist in giving a scientific insight to support desired future research work, not only associated with bone tissue engineering applications. It is expected that the analysis of alginate, gelatine and hydroxyapatite in terms of 3D bioprinting, clinical outcomes, scaffold architecture, and the regenerative medicine approach will enhance the research into bone tissue engineering in the near future. Continued studies into these research fields are highly recommended.

Rational design of hydrogels to enhance osteogenic potential

Bone tissue engineering (BTE) encompasses the field of biomaterials, cells, and bioactive molecules to successfully guide the growth and repair of bone tissue. Current BTE strategies rely on delivering osteogenic molecules or cells via scaffolding materials. However, growth factor- and stem cell-based treatments have several limitations, such as source restriction, low stability, difficulties in predicting long-term efficacy, and high costs, among others. These issues have promoted the development of material-based therapy with properties of accessibility, high stability, tunable efficacy, and low-cost production. Hydrogels are widely used in BTE applications because of their unique hydrophilic nature and tunable physicochemical properties to mimic the native bone environment. However, current hydrogel materials are not ideal candidates due to minimal osteogenic capability on their own. Therefore, recent studies of BTE hydrogels attempt to counterbalance these issues by modifying their biophysical properties. In this article, we review recent progress in the design of hydrogels to instruct osteogenic potential, and present strategies developed to precisely control its bone healing properties.