Effects of controlled dual growth factor delivery on bone regeneration following composite bone-muscle injury

Fabrication of polymeric microspheres for biomedical applications

Polymeric microspheres (PMs) have attracted great attention in the field of biomedicine in the last several decades due to their small particle size, special functionalities shown on the surface and high surface-to-volume ratio. However, how to fabricate PMs which can meet the clinical needs and transform laboratory achievements to industrial scale-up still remains a challenge. Therefore, advanced fabrication technologies are pursued. In this review, we summarize the technologies used to fabricate PMs, including emulsion-based methods, microfluidics, spray drying, coacervation, supercritical fluid and superhydrophobic surface-mediated method and their advantages and disadvantages. We also review the different structures, properties and functions of the PMs and their applications in the fields of drug delivery, cell encapsulation and expansion, scaffolds in tissue engineering, transcatheter arterial embolization and artificial cells. Moreover, we discuss existing challenges and future perspectives for advancing fabrication technologies and biomedical applications of PMs.

Electrospun nanofibers synthesized from polymers incorporated with bioactive compounds for wound healing

The development of innovative wound dressing materials is crucial for effective wound care. It's an active area of research driven by a better understanding of chronic wound pathogenesis. Addressing wound care properly is a clinical challenge, but there is a growing demand for advancements in this field. The synergy of medicinal plants and nanotechnology offers a promising approach to expedite the healing process for both acute and chronic wounds by facilitating the appropriate progression through various healing phases. Metal nanoparticles play an increasingly pivotal role in promoting efficient wound healing and preventing secondary bacterial infections. Their small size and high surface area facilitate enhanced biological interaction and penetration at the wound site. Specifically designed for topical drug delivery, these nanoparticles enable the sustained release of therapeutic molecules, such as growth factors and antibiotics. This targeted approach ensures optimal cell-to-cell interactions, proliferation, and vascularization, fostering effective and controlled wound healing. Nanoscale scaffolds have significant attention due to their attractive properties, including delivery capacity, high porosity and high surface area. They mimic the Extracellular matrix (ECM) and hence biocompatible. In response to the alarming rise of antibiotic-resistant, biohybrid nanofibrous wound dressings are gradually replacing conventional antibiotic delivery systems. This emerging class of wound dressings comprises biopolymeric nanofibers with inherent antibacterial properties, nature-derived compounds, and biofunctional agents. Nanotechnology, diminutive nanomaterials, nanoscaffolds, nanofibers, and biomaterials are harnessed for targeted drug delivery aimed at wound healing. This review article discusses the effects of nanofibrous scaffolds loaded with nanoparticles on wound healing, including biological (in vivo and in vitro) and mechanical outcomes.

Advances and Trends of Photoresponsive Hydrogels for Bone Tissue Engineering

The development of static hydrogels as an optimal choice for bone tissue engineering (BTE) remains a difficult challenge primarily due to the intricate nature of bone healing processes, continuous physiological functions, and pathological changes. Hence, there is an urgent need to exploit smart hydrogels with programmable properties that can effectively enhance bone regeneration. Increasing evidence suggests that photoresponsive hydrogels are promising bioscaffolds for BTE due to their advantages such as controlled drug release, cell fate modulation, and the photothermal effect. Here, we review the current advances in photoresponsive hydrogels. The mechanism of photoresponsiveness and its advanced applications in bone repair are also elucidated. Future research would focus on the development of more efficient, safer, and smarter photoresponsive hydrogels for BTE. This review is aimed at offering comprehensive guidance on the trends of photoresponsive hydrogels and shedding light on their potential clinical application in BTE.

Leveraging the Recent Advancements in GelMA Scaffolds for Bone Tissue Engineering: An Assessment of Challenges and Opportunities

The field of bone tissue engineering has seen significant advancements in recent years. Each year, over two million bone transplants are performed globally, and conventional treatments, such as bone grafts and metallic implants, have their limitations. Tissue engineering offers a new level of treatment, allowing for the creation of living tissue within a biomaterial framework. Recent advances in biomaterials have provided innovative approaches to rebuilding bone tissue function after damage. Among them, gelatin methacryloyl (GelMA) hydrogel is emerging as a promising biomaterial for supporting cell proliferation and tissue regeneration, and GelMA has exhibited exceptional physicochemical and biological properties, making it a viable option for clinical translation. Various methods and classes of additives have been used in the application of GelMA for bone regeneration, with the incorporation of nanofillers or other polymers enhancing its resilience and functional performance. Despite promising results, the fabrication of complex structures that mimic the bone architecture and the provision of balanced physical properties for both cell and vasculature growth and proper stiffness for load bearing remain as challenges. In terms of utilizing osteogenic additives, the priority should be on versatile components that promote angiogenesis and osteogenesis while reinforcing the structure for bone tissue engineering applications. This review focuses on recent efforts and advantages of GelMA-based composite biomaterials for bone tissue engineering, covering the literature from the last five years.

Nanosized Silk-Magnesium Complexes for Promotion of Angiogenic and Osteogenic Activities

Injectable hydrogels with osteogenic and angiogenetic properties are of interest in bone tissue engineering. Since the bioactivity of ions is concentration-dependent, nanosized silk-magnesium (Mg) complexes were previously developed and assembled into hydrogels with angiogenic capabilities but failed to control both osteogenic and angiogenetic activities effectively. Here, nanosized silk particles with different sizes were obtained by using ultrasonic treatment to control silk-Mg coordination and particle formation, resulting in silk-Mg hydrogels with different types of bioactivity. Fourier transform infrared and X-ray diffraction results revealed that different coordination intensities were present in the different complexes as a basis for the differences in activities. Slow Mg ion release was controlled by these nanosized silk-Mg complexes through degradation. With the same amount of Mg ions, the different silk-Mg complexes exhibited different angiogenic and osteogenic capacities. Complexes with both angiogenic and osteogenic capacities were developed by optimizing the sizes of the silk particles, resulting in faster and improved quality of bone formed in vivo than complexes with the same composition of silk and Mg but only angiogenic or osteogenic capacities. The biological selectivity of silk-Mg complexes should facilitate applications in tissue regeneration.

Hydrogel-Based Growth Factor Delivery Platforms: Strategies and Recent Advances

Growth factors play a crucial role in regulating a broad variety of biological processes and are regarded as powerful therapeutic agents in tissue engineering and regenerative medicine in the past decades. However, their application is limited by their short half-lives and potential side effects in physiological environments. Hydrogels are identified as having the promising potential to prolong the half-lives of growth factors and mitigate their adverse effects by restricting them within the matrix to reduce their rapid proteolysis, burst release, and unwanted diffusion. This review discusses recent progress in the development of growth factor-containing hydrogels for various biomedical applications, including wound healing, brain tissue repair, cartilage and bone regeneration, and spinal cord injury repair. In addition, the review introduces strategies for optimizing growth factor release including affinity-based delivery, carrier-assisted delivery, stimuli-responsive delivery, spatial structure-based delivery, and cellular system-based delivery. Finally, the review presents current limitations and future research directions for growth factor-delivering hydrogels.

The critical impact of traumatic muscle loss on fracture healing: Basic science and clinical aspects

Musculoskeletal trauma, specifically fractures, is a leading cause of patient morbidity and disability worldwide. In approximately 20% of cases with fracture and related traumatic muscle loss, bone healing is impaired leading to fracture nonunion. Over the past few years, several studies have demonstrated that bone and the surrounding muscle tissue interact not only anatomically and mechanically but also through biochemical pathways and mediators. Severe damage to the surrounding musculature at the fracture site causes an insufficiency in muscle-derived osteoprogenitor cells that are crucial for fracture healing. As an endocrine tissue, skeletal muscle produces many myokines that act on different bone cells, such as osteoblasts, osteoclasts, osteocytes, and mesenchymal stem cells. Investigating how muscle influences fracture healing at cellular, molecular, and hormonal levels provides translational therapeutic solutions to this clinical challenge. This review provides an overview about the contributions of surrounding muscle tissue in directing fracture healing. The focus of the review is on describing the interactions between bone and muscle in both healthy and fractured environments. We discuss current progress in identifying the bone-muscle molecular pathways and strategies to harness these pathways as cues for accelerating fracture healing. In addition, we review the existing challenges and research opportunities in the field.

[Advances in Molecular Regulatory Mechanisms of Jaw Repair and Reconstruction]

Jawbone injuries resulting from trauma, diseases, and surgical resections are commonly seen in clinical practice, necessitating precise and effective strategies for repair and reconstruction to restore both function and aesthetics. The precise and effective repair and the reconstruction of jawbone injuries pose a significant challenge in the field of oral and maxillofacial surgery, owing to the unique biomechanical characteristics and physiological functions of the jawbone. The natural repair process following jawbone injuries involves stages such as hematoma formation, inflammatory response, ossification, and bone remodeling. Bone morphogenetic proteins (BMPs), transforming growth factor beta (TGF-β), vascular endothelial growth factor (VEGF), and other growth factors play crucial roles in promoting jawbone regeneration. Cytokines such as interleukins and tumor necrosis factor play dual roles in regulating inflammatory response and bone repair. In recent years, significant progress in molecular biology research has been made in the field of jawbone repair and reconstruction. Tissue engineering technologies, including stem cell therapy, bioactive scaffolds, and growth factor delivery systems, have found important applications in jawbone repair. However, the intricate molecular regulatory mechanisms involved in the complex jawbone repair and reconstruction methods are not fully understood and still require further research. Future research directions will be focused on the precise control of these molecular processes and the development of more efficient combination therapeutic strategies to promote the effective and functional reconstruction of the jawbone. This review aims to examine the latest findings on the molecular regulatory mechanisms of the repair and reconstruction of jawbone injuries and the therapeutic strategies. The conclusions drawn in this article provide a molecular-level understanding of the repair of jawbone injuries and highlight potential directions for future research.

Piezoelectrically and Topographically Engineered Scaffolds for Accelerating Bone Regeneration

Bone regeneration remains a critical concern across diverse medical disciplines, because it is a complex process that requires a combinatorial approach involving the integration of mechanical, electrical, and biological stimuli to emulate the native cellular microenvironment. In this context, piezoelectric scaffolds have attracted considerable interest owing to their remarkable ability to generate electric fields in response to dynamic forces. Nonetheless, the application of such scaffolds in bone tissue engineering has been limited by the lack of a scaffold that can simultaneously provide both the intricate electromechanical environment and the biocompatibility of the native bone tissue. Here, we present a pioneering biomimetic scaffold that combines the unique properties of piezoelectric and topographical enhancement with the inherent osteogenic abilities of hydroxyapatite (HAp). Notably, the novelty of this work lies in the incorporation of HAp into polyvinylidene fluoride-co-trifluoro ethylene in a freestanding form, leveraging its natural osteogenic potential within a piezoelectric framework. Through comprehensive in vitro and in vivo investigations, we demonstrate the remarkable potential of these scaffolds to accelerate bone regeneration. Moreover, we demonstrate and propose three pivotal mechanisms─(i) electrical, (ii) topographical, and (iii) paracrine─that collectively contribute to the facilitated bone healing process. Our findings present a synergistically derived biomimetic scaffold design with wide-ranging prospects for bone regeneration as well as various regenerative medicine applications.

Biomedical applications of engineered heparin-based materials

Heparin is a negatively charged polysaccharide with various chain lengths and a hydrophilic backbone. Due to its fascinating chemical and physical properties, nontoxicity, biocompatibility, and biodegradability, heparin has been extensively used in different fields of medicine, such as cardiovascular and hematology. This review highlights recent and future advancements in designing materials based on heparin for various biomedical applications. The physicochemical and mechanical properties, biocompatibility, toxicity, and biodegradability of heparin are discussed. In addition, the applications of heparin-based materials in various biomedical fields, such as drug/gene delivery, tissue engineering, cancer therapy, and biosensors, are reviewed. Finally, challenges, opportunities, and future perspectives in preparing heparin-based materials are summarized.

Human nasal mucosa ectomesenchymal stem cells derived extracellular vesicles loaded omentum/chitosan composite scaffolds enhance skull defects regeneration

Engineered bone tissue that can promote osteogenic differentiation is considered an ideal substitute for materials to heal bone defects. Extracellular vesicle (EV)-based cell-free regenerative therapies represent an emerging promising alternative for bone tissue engineering. We hypothesized that EVs derived from human nasal mucosa-derived ectomesenchymal stem cells (hEMSCs) can promote bone tissue regeneration. Herein, hEMSCs were cultured with osteogenic induction medium or normal medium to generate two types of EVs. We first demonstrated that the two EVs exhibited strong potential to promote rat suture mesenchymal stem cell (SMSC) osteogenesis by transferring TG2 to SMSCs and regulating extracellular matrix (ECM) synthesis. Next, we developed a composite hydrogel made of porcine omentum and chitosan into which EVs were adsorbed to enable the effective delivery of EVs with sustained release kinetics. Implantation of the EV-loaded hydrogels in a critical-size rat cranial defect model significantly promoted bone regeneration. Therefore, we suggest that our hEMSC-derived EV-loading system can serve as a new therapeutic paradigm for promoting bone tissue regeneration in the clinic.

Sequential deacetylation/self-gelling chitin hydrogels and scaffolds functionalized with fucoidan for enhanced BMP-2 loading and sustained release

Bone morphogenetic protein 2 (BMP-2) is a potent osteoinductive factor that promotes bone formation. A major obstacle to the clinical application of BMP-2 is its inherent instability and complications caused by its rapid release from implants. Chitin based materials have excellent biocompatibility and mechanical properties, making them ideal for bone tissue engineering applications. In this study, a simple and easy method was developed to spontaneously form deacetylated β-chitin (DAC-β-chitin) gels at room temperature through a sequential deacetylation/self-gelation process. The structural transformation of β-chitin to DAC-β-chitin leads to the formation of self-gelling DAC-β-chitin, from which hydrogels and scaffolds were prepared. Gelatin (GLT) accelerated the self-gelation of DAC-β-chitin and increased the pore size and porosity of the DAC-β-chitin scaffold. The DAC-β-chitin scaffolds were then functionalized with a BMP-2-binding sulfate polysaccharide, fucoidan (FD). Compared with β-chitin scaffolds, FD-functionalized DAC-β-chitin scaffolds showed higher BMP-2 loading capacity and more sustainable release of BMP-2, and thus had better osteogenic activity for bone regeneration.

Reaming the intramedullary canal during tibial nailing does not affect in vivo intramuscular pH of the anterior tibialis

Many investigations have evaluated local and systemic consequences of intramedullary (IM) reaming and suggest that reaming may cause, or exacerbate, injury to the soft tissues adjacent to fractures. To date, no study has examined the effect on local muscular physiology as measured by intramuscular pH (IpH). Here, we observe in vivo IpH during IM reaming for tibia fractures.

Methods

Adults with acute tibia shaft fractures (level 1, academic, 2019-2021) were offered enrollment in an observational cohort. During IM nailing, a sterile, validated IpH probe was placed into the anterior tibialis (<5 cm from fracture, continuous sampling, independent research team). IpH before, during, and after reaming was averaged and compared through repeated measures ANOVA. As the appropriate period to analyze IpH during reaming is unknown, the analysis was repeated over periods of 0.5, 1, 2, 5, 10, and 15 minutes prereaming and postreaming time intervals.

Results

Sixteen subjects with tibia shaft fractures were observed during nailing. Average time from injury to surgery was 35.0 hours (SD, 31.8). Starting and ending perioperative IpH was acidic, averaging 6.64 (SD, 0.21) and 6.74 (SD, 0.17), respectively. Average reaming time lasted 15 minutes. Average IpH during reaming was 6.73 (SD, 0.15). There was no difference in IpH between prereaming, intrareaming, and postreaming periods. IpH did not differ regardless of analysis over short or long time domains compared with the duration of reaming.

Conclusions

Reaming does not affect IpH. Both granular and broad time domains were tested, revealing no observable local impact.

Hydrogel Drug Delivery Systems for Bone Regeneration

With the in-depth understanding of bone regeneration mechanisms and the development of bone tissue engineering, a variety of scaffold carrier materials with desirable physicochemical properties and biological functions have recently emerged in the field of bone regeneration. Hydrogels are being increasingly used in the field of bone regeneration and tissue engineering because of their biocompatibility, unique swelling properties, and relative ease of fabrication. Hydrogel drug delivery systems comprise cells, cytokines, an extracellular matrix, and small molecule nucleotides, which have different properties depending on their chemical or physical cross-linking. Additionally, hydrogels can be designed for different types of drug delivery for specific applications. In this paper, we summarize recent research in the field of bone regeneration using hydrogels as delivery carriers, detail the application of hydrogels in bone defect diseases and their mechanisms, and discuss future research directions of hydrogel drug delivery systems in bone tissue engineering.

An intelligent phase transformation system based on lyotropic liquid crystals for sequential biomolecule delivery to enhance bone regeneration

Endogenous repair of critical bone defects is typically hampered by inadequate vascularization in the early stages and insufficient bone regeneration later on. Therefore, drug delivery systems with the ability to couple angiogenesis and osteogenesis in a spatiotemporal manner are highly desirable for vascularized bone formation. Herein, we devoted to develop a liquid crystal formulation system (LCFS) attaining a controlled temporal release of angiogenic and osteoinductive bioactive molecules that could orchestrate the coupling of angiogenesis and osteogenesis in an optimal way. It has been demonstrated that the release kinetics of biomolecules depend on the hydrophobicity of the loaded molecules, making the delivery profile programmable and controllable. The hydrophilic deferoxamine (DFO) could be released rapidly within 5 days to activate angiogenic signaling, while the lipophilic simvastatin (SIM) showed a slow and sustained release for continuous osteogenic induction. Apart from its good biocompatibility with mesenchymal stem cells derived from rat bone marrow (rBMSCs), the DFO/SIM loaded LCFS could stimulate the formation of a vascular morphology in human umbilical vein endothelial cells (HUVECs) and the osteogenic differentiation of rBMSCs in vitro. The in vivo rat femoral defect models have witnessed the prominent angiogenic and osteogenic effects induced by the sequential presentation of DFO and SIM. This study suggests that the sequential release of DFO and SIM from the LCFS results in enhanced bone formation, offering a facile and viable treatment option for bone defects by mimicking the physiological process of bone regeneration.

Multiscale design of stiffening and ROS scavenging hydrogels for the augmentation of mandibular bone regeneration

Although biomimetic hydrogels play an essential role in guiding bone remodeling, reconstructing large bone defects is still a significant challenge since bioinspired gels often lack osteoconductive capacity, robust mechanical properties and suitable antioxidant ability for bone regeneration. To address these challenges, we first engineered molecular design of hydrogels (gelatin/polyethylene glycol diacrylate/2-(dimethylamino)ethyl methacrylate, GPEGD), where their mechanical properties were significantly enhanced via introducing trace amounts of additives (0.5 wt%). The novel hybrid hydrogels show high compressive strength (>700 kPa), stiff modulus (>170 kPa) and strong ROS-scavenging ability. Furthermore, to endow the GPEGD hydrogels excellent osteoinductions, novel biocompatible, antioxidant and BMP-2 loaded polydopamine/heparin nanoparticles (BPDAH) were developed for functionalization of the GPEGD gels (BPDAH-GPEGD). In vitro results indicate that the antioxidant BPDAH-GPEGD is able to deplete elevated ROS levels to protect cells viability against ROS damage. More importantly, the BPDAH-GPEGD hydrogels have good biocompatibility and promote the osteo differentiation of preosteoblasts and bone regenerations. At 4 and 8 weeks after implantation of the hydrogels in a mandibular bone defect, Micro-computed tomography and histology results show greater bone volume and enhancements in the quality and rate of bone regeneration in the BPDAH-GPEGD hydrogels. Thus, the multiscale design of stiffening and ROS scavenging hydrogels could serve as a promising material for bone regeneration applications.

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Trace additives of DMAEMA markedly enhanced the mechanical performances of the gelatin-based hydrogels through molecular induced multiple crosslinking structures.

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Molecular design strategy combined with bioactive nanocomposites have a synergistically effects on promoting ROS scavenging ability and osteoactivity of the biomimetic hydrogels.

Nanoparticles based composite coatings with tunable vascular endothelial growth factor and bone morphogenetic protein-2 release for bone regeneration

Bone healing is a complex cascade involving precisely coordinated spatiotemporal presentation of multiple growth factors (GFs), including osteogenic and angiogenic GFs, and each stage of bone healing requires varying types and content of GFs. In this study, we fabricated a composite nanocoating with tunable vascular endothelial growth factor (VEGF) and bone morphogenetic protein-2 (BMP-2) that was coated on the surface of a polydopamine (PDA)-decorated tertiary calcium phosphate (TCP) scaffold using VEGF-loaded chitosan/bovine serum albumin nanoparticles (CS/BSA-NPs) and BMP-2-loaded poly-L-lysine/oxidized alginate nanoparticles (PLL/OALG-NPs). It was found that VEGF could be efficiently released to promote vascularization in early bone repair stages due to the rapid biodegradation of CS/BSA-NPs, while bone formation can be promoted by a sustained release of BMP-2 from the slowly degrading PLL/OALG-NPs. The composite coating and TCP scaffold can be conjugated due to the excellent adhesive property of PDA. The composite coating can achieve the rapid release of VEGF and sustained release of BMP-2, which can activate GFs for accelerating bone healing.

Application of bioactive metal ions in the treatment of bone defects

The treatment of bone defects is an important problem in clinical practice. The rapid development of bone tissue engineering (BTE) may provide a new method for bone defect treatment. Metal ions have been widely studied in BTE and demonstrated a significant effect in promoting bone tissue growth. Different metal ions can be used to treat bone defects according to specific conditions, including promoting osteogenic activity, inhibiting osteoclast activity, promoting vascular growth, and exerting certain antibacterial effects. Multiple studies have confirmed that metal ions-modified composite scaffolds can effectively promote bone defect healing. By studying current extensive research on metal ions in the treatment of bone defects, this paper reviews the mechanism of metal ions in promoting bone tissue growth, analyzes the loading mode of metal ions, and lists some specific applications of metal ions in different types of bone defects. Finally, this paper summarizes the advantages and disadvantages of metal ions and analyzes the future research trend of metal ions in BTE. This article can provide some new strategies and methods for future research and applications of metal ions in the treatment of bone defects.

Functionalized multidimensional biomaterials for bone microenvironment engineering applications: Focus on osteoimmunomodulation

As bone biology develops, it is gradually recognized that bone regeneration is a pathophysiological process that requires the simultaneous participation of multiple systems. With the introduction of osteoimmunology, the interplay between the immune system and the musculoskeletal diseases has been the conceptual framework for a thorough understanding of both systems and the advancement of osteoimmunomodulaty biomaterials. Various therapeutic strategies which include intervention of the surface characteristics or the local delivery systems with the incorporation of bioactive molecules have been applied to create an ideal bone microenvironment for bone tissue regeneration. Our review systematically summarized the current research that is being undertaken in the field of osteoimmunomodulaty bone biomaterials on a case-by-case basis, aiming to inspire more extensive research and promote clinical conversion.

Multifunctional hydrogel enhances bone regeneration through sustained release of Stromal Cell-Derived Factor-1α and exosomes

Fracture nonunion remains a great challenge for orthopedic surgeons. Fracture repair comprises of three phases, the inflammatory, repair and remodeling stage. Extensive advancements have been made in the field of bone repair, including development of strategies to balance the M1/M2 macrophage populations, and to improve osteogenesis and angiogenesis. However, such developments focused on only one or the latter two phases, while ignoring the inflammatory phase during which cell recruitment occurs. In this study, we combined Stromal Cell-Derived Factor-1α (SDF-1α) and M2 macrophage derived exosomes (M2D-Exos) with a hyaluronic acid (HA)-based hydrogel precursor solution to synthesize an injectable, self-healing, adhesive HA@SDF-1α/M2D-Exos hydrogel. The HA hydrogel demonstrated good biocompatibility and hemostatic ability, with the 4% HA hydrogels displaying great antibacterial activity against gram-negative E. coli and gram-positive S. aureus and Methicillin-resistant Staphylococcus aureus (MRSA). Synchronously and sustainably released SDF-1α and M2D-Exos from the HA@SDF-1α/M2D-Exos hydrogel enhanced proliferation and migration of human bone marrow mesenchymal stem cell (HMSCs) and Human Umbilical Vein Endothelial Cells (HUVECs), promoting osteogenesis and angiogenesis both in vivo and in vitro. Overall, the developed HA@ SDF-1α/M2D-Exos hydrogel was compatible with the natural healing process of fractures and provides a new modality for accelerating bone repair by coupling osteogenesis, angiogenesis, and resisting infection at all stages.

Application of BMP in Bone Tissue Engineering

At present, bone nonunion and delayed union are still difficult problems in orthopaedics. Since the discovery of bone morphogenetic protein (BMP), it has been widely used in various studies due to its powerful role in promoting osteogenesis and chondrogenesis. Current results show that BMPs can promote healing of bone defects and reduce the occurrence of complications. However, the mechanism of BMP in vivo still needs to be explored, and application of BMP alone to a bone defect site cannot achieve good therapeutic effects. It is particularly important to modify implants to carry BMP to achieve slow and sustained release effects by taking advantage of the nature of the implant. This review aims to explain the mechanism of BMP action in vivo, its biological function, and how BMP can be applied to orthopaedic implants to effectively stimulate bone healing in the long term. Notably, implantation of a system that allows sustained release of BMP can provide an effective method to treat bone nonunion and delayed bone healing in the clinic.

Strategies for Enhancing Vascularization of Biomaterial-Based Scaffold in Bone Regeneration

Despite the recent advances in reconstructive orthopedics; fracture union is a challenge to bone regeneration. Concurrent angiogenesis is a complex process governed by events, delicately entwined with osteogenesis. However, poorly perfused scaffolds have lower success rates; necessitating the need for a better vascular component, which is important for the delivery of nutrients, oxygen, waste elimination, recruitment of cells for optimal bone repair. This review highlights the latest strategies to promote biomaterial-based scaffold vascularization by incorporation of cells, growth factors, inorganic ions, etc. into natural or synthetic polymers, ceramic materials, or composites of organic and inorganic compounds. Furthermore, it emphasizes structural modifications, biophysical stimuli, and natural molecules to fabricate scaffolds aiding the genesis of dense vascularization following their implantation to manifest a compatible regenerative microenvironment without graft rejection.

Proteoglycans and proteoglycan mimetics for tissue engineering

Proteoglycans play a crucial role in proper tissue morphology and function throughout the body that is defined by a combination of their core protein and the attached glycosaminoglycan chains. Although they serve a myriad of roles, the functions of extracellular proteoglycans can be generally sorted into four categories: modulation of tissue mechanical properties, regulation and protection of the extracellular matrix, sequestering of proteins, and regulation of cell signaling. The loss of proteoglycans can result in significant tissue disfunction, ranging from poor mechanical properties to uncontrolled inflammation. Because of the key roles they play in proper tissue function and due to their complex synthesis, the past two decades have seen significant research into the development of proteoglycan mimetic molecules to recapitulate the function of proteoglycans for therapeutic and tissue engineering applications. These strategies have ranged from semisynthetic graft copolymers to recombinant proteoglycan domains synthesized by genetically engineered cells. In this review, we highlight some of the important functions of extracellular proteoglycans, as well as the strategies developed to recapitulate these functions.

Growth Factor Immobilization Strategies for Musculoskeletal Disorders

PURPOSE OF REVIEW

Tissue regenerative solutions for musculoskeletal disorders have become increasingly important with a growing aged population. Current growth factor treatments often require high dosages with the potential for off-target effects. Growth factor immobilization strategies offer approaches towards alleviating these concerns. This review summarizes current growth factor immobilization techniques (encapsulation, affinity interactions, and covalent binding) and the effects of immobilization on growth factor loading, release, and bioactivity.

RECENT FINDINGS

The breadth of immobilization techniques based on encapsulation, affinity, and covalent binding offer multiple methods to improve the therapeutic efficacy of growth factors by controlling bioactivity and release. Growth factor immobilization strategies have evolved to more complex systems with the capacity to load and release multiple growth factors with spatiotemporal control. The advancements in immobilization strategies allow for development of new, complex musculoskeletal tissue treatment strategies with improved spatiotemporal control of loading, release, and bioactivity.

Local Application of Mineral-Coated Microparticles Loaded With VEGF and BMP-2 Induces the Healing of Murine Atrophic Non-Unions

Deficient angiogenesis and disturbed osteogenesis are key factors for the development of nonunions. Mineral-coated microparticles (MCM) represent a sophisticated carrier system for the delivery of vascular endothelial growth factor (VEGF) and bone morphogenetic protein (BMP)-2. In this study, we investigated whether a combination of VEGF- and BMP-2-loaded MCM (MCM + VB) with a ratio of 1:2 improves bone repair in non-unions. For this purpose, we applied MCM + VB or unloaded MCM in a murine non-union model and studied the process of bone healing by means of radiological, biomechanical, histomorphometric, immunohistochemical and Western blot techniques after 14 and 70 days. MCM-free non-unions served as controls. Bone defects treated with MCM + VB exhibited osseous bridging, an improved biomechanical stiffness, an increased bone volume within the callus including ongoing mineralization, increased vascularization, and a histologically larger total periosteal callus area consisting predominantly of osseous tissue when compared to defects of the other groups. Western blot analyses on day 14 revealed a higher expression of osteoprotegerin (OPG) and vice versa reduced expression of receptor activator of NF-κB ligand (RANKL) in bone defects treated with MCM + VB. On day 70, these defects exhibited an increased expression of erythropoietin (EPO), EPO-receptor and BMP-4. These findings indicate that the use of MCM for spatiotemporal controlled delivery of VEGF and BMP-2 shows great potential to improve bone healing in atrophic non-unions by promoting angiogenesis and osteogenesis as well as reducing early osteoclast activity.

Emerging zero-dimensional to four-dimensional biomaterials for bone regeneration

Bone is one of the most sophisticated and dynamic tissues in the human body, and is characterized by its remarkable potential for regeneration. In most cases, bone has the capacity to be restored to its original form with homeostatic functionality after injury without any remaining scarring. Throughout the fascinating processes of bone regeneration, a plethora of cell lineages and signaling molecules, together with the extracellular matrix, are precisely regulated at multiple length and time scales. However, conditions, such as delayed unions (or nonunion) and critical-sized bone defects, represent thorny challenges for orthopedic surgeons. During recent decades, a variety of novel biomaterials have been designed to mimic the organic and inorganic structure of the bone microenvironment, which have tremendously promoted and accelerated bone healing throughout different stages of bone regeneration. Advances in tissue engineering endowed bone scaffolds with phenomenal osteoconductivity, osteoinductivity, vascularization and neurotization effects as well as alluring properties, such as antibacterial effects. According to the dimensional structure and functional mechanism, these biomaterials are categorized as zero-dimensional, one-dimensional, two-dimensional, three-dimensional, and four-dimensional biomaterials. In this review, we comprehensively summarized the astounding advances in emerging biomaterials for bone regeneration by categorizing them as zero-dimensional to four-dimensional biomaterials, which were further elucidated by typical examples. Hopefully, this review will provide some inspiration for the future design of biomaterials for bone tissue engineering.

A 3D-printed bioactive polycaprolactone scaffold assembled with core/shell microspheres as a sustained BMP2-releasing system for bone repair

Integration of biological factors and hierarchical rigid scaffolds is of great interest in bone tissue engineering for fabrication of biomimetic constructs with high physical and biological performance for enhanced bone repair. Core/shell microspheres (CSMs) delivering bone morphogenetic protein-2 (BMP-2) and a strategy to integrate CSMs with 3D-printed scaffolds were developed herein to form a hybrid 3D system for bone repair. The scaffold was printed with polycaprolactone (PCL) and then coated with polydopamine. Shells of CSMs were electrosprayed with alginate. Cores were heparin-coated polylactic acid (PLA) microparticles fabricated via simple emulsion and heparin coating strategy. Assembly of microspheres and scaffolds was realized via a self-locking method with the assistance of controlled expansion of CSMs. The hybrid system was evaluated in the rat critical-sized bone defect model. CSMs released BMP-2 in a tunable manner and boosted osteogenic performance in vitro. CSMs were then successfully integrated inside the scaffolds. The assembled system effectively promoted osteogenesis in vitro and in vivo. These observations show the importance of how BMP-2 is delivered, and the core/shell microspheres represent effective BMP-2 carriers that could be integrated into scaffolds, together forming a hybrid system as a promising candidate for enhanced bone regeneration.

Spatiotemporal regulation of angiogenesis/osteogenesis emulating natural bone healing cascade for vascularized bone formation

Engineering approaches for growth factor delivery have been considerably advanced for tissue regeneration, yet most of them fail to provide a complex combination of signals emulating a natural healing cascade, which substantially limits their clinical successes. Herein, we aimed to emulate the natural bone healing cascades by coupling the processes of angiogenesis and osteogenesis with a hybrid dual growth factor delivery system to achieve vascularized bone formation. Basic fibroblast growth factor (bFGF) was loaded into methacrylate gelatin (GelMA) to mimic angiogenic signalling during the inflammation and soft callus phases of the bone healing process, while bone morphogenetic protein-2 (BMP-2) was bound onto mineral coated microparticles (MCM) to mimics osteogenic signalling in the hard callus and bone remodelling phases. An Initial high concentration of bFGF accompanied by a sustainable release of BMP-2 and inorganic ions was realized to orchestrate well-coupled osteogenic and angiogenic effects for bone regeneration. In vitro experiments indicated that the hybrid hydrogel markedly enhanced the formation of vasculature in human umbilical vein endothelial cells (HUVECs), as well as the osteogenic differentiation of mesenchymal stem cells (BMSCs). In vivo results confirmed the optimal osteogenic performance of our F/G-B/M hydrogel, which was primarily attributed to the FGF-induced vascularization. This research presents a facile and potent alternative for treating bone defects by emulating natural cascades of bone healing.

Effects of a novel self-assembling peptide scaffold on bone regeneration and controlled release of two growth factors

RADA16 is a self-assembling peptide material with good bioactivity. To improve the bioactivity of a material, some specific functional motifs can be added to its peptide sequence. Here, we report a self-assembling peptide nanogel, RADA16-RGD, that has better bioactivity than RADA16 and can simultaneously carry and control the release of two growth factors, VEGF and BMP-2, which have synergistic effects on bone formation. The peptide materials were characterized by transmission electron microscopy and scanning electron microscopy. The mechanical properties of the peptides were evaluated by the rheology test. The biocompatibility of the materials was evaluated via the use of the CCK-8 test, live/dead staining and confocal laser scanning microscopy. Osteogenesis capability in vitro was evaluated by means of ALP staining, extracellular matrix mineralization and detection of osteogenic markers. The controlled release of growth factors was examined by ELISA. The results showed that RADA16-RGD exhibited a better ability than RADA16 to promote cell proliferation, adhesion and bone formation. In addition, RADA16-RGD had good biocompatibility and exhibited effective controlled release of VEGF and BMP-2. More importantly, compared with RADA16-RGD loaded with single growth factor or without growth factors, RADA16-RGD loaded with two growth factors exhibited a stronger ability to promote cell proliferation and osteogenesis. This study provides a promising strategy for the application of self-assembling peptides to promote osteogenesis and controlled release of proteins.

Nanoscale mineralization of cell-laden methacrylated gelatin hydrogels using calcium carbonate-calcium citrate core-shell microparticles

Conventional biomaterials developed for bone regeneration fail to fully recapitulate the nanoscale structural organization and complex composition of the native bone microenvironment. Therefore, despite promoting osteogenic differentiation of stem cells, they fall short of providing the structural, biochemical, and mechanical stimuli necessary to drive osteogenesis for bone regeneration and function. To address this, we have recently developed a novel strategy to engineer bone-like tissue using a biomimetic approach to achieve rapid and controlled nanoscale mineralization of a cell-laden matrix in the presence of osteopontin, a non-collagenous protein, and a supersaturated solution of calcium and phosphate medium. Here, we build on this approach to engineer bone regeneration scaffolds comprising methacrylated gelatin (GelMA) hydrogels incorporated with calcium citrate core-shell microparticles as a sustained and reliable source of calcium ions for in situ mineralization. We demonstrate successful biomineralization of GelMA hydrogels by embedded calcium carbonate-calcium citrate core-shell microparticles with the resultant mineral chemistry, structure, and organization reminiscent of that of native bone. The biomimetic mineralization was further shown to promote osteogenic differentiation of encapsulated human mesenchymal stem cells even in the absence of other exogenous osteogenic induction factors. Ultimately, by combining the superior biological response engendered by biomimetic mineralization with the intrinsic tissue engineering advantages offered by GelMA, such as biocompatibility, biodegradability, and printability, we envision that our system offers great potential for bone regeneration efforts.

The Construction of Multi-Incorporated Polylactic Composite Nanofibrous Scaffold for the Potential Applications in Bone Tissue Regeneration

To improve the bone regeneration ability of pure polymer, varieties of bioactive components were incorporated to a biomolecular scaffold with different structures. In this study, polysilsesquioxane (POSS), pearl powder and dexamethasone loaded porous carbon nanofibers (DEX@PCNFs) were incorporated into polylactic (PLA) nanofibrous scaffold via electrospinning for the application of bone tissue regeneration. The morphology observation showed that the nanofibers were well formed through electrospinning process. The mineralization test of incubation in simulated body fluid (SBF) revealed that POSS incorporated scaffold obtained faster hydroxyapatite depositing ability than pristine PLA nanofibers. Importantly, benefitting from the bioactive components of pearl powder like bone morphogenetic protein (BMP), bone mesenchymal stem cells (BMSCs) cultured on the composite scaffold presented higher proliferation rate. In addition, by further incorporating with DEX@PCNFs, the alkaline phosphatase (ALP) level and calcium deposition were a little higher based on pearl powder. Consequently, the novel POSS, pearl powder and DEX@PCNFs multi-incorporated PLA nanofibrous scaffold can provide better ability to enhance the biocompatibility and accelerate osteogenic differentiation of BMSCs, which has potential applications in bone tissue regeneration.

Jawbones Scaffold Constructed by TGF-β1 and BMP-2 Loaded Chitosan Microsphere Combining with Alg/HA/ICol for Osteogenic-Induced Differentiation

Bone scaffolds based on multi-components are the leading trend to address the multifaceted prerequisites to repair various bone defects. Chitosan is the most useable biopolymer, having excellent biological applications. Therefore, in the present study, the chitosan microsphere was prepared by the ion-gel method; transforming growth factor β (TGF-β1) and bone morphogenetic protein 2 (BMP-2) were loaded onto it and then combined with alginate/hyaluronic acid/collagen (Alg/HA/ICol) to construct a jawbones scaffold. The Alg/HA/ICol scaffolds were characterized by FTIR and SEM, and the water content, porosity, tensile properties, biocompatibility, and osteogenic-induced differentiation ability of the Alg/HA/ICol jawbones scaffolds were studied. The results indicate that a three-dimensional porous jawbone scaffold was successfully constructed having 100-250 μm of pore size and >90% of porosity without cytotoxicity against adipose-derived stem cells (ADSCs). Its ALP quantification, osteocalcin expression, and Von Kossamineralized nodule staining was higher than the control group. The jawbones scaffold constructed by TGF-β1 and BMP-2 loaded chitosan microsphere combining with Alg/HA/ICol has potential biomedical application in the future.

Triple growth factor delivery promotes functional bone regeneration following composite musculoskeletal trauma

Successful bone healing in severe trauma depends on early revascularization to restore oxygen, nutrient, growth factor, and progenitor cell supply to the injury. Therapeutic angiogenesis strategies have therefore been investigated to promote revascularization following severe bone injuries; however, results have been inconsistent. This is the first study investigating the effects of dual angiogenic growth factors (VEGF and PDGF) with low-dose bone morphogenetic protein-2 (BMP-2; 2.5 µg) on bone healing in a clinically challenging composite bone-muscle injury model. Our hydrogel-based delivery systems demonstrated a more than 90% protein entrapment efficiency and a controlled simultaneous release of three growth factors over 28 days. Co-stimulation of microvascular fragment constructs with VEGF and PDGF promoted vascular network formation in vitro compared to VEGF or PDGF alone. In an in vivo model of segmental bone and volumetric muscle loss injury, combined VEGF (5 µg) and PDGF (7.5 µg or 15 µg) delivery with a low dose of BMP-2 significantly enhanced regeneration of vascularized bone compared to BMP-2 treatment alone. Notably, the regenerated bone mechanics reached ~60% of intact bone, a value that was previously only achieved by delivery of high-dose BMP-2 (10 µg) in this injury model. Overall, sustained delivery of VEGF, PDFG, and BMP-2 is a promising strategy to promote functional vascularized bone tissue regeneration following severe composite musculoskeletal injury. Although this study is conducted in a clinically relevant composite injury model in rats using a simultaneous release strategy, future studies are necessary to test the regenerative potential of spatiotemporally controlled delivery of triple growth factors on bone healing using large animal models. STATEMENT OF SIGNIFICANCE: Volumetric muscle loss combined with delayed union or non-union bone defect causes deleterious effects on bone regeneration even with the supplementation of bone morphogenetic protein-2 (BMP-2). In this study, the controlled delivery of dual angiogenic growth factors (vascular endothelial growth factor [VEGF] + Platelet-derived growth factor [PDGF]) increases vascular growth in vitro. Co-delivering VEGF+PDGF significantly increase the bone formation efficacy of low-dose BMP-2 and improves the mechanics of regenerated bone in a challenging composite bone-muscle injury model.

Biomaterials for protein delivery for complex tissue healing responses

Tissue repair requires a complex cascade of events mediated by a variety of cells, proteins, and matrix molecules; however, the healing cascade can be easily disrupted by numerous factors, resulting in impaired tissue regeneration. Recent advances in biomaterials for tissue regeneration have increased the ability to tailor the delivery of proteins and other biomolecules to injury sites to restore normal healing cascades and stimulate robust tissue repair. In this review, we discuss the evolution of the field toward creating biomaterials that precisely control protein delivery to stimulate tissue regeneration, with a focus on addressing complex and dynamic injury environments. We highlight biomaterials that leverage different mechanisms to deliver and present proteins involved in healing cascades, tissue targeting and mimicking strategies, materials that can be triggered by environmental cues, and integrated strategies that combine multiple biomaterial properties to improve protein delivery. Improvements in biomaterial design to address complex injury environments will expand our understanding of both normal and aberrant tissue repair processes and ultimately provide a better standard of patient care.

Prevascularized hydrogels with mature vascular networks promote the regeneration of critical-size calvarial bone defects in vivo

Adequate vascularization of scaffolds is a prerequisite for successful repair and regeneration of lost and damaged tissues. It has been suggested that the maturity of engineered vascular capillaries, which is largely determined by the presence of functional perivascular mural cells (or pericytes), plays a vital role in maintaining vessel integrity during tissue repair and regeneration. Here, we investigated the role of pericyte-supported-engineered capillaries in regenerating bone in a critical-size rat calvarial defect model. Prior to implantation, human umbilical vein endothelial cells and human bone marrow stromal cells (hBMSCs) were cocultured in a collagen hydrogel to induce endothelial cell morphogenesis into microcapillaries and hBMSC differentiation into pericytes. Upon implantation into the calvarial bone defects (8 mm), the prevascularized hydrogels showed better bone formation than either untreated controls or defects treated with autologous bone grafts (positive control). Bone formation parameters such as bone volume, coverage area, and vascularity were significantly better in the prevascularized hydrogel group than in the autologous bone group. Our results demonstrate that tissue constructs engineered with pericyte-supported vascular capillaries may approximate the regenerative capacity of autologous bone, despite the absence of osteoinductive or vasculogenic growth factors.

Sequential dual-drug delivery of BMP-2 and alendronate from hydroxyapatite-collagen scaffolds for enhanced bone regeneration

The clinical use of bioactive molecules in bone regeneration has been known to have side effects, which result from uncontrolled and supraphysiological doses. In this study, we demonstrated the synergistic effect of two bioactive molecules, bone morphogenic protein-2 (BMP-2) and alendronate (ALN), by releasing them in a sequential manner. Collagen-hydroxyapatite composite scaffolds functionalized using BMP-2 are loaded with biodegradable microspheres where ALN is encapsulated. The results indicate an initial release of BMP-2 for a few days, followed by the sequential release of ALN after two weeks. The composite scaffolds significantly increase osteogenic activity owing to the synergistic effect of BMP-2 and ALN. Enhanced bone regeneration was identified at eight weeks post-implantation in the rat 8-mm critical-sized defect. Our findings suggest that the sequential delivery of BMP-2 and ALN from the scaffolds results in a synergistic effect on bone regeneration, which is unprecedented. Therefore, such a system exhibits potential for the application of cell-free tissue engineering.