Vascularization Strategies in Bone Tissue Engineering

Calcium Phosphates: A Key to Next-Generation In Vitro Bone Modeling

The replication of bone physiology under laboratory conditions is a prime target behind the development of in vitro bone models. The model should be robust enough to elicit an unbiased response when stimulated experimentally, giving reproducible outcomes. In vitro bone tissue generation majorly requires the availability of cellular components, the presence of factors promoting cellular proliferation and differentiation, efficient nutrient supply, and a supporting matrix for the cells to anchor - gaining predefined topology. Calcium phosphates (CaP) are difficult to ignore while considering the above requirements of a bone model. Therefore, the current review focuses on the role of CaP in developing an in vitro bone model addressing the prerequisites of bone tissue generation. Special emphasis is given to the physico-chemical properties of CaP that promote osteogenesis, angiogenesis and provide sufficient mechanical strength for load-bearing applications. Finally, the future course of action is discussed to ensure efficient utilization of CaP in the in vitro bone model development field.

Advancing bone regeneration: Unveiling the potential of 3D cell models in the evaluation of bone regenerative materials

Bone, a rigid yet regenerative tissue, has garnered extensive attention for its impressive healing abilities. Despite advancements in understanding bone repair and creating treatments for bone injuries, handling nonunions and large defects remains a major challenge in orthopedics. The rise of bone regenerative materials is transforming the approach to bone repair, offering innovative solutions for nonunions and significant defects, and thus reshaping orthopedic care. Evaluating these materials effectively is key to advancing bone tissue regeneration, especially in difficult healing scenarios, making it a critical research area. Traditional evaluation methods, including two-dimensional cell models and animal models, have limitations in predicting accurately. This has led to exploring alternative methods, like 3D cell models, which provide fresh perspectives for assessing bone materials' regenerative potential. This paper discusses various techniques for constructing 3D cell models, their pros and cons, and crucial factors to consider when using these models to evaluate bone regenerative materials. We also highlight the significance of 3D cell models in the in vitro assessments of these materials, discuss their current drawbacks and limitations, and suggest future research directions. STATEMENT OF SIGNIFICANCE: This work addresses the challenge of evaluating bone regenerative materials (BRMs) crucial for bone tissue engineering. It explores the emerging role of 3D cell models as superior alternatives to traditional methods for assessing these materials. By dissecting the construction, key factors of evaluating, advantages, limitations, and practical considerations of 3D cell models, the paper elucidates their significance in overcoming current evaluation method shortcomings. It highlights how these models offer a more physiologically relevant and ethically preferable platform for the precise assessment of BRMs. This contribution is particularly significant for "Acta Biomaterialia" readership, as it not only synthesizes current knowledge but also propels the discourse forward in the search for advanced solutions in bone tissue engineering and regeneration.

Generating human bone marrow organoids for disease modeling and drug discovery

The bone marrow supports and regulates hematopoiesis, responding to physiological requirements for blood cell production over ontogeny and during pathological challenges. Interactions between hematopoietic cells and niche components are challenging to study mechanistically in the human context, but are important to delineate in order to explore the pathobiology of blood and bone marrow disorders. Organoids are proving transformative in many research settings, but an accurate human bone marrow model incorporating multiple hematopoietic and stromal elements has been lacking. This protocol describes a method to generate three-dimensional, multilineage bone marrow organoids from human induced pluripotent stem cells (hiPSCs), detailing the steps for the directed differentiation of hiPSCs using a series of cytokine cocktails and hydrogel embedding. Over 18 days of differentiation, hiPSCs yield the key lineages that are present in central myelopoietic bone marrow, organized in a well-vascularized architecture that resembles native hematopoietic tissues. This presents a robust, in vitro system that can model healthy and perturbed hematopoiesis in a scalable three-dimensional microenvironment. Bone marrow organoids also support the growth of immortalized cell lines and primary cells from healthy donors and patients with myeloid and lymphoid cancers, including cell types that are poorly viable in standard culture systems. Moreover, we discuss assays for the characterization of organoids, including interrogation of pathogenic remodeling using recombinant TGF-ß treatment, and methods for organoid engraftment with exogenous cells. This protocol can be readily adapted to specific experimental requirements, can be easily implemented by users with tissue culture experience and does not require access to specialist equipment.

Efforts to promote osteogenesis-angiogenesis coupling for bone tissue engineering

Repair of bone defects exceeding a critical size has been always a big challenge in clinical practice. Tissue engineering has exhibited great potential to effectively repair the defects with less adverse effect than traditional bone grafts, during which how to induce vascularized bone formation has been recognized as a critical issue. Therefore, recently many studies have been launched to attempt to promote osteogenesis-angiogenesis coupling. This review summarized comprehensively and explored in depth current efforts to ameliorate the coupling of osteogenesis and angiogenesis from four aspects, namely the optimization of scaffold components, modification of scaffold structures, loading strategies for bioactive substances, and employment tricks for appropriate cells. Especially, the advantages and the possible reasons for every strategy, as well as the challenges, were elaborated. Furthermore, some promising research directions were proposed based on an in-depth analysis of the current research. This paper will hopefully spark new ideas and approaches for more efficiently boosting new vascularized bone formations.

Towards Stem Cell Therapy for Critical-Sized Segmental Bone Defects: Current Trends and Challenges on the Path to Clinical Translation

The management and reconstruction of critical-sized segmental bone defects remain a major clinical challenge for orthopaedic clinicians and surgeons. In particular, regenerative medicine approaches that involve incorporating stem cells within tissue engineering scaffolds have great promise for fracture management. This narrative review focuses on the primary components of bone tissue engineering-stem cells, scaffolds, the microenvironment, and vascularisation-addressing current advances and translational and regulatory challenges in the current landscape of stem cell therapy for critical-sized bone defects. To comprehensively explore this research area and offer insights for future treatment options in orthopaedic surgery, we have examined the latest developments and advancements in bone tissue engineering, focusing on those of clinical relevance in recent years. Finally, we present a forward-looking perspective on using stem cells in bone tissue engineering for critical-sized segmental bone defects.

Construction of Magnesium Phosphate Chemical Conversion Coatings with Different Microstructures on Titanium to Enhance Osteogenesis and Angiogenesis

Titanium (Ti) and its alloys are widely used as hard tissue substitutes in dentistry and orthopedics, but their low bioactivity leads to undesirable osseointegration defects in the early osteogenic phase. Surface modification is an important approach to overcome these problems. In the present study, novel magnesium phosphate (MgP) coatings with controllable structures were fabricated on the surface of Ti using the phosphate chemical conversion (PCC) method. The effects of the microstructure on the physicochemical and biological properties of the coatings on Ti were researched. The results indicated that accelerators in PCC solution were important factors affecting the microstructure and properties of the MgP coatings. In addition, the coated Ti exhibited excellent hydrophilicity, high bonding strength, and good corrosion resistance. Moreover, the biological results showed that the MgP coatings could improve the spread, proliferation, and osteogenic differentiation of mouse osteoblast cells (MC3T3-E1) and vascular differentiation of human umbilical vein endothelial cells (HUVECs), indicating that the coated Ti samples had a great effect on promoting osteogenesis and angiogenesis. Overall, this study provided a new research idea for the surface modification of conventional Ti to enhance osteogenesis and angiogenesis in different bone types for potential biomedical applications.

Application of Deferoxamine in Tissue Regeneration Attributed to Promoted Angiogenesis

Deferoxamine, an iron chelator used to treat diseases caused by excess iron, has had a Food and Drug Administration-approved status for many years. A large number of studies have confirmed that deferoxamine can reduce inflammatory response and promote angiogenesis. Blood vessels play a crucial role in sustaining vital life by facilitating the delivery of immune cells, oxygen, and nutrients, as well as eliminating waste products generated during cellular metabolism. Dysfunction in blood vessels may contribute significantly to the development of life-threatening diseases. Anti-angiogenesis therapy and pro-angiogenesis/angiogenesis strategies have been frequently recommended for various diseases. Herein, we describe the mechanism by which deferoxamine promotes angiogenesis and summarize its application in chronic wounds, bone repair, and diseases of the respiratory system. Furthermore, we discuss the drug delivery system of deferoxamine for treating various diseases, providing constructive ideas and inspiration for the development of new treatment strategies.

Enhancing osteogenesis and angiogenesis functions for Ti-24Nb-4Zr-8Sn scaffolds with methacrylated gelatin and deferoxamine

Repair of large bone defects remains challenge for orthopedic clinical treatment. Porous titanium alloys have been widely fabricated by the additive manufacturing, which possess the elastic modulus close to that of human cortical bone, good osteoconductivity and osteointegration. However, insufficient bone regeneration and vascularization inside the porous titanium scaffolds severely limit their capability for repair of large-size bone defects. Therefore, it is crucially important to improve the osteogenic function and vascularization of the titanium scaffolds. Herein, methacrylated gelatin (GelMA) were incorporated with the porous Ti-24Nb-4Zr-8Sn (Ti2448) scaffolds prepared by the electron beam melting (EBM) method (Ti2448-GelMA). Besides, the deferoxamine (DFO) as an angiogenic agent was doped into the Ti2448-GelMA scaffold (Ti2448-GelMA/DFO), in order to promote vascularization. The results indicate that GelMA can fully infiltrate into the pores of Ti2448 scaffolds with porous cross-linked network (average pore size: 120.2 ± 25.1 μm). Ti2448-GelMA scaffolds facilitated the differentiation of MC3T3-E1 cells by promoting the ALP expression and mineralization, with the amount of calcium contents ∼2.5 times at day 14, compared with the Ti2448 scaffolds. Impressively, the number of vascular meshes for the Ti2448-GelMA/DFO group (∼7.2/mm2) was significantly higher than the control group (∼5.3/mm2) after cultivation for 9 h, demonstrating the excellent angiogenesis ability. The Ti2448-GelMA/DFO scaffolds also exhibited sustained release of DFO, with a cumulative release of 82.3% after 28 days. Therefore, Ti2448-GelMA/DFO scaffolds likely provide a new strategy to improve the osteogenesis and angiogenesis for repair of large bone defects.

Multifunctional MXene-Based Platforms for Soft and Bone Tissue Regeneration and Engineering

MXenes and their composites hold great promise in the field of soft and bone tissue regeneration and engineering (TRE). However, there are challenges that need to be overcome, such as ensuring biocompatibility and controlling the morphologies of MXene-based scaffolds. The future prospects of MXenes in TRE include enhancing biocompatibility through surface modifications, developing multifunctional constructs, and conducting in vivo studies for clinical translation. The purpose of this perspective about MXenes and their composites in soft and bone TRE is to critically evaluate their potential applications and contributions in this field. This perspective aims to provide a comprehensive analysis of the challenges, advantages, limitations, and future prospects associated with the use of MXenes and their composites for soft and bone TRE. By examining the existing literature and research, the review seeks to consolidate the current knowledge and highlight the key findings and advancements in MXene-based TRE. It aims to contribute to the understanding of MXenes' role in promoting soft and bone TRE, addressing the challenges faced in terms of biocompatibility, morphology control, and tissue interactions.

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.

Zinc-energized dynamic hydrogel accelerates bone regeneration via potentiating the coupling of angiogenesis and osteogenesis

Insufficient initial vascularization plays a pivotal role in the ineffectiveness of bone biomaterials for treating bone defects. Consequently, enhancing the angiogenic properties of bone repair biomaterials holds immense importance in augmenting the efficacy of bone regeneration. In this context, we have successfully engineered a composite hydrogel capable of promoting vascularization in the process of bone regeneration. To achieve this, the researchers first prepared an aminated bioactive glass containing zinc ions (AZnBg), and hyaluronic acid contains aldehyde groups (HA-CHO). The composite hydrogel was formed by combining AZnBg with gelatin methacryloyl (GelMA) and HA-CHO through Schiff base bonding. This composite hydrogel has good biocompatibility. In addition, the composite hydrogel exhibited significant osteoinductive activity, promoting the activity of ALP, the formation of calcium nodules, and the expression of osteogenic genes. Notably, the hydrogel also promoted umbilical vein endothelial cell migration as well as tube formation by releasing zinc ions. The results of in vivo study demonstrated that implantation of the composite hydrogel in the bone defect of the distal femur of rats could effectively stimulate bone generation and the development of new blood vessels, thus accelerating the bone healing process. In conclusion, the combining zinc-containing bioactive glass with hydrogels can effectively promote bone growth and angiogenesis, making it a viable option for the repair of critical-sized bone defects.

Integrating bioprinting, cell therapies and drug delivery towards in vivo regeneration of cartilage, bone and osteochondral tissue

The biological and biomechanical functions of cartilage, bone and osteochondral tissue are naturally orchestrated by a complex crosstalk between zonally dependent cells and extracellular matrix components. In fact, this crosstalk involves biomechanical signals and the release of biochemical cues that direct cell fate and regulate tissue morphogenesis and remodelling in vivo. Three-dimensional bioprinting introduced a paradigm shift in tissue engineering and regenerative medicine, since it allows to mimic native tissue anisotropy introducing compositional and architectural gradients. Moreover, the growing synergy between bioprinting and drug delivery may enable to replicate cell/extracellular matrix reciprocity and dynamics by the careful control of the spatial and temporal patterning of bioactive cues. Although significant advances have been made in this direction, unmet challenges and open research questions persist. These include, among others, the optimization of scaffold zonality and architectural features; the preservation of the bioactivity of loaded active molecules, as well as their spatio-temporal release; the in vitro scaffold maturation prior to implantation; the pros and cons of each animal model and the graft-defect mismatch; and the in vivo non-invasive monitoring of new tissue formation. This work critically reviews these aspects and reveals the state of the art of using three-dimensional bioprinting, and its synergy with drug delivery technologies, to pattern the distribution of cells and/or active molecules in cartilage, bone and osteochondral engineered tissues. Most notably, this work focuses on approaches, technologies and biomaterials that are currently under in vivo investigations, as these give important insights on scaffold performance at the implantation site and its interaction/integration with surrounding tissues.

Ushering in the era of regenerative surgery

Tissue loss, irrespective of etiology, often requires extensive reconstruction. In many instances, the need exceeds what current treatments and technologies modern medicine can offer. Tissue engineering has made immense strides within the past few decades due to advances in biologics, biomaterials, and manufacturing. The convergence of these three domains has created limitless potential for future surgical care. Unfortunately, there still exists a disconnect on how to best implant these 'replacement parts' and care for the patient. It is therefore vital to develop paradigms for the integration of advanced surgical and tissue engineering technologies. This paper explores the convergence between tissue engineering and reconstructive surgery. We will describe the clinical problem of tissue loss, discuss currently available solutions, address limitations, and propose processes for integrating surgery and tissue engineering, thereby ushering in the era of regenerative surgery.

Bone ingrowth into a porous structure is achieved by preceding fibrogenesis and vascularization

Porous structures are frequently used in surgical implants to strengthen the interlocking power produced by bone ingrowth. Therefore, we aimed to elucidate the mechanism underlying bone ingrowth into a porous structure accompanied by vascularization. A nonbioactive polyetheretherketone implant with a 3D-printed porous structure was prepared and implanted in a bone hole created in the tibias of rabbits. We observed bone ingrowth in the same individual specimens immediately and at 2, 4, 8, and 12 weeks post-implantation using in-vivo computed tomography (CT). Furthermore, a detailed evaluation with blood vessels of each specimen at 2, 4, and 12 weeks was performed with ex-vivo CT and histological specimen. Additional histological evaluation was performed using thin sections of an implant made with thermoplastic polyurethane having the same structure. As a result, the bone invasion began after four weeks, when the construction of fibrous tissue and the spread of new blood vessels within the voids matured. As the bone matured in the load-bearing area, new blood vessels outside the bone matrix regressed. This longitudinal evaluation study suggests that preceding fibrogenesis and vascularization may be key in developing bone ingrowth. STATEMENT OF SIGNIFICANCE: A porous structure is an essential structure for dental and orthopedic implants because it provides strong fixation through bone invasion. Although it was known that vascularization was involved in this, the details were not known. This in vivo study revealed that in order for bone ingrowth to begin, a preparatory period of approximately 4 weeks was required to establish blood flow inside and outside the implant. Furthermore, it was confirmed that by spreading the fibrous structure in advance, it has an advantageous effect on the migration of cells involved in the formation of bones and blood vessels. We pointed out that it is necessary to consider fibrogenesis and vascularization when creating future implants.

Hierarchical Composite Scaffold with Deferoxamine Delivery System to Promote Bone Regeneration via Optimizing Angiogenesis

A bone defect refers to the loss of bone tissue caused by trauma or lesion. Bone defects result in high morbidity and deformity rates worldwide. Autologous bone grafting has been widely applied in clinics as the gold standard of treatment; however, it has limitations. Hence, bone tissue engineering has been proposed and developed as a novel therapeutic strategy for treating bone defects. Rapid and effective vascularization is essential for bone regeneration. In this study, a hierarchical composite scaffold with deferoxamine (DFO) delivery system, DFO@GMs-pDA/PCL-HNTs (DGPN), is developed, focusing on vascularized bone regeneration. The hierarchical structure of DGPN imitates the microstructure of natural bone and interacts with the local extracellular matrix, facilitating cell adhesion and proliferation. The addition of 1 wt% of halloysite nanotubes (HNTs) improves the material properties. Hydrophilic and functional groups conferred by polydopamine (pDA) modifications strengthen the scaffold bioactivity. Gelatin microspheres (GMs) protect the pharmacological activity of DFO, achieving local application and sustained release for 7 days. DFO effectively promotes angiogenesis by activating the signaling pathway of hypoxia inducible factor-1 α. In addition, DFO synergizes with HNTs to promote osteogenic differentiation and matrix mineralization. These results indicate that DGPN promotes bone regeneration and accelerates cranial defect healing.

Adipose-Derived Stem Cells: Angiogenetic Potential and Utility in Tissue Engineering

Adipose tissue (AT) is a large and important energy storage organ as well as an endocrine organ with a critical role in many processes. Additionally, AT is an enormous and easily accessible source of multipotent cell types used in our day for all types of tissue regeneration. The ability of adipose-derived stem cells (ADSCs) to differentiate into other types of cells, such as endothelial cells (ECs), vascular smooth muscle cells, or cardiomyocytes, is used in tissue engineering in order to promote/stimulate the process of angiogenesis. Being a key for future successful clinical applications, functional vascular networks in engineered tissue are targeted by numerous in vivo and ex vivo studies. The article reviews the angiogenic potential of ADSCs and explores their capacity in the field of tissue engineering (TE).

Engineering an injectable gellan gum-based hydrogel with osteogenesis and angiogenesis for bone regeneration

Injectable hydrogels are currently a topic of great interest in bone tissue engineering, which could fill irregular bone defects in a short time and avoid traditional major surgery. Herein, we developed an injectable gellan gum (GG)-based hydrogel for bone defect repair by blending nano-hydroxyapatite (nHA) and magnesium sulfate (MgSO4). In order to acquire an injectable GG-based hydrogel with superior osteogenesis, nHA were blended into GG solution with an optimized proportion. For the aim of endowing this hydrogel capable of angiogenesis, MgSO4 was also incorporated. Physicochemical evaluation revealed that GG-based hydrogel containing 5% nHA (w/v) and 2.5 mM MgSO4 (GG/5%nHA/MgSO4) had appropriate sol-gel transition time, showed a porosity-like structure, and could release magnesium ions for at least 14 days. Rheological studies showed that the GG/5%nHA/MgSO4 hydrogel had a stable structure and repeatable self-healing properties. In-vitro results determined that GG/5%nHA/MgSO4 hydrogel presented superior ability on stimulating bone marrow mesenchymal stem cells (BMSCs) to differentiate into osteogenic linage and human umbilical vein endothelial cells (HUVECs) to generate vascularization. In-vivo, GG/5%nHA/MgSO4 hydrogel was evaluated via a rat cranial defect model, as shown by better new bone formation and more neovascularization invasion. Therefore, the study demonstrated that the new injectable hydrogel, is a favorable bioactive GG-based hydrogel, and provides potential strategies for robust therapeutic interventions to improve the repair of bone defect.

Bone Tissue Engineering Scaffolds: Materials and Methods

The wide development in biomedical, regenerative medicine, and surgical techniques has ensured that new technologies are developed to improve patient-specific treatment and care. Tissue engineering is a special field in biomedical engineering that works toward cell development using scaffolds. Bone tissue engineering is a separate branch of tissue engineering, in which the construction of bone, functionalities of bone, and bone tissue regeneration are studied in detail to repair or regenerate new functional bone tissues. In India alone, people suffering from bone diseases are extensive in numbers. Almost 15% to 20% of the population suffers from osteoporosis. Bone scaffolds are proving to be an excellent solution for osseous abnormalities or defect treatment. Scaffolds are three dimensional (3D) and mostly porous structures created to enhance new tissue growth. Bone scaffolds are specially designed to promote osteoinductive cell growth, expansion, and migration on their surface. This review article aims to provide an overview of possible bone scaffolding materials in practice, different 3D techniques to fabricate these scaffolds, and effective bone scaffold characteristics targeted by researchers to fabricate tissue-engineered bone scaffolds.

Advances in tissue engineering of gellan gum-based hydrogels

Gellan Gum (GG) is a large, naturally occurring, linear polysaccharide with a similar structure and biological properties to the extracellular matrix. It's appropriate as a matrix material for the development of different composite materials due to its biocompatibility, biodegradability, and injectability. Hydrogels made from GG have found various applications in the field of Tissue Engineering (TE) in recent years after being mixed with a variety of other organic and inorganic components. These composites are considered multifunctional developing biomaterials because of their impressive mechanical capabilities, biocompatibility, low cytotoxicity, etc. This review focuses on the emerging advances of GG-based hydrogels in TE, providing an overview of the applications of different types of GG-based composite materials in bone TE, cartilage TE, nervous TE, retina TE, and other fields. Moreover, the investigations of GG-based hydrogels as bioink components for 3D bioprinting in TE will be elucidated. This review offers general guidance for the development of biomaterials related to GG, as well as ideas for future clinical diagnosis and treatment.

Emerging strategies for tissue engineering in vascularized composite allotransplantation: A review

Vascularized composite allotransplantation (VCA), which can effectively improve quality of life, is a promising therapy for repair and reconstruction after face or body trauma. However, intractable issues are associated with VCA, such as the inevitable multiple immunogenicities of different tissues that cause severe rejection, the limited protocols available for clinical application, and the shortage of donor sources. The existing regimens used to extend the survival of patients receiving VCAs and suppress rejection are generally the lifelong application of immunosuppressive drugs, which have side effects. Consequently, studies aiming at tissue engineering methods for VCA have become a topic. In this review, we summarize the emerging therapeutic strategies for tissue engineering aimed to prolong the survival time of VCA grafts, delay the rejection and promote prevascularization and tissue regeneration to provide new ideas for future research on VCA treatment.

3D printed Si-CaP scaffold released SiO32- and Ca2+ to synergistically promote angiogenesis

BACKGROUND AND PURPOSE

Structuring scaffold with both osteogenic and angiogenesis capabilities is a challenge for bone tissue engineering. Powder structured Si-CaP materials have shown excellent osteogenic properties and induction of stem cell differentiation. Our research group have successful produced 3D printed Si-CaP scaffolds by DLP technology. This study aims to explore the angiogenic effects of SiO32- and Ca2+ released by 3D printed Si-CaP scaffold, and whether there is a synergistic effect between the two ions.

METHODS

The 3D printed Si-CaP scaffolds were immersed in endothelial cell medium solution for 24 h. The Si, Ca ion released was detected by Inductively coupled plasma-optical emission spectrometry. We used detected data as a standard to prepare the simulated solution to investigate the effect of SiO32-, Ca2+ separately. Experiment was divided into control group, Si ion group, Ca ion group and Si + Ca ion group. We evaluated different ionic effect on HUVECs viability, proliferation, migration, gene expression, and tube formation on different groups.

RESULTS

The concentration of SiO32- was detected as 15.71 ± 0.04 μg/mL, Ca2+ as 67.14 ± 0.95 μg/mL. Na2SiO3 and CaCl2 were used to prepare the simulated solution. There were no statistically difference between simulated solution from ion released by scaffold. Si + Ca group promoted the gene expression significantly compared with the control group, p < .01. Expression of vascular-associated protein in Si + Ca ion group was higher than that in Si ion group, Ca ion group and control group. Si + Ca ion group significantly enhanced endothelial cell on migration and tube formation assay.

CONCLUSION

The 3D printed Si-CaP scaffold can release effective physiological concentrations of Si, Ca ions. Si and Ca ions have a synergistic effect on promoting angiogenesis of HUVECs. 3D printed Si-CaP scaffold is promising in vascularized bone tissue engineering application.

Hydrogels and Bioprinting in Bone Tissue Engineering: Creating Artificial Stem-Cell Niches for In Vitro Models

Advances in bioprinting have enabled the fabrication of complex tissue constructs with high speed and resolution. However, there remains significant structural and biological complexity within tissues that bioprinting is unable to recapitulate. Bone, for example, has a hierarchical organization ranging from the molecular to whole organ level. Current bioprinting techniques and the materials employed have imposed limits on the scale, speed, and resolution that can be achieved, rendering the technique unable to reproduce the structural hierarchies and cell-matrix interactions that are observed in bone. The shift toward biomimetic approaches in bone tissue engineering, where hydrogels provide biophysical and biochemical cues to encapsulated cells, is a promising approach to enhancing the biological function and development of tissues for in vitro modeling. A major focus in bioprinting of bone tissue for in vitro modeling is creating dynamic microenvironmental niches to support, stimulate, and direct the cellular processes for bone formation and remodeling. Hydrogels are ideal materials for imitating the extracellular matrix since they can be engineered to present various cues whilst allowing bioprinting. Here, recent advances in hydrogels and 3D bioprinting toward creating a microenvironmental niche that is conducive to tissue engineering of in vitro models of bone are reviewed.

Effect of autologous platelet-rich plasma on new bone formation and viability of a Marburg bone graft

This study aimed to compare the new bone formation, the process of remodeling, and the viability of bone grafts, using a combination of platelet-rich plasma (PRP) and Marburg bone graft versus bone grafts without any additional elements. For this study, 48 rabbits (with 24 rabbits in each group) were used. Bone defects were made in the femur, and the bone graft used was the human femoral head prepared according to the Marburg Bone Bank. Rabbits were divided into the following groups: heat-treated bone graft (HTBG group) and HTBG with PRP (HTBG + PRP group). After 14, 30, and 60 days post-surgery, the assessment of the results involved X-ray, histopathological, and histomorphometric analyses. The greater new bone formation was detected in the HTBG + PRP group on the 14 and 30 day (p < 0.001). Furthermore, the group using bone grafts with PRP demonstrated notably enhanced remodeling, characterized by stronger bone integration, more significant graft remineralization, and a circular pattern of newly formed bone. The PRP-bone graft complex improves bone tissue repair in the bone defect in the initial stages of bone regeneration. PRP has been identified to enhance the remodeling process and amplify the osteoconductive and osteoinductive capabilities of HTBGs.

Vaginal reconstruction with a double-sided biomembrane-a preclinical experimental study on large animals

Tissue engineering techniques bring the promise of vaginal reconstruction with low invasiveness and fewer complications. However, existing biomaterial scaffolds remain limited in efficient vaginal recovery, focusing only on regenerating an epithelial layer, but muscle layers are missing or abnormal. The lack of a multi-tissue hierarchical structure in the reconstructed vagina leads to shrinking, stenosis, and fibrosis. Here, an acellular matrix named a double-sided biomembrane (DBM) is demonstrated for vaginal recovery. The regeneration of epithelial and muscle layers is achieved simultaneously since the smooth side of the DBM is helpful for guiding epithelial cell growth, while its loose and porous side guides muscle cell growth. In addition, the DBM demonstrates excellent mechanical properties similar to vaginal tissue, and hydrophilicity. Therefore, neovaginas were observed in the fourth and twelfth weeks after DBMs were transplanted to repair full-thickness vaginal defects (4 cm) that we established in large animals. The DBMs can effectively promote rapid epithelialization, the formation of large muscle bundles, higher rates of angiogenesis, and the restoration of physiological function in a neovagina. That is, the injured vagina achieves nearly complete recovery in anatomy and function, similar to a normal vagina. These preclinical results indicate that the DBM has prospects for vaginal injury repair.

Temperature-responsive PCL-PLLA nanofibrous tissue engineering scaffolds with memorized porous microstructure recovery

Biomaterial scaffolds in tissue engineering facilitate tissue regeneration and integration with the host. Poor healing outcomes arise from lack of cell and tissue infiltration, and ill-fitting interfaces between matrices or grafts, resulting in fibrous tissue formation, inflammation, and resorption. Existing tissue engineering scaffolds struggle to recover from deformation to fit irregularly shaped defects encountered in clinical settings without compromising their mechanical properties and favorable internal architecture. This study introduces a synthetic biomaterial scaffold composed of high molecular weight poly (L-lactic acid) (PLLA) and an interpenetrating network of poly (ε-caprolactone) (PCL), in a composition aiming to address the need for conformal fitting synthetic matrices which retain and recover their advantageous morphologies. The scaffold, known as thermosensitive memorized microstructure (TS-MMS), forms nanofibrous materials with memorized microstructures capable of recovery after deformation, including macropores and nanofibers. TS-MMS nanofibers, with 50-500 nm diameters, are formed via thermally induced phase separation (TIPS) of PLLA after in situ polymerization of PCL-diacrylate. A critical partial-melting temperature of TS-MMS at 52°C enables bulk deformation above this temperature, while retaining the nanofibrous and macroporous structures upon cooling to 37°C. Incorporation of drug-loaded poly (lactide-co-glycolide) (PLGA) nanoparticles directly into TS-MMS nanofibers during fabrication allows sustained release of a model drug for up to 40 days. Subcutaneous implantation in vivo using LysM-Cre;td-Tomato; Col1eGFP mice demonstrates successful cellularization and integration of deformed/recovered TS-MMS materials, surpassing the limitations of deformed PLLA scaffolds, to facilitate cell and vasculature infiltration requisite for successful bone regeneration. Additionally we demonstrated a method for embedding controlled release vehicles directly into the scaffold nanofibers; controlled release of simvastatin enhances vascularization and tissue maturation. TS-MMS scaffolds offer promising improvements in clinical handling and performance compared to existing biomaterial scaffolds.

Small intestinal submucosa promotes angiogenesis via the Hippo pathway to improve vaginal repair

Vaginal reconstruction has incorporated the use of gastrointestinal segments for decades, but the technique is inevitably associated with complications. Tissue-engineering techniques, however, have brought great hope for vaginal reconstruction. This study aimed to evaluate the utility of small intestinal submucosa (SIS) in reconstructing clinically significant large vaginal defects in a porcine model and to investigate the role of the Hippo pathway in the vascular remodeling process. The composition and mechanical properties of SIS were characterized. Full-thickness vaginal defects were established in 10 minipig donors, with 4 cm lengths removed and replaced by an equal sized SIS scaffolds. The neovaginas were subjected to macroscopic, histological, immunohistochemical and molecular evaluations at 4 and 12 weeks after the surgery. Four weeks after the operation, extracellular matrix reorganization and complete coverage of the surface of the luminal matrix by vaginal epithelium were observed, accompanied by the formation of a microvascular network and the regeneration of smooth muscles, albeit disorderly arranged. Twelve weeks after implantation, enhancements were seen in the formation of the multi-layered squamous epithelium, angiogenesis, and large muscle bundle formation in the vagina. Additionally, the expression levels of angiogenesis-related proteins, proliferation-related proteins and Hippo pathway-related proteins in the neovagina were significantly increased. These results indicate that SIS could be used to reconstruct large vaginal defects and that the vascular remodeling process is potentially regulated by the Hippo pathway.

Dual effect of aucubin on promoting VEGFR2 mediated angiogenesis and reducing RANKL-induced bone resorption

BACKGROUND

Angiogenesis is regarded as a critical role in bone repair and regeneration, involving in pathological bone disorders such as osteoporosis. Aucubin, an iridoid glycoside primarily derived from Eucommia ulmoides, is reported to inhibit osteoclast activity, enhance bone formation and promote angiogenesis in osteoporosis models. Our study is to further investigate the anti-osteoporosis effect of aucubin in transgenic medaka, and the pro-angiogenic effect of aucubin and its mechanism of action both in vivo and in vitro.

METHODS

The anti-osteoporosis effect of aucubin was confirmed by using RANKL-stimulated bone resorption transgenic medaka. The pro-angiogenic effect of aucubin in vivo was investigated using vascular endothelial growth factor (VEGF) tyrosine kinase inhibitor II (VRI)-induced vascular insufficient transgenic zebrafish model. Furthermore, endothelial cell proliferation, migration, tube formation and the mechanisms were evaluated to identify the pro-angiogenic effect of aucubin in normal and su5416-injured human umbilical vein endothelial cells (HUVECs).

RESULTS

Aucubin decreased the resorption of the mineralized bone matrix and centra degradation in heat-shocked transgenic col10α1:nlGFP/rankl:HSE:CFP medaka. Moreover, aucubin reversed VRI-induced vascular insufficiency in zebrafish through regulating flt1, kdr, kdrl, vegfaa, ang-1, ang-2, tie1 and tie2 mRNA expressions in Tg(fli1a:EGFP)y1 or AB wild type zebrafish. Aucubin promoted cell proliferation by upregulating p-mTOR, p-Src, p-MEK, p-Erk1/2, p-Akt and p-FAK in HUVECs. Furthermore, aucubin exhibited a pro-angiogenic effect on su5416-injured HUVECs by promoting their proliferation, migration, and tube formation through regulating the phosphorylation of VEGFR2, MEK, ERK and the ratio of Bcl2-Bax.

CONCLUSION

Aucubin could reduce bone resorption in RANKL-induced osteoporosis medaka by live imaging. Meanwhile, aucubin exhibited a protective effect in VRI-induced vascular insufficient zebrafish by regulating VEGF-VEGFR and Ang-Tie signaling pathways. Additionally, aucubin promoted the proliferation, migration and tube formation of HUVECs probably by mediating VEGFR2/MEK/ERK, Akt/mTOR and Src/FAK signalling pathways. This study further indicated the dual effect of aucubin on angiogenesis and osteogenesis which may be beneficial to its treatment of osteoporosis.

Recent Advances in Bioengineering Bone Revascularization Based on Composite Materials Comprising Hydroxyapatite

The natural healing process of bone is impaired in the presence of tumors, trauma, or inflammation, necessitating external assistance for bone regeneration. The limitations of autologous/allogeneic bone grafting are still being discovered as research progresses. Bone tissue engineering (BTE) is now a crucial component of treating bone injuries and actively works to promote vascularization, a crucial stage in bone repair. A biomaterial with hydroxyapatite (HA), which resembles the mineral makeup of invertebrate bones and teeth, has demonstrated high osteoconductivity, bioactivity, and biocompatibility. However, due to its brittleness and porosity, which restrict its application, scientists have been prompted to explore ways to improve its properties by mixing it with other materials, modifying its structural composition, improving fabrication techniques and growth factor loading, and co-cultivating bone regrowth cells to stimulate vascularization. This review scrutinizes the latest five-year research on HA composite studies aimed at amplifying vascularization in bone regeneration.

3D printing titanium grid scaffold facilitates osteogenesis in mandibular segmental defects

Bone fusion of defect broken ends is the basis of the functional reconstruction of critical maxillofacial segmental bone defects. However, the currently available treatments do not easily achieve this goal. Therefore, this study aimed to fabricate 3D-printing titanium grid scaffolds, which possess sufficient pores and basic biomechanical strength to facilitate osteogenesis in order to accomplish bone fusion in mandibular segmental bone defects. The clinical trial was approved and supervised by the Medical Ethics Committee of the Chinese PLA General Hospital on March 28th, 2019 (Beijing, China. approval No. S2019-065-01), and registered in the clinical trials registry platform (registration number: ChiCTR2300072209). Titanium grid scaffolds were manufactured using selective laser melting and implanted in 20 beagle dogs with mandibular segmental defects. Half of the animals were treated with autologous bone chips and bone substances incorporated into the scaffolds; no additional filling was used for the rest of the animals. After 18 months of observation, radiological scanning and histological analysis in canine models revealed that the pores of regenerated bone were filled with titanium grid scaffolds and bone broken ends were integrated. Furthermore, three patients were treated with similar titanium grid scaffold implants in mandibular segmental defects; no mechanical complications were observed, and similar bone regeneration was observed in the reconstructed patients' mandibles in the clinic. These results demonstrated that 3D-printing titanium grid scaffolds with sufficient pores and basic biomechanical strength could facilitate bone regeneration in large-segment mandibular bone defects.

Pathophysiological mechanism of acute bone loss after fracture

BACKGROUND

Acute bone loss after fracture is associated with various effects on the complete recovery process and a risk of secondary fractures among patients. Studies have reported similarities in pathophysiological mechanisms involved in acute bone loss after fractures and osteoporosis. However, given the silence nature of bone loss and bone metabolism complexities, the actual underlying pathophysiological mechanisms have yet to be fully elucidated.

AIM OF REVIEW

To elaborate the latest findings in basic research with a focus on acute bone loss after fracture. To briefly highlight potential therapeutic targets and current representative drugs. To arouse researchers' attention and discussion on acute bone loss after fracture.

KEY SCIENTIFIC CONCEPTS OF REVIEW

Bone loss after fracture is associated with immobilization, mechanical unloading, blood supply damage, sympathetic nerve regulation, and crosstalk between musculoskeletals among other factors. Current treatment strategies rely on regulation of osteoblasts and osteoclasts, therefore, there is a need to elucidate on the underlying mechanisms of acute bone loss after fractures to inform the development of efficacious and safe drugs. In addition, attention should be paid towards ensuring long-term skeletal health.

Stimuli-responsive hydrogels: smart state of-the-art platforms for cardiac tissue engineering

Biomedicine and tissue regeneration have made significant advancements recently, positively affecting the whole healthcare spectrum. This opened the way for them to develop their applications for revitalizing damaged tissues. Thus, their functionality will be restored. Cardiac tissue engineering (CTE) using curative procedures that combine biomolecules, biomimetic scaffolds, and cells plays a critical part in this path. Stimuli-responsive hydrogels (SRHs) are excellent three-dimensional (3D) biomaterials for tissue engineering (TE) and various biomedical applications. They can mimic the intrinsic tissues' physicochemical, mechanical, and biological characteristics in a variety of ways. They also provide for 3D setup, adequate aqueous conditions, and the mechanical consistency required for cell development. Furthermore, they function as competent delivery platforms for various biomolecules. Many natural and synthetic polymers were used to fabricate these intelligent platforms with innovative enhanced features and specialized capabilities that are appropriate for CTE applications. In the present review, different strategies employed for CTE were outlined. The light was shed on the limitations of the use of conventional hydrogels in CTE. Moreover, diverse types of SRHs, their characteristics, assembly and exploitation for CTE were discussed. To summarize, recent development in the construction of SRHs increases their potential to operate as intelligent, sophisticated systems in the reconstruction of degenerated cardiac tissues.

In Vitro and In Vivo Applications of Magnesium-Enriched Biomaterials for Vascularized Osteogenesis in Bone Tissue Engineering: A Review of Literature

Bone is a highly vascularized tissue, and the ability of magnesium (Mg) to promote osteogenesis and angiogenesis has been widely studied. The aim of bone tissue engineering is to repair bone tissue defects and restore its normal function. Various Mg-enriched materials that can promote angiogenesis and osteogenesis have been made. Here, we introduce several types of orthopedic clinical uses of Mg; recent advances in the study of metal materials releasing Mg ions (pure Mg, Mg alloy, coated Mg, Mg-rich composite, ceramic, and hydrogel) are reviewed. Most studies suggest that Mg can enhance vascularized osteogenesis in bone defect areas. Additionally, we summarized some research on the mechanisms related to vascularized osteogenesis. In addition, the experimental strategies for the research of Mg-enriched materials in the future are put forward, in which clarifying the specific mechanism of promoting angiogenesis is the crux.

Bioprinting: a focus on improving bioink printability and cell performance based on different process parameters

Three dimensional (3D) bioprinting is an emerging biofabrication technique that shows great potential in the field of tissue engineering, regenerative medicine and advanced drug delivery. Despite the current advancement of bioprinting technology, it faces several obstacles such as the challenge of optimizing the printing resolution of 3D constructs while retaining cell viability before, during, and after bioprinting. Therefore, it is of great significance to fully understand factors that influence the shape fidelity of printed structures and the performance of cells encapsulated in bioinks. This review presents a comprehensive analysis of bioprinting process parameters that influence bioink printability and cell performance, including bioink properties (composition, concentration, and component ratio), printing speed and pressure, nozzle charateristics (size, length, and geometry), and crosslinking parameters (crosslinker types, concentration, and crosslinking time). Key examples are provided to analyze how these parameters could be tailored to achieve the optimal printing resolution as well as cell performance. Finally, future prospects of bioprinting technology, including correlating process parameters to particular cell types with predefined applications, applying statistical analysis and artificial intelligence (AI)/machine learning (ML) technique in parameter screening, and optimizing 4D bioprinting process parameters, are highlighted.

Fibro-Gel: An All-Aqueous Hydrogel Consisting of Microfibers with Tunable Release Profile and its Application in Wound Healing

Injectable hydrogels are valuable tools in tissue engineering and regenerative medicine due to their unique advantages of injectability with minimal invasiveness and usability for irregularly shaped sites. However, it remains challenging to achieve scalable manufacturing together with matching physicochemical properties and on-demand drug release for a high level of control over biophysical and biomedical cues to direct endogenous cells. Here, the use of an injectable fibro-gel is demonstrated, a water-filled network of entangled hydrogel microfibers, whose physicochemical properties and drug release profiles can be tailored to overcome these shortcomings. This fibro-gel exhibits favorable in vitro biocompatibility and the capability to aid vascularization. The potential use of the fibro-gel for advancing tissue regeneration is explored with a mice excision skin model. Preliminary in vivo tests indicate that the fibro-gel promotes wound healing and new healthy tissue regeneration at a faster rate than a commercial gel. Moreover, it is demonstrated that the release of distinct drugs at different rates can further accelerate wound healing with higher efficiency, by using a two-layer fibro-gel model. The combination of injectability and tailorable properties of this fibro-gel offers a promising approach in biomedical fields such as therapeutic delivery, medical dressings, and 3D tissue scaffolds for tissue engineering.

Epicatechin gallate promotes vascularization in co-culture of human osteoblasts and outgrowth endothelial cells

Prevascularization is crucial for the survival of tissue-engineered bone and further bone repair/regeneration. Since epicatechin gallate (ECG), the most abundant flavanol in green tea, shows potential beneficial effects on endothelial cells and bone cells, we decided to investigate whether it promotes vascularization/angiogenesis and osteogenesis using a co-culture system containing human primary osteoblasts (POBs) and outgrowth endothelial cells (OECs). We found that treatment with ECG (1) significantly enhanced microvessel formation in co-culture of POB and OECs, (2) improved cell viability/proliferation and the angiogenic/osteogenic capacities of OEC/POBs, (3) significantly increased the levels of E-selectin, IL-6, TNF-α, IFN-γ, VEGF, and PDGF-BB in co-cultures of POB and OEC, and (4) upregulated HIF-1α, HIF-2α, NF-κB, iNOS, GLUT1, VEGF, and Ang1/2 but downregulated PHD1 in monocultures of OEC or POB. Our findings demonstrate that ECG promotes angiogenesis and osteogenesis (probably via HIF signaling) in co-cultures of OECs and POBs. ECG thus has potential applications in the promotion of angiogenesis/vascularization in many tissue constructs including those of bone.

A Drop-on-Demand Bioprinting Approach to Spatially Arrange Multiple Cell Types and Monitor Their Cell-Cell Interactions towards Vascularization Based on Endothelial Cells and Mesenchymal Stem Cells

Spheroids, organoids, or cell-laden droplets are often used as building blocks for bioprinting, but so far little is known about the spatio-temporal cellular interactions subsequent to printing. We used a drop-on-demand bioprinting approach to study the biological interactions of such building blocks in dimensions of micrometers. Highly-density droplets (approximately 700 cells in 10 nL) of multiple cell types were patterned in a 3D hydrogel matrix with a precision of up to 70 μm. The patterns were used to investigate interactions of endothelial cells (HUVECs) and adipose-derived mesenchymal stem cells (ASCs), which are related to vascularization. We demonstrated that a gap of 200 μm between HUVEC and ASC aggregates led to decreased sprouting of HUVECs towards ASCs and increased growth from ASCs towards HUVECs. For mixed aggregates containing both cell types, cellular interconnections of ASCs with lengths of up to approximately 800 µm and inhibition of HUVEC sprouting were observed. When ASCs were differentiated into smooth muscle cells (dASCs), separate HUVEC aggregates displayed decreased sprouting towards dASCs, whereas no cellular interconnections nor inhibition of HUVEC sprouting were detected for mixed dASCs/HUVEC aggregates. These findings demonstrate that our approach could be applied to investigate cell-cell interactions of different cell types in 3D co-cultures.

Functional Approaches in Promoting Vascularization and Angiogenesis in Bone Critical-Sized Defects via Delivery of Cells, Growth Factors, Drugs, and Particles

Critical-sized bone defects, or CSDs, are defined as bone defects that cannot be regenerated by themselves and require surgical intervention via employing specific biomaterials and a certain regenerative strategy. Although a variety of approaches can be used to treat CSDs, poor angiogenesis and vascularization remain an obstacle in these methods. The complex biological healing of bone defects depends directly on the function of blood flow to provide sufficient oxygen and nutrients and the removal of waste products from the defect site. The absence of vascularization can lead to non-union and delayed-union defect development. To overcome this challenge, angiogenic agents can be delivered to the site of injury to stimulate vessel formation. This review begins by introducing the treatment methods for CSDs. The importance of vascularization in CSDs is subsequently highlighted. Delivering angiogenesis agents, including relevant growth factors, cells, drugs, particles, cell secretion substances, their combination, and co-delivery to CSDs are fully explored. Moreover, the effects of such agents on new bone formation, followed by vessel formation in defect areas, are evaluated.

Human urine-derived stem cell exosomes delivered via injectable GelMA templated hydrogel accelerate bone regeneration

The key to critical bone regeneration in tissue engineering relies on an ideal bio-scaffold coated with a controlled release of growth factors. Gelatin methacrylate (GelMA) and Hyaluronic acid methacrylate (HAMA) have been a novel topic of interest in bone regeneration while introducing appropriate nano-hydroxyapatite (nHAP) to improve its mechanical properties. And the exosomes derived from human urine-derived stem cells (human ^USCEXOs) have also been reported to promote osteogenesis in tissue engineering. The present study aimed to design a new GelMA-HAMA/nHAP composite hydrogel as a drug delivery system. The ^USCEXOs were encapsulated and slow-released in the hydrogel for better osteogenesis. The characterization of the GelMA-based hydrogel showed excellent controlled release performance and appropriate mechanical properties. The in vitro studies showed that the ^USCEXOs/GelMA-HAMA/nHAP composite hydrogel could promote the osteogenesis of bone marrow mesenchymal stem cells (BMSCs) and the angiogenesis of endothelial progenitor cells (EPCs), respectively. Meanwhile, the in vivo results confirmed that this composite hydrogel could significantly promote the defect repair of cranial bone in the rat model. In addition, we also found that ^USCEXOs/GelMA-HAMA/nHAP composite hydrogel can promote the formation of H-type vessels in the bone regeneration area, enhancing the therapeutic effect. In conclusion, our findings suggested that this controllable and biocompatible ^USCEXOs/GelMA-HAMA/nHAP composite hydrogel may effectively promote bone regeneration by coupling osteogenesis and angiogenesis.

Silk Fibroin Bioink for 3D Printing in Tissue Regeneration: Controlled Release of MSC extracellular Vesicles

Sodium alginate (SA)-based hydrogels are often employed as bioink for three-dimensional (3D) scaffold bioprinting. They offer a suitable environment for cell proliferation and differentiation during tissue regeneration and also control the release of growth factors and mesenchymal stem cell secretome, which is useful for scaffold biointegration. However, such hydrogels show poor mechanical properties, fast-release kinetics, and low biological performance, hampering their successful clinical application. In this work, silk fibroin (SF), a protein with excellent biomechanical properties frequently used for controlled drug release, was blended with SA to obtain improved bioink and scaffold properties. Firstly, we produced a printable SA solution containing SF capable of the conformational change from Silk I (random coil) to Silk II (β-sheet): this transition is a fundamental condition to improve the scaffold's mechanical properties. Then, the SA-SF blends' printability and shape fidelity were demonstrated, and mechanical characterization of the printed hydrogels was performed: SF significantly increased compressive elastic modulus, while no influence on tensile response was detected. Finally, the release profile of Lyosecretome-a freeze-dried formulation of MSC-secretome containing extracellular vesicles (EV)-from scaffolds was determined: SF not only dramatically slowed the EV release rate, but also modified the kinetics and mechanism release with respect to the baseline of SA hydrogel. Overall, these results lay the foundation for the development of SA-SF bioinks with modulable mechanical and EV-release properties, and their application in 3D scaffold printing.

Biofabricating the vascular tree in engineered bone tissue

The development of tissue engineering strategies for treatment of large bone defects has become increasingly relevant, given the growing demand for bone substitutes. Native bone is composed of a dense vascular network necessary for the regulation of bone development, regeneration and homeostasis. A major obstacle in fabricating living, clinically relevant-sized bone mimics (1-10 cm³) is the limited supply of nutrients, including oxygen to the core of the construct. Therefore, strategies to support vascularization are pivotal for the development of tissue engineered bone constructs. Creating a functional bone construct integrated with a vascular network, capable of delivering the necessary nutrients for optimal tissue development is imperative for translation into the clinics. The vascular system is composed of a complex network that runs throughout the body in a tree-like hierarchical branching fashion. A significant challenge for tissue engineering approaches lies in mimicking the intricate, multi-scale structures consisting of larger vessels (macro-vessels) which interconnect with multiple sprouting vessels (microvessels) in a closed network. The advent of biofabrication has enabled complex, out of plane channels to be generated and has laid the groundwork for the creation of multi-scale vasculature in recent years. This review highlights the key state-of-the-art achievements for the development of vascular networks of varying scales in the field of biofabrication with a particular focus for its application in developing a functional tissue engineered bone construct. STATEMENT OF SIGNIFICANCE: There is a growing need for bone substitutes to overcome the limited supply of patient-derived bone. Bone tissue engineering aims to overcome this by combining stem cells with scaffolds to restore missing bone. The current bottleneck in upscaling is the lack of an integrated vascular network, required for the delivery of nutrients to cells. 3D bioprinting techniques has enabled the creation of complex hollow structures of varying dimensions that resemble native blood vessels. The convergence of multiple materials, cell types and fabrication approaches, opens the possibility of developing clinically-relevant sized vascularized bone constructs. This review provides an up-to-date insight of the technologies currently available for the generation of complex vascular networks, with a focus on their application in bone tissue engineering.

Bone regeneration of mouse critical-sized calvarial defects with human mesenchymal stem cell sheets co-expressing BMP2 and VEGF

Background/purpose

Over-dependence on existing synthetic scaffolds and insufficient osteoinductive and vasculogenic growth factors have limited the development of bone regeneration. The study aimed to assess the feasibility of using marrow-derived mesenchymal stem cells (BMSCs) cell sheets co-expressing bone morphogenetic proteins 2 (BMP2) and vascular endothelial growth factor (VEGF) for repairing critical-sized calvarial defects.

Materials and methods

BMSCs cell sheets were genetically engineered to express BMP2/VEGF alone or together. Alterations in osteogenic markers were examined by quantitative real-time PCR (qRT-PCR) and western blotting. A critical-sized calvarial bone defect model was used to investigate the osteogenesis effects of BMP2/VEGF cell sheets alone or in combination. The efficacy was assessed with micro-computed tomography (micro-CT) and histology.

Results

In vitro, the expression of BMP2 and VEGF through lentiviral transduction was confirmed by qRT-PCR and western blotting against BMP2 and VEGF. Lentiviral delivery of BMP2 and VEGF resulted in the upregulation of osteogenic markers. In vivo, in a critical-sized calvarial bone defect model, 3D-reconstructed micro-CT images revealed that treatment of the calvarial defects with the BMP2/VEGF cell sheet resulted in significantly greater amounts of newly formed bone at 8 weeks after surgery than treatment with cell sheets with single gene transduction or vehicle controls. The results were confirmed by histological assessment by H&E staining and Masson staining.

Conclusion

This study demonstrates that BMP2/VEGF co-expressing BMSCs sheets promote bone regeneration in critical-sized calvarial bone defects. The BMP2/VEGF cell sheets provide a functional bioactive scaffold for critical-size bone reconstruction.

Building Osteogenic Microenvironments with a Double-Network Composite Hydrogel for Bone Repair

The critical factor determining the in vivo effect of bone repair materials is the microenvironment, which greatly depends on their abilities to promote vascularization and bone formation. However, implant materials are far from ideal candidates for guiding bone regeneration due to their deficient angiogenic and osteogenic microenvironments. Herein, a double-network composite hydrogel combining vascular endothelial growth factor (VEGF)-mimetic peptide with hydroxyapatite (HA) precursor was developed to build an osteogenic microenvironment for bone repair. The hydrogel was prepared by mixing acrylated β-cyclodextrins and octacalcium phosphate (OCP), an HA precursor, with gelatin solution, followed by ultraviolet photo-crosslinking. To improve the angiogenic potential of the hydrogel, QK, a VEGF-mimicking peptide, was loaded in acrylated β-cyclodextrins. The QK-loaded hydrogel promoted tube formation of human umbilical vein endothelial cells and upregulated the expression of angiogenesis-related genes, such as Flt1 , Kdr , and VEGF , in bone marrow mesenchymal stem cells. Moreover, QK could recruit bone marrow mesenchymal stem cells. Furthermore, OCP in the composite hydrogel could be transformed into HA and release calcium ions facilitating bone regeneration. The double-network composite hydrogel integrated QK and OCP showed obvious osteoinductive activity. The results of animal experiments showed that the composite hydrogel enhanced bone regeneration in skull defects of rats, due to perfect synergistic effects of QK and OCP on vascularized bone regeneration. In summary, improving the angiogenic and osteogenic microenvironments by our double-network composite hydrogel shows promising prospects for bone repair.

In vivo prevascularization strategy enhances neovascularization of β-tricalcium phosphate scaffolds in bone regeneration

Background

Neovascularization is critical for bone regeneration. Numerous studies have explored prevascularization preimplant strategies, ranging from calcium phosphate cement (CPC) scaffolds to co-culturing CPCs with stem cells. The aim of the present study was to evaluate an alternative in vivo prevascularization approach, using preimplant-prepared macroporous beta-tricalcium phosphate (β-TCP) scaffolds and subsequent transplantation in bone defect model.

Methods

The morphology of β-TCPs was characterized by scanning electron microscopy. After 3 weeks of prevascularization within a muscle pouch at the lateral size of rat tibia, we transplanted prevascularized macroporous β-TCPs in segmental tibia defects, using blank β-TCPs as a control. Extent of neovascularization was determined by angiography and immunohistochemical (IHC) evaluations. Tibia samples were collected at different time points for biomechanical, radiological, and histological analyses. RT-PCR and western blotting were used to evaluate angio- and osteo-specific markers.

Results

With macroporous β-TCPs, we documented more vascular and supporting tissue invasion in the macroporous β-TCPs with prior in vivo prevascularization. Radiography, biomechanical, IHC, and histological analyses revealed considerably more vascularity and bone consolidation in β-TCP scaffolds that had undergone the prevascularization step compared to the blank β-TCP scaffolds. Moreover, the prevascularization treatment remarkably upregulated mRNA and protein expression of BMP2 and vascular endothelial growth factor (VEGF) during bone regeneration.

Conclusion

This novel in vivo prevascularization strategy successfully accelerated vascular formation to bone regeneration. Our findings indicate that prevascularized tissue-engineered bone grafts have promising potential in clinical applications.

The translational potential of this article

This study indicates a novel in vivo prevascularization strategy for growing vasculature on β-TCP scaffolds to be used for repair of large segmental bone defects, might serve as a promising tissue-engineered bone grafts in the future.

Injectable temperature-sensitive hydrogel system incorporating deferoxamine-loaded microspheres promotes H-type blood vessel-related bone repair of a critical size femoral defect

Insufficient vascularization is a major challenge in the repair of critical-sized bone defects. Deferoxamine (DFO) has been reported to play a potential role in promoting the formation of H-type blood vessels, a specialized vascular subtype with coupled angiogenesis and osteogenesis. However, whether DFO promotes the expression of H-type vessels in critical femoral defects with complete periosteal damage remains unknown. Moreover, stable drug loading systems need to be designed owing to the short half-life and high-dose toxic effects of DFO. In this study, we developed an injectable DFO-gelatin microspheres (GMs) hydrogel complex as a stable drug loading system for the treatment of critical femoral defects in rats. Our results showed that sustained release of DFO in critical femoral defects stimulated the generation of functional H-type vessels. The DFO-GMs hydrogel complex effectively promoted proliferation, formation, and migration of human umbilical vein endothelial cells in vitro. In vivo, the application of the DFO-GMs hydrogel complex expanded the distribution range and prolonged the expression time of H-type vessels in the defect area and was positively correlated with the number of osterix+ cells and new bone tissue. Topical application of the HIF-1α inhibitor PX-478 partially blocked the stimulation of H-type vessels by DFO, whereas the osteogenic potential of the latter was also weakened. Our results extended the local application of DFO and provided a theoretical basis for targeting H-type vessels to treat large femoral defects. STATEMENT OF SIGNIFICANCE: Abundant functional blood vessels are essential for bone repair. The H-type blood vessel is a functional subtype with angiogenesis and osteogenesis coupling potential. A drug loading system with long-term controlled release was first used to investigate the formation of H-type blood vessels in critical femoral defects and promotion of bone repair. Our results showed that the application of DFO-GMs hydrogel complex expanded the distribution range and expression time of H-type vessels, and was positively correlated with the number of osteoblasts and volume of new bone tissue. These results expanded the local application approach of DFO and provide a theoretical basis for targeting H-type vessels to treat large femoral defects.

Potential Use of 3D CORAGRAF-Loaded PDGF-BB in PLGA Microsphere Seeded Mesenchymal Stromal Cells in Enhancing the Repair of Calvaria Critical-Size Bone Defect in Rat Model

Our previous study evidenced that the 3D CORAGRAF loaded with PLGA microsphere constitutes PDGF-BB can support cell attachment and proliferation and can induce an osteogenic commitment of mesenchymal stromal cells in the in vitro condition. However, how this construct can perform in pathophysiological conditions in terms of repairing critical bone defects is yet to be understood. A study was therefore conducted to investigate the regeneration potential of calvaria critical-size defects using CORAGRAF + PLGA with PDGF-BB + mesenchymal stromal cells (MSCs) in a rat model. A 5 mm critical bone defect was created on calvaria of 40 male Sprague-Dawley rats. CORAGRAF incorporated either with or without PDGF-BB and seeded with rat bone-marrow-derived MSCs was implanted at the defect region. The bone regeneration potential of implanted constructs was assessed using micro-CT imaging and histological staining in weeks 4 and 8. The micro-CT images indicated a significant closure of defects in the cranial bone of the rats treated with 3D CORAGRAF + PLGA with PDGF-BB + MSCs on week 4 and 8 post-implantation. This finding, further supported with the histology outcome where the rat cranial defect treated with CORAGRAF + PLGA with PDGF-BB + MSCs indicated neo-bony ingrowth with organized and mature bone-like morphology as compared with other groups. The previous in vitro results substantiated with our pre-clinical findings demonstrate that the combination of CORAGRAF + PLGA with PDGF-BB + MSCs could be an ideal construct to support bone regeneration in critical bone defects. Hence, this construct can be further investigated for its safety and efficacy in large animal models, or it can be skipped to human trial prior for commercialization.

Self-Adhesive Hydrogel Biomimetic Periosteum to Promote Critical-Size Bone Defect Repair via Synergistic Osteogenesis and Angiogenesis

The periosteum plays an important role in the regeneration of critical-size bone defects, with functions of recruiting multiple cells, accelerating vascular network reconstruction, and guiding bone tissue regeneration. However, these functions cannot be easily implemented by simply simulating the periosteum via a material structure design or by loading exogenous cytokines. Herein, inspired by the periosteal function, we propose a biomimetic periosteum preparation strategy to enhance natural polymer hydrogel membranes using inorganic bioactive materials. The biomimetic periosteum having bone tissue self-adhesive functions and resembling an extracellular matrix was prepared using dopamine-modified gelatin and oxidized hyaluronan (GA/HA), and micro/nanobioactive glass (MNBG) was further incorporated into the hydrogel to fabricate an organic/inorganic co-crosslinked hydrogel membrane (GA/HA-BG). The addition of MNBG enhanced the stability of the natural polymer hydrogel membrane, resulting in a sustained degradation time, biomineralization, and long-term release of ions. The Ca²⁺ and SiO₄^4- ions released by bioactive glass were shown to recruit cells and promote the differentiation of bone marrow stromal cells into osteoblasts, initiating multicentric osteogenic behavior. Additionally, the bioactive ions were able to continuously stimulate the endogenous expression of vascular endothelial growth factor from human umbilical vein endothelial cells through the PI3K/Akt/HIF-1α pathway, which accelerated vascularization of the defect area and synergistically promoted the repair of bone defects. This organic-inorganic biomimetic periosteum has been proved to be effective and versatile in critical-size bone defect repair and is expected to provide a promising strategy for solving clinical issues.

In Vivo Bone Tissue Engineering Strategies: Advances and Prospects

Reconstruction of critical-sized bone defects remains a tremendous challenge for surgeons worldwide. Despite the variety of surgical techniques, current clinical strategies for bone defect repair demonstrate significant limitations and drawbacks, including donor-site morbidity, poor anatomical match, insufficient bone volume, bone graft resorption, and rejection. Bone tissue engineering (BTE) has emerged as a novel approach to guided bone tissue regeneration. BTE focuses on in vitro manipulations with seed cells, growth factors and bioactive scaffolds using bioreactors. The successful clinical translation of BTE requires overcoming a number of significant challenges. Currently, insufficient vascularization is the critical limitation for viability of the bone tissue-engineered construct. Furthermore, efficacy and safety of the scaffolds cell-seeding and exogenous growth factors administration are still controversial. The in vivo bioreactor principle (IVB) is an exceptionally promising concept for the in vivo bone tissue regeneration in a predictable patient-specific manner. This concept is based on the self-regenerative capacity of the human body, and combines flap prefabrication and axial vascularization strategies. Multiple experimental studies on in vivo BTE strategies presented in this review demonstrate the efficacy of this approach. Routine clinical application of the in vivo bioreactor principle is the future direction of BTE; however, it requires further investigation for overcoming some significant limitations.

Functionalization of Electrospun Nanofiber for Bone Tissue Engineering

Bone-tissue engineering is an alternative treatment for bone defects with great potential in which scaffold is a critical factor to determine the effect of bone regeneration. Electrospun nanofibers are widely used as scaffolds in the biomedical field for their similarity with the structure of the extracellular matrix (ECM). Their unique characteristics are: larger surface areas, porosity and processability; these make them ideal candidates for bone-tissue engineering. This review briefly introduces bone-tissue engineering and summarizes the materials and methods for electrospining. More importantly, how to functionalize electrospun nanofibers to make them more conducive for bone regeneration is highlighted. Finally, the existing deficiencies of functionalized electrospun nanofibers for promoting osteogenesis are proposed. Such a summary can lay the foundation for the clinical practice of functionalized electrospun nanofibers.

The osteo-angiogenic signaling crosstalk for bone regeneration: harmony out of complexity

In recent years it has been increasingly appreciated that blood vessels are not simply suppliers of nutrients and oxygen, but actually play an exquisite regulatory role in bone development and repair. A specialized kind of endothelium, named type H because of its high expression of CD31 and Endomucin, constitutes anatomically defined vessels in proximity of the epiphyseal growth plate. Type H endothelium regulates the proliferation and differentiation of both osteoblasts and osteoclasts through the secretion of angiocrine signals and is a hub for the bidirectional molecular crosstalk between the different cell populations of the osteogenic microenvironment. Type H vessels are a key target for current translational approaches aiming at coupling angiogenesis and osteogenesis for bone repair. Open questions remain about their presence and features in notstereotyped tissues, like engineered osteogenic grafts, and the opportunities for their clinical stimulation by pharmacological treatments.