The effect of the delivery of vascular endothelial growth factor and bone morphogenic protein-2 to osteoprogenitor cell populations on bone formation

Advances in Polysaccharides for Cartilage Tissue Engineering Repair: A Review

Cartilage repair has been a significant challenge in orthopedics that has not yet been fully resolved. Due to the absence of blood vessels and the almost cell-free nature of mature cartilage tissue, the limited ability to repair cartilage has resulted in significant socioeconomic pressures. Polysaccharide materials have recently been widely used for cartilage tissue repair due to their excellent cell loading, biocompatibility, and chemical modifiability. They also provide a suitable microenvironment for cartilage repair and regeneration. In this Review, we summarize the techniques used clinically for cartilage repair, focusing on polysaccharides, polysaccharides for cartilage repair, and the differences between these and other materials. In addition, we summarize the techniques of tissue engineering strategies for cartilage repair and provide an outlook on developing next-generation cartilage repair and regeneration materials from polysaccharides. This Review will provide theoretical guidance for developing polysaccharide-based cartilage repair and regeneration materials with clinical applications for cartilage tissue repair and regeneration.

Biofabrication of nanocomposite-based scaffolds containing human bone extracellular matrix for the differentiation of skeletal stem and progenitor cells

Autograft or metal implants are routinely used in skeletal repair. However, they fail to provide long-term clinical resolution, necessitating a functional biomimetic tissue engineering alternative. The use of native human bone tissue for synthesizing a biomimetic material ink for three-dimensional (3D) bioprinting of skeletal tissue is an attractive strategy for tissue regeneration. Thus, human bone extracellular matrix (bone-ECM) offers an exciting potential for the development of an appropriate microenvironment for human bone marrow stromal cells (HBMSCs) to proliferate and differentiate along the osteogenic lineage. In this study, we engineered a novel material ink (LAB) by blending human bone-ECM (B) with nanoclay (L, Laponite®) and alginate (A) polymers using extrusion-based deposition. The inclusion of the nanofiller and polymeric material increased the rheology, printability, and drug retention properties and, critically, the preservation of HBMSCs viability upon printing. The composite of human bone-ECM-based 3D constructs containing vascular endothelial growth factor (VEGF) enhanced vascularization after implantation in an ex vivo chick chorioallantoic membrane (CAM) model. The inclusion of bone morphogenetic protein-2 (BMP-2) with the HBMSCs further enhanced vascularization and mineralization after only seven days. This study demonstrates the synergistic combination of nanoclay with biomimetic materials (alginate and bone-ECM) to support the formation of osteogenic tissue both in vitro and ex vivo and offers a promising novel 3D bioprinting approach to personalized skeletal tissue repair.

Graphic abstract

Supplementary Information

The online version contains supplementary material available at 10.1007/s42242-023-00265-z.

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.

Experimental Early Stimulation of Bone Tissue Neo-Formation for Critical Size Elimination Defects in the Maxillofacial Region

A biomaterial is proposed for closing extensive bone defects in the maxillofacial region. The composition of the biomaterial includes high-molecular chitosan, chondroitin sulfate, hyaluronate, heparin, alginate, and inorganic nanostructured hydroxyapatite. The purpose of this study is to demonstrate morphological and histological early signs of reconstruction of a bone cavity of critical size. The studies were carried out on 84 white female rats weighing 200-250 g. The study group consisted of 84 animals in total, 40 in the experimental group and 44 in the control group. In all animals, three-walled bone defects measuring 0.5 × 0.4 × 0.5 cm3 were applied subperiosteally in the region of the angle of the lower jaw and filled in the experimental group using lyophilized gel mass of chitosan-alginate-hydroxyapatite (CH-SA-HA). In control animals, the bone cavities were filled with their own blood clots after bone trepanation and bleeding. The periods for monitoring bone regeneration were 3, 5, and 7 days and 2, 3, 4, 6, 8, and 10 weeks. The control of bone regeneration was carried out using multiple morphological and histological analyses. Results showed that the following process is an obligatory process and is accompanied by the binding and release of angiogenic implantation: the chitosan construct actively replaced early-stage defects with the formation of full-fledged new bone tissue compared to the control group. By the 7th day, morphological analysis showed that the formation of spongy bone tissue could be seen. After 2 weeks, there was a pronounced increase in bone volume (p < 0.01), and at 6 weeks after surgical intervention, the closure of the defect was 70-80%; after 8 weeks, it was 100% without violation of bone morphology with a high degree of mineralization. Thus, the use of modified chitosan after filling eliminates bone defects of critical size in the maxillofacial region, revealing early signs of bone regeneration, and serves as a promising material in reconstructive dentistry.

Macrophages-bone marrow mesenchymal stem cells crosstalk in bone healing

Bone healing is associated with many orthopedic conditions, including fractures and osteonecrosis, arthritis, metabolic bone disease, tumors and periprosthetic particle-associated osteolysis. How to effectively promote bone healing has become a keen topic for researchers. The role of macrophages and bone marrow mesenchymal stem cells (BMSCs) in bone healing has gradually come to light with the development of the concept of osteoimmunity. Their interaction regulates the balance between inflammation and regeneration, and when the inflammatory response is over-excited, attenuated, or disturbed, it results in the failure of bone healing. Therefore, an in-depth understanding of the function of macrophages and bone marrow mesenchymal stem cells in bone regeneration and the relationship between the two could provide new directions to promote bone healing. This paper reviews the role of macrophages and bone marrow mesenchymal stem cells in bone healing and the mechanism and significance of their interaction. Several new therapeutic ideas for regulating the inflammatory response in bone healing by targeting macrophages and bone marrow mesenchymal stem cells crosstalk are also discussed.

Varying degrees of spontaneous osteogenesis of Masquelet's induced membrane: experimental and clinical observations

BACKGROUND

Masquelet's induced membrane (IM) has osteogenesis activity, but IM spontaneous osteogenesis (SO) has not been described previously.

OBJECTIVES

To report on varying degrees of IMSO and analyze its possible causes.

METHODS

Twelve eight-week-old male Sprague-Dawley rats with 10 mm right femoral bone defects who received the first stage of IM technique (IMT) were used to observe the SO. In addition, clinical data from patients with bone defects who received the first stage of IMT with an interval of > 2 months post-operatively and exhibited SO between January 2012 and June 2020 were retrospectively analyzed. The SO was divided into four grades according to the amount and characteristics of the new bone formation.

RESULTS

At twelve weeks, grade II SO was observed in all rats, and more new bone was formed in the IM near the bone end forming an uneven margin. Histology revealed bone and cartilage foci in the new bone. Four of the 98 patients treated with the first stage of IMT exhibited IMSO, including one female and three males with a median age of 40.5 years (range 29-52 years). The bone defects were caused by severe fractures and infection in two cases and by infection or tumor in one case each. Partial or segmental defects occurred in two cases. The time interval between inserting a cement spacer and diagnosis of SO ranged from six months to nine years. Two cases were grade I, and one case each of grades III and IV.

CONCLUSION

Varying degrees of SO confirm the existence of the IMSO phenomenon. Bioactive bone tissue or local inflammation and a long time interval are the primary reasons underlying enhancement of the osteogenic activity of IM and leading to SO, which tends to take place as endochondral osteogenesis.

Sequential release of vascular endothelial growth factor-A and bone morphogenetic protein-2 from osteogenic scaffolds assembled by PLGA microcapsules: A preliminary study in vitro

Bone regeneration is a complex process sequentially regulated by multiple cytokines at different stages. Vascular endothelial growth factor-A (VEGF-A) and bone morphogenetic protein-2 (BMP-2) are the two most important factors involved in this process, and the combination of the two can achieve better bone regeneration by coupling angiogenesis and osteogenesis. In this study, poly(lactic-co-glycolic acid) (PLGA) microspheres with core-shell structure (microcapsules) encapsulating VEGF-A or BMP-2 were prepared by coaxial channel injection and continuous fluid technology. The sequential release of two cytokines by microcapsules with different PLGA molecular weight and shell thickness and its performance in vitro were explored. It was demonstrated that the molecular weight of PLGA significantly affected the degradation and release kinetics of microcapsules, while the thickness of the shell can regulate the release in a finer level. VEGF-A encapsulated microcapsules with low molecular weight can induce vascular endothelial cells to form lumens structures in vitro at an early stage. And BMP-2 encapsulated microcapsules could promote osteogenic differentiation, but the effect could be delayed when the microcapsules were prepared with PLGA of 150 kDa. In conclusion, the core-shell PLGA microcapsules in this study can sequentially release VEGF-A and BMP-2 at different stages to simulate natural bone repair.

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.

Validation of a compartmental model to predict drug release from porous structures produced by ScCO2 techniques

A global release model is proposed to study the drug release from porous materials for pharmaceutical applications. This model is defined by implementing a compartmental model where the release profile could be explained as the combination of mass transfer phenomena through three compartments as well as a desorption process or dissolution process from the support. This model was validated with five different systems produced with supercritical CO₂ (aerogels, membranes, and fibers), showing different release processes. Numerical results indicate that this compartmental approach can be useful to determine adsorption and desorption constants as well as mass transfer resistances within the material. Likewise, this model can predict lag phases and imbibition phenomena. Therefore, the development of compartmental models can be an alternative to traditional models to successfully predict the drug profile of porous materials, achieving a complete understanding of the involved phenomena regardless of the material characteristics.

Gelatin Methacryloyl Hydrogels for Musculoskeletal Tissue Regeneration

Musculoskeletal disorders are a significant burden on the global economy and public health. Hydrogels have significant potential for enhancing the repair of damaged and injured musculoskeletal tissues as cell or drug delivery systems. Hydrogels have unique physicochemical properties which make them promising platforms for controlling cell functions. Gelatin methacryloyl (GelMA) hydrogel in particular has been extensively investigated as a promising biomaterial due to its tuneable and beneficial properties and has been widely used in different biomedical applications. In this review, a detailed overview of GelMA synthesis, hydrogel design and applications in regenerative medicine is provided. After summarising recent progress in hydrogels more broadly, we highlight recent advances of GelMA hydrogels in the emerging fields of musculoskeletal drug delivery, involving therapeutic drugs (e.g., growth factors, antimicrobial molecules, immunomodulatory drugs and cells), delivery approaches (e.g., single-, dual-release system), and material design (e.g., addition of organic or inorganic materials, 3D printing). The review concludes with future perspectives and associated challenges for developing local drug delivery for musculoskeletal applications.

Novel Techniques and Future Perspective for Investigating Critical-Size Bone Defects

A critical-size bone defect is a challenging clinical problem in which a gap between bone ends will not heal and will become a nonunion. The current treatment is to harvest and transplant an autologous bone graft to facilitate bone bridging. To develop less invasive but equally effective treatment options, one needs to first have a comprehensive understanding of the bone healing process. Therefore, it is imperative to leverage the most advanced technologies to elucidate the fundamental concepts of the bone healing process and develop innovative therapeutic strategies to bridge the nonunion gap. In this review, we first discuss the current animal models to study critical-size bone defects. Then, we focus on four novel analytic techniques and discuss their strengths and limitations. These four technologies are mass cytometry (CyTOF) for enhanced cellular analysis, imaging mass cytometry (IMC) for enhanced tissue special imaging, single-cell RNA sequencing (scRNA-seq) for detailed transcriptome analysis, and Luminex assays for comprehensive protein secretome analysis. With this new understanding of the healing of critical-size bone defects, novel methods of diagnosis and treatment will emerge.

Silk Hydrogel for Tissue Engineering: A Review

AIM

This review aims to explore the importance of silk hydrogel and its potential in tissue engineering (TE).

BACKGROUND

Tissue engineering is a procedure that incorporates cells into the scaffold materials with suitable growth factors to regenerate injured tissue. For tissue formation in TE, the scaffold material plays a key role. Different forms of silk fibroin (SF), such as films, mats, hydrogels, and sponges, can be easily manufactured when SF is disintegrated into an aqueous solution. High precision procedures such as micropatterning and bioprinting of SF-based scaffolds have been used for enhanced fabrication.

REVIEW RESULTS

In this narrative review, SF physicochemical and mechanical properties have been presented. We have also discussed SF fabrication techniques like electrospinning, spin coating, freeze-drying, and physiochemical cross-linking. The application of SF-based scaffolds for skeletal, tissue, joint, muscle, epidermal, tissue repair, and tympanic membrane regeneration has also been addressed.

CONCLUSION

SF has excellent mechanical properties, tunability, biodegradability, biocompatibility, and bioresorbability.

CLINICAL SIGNIFICANCE

Silk hydrogels are an ideal scaffold matrix material that will significantly impact tissue engineering applications, given the rapid scientific advancements in this field.

Controlled growth factor delivery system with osteogenic-angiogenic coupling effect for bone regeneration

Objective

Bone regeneration involves a coordinated cascade of events that are regulated by several cytokines and growth factors, among which bone morphogenic protein-2 (BMP-2), vascular endothelial growth factor (VEGF) and fibroblast growth factor-2 (FGF-2) play important roles. In this study, we investigated the effects of dual release of the three growth factors on bone regeneration in femur defects.

Methods

A composite consisting of Gelatin microparticles loaded with VEGF/FGF-2 and poly(lactic-co-glycolic acid)-poly(ethylene glycol)-carboxyl (PLGA-PEG-COOH) microparticles loaded with BMP-2 encapsulated in a nano hydroxyapatite-poly actic-co-glycolic acid (nHA-PLGA) scaffold was prepared for the dual release of the growth factors.

Results

On the 14th day, decreased release rate of BMP-2 compared with FGF-2 and VEGF was observed. However, after 14 days, compared to FGF-2 and VEGF, BMP-2 showed an increased release rate. Controlled dual release of BMP-2 and VEGF, FGF-2 resulted in a significant osteogenic differentiation of bone mesenchymal stem cells (BMSCs). Moreover, effects of the composite scaffold on functional connection of osteoblast-vascular cells during bone development were evaluated. The synergistic effects of dual delivery of growth factors were shown to promote the expression of VEGF in BMSCs. Increased secretion of VEGF from BMSCs promoted the proliferation and angiogenic differentiation of human umbilical vein endothelial cells (HUVECs) in the co-culture system. At 12 weeks after implantation, blood vessel and bone formation were analyzed by micro-CT and histology. The composite scaffold significantly promoted the formation of blood vessels and new bone in femur defects.

Conclusions

These findings demonstrate that dual delivery of angiogenic factors and osteogenic factors from Gelatin and PLGA-PEG-COOH microparticles-based composite scaffolds exerted an osteogenic-angiogenic coupling effect on bone regeneration. This approach will inform on the development of appropriate designs of high-performance bioscaffolds for bone tissue engineering.

Bone Scaffolds: An Incorporation of Biomaterials, Cells, and Biofactors

Large injuries to bones are still one of the most challenging musculoskeletal problems. Tissue engineering can combine stem cells, scaffold biomaterials, and biofactors to aid in resolving this complication. Therefore, this review aims to provide information on the recent advances made to utilize the potential of biomaterials for making bone scaffolds and the assisted stem cell therapy and use of biofactors for bone tissue engineering. The requirements and different types of biomaterials used for making scaffolds are reviewed. Furthermore, the importance of stem cells and biofactors (growth factors and extracellular vesicles) in bone regeneration and their use in bone scaffolds and the key findings are discussed. Lastly, some of the main obstacles in bone tissue engineering and future trends are highlighted.

Multiple Ion Scaffold-Based Delivery Platform for Potential Application in Early Stages of Bone Regeneration

Bone has the intrinsic capacity to regenerate itself, as long as the damage is small, through the sequential stimulation of specific phases, such as angiogenesis followed by osteogenesis. However, when the damage is extensive it is unable to regenerate and bone tissue engineering is used as an alternative. In this study, we developed a platform to allow the triple ion delivery with sequential delivery capacity to potentially stimulate antibacterial, angiogenic and osteogenic processes. The scaffold-based platform consisted of alginate/hydroxyapatite (HA) microparticles embedded in alginate fibers. Firstly, microparticles were developed using different ratios of alginate:HA using the spraying method, resulting in a high reproducibility of the technique. Microparticle size between 100–300 µm and ratio 1:40 resulted in a more spherical morphology and were selected for their incorporation into alginate fiber. Different amounts of copper and cobalt were added with the microparticles and alginate fiber, respectively, were used as model ions which could eventually modulate and mimic antimicrobial and angiogenic processes. Moreover, calcium ion was also incorporated in both, in order to provide the system with potential osteogenic properties together with HA. The multiple delivery of copper, cobalt and calcium released were in the therapeutic range as measured by induced coupled plasma (ICP), providing a promising delivery strategy for tissue engineering.

Advanced Strategies of Biomimetic Tissue-Engineered Grafts for Bone Regeneration

The failure to repair critical-sized bone defects often leads to incomplete regeneration or fracture non-union. Tissue-engineered grafts have been recognized as an alternative strategy for bone regeneration due to their potential to repair defects. To design a successful tissue-engineered graft requires the understanding of physicochemical optimization to mimic the composition and structure of native bone, as well as the biological strategies of mimicking the key biological elements during bone regeneration process. This review provides an overview of engineered graft-based strategies focusing on physicochemical properties of materials and graft structure optimization from macroscale to nanoscale to further boost bone regeneration, and it summarizes biological strategies which mainly focus on growth factors following bone regeneration pattern and stem cell-based strategies for more efficient repair. Finally, it discusses the current limitations of existing strategies upon bone repair and highlights a promising strategy for rapid bone regeneration.

Strategies for inclusion of growth factors into 3D printed bone grafts

There remains a critical need to develop new technologies and materials that can meet the demands of treating large bone defects. The advancement of 3-dimensional (3D) printing technologies has allowed the creation of personalized and customized bone grafts, with specific control in both macro- and micro-architecture, and desired mechanical properties. Nevertheless, the biomaterials used for the production of these bone grafts often possess poor biological properties. The incorporation of growth factors (GFs), which are the natural orchestrators of the physiological healing process, into 3D printed bone grafts, represents a promising strategy to achieve the bioactivity required to enhance bone regeneration. In this review, the possible strategies used to incorporate GFs to 3D printed constructs are presented with a specific focus on bone regeneration. In particular, the strengths and limitations of different methods, such as physical and chemical cross-linking, which are currently used to incorporate GFs to the engineered constructs are critically reviewed. Different strategies used to present one or more GFs to achieve simultaneous angiogenesis and vasculogenesis for enhanced bone regeneration are also covered in this review. In addition, the possibility of combining several manufacturing approaches to fabricate hybrid constructs, which better mimic the complexity of biological niches, is presented. Finally, the clinical relevance of these approaches and the future steps that should be taken are discussed.

Functionalization of Synthetic Bone Substitutes

Bone substitutes have been applied to treat osseous defects for a long time. To prevent implant related infection (IRI) and enhance bone healing functionalized biomaterials, antibiotics and osteoinductive substances have been introduced. This study gives an overview of the current available surface-coated bone substitutes and provides an outlook for future perspectives.

Silk Fibroin as a Functional Biomaterial for Tissue Engineering

Tissue engineering (TE) is the approach to combine cells with scaffold materials and appropriate growth factors to regenerate or replace damaged or degenerated tissue or organs. The scaffold material as a template for tissue formation plays the most important role in TE. Among scaffold materials, silk fibroin (SF), a natural protein with outstanding mechanical properties, biodegradability, biocompatibility, and bioresorbability has attracted significant attention for TE applications. SF is commonly dissolved into an aqueous solution and can be easily reconstructed into different material formats, including films, mats, hydrogels, and sponges via various fabrication techniques. These include spin coating, electrospinning, freeze drying, physical, and chemical crosslinking techniques. Furthermore, to facilitate fabrication of more complex SF-based scaffolds with high precision techniques including micro-patterning and bio-printing have recently been explored. This review introduces the physicochemical and mechanical properties of SF and looks into a range of SF-based scaffolds that have been recently developed. The typical TE applications of SF-based scaffolds including bone, cartilage, ligament, tendon, skin, wound healing, and tympanic membrane, will be highlighted and discussed, followed by future prospects and challenges needing to be addressed.

Polymeric Tissue Adhesives

Polymeric tissue adhesives provide versatile materials for wound management and are widely used in a variety of medical settings ranging from minor to life-threatening tissue injuries. Compared to the traditional methods of wound closure (i.e., suturing and stapling), they are relatively easy to use, enable rapid application, and introduce minimal tissue damage. Furthermore, they can act as hemostats to control bleeding and provide a tissue-healing environment at the wound site. Despite their numerous current applications, tissue adhesives still face several limitations and unresolved challenges (e.g., weak adhesion strength and poor mechanical properties) that limit their use, leaving ample room for future improvements. Successful development of next-generation adhesives will likely require a holistic understanding of the chemical and physical properties of the tissue-adhesive interface, fundamental mechanisms of tissue adhesion, and requirements for specific clinical applications. In this review, we discuss a set of rational guidelines for design of adhesives, recent progress in the field along with examples of commercially available adhesives and those under development, tissue-specific considerations, and finally potential functions for future adhesives. Advances in tissue adhesives will open new avenues for wound care and potentially provide potent therapeutics for various medical applications.

The Effect of Autogenous Bone Graft Mixed With Recombinant Human Vascular Endothelial Growth Factor on Bone Regeneration

INTRODUCTION

Bone regeneration depends on vascularization in the pertaining site. This study aims to investigate autogenous bone grafts mixed with recombinant human vascular endothelial growth factor (rhVEGF) effect on bone regeneration in rat mandibular bone defect.

MATERIAL METHODS

Using 32 Wistar Albino rats, our experimental study consists of 4 groups: Group1 (control group), the defect was empty; Group 2, autogenous bone graft only; Group 3, gelatin sponge plus rhVEGF applications; Group 4, autogenous bone graft plus rhVEGF applications. The rats were sacrificed on the 28th day after the operation. New bone regeneration was analyzed histologically and immunohistochemically.

RESULTS

Our histological analyses revealed that new bone regeneration in Group 3 was enhanced in comparison to Group 1 and Group 2. However, autogenous bone grafts combined with rhVEGF provided the best outcome in conjunction with the increased remodeling of the new bone.

CONCLUSIONS

In the light of our results, it can be concluded that autogenous bone grafts in combination with rhVEGF can, potentially, enhance neovascularization and bone regeneration.

Progress and Prospects of Polymer-Based Drug Delivery Systems for Bone Tissue Regeneration

Despite the high regenerative capacity of bone tissue, there are some cases where bone repair is insufficient for a complete functional and structural recovery after damage. Current surgical techniques utilize natural and synthetic bone grafts for bone healing, as well as collagen sponges loaded with drugs. However, there are certain disadvantages associated with these techniques in clinical usage. To improve the therapeutic efficacy of bone tissue regeneration, a number of drug delivery systems based on biodegradable natural and synthetic polymers were developed and examined in in vitro and in vivo studies. Recent studies have demonstrated that biodegradable polymers play a key role in the development of innovative drug delivery systems and tissue engineered constructs, which improve the treatment and regeneration of damaged bone tissue. In this review, we discuss the most recent advances in the field of polymer-based drug delivery systems for the promotion of bone tissue regeneration and the physical-chemical modifications of polymers for controlled and sustained release of one or more drugs. In addition, special attention is given to recent developments on polymer nano- and microparticle-based drug delivery systems for bone regeneration.

Microbial Exopolysaccharides as Drug Carriers

Microbial exopolysaccharides are peculiar polymers that are produced by living organisms and protect them against environmental factors. These polymers are industrially recovered from the medium culture after performing a fermentative process. These materials are biocompatible and biodegradable, possessing specific and beneficial properties for biomedical drug delivery systems. They can have antitumor activity, they can produce hydrogels with different characteristics due to their molecular structure and functional groups, and they can even produce nanoparticles via a self-assembly phenomenon. This review studies the potential use of exopolysaccharides as carriers for drug delivery systems, covering their versatility and their vast possibilities to produce particles, fibers, scaffolds, hydrogels, and aerogels with different strategies and methodologies. Moreover, the main properties of exopolysaccharides are explained, providing information to achieve an adequate carrier selection depending on the final application.

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

The objective of this study was to investigate the controlled release of two growth factors (BMP-2 and VEGF) as a treatment strategy for bone healing in clinically challenging composite injuries, consisting of a femoral segmental bone defect and volumetric muscle loss. This is the first investigation of dual growth factor delivery in a composite injury model using an injectable delivery system consisting of heparin microparticles and alginate gel. The loading efficiency of growth factors into these biomaterials was found to be >90%, revealing a strong affinity of VEGF and BMP-2 to heparin and alginate. The system could achieve simultaneous or tunable release of VEGF and BMP-2 by varying the loading strategy. Single growth factor delivery (VEGF or BMP-2 alone) significantly enhanced vascular growth in vitro. However, no synergistic effect was observed for dual growth factor (BMP-2 + VEGF) delivery in vitro. Effective bone healing was achieved in all treatment groups (BMP-2, simultaneous or tunable delivery of BMP-2 and VEGF) in the composite injury model. The mechanics of the regenerated bone reached a maximum strength of ~52% of intact bone with tunable delivery of VEGF and BMP-2. Overall, simultaneous or tunable co-delivery of low-dose BMP-2 and VEGF failed to fully restore the mechanics of bone in this injury model. Given the severity of the composite injury, VEGF alone may not be sufficient to establish mature and stable blood vessels when compared with previous studies co-delivering BMP-2+VEGF enhanced bone tissue regeneration. Hence, future studies are warranted to develop an alternative treatment strategy focusing on better control over growth factor dose, spatiotemporal delivery, and additional growth factors to regenerate fully functional bone tissue. STATEMENT OF SIGNIFICANCE: We have developed an injectable delivery system consisting of heparin microparticles and an alginate hydrogel that is capable of delivering multiple growth factors in a tunable manner. We used this delivery system to deliver BMP-2 and VEGF in a rodent model of composite bone-muscle injury that mimics clinical type III open fractures. An advanced treatment strategy is necessary for these injuries in order to avoid the negative side effects of high doses of growth factors and because it has been shown that the addition of a muscle injury in this model attenuates the bone regenerative effect of BMP-2. This is the first study to test the effects of dual growth factor delivery (BMP-2/VEGF) on bone healing in a composite bone-muscle injury model and is expected to open up new directions in protein delivery for regenerative medicine.

3D bioprinting spatiotemporally defined patterns of growth factors to tightly control tissue regeneration

Therapeutic growth factor delivery typically requires supraphysiological dosages, which can cause undesirable off-target effects. The aim of this study was to 3D bioprint implants containing spatiotemporally defined patterns of growth factors optimized for coupled angiogenesis and osteogenesis. Using nanoparticle functionalized bioinks, it was possible to print implants with distinct growth factor patterns and release profiles spanning from days to weeks. The extent of angiogenesis in vivo depended on the spatial presentation of vascular endothelial growth factor (VEGF). Higher levels of vessel invasion were observed in implants containing a spatial gradient of VEGF compared to those homogenously loaded with the same total amount of protein. Printed implants containing a gradient of VEGF, coupled with spatially defined BMP-2 localization and release kinetics, accelerated large bone defect healing with little heterotopic bone formation. This demonstrates the potential of growth factor printing, a putative point of care therapy, for tightly controlled tissue regeneration.

A comprehensive review of mouse diaphyseal femur fracture models

Complications related to treatment of long bone fractures still stand as a major challenge for orthopaedic surgeons. Elucidation of the mechanisms of bone healing and development, and the subsequent alteration of these mechanisms to improve outcomes, typically requires animal models as an intermediary between in vitro and human clinical studies. Murine models are some of the most commonly used in translational research, and mouse fracture models are particularly diverse, offering a wide variety of customization with distinct benefits and limitations depending on the study. This review critically examines three common femur fracture models in the mouse, namely cortical hole, 3-point fracture (Einhorn), and segmental bone defect. We lay out the general procedure for execution of each model, evaluate the practical implications and important advantages/disadvantages of each and describe recent innovations. Furthermore, we explore the applications that each model is best adapted for in the context of the current state of murine orthopaedic research.

Natural Polymeric Scaffolds in Bone Regeneration

Despite considerable advances in microsurgical techniques over the past decades, bone tissue remains a challenging arena to obtain a satisfying functional and structural restoration after damage. Through the production of substituting materials mimicking the physical and biological properties of the healthy tissue, tissue engineering strategies address an urgent clinical need for therapeutic alternatives to bone autografts. By virtue of their structural versatility, polymers have a predominant role in generating the biodegradable matrices that hold the cells in situ to sustain the growth of new tissue until integration into the transplantation area (i.e., scaffolds). As compared to synthetic ones, polymers of natural origin generally present superior biocompatibility and bioactivity. Their assembly and further engineering give rise to a wide plethora of advanced supporting materials, accounting for systems based on hydrogels or scaffolds with either fibrous or porous architecture. The present review offers an overview of the various types of natural polymers currently adopted in bone tissue engineering, describing their manufacturing techniques and procedures of functionalization with active biomolecules, and listing the advantages and disadvantages in their respective use in order to critically compare their actual applicability potential. Their combination to other classes of materials (such as micro and nanomaterials) and other innovative strategies to reproduce physiological bone microenvironments in a more faithful way are also illustrated. The regeneration outcomes achieved in vitro and in vivo when the scaffolds are enriched with different cell types, as well as the preliminary clinical applications are presented, before the prospects in this research field are finally discussed. The collection of studies herein considered confirms that advances in natural polymer research will be determinant in designing translatable materials for efficient tissue regeneration with forthcoming impact expected in the treatment of bone defects.

Adjuvant Drug-Assisted Bone Healing: Advances and Challenges in Drug Delivery Approaches

Bone defects of critical size after compound fractures, infections, or tumor resections are a challenge in treatment. Particularly, this applies to bone defects in patients with impaired bone healing due to frequently occurring metabolic diseases (above all diabetes mellitus and osteoporosis), chronic inflammation, and cancer. Adjuvant therapeutic agents such as recombinant growth factors, lipid mediators, antibiotics, antiphlogistics, and proangiogenics as well as other promising anti-resorptive and anabolic molecules contribute to improving bone healing in these disorders, especially when they are released in a targeted and controlled manner during crucial bone healing phases. In this regard, the development of smart biocompatible and biostable polymers such as implant coatings, scaffolds, or particle-based materials for drug release is crucial. Innovative chemical, physico- and biochemical approaches for controlled tailor-made degradation or the stimulus-responsive release of substances from these materials, and more, are advantageous. In this review, we discuss current developments, progress, but also pitfalls and setbacks of such approaches in supporting or controlling bone healing. The focus is on the critical evaluation of recent preclinical studies investigating different carrier systems, dual- or co-delivery systems as well as triggered- or targeted delivery systems for release of a panoply of drugs.

Solvent-Free Approaches for the Processing of Scaffolds in Regenerative Medicine

The regenerative medicine field is seeking novel strategies for the production of synthetic scaffolds that are able to promote the in vivo regeneration of a fully functional tissue. The choices of the scaffold formulation and the manufacturing method are crucial to determine the rate of success of the graft for the intended tissue regeneration process. On one hand, the incorporation of bioactive compounds such as growth factors and drugs in the scaffolds can efficiently guide and promote the spreading, differentiation, growth, and proliferation of cells as well as alleviate post-surgical complications such as foreign body responses and infections. On the other hand, the manufacturing method will determine the feasible morphological properties of the scaffolds and, in certain cases, it can compromise their biocompatibility. In the case of medicated scaffolds, the manufacturing method has also a key effect in the incorporation yield and retained activity of the loaded bioactive agents. In this work, solvent-free methods for scaffolds production, i.e., technological approaches leading to the processing of the porous material with no use of solvents, are presented as advantageous solutions for the processing of medicated scaffolds in terms of efficiency and versatility. The principles of these solvent-free technologies (melt molding, 3D printing by fused deposition modeling, sintering of solid microspheres, gas foaming, and compressed CO2 and supercritical CO2-assisted foaming), a critical discussion of advantages and limitations, as well as selected examples for regenerative medicine purposes are herein presented.

Visible light mediated PVA-tyramine hydrogels for covalent incorporation and tailorable release of functional growth factors

PVA-Tyr hydrogel facilitated covalent incorporation can control release of pristine growth factors while retaining their native bioactivity.

3D printed porous titanium cages filled with simvastatin hydrogel promotes bone ingrowth and spinal fusion in rhesus macaques

Sustainable release of simvastatin from poloxamer 407 hydrogel in 3D-printed porous Ti6Al4V for spinal fusion in rhesus macaques.

Enhancement and orchestration of osteogenesis and angiogenesis by a dual-modular design of growth factors delivery scaffolds and 26SCS decoration

Preserving the bioactivity of growth factors (GFs) and mimicking their in vivo supply patterns are challenging in the development of GFs-based bone grafts. In this study, we develop a 2-N, 6-O-sulfated chitosan (26SCS) functionalized dual-modular scaffold composed of mesoporous bioactive glass (MBG) with hierarchical porous structures (module I) and GelMA hydrogel columns (module II) in situ fixed in hollowed channels of the module I, which is capable of realizing differentiated delivery modes for osteogenic rhBMP-2 and angiogenic VEGF. A combinational release profile consisting of a high concentration of VEGF initially followed by a decreasing concentration over time, and a slower/sustainable release of rhBMP-2 is realized by immobilizing rhBMP-2 in module I and embedding VEGF in module II. Systematic in vitro and in vivo studies prove that the two coupled processes of osteogenesis and angiogenesis are well-orchestrated and both enhanced ascribed to the specific GFs delivery modes and 26SCS decoration. 26SCS not only enhances the GFs' bioactivity but also decreases antagonism effects of noggin. This study highlights the importance of differentiating the delivery pattern of different GFs and likely sheds light on the future design of growth factor-based bone grafts.

Scaffold-mediated sequential drug/gene delivery to promote nerve regeneration and remyelination following traumatic nerve injuries

Neural tissue regeneration following traumatic injuries is often subpar. As a result, the field of neural tissue engineering has evolved to find therapeutic interventions and has seen promising outcomes. However, robust nerve and myelin regeneration remain elusive. One possible reason may be the fact that tissue regeneration often follows a complex sequence of events in a temporally-controlled manner. Although several other fields of tissue engineering have begun to recognise the importance of delivering two or more biomolecules sequentially for more complete tissue regeneration, such serial delivery of biomolecules in neural tissue engineering remains limited. This review aims to highlight the need for sequential delivery to enhance nerve regeneration and remyelination after traumatic injuries in the central nervous system, using spinal cord injuries as an example. In addition, possible methods to attain temporally-controlled drug/gene delivery are also discussed for effective neural tissue regeneration.

Nonvascularized Bone Graft Reconstruction of the Irradiated Murine Mandible: An Analogue of Clinical Head and Neck Cancer Treatment

Nonvascularized bone grafts (NBGs) represent a practical method of mandibular reconstruction that is precluded in head and neck cancer patients by the destructive effects of radiotherapy. Advances in tissue-engineering may restore NBGs as a viable surgical technique, but expeditious translation demands a small-animal model that approximates clinical practice. This study establishes a murine model of irradiated mandibular reconstruction using a segmental iliac crest NBG for the investigation of imperative bone healing strategies. Twenty-seven male isogenic Lewis rats were divided into 2 groups; control bone graft and irradiated bone graft (XBG). Additional Lewis rats served as graft donors. The XBG group was administered a fractionated dose of 35Gy. All rats underwent reconstruction of a segmental, critical-sized defect of the left hemi-mandible with a 5 mm NBG from the iliac crest, secured by a custom radiolucent plate. Following a 60-day recovery period, hemi-mandibles were evaluated for bony union, bone mineralization, and biomechanical strength (P < 0.05). Bony union rates were significantly reduced in the XBG group (42%) compared with controls (80%). Mandibles in the XBG group further demonstrated substantial radiation injury through significant reductions in all metrics of bone mineralization and biomechanical strength. These observations are consistent with the clinical sequelae of radiotherapy that limit NBGs to nonirradiated patients. This investigation provides a clinically relevant, quantitative model in which innovations in tissue engineering may be evaluated in the setting of radiotherapy to ultimately provide the advantages of NBGs to head and neck cancer patients and reconstructive surgeons.

Bone allografts combined with adipose-derived stem cells in an optimized cell/volume ratio showed enhanced osteogenesis and angiogenesis in a murine femur defect model

Critical sized defects, especially in long bones, pose one of the biggest problems in orthopedic surgery. By definition, these defects do not heal without further treatment. Different therapeutic options range from autologous bone grafts, for example, free vascularized bone grafts, to commercially available bone allografts. Disadvantages of these bone allografts are related to reduced osteogenesis, since they are solely composed of cell-free bone matrix. The purpose of this study was to investigate the cell seeding efficiency of human adipose-derived stem cells (hASCs) on human bone allografts in vitro and furthermore analyze these optimized seeded allografts in a critical sized defect model in vivo. Cancellous human bone allografts were colonized with human ASCs in vitro. Cell seeding efficiency was evaluated by Cell Counting Kit-8 assay. Thereafter, optimized hASC-seeded bone scaffolds were examined in a murine femur defect model, stabilized with the MouseExFix system. Subsequently, x-ray analysis and histology were performed. Examination of cell seeding efficiency revealed an optimum starting population of 84,600 cells per 100 mm³ scaffold. In addition, scaffolds seeded with hASCs showed increased osteogenesis compared with controls. Histological analysis revealed increased remodeling and elevated new bone formation within hASC-seeded scaffolds. Moreover, immunohistochemical stainings revealed increased proliferation, osteogenesis, and angiogenesis. In this study, we systemically optimized cell/volume ratio of two promising components of tissue engineering: hASCs and human bone allografts. These findings may serve as a basis for future translational studies. KEY MESSAGES: Bone tissue engineering. Mesenchymal stem cells derived from human adipose tissue (hASCs). Optimal cell/volume ratio of cell-seeded scaffolds. Increased osteogenesis and angiogenesis in vivo.

Biomaterial-assisted local and systemic delivery of bioactive agents for bone repair

Although bone tissues possess an intrinsic capacity for repair, there are cases where bone healing is either impaired or insufficient, such as fracture non-union, osteoporosis, osteomyelitis, and cancers. In these cases, treatments like surgical interventions are used, either alone or in combination with bioactive agents, to promote tissue repair and manage associated clinical complications. Improving the efficacy of bioactive agents often requires carriers, with biomaterials being a pivotal player. In this review, we discuss the role of biomaterials in realizing the local and systemic delivery of biomolecules to the bone tissue. The versatility of biomaterials enables design of carriers with the desired loading efficiency, release profile, and on-demand delivery. Besides local administration, systemic administration of drugs is necessary to combat diseases like osteoporosis, warranting bone-targeting drug delivery systems. Thus, chemical moieties with the affinity towards bone extracellular matrix components like apatite minerals have been widely utilized to create bone-targeting carriers with better biodistribution, which cannot be achieved by the drugs alone. Bone-targeting carriers combined with the desired drugs or bioactive agents have been extensively investigated to enhance bone healing while minimizing off-target effects. Herein, these advancements in the field have been systematically reviewed. STATEMENT OF SIGNIFICANCE: Drug delivery is imperative when surgical interventions are not sufficient to address various bone diseases/defects. Biomaterial-assisted delivery systems have been designed to provide drugs with the desired loading efficiency, sustained release, and on-demand delivery to enhance bone healing. By surveying recent advances in the field, this review outlines the design of biomaterials as carriers for the local and systemic delivery of bioactive agents to the bone tissue. Particularly, biomaterials that bear chemical moieties with affinity to bone are attractive, as they can present the desired bioactive agents to the bone tissue efficiently and thus enhance the drug efficacy for bone repair.

Enzymatically-degradable alginate hydrogels promote cell spreading and in vivo tissue infiltration

Enzymatically-degradable materials recapitulate the dynamic and reciprocal interactions between cells and their native microenvironment by allowing cells to actively shape the degradation process. In order to engineer a synthetic 3D environment enabling cells to orchestrate the degradation of the surrounding material, norbornene-modified alginate was crosslinked with two different peptide crosslinkers susceptible to cleavage by matrix metalloproteinases using UV-initiated thiol-ene chemistry. Resulting hydrogels were characterized for their initial mechanical and rheological properties, and their degradation behavior was measured by tracking changes in wet weight upon enzyme incubation. This process was found to be a function of the crosslinker type and enzyme concentration, indicating that degradation kinetics could be controlled and tuned. When mouse embryonic fibroblasts were encapsulated in 3D, cell number remained constant and viability was high in all materials, while cell spreading and extensive filopodia formation was observed only in the degradable gels, not in non-degradable controls. After implanting hydrogels into the backs of C57/Bl6 mice for 8 weeks, histological stainings of recovered gel remnants and surrounding tissue revealed higher tissue and cell infiltration into degradable materials compared to non-degradable controls. This alginate-based material platform with cell-empowered enzymatic degradation could prove useful in diverse tissue engineering contexts, such as regeneration and drug delivery.

Coaxial bioprinting of cell-laden vascular constructs using a gelatin–tyramine bioink

The study revealed that linear distribution of multiple vascular cells could be achieved using synthetic bioink with short gelling time and a coaxial printing system.

Materials Science and Design Principles of Growth Factor Delivery Systems in Tissue Engineering and Regenerative Medicine

Growth factors (GFs) are signaling molecules that direct cell development by providing biochemical cues for stem cell proliferation, migration, and differentiation. GFs play a key role in tissue regeneration, but one major limitation of GF-based therapies is dosage-related adverse effects. Additionally, the clinical applications and efficacy of GFs are significantly affected by the efficiency of delivery systems and other pharmacokinetic factors. Hence, it is crucial to design delivery systems that provide optimal activity, stability, and tunable delivery for GFs. Understanding the physicochemical properties of the GFs and the biomaterials utilized for the development of biomimetic GF delivery systems is critical for GF-based regeneration. Many different delivery systems have been developed to achieve tunable delivery kinetics for single or multiple GFs. The identification of ideal biomaterials with tunable properties for spatiotemporal delivery of GFs is still challenging. This review characterizes the types, properties, and functions of GFs, the materials science of widely used biomaterials, and various GF loading strategies to comprehensively summarize the current delivery systems for tunable spatiotemporal delivery of GFs aimed for tissue regeneration applications. This review concludes by discussing fundamental design principles for GF delivery vehicles based on the interactive physicochemical properties of the proteins and biomaterials.

Synergistic effects of dual growth factor delivery from composite hydrogels incorporating 2-N,6-O-sulphated chitosan on bone regeneration

A promising strategy to accelerate bone generation is to deliver a combination of certain growth factors to the integration site via a controlled spatial and temporal delivery mode. Here, a composite hydrogel incorporating poly(lactide-co-glycolide) (PLGA) microspheres was accordingly prepared to load and deliver the osteogenic rhBMP-2 and angiogenic rhVEGF₁₆₅ in the required manner. In addition, 2-N,6-O-sulphated chitosan (26SCS), which is a synergetic factor of growth factors, was incorporated in the composite hydrogel as well. The system showed a similar release behaviour of the two growth factors regardless of 26SCS inclusion. RhBMP-2 loaded in PLGA microspheres showed a sustained release over a period of 2 weeks, whereas rhVEGF₁₆₅ loaded in hydrogel eluted almost completely from the hydrogel over the first 16 days. Both growth factors retained their efficacy, as quantified with relevant in vitro assays. Moreover, an enhanced cell response was achieved upon the delivery of dual growth factors, compared to that obtained with a single factor. Furthermore, in the presence of 26SCS, the system revealed significantly upregulated alkaline phosphatase activity, human umbilical vein endothelial cell proliferation, sprouting, nitric oxide secretion, and angiogenic gene expression. This study highlighted that the composite hydrogel incorporated with 26SCS appears to constitute a promising approach to deliver multiple growth factors. From our findings, we could also conclude that rhBMP-2 can promote angiogenesis and that the mechanism is worthy of further study in subsequent research.

Growth Factor Delivery Systems for Tissue Engineering and Regenerative Medicine

Growth factors (GFs) are often a key component in tissue engineering and regenerative medicine approaches. In order to fully exploit the therapeutic potential of GFs, GF delivery vehicles have to meet a number of key design criteria such as providing localized delivery and mimicking the dynamic native GF expression levels and patterns. The use of biomaterials as delivery systems is the most successful strategy for controlled delivery and has been translated into different commercially available systems. However, the risk of side effects remains an issue, which is mainly attributed to insufficient control over the release profile. This book chapter reviews the current strategies, chemistries, materials and delivery vehicles employed to overcome the current limitations associated with GF therapies.

Bone mesenchymal stem cells co-expressing VEGF and BMP-6 genes to combat avascular necrosis of the femoral head

The aim of the present study was to investigate the potential of bone mesenchymal stem cells (BMSCs) treated with a combination of vascular endothelial growth factor (VEGF) and bone morphogenetic protein-6 (BMP-6) genes for the treatment of avascular necrosis of the femoral head (ANFH). Rat BMSCs were isolated and purified using a density gradient centrifugation method. The purity and characteristics of the BMSCs were detected by cell surface antigens identification using flow cytometry. The experimental groups were administered with one of the following adeno-associated virus (AAV) vector constructs: AAV-green fluorescent protein (AAV-GFP), AAV-BMP-6, AAV-VEGF or AAV-VEGF-BMP-6. The expression of VEGF and BMP-6 was detected by reverse transcription-quantitative polymerase chain reaction, western blotting and ELISA assays. The effects of VEGF and BMP-6 on BMSCs were evaluated by angiogenic and osteogenic assays. The transfected BMSCs were combined with a biomimetic synthetic scaffold poly lactide-co-glycolide (PLAGA) and they were then subcutaneously implanted into nude mice. After four weeks, the implants were analyzed with histology and subsequent immunostaining to evaluate the effects of BMSCs on blood vessel and bone formation in vivo. In the AAV-VEGF-BMP-6 group, the expression levels of VEGF and BMP-6 were significantly increased and human umbilical vein endothelial cells tube formation was significantly enhanced compared with other groups. Capillaries and bone formation in the AAV-VEGF-BMP-6 group was significantly higher compared with the other groups. The results of the present study suggest that BMSCs expressing both VEGF and BMP-6 induce an increase in blood vessels and bone formation, which provides theoretical support for ANFH gene therapy.