Fabrication of Vascularized Bone Flaps with Sustained Release of Recombinant Human Bone Morphogenetic Protein-2 and Arteriovenous Bundle

Recent advancement in vascularized tissue-engineered bone based on materials design and modification

Bone is one of the most vascular network-rich tissues in the body and the vascular system is essential for the development, homeostasis, and regeneration of bone. When segmental irreversible damage occurs to the bone, restoring its vascular system by means other than autogenous bone grafts with vascular pedicles is a therapeutic challenge. By pre-generating the vascular network of the scaffold in vivo or in vitro, the pre-vascularization technique enables an abundant blood supply in the scaffold after implantation. However, pre-vascularization techniques are time-consuming, and in vivo pre-vascularization techniques can be damaging to the body. Critical bone deficiencies may be filled quickly with immediate implantation of a supporting bone tissue engineered scaffold. However, bone tissue engineered scaffolds generally lack vascularization, which requires modification of the scaffold to aid in enhancing internal vascularization. In this review, we summarize the relationship between the vascular system and osteogenesis and use it as a basis to further discuss surgical and cytotechnology-based pre-vascularization strategies and to describe the preparation of vascularized bone tissue engineered scaffolds that can be implanted immediately. We anticipate that this study will serve as inspiration for future vascularized bone tissue engineered scaffold construction and will aid in the achievement of clinical vascularized bone.

Intelligent Vascularized 3D/4D/5D/6D-Printed Tissue Scaffolds

Blood vessels are essential for nutrient and oxygen delivery and waste removal. Scaffold-repairing materials with functional vascular networks are widely used in bone tissue engineering. Additive manufacturing is a manufacturing technology that creates three-dimensional solids by stacking substances layer by layer, mainly including but not limited to 3D printing, but also 4D printing, 5D printing and 6D printing. It can be effectively combined with vascularization to meet the needs of vascularized tissue scaffolds by precisely tuning the mechanical structure and biological properties of smart vascular scaffolds. Herein, the development of neovascularization to vascularization to bone tissue engineering is systematically discussed in terms of the importance of vascularization to the tissue. Additionally, the research progress and future prospects of vascularized 3D printed scaffold materials are highlighted and presented in four categories: functional vascularized 3D printed scaffolds, cell-based vascularized 3D printed scaffolds, vascularized 3D printed scaffolds loaded with specific carriers and bionic vascularized 3D printed scaffolds. Finally, a brief review of vascularized additive manufacturing-tissue scaffolds in related tissues such as the vascular tissue engineering, cardiovascular system, skeletal muscle, soft tissue and a discussion of the challenges and development efforts leading to significant advances in intelligent vascularized tissue regeneration is presented.

Effect of adipose-derived stem cells seeding and surgical prefabrication on composite scaffold vascularization

The study aimed to evaluate an angiogenic effect of adipose-derived stem cells (ASCs) seeding and surgical prefabrication (placing a vascular pedicle inside the scaffold) on developed composite scaffolds made of poly-ε-caprolactone (PCL), β-tricalcium phosphate (β-TCP), and poly (lactic-co-glycolic acid) (PLGA) (PCL+β-TCP+PLGA). Moreover, we aimed to compare our data with previously tested PCL scaffolds to assess whether the new material has better angiogenic properties. The study included 18 inbred male WAG rats. There were three scaffold groups (six animals each): with non-seeded PCL+β-TCP+PLGA scaffolds, with PCL+β-TCP+PLGA scaffolds seeded with ASCs and with PCL+β-TCP+PLGA scaffolds seeded with ASCs and osteogenic-induced. Each rat was implanted with two scaffolds in the inguinal region (one prefabricated and one non-prefabricated). After 2 months from implantation, the scaffolds were explanted, and vessel density was determined by histopathological examination. Prefabricated ASC-seeded PCL+β-TCP+PLGA scaffolds promoted greater vessel formation than non-seeded scaffolds (19.73 ± 5.46 vs 12.54 ± 0.81; p = .006) and those seeded with osteogenic-induced ASCs (19.73 ± 5.46 vs 11.87±2.21; p = .004). The developed composite scaffold promotes vessel formation more effectively than the previously described PCL scaffold.

Use of 3D-printed polylactic acid/bioceramic composite scaffolds for bone tissue engineering in preclinical in vivo studies: A systematic review

3D-printed composite scaffolds have emerged as an alternative to deal with existing limitations when facing bone reconstruction. The aim of the study was to systematically review the feasibility of using PLA/bioceramic composite scaffolds manufactured by 3D-printing technologies as bone grafting materials in preclinical in vivo studies. Electronic databases were searched using specific search terms, and thirteen manuscripts were selected after screening. The synthesis of the scaffolds was carried out using mainly extrusion-based techniques. Likewise, hydroxyapatite was the most used bioceramic for synthesizing composites with a PLA matrix. Among the selected studies, seven were conducted in rats and six in rabbits, but the high variability that exists regarding the experimental process made it difficult to compare them. Regarding the results, PLA/Bioceramic composite scaffolds have shown to be biocompatible and mechanically resistant. Preclinical studies elucidated the ability of the scaffolds to be used as bone grafts, allowing bone growing without adverse reactions. In conclusion, PLA/Bioceramics scaffolds have been demonstrated to be a promising alternative for treating bone defects. Nevertheless, more care should be taken when designing and performing in vivo trials, since the lack of standardization of the processes, which prevents the comparison of the results and reduces the quality of the information. STATEMENT OF SIGNIFICANCE: 3D-printed polylactic acid/bioceramic composite scaffolds have emerged as an alternative to deal with existing limitations when facing bone reconstruction. Since preclinical in vivo studies with animal models represent a mandatory step for clinical translation, the present manuscript analyzed and discussed not only those aspects related to the selection of the bioceramic material, the synthesis of the implants and their characterization. But provides a new approach to understand how the design and perform of clinical trials, as well as the selection of the analysis methods, may affect the obtained results, by covering authors' knowledgebase from veterinary medicine to biomaterial science. Thus, this study aims to systematically review the feasibility of using polylactic acid/bioceramic scaffolds as grafting materials in preclinical trials.

Continuously released Zn2+ in 3D-printed PLGA/β-TCP/Zn scaffolds for bone defect repair by improving osteoinductive and anti-inflammatory properties

Long-term nonunion of bone defects has always been a major problem in orthopedic treatment. Artificial bone graft materials such as Poly (lactic-co-glycolic acid)/β-tricalcium phosphate (PLGA/β-TCP) scaffolds are expected to solve this problem due to their suitable degradation rate and good osteoconductivity. However, insufficient mechanical properties, lack of osteoinductivity and infections after implanted limit its large-scale clinical application. Hence, we proposed a novel bone repair bioscaffold by adding zinc submicron particles to PLGA/β-TCP using low temperature rapid prototyping 3D printing technology. We first screened the scaffolds with 1 wt% Zn that had good biocompatibility and could stably release a safe dose of zinc ions within 16 weeks to ensure long-term non-toxicity. As designed, the scaffold had a multi-level porous structure of biomimetic cancellous bone, and the Young's modulus (63.41 ± 1.89 MPa) and compressive strength (2.887 ± 0.025 MPa) of the scaffold were close to those of cancellous bone. In addition, after a series of in vitro and in vivo experiments, the scaffolds proved to have no adverse effects on the viability of BMSCs and promoted their adhesion and osteogenic differentiation, as well as exhibiting higher osteogenic and anti-inflammatory properties than PLGA/β-TCP scaffold without zinc particles. We also found that this osteogenic and anti-inflammatory effect might be related to Wnt/β-catenin, P38 MAPK and NFkB pathways. This study lay a foundation for the follow-up study of bone regeneration mechanism of Zn-containing biomaterials. We envision that this scaffold may become a new strategy for clinical treatment of bone defects.

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.

Microcomputed Tomography-Based Analysis of Neovascularization within Bioengineered Vascularized Tissues

In the field of tissue engineering, evaluating newly formed vascular networks is considered a fundamental step in deciphering the processes underlying tissue development. Several common modalities exist to study vessel network formation and function. However, a proper methodology that allows through three-dimensional visualization of neovessels in a reproducible manner is required. Here, we describe in-depth exploration, visualization, and analysis of vessels within newly formed tissues by utilizing a contrast agent perfusion protocol and high-resolution microcomputed tomography. Bioengineered constructs consisting of porous, biocompatible, and biodegradable scaffolds are loaded with cocultures of adipose-derived microvascular endothelial cells (HAMECs) and dental pulp stem cells (DPSCs) and implanted in a rat femoral bundle model. After 14 days of in vivo maturation, we performed the optimized perfusion protocol to allow host penetrating vascular visualization and assessment within neotissues. Following high-resolution microCT scanning of DPSC:HAMEC explants, we performed the volumetric and spatial analysis of neovasculature. Eventually, the process was repeated with a previously published coculture system for prevascularization based on adipose-derived mesenchymal stromal cells (MSCs) and HAMECs. Overall, our approach allows a comprehensive understanding of vessel organization during engraftment and development of neotissues.

Regenerative Approaches for the Treatment of Large Bone Defects

A variety of engineered materials have gained acceptance in orthopedic practice as substitutes for autologous bone grafts, although the regenerative efficacy of these engineered grafts is still limited compared with that of transplanted native tissues. For bone defects greater than 4-5 cm, however, common bone grafting procedures are insufficient and more complicated surgical interventions are required to repair and regenerate the damaged or missing bone. In this review, we describe current grafting materials and surgical techniques for the reconstruction of large bone defects, followed by tissue engineering (TE) efforts to develop improved therapies. Particular emphasis is placed on graft vascularization, because for both autologous bone and engineered alternatives, achieving adequate vascular development within the regenerating bone tissues remains a significant challenge in the context of large bone defects. To this end, TE and surgical strategies to induce development of a vasculature within bone grafts are discussed. Impact statement This review aims to present an accessible and thorough overview of current orthopedic surgical techniques as well as bone tissue engineering and vascularization strategies that might one day offer improvements to clinical therapies for the repair of large bone defects. We consider the lessons that clinical orthopedic reconstructive practices can contribute to the push toward engineered bone.

A novel device for treatment of osteonecrosis of femoral head: Feasibility and preliminary efficacy of animal study

Background

Interruption of blood supply will lead to necrosis of body tissues, such as osteonecrosis of femoral head (ONFH). Vascularization has always been regarded as one of the biggest challenges in tissue engineering. In the current study, a novel device was proposed to reconstruct blood supply of necrotic femoral head.

Methods

Cryo-insult with liquid nitrogen method was adopted to establish the ONFH model. In experimental group, a novel scaffold carrying vascular bundle was implanted into the necrotic femoral head after decompression and the transplanted vascular bundles were anastomosed with the existing blood vessels around the hip. In control group, a traditional porous scaffold was inserted alone without vessels. Feasibility of this strategy was verified by animal experiments. Micro-CT analysis and histological evaluation were performed to investigate its preliminary efficacy.

Results

Feasibility of this innovative treatment strategy had been successfully verified in animal experiments. In the area of necrosis repair, more bone tissue grew into the scaffold in experimental group than the control group evaluated by Micro-CT (three months: 29.66% VS 20.35%, P＜0.05; six months: 30.47% VS 25.10%, P＜0.05) and histological analysis (24.71% VS 16.45%, P＜0.05 ​at three months; 31.01% VS 20.60%, P＜0.05 ​at six months). Implanted vascular bundles had the potential to branch out many branches in the osteonecrosis repair area to facilitate blood supply reconstruction and bone repair.

Conclusions

This study proposed a novel device with clinical application prospects in the treatment of ONFH. It has the potential to provide new possibilities for rebuilding the blood supply of femoral head and repairing osteonecrosis.

Translational potential statement

The novel device proposed in this study has the potential to be applied to the treatment of early femoral head necrosis.

New Bone Formation in the Whole Decellularized Cortical Bone Scaffold Using the Model of Revitalizing a Haversian System

ABSTRACT

Decellularized allogeneic bone chips act as scaffolds for bone tissue regeneration. Owing to their lack of osteogenic potentials compared to autologous bone graft, decellularized bone scaffolds (DBSs) have applied only to small partial bone defects in clinical settings. Furthermore, only decellularized cancellous bone chips have been limitedly used for the purpose of bone regeneration. The cortical bone has less porosity and less osteogenic materials such as bone morphogenetic proteins in comparison with cancellous bone. In this study, we tried to accelerate new bone formation within the decellularized cortical bone scaffold using a vascular pedicle as an in vivo bioreactor.Forty DBSs were divided into 4 groups with different conditionings (DBS+ demineralized bone matrix [DBM], DBS+DBM+mesenchymal stem cells, DBS+DBM+vascular pedicle, and DBS+DBM+vascular pedicle+mesenchymal stem cells) and implanted into the back of 5 rabbits. Half of the DBSs were examined at 8 weeks and the other half at 16 weeks to determine vascularization level and osteogenesis within each group. New bone formation and bone-forming cells related to osteogenesis were observed via histological staining. Inclusion of the vascular pedicle resulted in larger areas of bone regeneration. With time, osteon structures became more prominent in groups containing the vascular pedicle.In summary, vascularized DBSs combined with a vascular pedicle have shown promising results for bone regeneration, thereby representing potential therapeutic alternatives for autologous bone grafts or bone tissue free transfer in large or segmental bone defects. In addition, demineralized whole cortical bone matrix along with vascular pedicle and various bone inductive materials, such as DBM and recombinant human bone morphogenetic protein-2, may be an additional new option of an ideal osteoinductive system.

Combining a Vascular Bundle and 3D Printed Scaffold with BMP-2 Improves Bone Repair and Angiogenesis

Vascularization is currently considered the biggest challenge in bone tissue engineering due to necrosis in the center of large scaffolds. We established a new expendable vascular bundle model to vascularize a three-dimensional printed channeled scaffold with and without bone morphogenetic protein-2 (BMP-2) for improved healing of large segmental bone defects. Bone formation and angiogenesis in an 8 mm critical-sized bone defect in the rat femur were significantly promoted by inserting a bundle consisting of the superficial epigastric artery and vein into the central channel of a large porous polycaprolactone scaffold. Vessels were observed sprouting from the vascular bundle inserted in the central tunnel. Although the regenerated bone volume in the group receiving the scaffold and vascular bundle was similar to that of the healthy femur, the rate of union of the group was not satisfactory (25% at 8 weeks). BMP-2 delivery was found to promote not only bone formation but also angiogenesis in the critical-sized bone defects. Both insertion of the vascular bundle alone and BMP-2 loading alone induced comparable levels of angiogenesis and when used in combination, significantly greater vascular volume was observed. These findings suggest a promising new modality of treatment in large bone defects. Level of Evidence: Therapeutic level I.

Printing New Bones: From Print-and-Implant Devices to Bioprinted Bone Organ Precursors

Regenerating large bone defects remains a significant clinical challenge, motivating increased interest in additive manufacturing and 3D bioprinting to engineer superior bone graft substitutes. 3D bioprinting enables different biomaterials, cell types, and growth factors to be combined to develop patient-specific implants capable of directing functional bone regeneration. Current approaches to bioprinting such implants fall into one of two categories, each with their own advantages and limitations. First are those that can be 3D bioprinted and then directly implanted into the body and second those that require further in vitro culture after bioprinting to engineer more mature tissues prior to implantation. This review covers the key concepts, challenges, and applications of both strategies to regenerate damaged and diseased bone.

Bioinspired channeled, rhBMP-2-coated β-TCP scaffolds with embedded autologous vascular bundles for increased vascularization and osteogenesis of prefabricated tissue-engineered bone

To date, the recovery of large bone defects is a major clinical challenge despite the availability of numerous therapeutic procedures including tissue engineering. Although there is a pressing need for large tissue-engineered constructs, inadequate vascularization remains an insurmountable barrier for successful clinical translation. Considering that vascularization is a prerequisite for osteogenesis, we proposed an advanced design of large customized porous β-tricalcium phosphate (TCP) scaffolds with biomimetic vascular hierarchy which upon embedding of femoral axial vascular bundles significantly improved overall vascularity of the scaffolds. Such scaffolds also promoted osteogenesis when they were coated with recombinant bone morphogenetic protein-2 (rhBMP-2). Compared to the conventional TCP scaffolds (S), the newly designed multi-channeled β-TCP (CS) scaffolds led to adequate blood vessels and bone-like tissue formation throughout their porous hierarchy within 4 weeks of implantation. Especially, the scaffolds coated with rhBMP-2 and embedded with flow-through vascular bundle (FVB) were able to form more uniform vascularized bone within 2 weeks post-implantation. Based on the clinical, radiographic, angiographic and histological assessments, the newly designed multi-channeled scaffolds were found to be promising for successful recovery of large bone defects.

Key components of engineering vascularized 3-dimensional bioprinted bone constructs

Vascularization has a pivotal role in engineering successful tissue constructs. However, it remains a major hurdle of bone tissue engineering, especially in clinical applications for the treatment of large bone defects. Development of vascularized and clinically-relevant engineered bone substitutes with sufficient blood supply capable of maintaining implant viability and supporting subsequent host tissue integration remains a major challenge. Since only cells that are 100-200 µm from blood vessels can receive oxygen through diffusion, engineered constructs that are thicker than 400 µm face a challenging oxygenation problem. Following implantation in vivo, spontaneous ingrowth of capillaries in thick engineered constructs is too slow. Thus, it is critical to provide optimal conditions to support vascularization in engineered bone constructs. To achieve this, an in-depth understanding of the mechanisms of angiogenesis and bone development is required. In addition, it is also important to mimic the physiological milieu of native bone to fabricate more successful vascularized bone constructs. Numerous applications of engineered vascularization with cell-and/or microfabrication-based approaches seek to meet these aims. Three-dimensional (3D) printing promises to create patient-specific bone constructs in the future. In this review, we discuss the major components of fabricating vascularized 3D bioprinted bone constructs, analyze their related challenges, and highlight promising future trends.

Vascular bundle transplantation combined with porous bone substituted scaffold for the treatment of early-stage avascular necrosis of femoral head

Various reasons leading to disruption of blood supply will result in avascular femoral head necrosis. Decompression of lesion area, structural support to subchondral bone, and rebuilding of blood supply system are the keys for a successful hip preserve surgery. Reconstruction of local vascular network is always a huge challenge in clinical. Based on tantalum rod implantation and free vascularized fibular grafting, we propose the combined application of vascular bundle transplantation and porous bone substitute scaffold implantation as a potential novel treatment method. It may simultaneously achieve decompression, support and blood supply reconstruction. The hypothesis provides some new ideals and possibilities for solving this clinical problem of femoral head necrosis.