Large Animal Models of an In Vivo Bioreactor for Engineering Vascularized Bone

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.

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.

Standardized and axially vascularized calcium phosphate-based implants for segmental mandibular defects: A promising proof of concept

The reconstruction of massive segmental mandibular bone defects (SMDs) remains challenging even today; the current gold standard in human clinics being vascularized bone transplantation (VBT). As alternative to this onerous approach, bone tissue engineering strategies have been widely investigated. However, they displayed limited clinical success, particularly in failing to address the essential problem of quick vascularization of the implant. Although routinely used in clinics, the insertion of intrinsic vascularization in bioengineered constructs for the rapid formation of a feeding angiosome remains uncommon. In a clinically relevant model (sheep), a custom calcium phosphate-based bioceramic soaked with autologous bone marrow and perfused by an arteriovenous loop was tested to regenerate a massive SMD and was compared to VBT (clinical standard). Animals did not support well the VBT treatment, and the study was aborted 2 weeks after surgery due to ethical and animal welfare considerations. SMD regeneration was successful with the custom vascularized bone construct. Implants were well osseointegrated and vascularized after only 3 months of implantation and totally entrapped in lamellar bone after 12 months; a healthy yellow bone marrow filled the remaining space. STATEMENT OF SIGNIFICANCE: Regenerative medicine struggles with the generation of large functional bone volume. Among them segmental mandibular defects are particularly challenging to restore. The standard of care, based on bone free flaps, still displays ethical and technical drawbacks (e.g., donor site morbidity). Modern engineering technologies (e.g., 3D printing, digital chain) were combined to relevant surgical techniques to provide a pre-clinical proof of concept, investigating for the benefits of such a strategy in bone-related regenerative field. Results proved that a synthetic-biologics-free approach is able to regenerate a critical size segmental mandibular defect of 15 cm³ in a relevant preclinical model, mimicking real life scenarii of segmental mandibular defect, with a full physiological regeneration of the defect after 12 months.

Frontiers of Hydroxyapatite Composites in Bionic Bone Tissue Engineering

Bone defects caused by various factors may cause morphological and functional disorders that can seriously affect patient's quality of life. Autologous bone grafting is morbid, involves numerous complications, and provides limited volume at donor site. Hence, tissue-engineered bone is a better alternative for repair of bone defects and for promoting a patient's functional recovery. Besides good biocompatibility, scaffolding materials represented by hydroxyapatite (HA) composites in tissue-engineered bone also have strong ability to guide bone regeneration. The development of manufacturing technology and advances in material science have made HA composite scaffolding more closely related to the composition and mechanical properties of natural bone. The surface morphology and pore diameter of the scaffold material are more important for cell proliferation, differentiation, and nutrient exchange. The degradation rate of the composite scaffold should match the rate of osteogenesis, and the loading of cells/cytokine is beneficial to promote the formation of new bone. In conclusion, there is no doubt that a breakthrough has been made in composition, mechanical properties, and degradation of HA composites. Biomimetic tissue-engineered bone based on vascularization and innervation show a promising future.

3D-Printing for Critical Sized Bone Defects: Current Concepts and Future Directions

The management and definitive treatment of segmental bone defects in the setting of acute trauma, fracture non-union, revision joint arthroplasty, and tumor surgery are challenging clinical problems with no consistently satisfactory solution. Orthopaedic surgeons are developing novel strategies to treat these problems, including three-dimensional (3D) printing combined with growth factors and/or cells. This article reviews the current strategies for management of segmental bone loss in orthopaedic surgery, including graft selection, bone graft substitutes, and operative techniques. Furthermore, we highlight 3D printing as a technology that may serve a major role in the management of segmental defects. The optimization of a 3D-printed scaffold design through printing technique, material selection, and scaffold geometry, as well as biologic additives to enhance bone regeneration and incorporation could change the treatment paradigm for these difficult bone repair problems.

Synthetic materials in craniofacial regenerative medicine: A comprehensive overview

The state-of-the-art approach to regenerating different tissues and organs is tissue engineering which includes the three parts of stem cells (SCs), scaffolds, and growth factors. Cellular behaviors such as propagation, differentiation, and assembling the extracellular matrix (ECM) are influenced by the cell's microenvironment. Imitating the cell's natural environment, such as scaffolds, is vital to create appropriate tissue. Craniofacial tissue engineering refers to regenerating tissues found in the brain and the face parts such as bone, muscle, and artery. More biocompatible and biodegradable scaffolds are more commensurate with tissue remodeling and more appropriate for cell culture, signaling, and adhesion. Synthetic materials play significant roles and have become more prevalent in medical applications. They have also been used in different forms for producing a microenvironment as ECM for cells. Synthetic scaffolds may be comprised of polymers, bioceramics, or hybrids of natural/synthetic materials. Synthetic scaffolds have produced ECM-like materials that can properly mimic and regulate the tissue microenvironment's physical, mechanical, chemical, and biological properties, manage adherence of biomolecules and adjust the material's degradability. The present review article is focused on synthetic materials used in craniofacial tissue engineering in recent decades.

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.

Criteria, Challenges, and Opportunities for Acellularized Allogeneic/Xenogeneic Bone Grafts in Bone Repairing

As bone grafts become more commonly needed by patients and as donors become scarcer, acellularized bone grafts (ABGs) are becoming more popular for restorative purposes. While autogeneic grafts are reliable as a gold standard, allogeneic and xenogeneic ABGs have been shown to be of particular interest due to the limited availability of autogeneic resources and reduced patient well-being in long-term surgeries. Because of the complete similarity of their structures with native bone, excellent mechanical properties, high biocompatibility, and similarities of biological behaviors (osteoinductive and osteoconductive) with local bones, successful outcomes of allogeneic and xenogeneic ABGs in both in vitro and in vivo research have raised hopes of repairing patients' bone injuries in clinical applications. However, clinical trials have been delayed due to a lack of standardized protocols pertaining to acellularization, cell seeding, maintenance, and diversity of ABG evaluation criteria. This study sought to uncover these factors by exploring the bone structures, ossification properties of ABGs, sources, benefits, and challenges of acellularization approaches (physical, chemical, and enzymatic), cell loading, and type of cells used and effects of each of the above items on the regenerative technologies. To gain a perspective on the repair and commercialization of products before implementing new research activities, this study describes the differences between ABGs created by various techniques and methods applied to them. With a comprehensive understanding of ABG behavior, future research focused on treating bone defects could provide a better way to combine the treatment approaches needed to treat bone defects.

Progress of Periosteal Osteogenesis: The Prospect of In Vivo Bioreactor

Repairing large segment bone defects is still a clinical challenge. Bone tissue prefabrication shows great translational potentials and has been gradually accepted clinically. Existing bone reconstruction strategies, including autologous periosteal graft, allogeneic periosteal transplantation, xenogeneic periosteal transplantation, and periosteal cell tissue engineering, are all clinically valuable treatments and have made significant progress in research. Herein, we reviewed the research progress of these techniques and briefly explained the relationship among in vivo microenvironment, mechanical force, and periosteum osteogenesis. Moreover, we also highlighted the importance of the critical role of periosteum in osteogenesis and explained current challenges and future perspective.

Periosteal Flaps Enhance Prefabricated Engineered Bone Reparative Potential

The clinical translation of bone tissue engineering for reconstructing large bone defects has not advanced without hurdles. The in vivo bioreactor (IVB) concept may therefore bridge between bone tissue engineering and reconstructive surgery by employing the patient body for prefabricating new prevascularized tissues. Ideally, IVB should minimize the need for exogenous growth factors/cells. Periosteal tissues are promising for IVB approaches to prefabricate tissue-engineered bone (TEB) flaps. However, the significance of preserving the periosteal vascular supply has not been adequately investigated. This study assessed muscle IVB with and without periosteal/pericranial grafts and flaps for prefabricating TEB flaps to reconstruct mandibular defects in sheep. The sheep (n = 14) were allocated into 4 groups: muscle IVB (M group; n_M = 3), muscle + periosteal graft (MP group; n_MP = 4), muscle + periosteal flap (MVP group; n_MVP = 4), and control group (n_Control = 3). In the first surgery, alloplastic bone blocks were implanted in the brachiocephalic muscle (M) with a periosteal graft (MP) or with a vascularized periosteal flap (MVP). After 9 wk, the prefabricated TEB flaps were transplanted to reconstruct a mandibular angle defect. In the control group, the defects were reconstructed by non-prevascularized bone blocks. Computed tomography (CT) scans were performed after 13 wk and after 23 wk at termination, followed by micro-CT (µCT) and histological analyses. Both CT and µCT analysis revealed enhanced new bone formation and decreased residual biomaterial volume in the MVP group compared with control and MP groups, while the M group showed less new bone formation and more residual biomaterial. The histological analysis showed that most of the newly formed bone emerged from defect edges, but larger areas of new bone islands were found in MP and MVP groups. The MVP group showed enhanced vascularization and higher biomaterial remodeling rates. The periosteal flaps boosted the reconstructive potential of the prefabricated TEB flaps. The regenerative potential of the periosteum was manifested after the transplantation into the mechanically stimulated bony defect microenvironment.

Vascularization Strategies in Bone Tissue Engineering

Bone is a highly vascularized tissue, and its development, maturation, remodeling, and regeneration are dependent on a tight regulation of blood vessel supply. This condition also has to be taken into consideration in the context of the development of artificial tissue substitutes. In classic tissue engineering, bone-forming cells such as primary osteoblasts or mesenchymal stem cells are introduced into suitable scaffolds and implanted in order to treat critical-size bone defects. However, such tissue substitutes are initially avascular. Because of the occurrence of hypoxic conditions, especially in larger tissue substitutes, this leads to the death of the implanted cells. Therefore, it is necessary to devise vascularization strategies aiming at fast and efficient vascularization of implanted artificial tissues. In this review article, we present and discuss the current vascularization strategies in bone tissue engineering. These are based on the use of angiogenic growth factors, the co-implantation of blood vessel forming cells, the ex vivo microfabrication of blood vessels by means of bioprinting, and surgical methods for creating surgically transferable composite tissues.

Highly interconnected inverse opal extracellular matrix scaffolds enhance stem cell therapy in limb ischemia

The therapeutic effectiveness of cell transplantation in treatment of diseases and injuries is often limited by low cell retention, survivability, and engraftment. Extracellular matrix (ECM)-derived scaffolds are capable of controlling cell responses, thereby offering potential solutions to current challenges associated with cell therapy. However, it remains a technical challenge to produce ECM scaffolds with highly interconnected porous structure specifically required for cell transplantation. Here, we developed inverse opal porous extracellular matrix (ioECM) scaffolds through subcutaneous implantation of sacrificial templates assembled from polymer microspheres, followed by removal of the microsphere template and cellular content. Such highly interconnected porous ioECM scaffolds supported the anchorage, survival, viability, anti-apoptotic and paracrine activities of rat bone marrow mesenchymal stem cells (BMSCs), which further promoted endothelial cell migration and tube formation and viability. Upon transplantation into nude mouse critical limb ischemic model, ioECM promoted the engraftment of laden BMSCs, facilitated interconnected vascular network formation with accelerated recovery of blood perfusion and inhibited muscle atrophy and fibrosis. Our study demonstrates a unique strategy to engineer highly porous yet well-interconnected ECM scaffolds specifically for cell transplantation with marked improvement of survivability and vascularization, which offers an essential step toward the success of cell therapy and regenerative medicine. STATEMENT OF SIGNIFICANCE: Cell-based therapy has a good developing foreground applied in a variety of tissue regeneration. Extracellular matrix (ECM) scaffolds is an optimal choice for cell delivery duo to its superior biocompatibility and favorable immune responses. However, the current ECM scaffolds lacking of the controllable pore structure restrict the cell delivery efficiency and therapeutic outcome. Here, we fabricated highly interconnected inverse opal extracellular matrix (ioECM) scaffolds, which can enhance the effect of stem cell therapy in limb ischemic model by improving the survival, viability, and paracrine activities of stem cells. Our study provides reference value for the design and fabrication of ECM based biomaterials for cell transplantation.

Graphene Oxide Quantum Dots Promote Osteogenic Differentiation of Stem Cells from Human Exfoliated Deciduous Teeth via the Wnt/β-Catenin Signaling Pathway

Graphene oxide quantum dots (GOQDs) are a carbon nanomaterial with broad potential for application in the field of nanomaterial biomedicine. Stem cells from human exfoliated deciduous teeth (SHEDs) play an important role in tissue engineering and regenerative medicine. This study investigated the effects of GOQDs on SHED osteogenic differentiation. GOQDs were synthesized; then, the proliferation of SHEDs incubated in GOQDs at different concentrations was evaluated; and the live cells were observed. We observed that live SHEDs incubated in GOQDs emitted green fluorescence in the absence of chemical dyes, and 1, 10, and 50 μg/mL GOQDs significantly promoted SHED proliferation. Culture with the osteogenic induction medium containing 10 μg/mL GOQDs induced calcium nodule formation, increased alkaline phosphatase (ALP) activity, and upregulated SHED mRNA and protein levels of OCN, RUNX2, COL I, and β-catenin. With the addition of Dickkopf 1 (DKK-1) or β-catenin knockdown, expression levels of the above mRNAs and proteins were decreased in GOQD-treated SHEDs. In summary, at a concentration of 10 μg/mL, GOQDs promote SHED proliferation and osteogenic differentiation via the Wnt/β-catenin signaling pathway. This work provides new ideas and fundamental information on interactions between GOQDs and SHEDs that are relevant for the biomedical engineering field.

Tough magnesium phosphate-based 3D-printed implants induce bone regeneration in an equine defect model

One of the important challenges in bone tissue engineering is the development of biodegradable bone substitutes with appropriate mechanical and biological properties for the treatment of larger defects and those with complex shapes. Recently, magnesium phosphate (MgP) doped with biologically active ions like strontium (Sr²⁺) have shown to significantly enhance bone formation when compared with the standard calcium phosphate-based ceramics. However, such materials can hardly be shaped into large and complex geometries and more importantly lack the adequate mechanical properties for the treatment of load-bearing bone defects. In this study, we have fabricated bone implants through extrusion assisted three-dimensional (3D) printing of MgP ceramics modified with Sr²⁺ ions (MgPSr) and a medical-grade polycaprolactone (PCL) polymer phase. MgPSr with 30 wt% PCL (MgPSr-PCL30) allowed the printability of relevant size structures (>780 mm³) at room temperature with an interconnected macroporosity of approximately 40%. The printing resulted in implants with a compressive strength of 4.3 MPa, which were able to support up to 50 cycles of loading without plastic deformation. Notably, MgPSr-PCL30 scaffolds were able to promote in vitro bone formation in medium without the supplementation with osteo-inducing components. In addition, long-term in vivo performance of the 3D printed scaffolds was investigated in an equine tuber coxae model over 6 months. The micro-CT and histological analysis showed that implantation of MgPSr-PCL30 induced bone regeneration, while no bone formation was observed in the empty defects. Overall, the novel polymer-modified MgP ceramic material and extrusion-based 3D printing process presented here greatly improved the shape ability and load-bearing properties of MgP-based ceramics with simultaneous induction of new bone formation.

Agricultural Animals as Biomedical Models: Occupational Health and Safety Considerations

The use of agricultural animals in biomedical research is increasing. Their overall size and metabolic rate, organ size, longer gestation period, and other physiological similarities make them good candidates for animal models of human disease. There are a number of special considerations for use of traditional farm animals for biomedical research. Differences in physical plant infrastructure, handling equipment, training of personnel, and potential zoonoses are some of the important considerations when traditional farm animals are used in biomedical research. This article provides an overview of some of the special considerations for using traditional agricultural animals in biomedical research. With the growing need for improved translational research, it is reasonable to predict significant growth in these animal models.

Effects of graphene oxide and graphene oxide quantum dots on the osteogenic differentiation of stem cells from human exfoliated deciduous teeth

Graphene and its derivatives, graphene oxide (GO) and graphene oxide quantum dots (GOQDs), have recently attracted much attention as bioactive factors in differentiating stem cells towards osteoblastic lineage. The stem cells from human exfoliated deciduous teeth (SHEDs) possess the properties of self-renewal, extensive proliferation, and multiple differentiation potential, and have gradually become one of the most promising mesenchymal stem cells (MSCs) in bone tissue engineering. The purpose of this study was to explore the effects of GO and GOQDs on the osteogenic differentiation of SHEDs. In this study, GO and GOQDs facilitated SHED proliferation up to 7 days in vitro at the concentration of 1 μg/ml. Because of their excellent fluorescent properties, GOQD uptake by SHEDs was confirmed and distributed in the SHED cytoplasm. Calcium nodules formation, alkaline phosphatase (ALP) activity, and RNA and protein expression increased significantly in SHEDs treated with osteogenic induction medium containing GOQDs but decreased with osteogenic induction medium containing GO. Interestingly, the Wnt/β-catenin signaling pathway appeared to be involved in osteogenic differentiation of SHEDs induced with GOQDs. In summary, GO and GOQDs at the concentration of 1 μg/ml promoted SHED proliferation. GOQDs induced the osteogenic differentiation of SHEDs, whilst GO slightly inhibited it.

Embossed Membranes with Vascular Patterns Guide Vascularization in a 3D Tissue Model

The vascularization of three-dimensional (3D) tissue constructs is necessary for transporting nutrients and oxygen to the component cells. In this study, a vacuum forming method was applied to emboss a vascular pattern on an electrospun membrane so that guided vascular structures could develop within the construct. Two- or six-layer constructs of electrospun membranes seeded with endothelial cells and pericytes were stacked and subcutaneously implanted into mice. Blood vessel formation in the implanted constructs with six alternating layers of flat membranes and membranes embossed with a blood vessel pattern was observed after two weeks of implantation. The formation of blood vessels was observed along the embossed blood vessel pattern in the structure of the embossed membrane laminated at four weeks and eight weeks. Vascular endothelial growth factor (VEGF) and angiopoietin 1 (Ang-1) were highly expressed in the vascularized structures. Therefore, we demonstrated that a structure capable of producing a desired blood vessel shape with electrospun membranes embossed with a blood vessel pattern can be manufactured, and that a variety of structures can be manufactured using electrospun membranes in the tissue engineering era.

Biomaterials-aided mandibular reconstruction using in vivo bioreactors

Large mandibular defects are clinically challenging to reconstruct due to the complex anatomy of the jaw and the limited availability of appropriate tissue for repair. We envision leveraging current advances in fabrication and biomaterials to create implantable devices that generate bone within the patients themselves suitable for their own specific anatomical pathology. The in vivo bioreactor strategy facilitates the generation of large autologous vascularized bony tissue of customized geometry without the addition of exogenous growth factors or cells. To translate this technology, we investigated its success in reconstructing a mandibular defect of physiologically relevant size in sheep. We fabricated and implanted 3D-printed in vivo bioreactors against rib periosteum and utilized biomaterial-based space maintenance to preserve the native anatomical mandibular structure in the defect site before reconstruction. Nine weeks after bioreactor implantation, the ovine mandibles were repaired with the autologous bony tissue generated from the in vivo bioreactors. We evaluated tissues generated in bioreactors by radiographic, histological, mechanical, and biomolecular assays and repaired mandibles by radiographic and histological assays. Biomaterial-aided mandibular reconstruction was successful in a large superior marginal defect in five of six (83%) sheep. Given that these studies utilized clinically available biomaterials, such as bone cement and ceramic particles, this strategy is designed for rapid human translation to improve outcomes in patients with large mandibular defects.