Vascularization in bone tissue engineering: physiology, current strategies, major hurdles and future challenges

How to explain the beneficial effects of leukocyte- and platelet-rich fibrin

The survival of an organism relies on its ability to repair the damage caused by trauma, toxic agents, and inflammation. This process involving cell proliferation and differentiation is driven by several growth factors and is critically dependent on the organization of the extracellular matrix. Since autologous platelet concentrates (APCs) are fibrin matrices in which cells, growth factors, and cytokines are trapped and delivered over time, they are able to influence that response at different levels. The present review thoroughly describes the molecular components present in one of these APCs, leukocyte- and platelet-rich fibrin (L-PRF), and summarizes the level of evidence regarding the influence of L-PRF on anti-inflammatory reactions, analgesia, hemostasis, antimicrobial capacity, and its biological mechanisms on bone/soft tissue regeneration.

Establishing stable and highly osteogenic hiPSC-derived MSCs for 3D-printed bone graft through microenvironment modulation by CHIR99021-treated osteocytes

Human induced pluripotent stem cell (hiPSC)-derived mesenchymal stem cells (iMSCs) are ideal candidates for the production of standardised and scalable bioengineered bone grafts. However, stable induction and osteogenic differentiation of iMSCs pose challenges in the industry. We developed a precise differentiation method to produce homogeneous and fully differentiated iMSCs. In this study, we established a standardised system to prepare iMSCs with increased osteogenic potential and improved bioactivity by introducing a CHIR99021 (C91)-treated osteogenic microenvironment (COOME). COOME enhances the osteogenic differentiation and mineralisation of iMSCs via canonical Wnt signalling. Global transcriptome analysis and co-culturing experiments indicated that COOME increased the pro-angiogenesis/neurogenesis activity of iMSCs. The superior osteogenic differentiation and mineralisation abilities of COOME-treated iMSCs were also confirmed in a Bio3D module generated using a polycaprolactone (PCL) and cell-integrated 3D printing (PCI3D) system, which is the closest model to in vivo research. This COOME-treated iMSCs differentiation system offers a new perspective for generating highly osteogenic, bioactive, and anatomically matched grafts for clinical applications.

Statement of significance

Although human induced pluripotent stem cell-derived MSCs (iMSCs) are ideal seed cells for synthetic bone implants, the challenges of stable induction and osteogenic differentiation hinder their clinical application. This study established a standardised system for the scalable preparation of iMSCs with improved osteogenic potential by combining our precise iMSC differentiation method with the CHIR99021 (C91)-treated osteocyte osteogenic microenvironment (COOME) through the activation of canonical Wnt signalling. Moreover, COOME upregulated the pro-angiogenic and pro-neurogenic capacities of iMSCs, which are crucial for the integration of implanted bone grafts. The superior osteogenic ability of COOME-treated iMSCs was confirmed in Bio3D modules generated using PCL and cell-integrated 3D printing systems, highlighting their functional potential in vivo. This study contributes to tissue engineering by providing insights into the functional differentiation of iMSCs for bone regeneration.

Murine iPSC-Loaded Scaffold Grafts Improve Bone Regeneration in Critical-Size Bone Defects

In certain situations, bones do not heal completely after fracturing. One of these situations is a critical-size bone defect where the bone cannot heal spontaneously. In such a case, complex fracture treatment over a long period of time is required, which carries a relevant risk of complications. The common methods used, such as autologous and allogeneic grafts, do not always lead to successful treatment results. Current approaches to increasing bone formation to bridge the gap include the application of stem cells on the fracture side. While most studies investigated the use of mesenchymal stromal cells, less evidence exists about induced pluripotent stem cells (iPSC). In this study, we investigated the potential of mouse iPSC-loaded scaffolds and decellularized scaffolds containing extracellular matrix from iPSCs for treating critical-size bone defects in a mouse model. In vitro differentiation followed by Alizarin Red staining and quantitative reverse transcription polymerase chain reaction confirmed the osteogenic differentiation potential of the iPSCs lines. Subsequently, an in vivo trial using a mouse model (n = 12) for critical-size bone defect was conducted, in which a PLGA/aCaP osteoconductive scaffold was transplanted into the bone defect for 9 weeks. Three groups (each n = 4) were defined as (1) osteoconductive scaffold only (control), (2) iPSC-derived extracellular matrix seeded on a scaffold and (3) iPSC seeded on a scaffold. Micro-CT and histological analysis show that iPSCs grafted onto an osteoconductive scaffold followed by induction of osteogenic differentiation resulted in significantly higher bone volume 9 weeks after implantation than an osteoconductive scaffold alone. Transplantation of iPSC-seeded PLGA/aCaP scaffolds may improve bone regeneration in critical-size bone defects in mice.

Exosomal miR-423-5p Derived from Cerebrospinal Fluid Pulsation Stress-Stimulated Osteoblasts Improves Angiogenesis of Endothelial Cells via DUSP8/ERK1/2 Signaling Pathway

Exosomes secreted from osteoblasts (OBs) can regulate the angiogenesis of endothelial cells (ECs); however, whether cerebrospinal fluid pulsation (CSFP) stress, a special mechanical stimulation, can influence the cell's communication in the context of angiogenesis remains unknown. In this study, the effect of exosomes derived from CSFP stress-stimulated OBs on facilitating the angiogenesis of ECs was investigated. First, OBs were cultured in a CSFP bioreactor, and exosomes derived from OBs were isolated and identified. Cell Counting Kit 8 assay, transwell migration assay, wound healing migration assay, and tube formation assay were conducted to assess the effects of CSFP stress-stimulated OBs-derived exosomes (CSFP-Exos) on the angiogenesis of ECs. Then high-throughput RNA sequencing was used to determine the miRNA profiles of Non-CSFP stress-stimulated OBs-derived exosomes (NCSFP-Exos) and CSFP-Exos, and the luciferase reporter gene assay was performed to confirm the binging of miR-423-5p to DUSP8. In addition, the Matrigel plug assay was performed to explore whether exosomal miR-423-5p has the same effects in vivo. Our results suggested that CSFP-Exos can promote the angiogenesis of ECs, and miR-423-5p was enriched in CSFP-Exos. Moreover, miR-423-5p could promote the effect of angiogenesis via directly targeting dual-specificity phosphatase 8 (DUSP8), which inhibited the ERK1/2 signaling pathway. In conclusion, exosomal miR-423-5p derived from CSFP stress-stimulated OBs could promote the angiogenesis of ECs by the DUSP8/ERK1/2 signaling pathway.

Advances in 3D bioprinting for regenerative medicine applications

Biofabrication techniques allow for the construction of biocompatible and biofunctional structures composed from biomaterials, cells and biomolecules. Bioprinting is an emerging 3D printing method which utilizes biomaterial-based mixtures with cells and other biological constituents into printable suspensions known as bioinks. Coupled with automated design protocols and based on different modes for droplet deposition, 3D bioprinters are able to fabricate hydrogel-based objects with specific architecture and geometrical properties, providing the necessary environment that promotes cell growth and directs cell differentiation towards application-related lineages. For the preparation of such bioinks, various water-soluble biomaterials have been employed, including natural and synthetic biopolymers, and inorganic materials. Bioprinted constructs are considered to be one of the most promising avenues in regenerative medicine due to their native organ biomimicry. For a successful application, the bioprinted constructs should meet particular criteria such as optimal biological response, mechanical properties similar to the target tissue, high levels of reproducibility and printing fidelity, but also increased upscaling capability. In this review, we highlight the most recent advances in bioprinting, focusing on the regeneration of various tissues including bone, cartilage, cardiovascular, neural, skin and other organs such as liver, kidney, pancreas and lungs. We discuss the rapidly developing co-culture bioprinting systems used to resemble the complexity of tissues and organs and the crosstalk between various cell populations towards regeneration. Moreover, we report on the basic physical principles governing 3D bioprinting, and the ideal bioink properties based on the biomaterials' regenerative potential. We examine and critically discuss the present status of 3D bioprinting regarding its applicability and current limitations that need to be overcome to establish it at the forefront of artificial organ production and transplantation.

Tetrahedral framework nucleic acids for improving wound healing

Wounds are one of the most common health issues, and the cost of wound care and healing has continued to increase over the past decade. In recent years, there has been growing interest in developing innovative strategies to enhance the efficacy of wound healing. Tetrahedral framework nucleic acids (tFNAs) have emerged as a promising tool for wound healing applications due to their unique structural and functional properties. Therefore, it is of great significance to summarize the applications of tFNAs for wound healing. This review article provides a comprehensive overview of the potential of tFNAs as a novel therapeutic approach for wound healing. In this review, we discuss the possible mechanisms of tFNAs in wound healing and highlight the role of tFNAs in modulating key processes involved in wound healing, such as cell proliferation and migration, angiogenesis, and tissue regeneration. The targeted delivery and controlled release capabilities of tFNAs offer advantages in terms of localized and sustained delivery of therapeutic agents to the wound site. In addition, the latest research progress on tFNAs in wound healing is systematically introduced. We also discuss the biocompatibility and biosafety of tFNAs, along with their potential applications and future directions for research. Finally, the current challenges and prospects of tFNAs are briefly discussed to promote wider applications.

Modular, Vascularized Hypertrophic Cartilage Constructs for Bone Tissue Engineering Applications

Insufficient vascularization is a main barrier to creating engineered bone grafts for treating large and ischemic defects. Modular tissue engineering approaches have promise in this application because of the ability to combine tissue types and to localize microenvironmental cues to drive desired cell function. In direct bone formation approaches, it is challenging to maintain sustained osteogenic activity, since vasculogenic cues can inhibit tissue mineralization. This study harnessed the physiological process of endochondral ossification to create multiphase tissues that allowed concomitant mineralization and vessel formation. Mesenchymal stromal cells in pellet culture were differentiated toward a cartilage phenotype, followed by induction to chondrocyte hypertrophy. Hypertrophic pellets exhibited increased alkaline phosphatase activity, calcium deposition, and osteogenic gene expression relative to chondrogenic pellets. In addition, hypertrophic pellets secreted and sequestered angiogenic factors, and supported new blood vessel formation by co-cultured endothelial cells and undifferentiated stromal cells. Multiphase constructs created by combining hypertrophic pellets and vascularizing microtissues and maintained in unsupplemented basal culture medium were shown to support robust vascularization and sustained tissue mineralization. These results demonstrate a new in vitro strategy to produce multiphase engineered constructs that concomitantly support the generation of mineralize and vascularized tissue in the absence of exogenous osteogenic or vasculogenic medium supplements.

3D Printed Piezoelectric BaTiO3/Polyhydroxybutyrate Nanocomposite Scaffolds for Bone Tissue Engineering

Bone defects are a significant health problem worldwide. Novel treatment approaches in the tissue engineering field rely on the use of biomaterial scaffolds to stimulate and guide the regeneration of damaged tissue that cannot repair or regrow spontaneously. This work aimed at developing and characterizing new piezoelectric scaffolds to provide electric bio-signals naturally present in bone and vascular tissues. Mixing and extrusion were used to obtain nanocomposites made of polyhydroxybutyrate (PHB) as a matrix and barium titanate (BaTiO3) nanoparticles as a filler, at BaTiO3/PHB compositions of 5/95, 10/90, 15/85 and 20/80 (w/w%). The morphological, thermal, mechanical and piezoelectric properties of the nanocomposites were studied. Scanning electron microscopy analysis showed good nanoparticle dispersion within the polymer matrix. Considerable increases in the Young's modulus, compressive strength and the piezoelectric coefficient d31 were observed with increasing BaTiO3 content, with d31 = 37 pm/V in 20/80 (w/w%) BaTiO3/PHB. 3D printing was used to produce porous cubic-shaped scaffolds using a 90° lay-down pattern, with pore size ranging in 0.60-0.77 mm and good mechanical stability. Biodegradation tests conducted for 8 weeks in saline solution at 37 °C showed low mass loss (∼4%) for 3D printed scaffolds. The results obtained in terms of piezoelectric, mechanical and chemical properties of the nanocomposite provide a new promising strategy for vascularized bone tissue engineering.

In-vitro and in-vivo evaluation for the bio-natural Alginate/nano-Hydroxyapatite (Alg/n-HA) injectable hydrogel for critical size bone substitution

Currently, bio-natural injectable hydrogels are receiving a lot of attention due to their ability to control, adjust, and adapt to random bone defects, in addition, to their ability to mimic the composition of natural bones. From such a viewpoint, this study goal is to prepare and characterize the injectable hydrogels paste based on the natural alginate (Alg) derived from brown sea algae as a polysaccharide polymer, which coupled with nano biogenic-hydroxyapatite (n-HA) prepared from eggshells and enriched with valuable trace elements. The viscosity and mechanical properties of the paste were investigated. As well as the in-vitro study in terms of water absorption and biodegradability in the PBS, biocompatibility and the capability of the injectable Alginate/n-Hydroxyapatite (Alg/n-HA) to regenerate bone for the most suitable injectable form. The injectable hydrogel (BP -B sample) was chosen for the study as it had an appropriate setting time for injecting (13 mins), and suitable compressive strength reached 6.3 MPa. The in vivo study was also carried out including a post-surgery follow-up test of the newly formed bone (NB) in the defect area after 10 and 20 weeks using different techniques such as (SEM/EDX) and histological analysis, the density of the newly formed bone by Dual x-ray absorptiometry (DEXA), blood biochemistry and the radiology test. The results proved that the injectable hydrogels Alginate/n-Hydroxyapatite (Alg/n-HA) had an appreciated biodegradability and bioactivity, which allow the progress of angiogenesis, endochondral ossification, and osteogenesis throughout the defect area, which positively impacts the healing time and ensures the full restoration for the well-mature bone tissue that similar to the natural bone.

Physiologically engineered porous titanium/brushite scaffolds for critical-size bone defects: A design and manufacturing study

Repairing critical-size bone defects still represents a critical clinical challenge in the field of trauma surgery. This study focuses on a physiological design and manufacturing of porous composite scaffold (titanium Ti with 10 % mole iron doped brushite DCPD-Fe3+) which can mimic the biomechanical properties of natural cortical bone, specifically for the purpose of repairing critical-size defects. To achieve this, the principle of design of experiments (DOE) was applied for investigating the impact of sintering temperature, mineral ratio, and volume fraction of porosity on the mechanical properties of the fabricated scaffolds. The fabricated scaffolds had open porosity up to 60 %, with pore size approximately between 100 μm and 850 μm. The stiffness of the porous composite scaffolds varied between 3.30 GPa and 20.50 GPa, while the compressive strength ranged from approximately 130 MPa-165 MPa at sintering temperatures equal to or exceeding 1000 °C. Scaffolds with higher porosity and mineral content demonstrated lower stiffness values, resembling natural bone. Numerical simulation was employed by Ansys Workbench to investigate the stress and strain distribution of a critical size defect in mid-shaft femur which was designed to be replaced with the fabricated scaffold. The fabricated scaffolds showed flexible biomechanical behaviour at the bone/scaffold interface, generating lower stress levels and indicating a better match with the femoral shaft stiffness. The experimental and numerical findings demonstrated promising applications for manufacturing a patient-specific bone scaffold for critical and potentially large defects for reducing stress shielding and minimizing non-union risk.

Ultrasound standing wave spatial patterning of human umbilical vein endothelial cells for 3D micro-vascular networks formation

Generating functional and perfusable micro-vascular networks is an important goal for the fabrication of large and three-dimensional tissues. Up to now, the fabrication of micro-vascular networks is a complicated multitask involving several different factors such as time consuming, cells survival, micro-diameter vasculature and strict alignment. Here, we propose a technique combining multi-material extrusion and ultrasound standing wave forces to create a network structure of human umbilical vein endothelial cells within a mixture of calcium alginate and decellularized extracellular matrix. The functionality of the matured microvasculature networks was demonstrated through the enhancement of cell-cell adhesion, angiogenesis process, and perfusion tests with microparticles, FITC-dextran, and whole mouse blood. Moreover, animal experiments exhibited the implantability including that the pre-existing blood vessels of the host sprout towards the preformed vessels of the scaffold over time and the microvessels inside the implanted scaffold matured from empty tubular structures to functional blood-carrying microvessels in two weeks.

Calvaria defect regeneration via human periodontal ligament stem cells and prevascularized scaffolds in athymic rats

BACKGROUND

Vascularization plays an important role in dental and craniofacial regenerations. Human periodontal ligament stem cells (hPDLSCs) are a promising cell source and, when co-cultured with human umbilical vein endothelial cells (hUVECs), could promote vascularization. The objectives of this study were to develop a novel prevascularized hPDLSC-hUVEC-calcium phosphate construct, and investigate the osteogenic and angiogenic efficacy of this construct with human platelet lysate (hPL) in cranial defects in rats for the first time.

METHODS

hPDLSCs and hUVECs were co-cultured on calcium phosphate cement (CPC) scaffolds with hPL. Cell proliferation, angiogenic gene expression, angiogenesis, alkaline phosphatase activity, and cell-synthesized minerals were determined. Bone and vascular regenerations were investigated in rat critical-sized cranial defects in vivo.

RESULTS

hPDLSC-hUVEC-CPC-hPL group had 2-fold greater angiogenic expressions and cell-synthesized mineral synthesis than hPDLSC-hUVEC-CPC group (p < 0.05). Microcapillary-like structures were formed on scaffolds in vitro. hPDLSC-hUVEC-CPC-hPL group had more vessels than hPDLSC-hUVEC-CPC group (p < 0.05). In cranial defects in rats, hPDLSC-hUVEC-CPC-hPL group regenerated new bone amount that was 2.1 folds and 4.0 folds, respectively, that of hPDLSC-hUVEC-CPC group and CPC control (p < 0.05). New blood vessel density of hPDLSC-hUVEC-CPC-hPL group was 2 folds and 7.9 folds, respectively, that of hPDLSC-hUVEC-CPC group and CPC control (p < 0.05).

CONCLUSION

The hPL pre-culture method is promising to enhance bone regeneration via prevascularized CPC. Novel hPDLSC-hUVEC-CPC-hPL prevascularized construct increased new bone formation and blood vessel density by 4-8 folds over CPC control.

CLINICAL SIGNIFICANCE

Novel hPDLSC-hUVEC-hPL-CPC prevascularized construct greatly increased bone and vascular regeneration in vivo and hence is promising for a wide range of craniofacial applications.

A Bioglass-Poly(lactic-co-glycolic Acid) Scaffold@Fibrin Hydrogel Construct to Support Endochondral Bone Formation

Bone tissue engineering using stem cells to build bone directly on a scaffold matrix often fails due to lack of oxygen at the injury site. This may be avoided by following the endochondral ossification route; herein, a cartilage template is promoted first, which can survive hypoxic environments, followed by its hypertrophy and ossification. However, hypertrophy is so far only achieved using biological factors. This work introduces a Bioglass-Poly(lactic-co-glycolic acid@fibrin (Bg-PLGA@fibrin) construct where a fibrin hydrogel infiltrates and encapsulates a porous Bg-PLGA. The hypothesis is that mesenchymal stem cells (MSCs) loaded in the fibrin gel and induced into chondrogenesis degrade the gel and become hypertrophic upon reaching the stiffer, bioactive Bg-PLGA core, without external induction factors. Results show that Bg-PLGA@fibrin induces hypertrophy, as well as matrix mineralization and osteogenesis; it also promotes a change in morphology of the MSCs at the gel/scaffold interface, possibly a sign of osteoblast-like differentiation of hypertrophic chondrocytes. Thus, the Bg-PLGA@fibrin construct can sequentially support the different phases of endochondral ossification purely based on material cues. This may facilitate clinical translation by decreasing in-vitro cell culture time pre-implantation and the complexity associated with the use of external induction factors.

Injectable and Cell-Laden Hydrogel in the Contained Bone Defect Animal Model: A Systematic Review

BACKGROUND

Due to its high water content and biomimetic properties simulating extracellular matrix (ECM), hydrogels have been used as preferred cell culture and delivery systems. Similarly, cell-loaded hydrogels can be easily injected into target areas in a minimally invasive manner, minimizing surgical trauma, adapting to irregular shaped defects, and benefiting patients. In this study, we systematically reviewed multiple studies on hydrogel-based bone defect research and briefly summarized the progress of injectable and cell-loaded hydrogels in bone defect repair.

METHODS

A systematic search was conducted in the PubMed and Web of Science databases using selected search terms.

RESULTS

Initially, 185 articles were retrieved from the databases. After full-text screening based on inclusion and exclusion criteria, 26 articles were included in this systematic review. Data collected from each study included culture model, seed cell type and origin, cell concentration, scaffold material, scaffold shape, experimental animal and site, bioactive agents, and binding method. This injectable and cell-loaded hydrogel shows certain feasibility in bone tissue engineering applications.

CONCLUSION

Injectable and cell-loaded hydrogels have been widely applied in bone tissue engineering research. The future direction of bone tissue engineering for bone defect treatment involves the use of new hydrogel materials and biochemical stimulation.

Regeneration of Critical-Sized Mandibular Defects Using 3D-Printed Composite Scaffolds: A Quantitative Evaluation of New Bone Formation in In Vivo Studies

Mandibular tissue engineering aims to develop synthetic substitutes for the regeneration of critical size defects (CSD) caused by a variety of events, including tumor surgery and post-traumatic resections. Currently, the gold standard clinical treatment of mandibular resections (i.e., autologous fibular flap) has many drawbacks, driving research efforts toward scaffold design and fabrication by additive manufacturing (AM) techniques. Once implanted, the scaffold acts as a support for native tissue and facilitates processes that contribute to its regeneration, such as cells infiltration, matrix deposition and angiogenesis. However, to fulfil these functions, scaffolds must provide bioactivity by mimicking natural properties of the mandible in terms of structure, composition and mechanical behavior. This review aims to present the state of the art of scaffolds made with AM techniques that are specifically employed in mandibular tissue engineering applications. Biomaterials chemical composition and scaffold structural properties are deeply discussed, along with strategies to promote osteogenesis (i.e., delivery of biomolecules, incorporation of stem cells, and approaches to induce vascularization in the constructs). Finally, a comparison of in vivo studies is made by taking into consideration the amount of new bone formation (NB), the CSD dimensions, and the animal model.

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

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

Application of mesenchymal stem cell sheet for regeneration of craniomaxillofacial bone defects

Bone defects are among the most common damages in human medicine. Due to limitations and challenges in the area of bone healing, the research field has turned into a hot topic discipline with direct clinical outcomes. Among several available modalities, scaffold-free cell sheet technology has opened novel avenues to yield efficient osteogenesis. It is suggested that the intact matrix secreted from cells can provide a unique microenvironment for the acceleration of osteoangiogenesis. To the best of our knowledge, cell sheet technology (CST) has been investigated in terms of several skeletal defects with promising outcomes. Here, we highlighted some recent advances associated with the application of CST for the recovery of craniomaxillofacial (CMF) in various preclinical settings. The regenerative properties of both single-layer and multilayer CST were assessed regarding fabrication methods and applications. It has been indicated that different forms of cell sheets are available for CMF engineering like those used for other hard tissues. By tackling current challenges, CST is touted as an effective and alternative therapeutic option for CMF bone regeneration.

SAXS imaging reveals optimized osseointegration properties of bioengineered oriented 3D-PLGA/aCaP scaffolds in a critical size bone defect model

Healing large bone defects remains challenging in orthopedic surgery and is often associated with poor outcomes and complications. A major issue with bioengineered constructs is achieving a continuous interface between host bone and graft to enhance biological processes and mechanical stability. In this study, we have developed a new bioengineering strategy to produce oriented biocompatible 3D PLGA/aCaP nanocomposites with enhanced osseointegration. Decellularized scaffolds -containing only extracellular matrix- or scaffolds seeded with adipose-derived mesenchymal stromal cells were tested in a mouse model for critical size bone defects. In parallel to micro-CT analysis, SAXS tensor tomography and 2D scanning SAXS were employed to determine the 3D arrangement and nanostructure within the critical-sized bone. Both newly developed scaffold types, seeded with cells or decellularized, showed high osseointegration, higher bone quality, increased alignment of collagen fibers and optimal alignment and size of hydroxyapatite minerals.

Recent Advances in Cell Sheet Engineering: From Fabrication to Clinical Translation

Cell sheet engineering, a scaffold-free tissue fabrication technique, has proven to be an important breakthrough technology in regenerative medicine. Over the past two decades, the field has developed rapidly in terms of investigating fabrication techniques and multipurpose applications in regenerative medicine and biological research. This review highlights the most important achievements in cell sheet engineering to date. We first discuss cell sheet harvesting systems, which have been introduced in temperature-responsive surfaces and other systems to overcome the limitations of conventional cell harvesting methods. In addition, we describe several techniques of cell sheet transfer for preclinical (in vitro and in vivo) and clinical trials. This review also covers cell sheet cryopreservation, which allows short- and long-term storage of cells. Subsequently, we discuss the cell sheet properties of angiogenic cytokines and vasculogenesis. Finally, we discuss updates to various applications, from biological research to clinical translation. We believe that the present review, which shows and compares fundamental technologies and recent advances in cell engineering, can potentially be helpful for new and experienced researchers to promote the further development of tissue engineering in different applications.

Hyaluronic acid hydrogels support to generate integrated bone formation through endochondral ossification in vivo using mesenchymal stem cells

Engineered cartilage tissue from differentiated mesenchymal stem cells (MSCs) can generate bone in vivo through endochondral ossification (ECO). This ECO-mediated approach has the potential to circumvent the severe problems associated with conventional MSC-based bone tissue engineering techniques that lack mechanisms to induce angiogenesis. Hyaluronic acid (HA) is a key component in the cartilage extracellular matrix. However, the ECO-supporting properties of HA remain largely unclear. This study aimed to compare the ability of HA and collagen hydrogels to support in vitro differentiation of MSC-based hypertrophic cartilage tissues and to promote endochondral bone formation in vivo. Following the chondrogenic and hypertrophic differentiation in vitro, both HA and collagen constructs accumulated sulfated glycosaminoglycan (sGAG) and type 1, type II, and type X collagen. However, HA hydrogels exhibited a more uniform distribution of sGAG, type 1 collagen, type X collagen, and osteocalcin proteins; in addition, the cells embedded in the hydrogels had more rounded cell morphologies than those in the collagen constructs. At week 5 of in vitro culture, two to three constructs were implanted into a subcutaneous pocket in nude mice and harvested after 4 and 8 weeks. Both HA and collagen constructs promoted endochondral bone formation with vascularization and bone marrow development; however, the HA constructs fused to form integrated bone tissues and the bone marrow developed along the space between the two adhered grafts in all implanted pockets (n = 5). In the collagen constructs, the integration was observed in 40% of the pockets (n = 5). Microcomputer CT analysis revealed that the bone volume of HA constructs was larger than that of collagen constructs. In conclusion, compared to collagen hydrogels, HA hydrogels had superior potential to generate integrated bone with vascularization and bone marrow development. This study provides valuable insights for applying ECO-mediated bone tissue engineering approaches for the repair of critical-sized bone defects.

Periosteum as a covering vascular flap in posterior mandibular augmentation: A retrospective cohort study

OBJECTIVE

To analyze the impact of creating periosteal vascular flaps on the amount of bone augmentation following inlay bone grafting (IBG) and cortical autogenous tenting (CAT).

MATERIALS AND METHODS

This was a retrospective cohort study enrolling a sample cohort of patients presented to a private clinic in 2015 and 2019 for posterior mandibular ridge augmentation before dental implant placement. The predictor variables were surgical methods: CAT vs. CAT in conjunction with periosteal flap (CATP) vs. IBG vs. IBG in conjunction with periosteal flap (IBGP). The primary outcome variables were supra bundle bone (SBB) superior to the inferior alveolar canal (ΔH) and crestal width difference (ΔW) at a 4-month follow-up. Appropriate statistics were computed at 0.05 significance level.

RESULTS

A total of 29 cases (10 males and 19 females) with a mean age of 57.96±7.14 years were included. A total of 33 sites were augmented through CATP, 16 sites through IBGP, 33 sites through CAT, and 11 sites through IBG techniques. All patients healed uneventfully without permanent neurosensory changes, and adequate horizontal (ΔW:3.33±0.71 mm) and vertical (ΔH:5.10±2.04 mm) bone dimensions were restored that allowed implant placement. Using periosteal vascular flaps significantly increased bone augmentation in both vertical and horizontal dimensions (P < 0.01).

CONCLUSION

Periosteal vascular flaps can increase the efficacy of mandibular augmentation techniques and decrease post-surgical complications.

3D bioprinting of cartilaginous templates for large bone defect healing

Damaged or diseased bone can be treated using autografts or a range of different bone grafting biomaterials, however limitations with such approaches has motivated increased interest in developmentally inspired bone tissue engineering (BTE) strategies that seek to recapitulate the process of endochondral ossification (EO) as a means of regenerating critically sized defects. The clinical translation of such strategies will require the engineering of scaled-up, geometrically defined hypertrophic cartilage grafts that can be rapidly vascularised and remodelled into bone in mechanically challenging defect environments. The goal of this study was to 3D bioprint mechanically reinforced cartilaginous templates and to assess their capacity to regenerate critically sized femoral bone defects. Human mesenchymal stem/stromal cells (hMSCs) were incorporated into fibrin based bioinks and bioprinted into polycaprolactone (PCL) frameworks to produce mechanically reinforced constructs. Chondrogenic priming of such hMSC laden constructs was required to support robust vascularisation and graft mineralisation in vivo following their subcutaneous implantation into nude mice. With a view towards maximising their potential to support endochondral bone regeneration, we next explored different in vitro culture regimes to produce chondrogenic and early hypertrophic engineered grafts. Following their implantation into femoral bone defects within transiently immunosuppressed rats, such bioprinted constructs were rapidly remodelled into bone in vivo, with early hypertrophic constructs supporting higher levels of vascularisation and bone formation compared to the chondrogenic constructs. Such early hypertrophic bioprinted constructs also supported higher levels of vascularisation and spatially distinct patterns of new formation compared to BMP-2 loaded collagen scaffolds (here used as a positive control). In conclusion, this study demonstrates that fibrin based bioinks support chondrogenesis of hMSCs in vitro, which enables the bioprinting of mechanically reinforced hypertrophic cartilaginous templates capable of supporting large bone defect regeneration. These results support the use of 3D bioprinting as a strategy to scale-up the engineering of developmentally inspired templates for BTE. STATEMENT OF SIGNIFICANCE: Despite the promise of developmentally inspired tissue engineering strategies for bone regeneration, there are still challenges that need to be addressed to enable clinical translation. This work reports the development and assessment (in vitro and in vivo) of a 3D bioprinting strategy to engineer mechanically-reinforced cartilaginous templates for large bone defect regeneration using human MSCs. Using distinct in vitro priming protocols, it was possible to generate cartilage grafts with altered phenotypes. More hypertrophic grafts, engineered in vitro using TGF-β3 and BMP-2, supported higher levels of blood vessel infiltration and accelerated bone regeneration in vivo. This study also identifies some of the advantages and disadvantages of such endochondral bone TE strategies over the direct delivery of BMP-2 from collagen-based scaffolds.

Vascularization of a Bone Organoid Using Dental Pulp Stem Cells

Bone organoids offer a novel path for the reconstruction and repair of bone defects. We previously fabricated scaffold-free bone organoids using cell constructs comprising only bone marrow-derived mesenchymal stem cells (BMSCs). However, the cells in the millimetre-scale constructs were likely to undergo necrosis because of difficult oxygen diffusion and nutrient delivery. Dental pulp stem cells (DPSCs) are capable of differentiating into vascular endothelial lineages and have great vasculogenic potential under endothelial induction. Therefore, we hypothesized that DPSCs can serve as a vascular source to improve the survival of the BMSCs within the bone organoid. In this study, the DPSCs had greater sprouting ability, and the proangiogenic marker expressions were significantly greater than those of BMSCs. DPSCs were incorporated into the BMSC constructs at various ratios (5%-20%), and their internal structures and vasculogenic and osteogenic characteristics were investigated after endothelial differentiation. As a result, the DPSCs are differentiated into the CD31-positive endothelial lineage in the cell constructs. The incorporation of DPSCs significantly suppressed cell necrosis and improved the viability of the cell constructs. In addition, lumen-like structures were visualized by fluorescently labelled nanoparticles in the DPSC-incorporated cell constructs. The vascularized BMSC constructs were successfully fabricated using the vasculogenic ability of the DPSCs. Next, osteogenic induction was initiated in the vascularized BMSC/DPSC constructs. Compared with only BMSCs, constructs with DPSCs had increased mineralized deposition and a hollow structure. Overall, this study demonstrated that vascularized scaffold-free bone organoids were successfully fabricated by incorporating DPSCs into BMSC constructs, and the biomimetic biomaterial is promising for bone regenerative medicine and drug development.

Significance of mechanical loading in bone fracture healing, bone regeneration, and vascularization

In 1892, J.L. Wolff proposed that bone could respond to mechanical and biophysical stimuli as a dynamic organ. This theory presents a unique opportunity for investigations on bone and its potential to aid in tissue repair. Routine activities such as exercise or machinery application can exert mechanical loads on bone. Previous research has demonstrated that mechanical loading can affect the differentiation and development of mesenchymal tissue. However, the extent to which mechanical stimulation can help repair or generate bone tissue and the related mechanisms remain unclear. Four key cell types in bone tissue, including osteoblasts, osteoclasts, bone lining cells, and osteocytes, play critical roles in responding to mechanical stimuli, while other cell lineages such as myocytes, platelets, fibroblasts, endothelial cells, and chondrocytes also exhibit mechanosensitivity. Mechanical loading can regulate the biological functions of bone tissue through the mechanosensor of bone cells intraosseously, making it a potential target for fracture healing and bone regeneration. This review aims to clarify these issues and explain bone remodeling, structure dynamics, and mechano-transduction processes in response to mechanical loading. Loading of different magnitudes, frequencies, and types, such as dynamic versus static loads, are analyzed to determine the effects of mechanical stimulation on bone tissue structure and cellular function. Finally, the importance of vascularization in nutrient supply for bone healing and regeneration was further discussed.

Unveiling the Time Course Mechanism of Bone Fracture Healing by Transcriptional Profiles

BACKGROUND

Bone fracture healing is a time-consuming and high-priority orthopedic problem worldwide.

OBJECTIVE

Discovering the potential mechanism of bone healing at a time course and transcriptional level may better help manage bone fracture.

METHODS

In this study, we analyze a time-course bone fracture healing transcriptional dataset in a rat model (GSE592, GSE594, and GSE1371) of Gene Expression Omnibus (GEO). RNA was obtained from female Sprague-Dawley rats with a femoral fracture at the initial time (day 3) as well as early (week 1), middle (week 2), and late (week 4) time periods, with nonfracture rats used as control. Gene Ontology (GO) functional analysis and pathway examinations were performed for further measurements of GSEA and hub genes.

RESULTS

Results indicated that the four stages of bone fracture healing at the initial, early, middle, and late time periods represent the phases of hematoma formation, callus formation, callus molding, and mature lamellar bone formation, respectively. Extracellular organization was positively employed throughout the four stages. At the hematoma formation phase, the muscle contraction process was downregulated. Antibacterial peptide pathway was downregulated at all phases. The upregulation of Fn1 (initial, early, middle, and late time periods), Col3a1 (initial, early, and middle time periods), Col11a1 (initial and early time periods), Mmp9 (middle and late time periods), Mmp13 (early, middle, and late time periods) and the downregulation of RatNP-3b (initial, early, middle, and late time periods) were possible symbols for bone fracture healing and may be used as therapeutic targets.

CONCLUSION

These findings suggest some new potential pathways and genes in the process of bone fracture healing and further provide insights that can be used in targeted molecular therapy for bone fracture healing.

Microvessels derived from hiPSCs are a novel source for angiogenesis and tissue regeneration

The establishment of effective vascularization represents a key challenge in regenerative medicine. Adequate sources of vascular cells and intact vessel fragments have not yet been explored. We herein examined the potential application of microvessels induced from hiPSCs for rapid angiogenesis and tissue regeneration. Microvessels were generated from human pluripotent stem cells (iMVs) under a defined induction protocol and compared with human adipose tissue-derived microvessels (ad-MVs) to illustrate the similarity and differences of the alternative source. Then, the therapeutic effect of iMVs was detected by transplantation in vivo. The renal ischemia-reperfusion model and skin damage model were applied to explore the potential effect of vascular cells derived from iMVs (iMVs-VCs). Besides, the subcutaneous transplantation model and muscle injury model were established to explore the ability of iMVs for angiogenesis and tissue regeneration. The results revealed that iMVs had remarkable similarities to natural blood vessels in structure and cellular composition, and were potent for vascular formation and self-organization. The infusion of iMVs-VCs promoted tissue repair in the renal and skin damage model through direct contribution to the reconstruction of blood vessels and modulation of the immune microenvironment. Moreover, the transplantation of intact iMVs could form a massive perfused blood vessel and promote muscle regeneration at the early stage. The infusion of iMVs-VCs could facilitate the reconstruction and regeneration of blood vessels and modulation of the immune microenvironment to restore structures and functions of damaged tissues. Meanwhile, the intact iMVs could rapidly form perfused vessels and promote muscle regeneration. With the advantages of abundant sources and high angiogenesis potency, iMVs could be a candidate source for vascularization units for regenerative medicine.

Multifunctionalized carbon-fiber-reinforced polyetheretherketone implant for rapid osseointegration under infected environment

Carbon fiber reinforced polyetheretherketone (CFRPEEK) possesses a similar elastic modulus to that of human cortical bone and is considered as a promising candidate to replace metallic implants. However, the bioinertness and deficiency of antibacterial activities impede its application in orthopedic and dentistry. In this work, titanium plasma immersion ion implantation (Ti-PIII) is applied to modify CFRPEEK, achieving unique multi-hierarchical nanostructures and active sites on the surface. Then, hybrid polydopamine (PDA)@ZnO-EDN1 nanoparticles (NPs) are introduced to construct versatile surfaces with improved osteogenic and angiogenic properties and excellent antibacterial properties. Our study established that the modified CFRPEEK presented favorable stability and cytocompatibility. Compared with bare CFRPEEK, improved osteogenic differentiation of rat mesenchymal stem cells (BMSCs) and vascularization of human umbilical vein endothelial cells (HUVECs) are found on the functionalized surface due to the zinc ions and EDN1 releasing. In vitro bacteriostasis assay confirms that hybrid PDA@ZnO NPs on the functionalized surface provided an effective antibacterial effect. Moreover, the rat infected model corroborates the enhanced antibiosis and osteointegration of the functionalized CFRPEEK. Our findings indicate that the multilevel nanostructured PDA@ZnO-EDN1 coated CFRPEEK with enhanced antibacterial, angiogenic, and osteogenic capacity has great potential as an orthopedic/dental implant material for clinical application.

Biosynthesized Bandages Carrying Magnesium Oxide Nanoparticles Induce Cortical Bone Formation by Modulating Endogenous Periosteal Cells

Bone grafting is frequently conducted to treat bone defects caused by trauma and tumor removal, yet with significant medical and socioeconomic burdens. Space-occupying bone substitutes remain challenging in the control of osteointegration, and meanwhile activation of endogenous periosteal cells by using non-space-occupying implants to promote new bone formation becomes another therapeutic strategy. Here, we fabricated a magnesium-based artificial bandage with optimal micropatterns for activating periosteum-associated biomineralization. Collagen was self-assembled on the surface of magnesium oxide nanoparticles embedded electrospun fibrous membranes as a hierarchical bandage structure to facilitate the integration with periosteum in situ. After the implantation on the surface of cortical bone in vivo, magnesium ions were released to generate a pro-osteogenic immune microenvironment by activating the endogenous periosteal macrophages into M2 phenotype and, meanwhile, promote blood vessel formation and neurite outgrowth. In a cortical bone defect model, magnesium-based artificial bandage guided the surrounding newly formed bone tissue to cover the defected area. Taken together, our study suggests that the strategy of stimulating bone formation can be achieved with magnesium delivery to periosteum in situ and the proposed periosteal bandages act as a bioactive media for accelerating bone healing.

The Performance of Nonwoven PLLA Scaffolds of Different Thickness for Stem Cells Seeding and Implantation

The 3D reconstruction of 100 μm- and 600 μm-thick fibrous poly-L/L-lactide scaffolds was performed by confocal laser scanning microscopy and supported by scanning electron microscopy and showed that the density of the fibers on the side adjacent to the electrode is higher, which can affect cell diffusion, while the pore size is generally the same. Bone marrow mesenchymal stem cells cultured in a 600 μm-thick scaffold formed colonies and produced conditions for cell differentiation. An in vitro study of stem cells after 7 days revealed that cell proliferation and hepatocyte growth factor release in the 600 μm-thick scaffold were higher than in the 100 μm-thick scaffold. An in vivo study of scaffolds with and without stem cells implanted subcutaneously onto the backs of recipient mice was carried out to test their biodegradation and biocompatibility over a 0-3-week period. The cells seeded onto the 600 μm-thick scaffold promoted significant neovascularization in vivo. After 3 weeks, a significant number of donor cells persisted only on the inside of the 600 μm-thick scaffold. Thus, the use of bulkier matrices allows to prolong the effect of secretion of growth factors by stem cells during implantation. These 600 μm-thick scaffolds could potentially be utilized to repair and regenerate injuries with stem cell co-culture for vascularization of implant.

Preclinical Study of Human Bone Marrow-Derived Mesenchymal Stem Cells Using a 3-Dimensional Manufacturing Setting for Enhancing Spinal Fusion

Spinal fusion surgery is a surgical technique that connects one or more vertebrae at the same time to prevent movement between the vertebrae. Although synthetic bone substitutes or osteogenesis-inducing recombinant proteins were introduced to promote bone union, the rate of revision surgery is still high due to pseudarthrosis. To promote successful fusion after surgery, stem cells with or without biomaterials were introduced; however, conventional 2D-culture environments have resulted in a considerable loss of the innate therapeutic properties of stem cells. Therefore, we conducted a preclinical study applying 3D-spheroids of human bone marrow-dewrived mesenchymal stem cells (MSCs) to a mouse spinal fusion model. First, we built a large-scale manufacturing platform for MSC spheroids, which is applicable to good manufacturing practice (GMP). Comprehensive biomolecular examinations, which include liquid chromatography-mass spectrometry and bioinformatics could suggest a framework of quality control (QC) standards for the MSC spheroid product regarding the identity, purity, viability, and potency. In our animal study, the mass-produced and quality-controlled MSC spheroids, either undifferentiated or osteogenically differentiated were well-integrated into decorticated bone of the lumbar spine, and efficiently improved angiogenesis, bone regeneration, and mechanical stability with statistical significance compared to 2D-cultured MSCs. This study proposes a GMP-applicable bioprocessing platform and QC directions of MSC spheroids aiming for their clinical application in spinal fusion surgery as a new bone graft substitute.

Preparation of artificial vascularised tissue and the indirect determination of its void volume using μCT

The non-invasive determination of the vasculature volume would be very useful in many fields of medicine such as oncology and implantation. The purpose of this research was, therefore, to develop a methodology to investigate vascularisation in phantoms using microcomputed tomography (μCT) without having to visualise the single vessels. Epoxy resin and cotton candy were used to form the phantoms with microchannels. The size of the channels was measured via microscopy and the proportion of the void volume (PVV) was calculated. The phantoms were placed in contrast agent solutions of different concentrations and scanned in μCT. The mean CT numbers of the phantoms were calculated with the Amira software and displayed as a function of the determined PVV and the contrast agent concentration (CAC). The fabricated microchannels had the size of biological capillaries (diameter: 5 μm to 15 μm) and the phantoms showed a microchannel density of 5 to15 microchannels per mm². With an increasing CAC, the CT numbers increased significantly. Additionally, the phantoms with a higher PVV also had a higher CT number. The CT numbers and the PVV correlated moderately together, but significantly. The slope of the regression line increased with an increasing CAC.

Strategies to Promote Vascularization in 3D Printed Tissue Scaffolds: Trends and Challenges

Three-dimensional (3D) printing techniques for scaffold fabrication have shown promising advancements in recent years owing to the ability of the latest high-performance printers to mimic the native tissue down to submicron scales. Nevertheless, host integration and performance of scaffolds in vivo have been severely limited owing to the lack of robust strategies to promote vascularization in 3D printed scaffolds. As a result, researchers over the past decade have been exploring strategies that can promote vascularization in 3D printed scaffolds toward enhancing scaffold functionality and ensuring host integration. Various emerging strategies to enhance vascularization in 3D printed scaffolds are discussed. These approaches include simple strategies such as the enhancement of vascular in-growth from the host upon implantation by scaffold modifications to complex approaches wherein scaffolds are fabricated with their own vasculature that can be directly anastomosed or microsurgically connected to the host vasculature, thereby ensuring optimal integration. The key differences among the techniques, their pros and cons, and the future opportunities for utilizing each technique are highlighted here. The Review concludes with the current limitations and future directions that can help 3D printing emerge as an effective biofabrication technique to realize tissues with physiologically relevant vasculatures to ultimately accelerate clinical translation.

The additive effects of photobiomodulation and bioactive glasses on enhancing early angiogenesis

Early angiogenesis is important to facilitate biomaterials' osteogenic effects and avoid the bone regeneration failure for large-sized bone defects. Bioactive glasses (BG) have been widely utilized as a biomaterial for bone repair. However, the early angiogenesis of BG may be inadequate. In this study, we explored the effects of photobiomodulation (PBM) combined with BG on early angiogenesis to solve this bottleneck problem of insufficient early angiogenesis. In vitro, human umbilical vein endothelial cells (HUVECs) were cultured with BG extracts and treated with PBM using 1 J/cm2. The 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay, real-time reverse transcription-polymerase chain reaction (real-time RT-PCR) and tubule formation assay were utilized to detect HUVECs' proliferation, vascular growth factor genes expression and tubules formation. In vivo, bone defects at the femoral metaphysis in Sprague-Dawley rats were treated with BG particulates and PBM at 120 J/cm2. Immunohistochemical staining was applied to observe the vascular-like structure formation. In vitro results showed that PBM combined with BG significantly promoted HUVECs' proliferation, genes expression and mature tubules formation. On days 2, 4 and 7, the VEGF gene expression in BG+PBM group was 2.70-, 2.59- and 3.05-fold higher than control (P<0.05), and higher than PBM and BG groups (P<0.05). On days 4 and 7, the bFGF gene expression in BG+PBM group was 2.42- and 1.82-fold higher than control (P<0.05), and also higher than PBM and BG groups (P<0.05). Tube formation assay showed that mature tubules formed in BG+PBM and PBM groups after 4 hours. The tubules number in BG+PBM group was significantly higher than other groups (P<0.05). In vivo results further confirmed that PBM induced early angiogenesis. More vascular-like structures were observed in BG+PBM and PBM groups 2-week post surgery. In conclusion, with the optimum PBM fluence and BG concentration, PBM combined with BG exerted additive effects on enhancing early angiogenesis.

Titanium dioxide dental implants surfaces related oxidative stress in bone remodeling: a systematic review

Background

Titanium dioxide dental implants have a controversial effect on reactive oxygen species (ROS) production. ROS is necessary for cellular signal transmission and proper metabolism, but also has the ability to cause cell death as well as DNA, RNA, and proteins damage by excessive oxidative stress. This study aimed to systematically review the effect of titanium dioxide dental implant-induced oxidative stress and its role on the osteogenesis-angiogenesis coupling in bone remodeling.

Methods

This systematic review was performed conforming to preferred reporting items for systematic review and meta-analysis (PRISMA) model. Four different databases (PubMed, Science Direct, Scopus and Medline databases) as well as manual searching were adopted. Relevant studies from January 2000 till September 2021 were retrieved. Critical Appraisal Skills Programme (CASP) was used to assess the quality of the selected studies.

Results

Out of 755 articles, only 14 which met the eligibility criteria were included. Six studies found that titanium dioxide nanotube (TNT) reduced oxidative stress and promoted osteoblastic activity through its effect on Wnt, mitogen-activated protein kinase (MAPK) and forkhead box protein O1 (FoxO1) signaling pathways. On the other hand, three studies confirmed that titanium dioxide nanoparticles (TiO₂NPs) induce oxidative stress, reduce ostegenesis and impair antioxidant defense system as a significant negative correlation was found between decreased SIR3 protein level and increased superoxide (O₂ ^•-). Moreover, five studies proved that titanium implant alloy enhances the generation of ROS and induces cytotoxicity of osteoblast cells via its effect on NOX pathway.

Conclusion

TiO₂NPs stimulate a wide array of oxidative stress related pathways. Scientific evidence are in favor to support the use of TiO₂ nanotube-coated titanium implants to reduce oxidative stress and promote osteogenesis in bone remodeling. To validate the cellular and molecular cross talk in bone remodeling of the present review, well-controlled clinical trials with a large sample size are required.

Tissue Engineered Neurovascularization Strategies for Craniofacial Tissue Regeneration

Craniofacial tissue injuries, diseases, and defects, including those within bone, dental, and periodontal tissues and salivary glands, impact an estimated 1 billion patients globally. Craniofacial tissue dysfunction significantly reduces quality of life, and successful repair of damaged tissues remains a significant challenge. Blood vessels and nerves are colocalized within craniofacial tissues and act synergistically during tissue regeneration. Therefore, the success of craniofacial regenerative approaches is predicated on successful recruitment, regeneration, or integration of both vascularization and innervation. Tissue engineering strategies have been widely used to encourage vascularization and, more recently, to improve innervation through host tissue recruitment or prevascularization/innervation of engineered tissues. However, current scaffold designs and cell or growth factor delivery approaches often fail to synergistically coordinate both vascularization and innervation to orchestrate successful tissue regeneration. Additionally, tissue engineering approaches are typically investigated separately for vascularization and innervation. Since both tissues act in concert to improve craniofacial tissue regeneration outcomes, a revised approach for development of engineered materials is required. This review aims to provide an overview of neurovascularization in craniofacial tissues and strategies to target either process thus far. Finally, key design principles are described for engineering approaches that will support both vascularization and innervation for successful craniofacial tissue regeneration.

The Development Tendency of 3D-Printed Bioceramic Scaffolds for Applications Ranging From Bone Tissue Regeneration to Bone Tumor Therapy

Three-dimensional (3D) printing concept has been successfully employed in regenerative medicine to achieve individualized therapy due to its benefit of a rapid, accurate, and predictable production process. Traditional biocomposites scaffolds (SCF) are primarily utilised for bone tissue engineering; nevertheless, over the last few years, there has already been a dramatic shift in the applications of bioceramic (BCR) SCF. As a direct consequence, this study focused on the structural, degeneration, permeation, and physiological activity of 3D-printed BCR (3DP-B) SCF with various conformations and work systems (macros, micros, and nanos ranges), as well as their impacts on the mechanical, degeneration, porosity, and physiological activities. In addition, 3DP-B SCF are highlighted in this study for potential uses applied from bone tissue engineering (BTE) to bone tumor treatment. The study focused on significant advances in practical 3DP-B SCF that can be utilized for tumor treatment as well as bone tissue regeneration (BTR). Given the difficulties in treating bone tumors, these operational BCR SCF offer a lot of promise in mending bone defects caused by surgery and killing any remaining tumor cells to accomplish bone tumor treatment. Furthermore, a quick assessment of future developments in this subject was presented. The study not only summarizes recent advances in BCR engineering, but it also proposes a new therapeutic strategy focused on the extension of conventional ceramics' multifunction to a particular diagnosis.

Vascularization in tissue engineering: The architecture cues of pores in scaffolds

Vascularization is a key event and also still a challenge in tissue engineering. Many efforts have been devoted to the development of vascularization based on cells, growth factors, and porous scaffolds in the past decades. Among these efforts, the architecture features of pores in scaffolds played important roles for vascularization, which have attracted increasing attention. It has been known that the open macro pores in scaffolds could facilitate cell migration, nutrient, and oxygen diffusion, which then could promote new tissue formation and vascularization. The pore parameters are the important factors affecting cells response and vessel formation. Thus, this review will give an overview of the current advances in the effects of pore parameters on vascularization in tissue engineering, mainly including pore size, interconnectivity, pore size distribution, pore shape (channel structure), and the micro/nano-surface topography of pores.

Piezoelectric Signals in Vascularized Bone Regeneration

The demand for bone substitutes is increasing in Western countries. Bone graft substitutes aim to provide reconstructive surgeons with off-the-shelf alternatives to the natural bone taken from humans or animal species. Under the tissue engineering paradigm, biomaterial scaffolds can be designed by incorporating bone stem cells to decrease the disadvantages of traditional tissue grafts. However, the effective clinical application of tissue-engineered bone is limited by insufficient neovascularization. As bone is a highly vascularized tissue, new strategies to promote both osteogenesis and vasculogenesis within the scaffolds need to be considered for a successful regeneration. It has been demonstrated that bone and blood vases are piezoelectric, namely, electric signals are locally produced upon mechanical stimulation of these tissues. The specific effects of electric charge generation on different cells are not fully understood, but a substantial amount of evidence has suggested their functional and physiological roles. This review summarizes the special contribution of piezoelectricity as a stimulatory signal for bone and vascular tissue regeneration, including osteogenesis, angiogenesis, vascular repair, and tissue engineering, by considering different stem cell sources entailed with osteogenic and angiogenic potential, aimed at collecting the key findings that may enable the development of successful vascularized bone replacements useful in orthopedic and otologic surgery.

Local dual delivery therapeutic strategies: Using biomaterials for advanced bone tissue regeneration

Bone development is a complex process involving a vast number of growth factors and chemical substances. These factors include transforming growth factor-beta, platelet-derived growth factor, insulin-like growth factor, and most importantly, the bone morphogenetic protein, which exhibits excellent therapeutic value in bone repair. However, the spatial-temporal relationship in the expression of these factors during bone formation makes the bone repair a more complicated process to address. Thus, using a single therapeutic agent to address bone formation does not seem to provide a clinically effective option. Conversely, a dual delivery approach facilitating the co-delivery of agents has proved to be a dynamic alternative since such a strategy can provide more efficient spatial-temporal action. Such delivery systems can smartly target more than one pathway or differentiation lineage and thus offer more efficient bone regeneration. This review discusses various dual delivery strategies reported in the literature employed to achieve improved bone regeneration. These include concurrent use of different therapeutic agents (including growth factors and drugs), enhancing bone formation and cell recruitment, and improving the efficiency of bone healing.

Regenerative Engineering: Current Applications and Future Perspectives

Many pathologies, congenital defects, and traumatic injuries are untreatable by conventional pharmacologic or surgical interventions. Regenerative engineering represents an ever-growing interdisciplinary field aimed at creating biological replacements for injured tissues and dysfunctional organs. The need for bioengineered replacement parts is ubiquitous among all surgical disciplines. However, to date, clinical translation has been limited to thin, small, and/or acellular structures. Development of thicker tissues continues to be limited by vascularization and other impediments. Nevertheless, currently available materials, methods, and technologies serve as robust platforms for more complex tissue fabrication in the future. This review article highlights the current methodologies, clinical achievements, tenacious barriers, and future perspectives of regenerative engineering.

Coculture of Endothelial and Stromal Cells to Promote Concurrent Osteogenesis and Vasculogenesis

A key challenge in the treatment of large bone defects is the need to provide an adequate and stable vascular supply as new tissue develops. Bone tissue engineering applies selected biomaterials and cell types to create an environment that promotes tissue formation, maturation, and remodeling. Mesenchymal stromal cells (MSCs) have been widely used in these strategies because of their established effects on bone formation, and their ability to act as stabilizing pericytes that support vascular regeneration by endothelial cells (ECs). However, the creation of vascularized bone tissue in vitro requires coupling of osteogenesis and vasculogenesis in a three-dimensional (3D) biomaterial environment. In the present study, 3D fibrin hydrogels containing MSCs and ECs were prevascularized in vitro for 7 days to create an endothelial network in the matrix, and were subsequently cultured for a further 14 days under either continued vasculogenic stimulus, a combination of vasculogenic and osteogenic (hybrid) stimulus, or only osteogenic stimulus. It was found that ECs produced robust vessel networks in 3D fibrin matrices over 7 days of culture, and these networks continued to expand over the 14-day treatment period under vasculogenic conditions. Culture in hybrid medium resulted in maintenance of vessel networks for 14 days, while osteogenic culture abrogated vessel formation. These trends were mirrored in data representing overall cell viability and cell number in the 3D fibrin constructs. MSCs were found to colocalize with EC networks under vasculogenic and hybrid conditions, suggesting pericyte-like function. The bone marker alkaline phosphatase increased over time in hybrid and osteogenic media, but mineral deposition was evident only under purely osteogenic conditions. These results suggest that hybrid media compositions can support some aspects of multiphase tissue formation, but that alternative strategies are needed to obtain robust, concomitant vascularization, and osteogenesis in engineered tissues in vitro. Impact statement The combined use of mesenchymal stromal cells (MSCs) and endothelial cells to concomitantly produce mature bone and a nourishing vasculature is a promising tissue engineering approach to treating large bone defects. However, it is challenging to create and maintain vascular networks in the presence of osteogenic cues. This study used a 3D fibrin matrix to demonstrate that prevascularization of the construct can lead to maintenance of vessel structures over time, but that osteogenesis is compromised under these conditions. This work illuminates the capacity of MSCs to serve as both supportive pericytes and as osteoprogenitor cells, and motivates new strategies for coupling osteogenesis and vasculogenesis in engineered bone tissues.

Advancing models of neural development with biomaterials

Human pluripotent stem cells have emerged as a promising in vitro model system for studying the brain. Two-dimensional and three-dimensional cell culture paradigms have provided valuable insights into the pathogenesis of neuropsychiatric disorders, but they remain limited in their capacity to model certain features of human neural development. Specifically, current models do not efficiently incorporate extracellular matrix-derived biochemical and biophysical cues, facilitate multicellular spatio-temporal patterning, or achieve advanced functional maturation. Engineered biomaterials have the capacity to create increasingly biomimetic neural microenvironments, yet further refinement is needed before these approaches are widely implemented. This Review therefore highlights how continued progression and increased integration of engineered biomaterials may be well poised to address intractable challenges in recapitulating human neural development.

Restoration of critical defects in the rabbit mandible using osteoblasts and vascular endothelial cells co-cultured with vascular stent-loaded nano-composite scaffolds

The success of large bone defect repair with tissue engineering technology depends mainly on angiogenesis and osteogenesis. In this study, we prepared poly-caprolactone/nano-hydroxyapatite/beta-calcium phosphate (PCL/nHA/β-TCP) composite scaffolds loaded with poly-(lactic-co-glycolic acid)/nano-hydroxyapatite/collagen/heparin sodium (PLGA/nHA/Col/HS) nanofiber small vascular stent by electrospinning and hot press forming-particle leaching methods. Supramolecular electrostatic self-assembly technology was used to modify the surfaces of small vascular stents to aid in hydrophilicity and anticoagulation. The surfaces of composite scaffolds were modified with an Arg-Gly-Asp (RGD) short peptide by physical adsorption to supply cell adhesion sites. The scaffolds were then combined with rabbit bone marrow-derived osteoblasts (OBs) and rabbit bone marrow-derived vascular endothelial cells (RVECs) to construct large, biologically active vascularized tissue-engineered bone in vitro; this bone was then used to repair critical bone defects in rabbit mandibles. Mechanical and biocompatibility testing results showed that PCL/nHA/β-TCP composite scaffolds loaded with small vascular stents had good surface structure, mechanical properties, biocompatibility, and bone-regeneration induction potential. Twelve weeks after implantation, histological analysis and X-ray scans showed that the use of osteoblasts and vascular endothelial cells co-cultured with PCL/nHA/β-TCP scaffolds was sufficient to repair critical defects in rabbit mandibles.

Immobilization of bioactive vascular endothelial growth factor onto Ca-deficient hydroxyapatite-coated Mg by covalent bonding using polydopamine

BACKGROUND

Bone tissue engineering (BTE) is considered a promising technology for repairing bone defects. Mg2+ promotes osteogenesis, which makes Mg-based scaffolds popular for research on orthopedic implant materials. Angiogenesis plays an important role in the process of bone tissue repair and regeneration, and it is one of the important problems in BTE urgently needs to be solved.

METHODS

Mg was firstly coated with Ca-deficient hydroxyapatite (CDHA) via hydrothermal treatment, and polydopamine (DOPA) was then used as the connecting medium to immobilize vascular endothelial growth factor (VEGF) on the CDHA coating. The physicochemical properties of the coatings were characterized by SEM, EDS, XPS, FTIR and immersion experiment in SBF. The ahesion, proliferation, and angiogenesis potential of the coatings were determined in vitro.

RESULTS

The composite coating significantly improved the corrosion resistance of Mg and prohibited excessively high local alkalinity. VEGF could be firmly immobilized on Mg via polydopamine. The CCK-8, live/dead staining and adhesion test results showed that the VEGF-DOPA-CDHA coating exhibited excellent biocompatibility and could significantly improve the adhesion and proliferation of MC3T3-E1 cells on Mg. Microtubule formation, immunofluorescence and Quantitative Real-Time PCR (qRT-PCR) experiments showed that VEGF immobilized on Mg still possessed bioactivity in promoting the differentiation of rat mesenchymal stem cells into endothelial cells.

CONCLUSION

In this study, we enabled the angiogenic biological activity of Mg by immobilizing VEGF on Mg. Mg was successfully coated with a functional VEGF-DOPA-CDHA composite coating. The CDHA coating significantly increased the corrosion resistance of Mg and prohibited the negative effect of excessively high local alkalinity on the biological activity of VEGF. As an intermediate layer, the DOPA coating protects Mg, and DOPA provides a binding site for VEGF so that VEGF can be firmly immobilized on Mg and give Mg angiogenic bioactivity during the initial period of implantation.

THE TRANSLATIONAL POTENTIAL OF THIS ARTICLE

The treatment of large bone defect is still one of the orthopedic trauma diseases that are difficult to be completely treated in clinic. The development of tissue engineering technology provides a new option for the treatment of large bone defects. The regeneration of blood vessels is of great significance for the repair of bone defects. In this study, VEGF was connected on the surface of degradable magnesium by covalent bonding. Vascular biofunctionalized magnesium scaffolds are expected to regenerate bone tissue with blood transport and be used in the clinical treatment of large bone defects.

Induced Pluripotent Stem Cell-Derived Endothelial Networks Accelerate Vascularization But Not Bone Regeneration

Vascularization is critical for engineering mineralized tissues. It has been previously shown that biomaterials containing preformed endothelial networks anastomose to host vasculature following implantation. However, the networks alone may not increase regeneration. In addition, a clinically applicable source of cells for vascularization is needed. In this study, vascular networks were generated from endothelial cells (ECs) derived from human induced pluripotent stem cells (iPSCs). Network formation by iPSC-ECs within fibrin gels was investigated in a mesenchymal stem cells (MSCs) coculture spheroid model. Statistical design of experiments technique was evaluated for its predicting capability during the optimization of experimental parameters. The prevascularized units were combined with hydroxyapatite nanoparticles to develop a vascularized composite hydrogel that was implanted in a rodent critical-sized cranial defect model. Immunohistological staining for human-specific CD31 at week 1 indicated the presence and maintenance of the implanted vessels. At 8 weeks, the prevascularized systems resulted in higher vessel density over MSC-only scaffolds. The implanted vessels appeared to establish flow with host vasculature. While there was a slight increase in bone volume in the prevascularized bone construct compared to MSC-only bone constructs, there was not a profound increase in bone regeneration. These results show that scaffolds with network structures can be generated from ECs derived from iPSC and that the networks survive and inosculate with the host postimplantation in a bone model. Impact statement Vascularization is critical for engineering bone. Prevascularized scaffolds have been shown to improve postimplantation vascularization. Herein, vascularized networks were generated from induced pluripotent cells derived from endothelial cells. These vascularized units were combined with a fibrin/hydroxyapatite scaffold to develop a prevascularized construct for bone regeneration. Implantation of these scaffolds in a small animal cranial defect model resulted in network inosculation and increased vascularization, but exhibited only a limited effect on bone formation. This study provides insight into the challenges of generating vascularized bone.

Review on Computer-Aided Design and Manufacturing of Drug Delivery Scaffolds for Cell Guidance and Tissue Regeneration

In the last decade, additive manufacturing (AM) processes have updated the fields of biomaterials science and drug delivery as they promise to realize bioengineered multifunctional devices and implantable tissue engineering (TE) scaffolds virtually designed by using computer-aided design (CAD) models. However, the current technological gap between virtual scaffold design and practical AM processes makes it still challenging to realize scaffolds capable of encoding all structural and cell regulatory functions of the native extracellular matrix (ECM) of health and diseased tissues. Indeed, engineering porous scaffolds capable of sequestering and presenting even a complex array of biochemical and biophysical signals in a time- and space-regulated manner, require advanced automated platforms suitable of processing simultaneously biomaterials, cells, and biomolecules at nanometric-size scale. The aim of this work was to review the recent scientific literature about AM fabrication of drug delivery scaffolds for TE. This review focused on bioactive molecule loading into three-dimensional (3D) porous scaffolds, and their release effects on cell fate and tissue growth. We reviewed CAD-based strategies, such as bioprinting, to achieve passive and stimuli-responsive drug delivery scaffolds for TE and cancer precision medicine. Finally, we describe the authors' perspective regarding the next generation of CAD techniques and the advantages of AM, microfluidic, and soft lithography integration for enhancing 3D porous scaffold bioactivation toward functional bioengineered tissues and organs.

Angiogenic Aptamer-Modified Tetrahedral Framework Nucleic Acid Promotes Angiogenesis In Vitro and In Vivo

In a search for a solution to large-area soft and hard tissue defects, whether or not tissue regeneration or tissue-substitutes transplantation is used, the problems with angiogenesis need to be solved urgently. Thus, a new and efficient proangiogenic approach is needed. Nanoengineering systems have been considered one of the most promising approaches. In this study, we modify the tetrahedral framework nucleic acid (tFNA) for the first time with two different angiogenic DNA aptamers to form aptamer-tFNA nanostructures, tFNA-Apt02 and tFNA-AptVEGF, and the effects of them on angiogenesis both in vitro and in vivo are investigated. We develop new nanomaterials for enhancing angiogenesis to solve the problem of tissue engineering vascularization and ischemic diseases. The results of our study confirm that tFNA-Apt02 and tFNA-AptVEGF has a stronger ability to accelerate endothelial cell proliferation and migration, tubule formation, spheroid sprouting, and angiogenesis in vivo. We first demonstrate that the engineered novel tFNA-Apt02 and tFNA-AptVEGF have promoting effects on angiogenesis both in vitro and in vivo and provide a theoretical basis and opportunity for their application in tissues engineering vascularization and ischemic diseases.