Biomaterials for the Delivery of Growth Factors and Other Therapeutic Agents in Tissue Engineering Approaches to Bone Regeneration

rhBMP-2-loaded hydroxyapatite/beta-tricalcium phosphate microsphere/hydrogel composite promotes bone regeneration in a novel rat femoral nonunion model

Background

Nonunion following fracture treatment remains a significant clinical challenge, adversely affecting the patient's quality of life and imposing a substantial economic burden. The emergence of bone morphogenetic protein 2 (BMP-2) for bone regeneration represents a promising avenue, albeit limited by side effects such as inflammatory reactions primarily due to suboptimal drug delivery systems. This study focuses on NOVOSIS putty (NP), a novel biomaterial designed for the sustained release of BMP-2, aiming to mitigate these limitations and enhance bone healing.

Objective

This research aimed to evaluate the effectiveness of NP, a hydroxyapatite granules/β-tricalcium phosphate hydrogel composite (HA/β-TCP/hydrogel), as a BMP-2 carrier for promoting bone regeneration in a new rat nonunion model of long bone.

Methods

Using Sprague Dawley rats, a 2-mm silicone disk was interposed at the femoral fracture site, and intramedullary fixation with K-wire was performed to create a nonunion with a 2-mm bone defect. After 3 weeks, internal fixation with a plate, removal of the silicon disk, and refreshing the nonunion site were performed by implanting three different materials into the nonunion sites: allogenic iliac bone (IB), collagen sponge (CS) containing 10 μg of BMP-2, or NP containing 10 μg of BMP-2. Bone healing was evaluated weekly using micro-computed tomography (CT); ex vivo micro-Ct and histological evaluation were conducted at 6 weeks.

Results

At 6 weeks, NP demonstrated a significantly higher bone union rate (76.5%) compared with the CS group (35.3%, p = 0.037), and the IB group (6.3%, p < 0.0001). Bone mineral density (BMD) and bone volume/tissue volume (BV/TV) were also significantly higher in the NP group compared with the CS group (BMD, p < 0.0001; BV/TV, p = 0.031). Histological analysis showed the fracture gap in the NP group was filled with more trabecular bone and less fibrous tissue compared with the CS group.

Conclusion

The study confirms NP is a highly effective BMP-2 carrier, significantly improving bone union rates and new bone formation in nonunion fractures. The sustained release of BMP-2 from the hydrogel component reduced inflammatory responses and enhanced bone regeneration. NP can be a promising alternative to collagen-based BMP-2 delivery systems.

A systematic scoping review of the latest data on orthobiologics in the surgical treatment of non-union

INTRODUCTION

Recent studies have shown a growing concern regarding the cost-effectiveness and the lack of supporting data for the biologic agents that are being increasingly used in the orthopedic field. Our aim was to conduct a systematic scoping review of recent publications (last five years) on the use of orthobiologics to treat fracture non-union and summarize the latest available data.

PATIENTS AND METHODS

The inclusion criteria for this review were articles published in English, from 2016 to 2022, and focusing on the use of orthobiologics for the surgical treatment of non-union. Searches were conducted in March 2023 using Pubmed/MEDLINE and Embase. Studies on spinal fusion or gene therapy were excluded. Reviews, case reports with five cases or less, conference proceedings, preliminary reports, pediatric or non-human studies were excluded as well.

RESULTS

The search found 1807 articles, 15 were eligible after PRISMA checklist and exclusions. The evidence was heterogenous and there was only one level II RCT. Recent data suggests that bone morphogenic protein (BMP-2) products could be effective for septic and aseptic tibial non-unions. However, the evidence was not conclusive regarding BMP-7, plasma rich platelets (PRP), stem cells or demineralized bone matrix (DBM).

DISCUSSION

Every non-union case is different in terms of bone defect, biology, mechanical stability, surgical technique and host factors, which contributes to the conflicting reports on the efficacy of orthobiologics in the literature. We might never see a level 1, high powered and robust study defining the efficacy, safety profile and cost-effectiveness of such products.

LEVEL OF EVIDENCE

IV.

Local mRNA Delivery from Nanocomposites Made of Gelatin and Hydroxyapatite Nanoparticles

Local delivery of messenger ribonucleic acid (mRNA) is increasingly being advocated as a promising new strategy to enhance the performance of biomaterials. While extensive research has been dedicated to the complexation of these oligonucleotides into nanoparticles to facilitate systemic delivery, research on developing suitable biomaterial carriers for the local delivery of mRNA is still scarce. So far, mRNA-nanoparticles (mRNA-NPs) are mainly loaded into traditional polymeric hydrogels. Here, we show that calcium phosphate nanoparticles can be used for both reinforcement of nanoparticle-based hydrogels and the complexation of mRNA. mRNA was incorporated into lipid-coated calcium phosphate nanoparticles (LCPs) formulated with a fusogenic ionizable lipid in the outer layer of the lipid coat. Nanocomposites of gelatin and hydroxyapatite nanoparticles were prepared at various ratios. Higher hydroxyapatite nanoparticle content increased the viscoelastic properties of the nanocomposite but did not affect its self-healing ability. Combination of these nanocomposites with peptide, lipid, and the LCP mRNA formulations achieved local mRNA release as demonstrated by protein expression in cells in contact with the biomaterials. The LCP-based formulation was superior to the other formulations by showing less sensitivity to hydroxyapatite and the highest cytocompatibility.

Insight into the role of integrins and integrins-targeting biomaterials in bone regeneration

Features of the extracellular matrix, along with biochemical factors, have a momentous impress in making genes on and/or off. The interaction of cells and the extracellular matrix is mediated by integrins. Therefore, these molecules have pivotal roles in regulating cell behaviors. Integrins include a group of molecules with a variety of characteristics that can affect different molecular cascades. Considering the importance of these molecules in tissue regeneration after injury, it is necessary to know well the integrins involved in the process of connecting cells to the extracellular matrix in each tissue.With the increase in life expectancy, bone tissue engineering has received more attention from researchers. Integrins are critical components in osteoblast differentiation, survival, and bone mechanotransduction. During osteogenic differentiation in stem cells, specific integrins facilitate multiple signaling pathways through their cytoplasmic domain, leading to the induction of osteogenic differentiation. Also, due to the importance of using biomaterials in bone tissue engineering, efforts have been made to design and use biomaterials with maximum interaction with integrins. Notably, the use of RGD peptide or fibronectin for surface modification is a well-established and commonly employed approach to manipulate integrin activity.This review article looks into integrins' role in bone development and regeneration. It then goes on to explore the complex mechanisms by which integrins contribute to these processes. In addition, this review discusses the use of natural and synthetic biomaterials that target integrins to promote bone regeneration.

Application of gelatin-based composites in bone tissue engineering

Natural bone tissue has the certain function of self-regeneration and repair, but it is difficult to repair large bone damage. Recently, although autologous bone grafting is the "gold standard" for improving bone repair, it has high cost, few donor sources. Besides, allogeneic bone grafting causes greater immune reactions, which hardly meet clinical needs. The bone tissue engineering (BTE) has been developed to promote bone repair. Gelatin, due to its biocompatibility, receives a great deal of attention in the BTE research field. However, the disadvantages of natural gelatin are poor mechanical properties and single structural property. With the development of BTE, gelatin is often used in combination with a range of natural, synthetic polymers, and inorganic materials to achieve synergistic effects for the complex physiological process of bone repair. The review delves into the fundamental structure and unique properties of gelatin, as well as the excellent properties necessary for bone scaffold materials. Then this review explores the application of modified gelatin three-dimensional (3D) scaffolds with various structures in bone repair, including 3D fiber scaffolds, hydrogels, and nanoparticles. In addition, the review focuses on the excellent efficacy of composite bone tissue scaffolds consisting of modified gelatin, various natural or synthetic polymeric materials, as well as bioactive ceramics and inorganic metallic/non-metallic materials in the repair of bone defects. The combination of these gelatin-based composite scaffolds provides new ideas for the design of scaffold materials for bone repair with good biosafety.

Decellularized Tissue-Induced Cellular Recruitment for Tissue Engineering and Regenerative Medicine

Biomaterials that recapitulate the native in vivo microenvironment are promising to facilitate tissue repair and regeneration when used in combination with relevant growth factors (GFs), chemokines, cytokines, and other small molecules and cell sources. However, limitations with the use of exogenous factors and ex vivo cell expansion has prompted cell-/GF-free tissue engineering strategies. Additionally, conventional chemotaxis assays for studying cell migration behavior provide limited information, lack long-term stability, and fail to recapitulate physiologically relevant conditions. In this study, articular cartilage tissue-based biomaterials were developed via a rapid tissue decellularization protocol. The decellularized tissue was further processed into a hydrogel through solubilization and self-assembly. Chemotactic activity of the tissue-derived gel was investigated using sophisticated cellular migration assays. These tissue-derived extracellular matrix (ECM) biomaterials retain biochemical cues of native tissue and stimulate the chemotactic migration of hBMSCs in 2D and 3D cell migration models using a real-time chemotaxis assay. This strategy, in a way, developed a new paradigm in tissue engineering where cartilage tissue repair and regeneration can be approached with decellularized cartilage tissue in the place of an engineered matrix. This strategy can be further expanded for other tissue-based ECMs to develop cell-/GF-free tissue engineering and regenerative medicine strategies for recruiting endogenous cell populations to facilitate tissue repair and regeneration.

Investigation of a new implant surface modification using phosphorylated pullulan

Various implant surface treatment methods have been developed to achieve good osseointegration in implant treatment. However, some cases remain impossible to treat with implants because osseointegration is not obtained after implantation, and the implants fail. Thus, this study focused on phosphorylated pullulan because of its adhesiveness to titanium (Ti) and bone, high biocompatibility, and early replacement with bone. In this study, the response of bone-related cells to phosphorylated pullulan was evaluated to develop a new surface treatment method. Saos-2 (human osteosarcoma-derived osteoblast-like cells), MC3T3-E1 (mouse calvaria-derived osteoblast-like cells), and RAW264.7 (mouse macrophage-like cells) were used. In evaluating cellular responses, phosphorylated pullulan was added to the culture medium, and cell proliferation and calcification induction tests were performed. The proliferation and calcification of cells on the surface of Ti disks coated with phosphorylated pullulan were also evaluated. In addition, bone morphogenetic protein-2 (BMP-2), an osteogenic factor, was used to evaluate the role of phosphorylated pullulan as a drug carrier in inducing calcification on Ti disks. Phosphorylated pullulan tended to promote the proliferation of osteoblast-like cells and the formation of calcification on Ti disks coated with phosphorylated pullulan. Ti disks coated with phosphorylated pullulan loaded with BMP-2 enhanced calcification. Phosphorylated pullulan inhibited osteoclast-like cell formation. These results are due to the properties of phosphorylated pullulan, such as adhesiveness to titanium and drug-loading function. Therefore, phosphorylated pullulan effectively promotes bone regeneration when coated on titanium implants and is useful for developing a new surface treatment method.

An Innovative Design to Enhance Osteoinductive Efficacy and Biomechanical Behavior of a Titanium Dental Implant

The present study investigated the in vivo bone-forming efficacy of an innovative titanium (Ti) dental implant combined with a collagen sponge containing recombinant human bone morphogenetic protein-2 (BMP-2) in a pig model. Two different concentrations of BMP-2 (20 and 40 µg/mL) were incorporated into collagen sponges and placed at the bottom of Ti dental implants. The investigated implants were inserted into the edentulous ridge at the canine-premolar regions of Lanyu small-ear pigs, which were then euthanized at weeks 1, 2, 4, 8, and 12 post-implantation. Specimens containing the implants and surrounding bone tissue were collected for histological evaluation of their bone-to-implant contact (BIC) ratios and calculation of maximum torques using removal torque measurement. Analytical results showed that the control and BMP-2-loaded implants presented good implant stability and bone healing for all testing durations. After 1 week of healing, the BMP-2-loaded implants with a concentration of 20 µg/mL exhibited the highest BIC ratios, ranging from 58% to 76%, among all groups (p = 0.034). Additionally, they also possessed the highest removal torque values (50.1 ± 1.3 N-cm) throughout the 8-week healing period. The BMP-2-loaded implants not only displayed excellent in vivo biocompatibility but also presented superior osteoinductive performance. Therefore, these findings demonstrate that BMP-2 delivered through a collagen sponge can potentially enhance the early-stage osseointegration of Ti dental implants.

Bone-Regeneration Therapy Using Biodegradable Scaffolds: Calcium Phosphate Bioceramics and Biodegradable Polymers

Calcium phosphate-based synthetic bone is broadly used for the clinical treatment of bone defects caused by trauma and bone tumors. Synthetic bone is easy to use; however, its effects depend on the size and location of the bone defect. Many alternative treatment options are available, such as joint arthroplasty, autologous bone grafting, and allogeneic bone grafting. Although various biodegradable polymers are also being developed as synthetic bone material in scaffolds for regenerative medicine, the clinical application of commercial synthetic bone products with comparable performance to that of calcium phosphate bioceramics have yet to be realized. This review discusses the status quo of bone-regeneration therapy using artificial bone composed of calcium phosphate bioceramics such as β-tricalcium phosphate (βTCP), carbonate apatite, and hydroxyapatite (HA), in addition to the recent use of calcium phosphate bioceramics, biodegradable polymers, and their composites. New research has introduced potential materials such as octacalcium phosphate (OCP), biologically derived polymers, and synthetic biodegradable polymers. The performance of artificial bone is intricately related to conditions such as the intrinsic material, degradability, composite materials, manufacturing method, structure, and signaling molecules such as growth factors and cells. The development of new scaffold materials may offer more efficient bone regeneration.

Functional modulation of lysophosphatidic acid type 2 G-protein coupled receptor facilitates alveolar bone formation

Lipid biosynthesis is recently studied its functions in a range of cellular physiology including differentiation and regeneration. However, it still remains to be elucidated in its precise function. To reveal this, we evaluated the roles of lysophosphatidic acid (LPA) signaling in alveolar bone formation using the LPA type 2 receptor (LPAR2) antagonist AMG-35 (Amgen Compound 35) using tooth loss without periodontal disease model which would be caused by trauma and usually requires a dental implant to restore masticatory function. In this study, in vitro cell culture experiments in osteoblasts and periodontal ligament fibroblasts revealed cell type-specific responses, with AMG-35 modulating osteogenic differentiation in osteoblasts in vitro. To confirm the in vivo results, we employed a mouse model of tooth loss without periodontal disease. Five to 10 days after tooth extraction, AMG-35 facilitated bone formation in the tooth root socket as measured by immunohistochemistry for differentiation markers KI67, Osteocalcin, Periostin, RUNX2, transforming growth factor beta 1 (TGF-β1) and SMAD2/3. The increased expression and the localization of these proteins suggest that AMG-35 elicits osteoblast differentiation through TGF-β1 and SMAD2/3 signaling. These results indicate that LPAR2/TGF-β1/SMAD2/3 represents a new signaling pathway in alveolar bone formation and that local application of AMG-35 in traumatic tooth loss can be used to facilitate bone regeneration and healing for further clinical treatment.

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

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

Fabrication and optimization of phospholipids-based phase separation in-situ gel loaded with BMP-2 nanosized emulsion for bone defect

Introduction: The health, development, and/or survival of a newborn can be impacted by congenital abnormalities such as cleft lip (CLP) and palate, one of alveolar bone defects that emerge thru pregnancy. Therefore, the primary purpose of this study is to use phospholipids-based phase separation in-situ gel (PPSG) in combination with bone morphogenetic protein-2 nanoemulsion (BMP-2-NE) to aid repairing alveolar bone defects. Methods: To investigate how formulation parameters, such as the concentrations of BMP-2 aqueous solution, LauroglycolTM FCC, and Labrafac PG oil, affect NE qualities including droplet size and stability index, an l-optimal co-ordinate exchange statistical design was opted. Injectable PPSG with the best NE formulation was tested for viscosity characteristics, gel strength, water absorption, and in-vitro BMP-2 release. In rabbits, the percentage of BMP-2 that was still in the maxilla after 14 days was assessed. Results: Collected results revealed that the droplet size and stability index of optimal NE were discovered to be 68 2.0 nm and 96 1.3%, respectively. When mixed with water, optimal BMP-2 NE loaded PPSG became viscous and reached a gel strength of 41 s, which is adequate for injectable in-situ gels. In comparison to BMP-2 solution loaded in-situ gel, the in-vivo studies indicated that the newly created BMP-2 NE loaded PPSG produced a sustained and controlled release of BMP-2 that continued for 336 h (14 days). Further, 8% of the BMP-2 was still entrapped and not completely dissolved after 14 days, thus, created formulation allowed a higher percentage of BMP-2 to remain in rabbits' maxilla for longer time. Conclusion: PPSG that has been loaded with BMP-2 NE may therefore be a promising, fruitful, and less painful paradigm for the noninvasive therapy of CLP with significant effect and extended release.

Sequential Dual-Biofactor Release from the Scaffold of Mesoporous HA Microspheres and PLGA Matrix for Boosting Endogenous Bone Regeneration

The combined design of scaffold structure and multi-biological factors is a prominent strategy to promote bone regeneration. Herein, a composite scaffold of mesoporous hydroxyapatite (HA) microspheres loaded with the bone morphogenetic protein-2 (BMP-2) and a poly(DL-lactic-co-glycolic acid) (PLGA) matrix is constructed by 3D printing. Furthermore, the chemokine stromal cell-derived factor-1α (SDF-1α) is adsorbed on a scaffold surface to achieve the sequential release of the dual-biofactors. The results indicate that the rapid release of SDF-1α chemokine on the scaffold surface effectively recruits bone marrow-derived mesenchymal stem cells (BMSCs) to the target defect area, whereas the long-term sustained release of BMP-2 from the HA microspheres in the degradable PLGA matrix successfully triggers the osteogenic differentiation in the recruited BMSCs, significantly promoting bone regeneration and reconstruction. In addition, these structures/biofactors specially combining scaffold exhibit significantly better biological performance than that of other combined scaffolds, including the bare HA/PLGA scaffold, the scaffold loaded with SDF-1α or BMP-2 biofactor alone, and the scaffold with surface SDF-1α and BMP-2 dual-biofactors. The utilization of mesoporous HA, the assembly method, and sequential release of the two biofactors in the 3D printed composite scaffold present a new method for future design of high-performance bone repairing scaffolds.

Surface-modified Ti3C2 MXene nanosheets for mesenchymal stem cell osteogenic differentiation via photothermal conversion

In the field of bone tissue engineering, the practical application of growth factors is limited by various factors such as systemic toxicity, instability, and the potential to induce inflammation. To circumvent these limitations, the use of physical signals, such as thermal stimulation, to regulate stem cells has been proposed as a promising alternative. The present study aims to investigate the potential of the two-dimensional nanomaterial Ti₃C₂ MXene, which exhibits unique photothermal properties, to induce osteogenic differentiation of bone marrow-derived mesenchymal stem cells (BMSCs) via photothermal conversion. Surface modification of Ti₃C₂ MXene nanosheets with PVP (Ti₃C₂-PVP) was employed to enhance their colloidal stability in physiological solutions. Characterization and cellular experiments showed that Ti₃C₂-PVP nanosheets have favorable photothermal properties and biocompatibility. Our study demonstrated that the induction of photothermal stimulation by co-culturing Ti₃C₂-PVP nanosheets with BMSCs and subsequent irradiation with 808 nm NIR significantly promoted cell proliferation, adhesion and osteogenic differentiation of BMSCs. In conclusion, the results of this study suggest that Ti₃C₂-PVP is a promising material for bone tissue engineering applications as it can modulate the cellular functions of BMSCs through photothermal conversion.

Cell-instructive biomaterials in tissue engineering and regenerative medicine

The field of biomaterials aims to improve regenerative outcomes or scientific understanding for a wide range of tissue types and ailments. Biomaterials can be fabricated from natural or synthetic sources and display a plethora of mechanical, electrical, and geometrical properties dependent on their desired application. To date, most biomaterial systems designed for eventual translation to the clinic rely on soluble signaling moieties, such as growth factors, to elicit a specific cellular response. However, these soluble factors are often limited by high cost, convoluted synthesis, low stability, and difficulty in regulation, making the translation of these biomaterials systems to clinical or commercial applications a long and arduous process. In response to this, significant effort has been dedicated to researching cell-directive biomaterials which can signal for specific cell behavior in the absence of soluble factors. Cells of all tissue types have been shown to be innately in tune with their microenvironment, which is a biological phenomenon that can be exploited by researchers to design materials that direct cell behavior based on their intrinsic characteristics. This review will focus on recent developments in biomaterials that direct cell behavior using biomaterial properties such as charge, peptide presentation, and micro- or nano-geometry. These next generation biomaterials could offer significant strides in the development of clinically relevant medical devices which improve our understanding of the cellular microenvironment and enhance patient care in a variety of ailments.

Biopotentials of Collagen Scaffold Impregnated with Plant-Cell-Derived Epidermal Growth Factor in Defective Bone Healing

The combination of scaffolds with recombinant human epidermal growth factor (rhEGF) protein can enhance defective bone healing via synergistic activation to stimulate cellular growth, differentiation, and survival. We examined the biopotentials of an rhEGF-loaded absorbable collagen scaffold (ACS) using a mouse model of calvarial defects, in which the rhEGF was produced from a plant cell suspension culture system because of several systemic advantages. Here, we showed a successful and large-scale production of plant-cell-derived rhEGF protein (p-rhEGF) by introducing an expression vector that cloned with its cDNA under the control of rice α-amylase 3D promoter into rice calli (Oryza sativa L. cv. Dongjin). Implantation with p-rhEGF (5 μg)-loaded ACSs into critical-sized calvarial defects enhanced new bone formation and the expression of osteoblast-specific markers in the defected regions greater than implantation with ACSs alone did. The potency of p-rhEGF-induced bone healing was comparable with that of Escherichia coli-derived rhEGF protein. The exogenous addition of p-rhEGF increased the proliferation of human periodontal ligament cells and augmented the induction of interleukin 8, bone morphogenetic protein 2, and vascular endothelial growth factor in the cells. Collectively, this study demonstrates the successful and convenient production of p-rhEGF, as well as its potency to enhance ACS-mediated bone regeneration by activating cellular responses that are required for wound healing.

Human Fibroblast Growth Factor-Treated Adipose-Derived Stem Cells Facilitate Wound Healing and Revascularization in Rats with Streptozotocin-Induced Diabetes Mellitus

Diabetes mellitus contributes to 15-25% of all chronic foot ulcers. Peripheral vascular disease is a cause of ischemic ulcers and exacerbates diabetic foot disease. Cell-based therapies are viable options to restore damaged vessels and induce the formation of new vessels. Adipose-derived stem cells (ADSCs) have the potential for angiogenesis and regeneration because of their greater paracrine effect. Preclinical studies are currently using other forced enhancement techniques (e.g., genetic modification or biomaterials) to increase the efficacy of human ADSC (hADSC) autotransplantation. Unlike genetic modifications and biomaterials, many growth factors have been approved by the equivalent regulatory authorities. This study confirmed the effect of enhanced human ADSC (ehADSC)s with a cocktail of FGF and other pharmacological agents to promote wound healing in diabetic foot disease. In vitro, ehADSCs exhibited a long and slender spindle-shaped morphology and showed significantly increased proliferation. In addition, it was shown that ehADSCs have more functionalities in oxidative stress toleration, stem cell stemness, and mobility. In vivo, the local transplantation of 1.2 × 10⁶ hADSCs or ehADSCs was performed in animals with diabetes induced by STZ. The ehADSC group showed a statistically decreased wound size and increased blood flow compared with the hADSC group and the sham group. Human Nucleus Antigen (HNA) positive cells were observed in some ADSC-transplanted animals. The ehADSC group showed a relatively higher portion of HNA-positive animals than the hADSC group. The blood glucose levels showed no significant difference among the groups. In conclusion, the ehADSCs showed a better performance in vitro, compared with conventional hADSCs. Additionally, a topical injection of ehADSCs into diabetic wounds enhanced wound healing and blood flow, while improving histological markers suggesting revascularization.

Biomimetic hematoma delivers an ultra-low dose of rhBMP-2 to successfully regenerate large femoral bone defects in rats

Successful repair of large bone defects remains a clinical challenge. Following fractures, a bridging hematoma immediately forms as a crucial step that initiates bone healing. In larger bone defects the micro-architecture and biological properties of this hematoma are compromised, and spontaneous union cannot occur. To address this need, we developed an ex vivo Biomimetic Hematoma that resembles naturally healing fracture hematoma, using whole blood and the natural coagulants calcium and thrombin, as an autologous delivery vehicle for a very reduced dose of rhBMP-2. When implanted into a rat femoral large defect model, complete and consistent bone regeneration with superior bone quality was achieved with 10-20× less rhBMP-2 compared to that required with the collagen sponges currently used. Moreover, calcium and rhBMP-2 demonstrated a synergistic effect enhancing osteogenic differentiation, and fully restored mechanical strength 8 weeks after surgery. Collectively, these findings suggest the Biomimetic Hematoma provides a natural reservoir for rhBMP-2, and that retention of the protein within the scaffold rather than its sustained release might be responsible for more robust and rapid bone healing. Clinically, this new implant, using FDA-approved components, would not only reduce the risk of adverse events associated with BMPs, but also decrease treatment costs and nonunion rates.

Chitosan-coated and thymol-loaded polymeric semi-interpenetrating hydrogels: An effective platform for bioactive molecule delivery and bone regeneration in vivo

Thymol (2-isopropyl-5-methylphenol; Thy) is a monoterpene phenolic phytocompound with medicinal properties; however, its impact on osteogenesis is yet to be thoroughly investigated. Its distribution is often hampered because of its intricate hydrophobic structure, which reduces its bioavailability. In this study, we synthesized a drug delivery vehicle using semi-interpenetrating polymer network (SIPN) hydrogels containing sodium alginate and poly(2-ethyl-2-oxazoline) (SA/Pox) loaded with Thy at varying concentrations (100, 150, and 200 μM). Subsequently, they were coated with chitosan (CS) to increase bioactivity and for sustained and prolonged release of Thy. Thy-loaded CS-coated SIPN hydrogels (SA/Pox/CS-Thy) were developed using ionic gelation and polyelectrolyte-complexation techniques. The addition of CS to hydrogels enhanced their physicochemical and material properties. These hydrogels were cytofriendly toward mouse mesenchymal stem cells (mMSCs). When mMSCs were cultured on hydrogels, Thy stimulated osteoblastic differentiation, as evidenced by calcium deposits at the cellular level. The expression of RUNX2, a key bone transcriptional factor, and other differentiation biomarkers was significantly enhanced in mMSCs cultured on SA/Pox/CS-Thy hydrogels. Notably, Thy in the SA/Pox/CS hydrogels significantly activated the TGF-β/BMP signaling pathway, which is involved in osteogenesis. A rat tibial bone defect model system revealed that the incorporation of Thy into SA/Pox/CS hydrogels augmented bone regeneration. Thus, sustained and prolonged release of Thy from the SA/Pox/CS hydrogels promoted osteoblast differentiation in vitro and bone formation in vivo. These findings shed light on the effect of Thy bioavailability in fostering osteoblast differentiation and its prospective application in bone rejuvenation.

Recent progress in the manipulation of biochemical and biophysical cues for engineering functional tissues

Tissue engineering (TE) is currently considered a cutting-edge discipline that offers the potential for developing treatments for health conditions that negatively affect the quality of life. This interdisciplinary field typically involves the combination of cells, scaffolds, and appropriate induction factors for the regeneration and repair of damaged tissue. Cell fate decisions, such as survival, proliferation, or differentiation, critically depend on various biochemical and biophysical factors provided by the extracellular environment during developmental, physiological, and pathological processes. Therefore, understanding the mechanisms of action of these factors is critical to accurately mimic the complex architecture of the extracellular environment of living tissues and improve the efficiency of TE approaches. In this review, we recapitulate the effects that biochemical and biophysical induction factors have on various aspects of cell fate. While the role of biochemical factors, such as growth factors, small molecules, extracellular matrix (ECM) components, and cytokines, has been extensively studied in the context of TE applications, it is only recently that we have begun to understand the effects of biophysical signals such as surface topography, mechanical, and electrical signals. These biophysical cues could provide a more robust set of stimuli to manipulate cell signaling pathways during the formation of the engineered tissue. Furthermore, the simultaneous application of different types of signals appears to elicit synergistic responses that are likely to improve functional outcomes, which could help translate results into successful clinical therapies in the future.

A Review of 3D Polymeric Scaffolds for Bone Tissue Engineering: Principles, Fabrication Techniques, Immunomodulatory Roles, and Challenges

Over the last few years, biopolymers have attracted great interest in tissue engineering and regenerative medicine due to the great diversity of their chemical, mechanical, and physical properties for the fabrication of 3D scaffolds. This review is devoted to recent advances in synthetic and natural polymeric 3D scaffolds for bone tissue engineering (BTE) and regenerative therapies. The review comprehensively discusses the implications of biological macromolecules, structure, and composition of polymeric scaffolds used in BTE. Various approaches to fabricating 3D BTE scaffolds are discussed, including solvent casting and particle leaching, freeze-drying, thermally induced phase separation, gas foaming, electrospinning, and sol-gel techniques. Rapid prototyping technologies such as stereolithography, fused deposition modeling, selective laser sintering, and 3D bioprinting are also covered. The immunomodulatory roles of polymeric scaffolds utilized for BTE applications are discussed. In addition, the features and challenges of 3D polymer scaffolds fabricated using advanced additive manufacturing technologies (rapid prototyping) are addressed and compared to conventional subtractive manufacturing techniques. Finally, the challenges of applying scaffold-based BTE treatments in practice are discussed in-depth.

Four-Dimensional Printing and Shape Memory Materials in Bone Tissue Engineering

The repair of severe bone defects is still a formidable clinical challenge, requiring the implantation of bone grafts or bone substitute materials. The development of three-dimensional (3D) bioprinting has received considerable attention in bone tissue engineering over the past decade. However, 3D printing has a limitation. It only takes into account the original form of the printed scaffold, which is inanimate and static, and is not suitable for dynamic organisms. With the emergence of stimuli-responsive materials, four-dimensional (4D) printing has become the next-generation solution for biological tissue engineering. It combines the concept of time with three-dimensional printing. Over time, 4D-printed scaffolds change their appearance or function in response to environmental stimuli (physical, chemical, and biological). In conclusion, 4D printing is the change of the fourth dimension (time) in 3D printing, which provides unprecedented potential for bone tissue repair. In this review, we will discuss the latest research on shape memory materials and 4D printing in bone tissue repair.

Resolvin D1 improves allograft osteointegration and directly enhances osteoblasts differentiation

Introduction

Allografts are the most common bone grafts for repairing osseous defects. However, their use is associated with an increased risk for infections, donor disease transmission and osteointegration deficiency. Resolvin D1 (RvD1) is an endogenous lipid with a scientifically proven pivotal role in inflammation resolution and osteoclastogenesis inhibition. Yet, its biological relevance as a potential bone regenerative drug has been scarcely studied. Here, we aim to investigate the RvD1 effect on allograft osteointegration in the alveolar bone regeneration (ABR) murine model.

Methods

ABR model consisted of osseous defects that were generated by the extraction of the maxillary first molar in C57BL/6 mice. The sockets were filled with allograft and analyzed via RNA sequencing. Then they were locally injected with either RvD1 or saline via single or repeated administrations. The mice were sacrificed 2W after the procedure, and regenerated sites were analyzed using µCT and histology. First, MC3T3-E1 preosteoblasts were plated with IL-17 pro-inflammatory medium, and RANKL/OPG ratio was measured. Secondly, the MC3T3-E1 were cultured w/o RvD1, for 3W. Osteoblasts' markers were evaluated in different days, using qRT-PCR and Alizarin Red staining for calcified matrix.

Results

In vivo, neither allograft alone nor single RvD1 administration promote bone regeneration in comparison to the control of spontaneous healing and even triggered an elevation in NR1D1 and IL1RL1 expression, markers associated with inflammation and inhibition of bone cell differentiation. However, repeated RvD1 treatment increased bone content by 135.92% ± 45.98% compared to its specific control, repeated sham, and by 39.12% ± 26.3% when compared to the spontaneous healing control group (n=7/group). Histologically, repeated RvD1 reduced the number of TRAP-positive cells, and enhanced allograft osteointegration with new bone formation. In vitro, RvD1 rescued OPG expression and decreased RANKL/OPG ratio in IL-17 pro-inflammatory conditions. Furthermore, RvD1 increased the expression of RUNX2, OSX, BSP and OC/BGLAP2 and the mineralized extracellular matrix during MC3T3-E1 osteoblasts differentiation.

Conclusions

Repeated administrations of RvD1 promote bone regeneration via a dual mechanism: directly, via enhancement of osteoblasts' differentiation and indirectly, through reduction of osteoclastogenesis and RANKL/OPG ratio. This suggests that RvD1 may be a potential therapeutic bioagent for osseous regeneration following allograft implantation.

Achievements in Mesoporous Bioactive Glasses for Biomedical Applications

Nowadays, mesoporous bioactive glasses (MBGs) are envisaged as promising candidates in the field of bioceramics for bone tissue regeneration. This is ascribed to their singular chemical composition, structural and textural properties and easy-to-functionalize surface, giving rise to accelerated bioactive responses and capacity for local drug delivery. Since their discovery at the beginning of the 21st century, pioneering research efforts focused on the design and fabrication of MBGs with optimal compositional, textural and structural properties to elicit superior bioactive behavior. The current trends conceive MBGs as multitherapy systems for the treatment of bone-related pathologies, emphasizing the need of fine-tuning surface functionalization. Herein, we focus on the recent developments in MBGs for biomedical applications. First, the role of MBGs in the design and fabrication of three-dimensional scaffolds that fulfil the highly demanding requirements for bone tissue engineering is outlined. The different approaches for developing multifunctional MBGs are overviewed, including the incorporation of therapeutic ions in the glass composition and the surface functionalization with zwitterionic moieties to prevent bacterial adhesion. The bourgeoning scientific literature on MBGs as local delivery systems of diverse therapeutic cargoes (osteogenic/antiosteoporotic, angiogenic, antibacterial, anti-inflammatory and antitumor agents) is addressed. Finally, the current challenges and future directions for the clinical translation of MBGs are discussed.

Carrier systems for bone morphogenetic proteins: An overview of biomaterials used for dentoalveolar and maxillofacial bone regeneration

Different types of biomaterials have been used to fabricate carriers to deliver bone morphogenetic proteins (BMPs) in both dentoalveolar and maxillofacial bone regeneration procedures. Despite that absorbable collagen sponge (ACS) is considered the gold standard for BMP delivery, there is still some concerns regarding its use mainly due to its poor mechanical properties. To overcome this, novel systems are being developed, however, due to the wide variety of biomaterial combination, the heterogeneous assessment of newly formed tissue, and the intended clinical applications, there is still no consensus regarding which is more efficient in a particular clinical scenario. The combination of two or more biomaterials in different topological configurations has allowed specific controlled-release patterns for BMPs, improving their biological and mechanical properties compared with classical single-material carriers. However, more basic research is needed. Since the BMPs can be used in multiple clinical scenarios having different biological and mechanical needs, novel carriers should be developed in a context-specific manner. Thus, the purpose of this review is to gather current knowledge about biomaterials used to fabricate delivery systems for BMPs in both dentoalveolar and maxillofacial contexts. Aspects related with the biological, physical and mechanical characteristics of each biomaterial are also presented and discussed.

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Strategies for bone formation and regeneration are a major concern in dentistry.

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Topical delivery of bone morphogenetic proteins (BMPs) allows rapid bone formation.

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BMPs requires proper carrier system to allow controlled and sustained release.

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Carrier should also fulfill mechanical requirements of bone defect sites.

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By using complex composites, it would be possible to develop new carriers for BMPs.

Self-assembling peptide-laden electrospun scaffolds for guided mineralized tissue regeneration

OBJECTIVES

Electrospun scaffolds are a versatile biomaterial platform to mimic fibrillar structure of native tissues extracellular matrix, and facilitate the incorporation of biomolecules for regenerative therapies. Self-assembling peptide P11-4 has emerged as a promising strategy to induce mineralization; however, P11-4 application has been mostly addressed for early caries lesions repair on dental enamel. Here, to investigate P11-4's efficacy on bone regeneration, polymeric electrospun scaffolds were developed, and then distinct concentrations of P11-4 were physically adsorbed on the scaffolds.

METHODS

P11-4-laden and pristine (P11-4-free) electrospun scaffolds were immersed in simulated body fluid and mineral precipitation identified by SEM. Functional groups and crystalline phases were analyzed by FTIR and XRD, respectively. Cytocompatibility, mineralization, and gene expression assays were conducted using stem cells from human exfoliated deciduous teeth. To investigate P11-4-laden scaffolds potential to induce in vivo mineralization, an established rat calvaria critical-size defect model was used.

RESULTS

We successfully synthesized nanofibrous (∼ 500 nm fiber diameter) scaffolds and observed that functionalization with P11-4 did not affect the fibers' diameter. SEM images indicated mineral precipitation, while FTIR and XRD confirmed apatite-like formation and crystallization for P11-4-laden scaffolds. In addition, P11-4-laden scaffolds were cytocompatible, highly stimulated cell-mediated mineral deposition, and upregulated the expression of mineralization-related genes compared to pristine scaffolds. P11-4-laden scaffolds led to enhanced in vivo bone regeneration after 8 weeks compared to pristine PCL.

SIGNIFICANCE

Electrospun scaffolds functionalized with P11-4 are a promising strategy for inducing mineralized tissues regeneration in the craniomaxillofacial complex.

Hope for bone regeneration: The versatility of iron oxide nanoparticles

Although bone tissue has the ability to heal itself, beyond a certain point, bone defects cannot rebuild themselves, and the challenge is how to promote bone tissue regeneration. Iron oxide nanoparticles (IONPs) are a magnetic material because of their excellent properties, which enable them to play an active role in bone regeneration. This paper reviews the application of IONPs in bone tissue regeneration in recent years, and outlines the mechanisms of IONPs in bone tissue regeneration in detail based on the physicochemical properties, structural characteristics and safety of IONPs. In addition, a bibliometric approach has been used to analyze the hot spots and trends in the field in order to identify future directions. The results demonstrate that IONPs are increasingly being investigated in bone regeneration, from the initial use as magnetic resonance imaging (MRI) contrast agents to later drug delivery vehicles, cell labeling, and now in combination with stem cells (SCs) composite scaffolds. In conclusion, based on the current research and development trends, it is more inclined to be used in bone tissue engineering, scaffolds, and composite scaffolds.

Electrospun Biomimetic Nanofibrous Scaffolds: A Promising Prospect for Bone Tissue Engineering and Regenerative Medicine

Skeletal-related disorders such as arthritis, bone cancer, osteosarcoma, and osteoarthritis are among the most common reasons for mortality in humans at present. Nanostructured scaffolds have been discovered to be more efficient for bone regeneration than macro/micro-sized scaffolds because they sufficiently permit cell adhesion, proliferation, and chemical transformation. Nanofibrous scaffolds mimicking artificial extracellular matrices provide a natural environment for tissue regeneration owing to their large surface area, high porosity, and appreciable drug loading capacity. Here, we review recent progress and possible future prospective electrospun nanofibrous scaffolds for bone tissue engineering. Electrospun nanofibrous scaffolds have demonstrated promising potential in bone tissue regeneration using a variety of nanomaterials. This review focused on the crucial role of electrospun nanofibrous scaffolds in biological applications, including drug/growth factor delivery to bone tissue regeneration. Natural and synthetic polymeric nanofibrous scaffolds are extensively inspected to regenerate bone tissue. We focused mainly on the significant impact of nanofibrous composite scaffolds on cell adhesion and function, and different composites of organic/inorganic nanoparticles with nanofiber scaffolds. This analysis provides an overview of nanofibrous scaffold-based bone regeneration strategies; however, the same concepts can be applied to other organ and tissue regeneration tactics.

Development, In-Vitro Characterization and In-Vivo Osteoinductive Efficacy of a Novel Biomimetically-Precipitated Nanocrystalline Calcium Phosphate With Internally-Incorporated Bone Morphogenetic Protein-2

The repair of large-volume bone defects (LVBDs) remains a great challenge in the fields of orthopedics and maxillofacial surgery. Most clinically available bone-defect-filling materials lack proper degradability and efficient osteoinductivity. In this study, we synthesized a novel biomimetically-precipitated nanocrystalline calcium phosphate (BpNcCaP) with internally incorporated bone morphogenetic protein-2 (BpNcCaP + BMP-2) with an aim to develop properly degradable and highly osteoinductive granules to repair LVBDs. We first characterized the physicochemical properties of the granules with different incorporation amounts of BMP-2 using scanning electron microscopy, X-ray diffraction, Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy. We evaluated the cytotoxicity and cytocompatibility of BpNcCaP by assessing the viability and adhesion of MC3T3-E1 pre-osteoblasts using PrestoBlue assay, Rhodamine-Phalloidin and DAPI staining, respectively. We further assessed the in-vivo osteoinductive efficacy in a subcutaneous bone induction model in rats. In-vitro characterization data showed that the BpNcCaP + BMP-2 granules were comprised of hexagonal hydroxyapatite with an average crystallite size ranging from 19.7 to 25.1 nm and a grain size at 84.13 ± 28.46 nm. The vickers hardness of BpNcCaP was 32.50 ± 3.58 HV 0.025. BpNcCaP showed no obvious cytotoxicity and was favorable for the adhesion of pre-osteoblasts. BMP-2 incorporation rate could be as high as 65.04 ± 6.01%. In-vivo histomorphometric analysis showed that the volume of new bone induced by BpNcCaP exhibited a BMP-2 amount-dependent increasing manner. The BpNcCaP+50 μg BMP-2 exhibited significantly more degradation and fewer foreign body giant cells in comparison with BpNcCaP. These data suggested a promising application potential of BpNcCaP + BMP-2 in repairing LVBDs.

Chemical Bonding of Biomolecules to the Surface of Nano-Hydroxyapatite to Enhance Its Bioactivity

Hydroxyapatite (HA) is a significant constituent of bones or teeth and is widely used as an artificial bone graft. It is often used to replace the lost bones or in reconstructing alveolar bones before dental implantation. HA with biological functions finds its importance in orthopedic surgery and dentistry to increase the local concentration of calcium ions, which activate the growth and differentiation of mesenchymal stem cells (MSC). To make relevant use of HA in bone transplantation, the surfaces of orthopedic and dental implants are frequently coated with nanosized hydroxyapatite (nHA), but its low dispersibility and tendency to form aggregates, the purpose of the surface modification of bone implants is defeated. To overcome these drawbacks and to improve the histocompatibility of bone implants or to use nHA in therapeutic applications of implants in the treatment of bone diseases, various studies suggested the attachment of biomolecules (growth factors) or drugs through chemical bonding at the surface of nHA. The growth factors or drugs bonded physically at the surface of nHA are mostly unstable and burst released immediately. Therefore, reported studies suggested that the surface of nHA needs to be modified through the chemical bonding of biologically active molecules at the surface of bone implants such as proteins, peptides, or naturally occurring polysaccharides to prevent the aggregation of nHA and to get homogenous dispersion of nHA in solution. The role of irradiation in producing bioactive and antibacterial nHA through morphological variations in surfaces of nHA is also summarized by considering internal structures and the formation of reactive oxygen species on irradiation. This mini-review aims to highlight the importance of small molecules such as proteins, peptides, drugs, and photocatalysts in surface property modification of nHA to achieve stable, bioactive, and antibacterial nHA to act as artificial bone implants (scaffolds) in combination with biodegradable polymers.

Early Osteogenic Marker Expression in hMSCs Cultured onto Acid Etching-Derived Micro- and Nanotopography 3D-Printed Titanium Surfaces

Polyetheretherketone (PEEK) titanium composite (PTC) is a novel interbody fusion device that combines a PEEK core with titanium alloy (Ti6Al4V) endplates. The present study aimed to investigate the in vitro biological reactivity of human bone-marrow-derived mesenchymal stem cells (hBM-MSCs) to micro- and nanotopographies produced by an acid-etching process on the surface of 3D-printed PTC endplates. Optical profilometer and scanning electron microscopy were used to assess the surface roughness and identify the nano-features of etched or unetched PTC endplates, respectively. The viability, morphology and the expression of specific osteogenic markers were examined after 7 days of culture in the seeded cells. Haralick texture analysis was carried out on the unseeded endplates to correlate surface texture features to the biological data. The acid-etching process modified the surface roughness of the 3D-printed PTC endplates, creating micro- and nano-scale structures that significantly contributed to sustaining the viability of hBM-MSCs and triggering the expression of early osteogenic markers, such as alkaline phosphatase activity and bone-ECM protein production. Finally, the topography of 3D-printed PTC endplates influenced Haralick's features, which in turn correlated with the expression of two osteogenic markers, osteopontin and osteocalcin. Overall, these data demonstrate that the acid-etching process of PTC endplates created a favourable environment for osteogenic differentiation of hBM-MSCs and may potentially have clinical benefit.

Enhancing the function of PLGA-collagen scaffold by incorporating TGF-β1-loaded PLGA-PEG-PLGA nanoparticles for cartilage tissue engineering using human dental pulp stem cells

Since cartilage has a limited capacity for self-regeneration, treating cartilage degenerative disorders is a long-standing difficulty in orthopedic medicine. Researchers have scrutinized cartilage tissue regeneration to handle the deficiency of cartilage restoration capacity. This investigation proposed to compose an innovative nanocomposite biomaterial that enhances growth factor delivery to the injured cartilage site. Here, we describe the design and development of the biocompatible poly(lactide-co-glycolide) acid-collagen/poly(lactide-co-glycolide)-poly(ethylene glycol)-poly(lactide-co-glycolide) (PLGA-collagen/PLGA-PEG-PLGA) nanocomposite hydrogel containing transforming growth factor-β1 (TGF-β1). PLGA-PEG-PLGA nanoparticles were employed as a delivery system embedding TGF-β1 as an articular cartilage repair therapeutic agent. This study evaluates various physicochemical aspects of fabricated scaffolds by ¹HNMR, FT-IR, SEM, BET, and DLS methods. The physicochemical features of the developed scaffolds, including porosity, density, degradation, swelling ratio, mechanical properties, morphologies, BET, ELISA, and cytotoxicity were assessed. The cell viability was investigated with the MTT test. Chondrogenic differentiation was assessed via Alcian blue staining and RT-PCR. In real-time PCR testing, the expression of Sox-9, collagen type II, and aggrecan genes was monitored. According to the results, human dental pulp stem cells (hDPSCs) exhibited high adhesion, proliferation, and differentiation on PLGA-collagen/PLGA-PEG-PLGA-TGFβ1 nanocomposite scaffolds compared to the control groups. SEM images displayed suitable cell adhesion and distribution of hDPSCs throughout the scaffolds. RT-PCR assay data displayed that TGF-β1 loaded PLGA-PEG-PLGA nanoparticles puts forward chondroblast differentiation in hDPSCs through the expression of chondrogenic genes. The findings revealed that PLGA-collagen/PLGA-PEG-PLGA-TGF-β1 nanocomposite hydrogel can be utilized as a supportive platform to support hDPSCs differentiation by implementing specific physio-chemical features.

Nonlinear micro finite element models based on digital volume correlation measurements predict early microdamage in newly formed bone

Bone regeneration in critical-sized defects is a clinical challenge, with biomaterials under constant development aiming at enhancing the natural bone healing process. The delivery of bone morphogenetic proteins (BMPs) in appropriate carriers represents a promising strategy for bone defect treatment but optimisation of the spatial-temporal release is still needed for the regeneration of bone with biological, structural, and mechanical properties comparable to the native tissue. Nonlinear micro finite element (μFE) models can address some of these challenges by providing a tool able to predict the biomechanical strength and microdamage onset in newly formed bone when subjected to physiological or supraphysiological loads. Yet, these models need to be validated against experimental data. In this study, experimental local displacements in newly formed bone induced by osteoinductive biomaterials subjected to in situ X-ray computed tomography compression in the apparent elastic regime and measured using digital volume correlation (DVC) were used to validate μFE models. Displacement predictions from homogeneous linear μFE models were highly correlated to DVC-measured local displacements, while tissue heterogeneity capturing mineralisation differences showed negligible effects. Nonlinear μFE models improved the correlation and showed that tissue microdamage occurs at low apparent strains. Microdamage seemed to occur next to large cavities or in biomaterial-induced thin trabeculae, independent of the mineralisation. While localisation of plastic strain accumulation was similar, the amount of damage accumulated in these locations was slightly higher when including material heterogeneity. These results demonstrate the ability of the nonlinear μFE model to capture local microdamage in newly formed bone tissue and can be exploited to improve the current understanding of healing bone and mechanical competence. This will ultimately aid the development of BMPs delivery systems for bone defect treatment able to regenerate bone with optimal biological, mechanical, and structural properties.

3D-Printed Tubular Scaffolds Decorated with Air-Jet-Spun Fibers for Bone Tissue Applications

The fabrication of instructive materials to engineer bone substitute scaffolds is still a relevant challenge. Current advances in additive manufacturing techniques make possible the fabrication of 3D scaffolds with even more controlled architecture at micro- and submicrometric levels, satisfying the relevant biological and mechanical requirements for tissue engineering. In this view, integrated use of additive manufacturing techniques is proposed, by combining 3D printing and air-jet spinning techniques, to optimize the fabrication of PLA tubes with nanostructured fibrous coatings for long bone defects. The physicochemical characterization of the 3D tubular scaffolds was performed by scanning electron microscopy, thermogravimetric analysis, differential scanning calorimetry, profilometry, and mechanical properties. In vitro biocompatibility was evaluated in terms of cell adhesion, proliferation, and cell–material interactions, by using human fetal osteoblasts to validate their use as a bone growth guide. The results showed that 3D-printed scaffolds provide a 3D architecture with highly reproducible properties in terms of mechanical and thermal properties. Moreover, nanofibers are collected onto the surface, which allows forming an intricate and interconnected network that provides microretentive cues able to improve adhesion and cell growth response. Therefore, the proposed approach could be suggested to design innovative scaffolds with improved interface properties to support regeneration mechanisms in long bone treatment.

Bone Healing Materials in the Treatment of Recalcitrant Nonunions and Bone Defects

The usual treatment for bone defects and recalcitrant nonunions is an autogenous bone graft. However, due to the limitations in obtaining autogenous bone grafts and the morbidity associated with their procurement, various bone healing materials have been developed in recent years. The three main treatment strategies for bone defects and recalcitrant nonunions are synthetic bone graft substitutes (BGS), BGS combined with bioactive molecules, and BGS and stem cells (cell-based constructs). Regarding BGS, numerous biomaterials have been developed to prepare bone tissue engineering scaffolds, including biometals (titanium, iron, magnesium, zinc), bioceramics (hydroxyapatite (HA)), tricalcium phosphate (TCP), biopolymers (collagen, polylactic acid (PLA), polycaprolactone (PCL)), and biocomposites (HA/MONs@miR-34a composite coating, Bioglass (BG)-based ABVF-BG (antibiotic-releasing bone void filling) putty). Bone tissue engineering scaffolds are temporary implants that promote tissue ingrowth and new bone regeneration. They have been developed to improve bone healing through appropriate designs in terms of geometric, mechanical, and biological performance. Concerning BGS combined with bioactive molecules, one of the most potent osteoinductive growth factors is bone morphogenetic proteins (BMPs). In recent years, several natural (collagen, fibrin, chitosan, hyaluronic acid, gelatin, and alginate) and synthetic polymers (polylactic acid, polyglycolic acid, polylactic-coglycolide, poly(e-caprolactone) (PCL), poly-p-dioxanone, and copolymers consisting of glycolide/trimethylene carbonate) have been investigated as potential support materials for bone tissue engineering. Regarding BGS and stem cells (cell-based constructs), the main strategies are bone marrow stromal cells, adipose-derived mesenchymal cells, periosteum-derived stem cells, and 3D bioprinting of hydrogels and cells or bioactive molecules. Currently, significant research is being performed on the biological treatment of recalcitrant nonunions and bone defects, although its use is still far from being generalized. Further research is needed to investigate the efficacy of biological treatments to solve recalcitrant nonunions and bone defects.

Single Application of Low-Dose, Hydroxyapatite-Bound BMP-2 or GDF-5 Induces Long-Term Bone Formation and Biomechanical Stabilization of a Bone Defect in a Senile Sheep Lumbar Osteopenia Model

Effects of hydroxyapatite (HA) particles with bone morphogenetic BMP-2 or GDF-5 were compared in sheep lumbar osteopenia; in vitro release in phosphate-buffered saline (PBS) or sheep serum was assessed by ELISA. Lumbar (L) vertebral bone defects (Ø 3.5 mm) were generated in aged, osteopenic female sheep (n = 72; 9.00 ± 0.11 years; mean ± SEM). Treatment was: (a) HA particles (2.5 mg; L5); or (b) particles coated with BMP-2 (1 µg; 10 µg) or GDF-5 (5 µg; 50 µg; L4; all groups n = 6). Untouched vertebrae (L3) served as controls. Three and nine months post-therapy, bone formation was assessed by osteodensitometry, histomorphometry, and biomechanical testing. Cumulative 14-day BMP release was high in serum (76-100%), but max. 1.4% in PBS. In vivo induction of bone formation by HA particles with either growth factor was shown by: (i) significantly increased bone volume, trabecular and cortical thickness (overall increase HA + BMP vs. control close to the injection channel 71%, 110%, and 37%, respectively); (ii) partial significant effects for bone mineral density, bone formation, and compressive strength (increase 17%; 9 months; GDF-5). Treatment effects were not dose-dependent. Combined HA and BMPs (single low-dose) highly augment long-term bone formation and biomechanical stabilization in sheep lumbar osteopenia. Thus, carrier-bound BMP doses 20,000-fold to 1000-fold lower than previously applied appear suitable for spinal fusion/bone regeneration and improved treatment safety.

Modern Approaches to Acellular Therapy in Bone and Dental Regeneration

An approach called cell-free therapy has rapidly developed in regenerative medicine over the past decade. Understanding the molecular mechanisms and signaling pathways involved in the internal potential of tissue repair inspires the development of new strategies aimed at controlling and enhancing these processes during regeneration. The use of stem cell mobilization, or homing for regeneration based on endogenous healing mechanisms, prompted a new concept in regenerative medicine: endogenous regenerative medicine. The application of cell-free therapeutic agents leading to the recruitment/homing of endogenous stem cells has advantages in overcoming the limitations and risks associated with cell therapy. In this review, we discuss the potential of cell-free products such as the decellularized extracellular matrix, growth factors, extracellular vesicles and miRNAs in endogenous bone and dental regeneration.

Advancements in nucleic acids-based techniques for bone regeneration

The dynamic biology of bone involving an enormous magnitude of cellular interactions and signaling transduction provides ample biomolecular targets, which can be enhanced or repressed to mediate a rapid regeneration of the impaired bone tissue. The delivery of nucleic acids such as DNA and RNA can enhance the expression of osteogenic proteins. Members of the RNA interference pathway such as miRNA and siRNA can repress negative osteoblast differentiation regulators. Advances in nanomaterials have provided researchers with a plethora of delivery modules that can ensure proper transfection. Combining the nucleic acid carrying vectors with bone scaffolds has met with tremendous success in accomplishing bone formation. Recent years have witnessed the advent of CRISPR and DNA nanostructures in regenerative medicine. This review focuses on the delivery of nucleic acids and touches upon the prospect of CRISPR and DNA nanostructures for bone tissue engineering, emphasizing their potential in treating bone defects.

Hyaluronic acid hydrogels, as a biological macromolecule-based platform for stem cells delivery and their fate control: A review

Stem cell-based therapies offer numerous potentials to repair damaged or defective organs. The therapeutic outcomes of human studies, however, fall far short from what is expected. Enhancing stem cells local density and longevity would possibly maximize their healing potential. One promising strategy is to administer stem cells via injectable hydrogels. However, stem cells differentiation process is a delicate matter which is easily affected by various factors such as their interaction with their surrounding materials. Among various biomaterial options for hydrogels' production, hyaluronic acid (HA) has shown great promise. HA is a naturally occurring biological macromolecule, a polysaccharide of large molecular weight which is involved in cell proliferation, cell migration, angiogenesis, fetal development, and tissue function. In the current study we will discuss the applications, prospects, and challenges of HA-based hydrogels in stem cell delivery and fate control.

Biomaterials and Their Biomedical Applications: From Replacement to Regeneration

The history of biomaterials dates back to the mists of time: human beings had always used exogenous materials to facilitate wound healing and try to restore damaged tissues and organs. Nowadays, a wide variety of materials are commercially available and many others are under investigation to both maintain and restore bodily functions. Emerging clinical needs forced the development of new biomaterials, and lately discovered biomaterials allowed for the performing of new clinical applications. The definition of biomaterials as materials specifically conceived for biomedical uses was raised when it was acknowledged that they have to possess a fundamental feature: biocompatibility. At first, biocompatibility was mainly associated with biologically inert substances; around the 1970s, bioactivity was first discovered and the definition of biomaterials was consequently extended. At present, it also includes biologically derived materials and biological tissues. The present work aims at walking across the history of biomaterials, looking towards the scientific literature published on this matter. Finally, some current applications of biomaterials are briefly depicted and their future exploitation is hypothesized.

Long-term induction of endogenous BMPs growth factor from antibacterial dual network hydrogels for fast large bone defect repair

Osteoinductive, osteoconductive, and antibacterial properties of bone repair materials play important roles in regulating the successful bone regeneration. In the present work, we developed pH-sensitive gelatin methacryloyl (GelMA)-oxidized sodium alginate (OSA) hydrogels for dual-release of gentamicin sulfate (GS) and phenamil (Phe) to enhance the antibacterial activity and to promote large bone defect repair. Controlled release of GS was achieved through physical blending with GelMA-OSA solution before photo-polymeriaztion, while Phe was encapsulated into mesoporous silicate nanoparticles (MSN) within the hydrogels. In vitro antibacterial studies against Staphylococcus aureus and Escherichia coli indicated the broad-spectrum antibacterial property. Moreover, in vitro cell tests verified the synergistically enhanced osteogenic differentiation ability. Furthermore, in vivo studies revealed that the hydrogels significantly increased new bone formation in a critical-sized mouse cranial bone defect model. In summary, the novel dual-network hydrogels with both antibacterial and osteoinductive properties showed promising potential applications in bone tissue engineering.

Innovations in Craniofacial Bone and Periodontal Tissue Engineering - From Electrospinning to Converged Biofabrication

From a materials perspective, the pillars for the development of clinically translatable scaffold-based strategies for craniomaxillofacial (CMF) bone and periodontal regeneration have included electrospinning and 3D printing (biofabrication) technologies. Here, we offer a detailed analysis of the latest innovations in 3D (bio)printing strategies for CMF bone and periodontal regeneration and provide future directions envisioning the development of advanced 3D architectures for successful clinical translation. First, the principles of electrospinning applied to the generation of biodegradable scaffolds are discussed. Next, we present on extrusion-based 3D printing technologies with a focus on creating scaffolds with improved regenerative capacity. In addition, we offer a critical appraisal on 3D (bio)printing and multitechnology convergence to enable the reconstruction of CMF bones and periodontal tissues. As a future outlook, we highlight future directions associated with the utilization of complementary biomaterials and (bio)fabrication technologies for effective translation of personalized and functional scaffolds into the clinics.

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

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