Development of Organic/Inorganic Compatible and Sustainably Bioactive Composites for Effective Bone Regeneration

3D printed biopolymer/black phosphorus nanoscaffolds for bone implants: A review

Bone implantation is one of the recognized and effective means of treating bone defects, but osteoporosis and bone tumor-related bone abnormalities have a series of problems such as susceptibility to infection, difficulty in healing, and poor therapeutic effect, which poses a great challenge to clinical medicine. Three-dimensional things may be printed using 3D printing. Researchers can feed materials through the printer layer by layer to create the desired shape for a 3D structure. It is widely employed in the healing of bone defects, and it is an improved form of additive manufacturing technology with prospective future applications. This review's objective is to provide an overview of the findings reports pertaining to 3D printing biopolymers in recent years, provide an overview of biopolymer materials and their composites with black phosphorus for 3D printing bone implants, and the characterization methods of composite materials are also summarized. In addition, summarizes 3D printing methods based on ink printing and laser printing, pointing out their special features and advantages, and provide a combination strategy of photothermal therapy and bone regeneration materials for black phosphorus-based materials. Finally, the associations between bone implant materials and immune cells, the bio-environment, as well as the 3D printing bone implants prospects are outlined.

MicroRNA-200c Release from Gelatin-Coated 3D-Printed PCL Scaffolds Enhances Bone Regeneration

The fabrication of clinically relevant synthetic bone grafts relies on combining multiple biodegradable biomaterials to create a structure that supports the regeneration of defects while delivering osteogenic biomolecules that enhance regeneration. MicroRNA-200c (miR-200c) functions as a potent osteoinductive biomolecule to enhance osteogenic differentiation and bone formation; however, synthetic tissue-engineered bone grafts that sustain the delivery of miR-200c for bone regeneration have not yet been evaluated. In this study, we created novel, multimaterial, synthetic bone grafts from gelatin-coated 3D-printed polycaprolactone (PCL) scaffolds. We attempted to optimize the release of pDNA encoding miR-200c by varying gelatin types, concentrations, and polymer crosslinking materials to improve its functions for bone regeneration. We revealed that by modulating gelatin type, coating material concentration, and polymer crosslinking, we effectively altered the release rates of pDNA encoding miR-200c, which promoted osteogenic differentiation in vitro and bone regeneration in a critical-sized calvarial bone defect animal model. We also demonstrated that crosslinking the gelatin coatings on the PCL scaffolds with low-concentration glutaraldehyde was biocompatible and increased cell attachment. These results strongly indicate the potential use of gelatin-based systems for pDNA encoding microRNA delivery in gene therapy and further demonstrate the effectiveness of miR-200c for enhancing bone regeneration from synthetic bone grafts.

Gelatin Methacryloyl (GelMA)-Based Biomaterial Inks: Process Science for 3D/4D Printing and Current Status

Tissue engineering for injured tissue replacement and regeneration has been a subject of investigation over the last 30 years, and there has been considerable interest in using additive manufacturing to achieve these goals. Despite such efforts, many key questions remain unanswered, particularly in the area of biomaterial selection for these applications as well as quantitative understanding of the process science. The strategic utilization of biological macromolecules provides a versatile approach to meet diverse requirements in 3D printing, such as printability, buildability, and biocompatibility. These molecules play a pivotal role in both physical and chemical cross-linking processes throughout the biofabrication, contributing significantly to the overall success of the 3D printing process. Among the several bioprintable materials, gelatin methacryloyl (GelMA) has been widely utilized for diverse tissue engineering applications, with some degree of success. In this context, this review will discuss the key bioengineering approaches to identify the gelation and cross-linking strategies that are appropriate to control the rheology, printability, and buildability of biomaterial inks. This review will focus on the GelMA as the structural (scaffold) biomaterial for different tissues and as a potential carrier vehicle for the transport of living cells as well as their maintenance and viability in the physiological system. Recognizing the importance of printability toward shape fidelity and biophysical properties, a major focus in this review has been to discuss the qualitative and quantitative impact of the key factors, including microrheological, viscoelastic, gelation, shear thinning properties of biomaterial inks, and printing parameters, in particular, reference to 3D extrusion printing of GelMA-based biomaterial inks. Specifically, we emphasize the different possibilities to regulate mechanical, swelling, biodegradation, and cellular functionalities of GelMA-based bio(material) inks, by hybridization techniques, including different synthetic and natural biopolymers, inorganic nanofillers, and microcarriers. At the close, the potential possibility of the integration of experimental data sets and artificial intelligence/machine learning approaches is emphasized to predict the printability, shape fidelity, or biophysical properties of GelMA bio(material) inks for clinically relevant tissues.

Leveraging the Recent Advancements in GelMA Scaffolds for Bone Tissue Engineering: An Assessment of Challenges and Opportunities

The field of bone tissue engineering has seen significant advancements in recent years. Each year, over two million bone transplants are performed globally, and conventional treatments, such as bone grafts and metallic implants, have their limitations. Tissue engineering offers a new level of treatment, allowing for the creation of living tissue within a biomaterial framework. Recent advances in biomaterials have provided innovative approaches to rebuilding bone tissue function after damage. Among them, gelatin methacryloyl (GelMA) hydrogel is emerging as a promising biomaterial for supporting cell proliferation and tissue regeneration, and GelMA has exhibited exceptional physicochemical and biological properties, making it a viable option for clinical translation. Various methods and classes of additives have been used in the application of GelMA for bone regeneration, with the incorporation of nanofillers or other polymers enhancing its resilience and functional performance. Despite promising results, the fabrication of complex structures that mimic the bone architecture and the provision of balanced physical properties for both cell and vasculature growth and proper stiffness for load bearing remain as challenges. In terms of utilizing osteogenic additives, the priority should be on versatile components that promote angiogenesis and osteogenesis while reinforcing the structure for bone tissue engineering applications. This review focuses on recent efforts and advantages of GelMA-based composite biomaterials for bone tissue engineering, covering the literature from the last five years.

Advantages and limitations of using cell viability assays for 3D bioprinted constructs

Bioprinting shows promise for bioengineered scaffolds and three-dimensional (3D) disease models, but assessing the viability of embedded cells is challenging. Conventional assays are limited by the technical problems that derive from using multi-layered bioink matrices dispersing cells in three dimensions. In this study, we tested bioprinted osteogenic bioinks as a model system. Alginate- or gelatin-based bioinks were loaded with/without ceramic microparticles and osteogenic cells (bone tumor cells, with or without normal bone cells). Despite demonstrating 80%-90% viability through manual counting and live/dead staining, this was time-consuming and operator-dependent. Moreover, for the alginate-bioprinted scaffold, cell spheroids could not be distinguished from single cells. The indirect assay (alamarBlue), was faster but less accurate than live/dead staining due to dependence on hydrogel permeability. Automated confocal microscope acquisition and cell counting of live/dead staining was more reproducible, reliable, faster, efficient, and avoided overestimates compared to manual cell counting by optical microscopy. Finally, for 1.2 mm thick 3D bioprints, dual-photon confocal scanning with vital staining greatly improved the precision of the evaluation of cell distribution and viability and cell-cell interactions through thez-axis. In summary, automated confocal microscopy and cell counting provided superior accuracy for the assessment of cell viability and interactions in 3D bioprinted models compared to most commonly and currently used techniques.

Study on Synergistic Effects of Nanohydroxyapatite/High-Viscosity Carboxymethyl Cellulose Scaffolds Stimulated by LIPUS for Bone Defect Repair of Rats

Despite the self-healing capacity of bone, the regeneration of critical-size bone defects remains a major clinical challenge. In this study, nanohydroxyapatite (nHAP)/high-viscosity carboxymethyl cellulose (hvCMC, 6500 mPa·s) scaffolds and low-intensity pulsed ultrasound (HA-LIPUS) were employed to repair bone defects. First, hvCMC was prepared from ramie fiber, and the degree of substitution (DS), purity, and content of NaCl of hvCMC samples were 0.91, 99.93, and 0.017%, respectively. Besides, toxic metal contents were below the permissible limits for pharmaceutically used materials. Our results demonstrated that the hvCMC is suitable for pharmaceutical use. Second, nHAP and hvCMC were employed to prepare scaffolds by freeze-drying. The results indicated that the scaffolds were porous, and the porosity was 35.63 ± 3.52%. Subsequently, the rats were divided into four groups (n = 8) randomly: normal control (NC), bone defect (BD), bone defect treated with nHAP/hvCMC scaffolds (HA), and bone defect treated with nHAP/hvCMC scaffolds and stimulated by LIPUS (HA-LIPUS). After drilling surgery, nHAP/hvCMC scaffolds were implanted in the defect region of HA and HA-LIPUS rats. Meanwhile, HA-LIPUS rats were treated by LIPUS (1.5 MHz, 80 mW cm-2) irradiation for 2 weeks. Compared with BD rats, the maximum load and bone mineral density of HA-LIPUS rats were increased by 20.85 and 51.97%, respectively. The gene and protein results indicated that nHAP/hvCMC scaffolds and LIPUS promoted the bone defect repair and regeneration of rats significantly by activating Wnt/β-catenin and inhibiting OPG/RANKL signaling pathways. Overall, compared with BD rats, nHAP/hvCMC scaffolds and LIPUS promoted bone defect repair significantly. Furthermore, the research results also indicated that there are synergistic effects for bone defect repair between the nHAP/hvCMC scaffolds and LIPUS.

Aligned cryogel fibers incorporated 3D printed scaffold effectively facilitates bone regeneration by enhancing cell recruitment and function

Reconstructing extensive cranial defects represents a persistent clinical challenge. Here, we reported a hybrid three-dimensional (3D) printed scaffold with modification of QK peptide and KP peptide for effectively promoting endogenous cranial bone regeneration. The hybrid 3D printed scaffold consists of vertically aligned cryogel fibers that guide and promote cell penetration into the defect area in the early stages of bone repair. Then, the conjugated QK peptide and KP peptide further regulate the function of the recruited cells to promote vascularization and osteogenic differentiation in the defect area. The regenerated bone volume and surface coverage of the dual peptide-modified hybrid scaffold were significantly higher than the positive control group. In addition, the dual peptide-modified hybrid scaffold demonstrated sustained enhancement of bone regeneration and avoidance of bone resorption compared to the collagen sponge group. We expect that the design of dual peptide-modified hybrid scaffold will provide a promising strategy for bone regeneration.

Hydrogel-Based Growth Factor Delivery Platforms: Strategies and Recent Advances

Growth factors play a crucial role in regulating a broad variety of biological processes and are regarded as powerful therapeutic agents in tissue engineering and regenerative medicine in the past decades. However, their application is limited by their short half-lives and potential side effects in physiological environments. Hydrogels are identified as having the promising potential to prolong the half-lives of growth factors and mitigate their adverse effects by restricting them within the matrix to reduce their rapid proteolysis, burst release, and unwanted diffusion. This review discusses recent progress in the development of growth factor-containing hydrogels for various biomedical applications, including wound healing, brain tissue repair, cartilage and bone regeneration, and spinal cord injury repair. In addition, the review introduces strategies for optimizing growth factor release including affinity-based delivery, carrier-assisted delivery, stimuli-responsive delivery, spatial structure-based delivery, and cellular system-based delivery. Finally, the review presents current limitations and future research directions for growth factor-delivering hydrogels.

SLS 3D Printing To Fabricate Poly(vinyl alcohol)/Hydroxyapatite Bioactive Composite Porous Scaffolds and Their Bone Defect Repair Property

Poly(vinyl alcohol) (PVA) exhibits a wide range of potential applications in the biomedical field due to its favorable mechanical properties and biocompatibility. However, few studies have been carried out on selective laser sintering (SLS) of PVA due to its poor thermal processability. In this study, in order to impart PVA powder the excellent thermal processability, the molecular complexation technology was performed to destroy the strong hydrogen bonds in PVA and thus significantly reduced the PVA melting point and crystallinity to 190.9 °C and 27.9%, respectively. The modified PVA (MPVA) was then compounded with hydroxyapatite (HA) to prepare PVA/HA composite powders suitable for SLS 3D printing. The final SLS 3D-printed MPVA/HA composite porous scaffolds show high precision and interconnected pores with a porosity as high as 68.3%. The in vitro cell culture experiments revealed that the sintered composite scaffolds could significantly promote the adhesion and proliferation of osteoblasts and facilitate bone regeneration, and the quantitative real-time polymerase chain reaction results further demonstrate that the printed MPVA/20HA scaffold could significantly enhance the expression levels of both early osteogenic-specific marker of alkaline phosphatase stain and runt-related transcription factor 2. Meanwhile, in in vivo experiments, it is encouragingly found that the resultant MPVA/20HA SLS 3D-printed part has an obvious effect on promoting the growth of new bone tissue as well as a better bone regeneration capability. This work could provide a promising strategy for fabrication of PVA scaffolds through SLS 3D printing, exhibiting a great potential for clinical applications in bone tissue engineering.

GelMA-catechol coated FeHAp nanorods functionalized nanofibrous reinforced bio-instructive and mechanically robust composite hydrogel scaffold for bone tissue engineering

Critical bone defects complicate tissue graft-based surgeries, raising healthcare expenditures and underscoring scaffold-based tissue-engineering strategies to support bone reconstruction. Our study highlighted that the phase-compatible combination of inorganic nanorods, nanofibers, and hydrogels is promising for developing biomimetic and cell-instructive scaffolds since the bone matrix is a porous organic/inorganic composite. In brief, methacrylated gelatin (GelMA) was reacted with dopamine to form catechol-modified GeLMA (GelMA-C). The GelMA-C was nanocoated onto an iron-doped hydroxyapatite (FeHAp) nanorod via metal-catechol network coordination. The modified nanorod (FeHAp@GelMA-C) was loaded onto GelMA-based nanofibers. The nanorods loaded pre-fibers were electrospun onto GelMA solution and photochemically crosslinked to fabricate a fiber-reinforced hydrogel. The structural, mechanical, physicochemical, biocompatibility, swelling properties, osteogenic potential, and bone remodelling potential (using rat femoral defect model) of modified nanorods, simple hydrogel, and nanorod-loaded fiber-reinforced hydrogel were studied. The results supported that the interface interaction between GelMA-C/nanorods, nanorods/nanofibers, nanorods/hydrogels, and nanofiber/hydrogels significantly improved the microstructural and mechanical properties of the scaffold. Compared to pristine hydrogel, the nanorod-loaded fiber-reinforced scaffold better supported cellular responses, osteogenic differentiation, matrix mineralization, and accelerated bone regeneration. The nanorod-loaded fiber-reinforced hydrogel proved more biomimetic and cell-instructive for guided bone reconstruction.

Histomorphometric evaluation, SEM, and synchrotron analysis of the biological response of biodegradable and ceramic hydroxyapatite-based grafts: from the synthesis to the bed application

This study aimed to analyze the physicochemical and histological properties of nanostructured hydroxyapatite and alginate composites produced at different temperatures with and without sintering and implanted in rabbit tibiae. Hydroxyapatite-alginate (HA) microspheres (425-600 µm) produced at 90 and 5 °C without (HA90 and HA5) or with sintering at 1000 °C (HA90S and HA5S) were characterized and applied to evaluate thein vitrodegradation; also were implanted in bone defects on rabbit's tibiae (n= 12). The animals were randomly divided into five groups (blood clot, HA90S, HA5S, HA90, and HA5) and euthanized after 7 and 28 d. X-ray diffraction and Fourier-transform infrared analysis of the non-sintered biomaterials showed a lower crystallinity than sintered materials, being more degradablein vitroandin vivo. However, the sinterization of HA5 led to the apatite phase's decomposition into tricalcium phosphate. Histomorphometric analysis showed the highest (p< 0.01) bone density in the blood clot group, similar bone levels among HA90S, HA90, and HA5, and significantly less bone in the HA5S. HA90 and HA5 groups presented higher degradation and homogeneous distribution of the new bone formation onto the surface of biomaterial fragments, compared to HA90S, presenting bone only around intact microspheres (p< 0.01). The elemental distribution (scanning electron microscope and energy dispersive spectroscopy andμXRF-SR analysis) of Ca, P, and Zn in the newly formed bone is similar to the cortical bone, indicating bone maturity at 28 d. The synthesized biomaterials are biocompatible and osteoconductive. The heat treatment directly influenced the material's behavior, where non-sintered HA90 and HA5 showed higher degradation, allowing a better distribution of the new bone onto the surface of the biomaterial fragments compared to HA90S presenting the same level of new bone, but only on the surface of the intact microspheres, potentially reducing the bone-biomaterial interface.

Tunable Double-Network GelMA/Alginate Hydrogels for Platelet Lysate-Derived Protein Delivery

Hydrogels (gels) are attractive tools for tissue engineering and regenerative medicine due to their potential for drug delivery and ECM-like composition. In this study, we use rheology to characterize GelMA/alginate gels loaded with human platelet lysate (PL). We then characterize these gels from a physicochemical perspective and evaluate their ability to transport PL proteins, their pore size, and their rate of degradation. Finally, their biocompatibility is evaluated. We describe how alginate changes the mechanical behavior of the gels from elastic to viscoelastic after ionic (calcium-mediated) crosslinking. In addition, we report the release of ~90% of PL proteins from the gels and relate it to the degradation profile of the gels. Finally, we evaluated the biocompatibility of the gels. Thus, the developed gels represent attractive substrates for both cell studies and as bioactive materials.

Bone Morphogenic Protein and Mesenchymal Stem Cells to Regenerate Bone in Calvarial Defects: A Systematic Review

BACKGROUND

The use of bone morphogenic protein and mesenchymal stem cells has shown promise in promoting bone regeneration in calvarial defects. However, a systematic review of the available literature is needed to evaluate the efficacy of this approach.

METHODS

We comprehensively searched electronic databases using MeSH terms related to skull defects, bone marrow mesenchymal stem cells, and bone morphogenic proteins. Eligible studies included animal studies that used BMP therapy and mesenchymal stem cells to promote bone regeneration in calvarial defects. Reviews, conference articles, book chapters, and non-English language studies were excluded. Two independent investigators conducted the search and data extraction.

RESULTS

Twenty-three studies published between 2010 and 2022 met our inclusion criteria after a full-text review of the forty-five records found in the search. Eight of the 23 studies used mice as models, while 15 used rats. The most common mesenchymal stem cell was bone marrow-derived, followed by adipose-derived. BMP-2 was the most popular. Stem cells were embedded in Scaffold (13), Transduction (7), and Transfection (3), and they were delivered BMP to cells. Each treatment used 2 × 10⁴-1 × 10⁷ mesenchymal stem cells, averaging 2.26 × 10⁶. Most BMP-transduced MSC studies used lentivirus.

CONCLUSIONS

This systematic review examined BMP and MSC synergy in biomaterial scaffolds or alone. BMP therapy and mesenchymal stem cells in calvarial defects, alone, or with a scaffold regenerated bone. This method treats skull defects in clinical trials. The best scaffold material, therapeutic dosage, administration method, and long-term side effects need further study.

A multifunctional nanocomposite hydrogel with controllable release behavior enhances bone regeneration

Autologous and allogeneic bone grafts remain the gold standard for repairing bone defects. However, donor shortages and postoperative infections contribute to unsatisfactory treatment outcomes. Tissue engineering technology that utilizes biologically active composites to accelerate the healing and reconstruction of segmental bone defects has led to new ideas for in situ bone repair. Multifunctional nanocomposite hydrogels were constructed by covalently binding silver (Ag+) core-embedded mesoporous silica nanoparticles (Ag@MSN) to bone morphogenetic protein-2 (BMP-2), which was encapsulated into silk fibroin methacryloyl (SilMA) and photo-crosslinked to form an Ag@MSN-BMP-2/SilMA hydrogel to preserve the biological activity of BMP-2 and slow its release. More importantly, multifunctional Ag+-containing nanocomposite hydrogels showed antibacterial properties. These hydrogels possessed synergistic osteogenic and antibacterial effects to promote bone defect repair. Ag@MSN-BMP-2/SilMA exhibited good biocompatibility in vitro and in vivo owing to its interconnected porosity and improved hydrophilicity. Furthermore, the multifunctional nanocomposite hydrogel showed controllable sustained-release activity that promoted bone regeneration in repairing rat skull defects by inducing osteogenic differentiation and neovascularization. Overall, Ag@MSN-BMP-2/SilMA hydrogels enrich bone regeneration strategies and show great potential for bone regeneration.

Strategies of immobilizing BMP-2 with 3D-printed scaffolds to improve osteogenesis

The management and definitive treatment of critical-size bone defects in severe trauma, tumor resection and congenital malformation are troublesome for orthopedic surgeons and patients worldwide without recognized good treatment strategies. Researchers and clinicians are working to develop new strategies to treat these problems. This review aims to summarize the techniques used by additive manufacturing scaffolds loaded with BMP-2 to promote osteogenesis and to analyze the current status and trends in relevant clinical translation. Optimize composite scaffold design to enhance bone regeneration through printing technology, material selection, structure design and loading methods of BMP-2 to advance the clinical therapeutic bone repair field.

Quercetin-solid lipid nanoparticle-embedded hyaluronic acid functionalized hydrogel for immunomodulation to promote bone reconstruction

Bone defects are a persistent challenge in clinical practice. Although repair therapies based on tissue-engineered materials, which are known to have a crucial role in defective bone regeneration, have gathered increased attention, the current treatments for massive bone defects have several limitations. In the present study, based on the immunomodulatory inflammatory microenvironment properties of quercetin, we encapsulated quercetin-solid lipid nanoparticles (SLNs) in a hydrogel. Temperature-responsive poly(ε-caprolactone-co-lactide)-b-poly(ethylene glycol)-b-poly(ε-caprolactone-co-lactide) modifications were coupled to the main chain of hyaluronic acid hydrogel, constructing a novel, injectable bone immunomodulatory hydrogel scaffold. Extensive in vitro and in vivo data showed that this bone immunomodulatory scaffold forms an anti-inflammatory microenvironment by decreasing M1 polarization, while elevating the M2 polarization. Synergistic effects on angiogenesis and anti-osteoclastic differentiation were observed. These findings further proved that administering quercetin SLNs encapsulated in a hydrogel can aid bone defect reconstruction in rats, providing new insights for large-scale bone defect repair.

Drug delivery systems based on polyethylene glycol hydrogels for enhanced bone regeneration

Drug delivery systems composed of osteogenic substances and biological materials are of great significance in enhancing bone regeneration, and appropriate biological carriers are the cornerstone for their construction. Polyethylene glycol (PEG) is favored in bone tissue engineering due to its good biocompatibility and hydrophilicity. When combined with other substances, the physicochemical properties of PEG-based hydrogels fully meet the requirements of drug delivery carriers. Therefore, this paper reviews the application of PEG-based hydrogels in the treatment of bone defects. The advantages and disadvantages of PEG as a carrier are analyzed, and various modification methods of PEG hydrogels are summarized. On this basis, the application of PEG-based hydrogel drug delivery systems in promoting bone regeneration in recent years is summarized. Finally, the shortcomings and future developments of PEG-based hydrogel drug delivery systems are discussed. This review provides a theoretical basis and fabrication strategy for the application of PEG-based composite drug delivery systems in local bone defects.

Enhanced bone tissue regeneration with hydrogel-based scaffolds by embedding parathyroid hormone in mesoporous bioactive glass

OBJECTIVES

To evaluate hydrogel-based scaffolds embedded with parathyroid hormone (PTH)-loaded mesoporous bioactive glass (MBG) on the enhancement of bone tissue regeneration in vitro.

MATERIALS AND METHODS

MBG was produced via sol-gel technique followed by PTH solution imbibition. PTH-loaded MBG was blended into the hydrogels and submitted to a lyophilisation process associated with a chemical crosslinking reaction to the production of the scaffolds. Characterisation of the MBG and PTH-loaded MBG scaffolds, including the scanning electron microscope (SEM) connected with an X-ray detector (EDX), Fourier transform infrared (FTIR), compression strength, rheological measurements, swelling and degradation rates, and PTH release analysis, were performed. Also, bioactivity using simulated-body fluid (SBF), biocompatibility (MTT), and osteogenic differentiation analyses (von Kossa and Alizarin Red stainings, and μ-computed tomography, μCT) of the scaffolds were carried out.

RESULTS

SEM images demonstrated MBG particles dispersed into the hydrogel-based scaffold structure, which was homogeneously porous and well interconnected. EDX and FTIR revealed large amounts of carbon, oxygen, sodium, and silica in the scaffold composition. Bioactivity experiments revealed changes on sample surfaces over the analysed period, indicating the formation of carbonated hydroxyapatite; however, the chemical composition remained stable. PTH-loaded hydrogel-based scaffolds were biocompatible for stem cells from human-exfoliated deciduous teeth (SHED). A high quantity of calcium deposits on the extracellular matrix of SHED was found for PTH-loaded hydrogel-based scaffolds. μCT images showed MBG particles dispersed into the scaffolds' structure, and a porous, lamellar, and interconnected hydrogel architecture.

CONCLUSIONS

PTH-loaded hydrogel-based scaffolds demonstrated consistent morphology and physicochemical properties for bone tissue regeneration, as well as bioactivity, biocompatibility, and osteoinductivity in vitro. Thus, the scaffolds presented here are recommended for future studies on 3D printing.

CLINICAL RELEVANCE

Bone tissue regeneration is still a challenge for several approaches to oral and maxillofacial surgeries, though tissue engineering applying SHED, scaffolds, and osteoinductive mediators might help to overcome this clinical issue.

Preparation and In Vitro Osteogenic Evaluation of Biomimetic Hybrid Nanocomposite Scaffolds Based on Gelatin/Plasma Rich in Growth Factors (PRGF) and Lithium-Doped 45s5 Bioactive Glass Nanoparticles

Bone tissue engineering is an emerging technique for repairing large bone lesions. Biomimetic techniques expand the use of organic-inorganic spongy-like nanocomposite scaffolds and platelet concentrates. In this study, a biomimetic nanocomposite scaffold was prepared using lithium-doped bioactive-glass nanoparticles and gelatin/PRGF. First, sol-gel method was used to prepare bioactive-glass nanoparticles that contain 0, 1, 3, and 5%wt lithium. The lithium content was then optimized based on antibacterial and MTT testing. By freeze-drying, hybrid scaffolds comprising 5, 10, and 20% bioglass were made. On the scaffolds, human endometrial stem cells (hEnSCs) were cultured for adhesion (SEM), survival, and osteogenic differentiation. Alkaline phosphatase activity and osteopontin, osteocalcin, and Runx2 gene expression were measured. The effect of bioactive-glass nanoparticles and PRGF on nanocomposites' mechanical characteristics and glass-transition temperature (T _g) was also studied. An optimal lithium content in bioactive glass structure was found to be 3% wt. Nanoparticle SEM examination indicated grain deformation due to different sizes of lithium and sodium ions. Results showed up to 10% wt bioactive-glass and PRGF increased survival and cell adhesion. Also, Hybrid scaffolds revealed higher ALP-activity and OP, OC, and Runx2 gene expression. Furthermore, bioactive-glass has mainly increased ALP-activity and Runx2 expression, whereas PRGF increases the expression of OP and OC genes. Bioactive-glass increases scaffold modulus and T _g continuously. Hence, the presence of both bioactive-glass and nanocomposite scaffold improves the expression of osteogenic differentiation biomarkers. Subsequently, it seems that hybrid scaffolds based on biopolymers, Li-doped bioactive-glass, and platelet extracts can be a good strategy for bone repair.

Rubbing-Assisted Approach for Fabricating Oriented Nanobiomaterials

The highly-oriented structures in biological tissues play an important role in determining the functions of the tissues. In order to artificially fabricate oriented nanostructures similar to biological tissues, it is necessary to understand the oriented mechanism and invent the techniques for controlling the oriented structure of nanobiomaterials. In this review, the oriented structures in biological tissues were reviewed and the techniques for producing highly-oriented nanobiomaterials by imitating the oriented organic/inorganic nanocomposite mechanism of the biological tissues were summarized. In particular, we introduce a fabrication technology for the highly-oriented structure of nanobiomaterials on the surface of a rubbed polyimide film that has physicochemical anisotropy in order to further form the highly-oriented organic/inorganic nanocomposite structures based on interface interaction. This is an effective technology to fabricate one-directional nanobiomaterials by a biomimetic process, indicating the potential for wide application in the biomedical field.

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.

Engineering immunomodulatory and osteoinductive implant surfaces via mussel adhesion-mediated ion coordination and molecular clicking

Immune response and new tissue formation are important aspects of tissue repair. However, only a single aspect is generally considered in previous biomedical interventions, and the synergistic effect is unclear. Here, a dual-effect coating with immobilized immunomodulatory metal ions (e.g., Zn²⁺) and osteoinductive growth factors (e.g., BMP-2 peptide) is designed via mussel adhesion-mediated ion coordination and molecular clicking strategy. Compared to the bare TiO₂ group, Zn²⁺ can increase M2 macrophage recruitment by up to 92.5% in vivo and upregulate the expression of M2 cytokine IL-10 by 84.5%; while the dual-effect of Zn²⁺ and BMP-2 peptide can increase M2 macrophages recruitment by up to 124.7% in vivo and upregulate the expression of M2 cytokine IL-10 by 171%. These benefits eventually significantly enhance bone-implant mechanical fixation (203.3 N) and new bone ingrowth (82.1%) compared to the bare TiO₂ (98.6 N and 45.1%, respectively). Taken together, the dual-effect coating can be utilized to synergistically modulate the osteoimmune microenvironment at the bone-implant interface, enhancing bone regeneration for successful implantation.

Immune response and new tissue formation are important aspects of tissue repair but often only one aspect is considered in biomedical interventions. Here, the authors report on the use of a mussel-like surface coating to immobilize immune modulating metal ions and growth factors and demonstrated improved in vivo outcomes.

Mechanistically Scoping Cell-Free and Cell-Dependent Artificial Scaffolds in Rebuilding Skeletal and Dental Hard Tissues

Rebuilding mineralized tissues in skeletal and dental systems remains costly and challenging. Despite numerous demands and heavy clinical burden over the world, sources of autografts, allografts, and xenografts are far limited, along with massive risks including viral infections, ethic crisis, and so on. Per such dilemma, artificial scaffolds have emerged to provide efficient alternatives. To date, cell-free biomimetic mineralization (BM) and cell-dependent scaffolds have both demonstrated promising capabilities of regenerating mineralized tissues. However, BM and cell-dependent scaffolds have distinctive mechanisms for mineral genesis, which makes them methodically, synthetically, and functionally disparate. Herein, these two strategies in regenerative dentistry and orthopedics are systematically summarized at the level of mechanisms. For BM, methodological and theoretical advances are focused upon; and meanwhile, for cell-dependent scaffolds, it is demonstrated how scaffolds orchestrate osteogenic cell fate. The summary of the experimental advances and clinical progress will endow researchers with mechanistic understandings of artificial scaffolds in rebuilding hard tissues, by which better clinical choices and research directions may be approached.

Implantation of stem cells on synthetic or biological scaffolds: an overview of bone regeneration

Humans are exposed to a wide range of bone tissue injuries. In severe cases, bone damages could be only treated with transplantation of autologous or allogeneic grafting.In recent years, tissue engineering has become a promising strategy for repairing damaged organs and tissues, providing a great opportunity to cure several diseases. Bone tissue engineering consists of three components: scaffold, cells, and growth factors. Current bone tissue engineering strategies combine the use of stem cells with biologically active materials and gene therapy to mimic the natural microenvironment of bone. The combination of the scaffold with growth factors and extracellular matrix protein molecules can promote cell attachment, proliferation, and induce osteogenesis, which could provide signals for cell migration to begin the healing process during repair and bone formation.This article reviews the principles of bone regeneration and the most current developments of bone tissue engineering related to bone growth factors, the biologically active materials, such as bacterial cellulose, and stem cells.

Gelatin methacrylate hydrogel scaffold carrying resveratrol-loaded solid lipid nanoparticles for enhancement of osteogenic differentiation of BMSCs and effective bone regeneration

Critical-sized bone defects caused by traumatic fractures, tumour resection and congenital malformation are unlikely to heal spontaneously. Bone tissue engineering is a promising strategy aimed at developing in vitro replacements for bone transplantation and overcoming the limitations of natural bone grafts. In this study, we developed an innovative bone engineering scaffold based on gelatin methacrylate (GelMA) hydrogel, obtained via a two-step procedure: first, solid lipid nanoparticles (SLNs) were loaded with resveratrol (Res), a drug that can promote osteogenic differentiation and bone formation; these particles were then encapsulated at different concentrations (0.01%, 0.02%, 0.04% and 0.08%) in GelMA to obtain the final Res-SLNs/GelMA scaffolds. The effects of these scaffolds on osteogenic differentiation of bone marrow mesenchymal stem cells (BMSCs) and bone regeneration in rat cranial defects were evaluated using various characterization assays. Our in vitro and in vivo investigations demonstrated that the different Res-SLNs/GelMA scaffolds improved the osteogenic differentiation of BMSCs, with the ideally slow and steady release of Res; the optimal scaffold was 0.02 Res-SLNs/GelMA. Therefore, the 0.02 Res-SLNs/GelMA hydrogel is an appropriate release system for Res with good biocompatibility, osteoconduction and osteoinduction, thereby showing potential for application in bone tissue engineering.

[Study on the gelatin methacryloyl composite scaffold with exogenous transforming growth factor β 1 to promote the repair of skull defects]

Objective

To prepare a bone tissue engineering scaffold for repairing the skull defect of Sprague Dawley (SD) rats by combining exogenous transforming growth factor β ₁ (TGF-β ₁) with gelatin methacryloyl (GelMA) hydrogel.

Methods

Firstly, GelMA hydrogel composite scaffolds containing exogenous TGF-β ₁ at concentrations of 0, 150, 300, 600, 900, and 1 200 ng/mL (set to groups A, B, C, D, E, and F, respectively) were prepared. Cell counting kit 8 (CCK-8) method was used to detect the effect of composite scaffold on the proliferation of bone marrow mesenchymal stem cells (BMSCs) in SD rats. ALP staining, alizarin red staining, osteocalcin (OCN) immunofluorescence staining, and Western blot were used to explore the effect of scaffolds on osteogenic differentiation of BMSCs, and the optimal concentration of TGF-β ₁/GelMA scaffold was selected. Thirty-six 8-week-old SD rats were taken to prepare a 5 mm diameter skull bone defect model and randomly divided into 3 groups, namely the control group, the GelMA group, and the GelMA+TGF-β ₁ group (using the optimal concentration of TGF-β ₁/GelMA scaffold). The rats were sacrificed at 4 and 8 weeks after operation, and micro-CT, HE staining, and OCN immunohistochemistry staining were performed to observe the repair effect of skull defects.

Results

The CCK-8 method showed that the TGF-β ₁/GelMA scaffolds in each group had a promoting effect on the proliferation of BMSCs. Group D had the strongest effect, and the cell activity was significantly higher than that of the other groups ( P<0.05). The results of ALP staining, alizarin red staining, OCN immunofluorescence staining, and Western blot showed that the percentage of ALP positive area, the percentage of alizarin red positive area, and the relative expressions of ALP and OCN proteins in group D were significantly higher than those of the other groups ( P<0.05), the osteogenesis effect in group D was the strongest. Therefore, in vitroexperiments screened out the optimal concentration of TGF-β ₁/GelMA scaffold to be 600 ng/mL. Micro-CT, HE staining, and OCN immunohistochemistry staining of rat skull defect repair experiments showed that the new bone tissue and bone volume/tissue volume ratio in the TGF-β ₁+GelMA group were significantly higher than those in the GelMA group and control group at 4 and 8 weeks after operation ( P<0.05).

Conclusion

The TGF-β ₁/GelMA scaffold with a concentration of 600 ng/mL can significantly promote the osteogenic differentiation of BMSCs, can significantly promote bone regeneration at the skull defect, and can be used as a bioactive material for bone tissue regeneration.

Silk Biomaterials for Bone Tissue Engineering

Silk is a natural fibrous polymer with application potential in regenerative medicine. Increasing interest remains for silk materials in bone tissue engineering due to their characteristics in biocompatibility, biodegradability and mechanical properties. Plenty of the in vitro and in vivo studies confirmed the advantages of silk in accelerating bone regeneration. Silk is processed into scaffolds, hydrogels, and films to facilitate different bone regenerative applications. Bioactive factors such as growth factors and drugs, and stem cells are introduced to silk-based matrices to create friendly and osteogenic microenvironments, directing cell behaviors and bone regeneration. The recent progress in silk-based bone biomaterials is discussed and focused on different fabrication and functionalization methods related to osteogenesis. The challenges and potential targets of silk bone materials are highlighted to evaluate the future development of silk-based bone materials.

Strategies to Control or Mimic Growth Factor Activity for Bone, Cartilage, and Osteochondral Tissue Engineering

Growth factors play a critical role in tissue repair and regeneration. However, their clinical success is limited by their low stability, short half-life, and rapid diffusion from the delivery site. Supraphysiological growth factor concentrations are often required to demonstrate efficacy but can lead to adverse reactions, such as inflammatory complications and increased cancer risk. These issues have motivated the development of delivery systems that enable sustained release and controlled presentation of growth factors. This review specifically focuses on bioconjugation strategies to enhance growth factor activity for bone, cartilage, and osteochondral applications. We describe approaches to localize growth factors using noncovalent and covalent methods, bind growth factors via peptides, and mimic growth factor function with mimetic peptide sequences. We also discuss emerging and future directions to control spatiotemporal growth factor delivery to improve functional tissue repair and regeneration.

Synthesis of an organic-inorganic hybrid with short organic molecular chains by sol-gel chemistry

An oligomer with short organic molecular chains was successfully synthesized with KH550 and KH560. This oligomer was combined with tetraethyl orthosilicate and calcium chloride to prepare an organic-inorganic hybrid biological material (OI-BM) by sol-gel chemistry. The hybrid was fully characterized by a series of instrumental characterizations including nuclear magnetic resonance spectrometry, scanning electron microscopy, transmission electron microscopy, Fourier transform infrared spectroscopy, X-ray energy spectroscopy and inductively coupled plasma spectrometry. OI-BM presented elastic deformation under compression. The elastic modulus and ultimate stress of OI-BM were 0.4 ± 0.1 GPa and 23.0 ± 4.0 MPa, respectively, lower than those of 45S5 bioactive glass (45S5-BG), whereas the strain at failure and modulus of toughness of OI-BM was about 4.5 times and 4 times higher. The hybrid is easy to form due to the improved mechanical property, suggesting excellent machining properties. The hybrid OI-BM produced hydroxyapatite in 1 h in simulated body fluid due to its excellent bioactivity. CCK-8 assay further demonstrated the desirable cytocompatibility of the hybrid. Thus, the hybrid can be a potential material for satisfying the mechanical property requirement of an implant.

Hydroxyapatite Based Materials for Bone Tissue Engineering: A Brief and Comprehensive Introduction

Hydroxyapatite (HA) is widely used in bone tissue engineering for its bioactivity and biocompatibility, and a growing number of researchers are exploring ways to improve the physical properties and biological functions of hydroxyapatite. Up to now, HA has been used as inorganic building blocks for tissue engineering or as nanofillers to blend with polymers, furthermore, various methods such as ion doping or surface modification have been also reported to prepare functionalized HA. In this review, we try to give a brief and comprehensive introduction about HA-based materials, including ion-doped HA, HA/polymer composites and surface modified HA and their applications in bone tissue engineering. In addition, the prospective of HA is also discussed. This review may be helpful for researchers to get a general understanding about the development of hydroxyapatite based materials.

GelMA/bioactive silica nanocomposite bioinks for stem cell osteogenic differentiation

Leveraging 3D bioprinting for processing stem cell-laden biomaterials has unlocked a tremendous potential for fabricating living 3D constructs for bone tissue engineering. Even though several bioinks developed to date display suitable physicochemical properties for stem cell seeding and proliferation, they generally lack the nanosized minerals present in native bone bioarchitecture. To enable the bottom-up fabrication of biomimetic 3D constructs for bioinstructing stem cells pro-osteogenic differentiation, herein we developed multi-bioactive nanocomposite bioinks that combine the organic and inorganic building blocks of bone. For the organic component gelatin methacrylate (GelMA), a photocrosslinkable denaturated collagen derivative used for 3D bioprinting was selected due to its rheological properties display of cell adhesion moities to which bone tissue precursors such as human bone marrow derived mesenchymal stem cells (hBM-MSCs) can attach to. The inorganic building block was formulated by incorporating mesoporous silica nanoparticles functionalized with calcium, phosphate and dexamethasone (MSNCaPDex), which previously proven to induce osteogenic differentiation. The newly formulated photocrosslinkable nanocomposite GelMA bioink incorporating MSNCaPDex nanoparticles and laden with hBM-MSCs was sucessfully processed into a 3D bioprintable construct with structural fidelity and well dispersed nanoparticles throughout the hydrogel matrix. These nanocomposite constructs could induce the deposition of apatite in vitro, thus showing attractive bioactivity properties. Viability and differentiation studies showed that hBM-MSCs remained viable and exhibited osteogenic differentiation biomarkers when incorporated in GelMA/MSNCaPDex constructs and without requiring further biochemical nor mechanical stimuli. Overall, our nanocomposite bioink has demonstrated excellent processability via extrusion bioprinting into osteogenic constructs with potential application in bone tissue repair and regeneration.

Chemokine SDF1 Mediated Bone Regeneration Using Biodegradable Poly(D,L-lactide-co-glycolide) 3D Scaffolds and Bone Marrow-Derived Mesenchymal Stem Cells: Implication for the Development of an "Off-the-Shelf" Pharmacologically Active Construct

There is an increasing need for bone substitutes for reconstructive orthopedic surgery following removal of bone tumors. Despite the advances in bone regeneration, the use of autologous mesenchymal stem cells (MSC) presents a significant challenge, particularly for the treatment of large bone defects in cancer patients. This study aims at developing new chemokine-based technology to generate biodegradable scaffolds that bind pharmacologically active proteins for regeneration/repair of target injured tissues in patients. Primary MSC were cultured from the uninvolved bone marrow (BM) of cancer patients and further characterized for "stemness". Their ability to differentiate into an osteogenic lineage was studied in 2D cultures as well as on 3D macroporous PLGA scaffolds incorporated with biomacromolecules bFGF and homing factor chemokine stromal-cell derived factor-1 (SDF1). MSC from the uninvolved BM of cancer patients exhibited properties similar to that reported for MSC from BM of healthy individuals. Macroporous PLGA discs were prepared and characterized for pore size, architecture, functional groups, thermostability, and cytocompatibility by ESEM, FTIR, DSC, and CCK-8 dye proliferation assay, respectively. It was observed that the MSC+PLGA+bFGF+SDF1 construct cultured for 14 days supported significant cell growth, osteo-lineage differentiation with increased osteocalcin expression, alkaline phosphatase secretion, calcium mineralization, bone volume, and soluble IL6 compared to unseeded PLGA and PLGA+MSC, as analyzed by confocal microscopy, biochemistry, ESEM, microCT imaging, flow cytometry, and EDS. Thus, chemotactic biomacromolecule SDF1-guided tissue repair/regeneration ability of MSC from cancer patients opens up the avenues for development of "off-the-shelf" pharmacologically active construct for optimal repair of the target injured tissue in postsurgery cancer patients, bone defects, damaged bladder tissue, and radiation-induced skin/mucosal lesions.

Black phosphorus-based 2D materials for bone therapy

Since their discovery, Black Phosphorus (BP)-based nanomaterials have received extensive attentions in the fields of electromechanics, optics and biomedicine, due to their remarkable properties and excellent biocompatibility. The most essential feature of BP is that it is composed of a single phosphorus element, which has a high degree of homology with the inorganic components of natural bone, therefore it has a full advantage in the treatment of bone defects. This review will first introduce the source, physicochemical properties, and degradation products of BP, then introduce the remodeling process of bone, and comprehensively summarize the progress of BP-based materials for bone therapy in the form of hydrogels, polymer membranes, microspheres, and three-dimensional (3D) printed scaffolds. Finally, we discuss the challenges and prospects of BP-based implant materials in bone immune regulation and outlook the future clinical application.

Bioprinted Injectable Hierarchically Porous Gelatin Methacryloyl Hydrogel Constructs with Shape-Memory Properties

Direct injection of cell-laden hydrogels shows high potentials in tissue regeneration for translational therapy. The traditional cell-laden hydrogels are often used as bulk space fillers to tissue defects after injection, likely limiting their structural controllability. On the other hand, patterned cell-laden hydrogel constructs often necessitate invasive surgical procedures. To overcome these problems, herein, we report a unique strategy for encapsulating living human cells in a pore-forming gelatin methacryloyl (GelMA)-based bioink to ultimately produce injectable hierarchically macro-micro-nanoporous cell-laden GelMA hydrogel constructs through three-dimensional (3D) extrusion bioprinting. The hydrogel constructs can be fabricated into various shapes and sizes that are defect-specific. Due to the hierarchically macro-micro-nanoporous structures, the cell-laden hydrogel constructs can readily recover to their original shapes, and sustain high cell viability, proliferation, spreading, and differentiation after compression and injection. Besides, in vivo studies further reveal that the hydrogel constructs can integrate well with the surrounding host tissues. These findings suggest that our unique 3D-bioprinted pore-forming GelMA hydrogel constructs are promising candidates for applications in minimally invasive tissue regeneration and cell therapy.

Gelatin Methacrylate (GelMA)-Based Hydrogels for Cell Transplantation: an Effective Strategy for Tissue Engineering

Gelatin methacrylate (GelMA)-based hydrogels are gaining a great deal of attention as potentially implantable materials in tissue engineering applications because of their biofunctionality and mechanical tenability. Since different natural tissues respond differently to mechanical stresses, an ideal implanted material would closely match the mechanical properties of the target tissue. In this regard, applications employing GelMA hydrogels are currently limited by the low mechanical strength and biocompatibility of GelMA. Therefore, this review focuses on modifications made to GelMA hydrogels to make them more suitable for tissue engineering applications. A large number of reports detail rational synthetic processes for GelMA or describe the incorporation of various biomaterials into GelMA hydrogels to tune their various properties, e.g., physical strength, chemical properties, conductivity, and porosity, and to promote cell loading and accelerate tissue repair. A novel strategy for repairing tissue injuries, based on the transplantation of cell-loaded GelMA scaffolds, is examined and its advantages and challenges are summarized. GelMA-cell combinations play a critical and pioneering role in this process and could potentially accelerate the development of clinically relevant applications.

Design and evaluation of collagen-inspired mineral-hydrogel nanocomposites for bone regeneration

Bone loss due to trauma and tumors remains a serious clinical concern. Due to limited availability and disease transmission risk with autografts and allografts, calcium phosphate bone fillers and growth factor-based substitute bone grafts are currently used in the clinic. However, substitute grafts lack bone regeneration potential when used without growth factors. When used along with the added growth factors, they lead to unwanted side effects such as uncontrolled bone growth. Collagen-based hydrogel grafts available on the market fail to provide structural guidance to native cells due to high water-solubility and faster degradation. To overcome these limitations, we employed bioinspired material design and fabricated three different hydrogels with structural features similar to native collagen at multiple length-scales. These hydrogels fabricated using polyionic complexation of oppositely charged natural polysaccharides exhibited multi-scale architecture mimicking nanoscale banding pattern, and microscale fibrous structure of native collagen. All three hydrogels promoted biomimetic apatite-like mineral deposition in vitro elucidating crystalline structure on the surface while amorphous calcium phosphate inside the hydrogels resulting in mineral-hydrogel nanocomposites. When evaluated in a non-load bearing critical size mouse calvaria defect model, chitosan - kappa carrageenan mineral-hydrogel nanocomposites enhanced bone regeneration without added growth factors compared to empty defect as well as widely used marketed collagen scaffolds. Histological assessment of the regenerated bone revealed improved healing and tissue remodeling with mineral-hydrogel nanocomposites. Overall, these collagen-inspired mineral-hydrogel nanocomposites showed multi-scale hierarchical structure and can potentially serve as promising bioactive hydrogel to promote bone regeneration. STATEMENT OF SIGNIFICANCE: Hydrogels, especially collagen, are widely used in bone tissue engineering. Collagen fibrils play arguably the most important role during natural bone development. Its multi-scale hierarchical structure to form fibers from fibrils and electrostatic charges enable mineral sequestration, nucleation, and growth. However, bulk collagen hydrogels exhibit limited bone regeneration and are mostly used as carriers for highly potent growth factors such as bone morphogenic protein-2, which increase the risk of uncontrolled bone growth. Thus, there is an unmet clinical need for a collagen-inspired biomaterial that can recreate structural hierarchy, mineral sequestration ability, and stimulate recruitment of host progenitor cells to facilitate bone regeneration. Here, we propose collagen-inspired bioactive mineral-hydrogel nanocomposites as a growth factor-free approach to guide and enhance bone regeneration.

Human skeletal physiology and factors affecting its modeling and remodeling

Human skeleton is a living tissue that performs structural and metabolic functions. It is not only the largest storehouse for calcium and other essential ions but also a depot for toxic chemicals faced by human body throughout life. Skeletal modeling starts at conception and then throughout life undergoes constant remodeling to adopt its shape and strength according to human needs. With the passage of time, like other tissues in the body, bones also bear the brunt of life and in this life long process loses its strength and vitality. Multiple genetic and environmental factors play an integral part in its formation, strength, and decline.

Sustained protein therapeutics enabled by self-healing nanocomposite hydrogels for non-invasive bone regeneration

Bone tissue engineering based on stem cells, growth factors and bioactive scaffolds presents an appealing but challenging approach for rehabilitation of patients with bone defects. A versatile system with the capability for easy operation and precise protein delivery in specific locations is attractive for enhancing bone regeneration. Here, we develop a non-invasive delivery system based on injectable and self-healing nanocomposite hydrogels for sustained protein release, which has the potential to improve the current orthopedic strategy. Specifically, LAPONITE® (LAP) nanoplatelets are able to accelerate the gelation process through hydrogen bonds with polysaccharide matrices, endowing hydrogels with superior mechanical and rheological behaviors, along with better injectability and self-healing ability. Attractively, the strong static binding between LAP nanoplatelets and bone morphogenetic protein-2 (BMP-2) can form stable LAP@BMP-2 complexes. The results indicate that the complexes effectively preserve the intrinsic bioactivity of BMP-2 and prolong the release period for more than four weeks. Moreover, hydrogels incorporating with the LAP@BMP-2 complexes synergistically boost cell spreading, proliferation activity and osteogenesis, both in vitro and in vivo, compared with LAP or BMP-2 alone. Overall, this study proposes a valid platform for protein therapeutics and non-invasive bone repair.

In vivo study of polyurethane and tannin-modified hydroxyapatite composites for calvarial regeneration

Biomaterial mediated bone regeneration is an attractive strategy for bone defect treatment. Organic/inorganic composites have been well established as effective bone graft. Here, the bone regenerative effect of the composites made from tannic acid (TA) modified hydroxyapatite (HA) (THA) or TA & silver nanoparticles (Ag NPs) modified HA (Ag-THA) and polyurethane (PU) was evaluated on critical-sized calvarial defects in rats. The in vivo study indicates that PU/THA and PU/Ag-THA scaffolds exhibited acceptable biocompatibility and induced significantly enhanced bone mineral densities comparing with the blank control (CON) group as well as PU/HA group. The inclusion of TA on HA brought the composites with enhanced osteogenesis and angiogenesis, evidenced by osteocalcin (OCN) and vascular endothelial growth factor (VEGF) immunohistochemical staining. Tartrate resistant acid phosphatase (TRAP) staining showed high osteoclast activity along with osteogenesis, especially in PU/THA and PU/Ag-THA groups. However, further introduction of Ag NPs on HA depressed the angiogenesis of the composites, leading to even lower VEGF expression than that of CON group. This study once more proved that THA can serve as a better bone composite component that pure HA and can promote osteogenesis and angiogenesis. While, the introduction of antimicrobial Ag NPs on HA need to be controlled in some extent not to affect the angiogenesis of the composites.

Hierarchically Porous Hydroxyapatite Hybrid Scaffold Incorporated with Reduced Graphene Oxide for Rapid Bone Ingrowth and Repair

Hydroxyapatite (HA), the traditional bone tissue replacement material was widely used in the clinical treatment of bone defects because of its excellent biocompatibility. However, the processing difficulty and poor osteoinductive ability greatly limit the application of HA. Although many strategies have been reported to improve the machinability and osteointegration ability, the performance including mechanical strength, porosity, cell adhesion, etc. of material still can not meet the requirements. In this work, a soft template method was developed and a porous scaffold with hierarchical pore structure, nano surface morphology, suitable porosity and pore size, and good biomechanical strength was successfully prepared. The hierarchical pore structure is beneficial for cell adhesion, fluid transfer, and cell ingrowth. Moreover, the loaded reduced graphene oxide (rGO) can improve the adhesion and promote the proliferation and spontaneous osteogenic differentiation bone marrow mesenchymal stem cells. The scaffold is then crushed, degraded and wrapped by the newly formed bone and the newly formed bone gradually replaces the scaffold. The degradation rate of the scaffold well matches the rate of the new bone formation. The hierarchical porous HA/rGO composite scaffolds can greatly accelerate the bone ingrowth in the scaffold and bone repair in critical bone defects, thus providing a clinical potential candidate for large segment bone tissue engineering.

Three-Dimensional Printing of Biodegradable Piperazine-Based Polyurethane-Urea Scaffolds with Enhanced Osteogenesis for Bone Regeneration

Synthetic biodegradable polymeric scaffolds with uniformly interconnected pore structure, appropriate mechanical properties, excellent biocompatibility, and even enhanced osteogenesis ability are urgently required for in situ bone regeneration. In this study, for the first time, a series of biodegradable piperazine (PP)-based polyurethane-urea (P-PUU) scaffolds with a gradient of PP contents were developed by air-driven extrusion 3D printing technology. The P-PUU ink of 60 wt % concentration was demonstrated to have appropriate viscosity for scaffold fabrication. The 3D-printed P-PUU scaffolds exhibited an interconnected porous structure of about 450 μm in macropore size and about 75% in porosity. By regulating the contents of PP in P-PUU scaffolds, their mechanical properties could be moderated, and P-PUU1.4 scaffolds with the highest PP contents exhibited the highest compressive modulus (155.9 ± 5.7 MPa) and strength (14.8 ± 1.1 MPa). Moreover, both in vitro and in vivo biological results suggested that the 3D-printed P-PUU scaffolds possessed excellent biocompatibility and osteoconductivity to facilitate new bone formation. The small molecular PP itself was confirmed for the first time to regulate osteogenesis of osteoblasts in a dose-dependent manner and the optimum concentration for osteoconductivity was about ∼0.5 mM, which suggests that PP molecules, together with the mechanical behavior, nitrogen-contents, and hydrophilicity of P-PUUs, play an important role in enhancing the osteoconductive ability of P-PUU scaffolds. Therefore, the 3D-printed P-PUU scaffolds, with suitable interconnected pore structure, appropriate mechanical properties, and intrinsically osteoconductive ability, should provide a promising alternative for bone regeneration.

In situ bone regeneration enabled by a biodegradable hybrid double-network hydrogel

The biodegradable hybrid double-network hydrogel for stem cell-enhanced bone regeneration.

Synthesis and Properties of Gelatin Methacryloyl (GelMA) Hydrogels and Their Recent Applications in Load-Bearing Tissue

Photocrosslinked gelatin methacryloyl (GelMA) hydrogels have attracted great concern in the biomedical field because of their good biocompatibility and tunable physicochemical properties. Herein, different approaches to synthesize GelMA were introduced, especially, the typical method using UV light to crosslink the gelatin-methacrylic anhydride (MA) precursor was introduced in detail. In addition, the traditional and cutting-edge technologies to characterize the properties of GelMA hydrogels and GelMA prepolymer were also overviewed and compared. Furthermore, the applications of GelMA hydrogels in cell culture and tissue engineering especially in the load-bearing tissue (bone and cartilage) were summarized, followed by concluding remarks.