Bioactive hydrogels for bone regeneration

Hydrogel-Based Systems in Neuro-Vascularized Bone Regeneration: A Promising Therapeutic Strategy

Blood vessels and nerve fibers are distributed throughout the skeletal tissue, which enhance the development and function of each other and have an irreplaceable role in bone formation and remodeling. Despite significant progress in bone tissue engineering, the inadequacy of nerve-vascular network reconstruction remains a major limitation. This is partly due to the difficulty of integrating and regulating multiple tissue types with artificial materials. Thus, understanding the anatomy and underlying coupling mechanisms of blood vessels and nerve fibers within bone to further develop neuro-vascularized bone implant biomaterials is an extremely critical aspect in the field of bone regeneration. Hydrogels have good biocompatibility, controllable mechanical characteristics, and osteoconductive and osteoinductive properties, making them important candidates for research related to neuro-vascularized bone regeneration. This review reports the classification and physicochemical properties of hydrogels, with a focus on the application advantages and status of hydrogels for bone regeneration. The authors also highlight the effect of neurovascular coupling on bone repair and regeneration and the necessity of achieving neuro-vascularized bone regeneration. Finally, the recent progress and design strategies of hydrogel-based biomaterials for neuro-vascularized bone regeneration are discussed.

A submicron forest-like silicon surface promotes bone regeneration by regulating macrophage polarization

Introduction: Silicon is a major trace element in humans and a prospective supporting biomaterial to bone regeneration. Submicron silicon pillars, as a representative surface topography of silicon-based biomaterials, can regulate macrophage and osteoblastic cell responses. However, the design of submicron silicon pillars for promoting bone regeneration still needs to be optimized. In this study, we proposed a submicron forest-like (Fore) silicon surface (Fore) based on photoetching. The smooth (Smo) silicon surface and photoetched regular (Regu) silicon pillar surface were used for comparison in the bone regeneration evaluation. Methods: Surface parameters were investigated using a field emission scanning electron microscope, atomic force microscope, and contact angle instrument. The regulatory effect of macrophage polarization and succedent osteogenesis was studied using Raw264.7, MC3T3-E1, and rBMSCs. Finally, a mouse calvarial defect model was used for evaluating the promoting effect of bone regeneration on the three surfaces. Results: The results showed that the Fore surface can increase the expression of M2-polarized markers (CD163 and CD206) and decrease the expression of inflammatory cytokines, including interleukin-6 (IL-6) and tumor necrosis factor alpha (TNF-α). Fore surface can promote the osteogenesis in MC3T3-E1 cells and osteoblastic differentiation of rBMSCs. Furthermore, the volume fraction of new bone and the thickness of trabeculae on the Fore surface were significantly increased, and the expression of RANKL was downregulated. In summary, the upregulation of macrophage M2 polarization on the Fore surface contributed to enhanced osteogenesis in vitro and accelerated bone regeneration in vivo. Discussion: This study strengthens our understanding of the topographic design for developing future silicon-based biomaterials.

Surface-modified deproteinized human demineralized tooth matrix for bone regeneration: physicochemical characterization and osteoblast cell biocompatibility

Tooth presents an intriguing option as a bone graft due to its compositional similarity to bone. However, the deproteinized human demineralized tooth matrix (dpDTM), developed to overcome the limited availability of autologous tooth grafts, has suboptimal pore size and surface roughness. This study aimed to fabricate a surface-modified dpDTM using acid etching and collagen coating, followed by in vitro evaluation of physicochemical and biological properties. The dpDTM was modified into two protocols: Acid-modified dpDTM (A-dpDTM) and collagen-modified dpDTM (C-dpDTM). Results demonstrated that A-dpDTM and C-dpDTM had increased pore sizes and rougher surfaces compared to dpDTM. Collagen immobilization was evidenced by nitrogen presence exclusively in C-dpDTM. All groups had a Ca/P molar ratio of 1.67 and hydroxyapatite as the sole constituent, with 65-67% crystallinity. Degradation rates significantly increased to 30% and 20% for C-dpDTM and A-dpDTM, respectively, compared to 10% for dpDTM after 120 days. Cumulative collagen release of C-dpDTM on Day 30 was 45.16 µg/ml. Osteoblasts attachment and proliferation were enhanced on all scaffolds, especially C-dpDTM, which displayed the highest proliferation and differentiation rates. In conclusion, surface modified of dpDTM, including A-dpDTM and C-dpDTM, significantly enhances bioactivity by altering surface properties and promoting osteoblast activity, thereby demonstrating promise for bone regeneration applications.

Injectable phase-separated tetra-armed poly(ethylene glycol) hydrogel scaffold allows sustained release of growth factors to enhance the repair of critical bone defects

With the rising prevalence of bone-related injuries, it is crucial to improve treatments for fractures and defects. Tissue engineering offers a promising solution in the form of injectable hydrogel scaffolds that can sustain the release of growth factors like bone morphogenetic protein-2 (BMP-2) for bone repair. Recently, we discovered that tetra-PEG hydrogels (Tetra gels) undergo gel-gel phase separation (GGPS) at low polymer content, resulting in hydrophobicity and tissue affinity. In this work, we examined the potential of a newer class of gel, the oligo-tetra-PEG gel (Oligo gel), as a growth factor-releasing scaffold. We investigated the extent of GGPS occurring in the two gels and assessed their ability to sustain BMP-2 release and osteogenic potential in a mouse calvarial defect model. The Oligo gel underwent a greater degree of GGPS than the Tetra gel, exhibiting higher turbidity, hydrophobicity, and pore formation. The Oligo gel demonstrated sustained protein or growth factor release over a 21-day period from protein release kinetics and osteogenic cell differentiation studies. Finally, BMP-2-loaded Oligo gels achieved complete regeneration of critical-sized calvarial defects within 28 days, significantly outperforming Tetra gels. The easy formulation, injectability, and capacity for sustained release makes the Oligo gel a promising candidate therapeutic biomaterial.

Chitosan/MWCNTs nanocomposite coating on 3D printed scaffold of poly 3-hydroxybutyrate/magnetic mesoporous bioactive glass: A new approach for bone regeneration

The utilization of 3D printing has become increasingly common in the construction of composite scaffolds. In this study, magnetic mesoporous bioactive glass (MMBG) was incorporated into polyhydroxybutyrate (PHB) to construct extrusion-based 3D printed scaffold. After fabrication of the PHB/MMBG composite scaffolds, they were coated with chitosan (Cs) and chitosan/multi-walled carbon nanotubes (Cs/MWCNTs) solutions utilizing deep coating method. FTIR was conducted to confirm the presence of Cs and MWCNTs on the scaffolds' surface. The findings of mechanical analysis illustrated that presence of Cs/MWCNTs on the composite scaffolds increases compressive young modulus significantly, from 16.5 to 42.2 MPa. According to hydrophilicity evaluation, not only MMBG led to decrease the contact angle of pure PHB but also scaffolds surface modification utilization of Cs and MWCNTs, the contact angle decreased significantly from 82.34° to 54.15°. Furthermore, investigation of cell viability, cell metabolism and inflammatory cytokines such as interleukin-6 (IL-6) and tumor necrosis factor alpha (TNF-α) and interleukin-10 (IL-10) and transforming growth factor-beta (TGF-β) proved that the scaffolds not only do not stimulate the immune system, but also polarize macrophage cells from M1 phase to M2 phase. The present study highlights the suitability of 3D printed scaffold PHB/MMBG with Cs/MWCNTs coating for bone tissue engineering.

Enhanced bone regeneration in rat calvarial defects through BMP2 release from engineered poly(ethylene glycol) hydrogels

The clinical standard therapy for large bone defects, typically addressed through autograft or allograft donor tissue, faces significant limitations. Tissue engineering offers a promising alternative strategy for the regeneration of substantial bone lesions. In this study, we harnessed poly(ethylene glycol) (PEG)-based hydrogels, optimizing critical parameters including stiffness, incorporation of arginine-glycine-aspartic acid (RGD) cell adhesion motifs, degradability, and the release of BMP2 to promote bone formation. In vitro we demonstrated that human bone marrow derived stromal cell (hBMSC) proliferation and spreading strongly correlates with hydrogel stiffness and adhesion to RGD peptide motifs. Moreover, the incorporation of the osteogenic growth factor BMP2 into the hydrogels enabled sustained release, effectively inducing bone regeneration in encapsulated progenitor cells. When used in vivo to treat calvarial defects in rats, we showed that hydrogels of low and intermediate stiffness optimally facilitated cell migration, proliferation, and differentiation promoting the efficient repair of bone defects. Our comprehensive in vitro and in vivo findings collectively suggest that the developed hydrogels hold significant promise for clinical translation for bone repair and regeneration by delivering sustained and controlled stimuli from active signaling molecules.

Bioactive gelatin-sheets as novel biopapers to support prevascularization organized by laser-assisted bioprinting for bone tissue engineering

Despite significant advances in the management of patients with oral cancer, maxillofacial reconstruction after ablative surgery remains a clinical challenge. In bone tissue engineering, biofabrication strategies have been proposed as promising alternatives to solve issues associated with current therapies and to produce bone substitutes that mimic both the structure and function of native bone. Among them, laser-assisted bioprinting (LAB) has emerged as a relevant biofabrication method to print living cells and biomaterials with micrometric resolution onto a receiving substrate, also called 'biopaper'. Recent studies have demonstrated the benefits of prevascularization using LAB to promote vascularization and bone regeneration, but mechanical and biological optimization of the biopaper are needed. The aim of this study was to apply gelatin-sheet fabrication process to the development of a novel biopaper able to support prevascularization organized by LAB for bone tissue engineering applications. Gelatin-based sheets incorporating bioactive glasses (BGs) were produced using various freezing methods and crosslinking (CL) parameters. The different formulations were characterized in terms of microstructural, physical, mechanical, and biological properties in monoculture and coculture. Based on multi-criteria analysis, a rank scoring method was used to identify the most relevant formulations. The selected biopaper underwent additional characterization regarding its ability to support mineralization and vasculogenesis, its bioactivity potential andin vivodegradability. The biopaper 'Gel5wt% BG1wt%-slow freezing-CL160 °C 24 h' was selected as the best candidate, due to its suitable properties including high porosity (91.69 ± 1.55%), swelling ratio (91.61 ± 0.60%), Young modulus (3.97 × 104± 0.97 × 104Pa) but also its great cytocompatibility, osteogenesis and bioactivity properties. The preorganization of human umbilical vein endothelial cell using LAB onto this new biopaper led to the formation of microvascular networks. This biopaper was also shown to be compatible with 3D-molding and 3D-stacking strategies. This work allowed the development of a novel biopaper adapted to LAB with great potential for vascularized bone biofabrication.

A Dual Reversible Cross-Linked Hydrogel with Enhanced Mechanical Property and Capable of Proangiogenic and Osteogenic Activities for Bone Defect Repair

The clinical treatment of bone defects presents ongoing challenges. One promising approach is bone tissue engineering (BTE), wherein hydrogels have garnered significant attention. However, the application of hydrogels in BTE is severely limited due to their poor mechanical properties, as well as their inferior proangiogenic and osteogenic activities. To address these limitations, our develop a dual cross-linked alendronate (ALN)-Ca2+ /Mg2+ -doped sulfated hyaluronic acid (SHA@CM) hydrogel, using a one-step mixing injection molding method known as "three-in-one" approach. This approach enabled the simultaneous formation of Schiff-Base crosslinking and electric attraction-based crosslinking within the hydrogel. The Schiff-Base crosslinking contributed to the majority of the hydrogel's mechanical strength, while the electric attraction-based crosslinking served as a release reservoir for Ca2+ /Mg2+ and ALN, promoting enhanced osteogenic activities and providing additional mechanical reinforcement to the hydrogel. These experimental data demonstrates several favorable properties of the SHA@CM hydrogel, including satisfactory injectability, rapid gelation, self-healing capacity, and excellent cytocompatibility. Moreover, the presence of sulfated groups and Mg2+ within the SHA@CM hydrogel exhibited pro-angiogenic effects, while the controlled release of nanoparticles formed by Ca2+ /Mg2+ and ALN further enhanced the osteogenesis of the hydrogel. Overall, these results indicate that the SHA@CM hydrogel holds significant potential for the clinical translation of BTE.

Bone Tissue Engineering Scaffolds: Materials and Methods

The wide development in biomedical, regenerative medicine, and surgical techniques has ensured that new technologies are developed to improve patient-specific treatment and care. Tissue engineering is a special field in biomedical engineering that works toward cell development using scaffolds. Bone tissue engineering is a separate branch of tissue engineering, in which the construction of bone, functionalities of bone, and bone tissue regeneration are studied in detail to repair or regenerate new functional bone tissues. In India alone, people suffering from bone diseases are extensive in numbers. Almost 15% to 20% of the population suffers from osteoporosis. Bone scaffolds are proving to be an excellent solution for osseous abnormalities or defect treatment. Scaffolds are three dimensional (3D) and mostly porous structures created to enhance new tissue growth. Bone scaffolds are specially designed to promote osteoinductive cell growth, expansion, and migration on their surface. This review article aims to provide an overview of possible bone scaffolding materials in practice, different 3D techniques to fabricate these scaffolds, and effective bone scaffold characteristics targeted by researchers to fabricate tissue-engineered bone scaffolds.

Comparison of the ability of exosomes and ectosomes derived from adipose-derived stromal cells to promote cartilage regeneration in a rat osteochondral defect model

BACKGROUND

Extracellular vesicles (EVs) derived from mesenchymal stromal cells (MSCs) offer promising prospects for stimulating cartilage regeneration. The different formation mechanisms suggest that exosomes and ectosomes possess different biological functions. However, little attention has been paid to the differential effects of EV subsets on cartilage regeneration.

METHODS

Our study compared the effects of the two EVs isolated from adipose-derived MSCs (ASCs) on chondrocytes and bone marrow-derived MSCs (BMSCs) in vitro. Additionally, we loaded the two EVs into type I collagen hydrogels to optimize their application for the treatment of osteochondral defects in vivo.

RESULTS

In vitro experiments demonstrate that ASC-derived exosomes (ASC-Exos) significantly promoted the proliferation and migration of both cells more effectively than ASC-derived ectosomes (ASC-Ectos). Furthermore, ASC-Exos facilitated a stronger differentiation of BMSCs into chondrogenic cells than ASC-Ectos, but both inhibited chondrocyte apoptosis to a similar extent. In the osteochondral defect model of rats, ASC-Exos promoted cartilage regeneration in situ better than ASC-Ectos. At 8 weeks, the hydrogel containing exosomes group (Gel + Exo group) had higher macroscopic and histological scores, a higher value of trabecular bone volume fraction (BV/TV), a lower value of trabecular thickness (Tb.Sp), and a better remodeling of extracellular matrix than the hydrogel containing ectosomes group (Gel + Ecto group). At 4 and 8 weeks, the expression of CD206 and Arginase-1 in the Gel + Exo group was significantly higher than that in the Gel + Ecto group.

CONCLUSION

Our findings indicate that administering ASC-Exos may be a more effective EV strategy for cartilage regeneration than the administration of ASC-Ectos.

Engineering Innervated Musculoskeletal Tissues for Regenerative Orthopedics and Disease Modeling

Musculoskeletal (MSK) disorders significantly burden patients and society, resulting in high healthcare costs and productivity loss. These disorders are the leading cause of physical disability, and their prevalence is expected to increase as sedentary lifestyles become common and the global population of the elderly increases. Proper innervation is critical to maintaining MSK function, and nerve damage or dysfunction underlies various MSK disorders, underscoring the potential of restoring nerve function in MSK disorder treatment. However, most MSK tissue engineering strategies have overlooked the significance of innervation. This review first expounds upon innervation in the MSK system and its importance in maintaining MSK homeostasis and functions. This will be followed by strategies for engineering MSK tissues that induce post-implantation in situ innervation or are pre-innervated. Subsequently, research progress in modeling MSK disorders using innervated MSK organoids and organs-on-chips (OoCs) is analyzed. Finally, the future development of engineering innervated MSK tissues to treat MSK disorders and recapitulate disease mechanisms is discussed. This review provides valuable insights into the underlying principles, engineering methods, and applications of innervated MSK tissues, paving the way for the development of targeted, efficacious therapies for various MSK conditions.

Rheological insights on Carboxymethyl cellulose hydrogels

Hydrogels are copiously studied for tissue engineering, drug delivery, and bone regeneration owing to their water content, mechanical strength, and elastic behaviour. The preparation of stable and mechanically strengthened hydrogels without using toxic crosslinkers and expensive approaches is immensely challenging. In this study, we prepared Carboxymethyl cellulose based hydrogels with different polymer concentration via a less expensive physical crosslinking approach without using any toxic crosslinkers and evaluated their mechanical strength. In this hydrogel system, the carbopol concentration was fixed at 1 wt/v% and the Carboxymethyl cellulose concentration was varied between 1 and 5 wt/v%. In this hydrogel system, Carbopol serves as the crosslinker to bridge Carboxymethyl cellulose polymer through hydrogen bonds. Rheological analysis was employed in assessing the mechanical properties of the prepared hydrogel, in particular, the viscoelastic behaviour of the hydrogels. The viscoelastic nature and mechanical strength of the hydrogels increased with an increase in the Carboxymethyl cellulose polymer concentration. Further, our results suggested that gels with Carboxymethyl cellulose concentration between 3 wt/v % and 4 wt/v % with yield stresses of 58.83 Pa and 81.47 Pa, respectively, are potential candidates for use in transdermal drug delivery. The prepared hydrogels possessed high thermal stability and retained their gel network structure even at 50 °C. These findings are beneficial for biomedical applications in transdermal drug delivery and tissue engineering owing to the biocompatibility, stability, and mechanical strength of the prepared hydrogels.

Hyaluronic acid hydrogel for controlled release of heterobifunctional photocleavable linker-modified epidermal growth factor in wound healing

Peptide and protein drugs, such as epidermal growth factor (EGF), face challenges related to stability and bioavailability. Recently, hydrogels have emerged as promising carriers for these drugs. This study focuses on a light-responsive hydrogel-based drug delivery system for the controlled release of EGF in wound healing. A photocleavable (PC) linker was designed to bind EGF to the hydrogel matrix, enabling UV light-triggered release of EGF. Hydrogels have evolved from drug reservoirs to controlled release systems, and the o-nitrobenzyl-based PC linkers offer selective cleavage under UV irradiation. We used a thiol-ene crosslinked hyaluronic acid (HA) hydrogel matrix modified with the PC-linked EGF. The release of EGF from the HA hydrogel under UV irradiation was evaluated, along with in vitro and in vivo experiments to assess the controlled effect of EGF on wound healing. Our results indicate that the successful development of a light-responsive hydrogel-based system for precise temporal release of EGF enhances the therapeutic potential in wound healing. This study highlights the importance of incorporating stimulus-responsive functionalities into hydrogel-based drug delivery systems to optimize protein drugs in clinical applications.

Photo-cross-linked Hydrogels for Cartilage and Osteochondral Repair

Photo-cross-linked hydrogels, which respond to light and induce structural or morphological transitions, form a microenvironment that mimics the extracellular matrix of native tissue. In the last decades, photo-cross-linked hydrogels have been widely used in cartilage and osteochondral tissue engineering due to their good biocompatibility, ease of fabrication, rapid in situ gel-forming ability, and tunable mechanical and degradable properties. In this review, we systemically summarize the different types and physicochemical properties of photo-cross-linked hydrogels (including the materials and photoinitiators) and explore the biological properties modulated through the incorporation of additives, including cells, biomolecules, genes, and nanomaterials, into photo-cross-linked hydrogels. Subsequently, we compile the applications of photo-cross-linked hydrogels with a specific focus on cartilage and osteochondral repair. Finally, current limitations and future perspectives of photo-cross-linked hydrogels are also discussed.

Modified-Hydroxyapatite-Chitosan Hybrid Composite Interfacial Coating on 3D Polymeric Scaffolds for Bone Tissue Engineering

Three dimensional (3D) scaffolds have huge limitations due to their low porosity, mechanical strength, and lack of direct cell-bioactive drug contact. Whereas bisphosphonate drug has the ability to stimulate osteogenesis in osteoblasts and bone marrow mesenchymal stem cells (hMSC) which attracted its therapeutic use. However it is hard administration low bioavailability, and lack of site-specificity, limiting its usage. The proposed scaffold architecture allows cells to access the bioactive surface at their apex by interacting at the scaffold's interfacial layer. The interface of 3D polycaprolactone (PCL) scaffolds has been coated with alendronate-modified hydroxyapatite (MALD) enclosed in a chitosan matrix, to mimic the native environment and stupulate the through interaction of cells to bioactive layer. Where the mechanical strength will be provided by the skeleton of PCL. In the MALD composite's hydroxyapatite (HAP) component will govern alendronate (ALD) release behavior, and HAP presence will drive the increase in local calcium ion concentration increases hMSC proliferation and differentiation. In results, MALD show release of 86.28 ± 0.22. XPS and SEM investigation of the scaffold structure, shows inspiring particle deposition with chitosan over the interface. All scaffolds enhanced cell adhesion, proliferation, and osteocyte differentiation for over a week without in vitro cell toxicity with 3.03 ± 0.2 kPa mechanical strength.

Nanocore-Shell Bone Filler Contained Mesoporous Silica Modified with Hydroxyapatite Precursors; Wrapped in a Natural Metal-Phenolic Network

Various therapeutic strategies have been developed to address bone diseases caused by aging society and skeletal defects caused by trauma or accidental events. One such approach is using bone fillers, such as hydroxyapatite (HA) and bioactive glasses. Although they have provided effective osteogenesis, infection and inflammation due to the surgical procedure and uncontrolled ion release can hinder the efficiency of bone regeneration. In response to these challenges, immobilizing a neutral metal-phenolic network on the surface of osteoconductive nanoparticles would be the master key to achieving a gradual, controlled release during the mineralization period and reducing infection and inflammation through biological pathways. In this regard, a mesoporous silica nanocomposite modified by an HA precursor was synthesized to enhance bone regeneration. In addition, to improve the therapeutic effects, its surface was wrapped with a magnesium-phenolic network made from pomegranate extract, which can simultaneously produce anti-inflammatory and antibacterial effects. The obtained core-shell nanocomposite was characterized by its physicochemical properties, biocompatibility, and bioactivity. The in vitro studies revealed that the synthesized nanocomposite exhibits higher osteogenic activity than the control groups, as confirmed by alizarin red staining. Moreover, the nanocomposite maintained low toxicity as measured by the 3-(4,5-dimethylthiazol-2-yl)-5-(3-carboxymethoxyphenyl)-2-(4-sulfophenyl)-2H-tetrazolium (MTS) assay and increased antibacterial activity against Staphylococcus aureus and Escherichia coli compared with the control groups. Therefore, this research presents a promising strategy for bone regeneration, combining the advantages of mesoporous silica nanocomposite modified by an HA precursor with the beneficial effects of a magnesium-phenolic network.

Nano-residronate loaded κ-carrageenan-based injectable hydrogels for bone tissue regeneration

Bone tissue possesses intrinsic regenerative capabilities to address deformities; however, its ability to repair defects caused by severe fractures, tumor resections, osteoporosis, joint arthroplasties, and surgical reconsiderations can be hindered. To address this limitation, bone tissue engineering has emerged as a promising approach for bone repair and regeneration, particularly for large-scale bone defects. In this study, an injectable hydrogel based on kappa-carrageenan-co-N-isopropyl acrylamide (κC-co-NIPAAM) was synthesized using free radical polymerization and the antisolvent evaporation technique. The κC-co-NIPAAM hydrogel's cross-linked structure was confirmed using Fourier transform infrared spectra (FTIR) and nuclear magnetic resonance (1H NMR). The hydrogel's thermal stability and morphological behavior were assessed using thermogravimetric analysis (TGA) and scanning electron microscopy (SEM), respectively. Swelling and in vitro drug release studies were conducted at varying pH and temperatures, with minimal swelling and release observed at low pH (1.2) and 25 °C, while maximum swelling and release occurred at pH 7.4 and 37oC. Cytocompatibility analysis revealed that the κC-co-NIPAAM hydrogels were biocompatible, and hematoxylin and eosin (H&E) staining demonstrated their potential for tissue regeneration and enhanced bone repair compared to other experimental groups. Notably, digital x-ray examination using an in vivo bone defect model showed that the κC-co-NIPAAM hydrogel significantly improved bone regeneration, making it a promising candidate for bone defects.

Biomechanical Stimulation of Muscle Constructs Influences Phenotype of Bone Constructs by Modulating Myokine Secretion

Diabetes is a chronic metabolic disorder that can lead to diabetic myopathy and bone diseases. The etiology of musculoskeletal complications in such metabolic disorders and the interplay between the muscular and osseous systems are not well understood. Exercise training promises to prevent diabetic myopathy and bone disease and offer protection. Although the muscle-bone interaction is largely biomechanical, the muscle secretome has significant implications for bone biology. Uncoupling effects of biophysical and biochemical stimuli on the adaptive response of bone during exercise training may offer therapeutic targets for diabetic bone disease. Here, we have developed an in vitro model to elucidate the effects of mechanical strain on myokine secretion and its impact on bone metabolism decoupled from physical stimuli. We developed bone constructs using cross-linked gelatin, which facilitated osteogenic differentiation of osteoprogenitor cells. Then muscle constructs were made from fibrin, which enabled myoblast differentiation and myotube formation. We investigated the myokine expression by muscle constructs under strain regimens replicating endurance (END) and high-intensity interval training (HIIT) in hyperglycemic conditions. In monocultures, both regimens induced higher expression of Il15 and Igf1, whereas END supported more myoblast differentiation and myotube maturation than HIIT. When co-cultured with bone constructs, HIIT regimen increased Glut4 expression in muscle constructs more than END, supporting higher glucose uptake. Likewise, the muscle constructs under the HIIT regimen promoted a healthier and more matured bone phenotype than END. Under static conditions, myostatin (Mstn) expression was significantly downregulated in muscle constructs co-cultured with bone constructs compared with monocultures. Together, our in vitro co-culture system allowed orthogonal manipulation of mechanical strain on muscle constructs while facilitating bone-muscle biochemical cross-talk. Such systems can provide an individualized microenvironment that allows decoupled biomechanical manipulation, help identify molecular targets, and develop engineered therapies for metabolic bone disease. © 2023 The Authors. JBMR Plus published by Wiley Periodicals LLC. on behalf of American Society for Bone and Mineral Research.

Current application and modification strategy of marine polysaccharides in tissue regeneration: A review

Marine polysaccharides (MPs) are exceptional bioactive materials that possess unique biochemical mechanisms and pharmacological stability, making them ideal for various tissue engineering applications. Certain MPs, including agarose, alginate, carrageenan, chitosan, and glucan have been successfully employed as biological scaffolds in animal studies. As carriers of signaling molecules, scaffolds can enhance the adhesion, growth, and differentiation of somatic cells, thereby significantly improving the tissue regeneration process. However, the biological benefits of pure MPs composite scaffold are limited. Therefore, physical, chemical, enzyme modification and other methods are employed to expand its efficacy. Chemically, the structural properties of MPs scaffolds can be altered through modifications to functional groups or molecular weight reduction, thereby enhancing their biological activities. Physically, MPs hydrogels and sponges emulate the natural extracellular matrix, creating a more conducive environment for tissue repair. The porosity and high permeability of MPs membranes and nanomaterials expedite wound healing. This review explores the distinctive properties and applications of select MPs in tissue regeneration, highlighting their structural versatility and biological applicability. Additionally, we provide a brief overview of common modification strategies employed for MP scaffolds. In conclusion, MPs have significant potential and are expected to be a novel regenerative material for tissue engineering.

Photodynamic Therapy Minimally Affects HEMA-DMAEMA Hydrogel Viscoelasticity

Soft matter implants are a rapidly growing field in medicine for reconstructive surgery, aesthetic treatments, and regenerative medicine. Though these procedures are efficacious, all implants carry risks associated with microbial infection which are often aggressive. Preventative and responsive measures exist but are limited in applicability to soft materials. Photodynamic therapy (PDT) presents a means to perform safe and effective antimicrobial treatments in proximity to soft implants. HEMA-DMAEMA hydrogels are prepared with the photosensitizer methylene blue included at 10 and 100 µM in solution used for swelling over 2 or 4 days. Thirty minutes or 5 h of LED illumination at9.20 m W c m 2 $9.20\frac{{mW}}{{c{m}^2}}$ is then used for PDT-induced generation of reactive oxygen species in direct contact with hydrogels to test viable limits of treatment. Frequency sweep rheological measurements reveal minimal overall changes in terms of loss modulus and loss factor but a statistically significant drop in storage modulus for some PDT doses, though within the range of controls and biological variation. These mild impacts suggest the feasibility of PDT application for infection clearing in proximity to soft implants. Future investigation with additional hydrogel varieties and current implant models will further detail the safety of PDT in implant applications.

Biocomposite-based strategies for dental bone regeneration

OBJECTIVE

Because of the anatomical complexity of the oral and maxillofacial sites, repairing bone defects in these regions is very difficult. This review article aims to consider the application of biocomposites-based strategies for dental bone regeneration.

STUDY DESIGN

Research papers related to the topic, published over the last 20 years, were selected using the Web of Science, Pubmed, Scopus, and Google Scholar databases.

RESULTS

The strategies of monophasic, biphasic/multiphasic scaffolds, and biopolymer-based nanocomposite scaffolds containing nanomaterials compared with traditional methods used for bone regeneration, such as autografts, allografts, xenografts, and alloplasts are found to be superior because of their ability to overcome the issues (e.g., limited bone sources, pain, immune responses, high cost) related to the applications of the traditional methods.

CONCLUSIONS

In addition, additive manufacturing technologies were found to be highly advantageous for improving the efficacy of biocomposite scaffolds for treating dental bone defects.

Bioinspired mineralization of engineered living materials to promote osteogenic differentiation

In this work, Engineered Living Materials (ELMs), based on the combination of genetically-modified bacteria and mineral-reinforced organic matrices, and endowed with self-healing or regenerative properties and adaptation to specific biological environments were developed. Concretely, we produced ELMs combining human mesenchymal stem cells (hMSCs) and Lactococcus lactis (L. lactis), which was specifically programmed to deliver bone morphogenetic protein (BMP-2) upon external stimulation using nisin, into mineralized alginate matrices. The hybrid organic/inorganic matrix was built through a protocol, inspired by bone mineralization, in which alginate (Alg) assembly and apatite (HA) mineralization occurred simultaneously driven by calcium ions. Chemical composition, structure and reologhical properties of the hybrid 3D matrices were dedicately optimized prior the incorportation of the living entities. Then, the same protocol was reproduced in the presence of hMSC and engineered L. lactis that secrete BMP-2 resulting in 3D hybrid living hydrogels. hMSC viability and osteogenic differentiation in the absence and presence of the bacteria were evaluated by live/dead and quantitative real-time polymerase chain reaction (qPCR) and immunofluorescence assays, respectively. Results demonstrate that these 3D engineered living material support osteogenic differentiation of hMSCs due to the synergistic effect between HA and the growth factors BMP-2 delivered by L. lactis.

Hydrogels as Scaffolds in Bone-Related Tissue Engineering and Regeneration

Several years have passed since the medical and scientific communities leaned toward tissue engineering as the most promising field to aid bone diseases and defects resulting from degenerative conditions or trauma. Owing to their histocompatibility and non-immunogenicity, bone grafts, precisely autografts, have long been the gold standard in bone tissue therapies. However, due to issues associated with grafting, especially the surgical risks and soaring prices of the procedures, alternatives are being extensively sought and researched. Fibrous and non-fibrous materials, synthetic substitutes, or cell-based products are just a few examples of research directions explored as potential solutions. A very promising subgroup of these replacements involves hydrogels. Biomaterials resembling the bone extracellular matrix and therefore acting as 3D scaffolds, providing the appropriate mechanical support and basis for cell growth and tissue regeneration. Additional possibility of using various stimuli in the form of growth factors, cells, etc., within the hydrogel structure, extends their use as bioactive agent delivery platforms and acts in favor of their further directed development. The aim of this review is to bring the reader closer to the fascinating subject of hydrogel scaffolds and present the potential of these materials, applied in bone and cartilage tissue engineering and regeneration.

Bioinspired self-healing injectable nanocomposite hydrogels based on oxidized dextran and gelatin for growth-factor-free bone regeneration

Hydrogels with great biocompatibility, biodegradability, and mechanical properties, combined with osteoconductivity, osteoinductivity, and osteointegration as biomaterials for bone regeneration without adding exogenous growth factors and cells are highly appealing but challenging. Here, inspired by organic-inorganic analogues of natural bone tissue and the adhesion chemistry of mussels, nanocomposite hydrogels with self-healing, injectable, adhesive, antioxidant, and osteoinductive properties (termed GO-PHA-CPs) were constructed by Schiff base cross-linking between dopamine-modified gelatin (Gel-DA) and oxidized dextran (ODex). Furthermore, the hydrogel network was enhanced by the introduction of polydopamine-functionalized nanohydroxyapatite (PHA) by improving the interfacial compatibility between the rigid inorganic particles and the flexible hydrogel matrix. Bioactive cod peptides (CPs) with osteogenic activity from Atlantic cod were further incorporated into the nanocomposite hydrogel. As a result, the multicomponent nanocomposite hydrogel favored the adhesion and spreading of MC3T3-E1 cells. The increased ALP activity suggested that GO-PHA-CPs hydrogels contributed to the osteogenic differentiation of MC3T3-E1 cells. The suitability of GO-PHA-CPs hydrogels for enhancing bone regeneration in vivo was further confirmed by the rat femoral defect model. Our results indicate that the multifunctional GO-PHA-CPs nanocomposite hydrogels without growth factors are a promising and effective candidate material for bone regeneration.

Rational Design of Bioactive Materials for Bone Hemostasis and Defect Repair

Everyday unnatural events such as trauma, accidents, military conflict, disasters, and even medical malpractice create open wounds and massive blood loss, which can be life-threatening. Fractures and large bone defects are among the most common types of injuries. Traditional treatment methods usually involve rapid hemostasis and wound closure, which are convenient and fast but may result in various complications such as nerve injury, deep infection, vascular injury, and deep hematomas. To address these complications, various studies have been conducted on new materials that can be degraded in the body and reduce inflammation and abscesses in the surgical area. This review presents the latest research progress in biomaterials for bone hemostasis and repair. The mechanisms of bone hemostasis and bone healing are first introduced and then principles for rational design of biomaterials are summarized. After providing representative examples of hemostatic biomaterials for bone repair, future challenges and opportunities in the field are proposed.

Dual-crosslinked in-situ forming alginate/silk fibroin hydrogel with potential for bone tissue engineering

This study aimed to improve the mechanical and biological properties of alginate-based hydrogels. For this purpose, in-situ forming hydrogels were prepared by dual crosslinking of Alginate (Alg)/Oxidized Alginate (OAlg)/Silk Fibroin (SF) through simultaneous ionic gelation using CaCO3-GDL and Schiff-base reaction. The resulting hydrogels were characterized by FTIR, SEM, compressive modulus, and rheological tests. Compared to the physically-crosslinked alginate hydrogel, the compressive modulus of dual-crosslinked Alg/OAlg/SF hydrogel increased from 28 to 67 kPa, due to the covalent imine bond formation. Then, MTT and DAPI staining assays were performed to demonstrate the biocompatibility of hydrogel. Furthermore, the differentiation potential of bone marrow mesenchymal stem cells encapsulated in hydrogel scaffolds to bone tissue was tested by ALP activity, Alizarin Red staining, and real-time PCR. The overall results showed the potential of Alginate/Oxidized Alginate/Silk Fibroin hydrogel scaffold for bone tissue engineering applications.

Whey Protein Isolate/Calcium Silicate Hydrogels for Bone Tissue Engineering Applications-Preliminary In Vitro Evaluation

Whey protein isolate (WPI) hydrogels are attractive biomaterials for application in bone repair and regeneration. However, their main limitation is low mechanical strength. Therefore, to improve these properties, the incorporation of ceramic phases into hydrogel matrices is currently being performed. In this study, novel whey protein isolate/calcium silicate (WPI/CaSiO3) hydrogel biomaterials were prepared with varying concentrations of a ceramic phase (CaSiO3). The aim of this study was to investigate the effect of the introduction of CaSiO3 to a WPI hydrogel matrix on its physicochemical, mechanical, and biological properties. Our Fourier Transform Infrared Spectroscopy results showed that CaSiO3 was successfully incorporated into the WPI hydrogel matrix to create composite biomaterials. Swelling tests indicated that the addition of 5% (w/v) CaSiO3 caused greater swelling compared to biomaterials without CaSiO3 and ultimate compressive strength and strain at break. Cell culture experiments demonstrated that WPI hydrogel biomaterials enriched with CaSiO3 demonstrated superior cytocompatibility in vitro compared to the control hydrogel biomaterials without CaSiO3. Thus, this study revealed that the addition of CaSiO3 to WPI-based hydrogel biomaterials renders them more promising for bone tissue engineering applications.

Novel scaffold platforms for simultaneous induction osteogenesis and angiogenesis in bone tissue engineering: a cutting-edge approach

Despite the recent advances in the development of bone graft substitutes, treatment of critical size bone defects continues to be a significant challenge, especially in the elderly population. A current approach to overcome this challenge involves the creation of bone-mimicking scaffolds that can simultaneously promote osteogenesis and angiogenesis. In this context, incorporating multiple bioactive agents like growth factors, genes, and small molecules into these scaffolds has emerged as a promising strategy. To incorporate such agents, researchers have developed scaffolds incorporating nanoparticles, including nanoparticulate carriers, inorganic nanoparticles, and exosomes. Current paper provides a summary of the latest advancements in using various bioactive agents, drugs, and cells to synergistically promote osteogenesis and angiogenesis in bone-mimetic scaffolds. It also discusses scaffold design properties aimed at maximizing the synergistic effects of osteogenesis and angiogenesis, various innovative fabrication strategies, and ongoing clinical studies.

Immune homeostasis modulation by hydrogel-guided delivery systems: a tool for accelerated bone regeneration

Immune homeostasis is delicately mediated by the dynamic balance between effector immune cells and regulatory immune cells. Local deviations from immune homeostasis in the microenvironment of bone fractures, caused by an increased ratio of effector to regulatory cues, can lead to excessive inflammatory conditions and hinder bone regeneration. Therefore, achieving effective and localized immunomodulation of bone fractures is crucial for successful bone regeneration. Recent research has focused on developing localized and specific immunomodulatory strategies using local hydrogel-based delivery systems. In this review, we aim to emphasize the significant role of immune homeostasis in bone regeneration, explore local hydrogel-based delivery systems, discuss emerging trends in immunomodulation for enhancing bone regeneration, and address the limitations of current delivery strategies along with the challenges of clinical translation.

Injectable pH and Thermo-Responsive Hydrogel Scaffold with Enhanced Osteogenic Differentiation of Preosteoblasts for Bone Regeneration

Bone fractures are common in the geriatric population and pose a great economic burden worldwide. While traditional methods for repairing bone defects have primarily been autografts, there are several drawbacks limiting its use. Bone graft substitutes have been used as alternative strategies to improve bone healing. However, there remain several impediments to achieving the desired healing outcomes. Injectable hydrogels have become attractive scaffold materials for bone regeneration, given their high performance in filling irregularly sized bone defects and their ability to encapsulate cells and bioactive molecules and mimic the native ECM of bone. We investigated the use of an injectable chitosan-based hydrogel scaffold to promote the differentiation of preosteoblasts in vitro. The hydrogels were characterized by evaluating cell homogeneity, cell viability, rheological and mechanical properties, and differentiation ability of preosteoblasts in hydrogel scaffolds. Cell-laden hydrogel scaffolds exhibited shear thinning behavior and the ability to maintain shape fidelity after injection. The CNC-CS hydrogels exhibited higher mechanical strength and significantly upregulated the osteogenic activity and differentiation of preosteoblasts, as shown by ALP activity assays and histological analysis of hydrogel scaffolds. These results suggest that this injectable hydrogel is suitable for cell survival, can promote osteogenic differentiation of preosteoblasts, and structurally support new bone growth.

An engineered lamellar bone mimicking full-scale hierarchical architecture for bone regeneration

Lamellar bone, compactly and ingeniously organized in the hierarchical pattern with 6 ordered scales, is the structural motif of mature bone. Each hierarchical scale exerts an essential role in determining physiological behavior and osteogenic bioactivity of bone. Engineering lamellar bone with full-scale hierarchy remains a longstanding challenge. Herein, using bioskiving and mineralization, we attempt to engineer compact constructs resembling full-scale hierarchy of lamellar bone. Through systematically investigating the effect of mineralization on physicochemical properties and bioactivities of multi-sheeted collagen matrix fabricated by bioskiving, the hierarchical mimicry and hierarchy-property relationship are elucidated. With prolongation of mineralization, hierarchical mimicry and osteogenic bioactivity of constructs are performed in a bidirectional manner, i.e. first rising and then descending, which is supposed to be related with transformation of mineralization mechanism from nonclassical to classical crystallization. Construct mineralized 9 days can accurately mimic each hierarchical scale and efficiently promote osteogenesis. Bioinformatic analysis further reveals that this construct potently activates integrin α5-PI3K/AKT signaling pathway through mechanical and biophysical cues, and thereby repairing critical-sized bone defect. The present study provides a bioinspired strategy for completely resembling complex hierarchy of compact mineralized tissue, and offers a critical research model for in-depth studying the structure-function relationship of bone.

3D and 4D printing hydroxyapatite-based scaffolds for bone tissue engineering and regeneration

The osseous tissue can be classified as a nanocomposite that encompasses a complex interweaving of organic and inorganic matrices. This intricate amalgamation consists of a collagen component and a mineral phase that are intricately arranged to form elaborate and perforated configurations. Hydroxyapatite, whether synthesized artificially or obtained from natural sources, has garnered considerable attention as a composite material in the field of bone tissue engineering due to its striking resemblance to bone in terms of structure and characteristics. Hydroxyapatite (HA) constitutes the predominant ceramic biomaterial for biomedical applications due to its ability to replicate the mineral composition of vertebrate bone. Nonetheless, it is noteworthy that the present biomimetic substance exhibits unfavorable mechanical characteristics, characterized by insufficient tensile and compressive strength, thus rendering it unsuitable for effective employment in the field of bone tissue engineering. Due to its beneficial attributes, hydroxyapatite (HA) is frequently employed in conjunction with various polymers and crosslinkers as composites to enhance mechanical properties and overall efficacy of implantable biomaterials engineered. The restoration of skeletal defects through the use of customized replacements is an effective way to replace damaged or lost bone structures. This method not only restores the bones' original functions but also reinstates their initial aesthetic appearance. The utilization of hydroxyapatite-polymer composites within 3D-printed grafts necessitates meticulous optimization of both mechanical and biological properties, in order to ensure their suitability for employment in medical devices. The utilization of 3D-printing technology represents an innovative approach in the manufacturing of HA-based scaffolds, which offers advantageous prospects for personalized bone regeneration. The expeditious prototyping method, with emphasis on the application of 3D printing, presents a viable approach in the development of bespoke prosthetic implants, grounded on healthcare data sets. 4D printing approach is an evolved form of 3D printing that utilizes programmable materials capable of altering the intended shape of printed structures, contingent upon single or dual stimulating factors. These factors include aspects such as pH level, temperature, humidity, crosslinking degree, and leaching factors.

Injectable hydrogels for cartilage and bone tissue regeneration: A review

Annually, millions of patients suffer from irreversible injury owing to the loss or failure of an organ or tissue caused by accident, aging, or disease. The combination of injectable hydrogels and the science of stem cells have emerged to address this persistent issue in society by generating minimally invasive treatments to augment tissue function. Hydrogels are composed of a cross-linked network of polymers that exhibit a high-water retention capacity, thereby mimicking the wet environment of native cells. Due to their inherent mechanical softness, hydrogels can be used as needle-injectable stem cell carrier materials to mend tissue defects. Hydrogels are made of different natural or synthetic polymers, displaying a broad portfolio of eligible properties, which include biocompatibility, low cytotoxicity, shear-thinning properties as well as tunable biological and physicochemical properties. Presently, novel ongoing developments and native-like hydrogels are increasingly being used broadly to improve the quality of life of those with disabling tissue-related diseases. The present review outlines various future and in-vitro applications of injectable hydrogel-based biomaterials, focusing on the newest ongoing developments of in-situ forming injectable hydrogels for bone and cartilage tissue engineering purposes.

Advances and Prospects in Materials for Craniofacial Bone Reconstruction

The craniofacial region is composed of 23 bones, which provide crucial function in keeping the normal position of brain and eyeballs, aesthetics of the craniofacial complex, facial movements, and visual function. Given the complex geometry and architecture, craniofacial bone defects not only affect the normal craniofacial structure but also may result in severe craniofacial dysfunction. Therefore, the exploration of rapid, precise, and effective reconstruction of craniofacial bone defects is urgent. Recently, developments in advanced bone tissue engineering bring new hope for the ideal reconstruction of the craniofacial bone defects. This report, presenting a first-time comprehensive review of recent advances of biomaterials in craniofacial bone tissue engineering, overviews the modification of traditional biomaterials and development of advanced biomaterials applying to craniofacial reconstruction. Challenges and perspectives of biomaterial development in craniofacial fields are discussed in the end.

Biofabrication of functional bone tissue: defining tissue-engineered scaffolds from nature

Damage to bone leads to pain and loss of movement in the musculoskeletal system. Although bone can regenerate, sometimes it is damaged beyond its innate capacity. Research interest is increasingly turning to tissue engineering (TE) processes to provide a clinical solution for bone defects. Despite the increasing biomimicry of tissue-engineered scaffolds, significant gaps remain in creating the complex bone substitutes, which include the biochemical and physical conditions required to recapitulate bone cells' natural growth, differentiation and maturation. Combining advanced biomaterials with new additive manufacturing technologies allows the development of 3D tissue, capable of forming cell aggregates and organoids based on natural and stimulated cues. Here, we provide an overview of the structure and mechanical properties of natural bone, the role of bone cells, the remodelling process, cytokines and signalling pathways, causes of bone defects and typical treatments and new TE strategies. We highlight processes of selecting biomaterials, cells and growth factors. Finally, we discuss innovative tissue-engineered models that have physiological and anatomical relevance for cancer treatments, injectable stimuli gels, and other therapeutic drug delivery systems. We also review current challenges and prospects of bone TE. Overall, this review serves as guide to understand and develop better tissue-engineered bone designs.

Recent Advances in Bioengineering Bone Revascularization Based on Composite Materials Comprising Hydroxyapatite

The natural healing process of bone is impaired in the presence of tumors, trauma, or inflammation, necessitating external assistance for bone regeneration. The limitations of autologous/allogeneic bone grafting are still being discovered as research progresses. Bone tissue engineering (BTE) is now a crucial component of treating bone injuries and actively works to promote vascularization, a crucial stage in bone repair. A biomaterial with hydroxyapatite (HA), which resembles the mineral makeup of invertebrate bones and teeth, has demonstrated high osteoconductivity, bioactivity, and biocompatibility. However, due to its brittleness and porosity, which restrict its application, scientists have been prompted to explore ways to improve its properties by mixing it with other materials, modifying its structural composition, improving fabrication techniques and growth factor loading, and co-cultivating bone regrowth cells to stimulate vascularization. This review scrutinizes the latest five-year research on HA composite studies aimed at amplifying vascularization in bone regeneration.

Fabrication and characterisation of bioglass and hydroxyapatite-filled scaffolds

Tissue engineering is a continuously evolving field. One of the main lines of research in this field focuses on the replacement of bone defects with materials designed to interact with the cells of a living organism in order to provide the body with a structure on which new tissues can easily grow. Among the most commonly used materials are bioglasses, which are frequently used due to their versatility and good properties. This article discusses the results of the production of an injectable paste of Bioglass® 45S5 and hydroxyapatite on a 3D printed porous structure by additive manufacturing, using a thermoplastic (PLA). The results were evaluated in a specific application of the paste, so the mechanical and bioactive properties were studied to show the multiple possibilities of using this combination for its application in regenerative medicine and more specifically in bone implants.

3D Bioprinting in Otolaryngology: A Review

The evolution of tissue engineering and 3D bioprinting has allowed for increased opportunities to generate musculoskeletal tissue grafts that can enhance functional and aesthetic outcomes in otolaryngology-head and neck surgery. Despite literature reporting successes in the fabrication of cartilage and bone scaffolds for applications in the head and neck, the full potential of this technology has yet to be realized. Otolaryngology as a field has always been at the forefront of new advancements and technology and is well poised to spearhead clinical application of these engineered tissues. In this review, current 3D bioprinting methods are described and an overview of potential cell types, bioinks, and bioactive factors available for musculoskeletal engineering using this technology is presented. The otologic, nasal, tracheal, and craniofacial bone applications of 3D bioprinting with a focus on engineered graft implantation in animal models to highlight the status of functional outcomes in vivo; a necessary step to future clinical translation are reviewed. Continued multidisciplinary efforts between material chemistry, biological sciences, and otolaryngologists will play a key role in the translation of engineered, 3D bioprinted constructs for head and neck surgery.

Research Progress of Design Drugs and Composite Biomaterials in Bone Tissue Engineering

Bone, like most organs, has the ability to heal naturally and can be repaired slowly when it is slightly injured. However, in the case of bone defects caused by diseases or large shocks, surgical intervention and treatment of bone substitutes are needed, and drugs are actively matched to promote osteogenesis or prevent infection. Oral administration or injection for systemic therapy is a common way of administration in clinic, although it is not suitable for the long treatment cycle of bone tissue, and the drugs cannot exert the greatest effect or even produce toxic and side effects. In order to solve this problem, the structure or carrier simulating natural bone tissue is constructed to control the loading or release of the preparation with osteogenic potential, thus accelerating the repair of bone defect. Bioactive materials provide potential advantages for bone tissue regeneration, such as physical support, cell coverage and growth factors. In this review, we discuss the application of bone scaffolds with different structural characteristics made of polymers, ceramics and other composite materials in bone regeneration engineering and drug release, and look forward to its prospect.

Chitosan, Gelatin, and Collagen Hydrogels for Bone Regeneration

Hydrogels are versatile biomaterials characterized by three-dimensional, cross-linked, highly hydrated polymeric networks. These polymers exhibit a great variety of biochemical and biophysical properties, which allow for the diffusion of diverse molecules, such as drugs, active ingredients, growth factors, and nanoparticles. Meanwhile, these polymers can control chemical and molecular interactions at the cellular level. The polymeric network can be molded into different structures, imitating the structural characteristics of surrounding tissues and bone defects. Interestingly, the application of hydrogels in bone tissue engineering (BTE) has been gathering significant attention due to the beneficial bone improvement results that have been achieved. Moreover, essential clinical and osteoblastic fate-controlling advances have been achieved with the use of synthetic polymers in the production of hydrogels. However, current trends look towards fabricating hydrogels from biological precursors, such as biopolymers, due to the high biocompatibility, degradability, and mechanical control that can be regulated. Therefore, this review analyzes the concept of hydrogels and the characteristics of chitosan, collagen, and gelatin as excellent candidates for fabricating BTE scaffolds. The changes and opportunities brought on by these biopolymers in bone regeneration are discussed, considering the integration, synergy, and biocompatibility features.

3D Bioprinting of Hyaline Articular Cartilage: Biopolymers, Hydrogels, and Bioinks

The musculoskeletal system, consisting of bones and cartilage of various types, muscles, ligaments, and tendons, is the basis of the human body. However, many pathological conditions caused by aging, lifestyle, disease, or trauma can damage its elements and lead to severe disfunction and significant worsening in the quality of life. Due to its structure and function, articular (hyaline) cartilage is the most susceptible to damage. Articular cartilage is a non-vascular tissue with constrained self-regeneration capabilities. Additionally, treatment methods, which have proven efficacy in stopping its degradation and promoting regeneration, still do not exist. Conservative treatment and physical therapy only relieve the symptoms associated with cartilage destruction, and traditional surgical interventions to repair defects or endoprosthetics are not without serious drawbacks. Thus, articular cartilage damage remains an urgent and actual problem requiring the development of new treatment approaches. The emergence of biofabrication technologies, including three-dimensional (3D) bioprinting, at the end of the 20th century, allowed reconstructive interventions to get a second wind. Three-dimensional bioprinting creates volume constraints that mimic the structure and function of natural tissue due to the combinations of biomaterials, living cells, and signal molecules to create. In our case-hyaline cartilage. Several approaches to articular cartilage biofabrication have been developed to date, including the promising technology of 3D bioprinting. This review represents the main achievements of such research direction and describes the technological processes and the necessary biomaterials, cell cultures, and signal molecules. Special attention is given to the basic materials for 3D bioprinting-hydrogels and bioinks, as well as the biopolymers underlying the indicated products.

Nanogels for bone tissue engineering - from synthesis to application

Nanogels are cross-linked hydrogel nanoparticles with a three-dimensional, tunable porous structure that merges the best features of hydrogels and nanoparticles, including the ability to retain their hydrated nature and to swell and shrink in response to environmental changes. Nanogels have attracted increasing attention for use in bone tissue engineering as scaffolds for growth factor transport and cell adhesion. Their three-dimensional structures allow the encapsulation of a wide range of hydrophobic and hydrophilic drugs, enhance their half-life, and impede their enzymatic breakdown in vivo. Nanogel-based scaffolds are a viable treatment modality for enhanced bone regeneration. They act as carriers for cells and active ingredients capable of controlled release, enhanced mechanical support, and osteogenesis for enhanced bone tissue regeneration. However, the development of such nanogel constructs might involve combinations of several biomaterials to fabricate active ingredients that can control release, enhance mechanical support, and facilitate osteogenesis for more effective bone tissue regeneration. Hence, this review aims to highlight the potential of nanogel-based scaffolds to address the needs of bone tissue engineering.

Dual-Crosslinking of Gelatin-Based Hydrogels: Promising Compositions for a 3D Printed Organotypic Bone Model

Gelatin-based hydrogels have emerged as a popular scaffold material for tissue engineering applications. The introduction of variable crosslinking methods has shown promise for fabricating stable cell-laden scaffolds. In this work, we examine promising composite biopolymer-based inks for extrusion-based 3D bioprinting, using a dual crosslinking approach. A combination of carefully selected printable hydrogel ink compositions and the use of photoinduced covalent and ionic crosslinking mechanisms allows for the fabrication of scaffolds of high accuracy and low cytotoxicity, resulting in unimpeded cell proliferation, extracellular matrix deposition, and mineralization. Three selected bioink compositions were characterized and the respective cell-laden scaffolds were bioprinted. Temporal stability, morphology, swelling, and mechanical properties of the scaffolds were thoroughly studied and the biocompatibility of the constructs was assessed using rat mesenchymal stem cells while focusing on osteogenesis. Experimental results showed that the composition of 1% alginate, 4% gelatin, and 5% (w/v) gelatine methacrylate, was found to be optimal among the examined, with shape fidelity of 88%, large cell spreading area and cell viability at around 100% after 14 days. The large pore diameters that exceed 100 µm, and highly interconnected scaffold morphology, make these hydrogels extremely potent in bone tissue engineering and bone organoid fabrication.

Effects of Gamma Irradiation and Supercritical Carbon Dioxide Sterilization on Methacrylated Gelatin/Hyaluronan Hydrogels

Biopolymer hydrogels have become an important group of biomaterials in experimental and clinical use. However, unlike metallic or mineral materials, they are quite sensitive to sterilization. The aim of this study was to compare the effects of gamma irradiation and supercritical carbon dioxide (scCO₂) treatment on the physicochemical properties of different hyaluronan (HA)- and/or gelatin (GEL)-based hydrogels and the cellular response of human bone marrow-derived mesenchymal stem cells (hBMSC). Hydrogels were photo-polymerized from methacrylated HA, methacrylated GEL, or a mixture of GEL/HA. The composition and sterilization methods altered the dissolution behavior of the biopolymeric hydrogels. There were no significant differences in methacrylated GEL release but increased methacrylated HA degradation of gamma-irradiated samples. Pore size/form remained unchanged, while gamma irradiation decreased the elastic modulus from about 29 kPa to 19 kPa compared to aseptic samples. HBMSC proliferated and increased alkaline phosphatase activity (ALP) particularly in aseptic and gamma-irradiated methacrylated GEL/HA hydrogels alike, while scCO₂ treatment had a negative effect on both proliferation and osteogenic differentiation. Thus, gamma-irradiated methacrylated GEL/HA hydrogels are a promising base for multi-component bone substitute materials.

3D-bioimplants mimicking the structure and function of spine units for the treatment of spinal tuberculosis

Approximately 1-2% of the reported tuberculosis (TB) cases have skeletal system problems, particularly spinal TB. The complications of spinal TB involve the destruction of vertebral body (VB) and intervertebral disc (IVD) which consequently leads to kyphosis. This work aimed at utilizing different technologies to develop, for the first time, a functional spine unit (FSU) replacement to mimic the structure and function of the VB and IVD along with a good ability to treat spinal TB. 3D-printed scaffolds with different porous patterns (hexagonal or grid) were fabricated from biocompatible acrylonitrile butadiene styrene, and polylactic acid to replace damaged VB and IVD, respectively. The VB scaffold is filled with gelatine-based semi-IPN hydrogel containing mesoporous silica nanoparticles loaded with two antibiotics, rifampicin and levofloxacin, to act against TB. The IVD scaffold incorporates a gelatin hydrogel loaded with regenerative platelet-rich plasma and anti-inflammatory simvastatin-loaded mixed nanomicelles. The obtained results confirmed the superior mechanical strength of both 3D-printed scaffolds and loaded hydrogels as compared to normal bone and IVD with high in vitro (cell proliferation, anti-inflammation and anti-TB), and in vivo biocompatibility profiles. Moreover, the custom-designed replacements have achieved the expected prolonged release of antibiotics up to 60 days. Given the promising study findings, the utilization of the developed drug-eluting scaffold system can be extrapolated to treat not only spinal TB but also to resolve diverse backbone/spine problems that need a critical surgical process including degenerative IVD and its consequences like atherosclerosis, sliding or spondylolisthesis and severe traumatic bone fracture.

Incorporation of synthetic water-soluble curcumin polymeric drug within calcium phosphate cements for bone defect repairing

Modified macroporous structures and active osteogenic substances are necessary to overcome the limited bone regeneration capacity and low degradability of self-curing calcium phosphate cement (CPC). Curcumin (CUR), which possesses strong osteogenic activity and poor aqueous solubility/bioavailability, esterifies the side chains in hyaluronic acid (HA) to form a water-soluble CUR-HA macromolecule. In this study, we incorporated the CUR-HA and glucose microparticles (GMPs) into the CPC powder to fabricate the CUR-HA/GMP/CPC composite, which not only retained the good injectability and mechanical strength of bone cements, but also significantly increased the cement porosity and sustained release property of CUR-HA in vitro. CUR-HA incorporation greatly improved the differentiation ability of bone marrow mesenchymal stem cells (BMSCs) to osteoblasts by activating the RUNX family transcription factor 2/fibroblast growth factor 18 (RUNX2/FGF18) signaling pathway, increasing the expression of osteocalcin and enhancing the alkaline phosphatase activity. In addition, in vivo implantation of CUR-HA/GMP/CPC into femoral condyle defects dramatically accelerated the degradation rate of cement and boosted local vascularization and osteopontin protein expression, and consequently promoted rapid bone regeneration. Therefore, macroporous CPC based composite cement with CUR-HA shows a remarkable ability to repair bone defects and is a promising translational application of modified CPC in clinical practice.

Multifunctional dendrimer@nanoceria engineered GelMA hydrogel accelerates bone regeneration through orchestrated cellular responses

Bone defects in patients entail the microenvironment that needs to boost the functions of stem cells (e.g., proliferation, migration, and differentiation) while alleviating severe inflammation induced by high oxidative stress. Biomaterials can help to shift the microenvironment by regulating these multiple events. Here we report multifunctional composite hydrogels composed of photo-responsive Gelatin Methacryloyl (GelMA) and dendrimer (G3)-functionalized nanoceria (G3@nCe). Incorporation of G3@nCe into GelMA could enhance the mechanical properties of hydrogels and their enzymatic ability to clear reactive oxygen species (ROS). The G3@nCe/GelMA hydrogels supported the focal adhesion of mesenchymal stem cells (MSCs) and further increased their proliferation and migration ability (vs. pristine GelMA and nCe/GelMA). Moreover, the osteogenic differentiation of MSCs was significantly stimulated upon the G3@nCe/GelMA hydrogels. Importantly, the capacity of G3@nCe/GelMA hydrogels to scavenge extracellular ROS enabled MSCs to survive against H₂O₂-induced high oxidative stress. Transcriptome analysis by RNA sequencing identified the genes upregulated and the signalling pathways activated by G3@nCe/GelMA that are associated with cell growth, migration, osteogenesis, and ROS-metabolic process. When implanted subcutaneously, the hydrogels exhibited excellent tissue integration with a sign of material degradation while the inflammatory response was minimal. Furthermore, G3@nCe/GelMA hydrogels demonstrated effective bone regeneration capacity in a rat critical-sized bone defect model, possibly due to an orchestrated capacity of enhancing cell proliferation, motility and osteogenesis while alleviating oxidative stress.

Copper-containing chitosan-based hydrogels enabled 3D-printed scaffolds to accelerate bone repair and eliminate MRSA-related infection

Bone defect combined with drug-resistant bacteria-related infection is a thorny challenge in clinic. Herein, 3D-printed polyhydroxyalkanoates/β-tricalcium phosphate (PHA/β-TCP, PT) scaffolds were prepared by fused deposition modeling. Then copper-containing carboxymethyl chitosan/alginate (CA/Cu) hydrogels were integrated with the scaffolds via a facile and low-cost chemical crosslinking method. The resultant PT/CA/Cu scaffolds could not only promote proliferation but also osteogenic differentiation of preosteoblasts in vitro. Moreover, PT/CA/Cu scaffolds exhibited a strong antibacterial activity towards a broad-spectrum of bacteria including methicillin-resistant Staphylococcus aureus (MRSA) through inducing the intercellular generation of reactive oxygen species. In vivo experiments further demonstrated that PT/CA/Cu scaffolds significantly accelerated bone repair of cranial defects and efficiently eliminated MRSA-related infection, showing potential for application in infected bone defect therapy.

Gelatin hydrogel reinforced with mussel-inspired polydopamine-functionalized nanohydroxyapatite for bone regeneration

Developing high-strength hydrogels with biocompatibility and bone conductibility is still desirable for bone regeneration. The nanohydroxyapatite (nHA) was incorporated into a dopamine-modified gelatin (Gel-DA) hydrogel system to create a highly biomimetic native bone tissue microenvironment. In addition, to further increase the cross-linking density between nHA and Gel-DA, nHA was functionalized by mussel-inspired polydopamine (PDA). Compared with nHA, adding polydopamine functionalized nHA (PHA) increased the compressive strength of Gel-Da hydrogel from 449.54 ± 180.32 kPa to 611.18 ± 211.86 kPa without affecting its microstructure. Besides, the gelation time of Gel-DA hydrogels with PHA incorporation (GD-PHA) was controllable from 49.47 ± 7.93 to 88.11 ± 31.18 s, contributing to its injectable ability in clinical applications. In addition, the abundant phenolic hydroxyl group of PHA was beneficial to the cell adhesion and proliferation of Gel-DA hydrogels, leading to the excellent biocompatibility of Gel-PHA hydrogels. Notably, the GD-PHA hydrogels could accelerate the bone repair efficiency in the rat model of the femoral defect. In conclusion, our results suggest the Gel-PHA hydrogel with osteoconductivity, biocompatibility, and enhanced mechanical properties is a potential bone repair material.

Preparation of gum tragacanth/poly (vinyl alcohol)/halloysite hydrogel using electron beam irradiation with potential for bone tissue engineering

Nanocomposite hydrogels based on tyramine conjugated gum tragacanth, poly (vinyl alcohol) (PVA), and halloysite nanotubes (HNTs) were prepared by electron beam irradiation and characterized. The FTIR, ¹H NMR, and TGA results confirmed the chemical incorporation of HNTs into gum tragacanth. Gel content and swelling of hydrogels decreased with HNTs loading up to 20 % wt. The mechanical strength of hydrogels increased by increasing HNTs content up to 10 % with 371 kPa fracture stress at 0.95 fracture strain, compared to 312 kPa stress at 0.79 strain for gum tragacanth/PVA hydrogel. Hydrogel's biocompatibility and osteogenic activity were tested by seeding rabbit bone marrow mesenchymal stem cells. The cell viability was >85 % after 7 days of culture. In vitro secretion of ALP and calcium deposition on hydrogels in alizarin red assay after 21 days of culture indicated hydrogel potential for bone tissue engineering.