Mineralization of hydrogels for bone regeneration

An Elastin-Derived Composite Matrix for Enhanced Vascularized and Innervated Bone Tissue Reconstruction: From Material Development to Preclinical Evaluation

Despite progress in bone tissue engineering, reconstruction of large bone defects remains an important clinical challenge. Here, a biomaterial designed to recruit bone cells, endothelial cells, and neuronal fibers within the same matrix is developed, enabling bone tissue regeneration. The bioactive matrix is based on modified elastin-like polypeptides (ELPs) grafted with laminin-derived adhesion peptides IKVAV and YIGSR, and the SNA15 peptide for retention of hydroxyapatite (HA) particles. The composite matrix shows suitable porosity, interconnectivity, biocompatibility for endothelial cells, and the ability to support neurites outgrowth by sensory neurons. Subcutaneous implantation leads to the formation of osteoid tissue, characterized by the presence of bone cells, vascular networks, and neuronal structures, while minimizing inflammation. Using a rat femoral condyle defect model, longitudinal micro-CT analysis is performed, which demonstrates a significant increase in the volume of mineralized tissue when using the ELP-based matrix compared to empty defects and a commercially available control (Collapat). Furthermore, visible blood vessel networks and nerve fibers are observed within the lesions after a period of two weeks. By incorporating multiple key components that support cell growth, mineralization, and tissue integration, this ELP-based composite matrix provides a holistic and versatile solution to enhance bone tissue regeneration.

Injectable Hydrogels: A Paradigm Tailored with Design, Characterization, and Multifaceted Approaches

Biomaterials denoting self-healing and versatile structural integrity are highly curious in the biomedicine segment. The injectable and/or printable 3D printing technology is explored in a few decades back, which can alter their dimensions temporarily under shear stress, showing potential healing/recovery tendency with patient-specific intervention toward the development of personalized medicine. Thus, self-healing injectable hydrogels (IHs) are stunning toward developing a paradigm for tissue regeneration. This review comprises the designing of IHs, rheological characterization and stability, several benchmark consequences for self-healing IHs, their translation into tissue regeneration of specific types, applications of IHs in biomedical such as anticancer and immunomodulation, wound healing and tissue/bone regeneration, antimicrobial potentials, drugs, gene and vaccine delivery, ocular delivery, 3D printing, cosmeceuticals, and photothermal therapy as well as in other allied avenues like agriculture, aerospace, electronic/electrical industries, coating approaches, patents associated with therapeutic/nontherapeutic avenues, and numerous futuristic challenges and solutions.

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.

Multifunctional hydrogel/platelet-rich fibrin/nanofibers scaffolds with cell barrier and osteogenesis for guided tissue regeneration/guided bone regeneration applications

Periodontal defect seriously affects people's life health and quality. Guided tissue regeneration (GTR) and guided bone regeneration (GBR) have made great progress in periodontal disease treatment, but some deficiencies existed in commercial materials of GTR and GBR. For obtaining better therapeutic effects, multifunctional composite scaffolds containing different biological macromolecules were developed in this study. Chitosan/poly (γ-glutamic acid)/nano-hydroxyapatite hydrogels (CP/nHA) made by electrostatic interactions and lyophilization were filled in the bone defects to achieve osteogenesis. Platelet-rich fibrin (PRF) extracted from blood could accelerate bone formation by releasing various bioactive substances as middle layer of composite scaffolds. Polycaprolactone/gelatin nanofibers (PG) prepared by electrospinning were attached to the junction of soft and hard tissue, which could prevent fibrous tissue from infiltrating into bone defects. The composite scaffolds showed good morphology, biocompatibility, cell barriers and osteogenic differentiation in vitro. The excellent ability of bone formation was verified by implantation of triple-layered composite scaffolds into alveolar bone defects in rabbit in vivo. The hierarchical structure was conducive to personalized customization to meet the needs of different defects. All in all, the multifunctional scaffolds could play important roles of GTR and GBR in alveolar bone regeneration and provide good application prospect for bone repair in clinic.

Tri-Component Hydrogel as Template for Nanocrystalline Hydroxyapatite Deposition Using Alternate Soaking Method for Bone Tissue Engineering Applications

Composite hydrogels containing apatite-like particles can act as scaffolds for osteoblast proliferation, with applications in bone tissue engineering. In this respect, porous biocompatible hydrogels were obtained from chitosan, oxidized pullulan, and PVA in different ratios. The stability of the hydrogels was ensured both by covalent bonds between aldehyde groups of oxidized pullulan and free amino groups of chitosan, and by physical bonds formed during freeze-thaw cycles and lyophilization. The deposition of calcium phosphates was performed by alternate soaking of the porous hydrogels into solutions with calcium and phosphate ions, assuring a basic pH required for hydroxyapatite formation. The mineralized hydrogels were characterized using FTIR spectroscopy, scanning electron microscopy, X-ray diffraction, and thermogravimetric analysis, showing that inorganic particles containing between 80 and 92% hydroxyapatite were deposited in a high amount on the pore walls of the polymeric matrix. The composition of the organic matrix influenced the crystallization of calcium phosphates and the mechanical properties of the composite hydrogels. In vitro biological tests showed that mineralized hydrogels support the proliferation of MG-63 osteoblast-like cells to a greater extent compared to pristine hydrogels.

Thiol-Functionalized, Antioxidant, and Osteogenic Mesoporous Silica Nanoparticles for Osteoporosis

Osteoporosis is a chronic bone disorder characterized by decreased bone mass, leading to brittle bones and fractures. Oxidative stress has been identified as the most profound trigger for the initiation and progression of osteoporosis. Current treatment strategies do not induce new bone formation and fail to address a high level of reactive oxygen species (ROS). Mesoporous silica nanoparticles (MSNs) have been explored in bone tissue regeneration owing to their inherent osteogenic property, but they lack antioxidant and cell adhesion properties, required in such applications. We have developed thiolated, bioactive mesoporous silica nanoparticles (MSN-SH) to address this challenge. MSNs were fabricated using the Stöber method, and 11% of the surface was functionalized post-synthesis with thiol groups using MPTMS to obtain MSN-SH. The particle size measured by the dynamic light scattering technique was found to be around 300 nm. The surface morphology was investigated using HR-TEM, and their physical and chemical properties were characterized using various spectroscopic techniques. They exhibited more than 90% antioxidant activity, neutralized ROS formed in cells, and provided protection against ROS-induced cell damage. The cell viability assay in murine osteoblast precursor cells (MC3T3) showed that MSN-SH is cell-proliferative in nature with 140% cell viability. Osteogenic potential was evaluated by measuring the ALP activities, calcium deposition, and gene expression levels of osteogenic markers, such as RUNX2, ALP, OCN, and OPN, and results revealed that MSN-SH increases calcium deposition and induces osteogenesis through upregulation of osteogenic genes and markers without the involvement of any osteogenic supplements. Besides promoting osteogenesis, MSN-SH was found to inhibit osteoclastogenesis. The nanomaterial was found to be regenerative in nature, and it stimulated migration of osteoblast cells and caused a complete wound closure within 48 h. We were able to achieve a multifunctional nanomaterial by simply modifying the surface. MSNs have been explored for bone tissue engineering/osteoporosis as a composite system incorporating metals, like gold and cerium, or as a nanocarrier loaded with growth factors or active drugs. This study offers a simple and economical method to enhance the existing properties of MSNs and impart new activities by a single-step surface modification. It can be concluded that MSN-SH holds promise as a complementary and alternate treatment for osteoporosis along with the standardized therapy.

DNA hydrogels for bone regeneration

DNA-based biomaterials have been proposed for tissue engineering approaches due to their predictable assembly into complex morphologies and ease of functionalization. For bone tissue regeneration, the ability to bind Ca2+ and promote hydroxyapatite (HAP) growth along the DNA backbone combined with their degradation and release of extracellular phosphate, a known promoter of osteogenic differentiation, make DNA-based biomaterials unlike other currently used materials. However, their use as biodegradable scaffolds for bone repair remains scarce. Here, we describe the design and synthesis of DNA hydrogels, gels composed of DNA that swell in water, their interactions in vitro with the osteogenic cell lines MC3T3-E1 and mouse calvarial osteoblast, and their promotion of new bone formation in rat calvarial wounds. We found that DNA hydrogels can be readily synthesized at room temperature, and they promote HAP growth in vitro, as characterized by Fourier transform infrared spectroscopy, X-ray diffraction, scanning electron microscopy, atomic force microscopy, and transmission electron microscopy. Osteogenic cells remain viable when seeded on DNA hydrogels in vitro, as characterized by fluorescence microscopy. In vivo, DNA hydrogels promote the formation of new bone in rat calvarial critical size defects, as characterized by micro-computed tomography and histology. This study uses DNA hydrogels as a potential therapeutic biomaterial for regenerating lost bone.

Biodegradable silk fibroin scaffold doped with mineralized collagen induces bone regeneration in rat cranial defects

Compared with most nondegradable or slowly degradable bone repair materials, bioactive biodegradable porous scaffolds with certain mechanical strengths can promote the regeneration of both new bone and vasculature while the cavity created by their degradation can be replaced by the infiltration of new bone tissue. Mineralized collagen (MC) is the basic structural unit of bone tissue, and silk fibroin (SF) is a natural polymer with adjustable degradation rates and superior mechanical properties. In this study, a three-dimensional porous biomimetic composite scaffold with a two-component SF-MC system was constructed based on the advantages of both materials. The spherical mineral agglomerates of the MC were uniformly distributed on the surface and inside the SF skeleton, which ensured good mechanical properties while regulating the degradation rate of the scaffold. Second, the SF-MC scaffold had good osteogenic induction of bone marrow mesenchymal stem cells (BMSCs) and preosteoblasts (MC3T3-E1) and also promoted the proliferation of MC3T3-E1 cells. Finally, in vivo 5 mm cranial defect repair experiments confirmed that the SF-MC scaffold stimulated vascular regeneration and promoted new bone regeneration in vivo by means of in situ regeneration. Overall, we believe that this low-cost biomimetic biodegradable SF-MC scaffold with many advantages has some clinical translation prospects.

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

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

3D bioprinting of human mesenchymal stem cells-laden hydrogels incorporating MXene for spontaneous osteodifferentiation

Contemporary advances in three-dimensional (3D) bioprinting technologies have enabled the fabrication of tailored live 3D tissue mimetics. Furthermore, the development of advanced bioink materials has been highlighted to accurately reproduce the composition of a native extracellular matrix and mimic the intrinsic properties of laden cells. Recent research has shown that MXene is one of promising nanobiomaterials with osteogenic activity for bone grafts and scaffolds due to its unique atomic structure of three titanium layers between two carbon layers. In this study, the MXene-incorporated gelatin methacryloyl (GelMA) and hyaluronic acid methacryloyl (HAMA) (i.e., GelMA/HAMA-MXene) bioinks were prepared to explore if they have the potential to enable the spontaneous osteodifferentiation of human mesenchymal stem cells (hMSCs) when the hMSCs-laden GelMA/HAMA-MXene bioinks were 3D printed. The physicochemical and rheological characteristics of the GelMA/HAMA-MXene hydrogels were proven to be unprecedentedly favorable supportive matrices suited for the growth and survival of hMSCs. Furthermore, hMSCs were shown to spontaneously differentiate into osteoblasts within GelMA-HAMA/MXene composites to provide favorable microenvironments for osteogenesis. Therefore, our results suggest that the remarkable biofunctional advantages of the MXene-incorporated GelMA/HAMA bioink can be utilized in a wide range of strategies for the development of effective scaffolds in bone tissue regeneration.

Self-healing hybrid hydrogels with sustained bioactive components release for guided bone regeneration

Guided bone regeneration (GBR) is widely used in treating oral bone defects to exclude the influence of non-osteogenic tissue on the bone healing process. The traditional method of GBR with a titanium mesh to treat large-area bone defects is limited by the deficiency of increased trauma and costs to patients. Herein, a bi-layered scaffold for GBR composed of a fiber barrier layer and a self-healing hydrogel repair layer is successfully fabricated. The barrier layer is a fibrous membrane material with specific porosity constructed by electrospinning, while the functional layer is a self-healing hydrogel material formed by multiple dynamic covalent bonds. The system can provide an osteogenic microenvironment by preventing the infiltration of connective tissue to bone defects, maintain the stability of the osteogenic space through the self-healing property, and regulate the release of bioactive substances in the dynamic physical condition, which is beneficial to osteoblast proliferation, differentiation, and bone regeneration. This study focused on exploring the effects of different crosslinkers and bonding methods on the comprehensive properties of hydrogels. and proved that the hybrid scaffold system has good biocompatibility, cell barrier function and can enhance bone regeneration activity. Thereby it could be a promising clinical strategy for bone regeneration.

Effects of Calcium Carbonate Microcapsules and Nanohydroxyapatite on Properties of Thermosensitive Chitosan/Collagen Hydrogels

Thermosensitive chitosan/collagen hydrogels are osteoconductive and injectable materials. In this study, we aimed to improve these properties by adjusting the ratio of nanohydroxyapatite particles to calcium carbonate microcapsules in a β-glycerophosphate-crosslinked chitosan/collagen hydrogel. Two hydrogel systems with 2% and 5% nanohydroxyapatite particles were studied, each of which had varying microcapsule content (i.e., 0%, 1%, 2%, and 5%). Quercetin-incorporated calcium carbonate microcapsules were prepared. Calcium carbonate microcapsules and nanohydroxyapatite particles were then added to the hydrogel according to the composition of the studied system. The properties of the hydrogels, including cytotoxicity and biocompatibility, were investigated in mice. The calcium carbonate microcapsules were 2-6 µm in size, spherical, with rough and nanoporous surfaces, and thus exhibited a burst release of impregnated quercetin. The 5% nanohydroxyapatite system is a solid particulate gel that supports homogeneous distribution of microcapsules in the three-dimensional matrix of the hydrogels. Calcium carbonate microcapsules increased the mechanical and physical strength, viscoelasticity, and physical stability of the nanohydroxyapatite hydrogels while decreasing their porosity, swelling, and degradation rates. The calcium carbonate microcapsules-nanohydroxyapatite hydrogels were noncytotoxic and biocompatible. The properties of the hydrogel can be tailored by adjusting the ratio of calcium carbonate microcapsules to the nanohydroxyapatite particles. The 1% calcium carbonate microcapsules containing 5% nanohydroxyapatite particle-chitosan/collagen hydrogel exhibited mechanical and physical strength, permeability, and prolonged release profiles of quercetin, which were superior to those of the other studied systems and were optimal for promoting bone regeneration and delivering natural flavonoids.

Synthesis and characterization of growth factor free nanoengineered bioactive scaffolds for bone tissue engineering

BACKGROUND

To address the obstacles that come with orthopedic surgery for biological graft tissues, including immune rejections, bacterial infections, and weak osseointegration, bioactive nanocomposites have been used as an alternative for bone grafting since they can mimic the biological and mechanical properties of the native bone. Among them, PCL-PEG-PCL (PCEC) copolymer has gained much attention for bone tissue engineering as a result of its biocompatibility and ability for osteogenesis.

METHODS

Here, we designed a growth factor-free nanoengineered scaffold based on the incorporation of Fe₃O₄ and hydroxyapatite (HA) nanoparticles into the PCL-PEG-PCL/Gelatin (PCEC/Gel) nanocomposite. We characterized different formulations of nanocomposite scaffolds in terms of physicochemical properties. Also, the mechanical property and specific surface area of the prepared scaffolds, as well as their feasibility for human dental pulp stem cells (hDPSCs) adhesion were assessed.

RESULTS

The results of in vitro cell culture study revealed that the PCEC/Gel Fe₃O₄&HA scaffold could promote osteogenesis in comparison with the bare scaffold, which confirmed the positive effect of the Fe₃O₄ and HA nanoparticles in the osteogenic differentiation of hDPSCs.

CONCLUSION

The incorporation of Fe₃O₄ and HA with PCEC/gelatin could enhance osteogenic differentiation of hDPSCs for possible substitution of bone grafting tissue.

Self-Healing Injectable Hydrogels for Tissue Regeneration

Biomaterials with the ability to self-heal and recover their structural integrity offer many advantages for applications in biomedicine. The past decade has witnessed the rapid emergence of a new class of self-healing biomaterials commonly termed injectable, or printable in the context of 3D printing. These self-healing injectable biomaterials, mostly hydrogels and other soft condensed matter based on reversible chemistry, are able to temporarily fluidize under shear stress and subsequently recover their original mechanical properties. Self-healing injectable hydrogels offer distinct advantages compared to traditional biomaterials. Most notably, they can be administered in a locally targeted and minimally invasive manner through a narrow syringe without the need for invasive surgery. Their moldability allows for a patient-specific intervention and shows great prospects for personalized medicine. Injected hydrogels can facilitate tissue regeneration in multiple ways owing to their viscoelastic and diffusive nature, ranging from simple mechanical support, spatiotemporally controlled delivery of cells or therapeutics, to local recruitment and modulation of host cells to promote tissue regeneration. Consequently, self-healing injectable hydrogels have been at the forefront of many cutting-edge tissue regeneration strategies. This study provides a critical review of the current state of self-healing injectable hydrogels for tissue regeneration. As key challenges toward further maturation of this exciting research field, we identify (i) the trade-off between the self-healing and injectability of hydrogels vs their physical stability, (ii) the lack of consensus on rheological characterization and quantitative benchmarks for self-healing injectable hydrogels, particularly regarding the capillary flow in syringes, and (iii) practical limitations regarding translation toward therapeutically effective formulations for regeneration of specific tissues. Hence, here we (i) review chemical and physical design strategies for self-healing injectable hydrogels, (ii) provide a practical guide for their rheological analysis, and (iii) showcase their applicability for regeneration of various tissues and 3D printing of complex tissues and organoids.

Biomimetic biphasic curdlan-based scaffold for osteochondral tissue engineering applications - Characterization and preliminary evaluation of mesenchymal stem cell response in vitro

Osteochondral defects remain a huge problem in medicine today. Biomimetic bi- or multi-phasic scaffolds constitute a very promising alternative to osteochondral autografts and allografts. In this study, a new curdlan-based scaffold was designed for osteochondral tissue engineering applications. To achieve biomimetic properties, it was enriched with a protein component - whey protein isolate as well as a ceramic ingredient - hydroxyapatite granules. The scaffold was fabricated via a simple and cost-efficient method, which represents a significant advantage. Importantly, this technique allowed generation of a scaffold with two distinct, but integrated phases. Scanning electron microcopy and optical profilometry observations demonstrated that phases of biomaterial possessed different structural properties. The top layer of the biomaterial (mimicking the cartilage) was smoother than the bottom one (mimicking the subchondral bone), which is beneficial from a biological point of view because unlike bone, cartilage is a smooth tissue. Moreover, mechanical testing showed that the top layer of the biomaterial had mechanical properties close to those of natural cartilage. Although the mechanical properties of the bottom layer of scaffold were lower than those of the subchondral bone, it was still higher than in many analogous systems. Most importantly, cell culture experiments indicated that the biomaterial possessed high cytocompatibility towards adipose tissue-derived mesenchymal stem cells and bone marrow-derived mesenchymal stem cells in vitro. Both phases of the scaffold enhanced cell adhesion, proliferation, and chondrogenic differentiation of stem cells (revealing its chondroinductive properties in vitro) as well as osteogenic differentiation of these cells (revealing its osteoinductive properties in vitro). Given all features of the novel curdlan-based scaffold, it is worth noting that it may be considered as promising candidate for osteochondral tissue engineering applications.

Magnesium Oxide Nanoparticle Coordinated Phosphate-Functionalized Chitosan Injectable Hydrogel for Osteogenesis and Angiogenesis in Bone Regeneration

Natural polysaccharide (NPH)-based injectable hydrogels have shown great potential for critical-sized bone defect repair. However, their osteogenic, angiogenic, and mechanical properties are insufficient. Here, MgO nanoparticles (NPs) were incorporated into a newly synthesized water-soluble phosphocreatine-functionalized chitosan (CSMP) water solution to form an injectable hydrogel (CSMP-MgO) via supramolecular combination between phosphate groups in CSMP and magnesium in MgO NPs to circumvent these drawbacks of chitosan-based injectable hydrogels. Water-soluble chitosan deviate CSMP was first synthesized by grafting methacrylic anhydride and phosphocreatine into a chitosan chain in a one-step lyophilization process. The phosphocreatine in this hydrogel not only provides sites to combine with MgO NPs to form supramolecular binding but also serves as the reservoir to control Mg²⁺ release. As a result, the lyophilized CSMP-MgO hydrogels presented a porous structure with some small holes in the pore wall, and the pore diameters ranged from 50 to 100 μm. The CSMP-MgO injectable hydrogels were restricted from swelling in DI water (lowest swelling ratio was 16.0 ± 1.1 g/g) and presented no brittle failure during compression even at a strain above 85% (maximum compressive strength was 195.0 kPa) versus the control groups (28.0 and 41.3 kPa for CSMP and CSMP-MgO (0.5) hydrogels), with regulated Mg²⁺ release in a stable and sustained manner. The CSMP-MgO injectable hydrogels promoted in vitro calcium phosphate (hydroxyapatite (HA) and tetracalcium phosphate (TTCP)) deposition in supersaturated calcium phosphate solution and presented no cytotoxicity to MC3T3-E1 cells; the CSMP-MgO hydrogel promoted MC3T3-E1 cell osteogenic differentiation with upregulation of BSP, OPN, and Osterix osteogenic gene expression and mineralization and HUVEC tube formation. Among them, CSMP-MgO (5) presented most of these properties. Moreover, this hydrogel (CSMP-MgO (5)) showed an excellent ability to promote new bone formation in critical-sized calvarial defects in rats. Thus, the CSMP-MgO injectable hydrogel shows great promise for bone regeneration.

Influence of Biomimetically Mineralized Collagen Scaffolds on Bone Cell Proliferation and Immune Activation

Collagen, as the main component of connective tissue, is frequently used in various tissue engineering applications. In this study, porous sponge-like collagen scaffolds were prepared by freeze-drying and were then mineralized in a simulated body fluid. The mechanical stability was similar in both types of scaffolds, but the mineralized scaffolds (MCS) contained significantly more calcium, magnesium and phosphorus than the unmineralized scaffolds (UCS). Although the MCS contained a lower percentage (~32.5%) of pores suitable for cell ingrowth (113–357 μm in diameter) than the UCS (~70%), the number of human-osteoblast-like MG-63 cells on days 1, 3 and 7 after seeding was higher on MCS than on UCS, and the cells penetrated deeper into the MCS. The cell growth in extracts prepared by eluting the scaffolds for 7 days in a cell culture medium was also markedly higher in the MCS extracts, as indicated by real-time monitoring in the sensory xCELLigence system for 7 days. From this point of view, MCS are more promising for bone tissue engineering than UCS. However, MCS evoked a more pronounced inflammatory response than UCS, as indicated by the production of tumor necrosis factor-alpha (TNF-α) in macrophage-like RAW 264.7 cells in cultures on these scaffolds.

Multifunctional GelMA platforms with nanomaterials for advanced tissue therapeutics

Polymeric hydrogels are fascinating platforms as 3D scaffolds for tissue repair and delivery systems of therapeutic molecules and cells. Among others, methacrylated gelatin (GelMA) has become a representative hydrogel formulation, finding various biomedical applications. Recent efforts on GelMA-based hydrogels have been devoted to combining them with bioactive and functional nanomaterials, aiming to provide enhanced physicochemical and biological properties to GelMA. The benefits of this approach are multiple: i) reinforcing mechanical properties, ii) modulating viscoelastic property to allow 3D printability of bio-inks, iii) rendering electrical/magnetic property to produce electro-/magneto-active hydrogels for the repair of specific tissues (e.g., muscle, nerve), iv) providing stimuli-responsiveness to actively deliver therapeutic molecules, and v) endowing therapeutic capacity in tissue repair process (e.g., antioxidant effects). The nanomaterial-combined GelMA systems have shown significantly enhanced and extraordinary behaviors in various tissues (bone, skin, cardiac, and nerve) that are rarely observable with GelMA. Here we systematically review these recent efforts in nanomaterials-combined GelMA hydrogels that are considered as next-generation multifunctional platforms for tissue therapeutics. The approaches used in GelMA can also apply to other existing polymeric hydrogel systems.

Hydroxyapatite-decorated Fmoc-hydrogel as a bone-mimicking substrate for osteoclast differentiation and culture

Hydrogels are water-swollen networks with great potential for tissue engineering applications. However, their use in bone regeneration is often hampered due to a lack of materials' mineralization and poor mechanical properties. Moreover, most studies are focused on osteoblasts (OBs) for bone formation, while osteoclasts (OCs), cells involved in bone resorption, are often overlooked. Yet, the role of OCs is pivotal for bone homeostasis and aberrant OC activity has been reported in several pathological diseases, such as osteoporosis and bone cancer. For these reasons, the aim of this work is to develop customised, reinforced hydrogels to be used as material platform to study cell function, cell-material interactions and ultimately to provide a substrate for OC differentiation and culture. Here, Fmoc-based RGD-functionalised peptide hydrogels have been modified with hydroxyapatite nanopowder (Hap) as nanofiller, to create nanocomposite hydrogels. Atomic force microscopy showed that Hap nanoparticles decorate the peptide nanofibres with a repeating pattern, resulting in stiffer hydrogels with improved mechanical properties compared to Hap- and RGD-free controls. Furthermore, these nanocomposites supported adhesion of Raw 264.7 macrophages and their differentiation in 2D to mature OCs, as defined by the adoption of a typical OC morphology (presence of an actin ring, multinucleation, and ruffled plasma membrane). Finally, after 7 days of culture OCs showed an increased expression of TRAP, a typical OC differentiation marker. Collectively, the results suggest that the Hap/Fmoc-RGD hydrogel has a potential for bone tissue engineering, as a 2D model to study impairment or upregulation of OC differentiation. STATEMENT OF SIGNIFICANCE: Altered osteoclasts (OC) function is one of the major cause of bone fracture in the most commonly skeletal disorders (e.g. osteoporosis). Peptide hydrogels can be used as a platform to mimic the bone microenvironment and provide a tool to assess OC differentiation and function. Moreover, hydrogels can incorporate different nanofillers to yield hybrid biomaterials with enhanced mechanical properties and improved cytocompatibility. Herein, Fmoc-based RGD-functionalised peptide hydrogels were decorated with hydroxyapatite (Hap) nanoparticles to generate a hydrogel with improved rheological properties. Furthermore, they are able to support osteoclastogenesis of Raw264.7 cells in vitro as confirmed by morphology changes and expression of OC-markers. Therefore, this Hap-decorated hydrogel can be used as a template to successfully differentiate OC and potentially study OC dysfunction.

The influence of Ca/Mg ratio on autogelation of hydrogel biomaterials with bioceramic compounds

Hydrogels, which are versatile three-dimensional structures containing polymers and water, are very attractive for use in biomedical fields, but they suffer from rather weak mechanical properties. In this regard, biocompatible particles can be used to enhance their mechanical properties. The possibility of loading such particles with drugs (e.g. enzymes) makes them a particularly useful component in hydrogels. In this study, micro/nanoparticles containing various ratios of Ca²⁺/Mg²⁺ with sizes ranging from 1 to 8 μm were prepared and mixed with gellan gum (GG) solution to study the in-situ formation of hydrogel-particle composites. The particles provide multiple functionalities: 1) they efficiently crosslink GG to induce hydrogel formation through the release of the divalent cations (Ca²⁺/Mg²⁺) known to bind to GG polymer chains; 2) they enhance mechanical properties of the hydrogel from 2 up to 100 kPa; 3) the samples most efficiently promoting cell growth were found to contain two types of minerals: vaterite and hydroxymagnesite, which enhanced cells proliferation and hydroxyapatite formation. The results demonstrate that such composite materials are attractive candidates for applications in bone regeneration.

3D-Printing Biodegradable PU/PAAM/Gel Hydrogel Scaffold with High Flexibility and Self-Adaptibility to Irregular Defects for Nonload-Bearing Bone Regeneration

A three-dimensional (3D) printed biodegradable hydrogel scaffold with a strong self-expanding ability to conform to the contour of irregular bone defects and be closely adjacent to host tissues is reported herein. The scaffold has a triple cross-linked network structure consisting of photo-cross-linked polyacrylamide (PAAM) and polyurethane (PU) as the primary IPN network and chemical cross-linked gelatin (Gel) as the secondary network, which confers the scaffold with good mechanical properties. The addition of PU in the polymerization process of acrylamide (AAM) can improve the ultraviolet (UV) photocuring efficiency of the hydrogel and incorporate abundant hydrogen bonds between the PAAM copolymer chain and the PU chain. The results show that the hydrogel scaffold contains regular structures with smooth morphology, excellent dimensional stability, and uniform aperture. The degradation rate of the hydrogel scaffold is controllable through adjusting cross-linking agents and can be up to about 60% after degradation for 28 days. More importantly, the rapid self-inflating characteristic of the scaffold in water, that is, the volume of hydrogel scaffold can increase to about 8 times that of their own in an hour and can generate a slight compressive stress on the surrounding host tissue, thus stimulating the reconstruction and growth of new bone tissues. The in vitro experiment indicates that the scaffold is nontoxic and biocompatible. The in vivo experiment shows that the PU/PAAM/Gel chemically cross-linked scaffold displays the desirable osteogenic capability. This UV-curable 3D printed self-adaptive and degradable hydrogel scaffold holds great potential for nonload-bearing bone repair.

Biomimetic Mineralization of Tannic Acid-Supplemented HEMA/SBMA Nanocomposite Hydrogels

This study developed a tannic acid (TA)-supplemented 2-hydroxyethyl methacrylate-co-sulfobetaine methacrylate (HEMA-co-SBMA) nanocomposite hydrogel with mineralization and antibacterial functions. Initially, hybrid hydrogels were synthesized by incorporating SBMA into the HEMA network and the influence of SBMA on the chemical structure, water content, mechanical properties, and antibacterial characteristics of the hybrid HEMA/SBMA hydrogels was examined. Then, nanoclay (Laponite XLG) was introduced into the hybrid HEMA/SBMA hydrogels and the effects evaluated of the nanoclay on the chemical structure, water content, and mechanical properties of these supplemented hydrogels. The 50/50 hybrid HEMA/SBMA hydrogel with 30 mg/mL nanoclay showed outstanding mechanical properties (3 MPa) and water content (60%) compared to pure polyHEMA hydrogels. TA then went on to be incorporated into these hybrid nanocomposite hydrogels and its effects investigated on biomimetic mineralization. Scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX) showed that bone-like spheroidal precipitates with a Ca/P ratio of 1.67% were observed after 28 days within these mineralized hydrogels. These mineralized hydrogels demonstrated an almost 1.5-fold increase in compressive moduli compared to the hydrogels without mineralization. These multifunctional hydrogels display good mechanical and biomimetic properties and may have applications in bone regeneration therapies.

Designing Hydrogel-Based Bone-On-Chips for Personalized Medicine

The recent development of bone-on-chips (BOCs) holds the main advantage of requiring a low quantity of cells and material, compared to traditional In Vitro models. By incorporating hydrogels within BOCs, the culture system moved to a three dimensional culture environment for cells which is more representative of bone tissue matrix and function. The fundamental components of hydrogel-based BOCs, namely the cellular sources, the hydrogel and the culture chamber, have been tuned to mimic the hematopoietic niche in the bone aspirate marrow, cancer bone metastasis and osteo/chondrogenic differentiation. In this review, we examine the entire process of developing hydrogel-based BOCs to model In Vitro a patient specific situation. First, we provide bone biological understanding for BOCs design and then how hydrogel structural and mechanical properties can be tuned to meet those requirements. This is followed by a review on hydrogel-based BOCs, developed in the last 10 years, in terms of culture chamber design, hydrogel and cell source used. Finally, we provide guidelines for the definition of personalized pathological and physiological bone microenvironments. This review covers the information on bone, hydrogel and BOC that are required to develop personalized therapies for bone disease, by recreating clinically relevant scenarii in miniaturized devices.

Novel composite scaffolds based on alginate and Mg-doped calcium phosphate fillers: Enhanced hydroxyapatite formation under biomimetic conditions

In the present study, we synthesized hydroxyapatite (HAP) powders followed by the production of alginate based macroporous scaffolds with the aim to imitate the natural bone structure. HAP powders were synthesized by using a hydrothermal method, and after calcination, dominant phases in the powders, undoped and doped with Mg²⁺ were HAP and β-tricalcium phosphate, respectively. Upon mixing with Na-alginate, followed by gelation and freeze-dying, highly macroporous composite scaffolds were obtained with open and connected pores and uniformly dispersed mineral phase as determined by scanning electron microscopy. Mechanical properties of the scaffolds were influenced by the composition of calcium phosphate fillers being improved as Ca²⁺ concentration increased while Mg²⁺ concentration decreased. HAP formation within all scaffolds was investigated in simulated body fluid (SBF) during 28 days under static conditions while the best candidate (Mg substituted HAP filler, precursor solution with [Ca + Mg]/P molar ratio of 1.52) was investigated under more physiological conditions in a biomimetic perfusion bioreactor. The continuous SBF flow (superficial velocity of 400 μm/s) induced the formation of abundant HAP crystals throughout the scaffolds leading to improved mechanical properties to some extent as compared to the initial scaffolds. These findings indicated potentials of novel biomimetic scaffolds for use in bone tissue engineering.

2D phosphorene nanosheets, quantum dots, nanoribbons: synthesis and biomedical applications

Phosphorene, also known as black phosphorus (BP), is a two-dimensional (2D) material that has gained significant attention in several areas of current research. Its unique properties such as outstanding surface activity, an adjustable bandgap width, favorable on/off current ratios, infrared-light responsiveness, good biocompatibility, and fast biodegradation differentiate this material from other two-dimensional materials. The application of BP in the biomedical field has been rapidly emerging over the past few years. This article aimed to provide a comprehensive review of the recent progress on the unique properties and extensive medical applications for BP in bone, nerve, skin, kidney, cancer, and biosensing related treatment. The details of applications of BP in these fields were summarized and discussed.

Bifunctional hydrogel for potential vascularized bone tissue regeneration

Most of the synthetic polymer-based hydrogels lack the intrinsic properties needed for tissue engineering applications. Here, we describe a biomimetic approach to induce the mineralization and vascularization of poly(ethylene glycol) (PEG)-based hydrogel to template the osteogenic activities. The strategy involves the covalent functionalization of oligo[poly(ethylene glycol) fumarate] (OPF) with phosphate groups and subsequent treatment of phosphorylated-OPF (Pi-OPF) hydrogels with alkaline phosphatase enzyme (ALP) and calcium. Unlike previously reported studies for ALP induced mineralization, in this study, the base polymer itself was modified with the phosphate groups for uniform mineralization of hydrogels. In addition to improvement of mechanical properties, enhancement of MC3T3-E1 cell attachment and proliferation, and promotion of mesenchymal stem cells (MSC) differentiation were observed as the intrinsic benefits of such mineralization. Current bone tissue engineering (BTE) research endeavors are also extensively focused on vascular tissue regeneration due to its inherent advantages in bone regeneration. Taking this into account, we further functionalized the mineralized hydrogels with FG-4592, small hypoxia mimicking molecule. The functionalized hydrogels elicited upregulated in vitro angiogenic activities of human umbilical vein endothelial cells (HUVEC). In addition, when implanted subcutaneously in rats, enhanced early vascularization activities around the implantation site were observed as demonstrated by the immunohistochemistry results. This further leveraged the formation of calcified tissues at the implantation site at later time points evident through X-ray imaging. The overall results here show the perspectives of bifunctional OPF hydrogels for vascularized BTE.

Engineering next-generation bioinks with nanoparticles: moving from reinforcement fillers to multifunctional nanoelements

The application of additive manufacturing in the biomedical field has become a hot topic in the last decade owing to its potential to provide personalized solutions for patients. Different bioinks have been designed trying to obtain a unique concoction that addresses all the needs for tissue engineering and drug delivery purposes, among others. Despite the remarkable progress made, the development of suitable bioinks which combine printability, cytocompatibility, and biofunctionality is still a challenge. In this sense, the well-established synthetic and functionalization routes to prepare nanoparticles with different functionalities make them excellent candidates to be combined with polymeric systems in order to generate suitable multi-functional bioinks. In this review, we briefly discuss the most recent advances in the design of functional nanocomposite hydrogels considering their already evaluated or potential use as bioinks. The scientific development over the last few years is reviewed, focusing the discussion on the wide range of functionalities that can be incorporated into 3D bioprinted constructs through the addition of multifunctional nanoparticles in order to increase their regenerative potential in the field of tissue engineering.

Rational design of hydrogels to enhance osteogenic potential

Bone tissue engineering (BTE) encompasses the field of biomaterials, cells, and bioactive molecules to successfully guide the growth and repair of bone tissue. Current BTE strategies rely on delivering osteogenic molecules or cells via scaffolding materials. However, growth factor- and stem cell-based treatments have several limitations, such as source restriction, low stability, difficulties in predicting long-term efficacy, and high costs, among others. These issues have promoted the development of material-based therapy with properties of accessibility, high stability, tunable efficacy, and low-cost production. Hydrogels are widely used in BTE applications because of their unique hydrophilic nature and tunable physicochemical properties to mimic the native bone environment. However, current hydrogel materials are not ideal candidates due to minimal osteogenic capability on their own. Therefore, recent studies of BTE hydrogels attempt to counterbalance these issues by modifying their biophysical properties. In this article, we review recent progress in the design of hydrogels to instruct osteogenic potential, and present strategies developed to precisely control its bone healing properties.

Mineralization of Biomaterials for Bone Tissue Engineering

Mineralized biomaterials have been demonstrated to enhance bone regeneration compared to their non-mineralized analogs. As non-mineralized scaffolds do not perform as well as mineralized scaffolds in terms of their mechanical and surface properties, osteoconductivity and osteoinductivity, mineralization strategies are promising methods in the development of functional biomimetic bone scaffolds. In particular, the mineralization of three-dimensional (3D) scaffolds has become a promising approach for guided bone regeneration. In this paper, we review the major approaches used for mineralizing tissue engineering constructs. The resulting scaffolds provide minerals chemically similar to the inorganic component of natural bone, carbonated apatite, Ca5(PO4,CO3)3(OH). In addition, we discuss the characterization techniques that are used to characterize the mineralized scaffolds, such as the degree of mineralization, surface characteristics, mechanical properties of the scaffolds, and the chemical composition of the deposited minerals. In vitro cell culture studies show that the mineralized scaffolds are highly osteoinductive. We also summarize, based on literature examples, the applications of 3D mineralized constructs, as well as the rationale behind their use. The mineralized scaffolds have improved bone regeneration in animal models due to the enhanced mechanical properties and cell recruitment capability making them a preferable option for bone tissue engineering over non-mineralized scaffolds.

Effects of beta-tricalcium phosphate nanoparticles on the properties of a thermosensitive chitosan/collagen hydrogel and controlled release of quercetin

In the present study, an inorganic matrix of beta-tricalcium phosphate (bTCP) nanoparticles and quercetin was incorporated into an organic matrix of 2:1 (w/w) chitosan/collagen composite to fabricate thermosensitive bTCP-chitosan/collagen-quercetin hydrogels. A sol-gel transition of the hydrogels was stimulated by beta-glycerophosphate (bGP) and temperature changes at physiological temperature and pH levels. Thereafter, the effects of 1%-3% (w/v) bTCP on properties of the bTCP-bGP-2:1 (w/w) chitosan/collagen hydrogels were investigated. Notably, the incorporation of 1%-3% (w/v) bTCP in the hydrogels did not interfere with the gelation process and time of the hydrogels at physiological temperature and pH levels. The bTCP-hydrogels exhibited a porous structure, interconnecting pore architecture, and median pore size of 100-200 μm. The incorporation of 3% bTCP increased the mechanical strength but decreased the swelling and degradation rates, pore size, permeability, and quercetin release rate of the hydrogels. The hydrogels were noncytotoxic and able to support cell encapsulation. A sustained quercetin release profile of the 3% bTCP-hydrogel further suggested the applicability of the hydrogel as a delivery vehicle of natural flavonoids for bone regeneration.

Hybrid gelatin/oxidized chondroitin sulfate hydrogels incorporating bioactive glass nanoparticles with enhanced mechanical properties, mineralization, and osteogenic differentiation

Biopolymer based hydrogels are characteristic of their biocompatibility and capability of mimicking extracellular matrix structure to support cellular behavior. However, these hydrogels suffer from low mechanical properties, uncontrolled degradation, and insufficient osteogenic activity, which limits their applications in bone regeneration. In this study, we developed hybrid gelatin (Gel)/oxidized chondroitin sulfate (OCS) hydrogels that incorporated mesoporous bioactive glass nanoparticles (MBGNs) as bioactive fillers for bone regeneration. Gel-OCS hydrogels could be self-crosslinked in situ under physiological conditions in the presence of borax. The incorporation of MBGNs enhanced the crosslinking and accelerated the gelation. The gelation time decreased with increasing the concentration of MBGNs added. Incorporation of MBGNs in the hydrogels significantly improved the mechanical properties in terms of enhanced storage modulus and compressive strength. The injectability of the hydrogels was not significantly affected by the MBGN incorporation. Also, the proliferation and osteogenic differentiation of rat bone marrow mesenchymal stem cells in vitro and rat cranial defect restoration in vivo were significantly promoted by the hydrogels in the presence of MBGNs. The hybrid Gel-OCS/MBGN hydrogels show promising potential as injectable biomaterials or scaffolds for bone regeneration/repair applications given their tunable degradation and gelation behavior as well as favorable mechanical behavior and osteogenic activities.

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In situ self-crosslinking of hybrid Gel-OCS/MBGN hydrogels.

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Hybrid hydrogels are porous, injectable and bioadhesive.

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Incorporation of MBGNs enhances mechanical and mineralization properties of hydrogels.

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Osteogenic differentiation of BMSCs enhanced after incorporating MBGNs into hydrogels.

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The presence of MBGNs enhances in vivo rat cranial defect restoration.

Consecutive dephosphorylation by alkaline phosphatase-directed in situ formation of porous hydrogels of SF with nanocrystalline calcium phosphate ceramics for bone regeneration

Alkaline phosphatase, as an enzyme involved in the process of bone mineralization and regeneration, was incorporated into a solution of SF to induce its gelation and mineralization through consecutive dephosphorylation actions on different substrates. In these processes, alkaline phosphatase firstly worked on a small peptide of NapGFFYp by removing its hydrophilic phosphate group. The resulted NapGFFY performed supramolecular assembly in the solution of SF and synergistically induced the conformation transition of SF from random coil to β-sheet structures, leading to the formation of a stable SF hydrogel under physiological conditions. And then, the entrapped ALP within the SF-NY gel network retained its catalytic activity, released phosphate ions from glycerophosphate, and catalysed the formation of calcium phosphate minerals within the porous gel. Because of the mild conditions of these processes and good biocompatibility of the scaffold, the mineralized SF gel can work as a biomimetic scaffold to promote the osteogenic differentiation of rBMSCs and stimulate femoral defect regeneration in a rat model.

Recent Trends in Three-Dimensional Bioinks Based on Alginate for Biomedical Applications

Three-dimensional (3D) bioprinting is an appealing and revolutionary manufacturing approach for the accurate placement of biologics, such as living cells and extracellular matrix (ECM) components, in the form of a 3D hierarchical structure to fabricate synthetic multicellular tissues. Many synthetic and natural polymers are applied as cell printing bioinks. One of them, alginate (Alg), is an inexpensive biomaterial that is among the most examined hydrogel materials intended for vascular, cartilage, and bone tissue printing. It has also been studied pertaining to the liver, kidney, and skin, due to its excellent cell response and flexible gelation preparation through divalent ions including calcium. Nevertheless, Alg hydrogels possess certain negative aspects, including weak mechanical characteristics, poor printability, poor structural stability, and poor cell attachment, which may restrict its usage along with the 3D printing approach to prepare artificial tissue. In this review paper, we prepare the accessible materials to be able to encourage and boost new Alg-based bioink formulations with superior characteristics for upcoming purposes in drug delivery systems. Moreover, the major outcomes are discussed, and the outstanding concerns regarding this area and the scope for upcoming examination are outlined.

Marine-Inspired Enzymatic Mineralization of Dairy-Derived Whey Protein Isolate (WPI) Hydrogels for Bone Tissue Regeneration

Whey protein isolate (WPI) is a by-product from the production of cheese and Greek yoghurt comprising β-lactoglobulin (β-lg) (75%). Hydrogels can be produced from WPI solutions through heating; hydrogels can be sterilized by autoclaving. WPI hydrogels have shown cytocompatibility and ability to enhance proliferation and osteogenic differentiation of bone-forming cells. Hence, they have promise in the area of bone tissue regeneration. In contrast to commonly used ceramic minerals for bone regeneration, a major advantage of hydrogels is the ease of their modification by incorporating biologically active substances such as enzymes. Calcium carbonate (CaCO₃) is the main inorganic component of the exoskeletons of marine invertebrates. Two polymorphs of CaCO₃, calcite and aragonite, have shown the ability to promote bone regeneration. Other authors have reported that the addition of magnesium to inorganic phases has a beneficial effect on bone-forming cell growth. In this study, we employed a biomimetic, marine-inspired approach to mineralize WPI hydrogels with an inorganic phase consisting of CaCO₃ (mainly calcite) and CaCO₃ enriched with magnesium using the calcifying enzyme urease. The novelty of this study lies in both the enzymatic mineralization of WPI hydrogels and enrichment of the mineral with magnesium. Calcium was incorporated into the mineral formed to a greater extent than magnesium. Increasing the concentration of magnesium in the mineralization medium led to a reduction in the amount and crystallinity of the mineral formed. Biological studies revealed that mineralized and unmineralized hydrogels were not cytotoxic and promoted cell viability to comparable extents (approximately 74% of standard tissue culture polystyrene). The presence of magnesium in the mineral formed had no adverse effect on cell viability. In short, WPI hydrogels, both unmineralized and mineralized with CaCO₃ and magnesium-enriched CaCO₃, show potential as biomaterials for bone regeneration.

Enhancement of Biomimetic Enzymatic Mineralization of Gellan Gum Polysaccharide Hydrogels by Plant-Derived Gallotannins

Mineralization of hydrogel biomaterials with calcium phosphate (CaP) is considered advantageous for bone regeneration. Mineralization can be both induced by the enzyme alkaline phosphatase (ALP) and promoted by calcium-binding biomolecules, such as plant-derived polyphenols. In this study, ALP-loaded gellan gum (GG) hydrogels were enriched with gallotannins, a subclass of polyphenols. Five preparations were compared, namely three tannic acids of differing molecular weight (MW), pentagalloyl glucose (PGG), and a gallotannin-rich extract from mango kernel (Mangifera indica L.). Certain gallotannin preparations promoted mineralization to a greater degree than others. The various gallotannin preparations bound differently to ALP and influenced the size of aggregates of ALP, which may be related to ability to promote mineralization. Human osteoblast-like Saos-2 cells grew in eluate from mineralized hydrogels. Gallotannin incorporation impeded cell growth on hydrogels and did not impart antibacterial activity. In conclusion, gallotannin incorporation aided mineralization but reduced cytocompatibility.

Phosphate functionalization and enzymatic calcium mineralization synergistically enhance oligo[poly(ethylene glycol) fumarate] hydrogel osteoconductivity for bone tissue engineering

A current approach in bone tissue engineering is the implantation of polymeric scaffolds that promote osteoblast attachment and growth as well as biomineralization. One promising polymer is oligo[poly(ethylene glycol) fumarate] (OPF), a polyethylene glycol-based material that is biocompatible, injectable, and biodegradable, but in its native form does not support robust bone cell attachment or growth. To address this issue, this study evaluated the osteoconductivity of bis[02-(methacryloyloxy)ethyl] phosphate (BP) functionalized OPF hydrogels (OPF-BP) using MC3T3-E1 pre-osteoblast cells, both before and after enzymatic mineralization with a calcium solution. The inclusion of negatively charged functional groups allowed for the tailored uptake and release of calcium, while also altering the mechanical properties and surface topography of the hydrogel surface. In cell culture, OPF-BP hydrogels with 20 and 30% (w/w) BP optimized osteoblast attachment, proliferation, and differentiation after a 21-day in vitro period. In addition, the OPF-BP30 treatment, when mineralized with calcium, exhibited a 128% increase in osteocalcin expression when compared with the non-mineralized treatment. These findings suggest that phosphate functionalization and enzymatic calcium mineralization can act synergistically to enhance the osteoconductivity of OPF hydrogels, making this processed material an attractive candidate for bone tissue engineering applications.

Synthesis and characterization of osteoinductive visible light-activated adhesive composites with antimicrobial properties

Orthopedic surgical procedures based on the use of conventional biological graft tissues are often associated with serious post-operative complications such as immune rejection, bacterial infection, and poor osseointegration. Bioresorbable bone graft substitutes have emerged as attractive alternatives to conventional strategies because they can mimic the composition and mechanical properties of the native bone. Among these, bioactive glasses (BGs) hold great potential to be used as biomaterials for bone tissue engineering owing to their biomimetic composition and high biocompatibility and osteoinductivity. Here, we report the development of a novel composite biomaterial for bone tissue engineering based on the incorporation of a modified strontium- and lithium-doped 58S BG (i.e., BG-5/5) into gelatin methacryloyl (GelMA) hydrogels. We characterized the physicochemical properties of the BG formulation via different analytical techniques. Composite hydrogels were then prepared by directly adding BG-5/5 to the GelMA hydrogel precursor, followed by photocrosslinking of the polymeric network via visible light. We characterized the physical, mechanical, and adhesive properties of GelMA/BG-5/5 composites, as well as their in vitro cytocompatibility and osteoinductivity. In addition, we evaluated the antimicrobial properties of these composites in vitro, using a strain of methicillin-resistant Staphylococcus Aureus. GelMA/BG-5/5 composites combined the functional characteristics of the inorganic BG component, with the biocompatibility, biodegradability, and biomimetic composition of the hydrogel network. This novel biomaterial could be used for developing osteoinductive scaffolds or implant surface coatings with intrinsic antimicrobial properties and higher therapeutic efficacy.

TiO2 doped chitosan/hydroxyapatite/halloysite nanotube membranes with enhanced mechanical properties and osteoblast-like cell response for application in bone tissue engineering

The current therapeutic strategies for healing bone defects commonly suffer from the occurrence of bacterial contamination on the graft, resulting in nonunion in the segmental bone defects and the requirement for secondary surgery to remove or sterilize the primary graft. A membrane with enhanced anti-bacterial efficacy, mechanical strength and osteoconductivity would represent an improvement in the therapeutic strategy for guided bone regeneration. The present study aims to optimize the content of halloysite nanotubes (HNTs) and TiO₂ in the polymer matrix of chitosan (CTS) with a constant amount of nano-hydroxyapatite (5%) with the objective of mimicking the mechanical and biological microenvironment of the natural bone extracellular matrix with enhanced anti-bacterial efficacy. HNTs are a low-cost alternative to MWNCTs for enhancing the mechanical properties and anti-bacterial efficacy of the composite. From the first stage of the study, it was concluded that the membranes possessed enhanced mechanical properties and optimum biological properties at 7.5% (w/w) loading of HNTs in the composite. In the second stage of this investigation, we studied the effect of the addition of TiO₂ nanoparticles (NPs) and TiO₂ nanotubes (NTs) in small amounts to the CTS/n-HAP/HNT nanocomposite at 7.5% HNT loading, with an aim to augment the anti-bacterial efficacy and osteoconductivity of this mechanically strong membrane. The study revealed a significant enhancement in the anti-bacterial efficacy, osteoblast-like MG-63 cell proliferation and ALP expression with the addition of TiO₂ NTs. The CHH-TiT membrane successfully inhibited the S. aureus and E. coli growth within 16 hours and simultaneously assisted the enhanced proliferation of osteoblast-like cells on its surface. The study supports the potential exploitation of CHH-TiT (7.5% HNT & 0.2% TiO₂ NT) membranes as a template for guided bone tissue regeneration.

This two-stage study aims to optimize the amount of halloysite nanotubes and TiO₂ in a chitosan/nano-hydroxyapatite composite to tailor the mechanical and biological properties for application in bone tissue engineering.

Multifunctional Triple-Layered Composite Scaffolds Combining Platelet-Rich Fibrin Promote Bone Regeneration

There has been substantial progress made in the development of bone regeneration materials, driven by the deficiencies that exist in current clinical products, such as finite sources, donor site complications, and potential for disease transmission. To overcome these shortcomings, multifunctional scaffolds should be developed to integrate the relationship among osteoinduction, osteoconduction, and osseointegration. In this study, we fabricated polycaprolactone/gelatin (PG) nanofiber films by electrospinning, to act as barriers against connective tissue migration into bone defect sites; chitosan/poly (γ-glutamic acid)/hydroxyapatite (CPH) hydrogels were formed by electrostatic interaction and lyophilization, to exert osteoconduction; and platelet-rich fibrin (PRF) was extracted from rat abdominal aorta and combined with composite scaffolds, to promote bone induction through the release of growth factors. Hydrogels were immersed in simulated body fluid (SBF) for 1 month to investigate mineralization in vitro. Cytocompatibility, cell barrier effect, and osteogenic differentiation were also explored in vitro. The ability to effectively regenerate bone was analyzed by implantation of triple-layered composite scaffolds into rat calvarial defects in vivo. Size-matched hydrogel filled the defect site, and then, fresh PRF was applied to the hydrogel surface. Finally, P2G3 nanofiber films were applied and attached to the surrounding soft tissue. In short, we fabricated multifunctional triple-layered scaffolds by combining the advantages of osteoinduction, osteoconduction, and osseointegration, which could give full play to the role of PRF in bone regeneration and provide new and pragmatic concepts for bone tissue regeneration in clinical applications.

Tissue-engineered 3D models for elucidating primary and metastatic bone cancer progression

Malignant bone tumors are aggressive neoplasms which arise from bone tissue or as a result of metastasis. The most prevalent types of cancer, such as breast, prostate, and lung cancer, all preferentially metastasize to bone, yet the role of the bone niche in promoting cancer progression remains poorly understood. Tissue engineering has the potential to bridge this knowledge gap by providing 3D in vitro systems that can be specifically designed to mimic key properties of the bone niche in a more physiologically relevant context than standard 2D culture. Elucidating the crucial components of the bone niche that recruit metastatic cells, support tumor growth, and promote cancer-induced destruction of bone tissue would support efforts for preventing and treating these devastating malignancies. In this review, we summarize recent efforts focused on developing in vitro 3D models of primary bone cancer and bone metastasis using tissue engineering approaches. Such 3D in vitro models can enable the identification of effective therapeutic targets and facilitate high-throughput drug screening to effectively treat bone cancers. STATEMENT OF SIGNIFICANCE: Biomaterials-based 3D culture have been traditionally used for tissue regeneration. Recent research harnessed biomaterials to create 3D in vitro cancer models, with demonstrated advantages over conventional 2D culture in recapitulating tumor progression and drug response in vivo. However, previous work has been largely limited to modeling soft tissue cancer, such as breast cancer and brain cancer. Unlike soft tissues, bone is characterized with high stiffness and mineral content. Primary bone cancer affects mostly children with poor treatment outcomes, and bone is the most common site of cancer metastasis. Here we summarize emerging efforts on engineering 3D bone cancer models using tissue engineering approaches, and future directions needed to further advance this relatively new research area.

Non-Cytotoxic Agarose/Hydroxyapatite Composite Scaffolds for Drug Release

Healing of large bone defects requires implants or scaffolds that provide structural guidance for cell growth, differentiation, and vascularization. In the present work, an agarose-hydroxyapatite composite scaffold was developed that acts not only as a 3D matrix, but also as a release system. Hydroxyapatite (HA) was incorporated into the agarose gels in situ in various ratios by a simple procedure consisting of precipitation, cooling, washing, and drying. The resulting gels were characterized regarding composition, porosity, mechanical properties, and biocompatibility. A pure phase of carbonated HA was identified in the scaffolds, which had pore sizes of up to several hundred micrometers. Mechanical testing revealed elastic moduli of up to 2.8 MPa for lyophilized composites. MTT testing on Lw35human mesenchymal stem cells (hMSCs) and osteosarcoma MG-63 cells proved the biocompatibility of the scaffolds. Furthermore, scaffolds were loaded with model drug compounds for guided hMSC differentiation. Different release kinetic models were evaluated for adenosine 5'-triphosphate (ATP) and suramin, and data showed a sustained release behavior over four days.