Enhancing the Gelation and Bioactivity of Injectable Silk Fibroin Hydrogel with Laponite Nanoplatelets

Calcium Cross-Linked Cellulose Nanofibrils: Hydrogel Design for Local and Controlled Nitric Oxide Release

Nitric oxide (NO) holds promise for wound healing due to its antimicrobial properties and role in promoting vasodilation and tissue regeneration. The local delivery of NO to target cells or organs offers significant potential in numerous biomedical applications, especially when NO donors are integrated into nontoxic viscous matrices. This study presents the development of robust cellulose nanofibril (CNF) hydrogels designed to control the release of nitric oxide (NO) generated in situ from a NO-donor molecule (S-nitrosoglutathione, GSNO) obtained from the nitrosation of its precursor molecule glutathione (GSH). CNF, efficiently isolated from sugar cane bagasse, exhibited a high aspect ratio and excellent colloidal stability in water. Although depletion forces could be observed upon the addition of GSH, this effect did not significantly alter the morphology of the CNF network at low GSH concentrations (<20 mM). Ionic cross-linking with Ca2+ resulted in nontoxic and robust hydrogels (elastic moduli ranging from 300 to 3000 Pa) at low CNF solid content. The release rate of NO from GSNO decreased in CNF from 1.61 to 0.40 mmol. L-1·h-1 when the nanofibril content raised from 0.3 to 1.0 wt %. The stabilization effect monitored for 16 h was assigned to hydrogel mesh size, which was easily tailored by modifying the concentration of CNF in the initial suspension. These results highlight the potential of CNF-based hydrogels in biomedical applications requiring a precise NO delivery.

Enhanced Tissue Regeneration Through Immunomodulatory Extracellular Vesicles Encapsulated in Diversified Silk-Based Injectable Hydrogels

Mesenchymal stem cells (MSCs) immunologically trained using lipopolysaccharide (LPS) display enhanced immunomodulatory capabilities. Extracellular vesicles (EVs) derived from MSCs are widely used in regenerative medicine owing to their bioactive properties without the drawbacks of cell therapy. However, it remains unclear whether EVs derived from LPS-stimulated (trained) MSCs (L-EVs) inherit the enhanced reparative potential from their parent cells. Thus, this study first aims to explore the effect of immunological training on the bioactivity of L-EVs. LPS-trained bone marrow-derived MSCs (BMSCs) secrete more EVs, and these EVs significantly promote M2 macrophage polarization. Subsequently, hydrogel systems based on thixotropic injectable silk fibroin are prepared for in vivo EV delivery. These hydrogels have controllable gelation time and exhibit outstanding reparative effects on rat skin wounds and alveolar bone defects. Finally, it is revealed that L-EVs promote M2 macrophage polarization by inhibiting the nuclear translocation of PKM2. Overall, this study shows that the immunological training of BMSCs effectively improves the therapeutic effects of their EVs and provides a convenient and diversified EV delivery strategy using an injectable silk fibroin hydrogel. This strategy has broad clinical application prospects for tissue regeneration.

Nanoclay-Composite Hydrogels for Bone Tissue Engineering

Nanoclay-composite hydrogels represent a promising avenue for advancing bone tissue engineering. Traditional hydrogels face challenges in providing mechanical strength, biocompatibility, and bioactivity necessary for successful bone regeneration. The incorporation of nanoclay into hydrogel matrices offers a potential unique solution to these challenges. This review provides a comprehensive overview of the fabrication, physico-chemical/biological performance, and applications of nanoclay-composite hydrogels in bone tissue engineering. Various fabrication techniques, including in situ polymerization, physical blending, and 3D printing, are discussed. In vitro and in vivo studies evaluating biocompatibility and bioactivity have demonstrated the potential of these hydrogels for promoting cell adhesion, proliferation, and differentiation. Their applications in bone defect repair, osteochondral tissue engineering and drug delivery are also explored. Despite their potential in bone tissue engineering, nanoclay-composite hydrogels face challenges such as optimal dispersion, scalability, biocompatibility, long-term stability, regulatory approval, and integration with emerging technologies to achieve clinical application. Future research directions need to focus on refining fabrication techniques, enhancing understanding of biological interactions, and advancing towards clinical translation and commercialization. Overall, nanoclay-composite hydrogels offer exciting opportunities for improving bone regeneration strategies.

Strong, elastic and degradation-tolerated hydrogels composed of chitosan, silk fibroin and bioglass nanoparticles with factor-bestowed activity for bone tissue engineering

Polymer hydrogels intended for use in bone repair need to be strong, elastic, and capable of enduring degradation. However, many natural polymer hydrogels lack these essential properties and thus, are unsuitable for bone repair applications. Here, a new type of multi-network hydrogel with improved mechanical and degradation-resistant properties has been developed for use in bone repair. The hydrogel is composed of thiolated chitosan (TCH), silk fibroin (SF), and thiolated bioglass (TBG) nanoparticles (NPs). The multi-networks are built through sulfhydryl self-crosslinking, diepoxide crosslinker-involved linkages of amino or hydroxyl groups, and enzyme-mediated phenol hydroxyl crosslinking. Additionally, mesoporous TBG NPs serve as a vehicle for loading stromal cell-derived factor-1 (SDF-1) to provide the gel with cell-recruiting activity. The formulated TCH/SF/TBG hydrogels exhibit remarkably enhanced strength, elasticity, and improved degradation tolerance compared to some gels made from only TCH or SF. Furthermore, TCH/SF/TBG gels can support the growth of seeded cells and the deposition of matrix components. Some TCH/SF/TBG gels also demonstrate the ability to release SDF-1 in an approximately linear manner for a few weeks while retaining the chemotactic properties of the released SDF-1. Overall, the multi-network hydrogel has the potential as an in situ forming material for cell-recruiting bone repair and regeneration.

Structure, rheological properties, and biocompatibility of Laponite® cross-linked starch/polyvinyl alcohol hydrogels

Hydrogels, owing to their unique porous structures, hydrophilic properties, and biocompatibility, are being developed as scaffolds for bone grafts. However, the use of toxic initiators or cross-linking agents is a drawback. To overcome this, we developed Laponite®/cross-linked starch/polyvinyl alcohol (PVA) hydrogels prepared by one-step solution mixing. The structure, rheological properties, and biocompatibility of the hydrogels were investigated. Zeta potential, Fourier transform infrared, and X-ray diffraction analyses showed that hydrogen bonding and electrostatic interactions jointly maintained the structure of the cross-linked hydrogel systems. At a Laponite® concentration of 10 %, the hydrogel with a starch/PVA ratio of 2:2 exhibited a uniform porous structure, the highest storage modulus (G'), and the lowest degradation rate. At a starch/PVA ratio of 2:2, the G' increased; however, the degradation rate decreased with the increase in Laponite® content from 5 % to 20 %. These results indicate that the mechanical strength and degradation rate of the hydrogels could be adjusted by altering the starch/PVA ratio and the amount of Laponite®. In vitro cytotoxicity experiments showed that the Laponite®/starch/PVA (LSP) hydrogels were non-toxic to MC3T3-E1 cells. The starch/PVA ratio had no obvious effect on the proliferation of MC3T3-E1 cells, but an increase in Laponite® content significantly promoted cell proliferation. In summary, the results suggest that these LSP hydrogels have great potential for applications in bone tissue engineering.

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.

A novel anti-washout curing solution of calcium phosphate cement prepared via irradiation polymerization

The anti-washout ability of calcium phosphate cement (CPC) determines its effectiveness in clinical application. In the current research, the common method for improving the anti-washout ability of CPC is to add anti-washout polymer agents. Sodium polyacrylate powder is an excellent anti-washout agent but when bonded with CPC it basically degrades the anti-washout performance of CPC after γ-ray irradiation, and is widely used in the sterilization process of CPC products. Therefore, we propose a method for the preparation of a sodium polyacrylate solution through irradiation polymerization as curing solution for CPC. This method first uses γ-ray irradiation sterilization to improve the anti-washout ability of CPC directly. It not only avoids the adverse effects of γ-rays on anti-washout agents, but also the CPC blended using this sodium polyacrylate solution had good biological properties and injectability. It provides a new method for promoting the anti-washout properties of calcium phosphate cement, which is of great significance for expanding the clinical application of CPC.

Effect of Sericin Content on the Structural Characteristics and Properties of New Silk Nonwoven Fabrics

Recently, natural silk nonwoven fabrics have attracted attention in biomedical and cosmetic applications because of their excellent biocompatibility, mechanical properties, and easy preparation. Herein, silk nonwoven fabrics were prepared by carding silk filaments to improve their productivity, and the effect of sericin content on the structure and properties of silk nonwoven fabrics was investigated. Owing to the binding effect of sericin in silk, a natural silk nonwoven fabric was successfully prepared through carding, wetting, and hot press treatments. Sericin content affected the structural characteristics and properties of the silk nonwoven fabrics. As the sericin content increased, the silk nonwoven fabrics became more compact with reduced porosity and thickness. Further, with increasing sericin content, the crystallinity and elongation of the silk nonwoven fabrics decreased while the moisture regain and the maximum stress increased. The thermal stability of most silk nonwoven fabrics was not affected by the sericin content. However, silk nonwoven fabrics without sericin had a lower thermal decomposition temperature than other nonwoven fabrics. Regardless of the sericin content, all silk nonwoven fabrics exhibited optimal cell viability and are promising candidates for cosmetic and biomedical applications.

Application of Silk-Fibroin-Based Hydrogels in Tissue Engineering

Silk fibroin (SF) is an excellent protein-based biomaterial produced by the degumming and purification of silk from cocoons of the Bombyx mori through alkali or enzymatic treatments. SF exhibits excellent biological properties, such as mechanical properties, biocompatibility, biodegradability, bioabsorbability, low immunogenicity, and tunability, making it a versatile material widely applied in biological fields, particularly in tissue engineering. In tissue engineering, SF is often fabricated into hydrogel form, with the advantages of added materials. SF hydrogels have mostly been studied for their use in tissue regeneration by enhancing cell activity at the tissue defect site or counteracting tissue-damage-related factors. This review focuses on SF hydrogels, firstly summarizing the fabrication and properties of SF and SF hydrogels and then detailing the regenerative effects of SF hydrogels as scaffolds in cartilage, bone, skin, cornea, teeth, and eardrum in recent years.

Multi-leveled Nanosilicate Implants Can Facilitate Near-Perfect Bone Healing

Several studies have shown that nanosilicate-reinforced scaffolds are suitable for bone regeneration. However, hydrogels are inherently too soft for load-bearing bone defects of critical sizes, and hard scaffolds typically do not provide a suitable three-dimensional (3D) microenvironment for cells to thrive, grow, and differentiate naturally. In this study, we bypass these long-standing challenges by fabricating a cell-free multi-level implant consisting of a porous and hard bone-like framework capable of providing load-bearing support and a softer native-like phase that has been reinforced with nanosilicates. The system was tested with rat bone marrow mesenchymal stem cells in vitro and as a cell-free system in a critical-sized rat bone defect. Overall, our combinatorial and multi-level implant design displayed remarkable osteoconductivity in vitro without differentiation factors, expressing significant levels of osteogenic markers compared to unmodified groups. Moreover, after 8 weeks of implantation, histological and immunohistochemical assays indicated that the cell-free scaffolds enhanced bone repair up to approximately 84% following a near-complete defect healing. Overall, our results suggest that the proposed nanosilicate bioceramic implant could herald a new age in the field of orthopedics.

Safe Etching Route of Nb2SnC for the Synthesis of Two-Dimensional Nb2CTx MXene: An Electrode Material with Improved Electrochemical Performance

In this study, low-temperature synthesis of a Nb2SnC non-MAX phase was carried out via solid-state reaction, and a novel approach was introduced to synthesize 2D Nb2CTx MXenes through selective etching of Sn from Nb2SnC using mild phosphoric acid. Our work provides valuable insights into the field of 2D MXenes and their potential for energy storage applications. Various techniques, including XRD, SEM, TEM, EDS, and XPS, were used to characterize the samples and determine their crystal structures and chemical compositions. SEM images revealed a two-dimensional layered structure of Nb2CTx, which is consistent with the expected morphology of MXenes. The synthesized Nb2CTx showed a high specific capacitance of 502.97 Fg-1 at 1 Ag-1, demonstrating its potential for high-performance energy storage applications. The approach used in this study is low-cost and could lead to the development of new energy storage materials. Our study contributes to the field by introducing a unique method to synthesize 2D Nb2CTx MXenes and highlights its potential for practical applications.

Piezoresistive MXene/Silk fibroin nanocomposite hydrogel for accelerating bone regeneration by Re-establishing electrical microenvironment

The electrical microenvironment plays an important role in bone repair. However, the underlying mechanism by which electrical stimulation (ES) promotes bone regeneration remains unclear, limiting the design of bone microenvironment–specific electroactive materials. Herein, by simple co-incubation in aqueous suspensions at physiological temperatures, biocompatible regenerated silk fibroin (RSF) is found to assemble into nanofibrils with a β-sheet structure on MXene nanosheets, which has been reported to inhibit the restacking and oxidation of MXene. An electroactive hydrogel based on RSF and bioencapsulated MXene is thus prepared to promote efficient bone regeneration. This MXene/RSF hydrogel also acts as a piezoresistive pressure transducer, which can potentially be utilized to monitor the electrophysiological microenvironment. RNA sequencing is performed to explore the underlying mechanisms, which can activate Ca²⁺/CALM signaling in favor of the direct osteogenesis process. ES is found to facilitate indirect osteogenesis by promoting the polarization of M2 macrophages, as well as stimulating the neogenesis and migration of endotheliocytes. Consistent improvements in bone regeneration and angiogenesis are observed with MXene/RSF hydrogels under ES in vivo. Collectively, the MXene/RSF hydrogel provides a distinctive and promising strategy for promoting direct osteogenesis, regulating immune microenvironment and neovascularization under ES, leading to re-establish electrical microenvironment for bone regeneration.

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MXene nanosheets could direct the selective growth of silk nanofibrils.

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Prepared MXene/RSF hydrogel exhibited good conductivity and sensing ability.

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The electroactive hydrogel could promote osteogenic differentiation of BMSCs by activating the Ca²⁺/CALM signaling pathway.

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The conductive system created an osteoblast–macrophage–endotheliocyte virtuous circle for bone microenvironment.

Silk-Inorganic Nanoparticle Hybrid Hydrogel as an Injectable Bone Repairing Biomaterial

Silk fibroin is regarded as a promising biomaterial in various areas, including bone tissue regeneration. Herein, Laponite^® (LAP), which can promote osteogenic differentiation, was introduced into regenerated silk fibroin (RSF) to prepare an RSF/LAP hybrid hydrogel. This thixotropic hydrogel is injectable during the operation process, which is favorable for repairing bone defects. Our previous work demonstrated that the RSF/LAP hydrogel greatly promoted the osteogenic differentiation of osteoblasts in vitro. In the present study, the RSF/LAP hydrogel was found to have excellent biocompatibility and significantly improved new bone formation in a standard rat calvarial defect model in vivo. Additionally, the underlying biological mechanism of the RSF/LAP hydrogel in promoting osteogenic differentiation of bone marrow mesenchymal stem cells (BMSCs) was extensively explored. The results indicate that the RSF/LAP hydrogels provide suitable conditions for the adhesion and proliferation of BMSCs, showing good biocompatibility in vitro. With the increase in LAP content, the alkaline phosphatase (ALP) activity and mRNA and protein expression of the osteogenic markers of BMSCs improved significantly. Protein kinase B (AKT) pathway activation was found to be responsible for the inherent osteogenic properties of the RSF/LAP hybrid hydrogel. Therefore, the results shown in this study firmly suggest such an injectable RSF/LAP hydrogel with good biocompatibility (both in vitro and in vivo) would have good application prospects in the field of bone regeneration.

Clay-Based Nanocomposite Hydrogels for Biomedical Applications: A Review

In recent decades, new and improved materials have been developed with a significant interest in three-dimensional (3D) scaffolds that can cope with the diverse needs of the expanding biomedical field and promote the required biological response in multiple applications. Due to their biocompatibility, ability to encapsulate and deliver drugs, and capacity to mimic the extracellular matrix (ECM), typical hydrogels have been extensively investigated in the biomedical and biotechnological fields. The major limitations of hydrogels include poor mechanical integrity and limited cell interaction, restricting their broad applicability. To overcome these limitations, an emerging approach, aimed at the generation of hybrid materials with synergistic effects, is focused on incorporating nanoparticles (NPs) within polymeric gels to achieve nanocomposites with tailored functionality and improved properties. This review focuses on the unique contributions of clay nanoparticles, regarding the recent developments of clay-based nanocomposite hydrogels, with an emphasis on biomedical applications.

Nonmulberry silk fibroin-based biomaterials: Impact on cell behavior regulation and tissue regeneration

Silk fibroin (SF) is a promising biomaterial due to its good biocompatibility, easy availability, and high mechanical properties. Compared with mulberry silk fibroin (MSF), nonmulberry silk fibroin (NSF) isolated from typical nonmulberry silkworm silk exhibits unique arginine-glycine-aspartic acid (RGD) sequences with favorable cell adhesion enhancing effect. This inherent property probably makes the NSF more suitable for cell culture and tissue regeneration-related applications. Accordingly, various types of NSF-based biomaterials, such as particles, films, fiber mats, and 3D scaffolds, are constructed and their application potential in different biomedical fields is extensively investigated. Based on these promising NSF biomaterials, this review firstly makes a systematical comparison between the molecular structure and properties of MSF and typical NSF and highlights the unique properties of NSF. In addition, we summarize the effective fabrication strategies from degummed nonmulberry silk fibers to regenerated NSF-based biomaterials with controllable formats and their recent application progresses in cell behavior regulation and tissue regeneration. Finally, current challenges and future perspectives for the fabrication and application of NSF-based biomaterials are discussed. Related research and perspectives may provide valuable references for designing and modifying effective NSF-based and other natural biomaterials. STATEMENT OF SIGNIFICANCE: There exist many reviews about mulberry silk fibroin (MSF) biomaterials and their biomedical applications, while that about nonmulberry silk fibroin (NSF) biomaterials is scarce. Compared with MSF, NSF exhibits unique arginine-glycine-aspartic acid sequences with promising cell adhesion enhancing effect, which makes NSF more suitable for cell culture and tissue regeneration related applications. Focusing on these advanced NSF biomaterials, this review has systematically compared the structure and properties of MSF and NSF, and emphasized the unique properties of NSF. Following that, the effective construction strategies for NSF-based biomaterials are summarized, and their recent applications in cell behavior regulations and tissue regenerations are highlighted. Furthermore, current challenges and future perspectives for the fabrication and application of NSF-based biomaterials were discussed.

Protein-inorganic hybrid porous scaffolds for bone tissue engineering

Porous scaffolds hold promise in the treatment of bone defects for bone tissue engineering due to their interconnected porous structure and suitable mechanical properties. Herein, LAPONITE® (LAP), which is able to promote osteogenic differentiation, was introduced into regenerated silk fibroin (RSF) porous scaffolds. Due to hydrogen bonding and electrostatic interactions between RSF and LAP, RSF/LAP 3D porous scaffolds were successfully prepared. The pore size, porosity, and mechanical properties of the RSF/LAP 3D porous scaffolds were modulated during the preparation process. Evaluation of the proliferation of bone marrow mesenchymal stem cells (BMSCs) on the RSF/LAP 3D porous scaffolds in vitro indicated that the addition of LAP improved the adhesion and proliferation of cells. Additionally, alkaline phosphatase activity and osteospecific gene expression analysis showed that the RSF/LAP 3D porous scaffolds enhanced the osteogenic differentiation of BMSCs compared to the pristine RSF porous scaffolds, especially with a higher LAP content. The subcutaneous implantation of the RSF/LAP 3D porous scaffolds in rats demonstrated good histocompatibility in vivo. Therefore, RSF/LAP 3D porous scaffolds with good biocompatibility and biodegradability have good application prospects in the field of bone tissue engineering.

Preparation and Characterization of Natural Silk Fibroin Hydrogel for Protein Drug Delivery

In recent years, hydrogels have been widely used as drug carriers, especially in the area of protein delivery. The natural silk fibroin produced from cocoons of the Bombyx mori silkworm possesses excellent biocompatibility, significant bioactivity, and biodegradability. Therefore, silk fibroin-based hydrogels are arousing widespread interest in biomedical research. In this study, a process for extracting natural silk fibroin from raw silk textile yarns was established, and three aqueous solutions of silk fibroin with different molecular weight distributions were successfully prepared by controlling the degumming time. Silk fibroin was dispersed in the aqueous solution as “spherical” aggregate particles, and the smaller particles continuously accumulated into large particles. Finally, a silk fibroin hydrogel network was formed. A rheological analysis showed that as the concentration of the silk fibroin hydrogel increased its storage modulus increased significantly. The degradation behavior of silk fibroin hydrogel in different media verified its excellent stability, and the prepared silk fibroin hydrogel had good biocompatibility and an excellent drug-loading capacity. After the protein model drug BSA was loaded, the cumulative drug release within 12 h reached 80%. We hope that these investigations will promote the potential utilities of silk fibroin hydrogels in clinical medicine.

A Biomimetic Electrospun Membrane Supports the Differentiation and Maturation of Kidney Epithelium from Human Stem Cells

Podocytes derived from human induced pluripotent stem (hiPS) cells are enabling studies of kidney development and disease. However, many of these studies are carried out in traditional tissue culture plates that do not accurately recapitulate the molecular and mechanical features necessary for modeling tissue- and organ-level functionalities. Overcoming these limitations requires the design and application of tunable biomaterial scaffolds. Silk fibroin is an attractive biomaterial due to its biocompatibility and versatility, which include its ability to form hydrogels, sponge-like scaffolds, and electrospun fibers and membranes appropriate for tissue engineering and biomedical applications. In this study, we show that hiPS cells can be differentiated into post-mitotic kidney glomerular podocytes on electrospun silk fibroin membranes functionalized with laminin. The resulting podocytes remain viable and express high levels of podocyte-specific markers consistent with the mature cellular phenotype. The resulting podocytes were propagated for at least two weeks, enabling secondary cell-based applications and analyses. This study demonstrates for the first time that electrospun silk fibroin membrane can serve as a supportive biocompatible platform for human podocyte differentiation and propagation. We anticipate that the results of this study will pave the way for the use of electrospun membranes and other biomimetic scaffolds for kidney tissue engineering, including the development of co-culture systems and organs-on-chips microphysiological devices.

A dual functional chondro-inductive chitosan thermogel with high shear modulus and sustained drug release for cartilage tissue engineering

We report a chitosan-based nanocomposite thermogel with superior shear modulus resembling that of cartilage and dual pro-chondrogenic and anti-inflammatory functions. Two therapeutic agents, kartogenin (KGN) and diclofenac sodium (DS), are employed to promote chondrogenesis of stem cells and suppress inflammation, respectively. To extend the release time in a controlled manner, KGN is encapsulated in the uniform-sized starch microspheres and DS is loaded into the halloysite nanotubes. Both drug carriers are doped into the maleimide-modified chitosan hydrogel to produce a shear modulus of 167 ± 5 kPa that is comparable to that of articular cartilage (50-250 kPa). Owing to the hydrogel injectability and relatively suitable gelation time (5 ± 0.5 min) at 37 °C, this system potentially constitutes a manageable platform for clinical practice. Moreover, sustained linear drug release for over a month boosts chondro-differentiation of stem cells to eliminate the necessity for multiple administrations. Considering virtues such as thermogel strength and ability to co-deliver anti-inflammatory and chondro-inductive biomolecules continuously, the materials and strategy have promising potential in functional cartilage tissue engineering.

Recombinant Human Bone Morphogenic Protein-2 Immobilized Fabrication of Magnesium Functionalized Injectable Hydrogels for Controlled-Delivery and Osteogenic Differentiation of Rat Bone Marrow-Derived Mesenchymal Stem Cells in Femoral Head Necrosis Repair

Femoral head necrosis (FHN) is a clinically progressive disease that leads to overwhelming complications without an effective therapeutic approach. In recent decades, transplantation of mesenchymal stem cells (MSCs) has played a promising role in the treatment of FHN in the initial stage; however, the success rate is still low because of unsuitable cell carriers and abridged osteogenic differentiation of the transplanted MSCs. Biopolymeric-derived hydrogels have been extensively applied as effective cell carriers and drug vesicles; they provide the most promising contributions in the fields of tissue engineering and regenerative medicine. However, the clinical potential of hydrogels may be limited because of inappropriate gelation, swelling, mechanical characteristics, toxicity in the cross-linking process, and self-healing ability. Naturally, gelated commercial hydrogels are not suitable for cell injection and infiltration because of their static network structure. In this study, we designed a novel thermogelling injectable hydrogel using natural silk fibroin-blended chitosan (CS) incorporated with magnesium (Mg) substitutes to improve physical cross-linking, stability, and cell osteogenic compatibility. The presented observations demonstrate that the developed injectable hydrogels can facilitate the controlled delivery of immobilized recombinant human bone morphogenic protein-2 (rhBMP-2) and rat bone marrow-derived MSCs (rBMSCs) with greater cell encapsulation efficiency, compatibility, and osteogenic differentiation. In addition, outcomes of in vivo animal studies established promising osteoinductive, bone mineral density, and bone formation rate after implantation of the injectable hydrogel scaffolds. Therefore, the developed hydrogels have great potential for clinical applications of FHN therapy.

Converging 2D Nanomaterials and 3D Bioprinting Technology: State-of-the-Art, Challenges, and Potential Outlook in Biomedical Applications

The development of next-generation of bioinks aims to fabricate anatomical size 3D scaffold with high printability and biocompatibility. Along with the progress in 3D bioprinting, 2D nanomaterials (2D NMs) prove to be emerging frontiers in the development of advanced materials owing to their extraordinary properties. Harnessing the properties of 2D NMs in 3D bioprinting technologies can revolutionize the development of bioinks by endowing new functionalities to the current bioinks. First the main contributions of 2D NMS in 3D bioprinting technologies are categorized here into six main classes: 1) reinforcement effect, 2) delivery of bioactive molecules, 3) improved electrical conductivity, 4) enhanced tissue formation, 5) photothermal effect, 6) and stronger antibacterial properties. Next, the recent advances in the use of each certain 2D NMs (1) graphene, 2) nanosilicate, 3) black phosphorus, 4) MXene, 5) transition metal dichalcogenides, 6) hexagonal boron nitride, and 7) metal-organic frameworks) in 3D bioprinting technology are critically summarized and evaluated thoroughly. Third, the role of physicochemical properties of 2D NMSs on their cytotoxicity is uncovered, with several representative examples of each studied 2D NMs. Finally, current challenges, opportunities, and outlook for the development of nanocomposite bioinks are discussed thoroughly.

Nanosilicate-Functionalized Polycaprolactone Orchestrates Osteogenesis and Osteoblast-Induced Multicellular Interactions for Potential Endogenous Vascularized Bone Regeneration

Massive oral and maxillofacial bone defect regeneration remains a major clinical challenge due to the absence of functionalized bone grafts with ideal mechanical and proregeneration properties. In the present study, Laponite (LAP), a synthetic nanosilicate, is incorporated into polycaprolactone (PCL) to develop a biomaterial for bone regeneration. It is explored whether LAP-embedded PCL would accelerate bone regeneration by orchestrating osteoblasts to directly and indirectly induce bone regeneration processes. The results confirmed the presence of LAP in PCL, and LAP is distributed in the exfoliated structure without aggregates. Incorporation of LAP in PCL slightly improved the compressive properties. LAP-embedded PCL is biocompatible and exerts pronounced enhancements in cell viability, osteogenic differentiation, and extracellular matrix formation of osteoblasts. Furthermore, osteoblasts cultured on LAP-embedded PCL facilitate angiogenesis of vessel endothelial cells and alleviate osteoclastogenesis of osteoclasts in a paracrine manner. The addition of LAP to the PCL endows favorable bone formation in vivo. Based upon these results, LAP-embedded PCL shows great potential as an ideal bone graft that exerts both space-maintaining and vascularized bone regeneration synergistic effects and can be envisioned for oral and maxillofacial bone defect regeneration.

Bioactive Silk Fibroin-Based Hybrid Biomaterials for Musculoskeletal Engineering: Recent Progress and Perspectives

Musculoskeletal engineering has been considered as a promising approach to customize regenerated tissue (such as bone, cartilage, tendon, and ligament) via a self-healing performance. Recent advances have demonstrated the great potential of bioactive materials for regenerative medicine. Silk fibroin (SF), a natural polymer, is regarded as a remarkable bioactive material for musculoskeletal engineering thanks to its biocompatibility, biodegradability, and tunability. To improve tissue-engineering performance, silk fibroin is hybridized with other biomaterials to form silk-fibroin-based hybrid biomaterials, which achieve superior mechanical and biological performance. Herein, we summarize the recent development of silk-based hybrid biomaterials in musculoskeletal tissue with reasonable generalization and classification, mainly including silk fibroin-based inorganic and organic hybrid biomaterials. The applied inorganics are composed of calcium phosphate, graphene oxide, titanium dioxide, silica, and bioactive glass, while the polymers include polycaprolactone, collagen (or gelatin), chitosan, cellulose, and alginate. This article mainly focuses on the physical and biological performances both in vitro and in vivo study of several common silk-based hybrid biomaterials in musculoskeletal engineering. The timely summary and highlight of silk-fibroin-based hybrid biomaterials will provide a research perspective to promote the further improvement and development of silk fibroin hybrid biomaterials for improved musculoskeletal engineering.

Enzymatically crosslinked silk-nanosilicate reinforced hydrogel with dual-lineage bioactivity for osteochondral tissue engineering

Osteochondral defects are characterized by damage to both articular cartilage and subchondral bone. Various tissue engineering strategies have been developed for osteochondral defect repair. However, strong mechanical properties and dual-lineage (osteogenesis and chondrogenesis) bioactivity still pose challenges for current biomaterial design. Silicate nanoclay has been reported to improve the mechanical properties and biofunctionality of polymer systems, but its effect on in vitro dual-lineage differentiation or in vivo osteochondral regeneration has not been extensively investigated before. Here, a novel enzymatically crosslinked silk fibroin (SF)-Laponite (LAP) nanocomposite hydrogel was fabricated and evaluated for osteochondral regeneration. The incorporation of a small amount of LAP (1% w/v) accelerated the gelation process of SF and greatly enhanced the mechanical properties and hydrophilicity of the hydrogel. In vitro investigations showed that the developed SF-LAP hydrogel was biocompatible and was able to induce osteogenic and chondrogenic differentiation of bone marrow-derived mesenchymal stem cells (BMSCs), validated by Alizarin red/Alcian blue staining, qPCR, and immunofluorescent staining. During an 8-week implantation into rabbit full-thickness osteochondral defects, the SF-LAP hydrogel promoted the simultaneous and enhanced regeneration of cartilage and subchondral bone. The repaired tissue in the chondral region was constituted mainly of hyaline cartilage with typical chondrocyte morphology and cartilaginous extracellular matrix (ECM). These findings suggested that the SF-LAP nanocomposite hydrogel developed in this study served as a promising biomaterial for osteochondral regeneration due to its mechanical reinforcement and dual-lineage bioactivity.

De Novo Design of Functional Coassembling Organic-Inorganic Hydrogels for Hierarchical Mineralization and Neovascularization

Synthetic nanostructured materials incorporating both organic and inorganic components offer a unique, powerful, and versatile class of materials for widespread applications due to the distinct, yet complementary, nature of the intrinsic properties of the different constituents. We report a supramolecular system based on synthetic nanoclay (Laponite, Lap) and peptide amphiphiles (PAs, PAH3) rationally designed to coassemble into nanostructured hydrogels with high structural integrity and a spectrum of bioactivities. Spectroscopic and scattering techniques and molecular dynamic simulation approaches were harnessed to confirm that PAH3 nanofibers electrostatically adsorbed and conformed to the surface of Lap nanodisks. Electron and atomic force microscopies also confirmed an increase in diameter and surface area of PAH3 nanofibers after coassembly with Lap. Dynamic oscillatory rheology revealed that the coassembled PAH3-Lap hydrogels displayed high stiffness and robust self-healing behavior while gas adsorption analysis confirmed a hierarchical and heterogeneous porosity. Furthermore, this distinctive structure within the three-dimensional (3D) matrix provided spatial confinement for the nucleation and hierarchical organization of high-aspect ratio hydroxyapatite nanorods into well-defined spherical clusters within the 3D matrix. Applicability of the organic-inorganic PAH3-Lap hydrogels was assessed in vitro using human bone marrow-derived stromal cells (hBMSCs) and ex vivo using a chick chorioallantoic membrane (CAM) assay. The results demonstrated that the organic-inorganic PAH3-Lap hydrogels promote human skeletal cell proliferation and, upon mineralization, integrate with the CAM, are infiltrated by blood vessels, stimulate extracellular matrix production, and facilitate extensive mineral deposition relative to the controls.

Poly(vinyl alcohol) Hydrogels with Integrated Toughness, Conductivity, and Freezing Tolerance Based on Ionic Liquid/Water Binary Solvent Systems

In recent years, ionic conductive hydrogels have shown great potential for application in flexible sensors, energy storage devices, and actuators. However, developing facile and effective methods for fabricating such hydrogels remains a great challenge, especially for hydrogels that retain their properties in extreme environmental conditions, such as at subzero temperatures or storage in open-air conditions. Herein, a water-miscible ionic liquid (IL), such as 1-ethyl-3-methylimidazolium acetate (EMImAc), was introduced to form an IL/water binary solvent system for poly(vinyl alcohol) (PVA) to create ionic conductive PVA hydrogels. The physically crosslinked PVA/EMImAc/H₂O hydrogels showed better mechanical properties and transparency than the traditional PVA hydrogel prepared by the freeze-thaw method due to the formation of homogeneous and small PVA microcrystals in the EMImAc/H₂O binary solvent system. More importantly, the PVA/EMImAc/H₂O hydrogel exhibited significant anti-freezing and water-retaining properties because of the presence of the IL. The hydrogels remained flexible and conductive at temperatures as low as -50 °C and retained more than 90% of their weight after storage in open-air conditions for 2 weeks. In addition, the thermal stability of the hydrogel could be increased to 95 °C through the addition of Mg(II) ions. A multimodal sensor based on the PVA/EMImAc/H₂O/Mg(II) hydrogel showed high sensitivity and a quick response to changes in pressure, strain, and temperature, with both long-term stability and a wide working temperature range. This study may open a new route for the fabrication of functional PVA-based hydrogel electrolytes and provide a practical pathway for their use in multifunctional electronic and sensory device applications.

Silk Biomaterials for Bone Tissue Engineering

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

The role of lithium in the osteogenic bioactivity of clay nanoparticles

LAPONITE® clay nanoparticles are known to exert osteogenic effects on human bone marrow stromal cells (HBMSCs), most characteristically, an upregulation in alkaline phosphatase activity and increased calcium deposition. The specific properties of LAPONITE® that impart its bioactivity are not known. In this study the role of lithium, a LAPONITE® degradation product, was investigated through the use of lithium salts and lithium modified LAPONITE® formulations. In contrast to intact particles, lithium ions applied at concentrations equivalent to that present in LAPONITE®, failed to induce any significant increase in alkaline phosphatase (ALP) activity. Furthermore, no significant differences were observed in ALP activity with modified clay structures and the positive effect on osteogenic gene expression did not correlate with the lithium content of modified clays. These results suggest that other properties of LAPONITE® nanoparticles, and not their lithium content, are responsible for their bioactivity.

Processing, mechanical properties and bio-applications of silk fibroin-based high-strength hydrogels

Hydrogels are an attractive class of materials that possess similar structural and functional characteristics to wet biological tissues and demonstrate a diversity of applications in biomedical engineering. Silk fibroin (SF) is a unique natural polymer due to its fibrous protein nature, versatile formats, biocompatibility, tunable biodegradation and is thus a good hydrogel candidate for bio-applications. Compared to synthetic polymer hydrogels, poor mechanical performance is still a fatal drawback that hinders the application of SF hydrogels as structural materials. Researchers have attempted to develop strategies to construct silk fibroin-based high-strength hydrogels (SF-HSHs). Herein, we firstly provide an overview of the approaches of processing SF-HSHs with a focus on the physical/non-covalent crosslinking mechanisms. The examples of SF-HSHs are discussed in detail under four categories, including physical-crosslinked, dual-crosslinked, double network and composite hydrogels respectively. A brief section follows to elucidate on the gelation mechanisms of SF-HSHs before a description of the utility of SF-HSHs in biomedicine and devices is presented. Finally, the potential challenges and future development of SF-HSHs are briefly discussed. This review aims to enhance our understanding of the structure-mechanical property-function relationships of soft materials made from natural polymers and guide future research of silk fibroin-based hydrogels for biomedical applications. STATEMENT OF SIGNIFICANCE: Silk fibroin (SF) extracted from silk fibres is increasingly applied in the biomedical field, and SF hydrogel has been an emerging area for frontier bio-research. Since SF biopolymer has an intrinsic tendency to form regular β-sheet stacks, it can be processed into purely physically crosslinked hydrogels, thus avoiding the use of chemical crosslinkers. Nevertheless, akin to other natural polymers, lab-produced SF is variable (i.e. the molecular weight and distribution), and the gelation of SF hydrogel is challenging to control. In addition, hydrogels made from SF are usually weak and brittle, which hinders the wide use of this biofriendly and biodegradable hydrogel. Recently, there is a pressing need for high strength hydrogels from natural polymers for biomedical applications, and SF is proposed as a strong candidate. Therefore, we have studied the literature in the past 10 years and would like to focus on the gelation mechanism and mechanical strength of SF hydrogels for the review.

Ethanol-induced coacervation in aqueous gelatin solution for constructing nanospheres and networks: Morphology, dynamics and thermal sensitivity

Ethanol was used to induce coacervation in aqueous solutions of gelatin. Coacervation resulted from phase separation driven by ethanol as a poor solvent for gelatin, impacting aggregation of gelatin chains. Static coacervation was performed to investigate coacervate morphology, and gelatin concentration and ethanol temperature influenced the morphologies of the gelatin coacervates. High-concentration gelatin solutions (>4.8 wt%) treated with lower temperature ethanol (<25 °C) formed network morphologies, while low-concentration gelatin solution (<4.8 wt%) treated with ethanol near room temperature formed nanosphere assemblies. Dispersive nanospheres were obtained after treatment with higher temperature ethanol (~45 °C). Stirring the mixture of gelatin solution and ethanol resulted in dispersed nanospheres where the size was adjusted by changing the volume ratio of aqueous gelatin solution and ethanol (V_Gel:V_EtOH) and the gelatin concentration. Turbidity and absorbance measurements were carried out to further investigate coacervation dynamics. The cocervation system reached dynamic equilibrium according to the V_Gel:V_EtOH, suggesting phase separation and molecular arrangements were key. DLS results showed that reversible changes in coacervate radius could be attained by periodic heating and cooling cycles (25-60 °C). This work provides useful information for constructing gelatin-based materials using a facile coacervation method.

Silk-based hybrid microfibrous mats as guided bone regeneration membranes

LAPONITE® (LAP) nanoplatelets were incorporated within a regenerated silk fibroin (RSF) microfibrous mat via electrospinning, which exhibited better cell adhesion and proliferation of bone marrow mesenchymal stem cells (BMSCs) than the pristine RSF ones.

Mineralization of calcium phosphate induced by a silk fibroin film under different biological conditions

Silk fibroin films can have an important effect on the mineralization process of calcium phosphate in different biological environments. There was improvement of MSF with good biocompatibility that are promising in bone tissue engineering.

Fabrication of protease XIV-loaded microspheres for cell spreading in silk fibroin hydrogels

Due to their excellent mechanical strength and biocompatibility, silk fibroin(SF) hydrogels can serve as ideal scaffolds. However, their slow rate of natural degradation limits the space available for cell proliferation, which hinders their application. In this study, litchi-like calcium carbonate@hydroxyapatite (CaCO₃@HA) porous microspheres loaded with proteases from Streptomyces griseus (XIV) were used as drug carriers to regulate the biodegradation rate of SF hydrogels. The results showed that litchi-like CaCO₃@HA microspheres with different phase compositions could be prepared by changing the hydrothermal reaction time. The CaCO₃@HA microspheres controlled the release of Ca ions, which was beneficial for the osteogenic differentiation of mesenchymal stem cells (MSCs). The adsorption and release of protease XIV from the CaCO₃@HA microcarriers indicated that the loading and release amount can be controlled with the initial drug concentration. The weight loss test and SEM observation showed that the degradation of the fibroin hydrogel could be controlled by altering the amount of protease XIV-loaded CaCO₃@HA microspheres. A three-dimensional (3D) cell encapsulation experiment proved that incorporation of the SF hydrogel with protease XIV-loaded microspheres promoted cell dispersal and spreading, suggesting that the controlled release of protease XIV can regulate hydrogel degradation. SF hydrogels incorporated with protease XIV-loaded microspheres are suitable for cell growth and proliferation and are expected to serve as excellent bone tissue engineering scaffolds.

Nano-Silicate-Reinforced and SDF-1α-Loaded Gelatin-Methacryloyl Hydrogel for Bone Tissue Engineering

PURPOSE

Autologous bone grafts are the gold standard for treating bone defects. However, limited bone supply and morbidity at the donor site restrict its extensive use. Therefore, developing bone graft materials as an alternative to autologous grafts has gained considerable attention. Injectable hydrogels endowed with osteogenic potential have the ability to fill irregular bone defects using minimally invasive procedures and have thus been attracting researchers' attention. However, from a clinical perspective, most fabrication methods employed for the current injectable osteogenic hydrogels are difficult and inconvenient. In the current study, we fabricated an injectable osteogenic hydrogel using a simple and convenient strategy.

MATERIALS AND METHODS

Gelatin-methacryloyl (GelMA) pre-polymer was synthetized. Nano silicate (SN) and stromal cell-derived factor-1 alpha (SDF-1α) were introduced into the pre-polymer to achieve injectability, controlled release property, excellent osteogenic ability, and efficient stem cell homing.

RESULTS

The GelMA-SN-SDF-1α demonstrated excellent injectability via a 17-G needle at room temperature. The loaded SDF-1α exhibited a long-term controlled release pattern and efficiently stimulated MSC migration and homing. The GelMA-SN-SDF-1α hydrogel amplified cell spreading, migration, osteogenic-related biomarker expression, and matrix mineralization. The GelMA-SN-SDF-1α hydrogel filled critical-sized calvaria defects in rats and demonstrated excellent bone regeneration ability, as assessed using micro-CT scanning and histomorphometric staining.

CONCLUSION

The GelMA-SN-SDF-1α hydrogel provides a simple and convenient strategy for the fabrication of injectable osteogenic graft materials.

Bone Marrow Mesenchymal Stem Cell-Derived Small Extracellular Vesicles Promote Periodontal Regeneration

Bone marrow mesenchymal stem cell-derived small extracellular vesicles (BMSC-sEVs) can be used as a potential cell-free strategy for periodontal tissue regeneration, and we aim to investigate the effect and possible mechanism of BMSC-sEV in periodontal tissue regeneration in this study. The BMSC-sEV was isolated by the Exosome Isolation™ reagent and identified by transmission electron microscopy, nanoparticle tracking analysis, and Western blotting. The human periodontal ligament cells (hPDLCs) were cocultured with BMSC-sEV in vitro to detect the effects of BMSC-sEV on hPDLC migration, proliferation, and differentiation. The BMSC-sEV loaded by hydrogel was injected into experimental periodontitis rats to verify the therapeutic effect and possible mechanism. The results showed that BMSC-sEVs were 30-150 nm vesicles and expressed the exosome protein CD63 and tumor susceptibility 101 (TSG101), which could promote the migration, proliferation, osteogenic differentiation of hPDLCs. The BMSC-sEV-hydrogel had a therapeutic effect on periodontitis rats. After administration for 4-8 weeks, microcomputed tomography and histological analysis showed that alveolar bone loss, inflammatory infiltration, and collagen destruction in the BMSC-sEV-hydrogel group were less than that in the phosphate-buffered saline (PBS)-hydrogel group and periodontitis group. Further immunohistochemical and immunofluorescent staining revealed that the number of tartrate-resistant acid phosphatase-positive cells and the expression ratio of osteoprotegerin (OPG) and receptor-activator of nuclear factor kappa beta ligand (RANKL) in the BMSC-sEV-hydrogel group were lower than that in the PBS-hydrogel group and periodontitis group, while the expression of transforming growth factor-beta 1 (TGF-β1) and the ratio of macrophage type 2 and macrophage type 1 (M2/M1) were opposite. Therefore, BMSC-sEV can promote the regeneration of periodontal tissues, and that may be partly due to BMSC-sEV involvement in the OPG-RANKL-RANK signaling pathway to regulate the function of osteoclasts and affect the macrophage polarization and TGF-β1 expression to modulate the inflammatory immune response, thereby inhibiting the development of periodontitis and immune damage of periodontal tissue. Impact statement Bone marrow mesenchymal stem cell-derived small extracellular vesicles (BMSCs-sEVs) have been proven to have similar functions to BMSCs, such as promoting the regeneration of heart, liver, kidney, and bone tissue and regulating immune responses. BMSCs are candidate seed cells of periodontal regeneration, but it is unclear about the role of BMSC-sEV on periodontal regeneration. In this study, we explored the effects and possible mechanism of BMSC-sEV on periodontal regeneration. The results of this study provide the evidence of BMSC-sEV as a cell-free strategy for periodontal regeneration.

A facile approach to obtain highly tough and stretchable LAPONITE®-based nanocomposite hydrogels

LAPONITE® sheets have been widely used for the preparation of tough nanocomposite hydrogels for enticing applications; however, their inferior dispersion in aqueous media resulting from electrostatic interactions between the nanosheets remarkably limits further improvements in the mechanical performances of the nanocomposite hydrogels. Here, we show a simple approach to dramatically accelerate the dispersion of LAPONITE® sheets in water, and in turn further improve the mechanical performances of the resulting nanocomposite hydrogels. Upon addition of poly(acrylic acid) (PAA), the electrostatic interactions between the LAPONITE® sheets were effectively reduced due to the adsorption of PAA onto the positively charged edges of the LAPONITE® sheets, thereby accelerating the dispersion of the LAPONITE® sheets in water. On this basis, a series of polyacrylamide (PAAm) hydrogels with a high content of LAPONITE® sheets was prepared, showing excellent tensile strength, stretchability, and anti-fatigue properties. This study will be beneficial for the preparation of LAPONITE®-based nanocomposite hydrogels bearing excellent mechanical properties for new applications.

Synthesis of Injectable Shear-Thinning Biomaterials of Various Compositions of Gelatin and Synthetic Silicate Nanoplatelet

Injectable shear-thinning biomaterials (iSTBs) have great potential for in situ tissue regeneration through minimally invasive therapeutics. Previously, an iSTB was developed by combining gelatin with synthetic silicate nanoplatelets (SNPs) for potential application to hemostasis and endovascular embolization. Hence, iSTBs are synthesized by varying compositions of gelatin and SNPs to navigate their material, mechanical, rheological, and bioactive properties. All compositions (each component percentage; 1.5-4.5%/total solid ranges; 3-9%) tested are injectable through both 5 Fr general catheter and 2.4 Fr microcatheter by manual pressure. In the results, an increase in gelatin contents causes decrease in swellability, increase in freeze-dried hydrogel scaffold porosity, increase in degradability and injection force during iSTB fabrication. Meanwhile, the amount of SNPs in composite hydrogels is mainly required to decrease degradability and increase shear thinning properties of iSTB. Finally, in vitro and in vivo biocompatibility tests show that the 1.5-4.5% range gelatin-SNP iSTBs are not toxic to the cells and animals. All results demonstrate that the iSTB can be modulated with specific properties for unmet clinical needs. Understanding of mechanical and biological consequences of the changing gelatin-SNP ratios through this study will shed light on the biomedical applications of iSTB on specific diseases.

Composite Hydrogels in Three-Dimensional in vitro Models

3-dimensional (3D) in vitro models were developed in order to mimic the complexity of real organ/tissue in a dish. They offer new possibilities to model biological processes in more physiologically relevant ways which can be applied to a myriad of applications including drug development, toxicity screening and regenerative medicine. Hydrogels are the most relevant tissue-like matrices to support the development of 3D in vitro models since they are in many ways akin to the native extracellular matrix (ECM). For the purpose of further improving matrix relevance or to impart specific functionalities, composite hydrogels have attracted increasing attention. These could incorporate drugs to control cell fates, additional ECM elements to improve mechanical properties, biomolecules to improve biological activities or any combinations of the above. In this Review, recent developments in using composite hydrogels laden with cells as biomimetic tissue- or organ-like constructs, and as matrices for multi-cell type organoid cultures are highlighted. The latest composite hydrogel systems that contain nanomaterials, biological factors, and combinations of biopolymers (e.g., proteins and polysaccharide), such as Interpenetrating Networks (IPNs) and Soft Network Composites (SNCs) are also presented. While promising, challenges remain. These will be discussed in light of future perspectives toward encompassing diverse composite hydrogel platforms for an improved organ environment in vitro.

Physically Cross-Linked Silk Fibroin-Based Tough Hydrogel Electrolyte with Exceptional Water Retention and Freezing Tolerance

Flexible ionic conductive hydrogel is attracting significant interest as it could be one of the crucial components for multifunctional ionotronic devices. However, their features of inevitably drying out without package and freezing at subzero temperatures may greatly limit the applications of conventional hydrogels in specific situations. Here, we present an ionic conductive hydrogel with water retention and freezing tolerance that consists of silk fibroin, ionic liquid, water, and inorganic salt. It is discovered that the ionic liquid serves multiple purposes to prevent water evaporation, decrease the freezing point, provide the essential conductivity of the hydrogel, etc. As a binary mixed solvent, the ionic liquid/water mixture enhances both water retention and freezing tolerance of the hydrogel electrolyte. Based on the silk fibroin (SF)/1-ethyl-3-methylimidazolium acetate (EMImAc)/H₂O/KCl hydrogel electrolyte, the flexible fiberlike supercapacitor could still function well at a temperature as low as -50 °C and after being stored in the open air for a long time. It is anticipated that this hydrogel will prove useful in developing new applications operating under harsh environments.

Hydrogel-clay Nanocomposites as Carriers for Controlled Release

The present review aims to summarize the research efforts undertaken in the last few years in the development and testing of hydrogel-clay nanocomposites proposed as carriers for controlled release of diverse drugs. Their advantages, disadvantages and different compositions of polymers/biopolymers with diverse types of clays, as well as their interactions are discussed. Illustrative examples of studies regarding hydrogel-clay nanocomposites are detailed in order to underline the progressive researches on hydrogel-clay-drug pharmaceutical formulations able to respond to a series of demands for the most diverse applications. Brief descriptions of the different techniques used for the characterization of the obtained complex hybrid materials such as: swelling, TGA, DSC, FTIR, XRD, mechanical, SEM, TEM and biology tests, are also included. Enlightened by the presented data, we can suppose that hydrogel-clay nanocomposites will still be a challenging subject of global assiduous researches. We can dare to dream to an efficient drug delivery platform for the treatment of multiple affection concomitantly, these being undoubtedly like "a tree of life" bearing different kinds of fruits and leaves proper for human healing.

Copolymer/Clay Nanocomposites for Biomedical Applications

Nanoclays still hold a great strength in biomedical nanotechnology applications due to their exceptional properties despite the development of several new nanostructured materials. This article reviews the recent advances in copolymer/clay nanocomposites with a focus on health care applications. In general, the structure of clay comprises aluminosilicate layers separated by a few nanometers. Recently, nanoclay‐incorporated copolymers have attracted the interest of both researchers and industry due to their phenomenal properties such as barrier function, stiffness, thermal/flame resistance, superhydrophobicity, biocompatibility, stimuli responsiveness, sustained drug release, resistance to hydrolysis, outstanding dynamic mechanical properties including resilience and low temperature flexibility, excellent hydrolytic stability, and antimicrobial properties. Surface modification of nanoclays provides additional properties due to improved adhesion between the polymer matrix and the nanoclay, high surface free energy, a high degree of intercalation, or exfoliated morphology. The architecture of the copolymer/clay nanocomposites has great impact on biomedical applications, too, by providing various cues especially in drug delivery systems and regenerative medicine.