Smart Hydrogels for Bone Reconstruction via Modulating the Microenvironment

Advances of novel hydrogels in the healing process of alveolar sockets

Tooth extraction is a common oral surgical procedure that often leads to delayed alveolar socket healing due to the complexity of the oral microenvironment, which can hinder the patient's aesthetic and functional recovery. Effective alveolar socket healing requires a multidisciplinary approach. Recent advancements in materials science and bioengineering have facilitated the development of innovative strategies, with hydrogels emerging as ideal restorative materials for alveolar socket repair due to their superior properties. This review provides an overview of recent advances in hydrogels for alveolar socket healing, focusing on their classification, physical properties (e.g., mechanical strength, swelling behavior, degradation rate, and injectability), biological functions, and applications in relevant animal models. Specifically, the bone-regenerative and antimicrobial properties of hydrogels are highlighted. Furthermore, this review identifies future directions and addresses challenges associated with the clinical application of hydrogels in extraction socket healing.

B-Lymphoid Tyrosine Kinase Crosslinks Redox and Apoptosis Signaling Networks to Promote the Survival of Transplanted Bone Marrow Mesenchymal Stem Cells

Stress-induced apoptosis presents an obstacle to bone marrow mesenchymal stem cell (BMSC) transplantation to repair steroid-induced osteonecrosis of the femoral head (SONFH). Thus, appropriate intervention strategies should be explored to mitigate this. In our previous study, we discovered a new subgroup of BMSCs-the oxidative stress-resistant BMSCs (OSR-BMSCs)-which can survive the oxidative stress microenvironment in the osteonecrotic area, through a mechanism that currently remains unclear. In this study, we found that B-lymphoid tyrosine kinase (BLK) may be the crucial factor regulating the oxidative stress resistance of OSR-BMSCs, as it is highly expressed in these cells. Knockdown of BLK eliminated oxidative stress resistance, aggravated oxidative stress-induced apoptosis, reduced the survival of OSR-BMSCs in the oxidative stress microenvironment of the osteonecrotic area, and greatly weakened the transplantation efficacy of OSR-BMSCs for SONFH. By contrast, BLK was weakly expressed in oxidative stress-sensitive BMSCs (OSS-BMSCs). Overexpression of BLK in susceptible OSS-BMSCs allowed them to acquire oxidative stress resistance, inhibited oxidative stress-induced apoptosis, promoted their survival in the osteonecrotic area, and improved the transplantation efficacy of OSS-BMSCs for SONFH. Mechanistically, BLK concurrently activates redox and apoptotic signaling networks through its tyrosine kinase activity, which confers oxidative stress resistance to BMSCs and inhibits their stress-induced apoptosis of BMSCs. Herein, we report that OSR-BMSCs have intrinsic oxidative stress resistance that is conferred and mediated by BLK. This finding provides a potential new intervention strategy for improving the survival of transplanted BMSCs and the therapeutic efficacy of BMSC transplantation for SONFH.

Ultrasound-responsive smart biomaterials for bone tissue engineering

Bone defects resulting from trauma, tumors, or other injuries significantly impact human health and quality of life. However, current treatments for bone defects are constrained by donor shortages and immune rejection. Bone tissue engineering has partially alleviated the limitations of traditional bone repair methods. The development of smart biomaterials that can respond to external stimuli to modulate the biofunctions has become a prominent area of research. Ultrasound technology is regarded as an optimal "remote controller" and "trigger" for bone repair biomaterials. This review reports the comprehensive and systematic overview of ultrasound-responsive bone repair smart biomaterials. It presents the fundamental theories of bone repair, the definition of ultrasound, and its applications. Furthermore, the review summarizes the ultrasound effect mechanisms of biomaterials and their roles in bone repair, including detailed studies on anti-inflammation, immunomodulation, and cell therapy. Finally, the advantages of ultrasound-responsive smart biomaterials and their future prospects in this field are discussed.

Antioxidant scaffolds for enhanced bone regeneration: recent advances and challenges

Bone regeneration is integral to maintaining bone function and integrity in the body, as well as treating bone diseases, such as osteoporosis and defects. However, oxidative stress often poses a significant obstacle during bone regeneration, leading to cell damage, inflammatory responses, and subsequent impediment of normal bone tissue formation. Therefore, to maintain bone regeneration, antioxidant therapy is essential. Bone scaffolds, serving as a temporary support for bone tissue, can provide an ideal microenvironment for cell proliferation and differentiation, effectively promoting bone tissue formation. In recent years, with in-depth research on antioxidants and their mechanisms of action, the development and application of antioxidant bone scaffolds have shown tremendous potential. These antioxidant bone scaffolds not only promote osteogenic differentiation and angiogenesis, but also effectively inhibit the inflammatory response and osteoclast formation, significantly improving the efficiency of bone regeneration. Notably, with the rapid development of nanotechnology, nanozymes with multi-enzyme-like activities have been successfully constructed and encapsulated within bone scaffolds, leading to the proposal of multifunctional antioxidant strategies. Therefore, this review summarizes recent research progress, categorically introducing types of bone scaffolds and antioxidants, elucidating therapeutic strategies of antioxidant bone scaffolds, and identifying current challenges, aiming to provide valuable guidance for subsequent research.

DNA-based hydrogels for bone regeneration: A promising tool for bone organoids

DNA-based hydrogels stand out for bone regeneration due to their exceptional biocompatibility and programmability. These hydrogels facilitate the formation of spatial bone structures through bulk hydrogel fabricating, microsphere formatting, and 3D printing. Furthermore, the bone microenvironment can be finely tuned by leveraging the degradation products, nanostructure, targeting, and delivery capabilities inherent to DNA-based materials. In this review, we underscore the advantages of DNA-based hydrogels, detailing their composition, gelation techniques, and structure optimization. We then delineate three critical elements in the promotion of bone regeneration using DNA-based hydrogels: (i) osteogenesis driven by phosphate ions, plasmids, and oligodeoxynucleotides (ODNs) that enhance mineralization and promote gene and protein expression; (ii) vascularization facilitated by tetrahedral DNA nanostructures (TDNs) and aptamers, which boosts gene expression and targeted release; (iii) immunomodulation achieved through loaded factors, TDNs, and bound ions that stimulate macrophage polarization and exhibit antibacterial properties. With these advantages and properties, these DNA-based hydrogels can be used to construct bone organoids, providing an innovative tool for disease modeling and therapeutic applications in bone tissue engineering. Finally, we discuss the current challenges and future prospects, emphasizing the potential impacts and applications in regenerative medicine.

Thioether functionalized degradable poly(amino acids) and its calcium sulfate/calcium hydrogen phosphate composites: Reducing oxidative stress and promoting osteogenesis

The imbalance of redox homeostasis, especially the abnormal levels of reactive oxygen species (ROS), is a key obstacle in the bone repair process. Therefore, developing materials capable of scavenging ROS and modulating the microenvironment of bone defects is crucial for promoting bone repair. In this study, to endow poly(amino acids) (PAA) and its composites with anti-oxidative stress properties and enhanced osteogenic differentiation, we designed and prepared a calcium sulfate/calcium hydrogen phosphate/poly(amino acids) (PCDM) composite material with a thioether structure (-S-) in the molecular chain of PAA matrix through situ polymerization and physical blending method. The results showed that the thioether was successfully introduced into the polymer, and the intrinsic viscosities of the poly(amino acids) ranged from 0.27 to 0.73 dL/g. PCDM materials exhibited good mechanical properties, with a compressive strength ranging from 16.28 to 33.83 MPa. The degradation performance results showed that the composite materials had a weight loss of 23.9-35.3 % after four weeks. The antioxidant stress results showed that the PCDM composite materials scavenged 67.6 %-78.3 % of DPPH radicals after 24 h and 61.4 %-93.6 % of ABTS radicals after 4 h, effectively reducing ROS levels in mouse bone mesenchymal stem cells. The cytotoxicity and osteogenic differentiation results showed that the materials had cytocompatibility and could promote alkaline phosphatase secretion and mineralized nodule formation. In conclusion, PCDM materials might broaden the application of poly(amino acids) composites in bone defect repair by regulating the ROS microenvironment and promoting the osteogenic differentiation of stem cells.

Immuno-osteoinductive 3D printed hydrogel scaffolds with triple crosslinking and GA/EGCG release for bone healing

Bone defects, caused by trauma, osteomyelitis, or osteoporosis, represent a significant global health challenge in orthopedics. However, current bone repair strategies often neglect the critical role of the immune microenvironment, which can impede effective bone regeneration. To address this gap, we developed a 3D-printed triple crosslinked hydrogel scaffold incorporating slow-release glycopyrrolate (GA) and epigallocatechin gallate (EGCG), that it could promote bone regeneration by modulating the immune response. We evaluated their immunomodulatory and bone-regenerative effects through in vitro cellular experiments and rat cranial defect models. Results demonstrated that these scaffolds effectively modulated the immune microenvironment, reducing inflammation while promoting osteoblast differentiation and proliferation, thereby significantly enhancing new bone formation and density. In conclusion, our novel 3D-printed hydrogel scaffold offers a promising approach to bone defect repair through its unique combination of mechanical strength, immunomodulation, and osteogenesis. This study provides valuable insights into leveraging immunomodulatory agents for enhanced bone regeneration, highlighting potential clinical applications.

Smart core-shell microneedles for psoriasis therapy: In situ self-assembly of calcium ion-coordinated dexamethasone hydrogel

Psoriasis is a prevalent relapsing dermatological condition that often necessitates lifelong treatment. The distinctive thickening of the stratum corneum presents a challenge to drug penetration. The employment of microneedles has been demonstrated to enhance the transdermal drug delivery efficacy by creating multiple microchannels in the skin. Nevertheless, polymeric microneedles often encounter difficulties in meeting the requirements for sustained drug release. It is imperative to acknowledge that sustained-release hydrogel microneedles are invariably fabricated under harsh crosslinking conditions. In addressing these challenges, a core-shell microneedles (CSMNs) system was customized at a facile, accessible process, enabling the in situ formation of supramolecular microhydrogels within the skin. This concept was realized by leveraging the interaction between the therapeutic drug dexamethasone sodium phosphate (DexP) and calcium chloride (CaCl2), combined with the differential biphasic release technology (DexP HMNs). Upon insertion into the skin, the core of the microneedles rapidly released CaCl2, which diffused to the shell and formed a hydrogel with DexP, creating multiple reservoirs for the sustained release of DexP. In vitro transdermal permeation experiments demonstrated that DexP HMNs greatly prolonged the skin retention time of DexP. In the context of psoriasis treatment, DexP HMNs were demonstrated to be more effective than DexP CSMNs in inhibiting keratinocyte proliferation and significantly reducing the levels of inflammatory factors and immune cell infiltration at the lesion site. This study provides a new direction for the development of intelligent microneedle drug delivery systems for sustained drug release and enhanced management of chronic skin diseases.

Acid-Triggered Dual-Functional Hydrogel Platform for Enhanced Bone Regeneration

Stem cell implantation holds promise for enhancing bone repair, but risks of pathogen transmission and malignant cell transformation should not be ignored. Compared to stem cell implantation, recruitment of endogenous stem cells to injured sites is more critical for in situ bone regeneration. In this study, based on the acidic microenvironment of bone injury, an HG-AA1:1-SDF-1α composite hydrogel with a dual-control intelligent switch function is developed by incorporating stromal cell-derived factor (SDF-1α), arginine carbon dots (Arg-CDs), and calcium ions (Ca2+) into the oxidized hyaluronic acid/gelatin methacryloyl (HG) hydrogel. The acidic microenvironment triggers the first switch (Schiff base bond is broken between HG-AA1:1 and SDF-1α) of HG-AA1:1-SDF-1α composite hydrogel to continuously release SDF-1α. Compared to the neutral (pH 7.4) media, the cumulative release of SDF-1α in acidic (pH 5.5) media is ≈2.5 times higher, which enhances the migration and recruitment of endogenous mesenchymal stem cells (MSCs). The recruited MSCs immediately initiate the second switch and metabolize Arg-CDs into the bioactive nitric oxide (NO) in the presence of Ca2+, activating NO/cyclic guanosine monophosphate (cGMP) signaling pathway to promote angiogenesis. Therefore, the engineered HG-AA1:1-SDF-1α composite hydrogel shows promising potential to achieve "coupling osteogenesis and angiogenesis" for bone regeneration.

Accelerating repair of infected bone defects through post-reinforced injectable hydrogel mediated antibacterial/immunoregulatory microenvironment at bone-hydrogel interface

Functional injectable hydrogel (IH) is promising for infected bone defects (IBDs) repair, but how to endow it with desired antibacterial/immunoregulatory functions as well as avoid mechanical failures during its manipulation has posed as main challenges. Herein, rosmarinic acid (RosA), a natural product with antibacterial/immunoregulatory activities, was utilized to develop a FCR IH through forming phenylboronic acid ester bonds with 4-formylphenyl phenylboronic acid (4-FPBA) grafted chitosan (CS) (FC). After being applied to the IBD site, the FCR IH was then injected with tobramycin (Tob) solution, another alkaline antibacterial drug, to induce in situ crystallization of the FC, endowing the resultant FCRT hydrogel with adaptively enhanced mechanical strength and structural stability. Owing to the specific structural composition, the FCRT hydrogel could sustainedly release Tob and RosA molecules at the IBD interface, effectively eliminating in situ bacterial infection. In addition, the released RosA molecules also induced the M2 polarization of in situ macrophages (Mφ), which was identified to be related to the NF-κB and PI3K-AKT pathways, therefore promoting the osteogenic differentiation of in situ bone marrow stromal cells (BMSCs). Due to the simultaneous antibacterial/osteo-immunoregulatory microenvironment at the IBD interface, the repair of IBDs was proved to be greatly accelerated by the FCRT hydrogel.

Functionalized Bone Implant Inspired by Lattice Defense Strategy: Grid Management, Precise and Effective Multiple-Prevention of Osteomyelitis Recurrence and Promote Bone Regeneration

Osteomyelitis with a high recurrence rate. Timely-prevention can avoid severe consequence and death. However, conventional drug response-release has the disadvantages of unnecessary release and waste, causing ineffective prevention. Inspired by "Lattice-defense technology", gridding lesion areas and constructing a "Triggered-precise response-release system" may be an effective multiple-prevention method. Here, a new strict pH-triggered response drug controlled-release mechanism was proposed innovatively to construct a "Triggered-precise response-release system" and achieve multiple-effective prevention. PO4 3--Ce3+ strict pH responsive release system is prepared through simultaneous hydration reaction of solution-polymerization and compounded in bone-implant. The dispersed system only targets micro-interface contact areas, achieving gridded management of the lesion site. In a normal environment, Ce3+ is captured by PO4 3- and kept electrostatic-attraction balance, ensuring the zero-concentration Ce3+ release continuously. Once osteomyelitis recurs and pH decrease, H+ at the interface will combine with PO4 3- under electrostatic drive and disrupt potential balance, achieving the release of Ce3+ only when the infection recurs. In vivo experiments was confirmed effective prevention and excellent promote bone regeneration. The adoption of "Lattice defense technology" has achieved accuracy spatiotemporal of drug delivery. Even if other lesion sites unfortunately recur again, effective-prevention can be guaranteed. Bone-implant show great potential in preventing osteomyelitis.

Advanced Bioresponsive Drug Delivery Systems for Promoting Diabetic Vascularized Bone Regeneration

The treatment of bone defects in diabetes mellitus (DM) patients remains a major challenge since the diabetic microenvironments significantly impede bone regeneration. Many abnormal factors including hyperglycemia, elevated oxidative stress, increased inflammation, imbalanced osteoimmune, and impaired vascular system in the diabetic microenvironment will result in a high rate of impaired, delayed, or even nonhealing events of bone tissue. Stimuli-responsive biomaterials that can respond to endogenous biochemical signals have emerged as effective therapeutic systems to treat diabetic bone defects via the combination of microenvironmental regulation and enhanced osteogenic capacity. Following the natural bone healing processes, coupling of angiogenesis and osteogenesis by advanced bioresponsive drug delivery systems has proved to be of significant approach for promoting bone repair in DM. In this Review, we have systematically summarized the mechanisms and therapeutic strategies of DM-induced impaired bone healing, outlined the bioresponsive design for drug delivery systems, and highlighted the vascularization strategies for promoting bone regeneration. Accordingly, we then overview the recent advances in developing bioresponsive drug delivery systems to facilitate diabetic vascularized bone regeneration by remodeling the microenvironment and modulating multiple regenerative cues. Furthermore, we discuss the development of adaptable drug delivery systems with unique features for guiding DM-associated bone regeneration in the future.

MMP-9 responsive hydrogel promotes diabetic wound healing by suppressing ferroptosis of endothelial cells

Ferroptosis plays a crucial role in the progression of diabetic wounds, suggesting potential therapeutic strategies to target ferroptosis. Transient receptor potential ankyrin 1 (TRPA1) is a non-selective calcium channel that acts as a receptor for a variety of physical or chemical stimuli. Cinnamaldehyde (CA) is a specific TRPA1 agonist. In in vitro experiments, we observed that high glucose (HG) treatment induced endothelial cell ferroptosis, impairing cell function. CA successfully inhibited endothelial cell ferroptosis, improving migration, proliferation, and tube formation. Further mechanistic studies showed that CA-activated TRPA1-induced Ca2+ influx promoted the phosphorylation of calmodulin-dependent protein kinase II (CaMKII) and nuclear factor-E 2-related factor 2 (Nrf2) translocation, which contributed to the elevation of glutathione peroxidase 4 (GPX4), leading to the inhibition of endothelial cell ferroptosis. In addition, CA was incorporated into an MMP-9-responsive injectable duplex hybrid hydrogel (CA@HA-Gel), allowing its efficient sustained release into diabetic wounds in an inflammation-responsive manner. The results showed that CA@HA-Gel inhibited wound endothelial cell ferroptosis and significantly promoted diabetic wound healing. In summary, the results presented in this study emphasize the potential therapeutic application of CA@HA-Gel in the treatment of diseases associated with ferroptosis.

Implantable Multifunctional Micro-Oxygen Reservoir System for Promoting Vascular-Osteogenesis via Remodeling Regenerative Microenvironment

Hypoxia and reactive oxygen species (ROS) overaccumulation cause persistent oxidative stress and impair intrinsic regenerative potential upon tissue injury. For local tissue injury with hypoxia, such as bone fracture and defects, a localized-sufficient oxygen supply is highly desirable but remains challenging. Therefore, to explore a strategy and its intrinsic mechanism for supplying oxygen locally and remodeling the regenerative microenvironment, an innovative oxygenating hydrogel microsphere system with sustained oxygenation and antioxidant properties is introduced by loading CaO2@SiO2@PDA (CSP) nanoparticles. Specifically, the CSP nanoparticles exhibited broad-spectrum free radicals scavenging ability, along with prolonged controlled-release of oxygen once integrated into the gelatin methacrylate anhydride (GelMA) microspheres (CSP-GM). The CSP-GM with extra cellular matrix (ECM)-mimicking structures reconstructed living niches, promoting the adhesion and proliferation of bone marrow stromal cells (BMSCs). As a multifaceted microenvironment regulator, CSP-GM remodeled the regenerative microenvironment by synergistically producing oxygen and scavenging ROS, recovering mitochondrial homeostasis and antioxidant defenses of BMSCs, promoting angiogenesis and osteogenesis under hypoxia conditions via precisely modulating the Nrf2/HO-1 signaling pathway. The multiple pro-regenerative effects of the implantable functionalized micro-oxygen reservoir on bone repair are further corroborated by the enhanced vascularized bone formation in rat femoral defects, presenting a comprehensive and promising strategy for tissue repair.

Multifaceted role of nanocomposite hydrogels in diabetic wound healing: enhanced biomedical applications and detailed molecular mechanisms

The complex microenvironment of diabetic wounds, which is characterized by persistent hyperglycemia, excessive inflammatory responses, and hypoxic conditions, significantly impedes the efficacy of traditional hydrogels. Nanocomposite hydrogels, which combine the high-water content and biocompatibility of hydrogels with the unique functionalities of nanomaterials, offer a promising solution. These hydrogels exhibit enhanced antibacterial, antioxidant, and drug-release properties. Incorporating nanomaterials increases the mechanical strength and bioactivity of hydrogels, allowing for dynamic regulation of the wound microenvironment and promoting cell migration, proliferation, and angiogenesis, thereby accelerating wound healing. This review provides a comprehensive overview of the latest advances in nanocomposite hydrogels for diabetic wound treatment and discusses their advantages and molecular mechanisms at various healing stages. The study aims to provide a theoretical foundation and practical guidance for future research and clinical applications. Furthermore, it highlights the challenges related to the mechanical durability, antimicrobial performance, resistance issues, and interactions with the cellular environments of these hydrogels. Future directions include optimizing smart drug delivery systems and personalized medical approaches to enhance the clinical applicability of nanocomposite hydrogels.

Nanoparticle-Mediated Gene Delivery for Bone Tissue Engineering

Critical-sized bone defects represent an urgent clinical problem, necessitating innovative treatment approaches. Gene-activated grafts for bone tissue engineering have emerged as a promising solution. However, traditional gene delivery methods are constrained by limited osteogenic efficacy and safety concerns. Recently, organic and inorganic nanoparticle (NP) vectors have attracted significant attention in bone tissue engineering for their safe, stable, and controllable gene delivery. Targeted gene delivery guided by insights into bone healing mechanisms, coupled with the multifunctional design of NPs, is crucial for enhancing therapeutic outcomes. Here, the theoretical foundations underlying NP-mediated gene therapy for enhancing bone healing across different histological stages are elucidated. Furthermore, the distinct attributes of functionalized NP vectors are discussed, and cutting-edge strategies aimed at optimizing gene delivery efficiency throughout the therapeutic process are highlighted. Additionally, the review addresses the unresolved challenges and prospects of this technology. This review may contribute to the continued development and clinical application of NP-mediated gene delivery for treating critical-sized bone defects.

Targeting Osteoblast-Osteoclast Cross-Talk Bone Homeostasis Repair Microcarriers Promotes Intervertebral Fusion in Osteoporotic Rats

Balancing osteoblast-osteoclast (OB-OC) cross-talk is crucial for restoring bone tissue structure and function. Current clinical drugs targeting either osteogenesis or osteoclastogenesis fail to effectively regulate cross-talk, impeding efficient bone repair in osteoporosis patients. Ubiquitin-specific protease 26 (USP26) is shown to coordinate OB-OC cross-talk by independently regulating β-catenin and Iκb-α. However, effective drugs for activating USP26 are still lacking. Here, they constructed bone homeostasis repair microcarriers (BHRC) that encapsulate Usp26 mRNA-loaded lipid nanoparticles (mRNA@LNP) within MMPs-responsive GelMA hydrogel microspheres. These microcarriers target the osteoporotic microenvironment and regulate OB-OC cross-talk, thereby facilitating intervertebral fusion in osteoporotic rats. Results demonstrate that mRNA@LNP exhibits uniform particle size and high transfection efficiency, while GelMA hydrogel microspheres possess excellent biocompatibility and MMP responsiveness, providing favorable cell survival space and controllable release of mRNA@LNP. The released LNP upregulates USP26 protein expression, effectively promoting osteogenesis while suppressing osteoclast formation. In vivo experiments show that injecting BHRC into the defect site of intervertebral discs in osteoporotic rats significantly promotes tail vertebrae fusion by responding to the microenvironment and regulating cell-to-cell cross-talk. Thus, the BHRC holds great potential in regulating osteoporotic homeostasis, particularly in challenging bone defects such as intervertebral fusion in osteoporotic environments.

Phosphoramide Hydrogels as Biodegradable Matrices for Inkjet Printing and Their Nano-Hydroxyapatite Composites

Inkjet printing is a leading technology in the biofabrication of three-dimensional biomaterials, offering digital, noncontact deposition with micron-level precision. Among these materials, hydroxyapatite is widely recognized for its use in bone tissue engineering. However, most hydroxyapatite-laden inks are unsuitable for inkjet printing. To address this, we developed photocurable and biodegradable phosphoramide-based hydrogels containing thiol-functionalized polyethylene glycol via click chemistry. These hydrogels degrade into phosphates, the natural component of bone. The rheological properties of the inks are finely tuned through chemical design to meet the requirements of nanohydroxyapatite composite inks for piezoelectric inkjet printing. We demonstrated their printability using simple geometric patterns, showcasing a versatile and efficient solution for the precise inkjet printing of biomaterial composites.

Introductory Review of Soft Implantable Bioelectronics Using Conductive and Functional Hydrogels and Hydrogel Nanocomposites

Interfaces between implantable bioelectrodes and tissues provide critical insights into the biological and pathological conditions of targeted organs, aiding diagnosis and treatment. While conventional bioelectronics, made from rigid materials like metals and silicon, have been essential for recording signals and delivering electric stimulation, they face limitations due to the mechanical mismatch between rigid devices and soft tissues. Recently, focus has shifted toward soft conductive materials, such as conductive hydrogels and hydrogel nanocomposites, known for their tissue-like softness, biocompatibility, and potential for functionalization. This review introduces these materials and provides an overview of recent advances in soft hydrogel nanocomposites for implantable electronics. It covers material strategies for conductive hydrogels, including both intrinsically conductive hydrogels and hydrogel nanocomposites, and explores key functionalization techniques like biodegradation, bioadhesiveness, injectability, and self-healing. Practical applications of these materials in implantable electronics are also highlighted, showcasing their effectiveness in real-world scenarios. Finally, we discuss emerging technologies and future needs for chronically implantable bioelectronics, offering insights into the evolving landscape of this field.

Injectable hydrogels for bone regeneration with tunable degradability via peptide chirality modification

The degradability of hydrogels plays a pivotal role in bone regeneration, yet its precise effects on the bone repair process remain poorly understood. Traditional studies have been limited by the use of hydrogels with insufficient variation in degradation properties for thorough comparative analysis. Addressing this gap, our study introduces the development of matrix metalloproteinase (MMP)-responsive hydrogels engineered with a tunable degradation rate, specifically designed for bone regeneration applications. These innovative hydrogels are synthesized by integrating MMP-sensitive peptides, which exhibit chirality-transferred amino acids, with norbornene (NB)-modified 8-arm polyethylene glycol (PEG) macromers to form the hydrogel network. The degradation behavior of these hydrogels is manipulated through the chirality of the incorporated peptides, resulting in the classification into L, LD, and D hydrogels. Remarkably, the L hydrogel variant shows a significantly enhanced degradation rate, both in vitro and in vivo, which in turn fosters bone regeneration by promoting cell migration and upregulating osteogenic gene expression. This research highlights the fundamental role of hydrogel degradability in bone repair and lays the groundwork for the advancement of degradable hydrogel technologies for bone regeneration, offering new insights and potential for future biomaterials development.

Rational design of multifunctional hydrogels targeting the microenvironment of diabetic periodontitis

Periodontitis is a chronic inflammatory disease and is the primary contributor to adult tooth loss. Diabetes exacerbates periodontitis, accelerates periodontal bone resorption. Thus, effectively managing periodontitis in individuals with diabetes is a long-standing challenge. This review introduces the etiology and pathogenesis of periodontitis, and analyzes the bidirectional relationship between diabetes and periodontitis. In this review, we comprehensively summarize the four pathological microenvironments influenced by diabetic periodontitis: high glucose microenvironment, bacterial infection microenvironment, inflammatory microenvironment, and bone loss microenvironment. The hydrogel design strategies and latest research development tailored to the four microenvironments of diabetic periodontitis are mainly focused on. Finally, the challenges and potential solutions in the treatment of diabetic periodontitis are discussed. We believe this review will be helpful for researchers seeking novel avenues in the treatment of diabetic periodontitis.

MicroRNA-29c-tetrahedral framework nucleic acids: Towards osteogenic differentiation of mesenchymal stem cells and bone regeneration in critical-sized calvarial defects

Certain miRNAs, notably miR29c, demonstrate a remarkable capacity to regulate cellular osteogenic differentiation. However, their application in tissue regeneration is hampered by their inherent instability and susceptibility to degradation. In this study, we developed a novel miR29c delivery system utilising tetrahedral framework nucleic acids (tFNAs), aiming to enhance its stability and endocytosis capability, augment the efficacy of miR29c, foster osteogenesis in bone marrow mesenchymal stem cells (BMSCs), and significantly improve the repair of critical-sized bone defects (CSBDs). We confirmed the successful synthesis and biocompatibility of sticky ends-modified tFNAs (stFNAs) and miR29c-modified stFNAs (stFNAs-miR29c) through polyacrylamide gel electrophoresis, microscopy scanning, a cell counting kit-8 assay and so on. The mechanism and osteogenesis effects of stFNAs-miR29c were explored using immunofluorescence staining, western blotting, and reserve transcription quantitative real-time polymerase chain reaction. Additionally, the impact of stFNAs-miR29c on CSBD repair was assessed via micro-CT and histological staining. The nano-carrier, stFNAs-miR29c was successfully synthesised and exhibited exemplary biocompatibility. This nano-nucleic acid material significantly upregulated osteogenic differentiation-related markers in BMSCs. After 2 months, stFNAs-miR29c demonstrated significant bone regeneration and reconstruction in CSBDs. Mechanistically, stFNAs-miR29c enhanced osteogenesis of BMSCs by upregulating the Wnt signalling pathway, contributing to improved bone tissue regeneration. The development of this novel nucleic acid nano-carrier, stFNAs-miR29c, presents a potential new avenue for guided bone regeneration and bone tissue engineering research.

Autocatalytic bifunctional supramolecular hydrogels for osteoporotic bone repair

Conventional bone scaffolds, which are mainly ascribed to highly active osteoclasts and an inflammatory microenvironment with high levels of reactive oxygen species and pro-inflammatory factors, barely satisfy osteoporotic defect repair. Herein, multifunctional self-assembled supramolecular fiber hydrogels (Ce-Aln gel) consisting of alendronate (Aln) and cerium (Ce) ions were constructed for osteoporotic bone defect repair. Based on the reversible interaction and polyvalent cerium ions, the Ce-Aln gel, which was mainly composed of ionic coordination and hydrogen bonds, displayed good injectability and autocatalytic amplification of the antioxidant effect. In vitro studies showed that the Ce-Aln gel effectively maintained the biological function of osteoblasts by regulating redox homeostasis and improved the inflammatory microenvironment to enhance the inhibitory effect on osteoclasts. Ribonucleic acid (RNA) sequencing further revealed significant downregulation of various metabolic pathways, including apoptosis signaling, hypoxia metabolism and tumor necrosis factor-alpha (TNF-α) signaling via the nuclear factor kappa-B pathway after treatment with the Ce-Aln gel. In vivo experiments showed that the clinical drug-based Ce-Aln gel effectively promoted the tissue repair of osteoporotic bone defects by improving inflammation and inhibiting osteoclast formation at the defect. Notably, in vivo systemic osteoporosis was significantly ameliorated, highlighting the strong potential of clinical translation for precise therapy of bone defects.

The Exosome-Mediated Bone Regeneration: An Advanced Horizon Toward the Isolation, Engineering, Carrying Modalities, and Mechanisms

Exosomes, nanoparticles secreted by various cells, composed of a bilayer lipid membrane, and containing bioactive substances such as proteins, nucleic acids, metabolites, etc., have been intensively investigated in tissue engineering owing to their high biocompatibility and versatile biofunction. However, there is still a lack of a high-quality review on bone defect regeneration potentiated by exosomes. In this review, the biogenesis and isolation methods of exosomes are first introduced. More importantly, the engineered exosomes of the current state of knowledge are discussed intensively in this review. Afterward, the biomaterial carriers of exosomes and the mechanisms of bone repair elucidated by compelling evidence are presented. Thus, future perspectives and concerns are revealed to help devise advanced modalities based on exosomes to overcome the challenges of bone regeneration. It is totally believed this review will attract special attention from clinicians and provide promising ideas for their future works.

Recent advances on thermosensitive hydrogels-mediated precision therapy

Precision therapy has become the preferred choice attributed to the optimal drug concentration in target sites, increased therapeutic efficacy, and reduced adverse effects. Over the past few years, sprayable or injectable thermosensitive hydrogels have exhibited high therapeutic potential. These can be applied as cell-growing scaffolds or drug-releasing reservoirs by simply mixing in a free-flowing sol phase at room temperature. Inspired by their unique properties, thermosensitive hydrogels have been widely applied as drug delivery and treatment platforms for precision medicine. In this review, the state-of-the-art developments in thermosensitive hydrogels for precision therapy are investigated, which covers from the thermo-gelling mechanisms and main components to biomedical applications, including wound healing, anti-tumor activity, osteogenesis, and periodontal, sinonasal and ophthalmic diseases. The most promising applications and trends of thermosensitive hydrogels for precision therapy are also discussed in light of their unique features.

A Factor-Free Hydrogel with ROS Scavenging and Responsive Degradation for Enhanced Diabetic Bone Healing

In view of the increased levels of reactive oxygen species (ROS) that disturb the osteogenic differentiation of bone marrow mesenchymal stem cells (BMSCs), the repair of diabetic bone defects remains a great challenge. Herein, a factor-free hydrogel is reported with ROS scavenging and responsive degradation properties for enhanced diabetic bone healing. These hydrogels contain ROS-cleavable thioketal (TK) linkers and ultraviolet (UV)-responsive norbornene (NB) groups conjugated with 8-arm PEG macromers, which are formed via UV crosslinking-mediated gelation. Upon reacting with high levels of ROS in the bone defect microenvironment, ROS-cleavable TK linkers are destroyed, allowing the responsive degradation of hydrogels, which promotes the migration of BMSCs. Moreover, ROS levels are reduced through hydrogel-mediated ROS scavenging to reverse BMSC differentiation from adipogenic to osteogenic phenotype. As such, a favorable microenvironment is created after simultaneous ROS scavenging and hydrogel degradation, leading to the effective repair of bone defects in diabetic mouse models, even without the addition of growth factors. Thus, this study presents a responsive hydrogel platform that regulates ROS scavenging and stromal degradation in bone engineering.

Smart responsive in situ hydrogel systems applied in bone tissue engineering

The repair of irregular bone tissue suffers severe clinical problems due to the scarcity of an appropriate therapeutic carrier that can match dynamic and complex bone damage. Fortunately, stimuli-responsive in situ hydrogel systems that are triggered by a special microenvironment could be an ideal method of regenerating bone tissue because of the injectability, in situ gelatin, and spatiotemporally tunable drug release. Herein, we introduce the two main stimulus-response approaches, exogenous and endogenous, to forming in situ hydrogels in bone tissue engineering. First, we summarize specific and distinct responses to an extensive range of external stimuli (e.g., ultraviolet, near-infrared, ultrasound, etc.) to form in situ hydrogels created from biocompatible materials modified by various functional groups or hybrid functional nanoparticles. Furthermore, "smart" hydrogels, which respond to endogenous physiological or environmental stimuli (e.g., temperature, pH, enzyme, etc.), can achieve in situ gelation by one injection in vivo without additional intervention. Moreover, the mild chemistry response-mediated in situ hydrogel systems also offer fascinating prospects in bone tissue engineering, such as a Diels-Alder, Michael addition, thiol-Michael addition, and Schiff reactions, etc. The recent developments and challenges of various smart in situ hydrogels and their application to drug administration and bone tissue engineering are discussed in this review. It is anticipated that advanced strategies and innovative ideas of in situ hydrogels will be exploited in the clinical field and increase the quality of life for patients with bone damage.

Matrix Metalloproteinase-Responsive Hydrogel with On-Demand Release of Phosphatidylserine Promotes Bone Regeneration Through Immunomodulation

Inflammation-responsive hydrogels loaded with therapeutic factors are effective biomaterials for bone tissue engineering and regenerative medicine. In this study, a matrix metalloproteinase (MMP)-responsive injectable hydrogel is constructed by integrating an MMP-cleavable peptide (pp) into a covalent tetra-armed poly-(ethylene glycol) (PEG) network for precise drug release upon inflammation stimulation. To establish a pro-regenerative environment, phosphatidylserine (PS) is encapsulated into a scaffold to form the PEG-pp-PS network, which could be triggered by MMP to release a large amount of PS during the early stage of inflammation and retain drug release persistently until the later stage of bone repair. The hydrogel is found to be mechanically and biologically adaptable to the complex bone defect area. In vivo and in vitro studies further demonstrated the ability of PEG-pp-PS to transform macrophages into the anti-inflammatory M2 phenotype and promote osteogenic differentiation, thus, resulting in new bone regeneration. Therefore, this study provides a facile, safe, and promising cell-free strategy on simultaneous immunoregulation and osteoinduction in bone engineering.

Injectable, self-healing hydrogels based on gelatin, quaternized chitosan, and laponite as localized celecoxib delivery system for nucleus pulpous repair

Utilization of injectable hydrogels stands as a paradigm of minimally invasive intervention in the context of intervertebral disc degeneration treatment. Restoration of nucleus pulposus (NP) function exerts a profound influence in alleviating back pain. This study introduces an innovative class of injectable shear-thinning hydrogels, founded on quaternized chitosan (QCS), gelatin (GEL), and laponite (LAP) with the capacity for sustained release of the anti-inflammatory drug, celecoxib (CLX). First, synthesis of Magnesium-Aluminum-Layered double hydroxide (LDH) was achieved through a co-precipitation methodology, as a carrier for celecoxib and a source of Mg ions. Intercalation of celecoxib within LDH layers (LDH-CLX) was verified through a battery of analytical techniques, including FTIR, XRD, SEM, EDAX, TGA and UV-visible spectroscopy confirmed a drug loading efficiency of 39.22 ± 0.09 % within LDH. Then, LDH-CLX was loaded in the optimal GEL-QCS-LAP hydrogel under physiological conditions. Release behavior (15 days profile), mechanical properties, swelling ratio, and degradation rate of the resulting composite were evaluated. A G* of 15-47 kPa was recorded for the hydrogel at 22-40 °C, indicating gel stability in this temperature range. Self-healing properties and injectability of the composite were proved by rheological measurements. Also, ex vivo injection into intervertebral disc of sheep, evidenced in situ forming and NP cavity filling behavior of the hydrogel. Support of GEL-QCS-LAP/LDH-CLX (containing mg2+ ions) for viability and proliferation (from ~94 % on day 1 to ~134 % on day 7) of NP cells proved using MTT assay, DAPI and Live/Dead assays. The hydrogel could significantly upregulate secretion of glycosaminoglycan (GAG, from 4.68 ± 0.1 to 27.54 ± 1.0 μg/ml), when LHD-CLX3% was loaded. We conclude that presence of mg2+ ion and celecoxib in the hydrogel can lead to creation of a suitable environment that encourages GAG secretion. In conclusion, the formulated hydrogel holds promise as a minimally invasive candidate for degenerative disc repair.

N6-Methyladenosine in Cell-Fate Determination of BMSCs: From Mechanism to Applications

The methylation of adenosine base at the nitrogen-6 position is referred to as "N6-methyladenosine (m6A)" and is one of the most prevalent epigenetic modifications in eukaryotic mRNA and noncoding RNA (ncRNA). Various m6A complex components known as "writers," "erasers," and "readers" are involved in the function of m6A. Numerous studies have demonstrated that m6A plays a crucial role in facilitating communication between different cell types, hence influencing the progression of diverse physiological and pathological phenomena. In recent years, a multitude of functions and molecular pathways linked to m6A have been identified in the osteogenic, adipogenic, and chondrogenic differentiation of bone mesenchymal stem cells (BMSCs). Nevertheless, a comprehensive summary of these findings has yet to be provided. In this review, we primarily examined the m6A alteration of transcripts associated with transcription factors (TFs), as well as other crucial genes and pathways that are involved in the differentiation of BMSCs. Meanwhile, the mutual interactive network between m6A modification, miRNAs, and lncRNAs was intensively elucidated. In the last section, given the beneficial effect of m6A modification in osteogenesis and chondrogenesis of BMSCs, we expounded upon the potential utility of m6A-related therapeutic interventions in the identification and management of human musculoskeletal disorders manifesting bone and cartilage destruction, such as osteoporosis, osteomyelitis, osteoarthritis, and bone defect.

Advances and Trends of Photoresponsive Hydrogels for Bone Tissue Engineering

The development of static hydrogels as an optimal choice for bone tissue engineering (BTE) remains a difficult challenge primarily due to the intricate nature of bone healing processes, continuous physiological functions, and pathological changes. Hence, there is an urgent need to exploit smart hydrogels with programmable properties that can effectively enhance bone regeneration. Increasing evidence suggests that photoresponsive hydrogels are promising bioscaffolds for BTE due to their advantages such as controlled drug release, cell fate modulation, and the photothermal effect. Here, we review the current advances in photoresponsive hydrogels. The mechanism of photoresponsiveness and its advanced applications in bone repair are also elucidated. Future research would focus on the development of more efficient, safer, and smarter photoresponsive hydrogels for BTE. This review is aimed at offering comprehensive guidance on the trends of photoresponsive hydrogels and shedding light on their potential clinical application in BTE.

Smart Hydrogels for Tissue Regeneration

The rapid growth in the portion of the aging population has led to a consequent increase in demand for biomedical hydrogels, together with an assortment of challenges that need to be overcome in this field. Smart hydrogels can autonomously sense and respond to the physiological/pathological changes of the tissue microenvironment and continuously adapt the response according to the dynamic spatiotemporal shifts in conditions. This along with other favorable properties, make smart hydrogels excellent materials for employing toward improving the precision of treatment for age-related diseases. The key factor during the smart hydrogel design is on accurately identifying the characteristics of natural tissues and faithfully replicating the composition, structure, and biological functions of these tissues at the molecular level. Such hydrogels can accurately sense distinct physiological and external factors such as temperature and biologically active molecules, so they may in turn actively and promptly adjust their response, by regulating their own biological effects, thereby promoting damaged tissue repair. This review summarizes the design strategies employed in the creation of smart hydrogels, their response mechanisms, as well as their applications in field of tissue engineering; and concludes by briefly discussing the relevant challenges and future prospects.

Repair of Infected Bone Defects with Hydrogel Materials

Infected bone defects represent a common clinical condition involving bone tissue, often necessitating surgical intervention and antibiotic therapy. However, conventional treatment methods face obstacles such as antibiotic resistance and susceptibility to postoperative infections. Hydrogels show great potential for application in the field of tissue engineering due to their advantageous biocompatibility, unique mechanical properties, exceptional processability, and degradability. Recent interest has surged in employing hydrogels as a novel therapeutic intervention for infected bone repair. This article aims to comprehensively review the existing literature on the anti-microbial and osteogenic approaches utilized by hydrogels in repairing infected bones, encompassing their fabrication techniques, biocompatibility, antimicrobial efficacy, and biological activities. Additionally, the potential opportunities and obstacles in their practical implementation will be explored. Lastly, the limitations presently encountered and the prospective avenues for further investigation in the realm of hydrogel materials for the management of infected bone defects will be deliberated. This review provides a theoretical foundation and advanced design strategies for the application of hydrogel materials in the treatment of infected bone defects.

Photo-Controllable Smart Hydrogels for Biomedical Application: A Review

Nowadays, smart hydrogels are being widely studied by researchers because of their advantages such as simple preparation, stable performance, response to external stimuli, and easy control of response behavior. Photo-controllable smart hydrogels (PCHs) are a class of responsive hydrogels whose physical and chemical properties can be changed when stimulated by light at specific wavelengths. Since the light source is safe, clean, simple to operate, and easy to control, PCHs have broad application prospects in the biomedical field. Therefore, this review timely summarizes the latest progress in the PCHs field, with an emphasis on the design principles of typical PCHs and their multiple biomedical applications in tissue regeneration, tumor therapy, antibacterial therapy, diseases diagnosis and monitoring, etc. Meanwhile, the challenges and perspectives of widespread practical implementation of PCHs are presented in biomedical applications. This study hopes that PCHs will flourish in the biomedical field and this review will provide useful information for interested researchers.

Multi-stimuli-responsive Hydrogels for Therapeutic Systems: An Overview on Emerging Materials, Devices, and Drugs

The rising interest in hydrogels nowadays is due to their usefulness in physiological conditions as multi-stimuli-responsive hydrogels. To reply to the prearranged stimuli, including chemical triggers, light, magnetic field, electric field, ionic strength, temperature, pH, and glucose levels, dual/multi-stimuli-sensitive gels/hydrogels display controllable variations in mechanical characteristics and swelling. Recent attention has focused on injectable hydrogel-based drug delivery systems (DDS) because of its promise to offer regulated, controlled, and targeted medication release to the tumor site. These technologies have great potential to improve treatment outcomes and lessen side effects from prolonged chemotherapy exposure.

3D-Printed Flat-Bone-Mimetic Bioceramic Scaffolds for Cranial Restoration

The limitations of autologous bone grafts necessitate the development of advanced biomimetic biomaterials for efficient cranial defect restoration. The cranial bones are typical flat bones with sandwich structures, consisting of a diploe in the middle region and 2 outer compact tables. In this study, we originally developed 2 types of flat-bone-mimetic β-tricalcium phosphate bioceramic scaffolds (Gyr-Comp and Gyr-Tub) by high-precision vat-photopolymerization-based 3-dimensional printing. Both scaffolds had 2 outer layers and an inner layer with gyroid pores mimicking the diploe structure. The outer layers of Gyr-Comp scaffolds simulated the low porosity of outer tables, while those of Gyr-Tub scaffolds mimicked the tubular pore structure in the tables of flat bones. The Gyr-Comp and Gyr-Tub scaffolds possessed higher compressive strength and noticeably promoted in vitro cell proliferation, osteogenic differentiation, and angiogenic activities compared with conventional scaffolds with cross-hatch structures. After implantation into rabbit cranial defects for 12 weeks, Gyr-Tub achieved the best repairing effects by accelerating the generation of bone tissues and blood vessels. This work provides an advanced strategy to prepare biomimetic biomaterials that fit the structural and functional needs of efficacious bone regeneration.

Mechanoluminescent Materials Enable Mechanochemically Controlled Atom Transfer Radical Polymerization and Polymer Mechanotransduction

Organic mechanophores have been widely adopted for polymer mechanotransduction. However, most examples of polymer mechanotransduction inevitably experience macromolecular chain rupture, and few of them mimic mussel's mechanochemical regeneration, a mechanically mediated process from functional units to functional materials in a controlled manner. In this paper, inorganic mechanoluminescent (ML) materials composed of CaZnOS-ZnS-SrZnOS: Mn2+ were used as a mechanotransducer since it features both piezoelectricity and mechanolunimescence. The utilization of ML materials in polymerization enables both mechanochemically controlled radical polymerization and the synthesis of ML polymer composites. This procedure features a mechanochemically controlled manner for the design and synthesis of diverse mechanoresponsive polymer composites.

Functional anti-bone tumor biomaterial scaffold: construction and application

Bone tumors, including primary bone tumors and bone metastases, have been plagued by poor prognosis for decades. Although most tumor tissue is removed, clinicians are still confronted with the dilemma of eliminating residual cancer cells and regenerating defective bone tissue after surgery. Therefore, functional biomaterial scaffolds are considered to be the ideal candidates to bridge defective tissues and restrain cancer recurrence. Through functionalized structural modifications or coupled therapeutic agents, they provide sufficient mechanical strength and osteoinductive effects while eliminating cancer cells. Numerous novel approaches such as photodynamic, photothermal, drug-conjugated, and immune adjuvant-assisted therapies have exhibited remarkable efficacy against tumors while exhibiting low immunogenicity. This review summarizes the progress of research on biomaterial scaffolds based on different functionalization strategies in bone tumors. We also discuss the feasibility and advantages of the combined application of multiple functionalization strategies. Finally, potential obstacles to the clinical translation of anti-tumor bone bioscaffolds are highlighted. This review will provide valuable references for future advanced biomaterial scaffold design and clinical bone tumor therapy.

Nano-crosslinked dynamic hydrogels for biomedical applications

Hydrogels resemble natural extracellular matrices and have been widely studied for biomedical applications. Nano-crosslinked dynamic hydrogels combine the injectability and self-healing property of dynamic hydrogels with the versatility of nanomaterials and exhibit unique advantages. The incorporation of nanomaterials as crosslinkers can improve the mechanical properties (strength, injectability, and shear-thinning properties) of hydrogels by reinforcing the skeleton and endowing them with multifunctionality. Nano-crosslinked functional hydrogels that can respond to external stimuli (such as pH, heat, light, and electromagnetic stimuli) and have photothermal properties, antimicrobial properties, stone regeneration abilities, or tissue repair abilities have been constructed through reversible covalent crosslinking strategies and physical crosslinking strategies. The possible cytotoxicity of the incorporated nanomaterials can be reduced. Nanomaterial hydrogels show excellent biocompatibility and can facilitate cell proliferation and differentiation for biomedical applications. This review introduces different nano-crosslinked dynamic hydrogels in the medical field, from fabrication to application. In this review, nanomaterials for dynamic hydrogel fabrication, such as metals and metallic oxides, nanoclays, carbon-based nanomaterials, black phosphorus (BP), polymers, and liposomes, are discussed. We also introduce the dynamic crosslinking method commonly used for nanodynamic hydrogels. Finally, the medical applications of nano-crosslinked hydrogels are presented. We hope that this summary will help researchers in the related research fields quickly understand nano-crosslinked dynamic hydrogels to develop more preparation strategies and promote their development and application.