Self-Healing Injectable Hydrogels for Tissue Regeneration

Brittle and ductile yielding in soft materials

Many soft materials yield under mechanical loading, but how this transition from solid-like behavior to liquid-like behavior occurs can vary significantly. Understanding the physics of yielding is of great interest for the behavior of biological, environmental, and industrial materials, including those used as inks in additive manufacturing and muds and soils. For some materials, the yielding transition is gradual, while others yield abruptly. We refer to these behaviors as being ductile and brittle. The key rheological signatures of brittle yielding include a stress overshoot in steady-shear-startup tests and a steep increase in the loss modulus during oscillatory amplitude sweeps. In this work, we show how this spectrum of yielding behaviors may be accounted for in a continuum model for yield stress materials by introducing a parameter we call the brittility factor. Physically, an increased brittility decreases the contribution of recoverable deformation to plastic deformation, which impacts the rate at which yielding occurs. The model predictions are successfully compared to results of different rheological protocols from a number of real yield stress fluids with different microstructures, indicating the general applicability of the phenomenon of brittility. Our study shows that the brittility of soft materials plays a critical role in determining the rate of the yielding transition and provides a simple tool for understanding its effects under various loading conditions.

Activating the healing process: three-dimensional culture of stem cells in Matrigel for tissue repair

BACKGROUND

To establish a strategy for stem cell-related tissue regeneration therapy, human gingival mesenchymal stem cells (hGMSCs) were loaded with three-dimensional (3D) bioengineered Matrigel matrix scaffolds in high-cell density microtissues to promote local tissue restoration.

METHODS

The biological performance and stemness of hGMSCs under 3D culture conditions were investigated by viability and multidirectional differentiation analyses. A Sprague‒Dawley (SD) rat full-thickness buccal mucosa wound model was established, and hGMSCs/Matrigel were injected into the submucosa of the wound. Autologous stem cell proliferation and wound repair in local tissue were assessed by histomorphometry and immunohistochemical staining.

RESULTS

Three-dimensional suspension culture can provide a more natural environment for extensions and contacts between hGMSCs, and the viability and adipogenic differentiation capacity of hGMSCs were significantly enhanced. An animal study showed that hGMSCs/Matrigel significantly accelerated soft tissue repair by promoting autologous stem cell proliferation and enhancing the generation of collagen fibers in local tissue.

CONCLUSION

Three-dimensional cell culture with hydrogel scaffolds, such as Matrigel, can effectively improve the biological function and maintain the stemness of stem cells. The therapeutic efficacy of hGMSCs/Matrigel was confirmed, as these cells could effectively stimulate soft tissue repair to promote the healing process by activating the host microenvironment and autologous stem cells.

Deferoxamine-Loaded Injectable Chitosan-Grafted Chlorogenic Acid/Oxidized Hyaluronic Acid Hybrid Hydrogel with Antibacterial, Anti-inflammatory, and Angiogenesis-Promoting Properties for Diabetic Wound Repair

Diabetic chronic wounds are notoriously difficult to heal as a result of their susceptibility to infection. To address this issue, we constructed an innovated and adaptable solution in the form of injectable chitosan (CS) hydrogel, denoted as CCOD, with enhanced antibacterial and anti-inflammatory properties. This hydrogel is created through a Schiff base reaction that combines chitosan-grafted chlorogenic acid (CS-CGA) and oxidized hyaluronic acid (OHA) with deferoxamine (DFO) as a model drug. The combination of CS and CGA has demonstrated excellent antibacterial and anti-inflammatory properties, while grafting played a pivotal role in making these positive effects stable. These unique features make it possible to customize injectable hydrogel and fit any wound shape, allowing for more effective and personalized treatment of complex bacterial infections. Furthermore, the hydrogel system is not only effective against inflammation and bacterial infections but also possesses antioxidant and angiogenic abilities, making it an ideal solution for the repair of chronic wounds that have been previously thought of as unmanageable.

Injectable, self-healing and degradable dynamic hydrogels with tunable mechanical properties and stability by thermal-induced micellization

Dynamic hydrogels possessing injectable, degradable and self-healing abilities have attracted considerable attention in the biomedical field in recent years, but it is difficult to tune the mechanical properties and stability of conventional dynamic hydrogels. In this work, we synthesized ABA-triblock copolymers via RAFT polymerization, where the A block consisted of thermo-sensitive poly(N-isopropylacrylamide-co-diacetone acrylamide) and the B block was hydrophilic poly(acrylamide). Subsequently, dynamic hydrogels were obtained based on the acylhydrazone bonds between the triblock copolymers and adipic acid dihydrazide (ADH). The obtained hydrogels exhibited injectable and self-healable abilities. In response to the thermal-induced micellization of their temperature-responsive blocks, the mechanical strength of the hydrogels not only increased, but also they exhibited high stability even at pH 2.0. Moreover, the hydrogel in the stable state could be degraded by the fracture of its trithiocarbonate groups. In addition, the hydrogels exhibited good cytocompatibility and controlled release behavior for doxorubicin (DOX). Considering these attractive tunable properties, these dynamic hydrogels show various potential applications in the biomedical field, such as drug carriers and cell or tissue engineering scaffolds.

Stimuli‐Responsive Hydrogels for Antibacterial Applications

Hydrogels have emerged as promising candidates for biomedical applications, especially in the field of antibacterial therapeutics, due to their unique structural properties, highly tunable physicochemical properties, and excellent biocompatibility. The integration of stimuli-responsive functions into antibacterial hydrogels holds the potential to enhance their antibacterial properties and therapeutic efficacy, dynamically responding to different external or internal stimuli, such as pH, temperature, enzymes, and light. Therefore, this review describes the applications of hydrogel dressings responsive to different stimuli in antibacterial therapy. The collaborative interaction between stimuli-responsive hydrogels and antibacterial materials is discussed. This synergistic approach, in contrast to conventional antibacterial materials, not only amplifies the antibacterial effect but also alleviates adverse side effects and diminishes the incidence of multiple infections and drug resistance. The review provides a comprehensive overview of the current challenges and outlines future research directions for stimuli-responsive antibacterial hydrogels. It underscores the imperative for ongoing interdisciplinary research aimed at unraveling the mechanisms of wound healing. This understanding is crucial for optimizing the design and implementation of stimuli-responsive antibacterial hydrogels. Ultimately, this review aims to offer scientific guidance for the development and practical clinical application of stimuli-responsive antibacterial hydrogel dressings.

Porous Hydrogels for Immunomodulatory Applications

Cancer immunotherapy relies on the insight that the immune system can be used to defend against malignant cells. The aim of cancer immunotherapy is to utilize, modulate, activate, and train the immune system to amplify antitumor T-cell immunity. In parallel, the immune system response to damaged tissue is also crucial in determining the success or failure of an implant. Due to their extracellular matrix mimetics and tunable chemical or physical performance, hydrogels are promising platforms for building immunomodulatory microenvironments for realizing cancer therapy and tissue regeneration. However, submicron or nanosized pore structures within hydrogels are not favorable for modulating immune cell function, such as cell invasion, migration, and immunophenotype. In contrast, hydrogels with a porous structure not only allow for nutrient transportation and metabolite discharge but also offer more space for realizing cell function. In this review, the design strategies and influencing factors of porous hydrogels for cancer therapy and tissue regeneration are first discussed. Second, the immunomodulatory effects and therapeutic outcomes of different porous hydrogels for cancer immunotherapy and tissue regeneration are highlighted. Beyond that, this review highlights the effects of pore size on immune function and potential signal transduction. Finally, the remaining challenges and perspectives of immunomodulatory porous hydrogels are discussed.

An Injectable, Self-Healing, Adhesive Multifunctional Hydrogel Promotes Bacteria-Infected Wound Healing

Bacterial infections have a serious impact on public health. It is urgent to develop antibacterial hydrogels with good biocompatibility to reduce the use of antibiotics. In this study, poly(lipoic acid-co-sodium lipoate)-phytic acid (P(LA-SL)-PA) hydrogels are prepared by a simple mixture of the natural small molecules lipoic acid (LA) and phytic acid (PA) in a mild and green reaction environment. The crosslinking network is constructed through the connection of covalent disulfide bonds as well as the hydrogen bonds, which endow the injectable and self-healing properties. The P(LA-SL)-PA hydrogels exhibit an adjustable compression modulus and adhesion. The in vitro agar plates assay indicates that the antibacterial rate of hydrogels against Escherichia coli and Staphylococcus aureus is close to 95%. In the rat-infected wound model, the P(LA-SL)-PA hydrogels adhere closely to the tissue and promote epithelialization and collagen deposition with a significant effect on wound healing. These results prove that the P(LA-SL)-PA hydrogels could act as effective wound dressings for promoting the healing of infected wounds.

Biodegradable chitin nanofiber-alginate dialdehyde hydrogel: An injectable, self-healing scaffold for anti-tumor drug delivery

Injectable hydrogels fabricated from natural polymers have attracted increasing attentions for their potential in biomedical application owing to the biocompatibility and biodegradability. A new class of natural polymer based self-healing hydrogel is constructed through dynamic covalent bonds. The injectable self-healing hydrogels are fabricated by introducing alginate aldehyde to form Schiff base bonds with the chitin nanofibers. These hydrogels demonstrate excellent self-healing properties, injectability, and pH-responsive sol-gel transition behaviors. As a result, they can serve as carriers to allow an effective encapsulation of doxorubicin (DOX) for drug delivery. Furthermore, these hydrogels exhibit excellent biocompatibility and degradability in vitro and in vivo. The sustained release of DOX from the hydrogels effectively suppresses tumor growth in animal models without causing significant systemic toxicity, suggesting their potential application in anti-tumor therapies.

Self-Healing Hydrogel Bioelectronics

Hydrogels have emerged as powerful building blocks to develop various soft bioelectronics because of their tissue-like mechanical properties, superior bio-compatibility, the ability to conduct both electrons and ions, and multiple stimuli-responsiveness. However, hydrogels are vulnerable to mechanical damage, which limits their usage in developing durable hydrogel-based bioelectronics. Self-healing hydrogels aim to endow bioelectronics with the property of repairing specific functions after mechanical failure, thus improving their durability, reliability, and longevity. This review discusses recent advances in self-healing hydrogels, from the self-healing mechanisms, material chemistry, and strategies for multiple properties improvement of hydrogel materials, to the design, fabrication, and applications of various hydrogel-based bioelectronics, including wearable physical and biochemical sensors, supercapacitors, flexible display devices, triboelectric nanogenerators (TENGs), implantable bioelectronics, etc. Furthermore, the persisting challenges hampering the development of self-healing hydrogel bioelectronics and their prospects are proposed. This review is expected to expedite the research and applications of self-healing hydrogels for various self-healing bioelectronics.

From Basic Science to Clinical Practice: A Review of Current Periodontal/Mucogingival Regenerative Biomaterials

Periodontitis is a dysbiosis-driven inflammatory disease affecting the tooth-supporting tissues, characterized by their progressive resorption, which can ultimately lead to tooth loss. A step-wise therapeutic approach is employed for periodontitis. After an initial behavioral and non-surgical phase, intra-bony or furcation defects may be amenable to regenerative procedures. This review discusses the regenerative technologies employed for periodontal regeneration, highlighting the current limitations and future research areas. The search, performed on the MEDLINE database, has identified the available biomaterials, including biologicals (autologous platelet concentrates, hydrogels), bone grafts (pure or putty), and membranes. Biologicals and bone grafts have been critically analyzed in terms of composition, mechanism of action, and clinical applications. Although a certain degree of periodontal regeneration is predictable in intra-bony and class II furcation defects, complete defect closure is hardly achieved. Moreover, treating class III furcation defects remains challenging. The key properties required for functional regeneration are discussed, and none of the commercially available biomaterials possess all the ideal characteristics. Therefore, research is needed to promote the advancement of more effective and targeted regenerative therapies for periodontitis. Lastly, improving the design and reporting of clinical studies is suggested by strictly adhering to the Consolidated Standards of Reporting Trials (CONSORT) 2010 statement.

Injectable-Hydrogel-Based Tissue Sealant for Hemostasis, Bacteria Inhibition, and Pro-Angiogenesis in Organ Bleeding Wounds and Therapeutic Outcome Monitoring Via NIR-II Optical Imaging

Sudden hemorrhage stemming from internal organ wounds poses a grave and potentially fatal risk if left untreated. Injectable-hydrogel-based tissue sealants featuring multiple actions, including fit-to-shape in situ gelation, rapid hemostasis, pro-angiogenic, anti-bacterial and outcome tracking, are ideal for the management of organ trauma wounds. Herein, an injectable-hydrogel tissue sealant AN@CD-PEG&TQ which consists of four-arm 4-arm poly(ethylene glycol) (PEG-SC) succinimidyl carbonate), AN@CD nanoprobe, and two bioactive peptides (anti-microbial peptide Tet213 and pro-angiogenic peptide QK) is developed. Among them, AN@CD nanoparticles form through host/guest complexation of amino-group-containing β-cyclodextrin and adamantyl group, enabling in situ biomarker (NO)-activatable optoacoustic/NIR-II: Near-infrared second biological window fluorescent imaging. The ample ─NH2 groups on the surface of AN@CD readily engage in rapid cross-linking with succinimidyl ester groups located at the ends of four-arm PEG-SC. This cross-linking expedites the gelation process without necessitating additional initiators or cross-linking agents; thus, significantly enhancing both hydrogel's application convenience and biocompatibility. Bioactive peptides (Tet213 and QK) safeguard against possible bacterial infections, facilitate angiogenesis, and eventually, improve organ wounds healing. This hydrogel-based tissue sealant demonstrates superior therapeutic and bioimaging performance in various mouse models including liver hemorrhage, gastric perforation, and bacterial-infected skin wound mouse models, highlighting its potential as a high-performance wound sealant for organ bleeding wound management.

Synergistic Drug-Loaded Shear-Thinning Star Polymer Hydrogel Facilitates Gastrointestinal Lesion Resection and Promotes Wound Healing

Easy injection, long-lasting barrier, and drug loading are the critical properties of submucosal injection materials for endoscopic surgery. However, conventional injectable polymers face challenges in simultaneously attaining these properties due to the inherent conflict between injectability and in situ stability. Here, a multi-arm star polymer hydrogel (denoted as βCP hydrogel) with long-lasting submucosal barrier (exceeding 120 min), rapid hemostasis, and sustained antibacterial properties is successfully developed by grafting poly(oligo(ethylene glycol) methyl ether methacrylate) (PEGMA) side-chains from β-CD via atom transfer radical polymerization (ATRP). During the onset of shearing, βCP hydrogel experiences the unwinding of polymer side-chains between neighboring star polymers, which facilitates the process of endoscopic injectability. After submucosal injection, βCP hydrogel undergoes the winding of polymer side-chains, thereby establishing a long-lasting barrier cushion. Meanwhile, owing to its distinctive structures with a hydrophobic inner cavity and an outer layer of hydrophilic polymer side-chains, βCP hydrogel enables simultaneous loading and on-demand release of diverse categories of drugs. This unique performance can adapt to the diverse demands during different stages of wound healing in a porcine endoscopic surgery model. These results indicate an appealing prospect for new application of star polymers as a good submucosal injection material in endoscopic treatments.

Enhancing calvarial defects repair with PDGF-BB mimetic peptide hydrogels

Addressing bone defects represents a significant challenge to public health. Localized delivery of growth factor has emerged as promising approach for bone regeneration. However, the clinical application of Platelet-Derived Growth Factor (PDGF) is hindered by its high cost and short half-life. In this work, we introduce the application of PDGF-mimicking peptide (PMP1) hydrogels for calvarial defect restoration, showcasing their remarkable effectiveness. Through osteogenic differentiation assays and q-PCR analyses, we demonstrate PMP1's substantial capacity to enhance osteogenic differentiation of bone marrow mesenchymal stem cell (BMSC), leading to increased expression of crucial osteogenic genes. Further molecular mechanistic investigations reveal PMP1's activation of the PI3K-AKT-mTOR signaling pathway, a key element of its osteogenic effect. In vivo experiments utilizing a rat calvaria critical-sized defect model underscore the hydrogels' exceptional ability to accelerate new bone formation, thereby significantly advancing the restoration of calvaria defects. This research provides a promising bioactive material for bone tissue regeneration.

Injectable Thermo-Responsive Peptide Hydrogels and Its Enzyme Triggered Dynamic Self-Assembly

Endogenous stimuli-responsive injectable hydrogels hold significant promise for practical applications due to their spatio-temporal controllable drug delivery. Herein, we report a facile strategy to construct a series of in situ formation polypeptide hydrogels with thermal responsiveness and enzyme-triggered dynamic self-assembly. The thermo-responsive hydrogels are from the diblock random copolymer mPEG-b-P(Glu-co-Tyr). The L-glutamic acid (Glu) segments with different γ-alkyl groups, including methyl, ethyl, and n-butyl, offer specific secondary structure, facilitating the formation of hydrogel. The L-tyrosine (Tyr) residues not only provide hydrogen-bond interactions and thus adjust the sol-gel transition temperatures, but also endow polypeptide enzyme-responsive properties. The PTyr segments could be phosphorylated, and the phosphotyrosine copolymers were amphiphilies, which could readily self-assemble into spherical aggregates and transform into sheet-like structures upon dephosphorylation by alkaline phosphatase (ALP). P(MGlu-co-Tyr/P) and P(MGlu-co-Tyr) copolymers showed good compatibility with both MC3T3-E1 and Hela cells, with cell viability above 80% at concentrations up to 1000 μg/mL. The prepared injectable polypeptide hydrogel and its enzyme-triggered self-assemblies show particular potential for biomedical applications.

Dynamic injectable tissue adhesives with strong adhesion and rapid self-healing for regeneration of large muscle injury

Wounds often necessitate the use of instructive biomaterials to facilitate effective healing. Yet, consistently filling the wound and retaining the material in place presents notable challenges. Here, we develop a new class of injectable tissue adhesives by leveraging the dynamic crosslinking chemistry of Schiff base reactions. These adhesives demonstrate outstanding mechanical properties, especially in regard to stretchability and self-healing capacity, and biodegradability. Furthermore, they also form robust adhesion to biological tissues. Their therapeutic potential was evaluated in a rodent model of volumetric muscle loss (VML). Ultrasound imaging confirmed that the adhesives remained within the wound site, effectively filled the void, and degraded at a rate comparable to the healing process. Histological analysis indicated that the adhesives facilitated muscle fiber and blood vessel formation, and induced anti-inflammatory macrophages. Notably, the injured muscles of mice treated with the adhesives displayed increased weight and higher force generation than the control groups. This approach to adhesive design paves the way for the next generation of medical adhesives in tissue repair.

Injectable bioactive poly(propylene fumarate) and polycaprolactone based click chemistry bone cement for spinal fusion in rabbits

Degenerative spinal pathology is a widespread medical issue, and spine fusion surgeries are frequently performed. In this study, we fabricated an injectable bioactive click chemistry polymer cement for use in spinal fusion and bone regrowth. Taking advantages of the bioorthogonal click reaction, this cement can be crosslinked by itself eliminating the addition of a toxic initiator or catalyst, nor any external energy sources like UV light or heat. Furthermore, nano-hydroxyapatite (nHA) and microspheres carrying recombinant human bone morphogenetic protein-2 (rhBMP-2) and recombinant human vascular endothelial growth factor (rhVEGF) were used to make the cement bioactive for vascular induction and osteointegration. After implantation into a rabbit posterolateral spinal fusion (PLF) model, the cement showed excellent induction of new bone formation and bridging bone, achieving results comparable to autograft control. This is largely due to the osteogenic properties of nano-hydroxyapatite (nHA) and the released rhBMP-2 and rhVEGF growth factors. Since the availability of autograft sources is limited in clinical settings, this injectable bioactive click chemistry cement may be a promising alternative for spine fusion applications in addressing various spinal conditions.

Tough, self-healing and injectable dynamic nanocomposite hydrogel based on gelatin and sodium alginate

Biomacromolecules based injectable and self-healing hydrogels possessing high mechanical properties have widespread potential in biomedical field. However, dynamic features are usually inversely proportional to toughness. It is challenging to simultaneously endow these properties to the dynamic hydrogels. Here, we fabricated an injectable nanocomposite hydrogel (CS-NPs@OSA-l-Gtn) stimultaneously possessing excellent autonomous self-healing performance and high mechanical strength by doping chitosan nanoparticles (CS-NPs) into dynamic polymer networks of oxidized sodium alginate (OSA) and gelatin (Gtn) in the presence of borax. The synergistic effect of the multiple reversible interactions combining dynamic covalent bonds (i.e., imine bond and borate ester bond) and noncovalent interactions (i.e., electrostatic interaction and hydrogen bond) provide effective energy dissipation to endure high fatigue resistance and cyclic loading. The dynamic hydrogel exhibited excellent mechanical properties like maximum 2.43 MPa compressive strength, 493.91 % fracture strain, and 89.54 kJ/m3 toughness. Moreover, the integrated hydrogel after injection and self-healing could withstand 150 successive compressive cycles. Besides, the bovine serum albumin embedded in CS-NPs could be sustainably released from the nanocomposite hydrogel for 12 days. This study proposes a novel strategy to synthesize an injectable and self-healing hydrogel combined with excellent mechanical properties for designing high-strength natural carriers with sustained protein delivery.

Spermidine-Functionalized Injectable Hydrogel Reduces Inflammation and Enhances Healing of Acute and Diabetic Wounds In Situ

The inflammatory response is a key factor affecting tissue regeneration. Inspired by the immunomodulatory role of spermidine, an injectable double network hydrogel functionalized with spermidine (DN-SPD) is developed, where the first and second networks are formed by dynamic imine bonds and non-dynamic photo-crosslinked bonds respectively. The single network hydrogel before photo-crosslinking exhibits excellent injectability and thus can be printed and photo-crosslinked in situ to form double network hydrogels. DN-SPD hydrogel has demonstrated desirable mechanical properties and tissue adhesion. More importantly, an "operando" comparison of hydrogels loaded with spermidine or diethylenetriamine (DETA), a sham molecule resembling spermidine, has shown similar physical properties, but quite different biological functions. Specifically, the outcomes of 3 sets of in vivo animal experiments demonstrate that DN-SPD hydrogel can not only reduce inflammation caused by implanted exogenous biomaterials and reactive oxygen species but also promote the polarization of macrophages toward regenerative M2 phenotype, in comparison with DN-DETA hydrogel. Moreover, the immunoregulation by spermidine can also translate into faster and more natural healing of both acute wounds and diabetic wounds. Hence, the local administration of spermidine affords a simple but elegant approach to attenuate foreign body reactions induced by exogenous biomaterials to treat chronic refractory wounds.

In Situ Bioprinting: Process, Bioinks, and Applications

Traditional tissue engineering methods face challenges, such as fabrication, implantation of irregularly shaped scaffolds, and limited accessibility for immediate healthcare providers. In situ bioprinting, an alternate strategy, involves direct deposition of biomaterials, cells, and bioactive factors at the site, facilitating on-site fabrication of intricate tissue, which can offer a patient-specific personalized approach and align with the principles of precision medicine. It can be applied using a handled device and robotic arms to various tissues, including skin, bone, cartilage, muscle, and composite tissues. Bioinks, the critical components of bioprinting that support cell viability and tissue development, play a crucial role in the success of in situ bioprinting. This review discusses in situ bioprinting techniques, the materials used for bioinks, and their critical properties for successful applications. Finally, we discuss the challenges and future trends in accelerating in situ printing to translate this technology in a clinical settings for personalized regenerative medicine.

Dual-Modified Hyaluronic Acid for Tunable Double Cross-Linked Hydrogel Adhesives

Conventional techniques for the closure of wounds, such as sutures and staples, have significant drawbacks that can negatively impact wound healing. Tissue adhesives have emerged as promising alternatives, but poor adhesion, low mechanical properties, and toxicity have hindered their widespread clinical adoption. In this work, a dual modified, aldehyde and methacrylate hyaluronic acid (HA) biopolymer (HA-MA-CHO) has been synthesized through a simplified route for use as a double cross-linked network (DCN) hydrogel (HA-MA-CHO-DCN) adhesive for the effective closure and sealing of wounds. HA-MA-CHO-DCN cross-links in two stages: initial cross-linking of the aldehyde functionality (CHO) of HA-MA-CHO using a disulfide-containing cross-linker, 3,3'-dithiobis (propionic hydrazide) (DTPH), leading to the formation of a self-healing injectable gel, followed by further cross-linking via ultraviolet (UV) initiated polymerization of the methacrylate (MA) functionality. This hydrogel adhesive shows a stable swelling behavior and remarkable versatility as the storage modulus (G') has shown to be highly tunable (103-105 Pa) for application to many different wound environments. The new HA-MA-CHO-DCN hydrogel showed excellent adhesive properties by surpassing the burst pressure and lap-shear strength for the widely used bovine serum albumin-glutaraldehyde (BSAG) glue while maintaining excellent cell viability.

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

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

Development of an Injectable Biphasic Hyaluronic Acid-Based Hydrogel With Stress Relaxation Properties for Cartilage Regeneration

Biomimetic stress-relaxing hydrogels with reversible crosslinks attract significant attention for stem cell tissue regeneration compared with elastic hydrogels. However, stress-relaxing hyaluronic acid (HA)-based hydrogels fabricated using conventional technologies lack stability, biocompatibility, and mechanical tunability. Here, it is aimed to address these challenges by incorporating calcium or phosphate components into the HA backbone, which allows reversible crosslinking of HA with alginate to form interpenetrating networks, offering stability and mechanical tunability for mimicking cartilage. Diverse stress-relaxing hydrogels (τ1/2; SR50, 60-2000 s) are successfully prepared at ≈3 kPa stiffness with self-healing and shear-thinning abilities, favoring hydrogel injection. In vitro cell experiments with RNA sequencing analysis demonstrate that hydrogels tune chondrogenesis in a biphasic manner (hyaline or calcified) depending on the stress-relaxation properties and phosphate components. In vivo studies confirm the potential for biphasic chondrogenesis. These results indicate that the proposed stress-relaxing HA-based hydrogel with biphasic chondrogenesis (hyaline or calcified) is a promising material for cartilage regeneration.

Facile fabrication of a novel, photodetachable salecan-based hydrogel dressing with self-healing, injectable, and antibacterial properties based on metal coordination

Achieving the controllable detachment of polysaccharide-based wound dressings is challenging. In this study, a novel, photodetachable salecan-based hydrogel dressing with injectable, self-healing, antibacterial, and wound healing properties was developed using a green and facile approach. A salecan hydrogel with a uniform porous structure and water content of 90.4 % was prepared by simply mixing salecan and an Fe3+-citric acid complexing solution in an acidic D-(+)-glucono-1,5-lactone environment. Metal coordinate interactions were formed between the released Fe3+ ions and carboxyl groups on the salecan polysaccharide, inducing homogeneous gelation. Benefiting from this dynamic and reversible crosslinking, the salecan hydrogel exhibited self-healing and injectable behavior, facilitating the formation of the desired shapes in situ. The exposure of Fe3+-citric acid to UV light (365 nm) resulted in the reduction of Fe3+ to Fe2+ through photochemical reactions, enabling phototriggered detachment. Moreover, the hydrogel exhibited excellent biocompatibility and satisfactory antibacterial efficacy against Escherichia coli and Staphylococcus aureus of 72.5 % and 85.3 %, respectively. The adhesive strength of the salecan hydrogel to porcine skin was 1.06 ± 0.12 kPa. In vivo wound healing experiments further highlighted the advantages of the prepared hydrogel in alleviating the degree of wound inflammation and promoting tissue regeneration within 12 days.

Sprayable Zwitterionic Antibacterial Hydrogel With High Mechanical Resilience and Robust Adhesion for Joint Wound Treatment

Wound healing in movable parts, including the joints and neck, remains a critical challenge due to frequent motions and poor flexibility of dressings, which may lead to mismatching of mechanical properties and poor fitting between dressings and wounds; thus, increasing the risk of bacterial infection. This study proposes a sprayable zwitterionic antibacterial hydrogel with outstanding flexibility and desirable adhesion. This hydrogel precursor is fabricated by combining zwitterionic sulfobetaine methacrylate (SBMA) with poly(sulfobetaine methacrylate-co-dopamine methacrylamide)-modified silver nanoparticles (PSBDA@AgNPs) through robust electrostatic interactions. About 150 s of exposure to UV light, the SBMA monomer polymerizes to form PSB chains entangled with PSBDA@AgNPs, transformed into a stable and adhesion PSB-PSB@Ag hydrogel at the wound site. The resulting hydrogel has adhesive strength (15-38 kPa), large tensile strain (>400%), suitable shape adaptation, and excellent mechanical resilience. Moreover, the hydrogel displays pH-responsive behavior; the acidic microenvironment at the infected wound sites prompts the hydrogel to rapidly release AgNPs and kill bacteria. Further, the healing effect of the hydrogel is demonstrated on the rat neck skin wound, showing improved wound closing rate due to reduced inflammation and enhanced angiogenesis. Overall, the sprayable zwitterionic antibacterial hydrogel has significant potential to promote joint skin wound healing.

Mussel inspired sequential protein delivery based on self-healing injectable nanocomposite hydrogel

Polysaccharide based self-healing and injectable hydrogels with reversible characteristics have widespread potential in protein drug delivery. However, it is a challenge to design the dynamic hydrogel for sequential release of protein drugs. Herein, we developed a novel mussel inspired sequential protein delivery dynamic polysaccharide hydrogel. The nanocomposite hydrogel can be fabricated through doping polydopamine nanoparticles (PDA NPs) into reversible covalent bond (imine bonds) crosslinked polymer networks of oxidized hyaluronic acid (OHA) and carboxymethyl chitosan (CEC), named PDA NPs@OHA-l-CEC. Besides multiple capabilities (i.e., injection, self-healing, and biodegradability), the nanocomposite hydrogel can achieve sustained and sequential protein delivery of vascular endothelial growth factor (VEGF) and bovine serum albumin (BSA). PDA NPs doped in hydrogel matrix serve dual roles, acting as secondary protein release structures and form dynamic non-covalent interactions (i.e., hydrogen bonds) with polysaccharides. Moreover, by adjusting the oxidation degree of OHA, the hydrogels with different crosslinking density could control overall protein release rate. Analysis of different release kinetic models revealed that Fickian diffusion drove rapid VEGF release, while the slower BSA release followed a Super Case II transport mechanism. The novel biocompatible system achieved sequential release of protein drugs has potentials in multi-stage synergistic drug deliver based on dynamic hydrogel.

Seamless Integration of Conducting Hydrogels in Daily Life: From Preparation to Wearable Application

Conductive hydrogels (CHs) have received significant attention for use in wearable devices because they retain their softness and flexibility while maintaining high conductivity. CHs are well suited for applications in skin-contact electronics and biomedical devices owing to their high biocompatibility and conformality. Although highly conductive hydrogels for smart wearable devices are extensively researched, a detailed summary of the outstanding results of CHs is required for a comprehensive understanding. In this review, the recent progress in the preparation and fabrication of CHs is summarized for smart wearable devices. Improvements in the mechanical, electrical, and functional properties of high-performance wearable devices are also discussed. Furthermore, recent examples of innovative and highly functional devices based on CHs that can be seamlessly integrated into daily lives are reviewed.

Thermoresponsive keratin-methylcellulose self-healing injectable hydrogel accelerating full-thickness wound healing by promoting rapid epithelialization

Chronic wounds suffer from impaired healing due to microbial attack and poor vascular growth. Thermoresponsive hydrogels gained attention in wound dressing owing to their gelation at physiological temperature enabling them to take the shape of asymmetric wounds. The present study delineates the development of thermoresponsive hydrogel (MCK), from hair-derived keratin (K) and methylcellulose (MC) in the presence of sodium sulfate. The gelation temperature (Tg) of this hydrogel is in the range of 30 °C to 33 °C. Protein-polymer interaction leading to thermoreversible sol-gel transition involved in MCK blends has been analyzed and confirmed by FTIR, XRD, and thermal studies. Keratin, has introduced antioxidant properties to the hydrogel imparted cytocompatibility towards human dermal fibroblasts (HDFs) as evidenced by both MTT and live dead assays. In vitro wound healing assessment has been shown by enhanced migration of HDFs in the presence of MCK hydrogel compared to the control. Also, CAM assay and CD31 expression by the Wistar rat model has shown increased blood vessel branching after the implantation of MCK hydrogel. Further, in vivo study, demonstrated MCK efficacy of hydrogel in accelerating full-thickness wounds with minimal scarring in Wistar rats, re-epithelialization, and reinstatement of the epidermal-dermal junction thereby exhibiting clinical relevance for chronic wounds.

Remodelers of the vascular microenvironment: The effect of biopolymeric hydrogels on vascular diseases

Vascular disease is the leading health problem worldwide. Vascular microenvironment encompasses diverse cell types, including those within the vascular wall, blood cells, stromal cells, and immune cells. Initiation of the inflammatory state of the vascular microenvironment and changes in its mechanics can profoundly affect vascular homeostasis. Biomedical materials play a crucial role in modern medicine, hydrogels, characterized by their high-water content, have been increasingly utilized as a three-dimensional interaction network. In recent times, the remarkable progress in utilizing hydrogels and understanding vascular microenvironment have enabled the treatment of vascular diseases. In this review, we give an emphasis on the utilization of hydrogels and their advantages in the various vascular diseases including atherosclerosis, aneurysm, vascular ulcers of the lower limbs and myocardial infarction. Further, we highlight the importance and advantages of hydrogels as artificial microenvironments.

Liposome-integrated hydrogel hybrids: Promising platforms for cancer therapy and tissue regeneration

Drug delivery platforms have gracefully emerged as an indispensable component of novel cancer chemotherapy, bestowing targeted drug distribution, elevating therapeutic effects, and reducing the burden of unwanted side effects. In this context, hybrid delivery systems artfully harnessing the virtues of liposomes and hydrogels bring remarkable benefits, especially for localized cancer therapy, including intensified stability, excellent amenability to hydrophobic and hydrophilic medications, controlled liberation behavior, and appropriate mucoadhesion to mucopenetration shift. Moreover, three-dimensional biocompatible liposome-integrated hydrogel networks have attracted unprecedented interest in tissue regeneration, given their tunable architecture and physicochemical properties, as well as enhanced mechanical support. This review elucidates and presents cutting-edge developments in recruiting liposome-integrated hydrogel systems for cancer treatment and tissue regeneration.

Chitin: A versatile biopolymer-based functional therapy for cartilage regeneration

Chitin is the second most abundant biopolymer and its inherent biological characteristics make it ideal to use for tissue engineering. For many decades, its properties like non-toxicity, abundant availability, ease of modification, biodegradability, biocompatibility, and anti-microbial activity have made chitin an ideal biopolymer for drug delivery. Research studies have also shown many potential benefits of chitin in the formulation of functional therapy for cartilage regeneration. Chitin and its derivatives can be processed into 2D/3D scaffolds, hydrogels, films, exosomes, and nano-fibers, which make it a versatile and functional biopolymer in tissue engineering. Chitin is a biomimetic polymer that provides targeted delivery of mesenchymal stem cells, especially of chondrocytes at the injected donor sites to accelerate regeneration by enhancing cell proliferation and differentiation. Due to this property, chitin is considered an interesting polymer that has a high potential to provide targeted therapy in the regeneration of cartilage. Our paper presents an overview of the method of extraction, structure, properties, and functional role of this versatile biopolymer in tissue engineering, especially cartilage regeneration.

Self-Assembled Nanocomposite Hydrogels as Carriers for Demineralized Bone Matrix Particles and Enhanced Bone Repair

Demineralized bone matrix (DBM) has been widely used as an allogeneic alternative to autologous bone graft for bone repair. However, more extensive use of DBM is limited due to its particulate nature after demineralization and rapid particle dispersion following irrigation, resulting in unpredictable osteoinductivity. Here, a new design of injectable hydrogel carriers for DBM that combine self-healing ability and osteogenic properties based on the self-assembly of guanidinylated hyaluronic acid and silica-rich nanoclays is reported. The nanoclays serve as reversible linkages to form a dynamic hydrogel network with the guanidine moieties on the polymer chains. Gelation kinetics and mechanical properties can be controlled by altering nanoclay content in the hydrogel. The resulting hydrogel exerts self-healing ability due to its dynamic crosslinks and well retains its overall performance with high DBM loading. The hydrogel exhibits great cytocompatibility and osteogenic effects mediated by the nanoclays. In vivo delivery of DBM using the nanocomposite hydrogel further demonstrates robust bone regeneration in a mouse calvarial defect model in comparison to DBM delivered with aqueous HA. This work suggests a promising hydrogel platform for many applications including therapeutic delivery and tissue engineering.

Advancements in Regenerative Hydrogels in Skin Wound Treatment: A Comprehensive Review

This state-of-the-art review explores the emerging field of regenerative hydrogels and their profound impact on the treatment of skin wounds. Regenerative hydrogels, composed mainly of water-absorbing polymers, have garnered attention in wound healing, particularly for skin wounds. Their unique properties make them well suited for tissue regeneration. Notable benefits include excellent water retention, creating a crucially moist wound environment for optimal healing, and facilitating cell migration, and proliferation. Biocompatibility is a key feature, minimizing adverse reactions and promoting the natural healing process. Acting as a supportive scaffold for cell growth, hydrogels mimic the extracellular matrix, aiding the attachment and proliferation of cells like fibroblasts and keratinocytes. Engineered for controlled drug release, hydrogels enhance wound healing by promoting angiogenesis, reducing inflammation, and preventing infection. The demonstrated acceleration of the wound healing process, particularly beneficial for chronic or impaired healing wounds, adds to their appeal. Easy application and conformity to various wound shapes make hydrogels practical, including in irregular or challenging areas. Scar minimization through tissue regeneration is crucial, especially in cosmetic and functional regions. Hydrogels contribute to pain management by creating a protective barrier, reducing friction, and fostering a soothing environment. Some hydrogels, with inherent antimicrobial properties, aid in infection prevention, which is a crucial aspect of successful wound healing. Their flexibility and ability to conform to wound contours ensure optimal tissue contact, enhancing overall treatment effectiveness. In summary, regenerative hydrogels present a promising approach for improving skin wound healing outcomes across diverse clinical scenarios. This review provides a comprehensive analysis of the benefits, mechanisms, and challenges associated with the use of regenerative hydrogels in the treatment of skin wounds. In this review, the authors likely delve into the application of rational design principles to enhance the efficacy and performance of hydrogels in promoting wound healing. Through an exploration of various methodologies and approaches, this paper is poised to highlight how these principles have been instrumental in refining the design of hydrogels, potentially revolutionizing their therapeutic potential in addressing skin wounds. By synthesizing current knowledge and highlighting potential avenues for future research, this review aims to contribute to the advancement of regenerative medicine and ultimately improve clinical outcomes for patients with skin wounds.

Phenylboronate-salicylate ester cross-linked self-healing hydrogel composed of modified hyaluronan at physiological pH

Several hydrogels with boronate/diol ester cross-linking have been reported. However, multiple synthetic steps or expensive reagents are required to modify some diol moieties into polymers. Therefore, diol-modified polymers, which are easily and inexpensively prepared via a single-step process, are required for the formation of boronate esters. This study reports a novel hydrogel composed of phenylboronic acid-modified hyaluronic acid and salicylic acid-modified hyaluronic acid. This hydrogel is injectable, can self-heal at physiological pH, and can be easily and inexpensively prepared. The polymer system behaved as a sol at pH 12.0 and a weak gel at pH 9.4 and 11.2, whereas it behaved as a gel over a wide pH range of 4.0-8.2. The viscoelasticity of the system decreased in response to sugar at pH 7.3. Thus, salicylic acid can be considered a promising diol moiety for hydrogel formation via boronate ester cross-linking.

Hybrid Hydrogels for Immunoregulation and Proangiogenesis through Mild Heat Stimulation to Accelerate Whole-Process Diabetic Wound Healing

Intense and persistent oxidative stress, excessive inflammation, and impaired angiogenesis severely hinder diabetic wound healing. Bioactive hydrogel dressings with immunoregulatory and proangiogenic properties have great promise in treating diabetic wounds. However, the therapeutic effects of dressings always depend on drugs with side effects, expensive cytokines, and cell therapies. Herein, a novel dynamic borate-bonds crosslinked hybrid multifunctional hydrogel dressings with photothermal properties are developed to regulate the microenvironment of diabetic wound sites and accelerate the whole process of its healing without additional medication. The hydrogel is composed of phenylboronic acid-modified chitosan and hyaluronic acid (HA) crosslinked by tannic acid (TA) through borate bonds and Prussian blue nanoparticles (PBNPs) with photothermal response characteristics are embedded in the polymer networks. The results indicate hydrogels show inherent broad-spectrum antioxidative activities through the integrated interaction of borate bonds, TA, and PBNPs. Meanwhile, combined with the regulation of macrophage phenotype by HA, the inflammatory microenvironment of diabetic wounds is transformed. Moreover, the angiogenesis is then enhanced by the mild photothermal effect of PBNPs, followed by promoted epithelialization and collagen deposition. In summary, this hybrid hydrogel system accelerates all stages of wound repair through antioxidative stress, immunomodulation, and proangiogenesis, showing great potential applications in diabetic wound management.

Effect of ion distribution on stress relaxation in polyelectrolyte complex gels

Solutions of polyelectrolytes consisting of polycations and polyanions in equal proportions were studied in the present work. Due to the physical cross-links formed by the charged groups, physical gels were formed in such systems. The mechanical properties and structure of the obtained gels depending on the charge arrangement along the backbone and the dimensionless Bjerrum length λ were investigated. The response of the systems to a uniaxial affine deformation was studied first. It was found that the systems can be divided into three groups depending on the charge arrangement: showing an almost elastic response; showing a viscoelastic response with a very long relaxation time; and showing a weak viscoelastic response with a short relaxation time. Interestingly, no stable aggregates were formed in the systems with the charges located on spacers, probably because of the increased mobility of the charges in such systems. The obtained stress relaxation curves had different functional forms, indicating that the relaxation has at least two characteristic times, which are different for different systems. In order to understand the molecular nature of the observed mechanical response, the temporal evolution of the network structure of a system showing a viscoelastic response with a very long relaxation time was studied; the aggregates were found to be dynamic, which leads to the relaxation of the "subchains" conformation.

Guanosine-based hydrogel as a supramolecular scaffold for template-assisted macrocyclization

The G-quartet-like supramolecular assembly present in guanosine hydrogel templates macrocyclization between bis-azide and bis-alkyne fragments. The resulting macrocycle enhances viscoelastic properties, and strengthens the hydrogel network. This approach holds potential for the in situ synthesis of drugs and their simultaneous delivery in a stimuli-responsive manner.

Photoluminescent Self-Healable Waterborne Polyurethane/Mo and S Codoped Graphitic Carbon Nitride Nanocomposite with Bioimaging and Encryption Capability

Creating polymers that combine various functions within a single system expands the potential applications of such polymeric materials. However, achieving polymer materials that possess simultaneously elevated strength, toughness, and self-healing capabilities, along with special properties, remains a significant challenge. The present study demonstrates the preparation of S and Mo codoped graphitic carbon nitride (g-C3N4) (Mo@S-CN) nanohybrid and the fabrication of self-healing waterborne polyurethane (SHWPU)/Mo@S-CN (SHWPU/NS) nanocomposites for advanced applications. Mo@S-CN is an intriguing combination of g-C3N4 nanosheets and molybdenum oxide (MoOx) nanorods, forming a complex lamellar structure. This unique arrangement significantly improves the inborn properties of SHWPU to an impressive degree, especially mechanical strength (28.37-34.11 MPa), fracture toughness (73.65-140.98 MJ m-2), and thermal stability (340.17-348.01 °C), and introduces fluorescence activity into the matrix. Interestingly, a representative SHWPU/NS0.5 film is so tough that a dumbbell of 15 kg, which is 53,003 times heavier than the weight of the film, can be successfully lifted without any significant crack. Remarkably, fluorescence activity is developed because of electronic excitations occurring within the repeating polymeric tris-triazine units of the Mo@S-CN nanohybrid. This fascinating feature was effectively harnessed by assessing the usability of aqueous dispersions of the Mo@S-CN nanohybrid and photoluminescent SHWPU/NS nanocomposites as sustainable stains for bioimaging of human dermal fibroblast cells and anticounterfeiting materials, respectively. The in vitro fluorescence tagging test showed blue emission from 365 nm excitation, green emission from 470 nm excitation, and red emission from 545 nm excitation. Most importantly, in vitro hemocompatibility assessment, in vitro cytocompatibility, cell proliferation assessment, and cellular morphology assessment supported the biocompatibility nature of the Mo@S-CN nanohybrid and SHWPU/NS nanocomposites. Thus, these materials can be used for advanced applications including bioimaging.

Versatile Mechanically Tunable Hydrogels for Therapeutic Delivery Applications

Hydrogels provide a versatile platform for biomedical material fabrication that can be structurally and mechanically fine-tuned to various tissues and applications. Applications of hydrogels in biomedicine range from highly dynamic injectable hydrogels that can flow through syringe needles and maintain or recover their structure after extrusion to solid-like wound-healing patches that need to be stretchable while providing a selective physical barrier. In this study, a toolbox is designed using thermo-responsive poly(N-isopropylacrylamide) (PNIPAM) polymeric matrices and nanocelluloses as reinforcing agent to obtain biocompatible hydrogels with altering mechanical properties, from a liquid injectable to a solid-like elastic hydrogel. The liquid hydrogels possess low viscosity and shear-thinning properties at 25 °C, which allows facile injection at room temperature, while they become viscoelastic gels at body temperature. In contrast, the covalently cross-linked solid-like hydrogels exhibit enhanced viscoelasticity. The liquid hydrogels are biocompatible and are able to delay the in vitro release and maintain the bioactivity of model drugs. The antimicrobial agent loaded solid-like hydrogels are effective against typical wound-associated pathogens. This work presents a simple method of tuning hydrogel mechanical strength to easily adapt to applications in different soft tissues and broaden the potential of renewable bio-nanoparticles in hybrid biomaterials with controlled drug release capabilities.

One-Pot Fabrication of Supramolecular Synthetic Protein Hydrogel with Tissue-like Integrated Dynamic Features

Protein-incorporated soft networks have received remarkable attention during the past several years. They possess desirable properties similar to native tissues and organs and exhibit unique advantages in applications. However, fabrication of protein-based hydrogels usually suffers from complex protein mutation and modification or chemical synthesis, which limited the scale and yield of production. Meanwhile, the lack of rationally designed noncovalent interactions in networks may result in a deficiency of the dynamic features of materials. Therefore, a highly efficient method is needed to include supramolecular interactions into protein hydrogel to generate a highly dynamic hydrogel possessing integrated tissue-like properties. Here, we report the design and construction of native protein-based supramolecular synthetic protein hydrogels through a simple and efficient one-pot polymerization of acrylamide and ligand monomers in the presence of a ligand-binding protein. The supramolecular interactions in the network yield integrated dynamic properties, including remarkable stretchability over 10,000% of their original length, ultrafast self-healing abilities within 3-4 s, tissue-like fast stress relaxation, satisfactory ability of adhesion to different living and nonliving substrates, injectability, and high biocompatibility. Furthermore, this material demonstrated potential as a biosensor to monitor small finger movements. This strategy provides a new avenue for fabricating synthetic protein hydrogels with integrated features.

Double-Network Hydrogel with Strengthened Mechanical Property for Controllable Release of Antibacterial Peptide

Developing double-network (DN) hydrogels with high mechanical properties and antibacterial efficacy to combat multidrug-resistant bacterial infections and serve as scaffolds for cell culture still remains an ongoing challenge. In this study, an ion-responsive antibacterial peptide (AMP) (C16-WIIIKKK, termed as IK7) was synergistically combined with a photoresponsive gelatin methacryloyl (GelMA) polymer to fabricate a biocompatible DN hydrogel. The GelMA-IK7 DN hydrogel showed enhanced mechanical properties in contrast to the individual IK7 and GelMA hydrogels and demonstrated substantial antibacterial efficacy. Further investigations revealed that the DN hydrogel effectively inhibited bacterial growth by the controlled and sustained release of the IK7 peptide. In addition, the formation of the DN hydrogel was also found to protect AMP IK7 from rapid degradation by proteinase K. Our findings suggested that the developed GelMA-IK7 DN hydrogel holds great potential for next-generation antibacterial hydrogels for three-dimensional cell culture and tissue regeneration.

Injectable Hydrogels for Nervous Tissue Repair-A Brief Review

The repair of nervous tissue is a critical research field in tissue engineering because of the degenerative process in the injured nervous system. In this review, we summarize the progress of injectable hydrogels using in vitro and in vivo studies for the regeneration and repair of nervous tissue. Traditional treatments have not been favorable for patients, as they are invasive and inefficient; therefore, injectable hydrogels are promising for the treatment of damaged tissue. This review will contribute to a better understanding of injectable hydrogels as potential scaffolds and drug delivery system for neural tissue engineering applications.

The synergetic effect of alginate-derived hydrogels and metal-phenolic nanospheres for chronic wound therapy

Management of diabetic wounds presents a global health challenge due to elevated levels of ROS in the wound microenvironment, persistent dysregulation of inflammation modulation, and limitations in commercially available dressings. Addressing this issue, we have developed a pH-responsive and glucose-sensitive multifunctional hydrogel dressing that dynamically responds to the wound microenvironment and enables on-demand drug release. The dressing incorporates a matrix material based on aminophenylboronic acid-functionalized alginate and a polyhydroxy polymer, alongside an enhancer phase consisting of self-assembled metal-phenol coordination nanospheres formed by tannic acid and iron ions. Using the dynamic borate ester bonds and catechol-metal ion coordination bonds, the dressing exhibits remarkable shape adaptability, self-healing capability, tissue adhesiveness, antioxidant activity, and photothermal responsiveness, without additional curatives or crosslinking agents. As a wound dressing, it elicits macrophage polarization towards an anti-inflammatory phenotype while maintaining long-lasting antimicrobial effects. In a diabetic mouse model of full-thickness wound infections, it effectively mitigated inflammation and vascular damage, significantly expediting the wound healing process with a commendable 97.7% wound closure rate. This work provides a new direction for developing multifunctional smart hydrogel dressings that can accelerate diabetic wound healing for human health.

Compound Microalgae-Type Biofunctional Hydrogel for Wound Repair during Full-Thickness Skin Injuries

A dual biofunctional hydrogel (HQCS-SP) wound dressing, offering antibacterial properties and a biological response, was innovatively designed and developed to repair full-layer skin defects. The HQCS-SP hydrogel creates an artificial matrix that facilitates cell recruitment, extracellular matrix deposition, exhibiting exceptional tissue affinity, robust self-healing, effective hemostatic capabilities and accelerates wound healing. It is synthesized by crosslinking modified chitosan (HQCS) with spirulina protein (SP) and Fe3+. The HQCS provides antibacterial, antioxidant, good tissue affinity and excellent hemostasis performance. The incorporation of SP not only reinforces the antioxidant, antibacterial, anti-inflammatory, and pro-angiogenesis effects but also participates in the regulation of signal pathways and promotes wound healing. Therefore, this study offers a new visual angle for the design of advanced functional trauma dressings with great application potential in the bio-medical field.

A novel injectable hydrogel prepared from phenylboronic acid modified gelatin and oxidized-dextran for bone tissue engineering

Complicated fractures have always been challenging in orthopaedics. Designing a multifunctional biomaterial that can contribute to the treatment of fractures using a simple operation remains challenging. Here, we developed a trinity hydrogel system consisting of hydrogel prepared from phenylboronic acid modified gelatin and oxidized-dextran, lithium and cobalt co-doped mesoporous bioactive glass nanoparticles (MBGNs), and irisin. This hydrogel material exhibits considerable injectability, fat-to-shape, and self-healing characteristics. In addition, compared to hydrogel prepared from gelatin and oxidized-dextran, the hydrogel material presented a noticeable enhancement in compression stress and adhesion strength towards porcine bone fragments, which enables it more effectively splice bone fragments during surgery. Based on the various interactions between irisin and the hydrogel network, the system exhibited a clear sustained release of irisin. Based on the results of in vitro cell tests, the hydrogel material showed good cytocompatibility. And it also considerably enhanced the in vitro pro-osteogenic and pro-angiogenic capacities of bone marrow mesenchymal stromal cells (BMSCs) and human umbilical vein endothelial cells (HUVECs). In vivo experimental results indicated that this hydrogel considerably improved the repair of cranial defects in rats. The current study provides a feasible strategy for the treatment of bone fractures and stimulation of fracture healing.

Multifunctional self-healing peptide hydrogel for wound healing

The complete healing of wounds remains a challenge in clinical care. In addition, various complications such as inflammation and infection that may occur during skin wound healing can impede the healing process. Here, we constructed a multifunctional self-repairing hydrogel by utilizing Schiff base bonds. This hydrogel exhibited good self-healing properties and could cope with destructive external influences. The self-healing hydrogel was injectable, ensuring that the hydrogel dressing adhered to the wound. Carboxymethyl chitosan and oxidized chondroitin sulfate demonstrated good biocompatibility and multiple bioactivities and were successfully used to prepare self-healing hydrogels. Meanwhile, the SIKVAV biopeptide was less expensive and more morphologically stable than vascular endothelial growth factor and had a high pro-angiogenic activity. Thus, the SIKVAV biopeptide was cross-linked to the oxidized chondroitin sulfate of the hydrogel through covalent bonding to avoid rapid biopeptide degradation, achieving a slow release of the drug. This peptide hydrogel exhibited good biocompatibility and antimicrobial properties; moreover, experiments conducted on mice revealed that it could effectively promote angiogenesis and skin tissue repair. These findings suggest that the injectable self-repairing peptide hydrogel may facilitate skin wound healing and other applications.

Glycosaminoglycans' Ability to Promote Wound Healing: From Native Living Macromolecules to Artificial Biomaterials

Glycosaminoglycans (GAGs) are important for the occurrence of signaling molecules and maintenance of microenvironment within the extracellular matrix (ECM) in living tissues. GAGs and GAG-based biomaterial approaches have been widely explored to promote in situ tissue regeneration and repair by regulating the wound microenvironment, accelerating re-epithelialization, and controlling ECM remodeling. However, most approaches remain unacceptable for clinical applications. To improve insights into material design and clinical translational applications, this review highlights the innate roles and bioactive mechanisms of native GAGs during in situ wound healing and presents common GAG-based biomaterials and the adaptability of application scenarios in facilitating wound healing. Furthermore, challenges before the widespread commercialization of GAG-based biomaterials are shared, to ensure that future designed and constructed GAG-based artificial biomaterials are more likely to recapitulate the unique and tissue-specific profile of native GAG expression in human tissues. This review provides a more explicit and clear selection guide for researchers designing biomimetic materials, which will resemble or exceed their natural counterparts in certain functions, thereby suiting for specific environments or therapeutic goals.

Engineered keratin/bFGF hydrogel to promote diabetic wound healing in rats

Keratin materials are promising in wound healing acceleration, however, it is a challenge for the keratin to efficiently therapy the impaired wound healing, such as diabetic foot ulcers. Here, we report a keratin/bFGF hydrogel for skin repair of chronic wounds in diabetic rats based on their characteristics of extracellular matrix and growth factor degradation in diabetic ulcer. Recombinant keratin 31 (K31), the most abundant keratin in human hair, exhibited the highly efficient performances in cell adhesion, proliferation and migration. More importantly, the introduction of bFGF into K31 hydrogel significantly enhances the properties of cell proliferation, wound closure acceleration, angiogenesis and skin appendages regeneration. Furthermore, the combination of K31 and bFGF can promote epithelial-mesenchymal transition by inhibiting the expression of E-cadherin and promoting the expression of vimentin and fibronectin. These findings demonstrate the engineered K31/bFGF hydrogel as a promising therapeutic agent for diabetic wound healing.

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.

Progress of polysaccharide-based tissue adhesives

Recently, polymer-based tissue adhesives (TAs) have gained the attention of scientists and industries as alternatives to sutures for sealing and closing wounds or incisions because of their ease of use, low cost, minimal tissue damage, and short application time. However, poor mechanical properties and weak adhesion strength limit the application of TAs, although numerous studies have attempted to develop new TAs with enhanced performance. Therefore, next-generation TAs with improved multifunctional properties are required. In this review, we address the requirements of polymeric TAs, adhesive characteristics, adhesion strength assessment methods, adhesion mechanisms, applications, advantages and disadvantages, and commercial products of polysaccharide (PS)-based TAs, including chitosan (CS), alginate (AL), dextran (DE), and hyaluronic acid (HA). Additionally, future perspectives are discussed.

Mechanical forces amplify TCR mechanotransduction in T cell activation and function

Adoptive T cell immunotherapies, including engineered T cell receptor (eTCR) and chimeric antigen receptor (CAR) T cell immunotherapies, have shown efficacy in treating a subset of hematologic malignancies, exhibit promise in solid tumors, and have many other potential applications, such as in fibrosis, autoimmunity, and regenerative medicine. While immunoengineering has focused on designing biomaterials to present biochemical cues to manipulate T cells ex vivo and in vivo, mechanical cues that regulate their biology have been largely underappreciated. This review highlights the contributions of mechanical force to several receptor-ligand interactions critical to T cell function, with central focus on the TCR-peptide-loaded major histocompatibility complex (pMHC). We then emphasize the role of mechanical forces in (i) allosteric strengthening of the TCR-pMHC interaction in amplifying ligand discrimination during T cell antigen recognition prior to activation and (ii) T cell interactions with the extracellular matrix. We then describe approaches to design eTCRs, CARs, and biomaterials to exploit TCR mechanosensitivity in order to potentiate T cell manufacturing and function in adoptive T cell immunotherapy.