Self-Healing Hydrogels: The Next Paradigm Shift in Tissue Engineering?

Cell-adhesive double network self-healing hydrogel capable of cell and drug encapsulation: New platform to construct biomimetic environment with bottom-up approach

This study presents the development and characterization of a novel double-network self-healing hydrogel based on N-carboxyethyl chitosan (CEC) and oxidized dextran (OD) with the incorporation of crosslinked collagen (CEC-OD/COL-GP) to enhance its biological and physicochemical properties. The hydrogel formed via dynamic imine bond formation exhibited efficient self-healing within 30 min, and a compressive modulus recovery of 92 % within 2 h. In addition to its self-healing ability, CEC-OD/COL-GP possesses unique physicochemical characteristics including transparency, injectability, and adhesiveness to various substrates and tissues. Cell encapsulation studies confirmed the biocompatibility and suitability of the hydrogel as a cell-culture scaffold, with the presence of a collagen network that enhances cell adhesion, spreading, long-term cell viability, and proliferation. Leveraging their unique properties, we engineered assemblies of self-healing hydrogel modules for controlled spatiotemporal drug delivery and constructed co-culture models that simulate angiogenesis in tumor microenvironments. Overall, the CEC-OD/COL-GP hydrogel is a versatile and promising material for biomedical applications, offering a bottom-up approach for constructing complex structures with self-healing capabilities, controlled drug release, and support for diverse cell types in 3D environments. This hydrogel platform has considerable potential for advancements in tissue engineering and therapeutic interventions.

Strategies of functionalized GelMA-based bioinks for bone regeneration: Recent advances and future perspectives

Gelatin methacryloyl (GelMA) hydrogels is a widely used bioink because of its good biological properties and tunable physicochemical properties, which has been widely used in a variety of tissue engineering and tissue regeneration. However, pure GelMA is limited by the weak mechanical strength and the lack of continuous osteogenic induction environment, which is difficult to meet the needs of bone repair. Moreover, GelMA hydrogels are unable to respond to complex stimuli and therefore are unable to adapt to physiological and pathological microenvironments. This review focused on the functionalization strategies of GelMA hydrogel based bioinks for bone regeneration. The synthesis process of GelMA hydrogel was described in details, and various functional methods to meet the requirements of bone regeneration, including mechanical strength, porosity, vascularization, osteogenic differentiation, and immunoregulation for patient specific repair, etc. In addition, the response strategies of smart GelMA-based bioinks to external physical stimulation and internal pathological microenvironment stimulation, as well as the functionalization strategies of GelMA hydrogel to achieve both disease treatment and bone regeneration in the presence of various common diseases (such as inflammation, infection, tumor) are also briefly reviewed. Finally, we emphasized the current challenges and possible exploration directions of GelMA-based bioinks for bone regeneration.

Natural polymer starch-based materials for flexible electronic sensor development: A review of recent progress

In response to the burgeoning interest in the development of highly conformable and resilient flexible electronic sensors capable of transducing diverse physical stimuli, this review investigates the pivotal role of natural polymers, specifically those derived from starch, in crafting sustainable and biocompatible sensing materials. Expounding on cutting-edge research, the exploration delves into innovative strategies employed to leverage the distinctive attributes of starch in conjunction with other polymers for the fabrication of advanced sensors. The comprehensive discussion encompasses a spectrum of starch-based materials, spanning all-starch-based gels to starch-based soft composites, meticulously scrutinizing their applications in constructing resistive, capacitive, piezoelectric, and triboelectric sensors. These intricately designed sensors exhibit proficiency in detecting an array of stimuli, including strain, temperature, humidity, liquids, and enzymes, thereby playing a pivotal role in the continuous and non-invasive monitoring of human body motions, physiological signals, and environmental conditions. The review highlights the intricate interplay between material properties, sensor design, and sensing performance, emphasizing the unique advantages conferred by starch-based materials, such as self-adhesiveness, self-healability, and re-processibility facilitated by dynamic bonding. In conclusion, the paper outlines current challenges and future research opportunities in this evolving field, offering valuable insights for prospective investigations.

Enhancing regenerative medicine with self-healing hydrogels: A solution for tissue repair and advanced cyborganic healthcare devices

Considering the global burden related to tissue and organ injuries or failures, self-healing hydrogels may be an attractive therapeutic alternative for the future. Self-healing hydrogels are highly hydrated 3D structures with the ability to self-heal after breaking, this property is attributable to a variety of dynamic non-covalent and covalent bonds that are able to re-linking within the matrix. Self-healing ability specially benefits minimal invasive medical treatments with cell-delivery support. Moreover, those tissue-engineered self-healing hydrogels network have demonstrated effectiveness for myriad purposes; for instance, they could act as delivery-platforms for different cargos (drugs, growth factors, cells, among others) in tissues such as bone, cartilage, nerve or skin. Besides, self-healing hydrogels have currently found their way into new and novel applications; for example, with the development of the self-healing adhesive hydrogels, by merely aiding surgical closing processes and by providing biomaterial-tissue adhesion. Furthermore, conductive hydrogels permit the stimuli and monitoring of natural electrical signals, which facilitated a better fitting of hydrogels in native tissue or the diagnosis of various health diseases. Lastly, self-healing hydrogels could be part of cyborganics - a merge between biology and machinery - which can pave the way to a finer healthcare devices for diagnostics and precision therapies.

Chemically Programmed Hydrogels for Spatiotemporal Modulation of the Cardiac Pathological Microenvironment

After myocardial infarction (MI), sustained ischemic events induce pathological microenvironments characterized by ischemia-hypoxia, oxidative stress, inflammatory responses, matrix remodeling, and fibrous scarring. Conventional clinical therapies lack spatially targeted and temporally responsive modulation of the infarct microenvironment, leading to limited myocardial repair. Engineered hydrogels have a chemically programmed toolbox for minimally invasive localization of the pathological microenvironment and personalized responsive modulation over different pathological periods. Chemically programmed strategies for crosslinking interactions, interfacial binding, and topological microstructures in hydrogels enable minimally invasive implantation and in situ integration tailored to the myocardium. This enhances substance exchange and signal interactions within the infarcted microenvironment. Programmed responsive polymer networks, intelligent micro/nanoplatforms, and biological therapeutic cues contribute to the formation of microenvironment-modulated hydrogels with precise targeting, spatiotemporal control, and on-demand feedback. Therefore, this review summarizes the features of the MI microenvironment and chemically programmed schemes for hydrogels to conform, integrate, and modulate the cardiac pathological microenvironment. Chemically programmed strategies for oxygen-generating, antioxidant, anti-inflammatory, provascular, and electrointegrated hydrogels to stimulate iterative and translational cardiac tissue engineering are discussed.

Proanthocyanidins-based tandem dynamic covalent cross-linking hydrogel for diabetic wound healing

Wound healing in diabetic patients presents significant challenges in clinical wound care due to high oxidative stress, excessive inflammation, and a microenvironment prone to infection. In this study, we successfully developed a multifunctional tandem dynamic covalently cross-linked hydrogel dressing aimed at diabetic wound healing. This hydrogel was constructed using cyanoacetic acid functionalized dextran (Dex-CA), 2-formylbenzoylboric acid (2-FPBA) and natural oligomeric proanthocyanidins (OPC), catalyzed by histidine. The resulting Dex-CA/OPC/2-FPBA (DPOPC) hydrogel can be dissolved triggered by cysteine, thereby achieving "controllable and non-irritating" dressing change. Furthermore, the incorporation of OPC as a hydrogel building block endowed the hydrogel with antioxidant and anti-inflammatory properties. The cross-linked network of the DPOPC hydrogel circumvents the burst release of OPC, enhancing its biosafety. In vivo studies demonstrated that the DPOPC hydrogel significantly accelerated the wound healing process in diabetic mice compared to a commercial hydrogel, achieving an impressive wound closure rate of 98 % by day 14. The DPOPC hydrogel effectively balanced the disrupted inflammatory state during the healing process. This dynamic hydrogel based on natural polyphenols is expected to be an ideal candidate for dressings intended for chronic wounds.

Triple-Responsive, Multimodal, Visual Electronic Skin toward All-in-One Health Management for Gestational Diabetes Mellitus

Gestational diabetes mellitus (GDM) is one of the most common metabolic disorders during pregnancy, leading to serious complications for pregnant women and a threat to life safety of infants. Therefore, it is particularly important to establish a multipurpose monitoring pathway to important physiological indicators of pregnant women. In this work, three kinds of double network hydrogels are prepared with poly(vinyl alcohol) (PVA), borax, and cellulose ethers with varying substituents of methyl (methyl cellulose, MC), hydroxypropyl (hydroxypropyl cellulose, HPC), or both (hydroxypropyl methyl cellulose, HPMC), respectively. The corresponding toughness (143.9, 102.3, and 135.9 kJ cm-3) and conductivity (0.69, 0.45, and 0.51 S m-1) of the hydrogels demonstrate that PB-MC was endowed with the prominent performance. Molecular dynamics simulations further revealed the essence that hydrogen bond interactions between PVA and cellulose ethers play a critical role in regulating the structure and properties of hydrogels. Thermochromic capsule powders (TCPs) were subsequently doped in to achieve a composite hydrogel (TCPs@PB-MC) to indicate the change in human body temperature. Furthermore, the process of the TCPs@PB-MC response to glucose, pH, and temperature was tracked in-depth through the electrochemical window. This work provides a novel strategy for all-in-one health management of GDM.

Exosomes encapsulated in hydrogels for effective central nervous system drug delivery

The targeted delivery of pharmacologically active molecules, metabolites, and growth factors to the brain parenchyma has become one of the major challenges following the onset of neurodegeneration and pathological conditions. The therapeutic effect of active biomolecules is significantly impaired after systemic administration in the central nervous system (CNS) because of the blood-brain barrier (BBB). Therefore, the development of novel therapeutic approaches capable of overcoming these limitations is under discussion. Exosomes (Exo) are nano-sized vesicles of endosomal origin that have a high distribution rate in biofluids. Recent advances have introduced Exo as naturally suitable bio-shuttles for the delivery of neurotrophic factors to the brain parenchyma. In recent years, many researchers have attempted to regulate the delivery of Exo to target sites while reducing their removal from circulation. The encapsulation of Exo in natural and synthetic hydrogels offers a valuable strategy to address the limitations of Exo, maintaining their integrity and controlling their release at a desired site. Herein, we highlight the current and novel approaches related to the application of hydrogels for the encapsulation of Exo in the field of CNS tissue engineering.

Advances in Hydrogels for Periodontitis Treatment

Periodontitis is a common condition characterized by a bacterial infection and the disruption of the body's immune-inflammatory response, which causes damage to the teeth and supporting tissues and eventually results in tooth loss. Current therapy involves the systemic and local administration of antibiotics. However, the existing treatments cannot exert effective, sustained release and maintain an effective therapeutic concentration of the drug at the lesion site. Hydrogels are used to treat periodontitis due to their low cytotoxicity, exceptional water retention capability, and controlled drug release profile. Hydrogels can imitate the extracellular matrix of periodontal cells while offering suitable sites to load antibiotics. This article reviews the utilization of hydrogels for periodontitis therapy based on the pathogenesis and clinical manifestations of the disease. Additionally, the latest therapeutic strategies for smart hydrogels and the main techniques for hydrogel preparation have been discussed. The information will aid in designing and preparing future hydrogels for periodontitis treatment.

Unleashing the power of chlorogenic acid: exploring its potential in nutrition delivery and the food industry

In the contemporary era, heightened emphasis on health and safety has emerged as a paramount concern among individuals with food. The concepts of "natural" and "green" have progressively asserted dominance in the food consumption market. Consequently, through continuous exploration and development, an escalating array of natural bioactive ingredients is finding application in both nutrition delivery and the broader food industry. Chlorogenic acid (CGA), a polyphenolic compound widely distributed in various plants in nature, has garnered significant attention. Abundant research underscores CGA's robust biological activity, showcasing notable preventive and therapeutic efficacy across diverse diseases. This article commences with a comprehensive overview, summarizing the dietary sources and primary biological activities of CGA. These encompass antioxidant, anti-inflammatory, antibacterial, anti-cancer, and neuroprotective activities. Next, a comprehensive overview of the current research on nutrient delivery systems incorporating CGA is provided. This exploration encompasses nanoparticle, liposome, hydrogel, and emulsion delivery systems. Additionally, the article explores the latest applications of CGA in the food industry. Serving as a cutting-edge theoretical foundation, this paper contributes to the design and development of CGA in the realms of nutrition delivery and the food industry. Finally, the article presents informed speculations and considerations for the future development of CGA.

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.

ROS-Responsive Injectable Hydrogel Loaded with SLC7A11-modRNA Inhibits Ferroptosis and Mitigates Intervertebral Disc Degeneration in Rats

Intervertebral disc degeneration (IVDD) is the primary cause of low back pain, with oxidative stress being a recognized factor that causes its development. Presently, low back pain imposes a significant global economic burden. However, the effectiveness of treatments for IVDD remains extremely limited. Therefore, this study aims to explore innovative and effective IVDD treatments by focusing on oxidative stress as a starting point. In this study, an injectable reactive oxygen species-responsive hydrogel (PVA-tsPBA@SLC7A11 modRNA) is developed, designed to achieve rapid loading and selective release of chemically synthesized modified mRNA (modRNA). SLC7A11 modRNA is specifically used to upregulate the expression of the ferroptosis marker SLC7A11. The local injection of PVA-tsPBA@SLC7A11 modRNA into the degenerated intervertebral disc (IVD) results in the cleavage of PVA-tsPBA, leading to the release of enclosed SLC7A11 modRNA. The extent of SLC7A11 modRNA release is directly proportional to the severity of IVDD, ultimately ameliorating IVDD by inhibiting ferroptosis in nucleus pulposus cells (NPCs). This study proposes an innovative system of PVA-tsPBA hydrogel-encapsulated modRNA, representing a potential novel treatment strategy for patients with early-stage IVDD.

Hyaluronic acid hydrogels with excellent self-healing capacity and photo-enhanced mechanical properties for wound healing

A continuously stable moist healing environment is immensely beneficial for wound healing, which can be availably achieved by providing an in situ hydrogel with enough strength resembling skin tissue and self-healing ability. Herein, through a dual-crosslinking strategy, hyaluronic acid-based hydrogels with excellent self-healing capacity and enhanced mechanical properties are fabricated via the acylhydrazone linkages and subsequent photocrosslinking based on hydrazide-modified sodium hyaluronate and aldehyde-modified maleic sodium hyaluronate. The hydrogels demonstrate the fast gelation process (< 1 min), the controlled swelling behaviors, and the good biocompatibility. Notably, they possess enhanced mechanical strength similar to the human dermis (∼ 2.2 kPa). Also, they can self-heal rapidly with a self-healing efficiency of ∼90 % at 6 h. Based on this, the hyaluronic acid-based hydrogels, without any biological factors involved, can facilitate the full-thickness skin wound reconstruction process by accelerating the three phases of the wound repair, including reducing wound inflammation in the inflammatory phase, promoting angiogenesis in the proliferative phase, and promoting the deposition and reconstruction of collagen in the remodeling phase. The produced hyaluronic acid hydrogel can serve as an ideal candidate for wound healing.

Black Phosphorus-Based Dynamic Self-Healing Hydrogel to Integrate Demineralized Bone Matrix and Noggin-Targeting siRNA for Synergistic Osteogenesis

Although a demineralized bone matrix (DBM) is often used as an alternative to an autologous bone graft, its clinical application is still hampered by easy dispersion of DBM particles and insufficient osteoinductivity in the defect site. Herein, we designed a self-healing hydrogel for DBM that can rapidly restore its structural integrity after damage based on amino-rich black phosphorus (BP) nanosheets and aldehyde-functionalized hyaluronic acid (AHA). Given the increased expression of bone morphogenetic protein (BMP) antagonists by DBM stimulation, the osteogenic potency of DBM in the hydrogel carrier was further enhanced by abrogating the BMP antagonism. The BP/AHA hydrogel provided dynamic polymer-nanosheet networks that combine injectability, modability, and physical stability with high DBM loading, where the BP nanosheets served as osteogenic cross-linkers to promote biomineralization and deliver siRNA to suppress undesirable expression of BMP antagonist noggin by DBM. As a result, the BP/AHA hydrogel integrated with DBM and noggin-targeting siRNA synergistically promoted osteogenic differentiation of mesenchymal stem cells by enhancing BMP/Smad signaling. This work demonstrates a promising strategy to improve the efficacy of bone regeneration using bone graft.

Exosome-coated oxygen nanobubble-laden hydrogel augments intracellular delivery of exosomes for enhanced wound healing

Wound healing is an obvious clinical concern that can be hindered by inadequate angiogenesis, inflammation, and chronic hypoxia. While exosomes derived from adipose tissue-derived stem cells have shown promise in accelerating healing by carrying therapeutic growth factors and microRNAs, intracellular cargo delivery is compromised in hypoxic tissues due to activated hypoxia-induced endocytic recycling. To address this challenge, we have developed a strategy to coat oxygen nanobubbles with exosomes and incorporate them into a polyvinyl alcohol/gelatin hybrid hydrogel. This approach not only alleviates wound hypoxia but also offers an efficient means of delivering exosome-coated nanoparticles in hypoxic conditions. The self-healing properties of the hydrogel, along with its component, gelatin, aids in hemostasis, while its crosslinking bonds facilitate hydrogen peroxide decomposition, to ameliorate wound inflammation. Here, we show the potential of this multifunctional hydrogel for enhanced healing, promoting angiogenesis, facilitating exosome delivery, mitigating hypoxia, and inhibiting inflammation in a male rat full-thickness wound model.

Skin-adhesive and self-healing diagnostic wound dressings for diabetic wound healing recording and electrophysiological signal monitoring

Performing efficient wound management is essential for infected diabetic wounds due to the complex pathology. Flexible electronics have been recognized as one of the promising solutions for wound management. Herein, a kind of skin-adhesive and self-healing flexible bioelectronic was developed, which could be employed as a diagnostic wound dressing to record diabetic wound healing and monitor electrophysiological signals of the patients. The flexible substrate of diagnostic wound dressings showed excellent tissue adhesive (to various substrates including biological samples), self-healing (fracture strength restores by 96%), and intrinsic antibacterial properties (antibacterial ratio >96% against multidrug-resistant bacteria). The diagnostic wound dressings could record the glucose level (1-30 mM), pH values (4-7), and body temperature (18.8-40.0 °C) around the infected diabetic wounds. Besides, the dressings could help optimize treatment strategies based on electrophysiological signals of patients monitored in real-time. This study contributes to developing flexible bioelectronics for the diagnosis and management of diabetic wounds.

Shape Memory Hydrogels for Biomedical Applications

The ability of shape memory polymers to change shape upon external stimulation makes them exceedingly useful in various areas, from biomedical engineering to soft robotics. Especially, shape memory hydrogels (SMHs) are well-suited for biomedical applications due to their inherent biocompatibility, excellent shape morphing performance, tunable physiochemical properties, and responsiveness to a wide range of stimuli (e.g., thermal, chemical, electrical, light). This review provides an overview of the unique features of smart SMHs from their fundamental working mechanisms to types of SMHs classified on the basis of applied stimuli and highlights notable clinical applications. Moreover, the potential of SMHs for surgical, biomedical, and tissue engineering applications is discussed. Finally, this review summarizes the current challenges in synthesizing and fabricating reconfigurable hydrogel-based interfaces and outlines future directions for their potential in personalized medicine and clinical applications.

Novel Photothermal Graphene-Based Hydrogels in Biomedical Applications

In the last decade, photothermal therapy (PTT) has attracted tremendous attention because it is non-invasive, shows high efficiency and antibacterial activity, and minimizes drug side effects. Previous studies demonstrated that PTT can effectively inhibit the growth of bacteria and promotes cell proliferation, accelerating wound healing and tissue regeneration. Among different NIR-responsive biomaterials, graphene-based hydrogels with photothermal properties are considered as the best candidates for biomedical applications, due to their excellent properties. This review summarizes the current advances in the development of innovative graphene-based hydrogels for PTT-based biomedical applications. Also, the information about photothermal properties and the potential applications of graphene-based hydrogels in biomedical therapies are provided. These findings provide a great potential for supporting their applications in photothermal biomedicine.

Putting Hybrid Nanomaterials to Work for Biomedical Applications

Hybrid nanomaterials have found use in many biomedical applications. This article provides a comprehensive review of the principles, techniques, and recent advancements in the design and fabrication of hybrid nanomaterials for biomedicine. We begin with an introduction to the general concept of material hybridization, followed by a discussion of how this approach leads to materials with additional functionality and enhanced performance. We then highlight hybrid nanomaterials in the forms of nanostructures, nanocomposites, metal-organic frameworks, and biohybrids, including their fabrication methods. We also showcase the use of hybrid nanomaterials to advance biomedical engineering in the context of nanomedicine, regenerative medicine, diagnostics, theranostics, and biomanufacturing. Finally, we offer perspectives on challenges and opportunities.

A High-Stretching, Rapid-Self-Healing, and Printable Composite Hydrogel Based on Poly(Vinyl Alcohol), Nanocellulose, and Sodium Alginate

Hydrogels with excellent flexibility, conductivity, and controllable mechanical properties are the current research hotspots in the field of biomaterial sensors. However, it is difficult for hydrogel sensors to regain their original function after being damaged, which limits their practical applications. Herein, a composite hydrogel (named SPBC) of poly(vinyl alcohol) (PVA)/sodium alginate (SA)/cellulose nanofibers (CNFs)/sodium borate tetrahydrate was synthesized, which has good self-healing, electrical conductivity, and excellent mechanical properties. The SPBC0.3 hydrogel demonstrates rapid self-healing (<30 s) and achieves mechanical properties of 33.92 kPa. Additionally, it exhibits high tensile strain performance (4000%). The abundant internal ions and functional groups of SPBC hydrogels provide support for the good electrical conductivity (0.62 S/cm) and electrical response properties. In addition, the SPBC hydrogel can be attached to surfaces such as fingers and wrists to monitor human movements in real time, and its good rheological property supports three-dimensional (3D) printing molding methods. In summary, this study successfully prepared a self-healing, conductive, printable, and mechanically superior SPBC hydrogel. Its suitability for 3D-printing personalized fabrication and outstanding sensor properties makes it a useful reference for hydrogels in wearable devices and human motion monitoring.

Engineered Gold and Silica Nanoparticle-Incorporated Hydrogel Scaffolds for Human Stem Cell-Derived Cardiac Tissue Engineering

Electrically conductive biomaterials and nanomaterials have demonstrated great potential in the development of functional and mature cardiac tissues. In particular, gold nanomaterials have emerged as promising candidates due to their biocompatibility and ease of fabrication for cardiac tissue engineering utilizing rat- or stem cell-derived cardiomyocytes (CMs). However, despite significant advancements, it is still not clear whether the enhancement in cardiac tissue function is primarily due to the electroconductivity features of gold nanoparticles or the structural changes of the scaffold resulting from the addition of these nanoparticles. To address this question, we developed nanoengineered hydrogel scaffolds comprising gelatin methacrylate (GelMA) embedded with either electrically conductive gold nanorods (GNRs) or nonconductive silica nanoparticles (SNPs). This enabled us to simultaneously assess the roles of electrically conductive and nonconductive nanomaterials in the functionality and fate of human-induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs). Our studies revealed that both GNR- and SNP-incorporated hydrogel scaffolds exhibited excellent biocompatibility and similar cardiac cell attachment. Although the expression of sarcomere alpha-actinin did not significantly differ among the conditions, a more organized sarcomere structure was observed within the GNR-embedded hydrogels compared to the nonconductive nanoengineered scaffolds. Furthermore, electrical coupling was notably improved in GNR-embedded scaffolds, as evidenced by the synchronous calcium flux and enhanced calcium transient intensity. While we did not observe a significant difference in the gene expression profile of human cardiac tissues formed on the conductive GNR- and nonconductive SNP-incorporated hydrogels, we noticed marginal improvements in the expression of some calcium and structural genes in the nanomaterial-embedded hydrogel groups as compared to the control condition. Given that the cardiac tissues formed atop the nonconductive SNP-based scaffolds (used as the control for conductivity) also displayed similar levels of gene expression as compared to the conductive hydrogels, it suggests that the electrical conductivity of nanomaterials (i.e., GNRs) may not be the sole factor influencing the function and fate of hiPSC-derived cardiac tissues when cells are cultured atop the scaffolds. Overall, our findings provide additional insights into the role of electrically conductive gold nanoparticles in regulating the functionalities of hiPSC-CMs.

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.

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.

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.

3D Printing of Self-Healing Materials

This review article presents a comprehensive overview of the latest advances in the field of 3D printable structures with self-healing properties. Three-dimensional printing (3DP) is a versatile technology that enables the rapid manufacturing of complex geometric structures with precision and functionality not previously attainable. However, the application of 3DP technology is still limited by the availability of materials with customizable properties specifically designed for additive manufacturing. The addition of self-healing properties within 3D printed objects is of high interest as it can improve the performance and lifespan of structural components, and even enable the mimicking of living tissues for biomedical applications, such as organs printing. The review will discuss and analyze the most relevant results reported in recent years in the development of self-healing polymeric materials that can be processed via 3D printing. After introducing the chemical and physical self-healing mechanism that can be exploited, the literature review here reported will focus in particular on printability and repairing performances. At last, actual perspective and possible development field will be critically discussed.

Engineering pH-Responsive, Self-Healing Vesicle-Type Artificial Tissues with Higher-Order Cooperative Functionalities

Multicellular organisms demonstrate a hierarchical organization where multiple cells collectively form tissues, thereby enabling higher-order cooperative functionalities beyond the capabilities of individual cells. Drawing inspiration from this biological organization, assemblies of multiple protocells are developed to create novel functional materials with emergent higher-order cooperative functionalities. This paper presents new artificial tissues derived from multiple vesicles, which serve as protocellular models. These tissues are formed and manipulated through non-covalent interactions triggered by a salt bridge. Exhibiting pH-sensitive reversible formation and destruction under neutral conditions, these artificial vesicle tissues demonstrate three distinct higher-order cooperative functionalities: transportation of large cargoes, photo-induced contractions, and enhanced survivability against external threats. The rapid assembly and disassembly of these artificial tissues in response to pH variations enable controlled mechanical task performance. Additionally, the self-healing property of these artificial tissues indicates robustness against external mechanical damage. The research suggests that these vesicles can detect specific pH environments and spontaneously assemble into artificial tissues with advanced functionalities. This leads to the possibility of developing intelligent materials with high environmental specificity, particularly for applications in soft robotics.

Rapid Self-Healing Hydrogel with Ultralow Electrical Hysteresis for Wearable Sensing

Self-healing hydrogels are in high demand for wearable sensing applications due to their remarkable deformability, high ionic and electrical conductivity, self-adhesiveness to human skin, as well as resilience to both mechanical and electrical damage. However, these hydrogels face challenges such as delayed healing times and unavoidable electrical hysteresis, which limit their practical effectiveness. Here, we introduce a self-healing hydrogel that exhibits exceptionally rapid healing with a recovery time of less than 0.12 s and an ultralow electrical hysteresis of less than 0.64% under cyclic strains of up to 500%. This hydrogel strikes an ideal balance, without notable trade-offs, between properties such as softness, deformability, ionic and electrical conductivity, self-adhesiveness, response and recovery times, durability, overshoot behavior, and resistance to nonaxial deformations such as twisting, bending, and pressing. Owing to this unique combination of features, the hydrogel is highly suitable for long-term, durable use in wearable sensing applications, including monitoring body movements and electrophysiological activities on the skin.

Gelatin/Polyacrylamide-Based Antimicrobial and Self-Healing Hydrogel Film for Wound Healing Application

In this study, a self-healing, adhesive, and superabsorbent film made of gelatin, poly(acrylamide), and boric acid (GelAA) was successfully synthesized using a free radical reaction mechanism. The optimized film showed a remarkable 2865 ± 42% water absorptivity and also exhibited excellent self-healing behavior. The GelAA films were further loaded with silver nanoclusters (AgNCs) and ursodeoxycholic acid (UDC) (loading efficiency = 10%) to develop UDC/Ag/GelAA films. The loading of AgNCs in UDC/Ag/GelAA films helped in exhibiting 99.99 ± 0.01% antibacterial activity against both Gram-positive and Gram-negative bacteria, making them very effective against bacterial infections. Additionally, UDC/Ag/GelAA films had 77.19 ± 0.52% porosity and showed 90% of UDC release in 30 h, which helps in improving the cell proliferation. Our research provides an easy but highly effective process for synthesizing a hydrogel film, which is an intriguing choice for wound healing applications without the use of antibiotics.

Recent Advances and Developments in Injectable Conductive Polymer Gels for Bioelectronics

Soft matter bioelectronics represents an emerging and interdisciplinary research frontier aiming to harness the synergy between biology and electronics for advanced diagnostic and healthcare applications. In this context, a whole family of soft gels have been recently developed with self-healing ability and tunable biological mimetic features to act as a tissue-like space bridging the interface between the electronic device and dynamic biological fluids and body tissues. This review article provides a comprehensive overview of electroactive polymer gels, formed by noncovalent intermolecular interactions and dynamic covalent bonds, as injectable electroactive gels, covering their synthesis, characterization, and applications. First, hydrogels crafted from conducting polymers (poly(3,4-ethylene-dioxythiophene) (PEDOT), polyaniline (PANi), and polypyrrole (PPy))-based networks which are connected through physical interactions (e.g., hydrogen bonding, π-π stacking, hydrophobic interactions) or dynamic covalent bonds (e.g., imine bonds, Schiff-base, borate ester bonds) are addressed. Injectable hydrogels involving hybrid networks of polymers with conductive nanomaterials (i.e., graphene oxide, carbon nanotubes, metallic nanoparticles, etc.) are also discussed. Besides, it also delves into recent advancements in injectable ionic liquid-integrated gels (iongels) and deep eutectic solvent-integrated gels (eutectogels), which present promising avenues for future research. Finally, the current applications and future prospects of injectable electroactive polymer gels in cutting-edge bioelectronic applications ranging from tissue engineering to biosensing are outlined.

Programmed microalgae-gel promotes chronic wound healing in diabetes

Chronic diabetic wounds are at lifelong risk of developing diabetic foot ulcers owing to severe hypoxia, excessive reactive oxygen species (ROS), a complex inflammatory microenvironment, and the potential for bacterial infection. Here we develop a programmed treatment strategy employing live Haematococcus (HEA). By modulating light intensity, HEA can be programmed to perform a variety of functions, such as antibacterial activity, oxygen supply, ROS scavenging, and immune regulation, suggesting its potential for use in programmed therapy. Under high light intensity (658 nm, 0.5 W/cm2), green HEA (GHEA) with efficient photothermal conversion mediate wound surface disinfection. By decreasing the light intensity (658 nm, 0.1 W/cm2), the photosynthetic system of GHEA can continuously produce oxygen, effectively resolving the problems of hypoxia and promoting vascular regeneration. Continuous light irradiation induces astaxanthin (AST) accumulation in HEA cells, resulting in a gradual transformation from a green to red hue (RHEA). RHEA effectively scavenges excess ROS, enhances the expression of intracellular antioxidant enzymes, and directs polarization to M2 macrophages by secreting AST vesicles via exosomes. The living HEA hydrogel can sterilize and enhance cell proliferation and migration and promote neoangiogenesis, which could improve infected diabetic wound healing in female mice.

ROS/pH dual responsive PRP-loaded multifunctional chitosan hydrogels with controlled release of growth factors for skin wound healing

Platelet-rich plasma (PRP) contains a variety of growth factors (GFs) and has been used in the treatment of a variety of diseases, including skin lesions. In particular, PRP with low immunogenicity will be more widely used. However, the explosive release of GFs limits its further application. In order to achieve controlled release of GFs, a multifunctional and reactive oxygen species (ROS)/pH dual responsive hydrogel was developed to load PRP derived from human cord blood for the treatment of skin wound healing. Based on the hydrogen bond and Schiff base interaction, carboxymethyl chitosan (CMCS), oxidized dextran (Odex) and oligomeric procyanidins (OPC) were crosslinked to form CMCS/Odex/OPC/PRP hydrogel with good injectability, self-healing, adhesion, ROS scavenging, antibacterial activity, controlled and sustained release of GFs. In vitro cell experiments suggested that this hydrogel possessed excellent biocompatibility and could promote the proliferation and migration of L929. In vivo healing of full-layer skin wounds further indicated that the prepared hydrogel could regulate inflammation and promote epithelialization, collagen deposition, and angiogenesis. In summary, this present study demonstrates that CMCS/Odex/OPC/PRP hydrogel may serve as a promising multifunctional dressing for skin wound healing.

Self-healable, Tolerant Superaerophobic Coating for Improving Electrochemical Hydrogen Production

Gas-evolving electrodes often suffer from the blocking of catalytic active sites-due to unwanted and unavoidable adhesion of generated gas bubbles, which elevates the overpotential for the electrochemical hydrogen evolution reaction (HER)- by raising the resistance of the electrode. Here, a catalyst-free and self-healable superaerophobic coating having ultra-low bubble adhesion is introduced for achieving significantly depleted overpotentials of 209 and 506 mV at both low (50 mA cm-2 ) and high (500 mA cm-2 ) current densities, respectively, compared to a bare nickel-foam electrode. The optimized coating ensured an early detachment of the generated tiny (0.8 ± 0.1 mm) gas bubble-and thus, prevented the undesired rise in resistance of the coated electrode. The systematic association of physical (i.e., ionic interactions, H-bonding, etc.) cross-linkage, β-amino ester type covalent cross-linkage and reinforced halloysite nano clay enables the design of such functional material embedded with essential characteristics-including improved mechanical (toughness of 63.7 kJ m-3 , and tensile modulus of 26 kPa) property and chemical (extremes of pH (1 and 14), salinity, etc.) stability, rapid (<10 min) self-healing ability (even at alkaline condition) and desired bubble-wettability (bubble contact angle of 158.2 ± 0.2° ) with ultralow force (4.2 ± 0.4 µN) of bubble adhesion.

Novel Injectable, Self-Healing, Long-Effective Bacteriostatic, and Healed-Promoting Hydrogel Wound Dressing and Controlled Drug Delivery Mechanisms

Multivalent ion cross-linking has been used to form hydrogels between sodium alginate (SA) and hyaluronic acid (HA) in previous studies. However, more stable and robust covalent cross-linking is rarely reported. Herein, we present a facile approach to fabricate a SA and HA hydrogel for wound dressings with injectable, good biocompatibility, and high ductility. HA was first reacted with ethylenediamine to graft an amino group. Then, it was cross-linked with oxidized SA with dialdehyde to form hydrogel networks. The dressing can effectively promote cell migration and wound healing. To increase the antibacterial property of the dressing, we successfully loaded tetracycline hydrochloride into the hydrogel as a model drug. The drug can be released slowly in the alkaline environment of chronic wounds, and the hydrogel releases drugs again in the more acidic environment with wound healing, achieving a long-term antibacterial effect. In addition, one-dimensional partial differential equations based on Fickian diffusion with time-varying diffusion coefficients and hydrogel thicknesses were used to model the entire complex drug release process and to predict drug release.

Supramolecular hydrogels for wound repair and hemostasis

The unique network characteristics and stimuli responsiveness of supramolecular hydrogels have rendered them highly advantageous in the field of wound dressings, showcasing unprecedented potential. However, there are few reports on a comprehensive review of supramolecular hydrogel dressings for wound repair and hemostasis. This review first introduces the major cross-linking methods for supramolecular hydrogels, which includes hydrogen bonding, electrostatic interactions, hydrophobic interactions, host-guest interactions, metal ligand coordination and some other interactions. Then, we review the advanced materials reported in recent years and then summarize the basic principles of each cross-linking method. Next, we classify the network structures of supramolecular hydrogels before outlining their forming process and propose their potential future directions. Furthermore, we also discuss the raw materials, structural design principles, and material characteristics used to achieve the advanced functions of supramolecular hydrogels, such as antibacterial function, tissue adhesion, substance delivery, anti-inflammatory and antioxidant functions, cell behavior regulation, angiogenesis promotion, hemostasis and other innovative functions in recent years. Finally, the existing problems as well as future development directions of the cross-linking strategy, network design, and functions in wound repair and hemostasis of supramolecular hydrogels are discussed. This review is proposed to stimulate further exploration of supramolecular hydrogels on wound repair and hemostasis by researchers in the future.

State-of-the-Art Advances and Current Applications of Gel-Based Membranes

Gel-based membranes, a fusion of polymer networks and liquid components, have emerged as versatile tools in a variety of technological domains thanks to their unique structural and functional attributes. Historically rooted in basic filtration tasks, recent advancements in synthetic strategies have increased the mechanical strength, selectivity, and longevity of these membranes. This review summarizes their evolution, emphasizing breakthroughs that have positioned them at the forefront of cutting-edge applications. They have the potential for desalination and pollutant removal in water treatment processes, delivering efficiency that often surpasses conventional counterparts. The biomedical field has embraced them for drug delivery and tissue engineering, capitalizing on their biocompatibility and tunable properties. Additionally, their pivotal role in energy storage as gel electrolytes in batteries and fuel cells underscores their adaptability. However, despite monumental progress in gel-based membrane research, challenges persist, particularly in scalability and long-term stability. This synthesis provides an overview of the state-of-the-art applications of gel-based membranes and discusses potential strategies to overcome current limitations, laying the foundation for future innovations in this dynamic field.

A Sol-Gel Transition and Self-Healing Hydrogel Triggered via Photodimerization of Coumarin

Reversible chemical covalency provides a path to materials that can degrade and recombine with appropriate stimuli and which can be used for tissue regeneration and repair. However, designing and preparing efficient and quickly self-healing materials has always been a challenge. The preparation strategies of photoresponsive gels attract a lot of attention due to their precise spatial and temporal control and their predetermined response to light stimulation. In this work, the linear copolymer PAC was synthesized via precipitation polymerization of acrylic acid and 7-(2-acrylate-ethoxylated)-4-methylcoumarin. The coumarin groups on the copolymer PAC side chains provide a reversible chemical cross-linking via photostimulation, which achieves reversible regulation of the gel network structure. The concentration of 18 wt% PAC solution produces gelation under irradiation with 365 nm. In contrast, PAC gel is restored to soluble copolymers under irradiation with 254 nm. Meanwhile, the mechanical and self-healing properties of the gel were also explored. It is demonstrated that the cracks of the gel can be repaired simply, quickly, and efficiently. Furthermore, the PAC copolymer shows an excellent adhesion property based on the reversible sol-gel transition. Thus, the PAC gel has considerable potential for applications in engineering and biomedical materials.

Exploration of Variable Solvent Directed Self-Healable Supramolecular M(II)-Metallogels (M = Co, Ni, Zn) of Azelaic Acid: Investigating Temperature-Dependent Ion Conductivity and Antibacterial Efficiency

Molecular self-assembly assisted self-healing supramolecular metallogels of azelaic acid with cobalt(II)-, nickel(II)-, and zinc(II)-based metal acetate salts were successfully fabricated. Individually, N,N'-dimethylformamide and dimethyl sulfoxide were immobilized within these distinctly synthesized soft-scaffolds of metallogels to attain their semisolid viscoelastic nature. Rheological experiments such as amplitude sweep, frequency sweep, and thixotropic measurements were executed for these metallogels to ratify their gel features. The different extents of supramolecular interactions operating within these solvent-directed metallogels were clearly reflected in terms of their distinct morphological patterns as investigated through field emission scanning electron microscopy. Comparative infrared (IR) spectral properties of metallogels along with individual metal salts and azelaic acid were analyzed. These experimental data clearly depict the significant shifting of Fourier transform (FT)-IR peaks of xerogel samples of different metallogels from the gel-forming precursors. The networks present within the soft-scaffold are evidently illustrated by the electrospray ionization-mass experimental data. The temperature-dependent ionic conductivity studies with these solvent-directed versatile metallogel systems were investigated through impedance spectroscopy. The temperature-dependent impedance spectroscopic studies clearly demonstrate that the ion-transportation within the gel matrix depends not only on the types of cations but also on the dielectric properties of the immobilized solvents. The antipathogenic effect of these metallogel systems has also been explored by testing their effectiveness against human pathogenic Gram-negative bacteria Klebsiella pneumoniae (MTCC 109) and Vibrio parahemolyticus, and Gram-positive bacteria like Bacillus cereus (MTCC 1272). These gel soft-scaffolds show no significant cytotoxicity against both the human neuroblastoma cell line-SH-SY5Y and the human embryonic kidney cell line-HEK 293.

Temporary Consolidation of Marine Artifact Based on Polyvinyl Alcohol/Tannic Acid Reversible Hydrogel

Underwater artefacts are vulnerable to damage and loss of archaeological information during the extraction process. To solve this problem, it is necessary to apply temporary consolidation materials to fix the position of marine artifacts. A cross-linked network hydrogel composed of polyvinyl alcohol (PVA), tannic acid (TA), borax, and calcium chloride has been created. Four hydrogels with varying concentrations of tannic acid were selected to evaluate the effect. The hydrogel exhibited exceptional strength, high adhesion, easy removal, and minimal residue. The PVA/TA hydrogel and epoxy resin were combined to extract waterlogged wooden artifacts and marine archaeological ceramics from a 0.4 m deep tank. This experiment demonstrates the feasibility of using hydrogel for the extraction of marine artifacts.

Mechanical characterization and design of biomaterials for nucleus pulposus replacement and regeneration

Biomaterials for nucleus pulposus (NP) replacement and regeneration have great potential to restore normal biomechanics in degenerated intervertebral discs following nucleotomy. Mechanical characterizations are essential for assessing the efficacy of biomaterial implants for clinical applications. While traditional compression tests are crucial to quantify various modulus values, relaxation behaviors and fatigue resistance, rheological measurements should also be conducted to investigate the viscoelastic properties, injectability, and overall stability upon deformation. To recapitulate the physiological in vivo environment, the use of spinal models is necessary to evaluate the risk of implant extrusion and the restoration of biomechanics under different loading conditions. When designing devices for NP replacement, injectable materials are ideal to fully fill the nucleus cavity and prevent implant migration. In addition to achieving biocompatibility and desirable mechanical characteristics, biomaterial implants should be optimized to avoid implant extrusion or re-herniation post-operatively. This review discusses the most commonly used testing protocols for assessing mechanical properties of biomaterial implants and serves as reference material for enabling researchers to characterize NP implants through a unified approach whereby newly developed biomaterials may be compared and contrasted to existing devices.

Morphological Evaluation of Supramolecular Soft Materials Obtained through Co-Assembly Processes

Low-molecular-weight gelators (LMWGs) are compounds with an intrinsic tendency to self-assemble forming various supramolecular architectures via non-covalent interactions. Considering that the development of supramolecular assemblies through the synergy of molecules is not entirely understood at the molecular level, this study introduced a Fmoc-short peptide and four Fmoc-amino acids as building blocks for the self-assembly/co-assembly process. Hence, we investigated the formation of supramolecular gels starting from the molecular aggregation following two triggering approaches: solvent/co-solvent method and pH switch. The complex morphological analysis (POM, AFM, and STEM) offered an insight into the spontaneous formation of well-ordered nanoaggregates. Briefly, POM and AFM images demonstrated that self-assembled gels present various morphologies like dendrimer, spherulite, and vesicle, whereas all co-assembled supramolecular systems exhibit fibrillar morphologies as a result of the interaction between co-partners of each system. STEM study has confirmed that the molecules interact and join together, finally forming a fibrous network, an aspect seen in both self-assembled and co-assembled gels. XRD allowed the determination of the molecular arrangement. The study emphasized that the Fmoc motif protected the amino groups and facilitated gelation through additional π-π interactions.

Injectable and self-healing dual crosslinked gelatin/kappa-carrageenan methacryloyl hybrid hydrogels via host-guest supramolecular interaction for wound healing

Injectable hydrogels based on natural polymers have shown great potential for various tissue engineering applications, such as wound healing. However, poor mechanical properties and weak self-healing ability are still major challenges. In this work, we introduce a host-guest (HG) supramolecular interaction between acrylate-β-cyclodextrin (Ac-β-CD) conjugated on methacrylated kappa-carrageenan (MA-κ-CA) and aromatic residues on gelatin to provide self-healing characteristics. We synthesize an MA-κ-CA to conjugate Ac-β-CD and fabricate dual crosslinked hybrid hydrogels with gelatin to mimic the native extracellular matrix (ECM). The dual crosslinking occurs on the MA-κ-CA backbone through the addition of KCl and photocrosslinking process, which enhances mechanical strength and stability. The hybrid hydrogels exhibit shear-thinning, self-healing, and injectable behavior, which apply easily under a minimally invasive manner and contribute to shear stress during the injection. In-vitro studies indicate enhanced cell viability. Furthermore, scratch assays are performed to examine cell migration and cell-cell interaction. It is envisioned that the combination of self-healing and injectable dual crosslinked hybrid hydrogels with HG interactions display a promising and functional biomaterial platform for wound healing applications.

Injectable hydrogels: An emerging therapeutic strategy for cartilage regeneration

The impairment of articular cartilage due to traumatic incidents or osteoarthritis has posed significant challenges for healthcare practitioners, researchers, and individuals suffering from these conditions. Due to the absence of an approved treatment strategy for the complete restoration of cartilage defects to their native state, the tissue condition often deteriorates over time, leading to osteoarthritic (OA). However, recent advancements in the field of regenerative medicine have unveiled promising prospects through the utilization of injectable hydrogels. This versatile class of biomaterials, characterized by their ability to emulate the characteristics of native articular cartilage, offers the distinct advantage of minimally invasive administration directly to the site of damage. These hydrogels can also serve as ideal delivery vehicles for a diverse range of bioactive agents, including growth factors, anti-inflammatory drugs, steroids, and cells. The controlled release of such biologically active molecules from hydrogel scaffolds can accelerate cartilage healing, stimulate chondrogenesis, and modulate the inflammatory microenvironment to halt osteoarthritic progression. The present review aims to describe the methods used to design injectable hydrogels, expound upon their applications as delivery vehicles of biologically active molecules, and provide an update on recent advances in leveraging these delivery systems to foster articular cartilage regeneration.

Nanocellulose-based porous materials: Regulation and pathway to commercialization in regenerative medicine

We review the recent progress that have led to the development of porous materials based on cellulose nanostructures found in plants and other resources. In light of the properties that emerge from the chemistry, shape and structural control, we discuss some of the most promising uses of a plant-based material, nanocellulose, in regenerative medicine. Following a brief discussion about the fundamental aspects of self-assembly of nanocellulose precursors, we review the key strategies needed for material synthesis and to adjust the architecture of the materials (using three-dimensional printing, freeze-casted porous materials, and electrospinning) according to their uses in tissue engineering, artificial organs, controlled drug delivery and wound healing systems, among others. For this purpose, we map the structure-property-function relationships of nanocellulose-based porous materials and examine the course of actions that are required to translate innovation from the laboratory to industry. Such efforts require attention to regulatory aspects and market pull. Finally, the key challenges and opportunities in this nascent field are critically reviewed.

Polymer Hydrogels and Frontal Polymerization: A Winning Coupling

Polymer hydrogels are 3D networks consisting of hydrophilic crosslinked macromolecular chains, allowing them to swell and retain water. Since their invention in the 1960s, they have become an outstanding pillar in the design, development, and application of engineered polymer systems suitable for biomedical and pharmaceutical applications (such as drug or cell delivery, the regeneration of hard and soft tissues, wound healing, and bleeding prevention, among others). Despite several well-established synthetic routes for developing polymer hydrogels based on batch polymerization techniques, about fifteen years ago, researchers started to look for alternative methods involving simpler reaction paths, shorter reaction times, and lower energy consumption. In this context, frontal polymerization (FP) has undoubtedly become an alternative and efficient reaction model that allows for the conversion of monomers into polymers via a localized and propagating reaction-by means of exploiting the formation and propagation of a "hot" polymerization front-able to self-sustain and propagate throughout the monomeric mixture. Therefore, the present work aims to summarize the main research outcomes achieved during the last few years concerning the design, preparation, and application of FP-derived polymeric hydrogels, demonstrating the feasibility of this technique for the obtainment of functional 3D networks and providing the reader with some perspectives for the forthcoming years.

A review on self-healing featured soft robotics

Soft robots are becoming more popular because they can solve issues stiff robots cannot. Soft component and system design have seen several innovations recently. Next-generation robot-human interactions will depend on soft robotics. Soft material technologies integrate safety at the material level, speeding its integration with biological systems. Soft robotic systems must be as resilient as biological systems in unexpected, uncontrolled situations. Self-healing materials, especially polymeric and elastomeric ones, are widely studied. Since most currently under-development soft robotic systems are composed of polymeric or elastomeric materials, this finding may provide immediate assistance to the community developing soft robots. Self-healing and damage-resilient systems are making their way into actuators, structures, and sensors, even if soft robotics remains in its infancy. In the future, self-repairing soft robotic systems composed of polymers might save both money and the environment. Over the last decade, academics and businesses have grown interested in soft robotics. Despite several literature evaluations of the soft robotics subject, there seems to be a lack of systematic research on its intellectual structure and development despite the rising number of articles. This article gives an in-depth overview of the existing knowledge base on damage resistance and self-healing materials' fundamental structure and classifications. Current uses, problems with future implementation, and solutions to those problems are all included in this overview. Also discussed are potential applications and future directions for self-repairing soft robots.

Mechanically Tunable Hydrogels with Self-Healing and Shape Memory Capabilities from Thermo-Responsive Amino Acid-Derived Vinyl Polymers

In this study, we report the fabrication and characterization of self-healing and shape-memorable hydrogels, the mechanical properties of which can be tuned via post-polymerization crosslinking. These hydrogels were constructed from a thermo-responsive poly(N-acryloyl glycinamide) (NAGAm) copolymer containing N-acryloyl serine methyl ester (NASMe) units (5 mol%) that were readily synthesized via conventional radical copolymerization. This transparent and free-standing hydrogel is produced via multiple hydrogen bonds between PNAGAm chains by simply dissolving the polymer in water at a high temperature (~90 °C) and then cooling it. This hydrogel exhibited moldability and self-healing properties. The post-polymerization crosslinking of the amino acid-derived vinyl copolymer network with glutaraldehyde, which acts as a crosslinker between the hydroxy groups of the NASMe units, tuned mechanical properties such as viscoelasticity and tensile strength. The optimal crosslinker concentration efficiently improved the viscoelasticity. Moreover, these hydrogels exhibited shape fixation (~60%)/memory (~100%) behavior owing to the reversible thermo-responsiveness (upper critical solution temperature-type) of the PNAGAm units. Our multifunctional hydrogel, with moldable, self-healing, mechanical tunability via post-polymerization crosslinking, and shape-memorable properties, has considerable potential for applications in engineering and biomedical materials.

Designing self-healing hydrogels for biomedical applications

Self-healing hydrogels have emerged as the most promising alternatives to conventional brittle hydrogels used in the biomedical field due to the features of long-term stability and durability. However, the incompatibility between the fast self-healing property and enough mechanical strength of hydrogels remains a challenge. Therefore, hydrogels that possess not only mechanical toughness but also autonomous self-healing capacity are sought after. This review presents a comprehensive summary of the latest self-healing mechanisms. Specifically, we review various systems based on dynamic bonds, ranging from dynamic covalent bonds to non-covalent bonds. Additionally, this review presents different characterization methods for self-healing hydrogels, and also highlights their potential applications in the biomedical field, such as tissue engineering, drug delivery, cell therapy, and wound dressing. Furthermore, this review aims to provide valuable guidance for constructing diverse self-healing hydrogels with tailored functions.

An Ultrahighly Stretchable and Recyclable Starch-Based Gel with Multiple Functions

With the development of various gel-based flexible sensors, novel gels with multiple integrated and efficient properties, particularly recyclability, have been developed. Herein, a starch-based ADM (amylopectin (AP)-poly(3-[dimethyl-[2-(2-methylprop-2- enoyloxy)ethyl]azaniumyl]propane-1-sulfonate) (PDMAPS)-MXene) gel is prepared by a facile "cooking" strategy accompanying the gelatinization of AP and polymerization reaction of zwitterionic monomers. Reversible crosslinking in the gel occurs through hydrogen bonding and electrostatic interactions. The ADM gel exhibits high stretchability (≈2700%, after one month), swift self-healing performance, self-adhesive properties, favorable freezing resistance, and satisfactory moisturizing properties (≥30 days). Interestingly, the ADM gel can be recycled and reused by a "kneading" method and "dissolution-dialysis" process, respectively. Furthermore, the ADM gel can be assembled as a strain sensor with a broad working strain range (≈800%) and quick response time (response time 211 ms and recovery time 253 ms, under 10% strain) to detect various macro- and micro-human-motions, even under harsh conditions such as pronunciation and handwriting. The ADM gel can also be used as a humidity sensor to investigate humidity and human respiratory status, suggesting its practical application in personal health management. This study provides a novel strategy for the preparation of high-performance recycled gels and flexible sensors.

Tissue engineered skin substitutes: A comprehensive review of basic design, fabrication using 3D printing, recent advances and challenges

The multi-layered skin structure includes the epidermis, dermis and hypodermis, which forms a sophisticated tissue composed of extracellular matrix (ECM). The wound repair is a well-orchestrated process when the skin is injured. However, this natural wound repair will be ineffective for large surface area wounds. Autografts-based treatment is efficient but, additional pain and secondary healing of the patient limits its successful application. Therefore, there is a substantial need for fabricating tissue-engineered skin constructs. The development of a successful skin graft requires a fundamental understanding of the natural skin and its healing process, as well as design criteria for selecting a biopolymer and an appropriate fabrication technique. Further, the fabrication of an appropriate skin graft needs to meet physicochemical, mechanical, and biological properties equivalent to the natural skin. Advanced 3D bioprinting provides spatial control of the placement of functional components, such as biopolymers with living cells, which can satisfy the prerequisites for the preparation of an ideal skin graft. In this view, here we elaborate on the basic design requirements, constraints involved in the fabrication of skin graft and choice of ink, the probable solution by 3D bioprinting technique, as well as their latest advancements, challenges, and prospects.

A Bioinspired Self-Healing Conductive Hydrogel Promoting Peripheral Nerve Regeneration

The development of self-healing conductive hydrogels is critical in electroactive nerve tissue engineering. Typical conductive materials such as polypyrrole (PPy) are commonly used to fabricate artificial nerve conduits. Moreover, the field of tissue engineering has advanced toward the use of products such as hyaluronic acid (HA) hydrogels. Although HA-modified PPy films are prepared for various biological applications, the cell-matrix interaction mechanisms remain poorly understood; furthermore, there are no reports on HA-modified PPy-injectable self-healing hydrogels for peripheral nerve repair. Therefore, in this study, a self-healing electroconductive hydrogel (HASPy) from HA, cystamine (Cys), and pyrrole-1-propionic acid (Py-COOH), with injectability, biodegradability, biocompatibility, and nerve-regenerative capacity is constructed. The hydrogel directly targets interleukin 17 receptor A (IL-17RA) and promotes the expression of genes and proteins relevant to Schwann cell myelination mainly by activating the interleukin 17 (IL-17) signaling pathway. The hydrogel is injected directly into the rat sciatic nerve-crush injury sites to investigate its capacity for nerve regeneration in vivo and is found to promote functional recovery and remyelination. This study may help in understanding the mechanism of cell-matrix interactions and provide new insights into the potential use of HASPy hydrogel as an advanced scaffold for neural regeneration.