Principles and Design of Bionic Hydrogel Adhesives for Skin Wound Treatment

Over millions of years of evolution, nature has developed a myriad of unique features that have inspired the design of adhesives for wound healing. Bionic hydrogel adhesives, capable of adapting to the dynamic movements of tissues, possess superior biocompatibility and effectively promote the healing of both external and internal wounds. This paper provides a systematic review of the design and principles of these adhesives, focusing on the treatment of skin wounds, and explores the feasibility of incorporating nature-inspired properties into their design. The adhesion mechanisms of bionic adhesives are analyzed from both chemical and physical perspectives. Materials from natural and synthetic polymers commonly used as adhesives are detailed regarding their biocompatibility and degradability. The multifunctional design elements of hydrogel adhesives for skin trauma treatment, such as self-healing, drug release, responsive design, and optimization of mechanical and physical properties, are further explored. The aim is to overcome the limitations of conventional treatments and offer a safer, more effective solution for the application of bionic wound dressings.

Overview of Dynamic Bond Based Hydrogels for Reversible Adhesion Processes

Polymeric hydrogels are soft materials with a three-dimensional (3D) hydrophilic network capable of retaining and absorbing large amounts of water or biological fluids. Due to their customizable properties, these materials are extensively studied for developing matrices for 3D cell culture scaffolds, drug delivery systems, and tissue engineering. However, conventional hydrogels still exhibit many drawbacks; thus, significant efforts have been directed towards developing dynamic hydrogels that draw inspiration from organisms' natural self-repair abilities after injury. The self-healing properties of these hydrogels are closely associated with their ability to form, break, and heal dynamic bonds in response to various stimuli. The primary objective of this review is to provide a comprehensive overview of dynamic hydrogels by examining the types of chemical bonds associated with them and the biopolymers utilized, and to elucidate the chemical nature of dynamic bonds that enable the modulation of hydrogels' properties. While dynamic bonds ensure the self-healing behavior of hydrogels, they do not inherently confer adhesive properties. Therefore, we also highlight emerging approaches that enable dynamic hydrogels to acquire adhesive properties.

Bioactive injectable mucoadhesive thermosensitive natural polymeric hydrogels for oral bone and periodontal regeneration

Periodontitis is an inflammation-related condition, caused by an infectious microbiome and host defense that causes damage to periodontium. The natural processes of the mouth, like saliva production and eating, significantly diminish therapeutic medication residency in the region of periodontal disease. Furthermore, the complexity and diversity of pathological mechanisms make successful periodontitis treatment challenging. As a result, developing enhanced local drug delivery technologies and logical therapy procedures provides the foundation for effective periodontitis treatment. Being biocompatible, biodegradable, and easily administered to the periodontal tissues, hydrogels have sparked substantial an intense curiosity in the discipline of periodontal therapy. The primary objective of hydrogel research has changed in recent years to intelligent thermosensitive hydrogels, that involve local adjustable sol-gel transformations and regulate medication release in reaction to temperature, we present a thorough introduction to the creation and efficient construction of new intelligent thermosensitive hydrogels for periodontal regeneration. We also address cutting-edge smart hydrogel treatment options based on periodontitis pathophysiology. Furthermore, the problems and prospective study objectives are reviewed, with a focus on establishing effective hydrogel delivery methods and prospective clinical applications.

Lysine-Triggered Polymeric Hydrogels with Self-Adhesion, Stretchability, and Supportive Properties

Hydrogels, recognized for their flexibility and diverse characteristics, are extensively used in medical fields such as wearable sensors and soft robotics. However, many hydrogel sensors derived from biomaterials lack mechanical strength and fatigue resistance, emphasizing the necessity for enhanced formulations. In this work, we utilized acrylamide and polyacrylamide as the primary polymer network, incorporated chemically modified poly(ethylene glycol) (DF-PEG) as a physical crosslinker, and introduced varying amounts of methacrylated lysine (LysMA) to prepare a series of hydrogels. This formulation was labeled as poly(acrylamide)-DF-PEG-LysMA, abbreviated as pADLx, with x denoting the weight/volume percentage of LysMA. We observed that when the hydrogel contained 2.5% w/v LysMA (pADL2.5), compared to hydrogels without LysMA (pADL0), its stress increased by 642 ± 76%, strain increased by 1790 ± 95%, and toughness increased by 2037 ± 320%. Our speculation regarding the enhanced mechanical performance of the pADL2.5 hydrogel revolves around the synergistic effects arising from the co-polymerization of LysMA with acrylamide and the formation of multiple intermolecular hydrogen bonds within the network structures. Moreover, the acid, amine, and amide groups present in the LysMA molecules have proven to be instrumental contributors to the self-adhesion capability of the hydrogel. The validation of the pADL2.5 hydrogel's exceptional mechanical properties through rigorous tensile tests further underscores its suitability for use in strain sensors. The outstanding stretchability, adhesive strength, and fatigue resistance demonstrated by this hydrogel affirm its potential as a key component in the development of robust and reliable strain sensors that fulfill practical requirements.

Development of a new kappa-carrageenan hydrogel system to study benthic diatom vertical movements

Benthic diatom vertical movement has been investigated mainly through indirect measurements based on chlorophyll a fluorescence and spectral reflectance signals. The presence of sediment hinders direct imaging and grazers activity renders the work under controlled conditions very difficult. This study provides a tool to study diatoms movement in a 3D hydrogel matrix. Synthetic and natural hydrogels were tested to find the best 3D transparent scaffold where diatoms could grow and freely move in all directions. Polyamidoamines (PAAm) hydrogels were no-cytocompatible and hyaluronic acid (HA) only allowed diatoms to survive for 2-days. Natural hydrogels made of gelatin/Na-alginate, Na-alginate and kappa-carrageenan (KC) were cytocompatible, with KC showing the best properties for diatom growth and movement on a long term (up to 2 months). Comparing Nitzschia spathulata, Gyrosigma limosum and Navicula phyllepta growth in liquid media vs in KC gels, we found that diatoms reached a significantly higher final biomass in the hydrogel condition. Hydrogels were also useful to isolate large size diatom species e.g., Nitzschia elongata, that did not survive in suspension. Finally, we showed three ways to study diatom species-specific movement in KC hydrogels: 1) controlled species mix; 2) natural diatom assemblages with grazers; and 3) natural diatom assemblages without grazers. With our system, single diatoms could be imaged, identified, and counted. In addition, different stimuli, e.g., light intensity and light composition can be applied and their effects on movement and physiology studied without being masked by sediment or impaired by meiofauna.

Nanoarchitectonics of Bimetallic Cu-/Co-Doped Nitrogen-Carbon Nanozyme-Functionalized Hydrogel with NIR-Responsive Phototherapy for Synergistic Mitigation of Drug-Resistant Bacterial Infections

Superbug infections and transmission have become major challenges in the contemporary medical field. The development of novel antibacterial strategies to efficiently treat bacterial infections and conquer the problem of antimicrobial resistance (AMR) is extremely important. In this paper, a bimetallic CuCo-doped nitrogen-carbon nanozyme-functionalized hydrogel (CuCo/NC-HG) has been successfully constructed. It exhibits photoresponsive-enhanced enzymatic effects under near-infrared (NIR) irradiation (808 nm) with strong peroxidase (POD)-like and oxidase (OXD)-like activities. Upon NIR irradiation, CuCo/NC-HG possesses photodynamic activity for producing singlet oxygen(1O2), and it also has a high photothermal conversion effect, which not only facilitates the elimination of bacteria but also improves the efficiency of reactive oxygen species (ROS) production and accelerates the consumption of GSH. CuCo/NC-HG shows a lower hemolytic rate and better cytocompatibility than CuCo/NC and possesses a positive charge and macroporous skeleton for restricting negatively charged bacteria in the range of ROS destruction, strengthening the antibacterial efficiency. Comparatively, CuCo/NC and CuCo/NC-HG have stronger bactericidal ability against methicillin-resistant Staphylococcus aureus (MRSA) and ampicillin-resistant Escherichia coli (AmprE. coli) through destroying the cell membranes with a negligible occurrence of AMR. More importantly, CuCo/NC-HG plus NIR irradiation can exhibit satisfactory bactericidal performance in the absence of H2O2, avoiding the toxicity from high-concentration H2O2. In vivo evaluation has been conducted using a mouse wound infection model and histological analyses, and the results show that CuCo/NC-HG upon NIR irradiation can efficiently suppress bacterial infections and promote wound healing, without causing inflammation and tissue adhesions.

Current status of development and biomedical applications of peptide-based antimicrobial hydrogels

Microbial contamination poses a serious threat to human life and health. Through the intersection of material science and modern medicine, advanced bionic hydrogels have shown great potential for biomedical applications due to their unique bioactivity and ability to mimic the extracellular matrix environment. In particular, as a promising antimicrobial material, the synthesis and practical biomedical applications of peptide-based antimicrobial hydrogels have drawn increasing research interest. The synergistic effect of peptides and hydrogels facilitate the controlled release of antimicrobial agents and mitigation of their biotoxicity while achieving antimicrobial effects and protecting the active agents from degradation. This review reports on the progress and trends of researches in the last five years and provides a brief outlook, aiming to provide theoretical background on peptide-based antimicrobial hydrogels and make suggestions for future related work.

Photothermal-promoted multi-functional gallic acid grafted chitosan hydrogel containing tannic acid miniaturized particles for peri-implantitis

Peri-implantitis, a leading cause of implant failure, currently lacks effective therapeutic strategies. Given that bacterial infection and reactive oxygen species overabundance serve as primary pathogenic and triggering factors, respectively, an adhesive hydrogel has been created for in-situ injection. The hydrogel is a gallic acid-grafted chitosan (CS-GA) hydrogel containing tannic acid miniaturized particles (TAMP). This provides antibacterial and antioxidant properties. Therefore, this study aims to evaluate the potential role of this hydrogel in preventing and treating peri-implantitis via several experiments. It undergoes rapid formation within a span of over 20 s via an oxidative crosslinking reaction catalyzed by horseradish peroxidase and hydrogen peroxide, demonstrating robust adhesion, superior cell compatibility, and a sealing effect. Furthermore, the incorporation of TAMP offer photothermal properties to the hydrogel, enabling it to enhance the viability, migration, and antioxidant activity of co-cultured human gingival fibroblasts when subjected 0.5 W/cm2 808 nm near-infrared (NIR) irradiation. At higher irradiation power, the hydrogel exhibits progressive improvements in its antibacterial efficacy against Porphyromonas gingivalis and Fusobacterium nucleatum. It attains rates of 83.11 ± 5.42 % and 83.48 ± 6.855 %, respectively, under 1 W/cm2 NIR irradiation. In summary, the NIR-controlled CS-GA/TAMP hydrogel, exhibiting antibacterial and antioxidant properties, represents a promising approach for the prophylaxis and management of peri-implantitis.

Self-healing hydrogels based on biological macromolecules in wound healing: A review

The complete healing of skin wounds has been a challenge in clinical treatment. Self-healing hydrogels are special hydrogels formed by distinctive physicochemically reversible bonds, and they are considered promising biomaterials in the biomedical field owing to their inherently good drug-carrying capacity as well as self-healing and repair abilities. Moreover, natural polymeric materials have received considerable attention in skin tissue engineering owing to their low cytotoxicity, low immunogenicity, and excellent biodegradation rates. In this paper, we review recent advances in the design of self-healing hydrogels based on natural polymers for skin-wound healing applications. First, we outline a variety of natural polymers that can be used to construct self-healing hydrogel systems and highlight the advantages and disadvantages of different natural polymers. We then describe the principle of self-healing hydrogels in terms of two different crosslinking mechanisms-physical and chemical-and dissect their performance characteristics based on the practical needs of skin-trauma applications. Next, we outline the biological mechanisms involved in the healing of skin wounds and describe the current application strategies for self-healing hydrogels based on these mechanisms. Finally, we analyze and summarize the challenges and prospects of natural-material-based self-healing hydrogels for skin applications.

Bioadhesives with Antimicrobial Properties

Bioadhesives with antimicrobial properties enable easier and safer treatment of wounds as compared to the traditional methods such as suturing and stapling. Composed of natural or synthetic polymers, these bioadhesives seal wounds and facilitate healing while preventing infections through the activity of locally released antimicrobial drugs, nanocomponents, or inherently antimicrobial polers. Although many different materials and strategies are employed to develop antimicrobial bioadhesives, the design of these biomaterials necessitates a prudent approach as achieving all the required properties including optimal adhesive and cohesive properties, biocompatibility, and antimicrobial activity can be challenging. Designing antimicrobial bioadhesives with tunable physical, chemical, and biological properties will shed light on the path for future advancement of bioadhesives with antimicrobial properties. In this review, the requirements and commonly used strategies for developing bioadhesives with antimicrobial properties are discussed. In particular, different methods for their synthesis and their experimental and clinical applications on a variety of organs are reviewed. Advances in the design of bioadhesives with antimicrobial properties will pave the way for a better management of wounds to increase positive clinical outcomes.

Bioadhesive and Injectable Hydrogels and Their Correlation with Mesenchymal Stem Cells Differentiation for Cartilage Repair: A Mini-Review

Injectable bioadhesive hydrogels, known for their capacity to carry substances and adaptability in processing, offer great potential across various biomedical applications. They are especially promising in minimally invasive stem cell-based therapies for treating cartilage damage. This approach harnesses readily available mesenchymal stem cells (MSCs) to differentiate into chondrocytes for cartilage regeneration. In this review, we investigate the relationship between bioadhesion and MSC differentiation. We summarize the fundamental principles of bioadhesion and discuss recent trends in bioadhesive hydrogels. Furthermore, we highlight their specific applications in conjunction with stem cells, particularly in the context of cartilage repair. The review also encompasses a discussion on testing methods for bioadhesive hydrogels and direct techniques for differentiating MSCs into hyaline cartilage chondrocytes. These approaches are explored within both clinical and laboratory settings, including the use of genetic tools. While this review offers valuable insights into the interconnected aspects of these topics, it underscores the need for further research to fully grasp the complexities of their relationship.

Bionic ordered structured hydrogels: structure types, design strategies, optimization mechanism of mechanical properties and applications

Natural organisms, such as lobsters, lotus, and humans, exhibit exceptional mechanical properties due to their ordered structures. However, traditional hydrogels have limitations in their mechanical and physical properties due to their disordered molecular structures when compared with natural organisms. Therefore, inspired by nature and the properties of hydrogels similar to those of biological soft tissues, researchers are increasingly focusing on how to investigate bionic ordered structured hydrogels and render them as bioengineering soft materials with unique mechanical properties. In this paper, we systematically introduce the various structure types, design strategies, and optimization mechanisms used to enhance the strength, toughness, and anti-fatigue properties of bionic ordered structured hydrogels in recent years. We further review the potential applications of bionic ordered structured hydrogels in various fields, including sensors, bioremediation materials, actuators, and impact-resistant materials. Finally, we summarize the challenges and future development prospects of bionic ordered structured hydrogels in preparation and applications.

Modern Approaches in Wounds Management

Wound management represents a well-known continuous challenge and concern of the global healthcare systems worldwide. The challenge is on the one hand related to the accurate diagnosis, and on the other hand to establishing an effective treatment plan and choosing appropriate wound care products in order to maximize the healing outcome and minimize the financial cost. The market of wound dressings is a dynamic field which grows and evolves continuously as a result of extensive research on developing versatile formulations with innovative properties. Hydrogels are one of the most attractive wound care products which, in many aspects, are considered ideal for wound treatment and are widely exploited for extension of their advantages in healing process. Smart hydrogels (SHs) offer the opportunities of the modulation physico-chemical properties of hydrogels in response to external stimuli (light, pressure, pH variations, magnetic/electric field, etc.) in order to achieve innovative behavior of their three-dimensional matrix (gel-sol transitions, self-healing and self-adapting abilities, controlled release of drugs). The SHs response to different triggers depends on their composition, cross-linking method, and manufacturing process approach. Both native or functionalized natural and synthetic polymers may be used to develop stimuli-responsive matrices, while the mandatory characteristics of hydrogels (biocompatibility, water permeability, bioadhesion) are preserved. In this review, we briefly present the physiopathology and healing mechanisms of chronic wounds, as well as current therapeutic approaches. The rational of using traditional hydrogels and SHs in wound healing, as well as the current research directions for developing SHs with innovative features, are addressed and discussed along with their limitations and perspectives in industrial-scale manufacturing.

Current state of the art and future directions for implantable sensors in medical technology: Clinical needs and engineering challenges

Implantable sensors have revolutionized the way we monitor biophysical and biochemical parameters by enabling real-time closed-loop intervention or therapy. These technologies align with the new era of healthcare known as healthcare 5.0, which encompasses smart disease control and detection, virtual care, intelligent health management, smart monitoring, and decision-making. This review explores the diverse biomedical applications of implantable temperature, mechanical, electrophysiological, optical, and electrochemical sensors. We delve into the engineering principles that serve as the foundation for their development. We also address the challenges faced by researchers and designers in bridging the gap between implantable sensor research and their clinical adoption by emphasizing the importance of careful consideration of clinical requirements and engineering challenges. We highlight the need for future research to explore issues such as long-term performance, biocompatibility, and power sources, as well as the potential for implantable sensors to transform healthcare across multiple disciplines. It is evident that implantable sensors have immense potential in the field of medical technology. However, the gap between research and clinical adoption remains wide, and there are still major obstacles to overcome before they can become a widely adopted part of medical practice.

Aquatic Diatoms-Inspired Universal Adhesive Coacervates Triggered by Water

Adhesives with strong underwater adhesion performance are urgently needed in diverse areas. However, designing adhesives with long-term stability to diverse materials underwater in a facile way is challenging. Here, inspired by aquatic diatoms, a series of novel biomimetic universal adhesives is reported that shows tunable performance with robust and long-lasting stable underwater adhesion to various substrates, including wet biological tissues. The versatile and robust wet-contact adhesives are pre-polymerized by N-[tris(hydroxymethyl)methyl]acrylamide, n-butyl acrylate, and methylacrylic acid in dimethyl sulfoxide and spontaneously coacervated in water triggered by solvent exchange. The synergistic interaction between hydrogen bonding and hydrophobic interaction allows the hydrogels with instant and strong adhesion to various substrate surfaces. The slowly formed covalent bonds enhance cohesion and adhesion strength in hours. The spatial and timescale-dependent adhesion mechanism endows the adhesives with strong and long-lasting stable underwater adhesion to be coupled with fault-tolerant convenient surgical operations.

Pullulan/Poly(vinyl alcohol) Hydrogels Loaded with Calendula officinalis Extract: Design and In Vitro Evaluation for Wound Healing Applications

The therapeutic efficiency of plant extracts has been limited by their poor pharmaceutical availability. Hydrogels have promising potential to be applied as wound dressings due to their high capacity to absorb exudates and their enhanced performance in loading and releasing plant extracts. In this work, pullulan/poly (vinyl alcohol) (P/PVA) hydrogels were first prepared using an eco-friendly method based on both a covalent and physical cross-linking approach. Then, the hydrogels were loaded with the hydroalcoholic extract of Calendula officinalis by a simple post-loading immersion method. Different loading capacities were investigated in terms of the physico-chemical properties, chemical composition, mechanical properties, and water absorption. The hydrogels exhibited high loading efficiency due to the hydrogen bonding interactions between polymer and extract. The water retention capacity as well as the mechanical properties decreased with the increase in the extract amount in hydrogel. However, higher amounts of extract in the hydrogel improved the bioadhesiveness. The release of extract from hydrogels was controlled by the Fickian diffusion mechanism. Extract-loaded hydrogels expressed high antioxidant activity, reaching 70% DPPH radical scavenging after 15 min immersion in buffer solution at pH 5.5. Additionally, loaded hydrogels showed a high antibacterial activity against Gram-positive and Gram-negative bacteria and were non-cytotoxic against HDFa cells.

Carbon Dot-Based Hydrogels: Preparations, Properties, and Applications

Hydrogels have extremely high moisture content, which makes it very soft and excellently biocompatible. They have become an important soft material and have a wide range of applications in various fields such as biomedicine, bionic smart material, and electrochemistry. Carbon dot (CD)-based hydrogels are based on carbon dots (CDs) and auxiliary substances, forming a gel material with comprehensive properties of individual components. CDs embedding in hydrogels could not only solve their aggregation-caused quenching (ACQ) effect, but also manipulate the properties of hydrogels and even bring some novel properties, achieving a win-win situation. In this review, the preparation methods, formation mechanism, and properties of CD-based hydrogels, and their applications in biomedicine, sensing, adsorption, energy storage, and catalysis -are summarized. Finally, a brief discussion on future research directions of CD-based hydrogels will be given.

Fabrication and desired properties of conductive hydrogel dressings for wound healing

Conductive hydrogels are platforms recognized as constituting promising materials for tissue engineering applications. This is because such conductive hydrogels are characterized by the inherent conductivity properties while retaining favorable biocompatibility and mechanical properties. These conductive hydrogels can be particularly useful in enhancing wound healing since their favorable conductivity can promote the transport of essential ions for wound healing via the imposition of a so-called transepithelial potential. Other valuable properties of these conductive hydrogels, such as wound monitoring, stimuli-response etc., are also discussed in this study. Crucially, the properties of conductive hydrogels, such as 3D printability and monitoring properties, suggest the possibility of its use as an alternative wound dressing to traditional dressings such as bandages. This review, therefore, seeks to comprehensively explore the functionality of conductive hydrogels in wound healing, types of conductive hydrogels and their preparation strategies and crucial properties of hydrogels. This review will also assess the limitations of conductive hydrogels and future perspectives, with an emphasis on the development trend for conductive hydrogel uses in wound dressing fabrication for subsequent clinical applications.

Multifunctional, Adhesive, and PDA-Coated Bioactive Glass Reinforced Composite Hydrogel for Regenerative Wound Healing

Effective wound management imposes several challenges in clinical outcomes due to the complexity of the wound microenvironment, bacterial infections, impaired angiogenesis, aggravated inflammation, and enduring pain. In addition, adhesion on wet biological tissue is another extremely challenging task. Addressing all the issues is necessary for an effective wound healing process. Herein, we developed a unique multifunctional, adhesive composite hydrogel composed of gelatin, chitosan, polydopamine-coated bioactive glass (BG), and curcumin-capped silver nanoparticles (Cur-AgNPs) to target the multifaceted complexity of the wound. The PDA-coated BG serves multiple purposes: (1) adhesivity: catechol groups of PDA and Ca ion released from BG chelate the group present in the hydrogel network and surrounding tissues, (2) angiogenesis: promotes vascularization due to the release of Si from BG, and (3) BG also serves as the "reservoir" for the pain-relieving diclofenac sodium drug with a sustained-release behavior. Cur-AgNPs provide excellent bactericidal and anti-inflammatory properties to the composite hydrogel. In situ application of the composite hydrogel could serve the purpose of a "skin biomimetic" and work as a barrier along with bactericidal properties to inhibit the microbial growth. The multifunctional composite hydrogel (MCH) targeted multiple aspects of wound repair including pain alleviation, elimination of microbes (up to 99%), reduced inflammation, high adhesivity, and increased angiogenesis for effective skin regeneration. The MCH showed excellent wound healing potential as significant wound closure was observed at day 7 and also significantly upregulated the expression of crucial genes involved in the skin regeneration process along with increasing vascularization in the wound area.

Injectable hydrogels of enzyme-catalyzed cross-linked tyramine-modified gelatin for drug delivery

Enzymatically catalyzed cross-linking is a hydrogel fabrication method that generally is considered to have lower cytotoxicity than traditional chemical cross-linking methods. In order to optimize the properties of injectable hydrogels and expand their applications, an enzyme-catalyzed cross-linked injectable hydrogel was designed. The tyramine-modified gelatin (G-T) was formed into a stable injectable hydrogel by the combination of horseradish peroxidase (HRP) and hydrogen peroxide (H2O2) catalysis. 1H NMR spectroscopy was used to demonstrate the successful modification of gelatin by tyramine. The surface morphology of the prepared hydrogels was characterized jointly by atomic force microscopy (AFM) and scanning electron microscopy (SEM). Rheological tests demonstrated the tunable mechanical strength, formation kinetics, shear thinning and good self-recovery properties of the hydrogels. In addition, the hydrogels can be formed into various shapes by injection. The hydrogel network structure is complex and interlaced, as such it is suitable to encapsulate drugs for controlled release. The drug release from the prepared hydrogels followed the Peppas–Sahlin model and belonged to Fickian diffusion. This study constructed injectable hydrogels through the enzyme-catalyzed cross-linking of modified gelatin and applied the hydrogels for drug release, which is expected to expand the application in biomedical fields.

Liquid Metal Patterning and Unique Properties for Next-Generation Soft Electronics

Room-temperature liquid metal (LM)-based electronics is expected to bring advancements in future soft electronics owing to its conductivity, conformability, stretchability, and biocompatibility. However, various difficulties arise when patterning LM because of its rheological features such as fluidity and surface tension. Numerous attempts are made to overcome these difficulties, resulting in various LM-patterning methods. An appropriate choice of patterning method based on comprehensive understanding is necessary to fully utilize the unique properties. Therefore, the authors aim to provide thorough knowledge about patterning methods and unique properties for LM-based future soft electronics. First, essential considerations for LM-patterning are investigated. Then, LM-patterning methods-serial-patterning, parallel-patterning, intermetallic bond-assisted patterning, and molding/microfluidic injection-are categorized and investigated. Finally, perspectives on LM-based soft electronics with unique properties are provided. They include outstanding features of LM such as conformability, biocompatibility, permeability, restorability, and recyclability. Also, they include perspectives on future LM-based soft electronics in various areas such as radio frequency electronics, soft robots, and heterogeneous catalyst. LM-based soft devices are expected to permeate the daily lives if patterning methods and the aforementioned features are analyzed and utilized.

Dramatic improvement in the mechanical properties of polydopamine/polyacrylamide hydrogel mediated human amniotic membrane

Human amniotic membrane (hAM) is a promising material for tissue engineering due to several benefits, including desirable biocompatibility, stem cell source, antibacterial activity, etc. However, because of its low elasticity, the clinical application of hAM is severely restricted. To solve this issue, we employed polydopamine/polyacrylamide (PDA/PAM) hydrogels to toughen hAM. The test results indicated that the PDA/PAM hydrogel can enhance the toughness of hAM dramatically due to the formation of abundant chemical bonds and the strong mechanical properties of the hydrogel itself. Compared to pure hAM, the break elongation and tensile strength of PDA/PAM-toughened hAM rose by 154.15 and 492.31%, respectively. And most importantly, the fracture toughness was almost 15 times higher than untreated hAM. In addition, the cytotoxicity of the PDA/PAM-coated hAM was not detected due to the superior biocompatibility of the chemicals used in the study. Treating hAM with adhesive hydrogels to increase its mechanical characteristics will further promote the application of hAM as a tissue engineering material.

Application of Polymer Hydrogels in the Prevention of Postoperative Adhesion: A Review

Postoperative adhesion is a common post-surgery complication formed between the surface of the body cavity, ranging from a layer of connective tissue to a fibrous bridge containing blood vessels and nerve tissue. Despite achieving a lot of progress, the mechanisms of adhesion formation still need to be further studied. In addition, few current treatments are consistently effective in the prevention of postoperative adhesion. Hydrogel is a kind of water-expanding crosslinked hydrophilic polymer network generated by a simple reaction of one or more monomers. Due to the porous structure, hydrogels can load different drugs and control the drug release kinetics. Evidence from existing studies has confirmed the feasibility and superiority of using hydrogels to counter postoperative adhesions, primarily due to their outstanding antifouling ability. In this review, the current research status of hydrogels as anti-adhesion barriers is summarized, the character of hydrogels in the prevention of postoperative adhesion is briefly introduced, and future research directions are discussed.

Advances in Hemostatic Hydrogels That Can Adhere to Wet Surfaces

Currently, uncontrolled bleeding remains a serious problem in emergency, surgical and battlefield environments. Despite the specific properties of available hemostatic agents, sealants, and adhesives, effective hemostasis under wet and dynamic conditions remains a challenge. In recent years, polymeric hydrogels with excellent hemostatic properties have received much attention because of their adjustable mechanical properties, high porosity, and biocompatibility. In this review, to investigate the role of hydrogels in hemostasis, the mechanisms of hydrogel hemostasis and adhesion are firstly elucidated, the adhesion design strategies of hemostatic hydrogels in wet environments are briefly introduced, and then, based on a comprehensive literature review, the studies and in vivo applications of wet-adhesive hemostatic hydrogels in different environments are summarized, and the improvement directions of such hydrogels in future studies are proposed.

Piper aduncum Essential Oil Rich in Dillapiole: Development of Hydrogel-Thickened Nanoemulsion and Nanostructured Lipid Carrier Intended for Skin Delivery

The essential oil extracted from the leaves of Piper aduncum, an aromatic plant from the Amazon region, is rich in dillapiole and presents anti-inflammatory activity. In this study, nanoemulsions (NE) and nanostructured lipid carriers (NLC), which are biocompatible nanostructured systems of a lipid nature, were prepared by high-pressure homogenization for the yet unexplored skin delivery of dillapiole. The addition of hydroxyethylcellulose produced hydrogel-thickened NE or NLC in view to improving the viscosity and skin adherence of the nanoformulations. Formulations were characterized with respect to dillapiole content, droplet size, polydispersity index, zeta potential, morphology, rheological behavior, bioadhesion, skin permeation profile, and in vitro irritancy (HET-CAM). The formulations developed presented spherical, homogeneous nanometric particle size (around 130 nm), narrow polydispersity index (<0.3), and negative zeta potential (around −40 mV). Dillapiole content was slightly lower in NLC compared to NE since the production process involves heating. The hydrogels containing nanocarriers showed pseudoplastic behavior with bioadhesive characteristics. The developed formulations exhibited a controlled release profile, dillapiole delivery up to the dermis, the layer of interest for anti-inflammatory potential, and low irritant potential in the chorioallantoic membrane (HET-CAM). Both hydrogels-thickened NE and NLC seemed to be promising formulations for skin delivery of Piper aduncum essential oil.

Quinones as an Efficient Molecular Scaffold in the Antibacterial/Antifungal or Antitumoral Arsenal

Quinone-based compounds constitute several general classes of antibiotics that have long shown unwavering efficiency against both Gram-positive and Gram-negative microbial infections. These quinone-based antibiotics are increasingly popular due to their natural origins and are used in natural beverages from herbs or plants in African, Chinese and Indian traditional medicines to treat and prevent various diseases. Quinone-based antibiotics display different bioactive profiles depending on their structures and exert specific biocidal and anti-biofilm properties, and based on recent literature, will be discussed herein.

Multifunctional all-in-one adhesive hydrogel for the treatment of perianal infectious wounds

Postoperative wound of perianal infectious disease represents a common but unique refractory wound in clinical practice. The reasons that hinder the wound healing process include not only the severe bacterial infection of the wound itself and the narrow and deep shape of the wound, but also its frequent bacterial contact. Therefore, the development of biofunctional dressings to aid in therapy is essential. In this study, we synthesized a new type of dressing comprising a hydrogel host based on the Schiff base principle and catechol groups between polydopamine, oxidized dextran, and quaternized chitosan, and then loaded it with traditional Chinese medicine molecules. These formed an integrated hydrogel for accelerated wound repair in a perianal infection model. The prepared hydrogels exhibited excellent wet tissue adhesion, antifouling, morphological variability, suitable swelling properties, and complete degradability, as well as remarkable contact antibacterial ability and the ability to rapidly scavenge free radicals. Hemostatic experiments showed excellent hemostatic properties, as the integrated hydrogel could instantly gel to seal the hemorrhage. Hemocompatibility and in vitro cell experiments showed that the integrated hydrogel had good biosafety and significantly promoted cell proliferation, which in turn accelerated the repair of infected whole cortexes in rats. A histomorphological evaluation showed that the integrated hydrogel promoted the recovery of normal anatomical tissue in rats by promoting the formation of collagen fibers and inhibiting inflammation. The results showed that this multifunctional integrated hydrogel has great potential for the treatment of continuously infected skin regeneration, providing a promising therapeutic strategy for postoperative wound healing in perianal infectious diseases.

Biomass-based hydrogels with high ductility, self-adhesion and conductivity inspired by starch paste for strain sensing

Currently, hydrogel sensors for health monitoring require external tapes, bandages or adhesives to immobilize them on the surface of human skin. However, these external fixation methods easily lead to skin allergic reactions and the decline of monitoring accuracy. A simple strategy to solve this problem is to endow hydrogel sensors with good adhesion. Inspired by the starch paste adhesion mechanism, a biomass-based hydrogel with good conductivity and high repetitive adhesion strength was prepared by introducing modified starch into polyacrylic acid hydrogel system. The properties of biomass-based hydrogels could be controlled by changing the proportion of amylose and amylopectin. The biomass-based hydrogel exhibited a variety of excellent properties, including good stretchability (1290 %), high adhesion strength (pig skin: 46.51 kPa) and conductivity (2.3 S/m). Noticeably, the repeated adhesive strength of biomass-based hydrogel did not decrease with the increase of adhesion times. The strain sensor based on the biomass-based hydrogel could accurately monitor the large-scale and small movements of the human body, and had broad application prospects in the field of flexible wearable devices.

Development of Biodegradable Osteopromotive Citrate-Based Bone Putty

The burden of bone fractures demands development of effective biomaterial solutions, while additional acute events such as noncompressible bleeding further motivate the search for multi-functional implants to avoid complications including osseous hemorrhage, infection, and nonunion. Bone wax has been widely used in orthopedic bleeding control due to its simplicity of use and conformation to irregular defects; however, its nondegradability results in impaired bone healing, risk of infection, and significant inflammatory responses. Herein, a class of intrinsically fluorescent, osteopromotive citrate-based polymer/hydroxyapatite (HA) composites (BPLP-Ser/HA) as a highly malleable press-fit putty is designed. BPLP-Ser/HA putty displays mechanics replicating early nonmineralized bone (initial moduli from ≈2-500 kPa), hydration induced mechanical strengthening in physiological conditions, tunable degradation rates (over 2 months), low swelling ratios (<10%), clotting and hemostatic sealing potential (resistant to blood pressure for >24 h) and significant adhesion to bone (≈350-550 kPa). Simultaneously, citrate's bioactive properties result in antimicrobial (≈100% and 55% inhibition of S. aureus and E. coli) and osteopromotive effects. Finally, BPLP-Ser/HA putty demonstrates in vivo regeneration in a critical-sized rat calvaria model equivalent to gold standard autograft. BPLP-Ser/HA putty represents a simple, off-the-shelf solution to the combined challenges of acute wound management and subsequent bone regeneration.

Poloxamer-Based Hydrogel as Drug Delivery System: How Polymeric Excipients Influence the Chemical-Physical Properties

Thermogelling amphiphilic block copolymers have been widely investigated in the development of pharmaceutical drug carriers. In particular, thermosensitive gels based on poloxamer 407 (P407) have great potential for periodontal disease treatment, thanks to their ability to be liquid at room temperature and become viscous gels at body temperature. However, some problems, related to short in situ residence time, reduce their feasible clinical use. Thus, in order to improve the effective applicability of these materials, we studied how P407 thermogels are affected by the pH and by the inclusion of different hydrophilic polymers, used as excipients for increasing the gel stiffness. For this scope, a complete chemical-physical characterization of the synthesized gels is provided, in terms of determination of sol-gel transition temperature, viscosity and erosion degree. The data are correlated according to a statistical multivariate approach based on Principal Component Analysis and their mucoadhesion properties are also tested by Tapping mode-Atomic Force Microscopy (TM-AFM) imaging. Finally, we studied how the different P407 formulations are able to influence the release pathway of two antibacterial drugs (i.e., chlorhexidine digluconate and doxycycline hyclate) largely used in oral diseases.

Janus mucosal dressing with a tough and adhesive hydrogel based on synergistic effects of gelatin, polydopamine, and nano-clay

There are many problems and challenges related to the treatment of highly prevalent oral mucosal diseases and oral drug delivery because of a large amount of saliva present in the oral cavity, the accompanying oral movements, and unconscious swallowing in the mouth. Therefore, an ideal oral dressing should possess stable adhesion and superior tough strength in the oral cavity. However, this fundamental requirement greatly limits the use of synthetic adhesive dressings for oral dressings. Here, we developed a mussel-inspired Janus gelatin-polydopamine-nano-clay (GPC) hydrogel with controlled adhesion and toughness through the synergistic physical and chemical interaction of gelatin (Gel), nano-clay, and dopamine (DA). The hydrogel not only exhibits strong wet adhesion force (63 kPa) but also has high toughness (1026 ± 100 J m^-3). Interfacial adhesion of hydrogels is achieved by modulating the interaction of catechol groups of the hydrogel with specific functional groups (e.g., NH₂, SH, OH, and COOH) on the tissue surface. The matrix dissipation of the hydrogel is regulated by physical crosslinking of gelatin, chemical crosslinking of gelatin with polydopamine (Michael addition and Schiff base formation), and nano-clay-induced constraint of the molecular chain. In addition, the GPC hydrogel shows high cell affinity and favors cell adhesion and proliferation. The hydrogel's instant and strong mucoadhesive properties provide a long-lasting therapeutic effect of the drug, thereby enhancing the healing of oral ulcers. Therefore, mussel-inspired wet-adhesion Janus GPC hydrogels can be used as a platform for mucosal dressing and drug delivery systems. STATEMENT OF SIGNIFICANCE: It is a great challenge to treat oral mucosal diseases due to the large amount of saliva present in the oral cavity, the accompanying oral movements, unconscious swallowing, and flushing of drugs in the mouth. To overcome the significant limitations of clinical bioadhesives, such as weakness, toxicity, and poor usage, in the present study, we developed a simple method through the synergistic effects of gelatin, polydopamine, and nano-clay to prepare an optimal mucosal dressing (Janus GPC) that integrates Janus, adhesion, toughness, and drug release property. It fits effectively in the mouth, resists saliva flushing and oral movements, provides oral drug delivery, and reduces patient discomfort. The Janus GPC adhesive hydrogels have great commercial potential to support further the development of innovative therapies for oral mucosal diseases.

Does the Freeze-Thaw Technique Affect the Properties of the Alginate/Chitosan Glutamate Gels with Posaconazole as a Model Antifungal Drug?

Hydrogels are semi-solid systems with high flexibility, which, due to holding large amounts of water, are similar to natural tissues and are very useful in the field of biomedical applications. Despite the wide range of polymers available to form hydrogels, novel techniques utilized to obtain hydrogels with adequate properties are still being developed. The aim of this study was to evaluate the impact of the freeze–thaw technique on the properties of cryogels based on sodium alginate and chitosan glutamate with posaconazole as a model antifungal substance. The effect of the freezing and thawing process on the physicochemical, rheological, textural and bioadhesive properties of prepared cryogels was examined. Additionally, the antifungal activity against Candida albicans, Candida parapsilosis and Candida krusei of designed formulations was examined. It was shown that the freeze–thaw technique significantly improved viscosity, bioadhesiveness, textural properties and prolonged the in vitro posaconazole release. Moreover, alginate/chitosan glutamate cryogels exhibited higher values of inhibition zone in C. parapsilosis culture than traditional hydrogel formulations.

Body Temperature Enhanced Adhesive, Antibacterial and Recyclable Ionic Hydrogel for Epidermal Electrophysiological Monitoring

Hydrogel-based epidermal electrodes have attracted widespread attention in health monitoring and human-machine interfaces for their good biocompatibility, skin-matched Young's modulus, and stable in situ electrophysiological recording performance. However, it is difficult to make the exact conformal attachment between skin and electrodes because of the hair, wrinkles as well as complex, curved contours of the skin. This also results in signal distortion and large noise. Here, a body temperature enhanced skin-adhesive epidermal electrode is proposed based on non-covalent cross-linked network ionic hydrogel. The ionic hydrogel is fabricated by the polyvinyl alcohol (PVA), branched polyethyleneimine (b-PEI) and calcium chloride (CaCl₂ ), which demonstrates impressive performances including ultra-stretchability of 1291%, great adhesion to skin and other non-biological materials, stable conductivity of 3.09 S/m, recyclability and outstanding antibacterial ability, simultaneously. Specifically, the adhesion of the ionic hydrogel behaves as temperature-sensitive and could be enhanced by body temperature. Furthermore, the ionic hydrogel is utilized as epidermal electrodes, which displays seductive capability to record multifarious electrophysiological signals with high signal-to-noise ratio and ultra-low detection limit, including electrocardiogram (ECG), electromyogram (EMG) and electroencephalogram (EEG). The as-proposed body temperature enhanced skin-adhesive ionic hydrogel brings intelligent functions and broadens the way for epidermal electronics, promoting the development of healthcare electronics. This article is protected by copyright. All rights reserved.

Cutting-Edge Progress in Stimuli-Responsive Bioadhesives: From Synthesis to Clinical Applications

With the advent of "intelligent" materials, the design of smart bioadhesives responding to chemical, physical, or biological stimuli has been widely developed in biomedical applications to minimize the risk of wounds reopening, chronic pain, and inflammation. Intelligent bioadhesives are free-flowing liquid solutions passing through a phase shift in the physiological environment due to stimuli such as light, temperature, pH, and electric field. They possess great merits, such as ease to access and the ability to sustained release as well as the spatial transfer of a biomolecule with reduced side effects. Tissue engineering, wound healing, drug delivery, regenerative biomedicine, cancer therapy, and other fields have benefited from smart bioadhesives. Recently, many disciplinary attempts have been performed to promote the functionality of smart bioadhesives and discover innovative compositions. However, according to our knowledge, the development of multifunctional bioadhesives for various biomedical applications has not been adequately explored. This review aims to summarize the most recent cutting-edge strategies (years 2015-2021) developed for stimuli-sensitive bioadhesives responding to external stimuli. We first focus on five primary categories of stimuli-responsive bioadhesive systems (pH, thermal, light, electric field, and biomolecules), their properties, and limitations. Following the introduction of principal criteria for smart bioadhesives, their performances are discussed, and certain smart polymeric materials employed in their creation in 2015 are studied. Finally, advantages, disadvantages, and future directions regarding smart bioadhesives for biomedical applications are surveyed.

Electrically Programmable Interfacial Adhesion for Ultrastrong Hydrogel Bonding

Adjustable interfacial adhesion is of great significance in smart-hydrogel-related engineering fields. This study presents an electroadhesion strategy for universal and ultrastrong hydrogel bonding with electrically programmable strength. An ionic hydrogel containing lithium ions is designed to achieve hydrated-ion-diffusion-mediated interfacial adhesion, where external electric fields are employed to precisely control spatiotemporal dynamics of the ion diffusion across ionic adhesion region (IAR). The hydrogel can realize a universal, ultrastrong, efficient, tough, reversible, and environmentally tolerant electroadhesion to diverse hydrogels, whose peak adhesion strength and interfacial adhesion toughness are as high as 1.2 MPa and 3750 J m^-2 , respectively. With a mechanoelectric coupling model, the dominant role of the hydrated ions in IAR played in the interfacial electroadhesion is further quantitatively revealed. The proposed strategy opens a door for developing high-performance adhesion hydrogels with electrically programmable functions, which are indispensable for various emerging fields like flexible electronics and soft robotics.

Mechanically tuneable physical nanocomposite hydrogels from polyelectrolyte complex templated silica nanoparticles for anionic therapeutic delivery

Hydrogels have shown great promise for drug delivery and tissue engineering but can be limited in practical applications by poor mechanical performance. The incorporation of polymer grafted silica nanoparticles as chemical or physical crosslinkers in in situ polymerised nanocomposite hydrogels has been widely researched to enhance their mechanical properties. Despite the enhanced mechanical stiffness, tensile strength, and self-healing properties, there remains a need for the development of simpler and modular approaches to obtain nanocomposite hydrogels. Herein, we report a facile protocol for the polyelectrolyte complex (PEC) templated synthesis of organic-inorganic hybrid poly(ethylenimine) functionalised silica nanoparticles (PEI-SiNPs) and their use as multifunctional electrostatic crosslinkers with hyaluronic acid (HA) to form nanocomposite hydrogels. Upon mixing, electrostatic interactions between cationic PEI-SiNPs and anionic HA resulted in the formation of a coacervate nanocomposite hydrogel with enhanced mechanical stiffness that can be tuned by varying the ratios of PEI-SiNPs and HA present. The reversible electrostatic interactions within the hydrogel networks also enabled self-healing and thixotropic properties. The excess positive charge present within the PEI-SiNPs facilitated high loading and retarded the release of the anionic anti-cancer drug methotrexate from the nanocomposite hydrogel. Furthermore, the electrostatic complexation of PEI-SiNP and HA was found to mitigate haemotoxicity concerns associated with the use of high molecular weight PEI. The method presented herein offers a simpler and more versatile strategy for the fabrication of coacervate nanocomposite hydrogels with tuneable mechanical stiffness and self-healing properties for drug delivery applications.

Functional Hydrogels for Treatment of Chronic Wounds

Chronic wounds severely affect 1-2% of the population in developed countries. It has been reported that nearly 6.5 million people in the United States suffer from at least one chronic wound in their lifetime. The treatment of chronic wounds is critical for maintaining the physical and mental well-being of patients and improving their quality of life. There are a host of methods for the treatment of chronic wounds, including debridement, hyperbaric oxygen therapy, ultrasound, and electromagnetic therapies, negative pressure wound therapy, skin grafts, and hydrogel dressings. Among these, hydrogel dressings represent a promising and viable choice because their tunable functional properties, such as biodegradability, adhesivity, and antimicrobial, anti-inflammatory, and pre-angiogenic bioactivities, can accelerate the healing of chronic wounds. This review summarizes the types of chronic wounds, phases of the healing process, and key therapeutic approaches. Hydrogel-based dressings are reviewed for their multifunctional properties and their advantages for the treatment of chronic wounds. Examples of commercially available hydrogel dressings are also provided to demonstrate their effectiveness over other types of wound dressings for chronic wound healing.

Mussel-inspired adhesive gelatin-polyacrylamide hydrogel wound dressing loaded with tetracycline hydrochloride to enhance complete skin regeneration

Even though the global wound care market size was valued at USD 19.83 billion in 2020, it is still a challenge to develop a hydrogel-based wound dressing with a good mechanical property, adhesiveness and antibacterial property. This study established and validated a mussel-inspired adhesive hydrogel wound dressing with antibacterial activity by dispersing tetracycline hydrochloride into hydrogel based polydopamine, gelatin and polyacrylamide. A tough hydrogel with a fracture stress of 0.42 MPa was prepared by changing the contents of the gelatin and polyacrylamide. With the addition of polydopamine and tetracycline hydrochloride, the hydrogel was endowed with an adhesive property (with a tissue adhesive strength of 4.13 kPa) and antibacterial activity against both Escherichia coli and Staphylococcus aureus. Finally, a rat full-thickness skin defect wound model was used to evaluate the performance of the hydrogels in wound repair. The hydrogel group showed a significantly reduced wound area (95.72%) compared with the blank group (86.34%) on day 14. The hydrogel promoted the collagen deposition, weakened the inflammatory response and enhanced wound healing. Therefore, the hydrogel with multifunctional properties is a promising candidate for complete skin regeneration.

A review of recent advances in metal ion hydrogels: mechanism, properties and their biological applications

The mechanisms, common properties and biological applications of different types of metal ion hydrogels are summarized.