Bioinspired Self-Healing Hydrogel Based on Benzoxaborole-Catechol Dynamic Covalent Chemistry for 3D Cell Encapsulation

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.

Boronate Ester Hydrogels for Biomedical Applications: Challenges and Opportunities

Boronate ester (BE) hydrogels are increasingly used for biomedical applications. The dynamic nature of these molecular networks enables bond rearrangement, which is associated with viscoelasticity, injectability, printability, and self-healing, among other properties. BEs are also sensitive to pH, redox reactions, and the presence of sugars, which is useful for the design of stimuli-responsive materials. Together, BE hydrogels are interesting scaffolds for use in drug delivery, 3D cell culture, and biofabrication. However, designing stable BE hydrogels at physiological pH (≈7.4) remains a challenge, which is hindering their development and biomedical application. In this context, advanced chemical insights into BE chemistry are being used to design new molecular solutions for material fabrication. This review article summarizes the state of the art in BE hydrogel design for biomedical applications with a focus on the materials chemistry of this class of materials. First, we discuss updated knowledge in BE chemistry including details on the molecular mechanisms associated with BE formation and breakage. Then, we discuss BE hydrogel formation at physiological pH, with an overview of the main systems reported to date along with new perspectives. A last section covers several prominent biomedical applications of BE hydrogels, including drug delivery, 3D cell culture, and bioprinting, with critical insights on the design relevance, limitations and potential.

A Lignin Silver Nanoparticles/Polyvinyl Alcohol/Sodium Alginate Hybrid Hydrogel with Potent Mechanical Properties and Antibacterial Activity

Antibacterial hydrogels have attracted significant attention due to their diverse applications, efficient antimicrobial properties, and adaptability to various environments and requirements. However, their relatively fragile structure, coupled with the potential for environmental toxicity when exposed to their surroundings for extended periods, may significantly limit their practical application potential. In this work, a composite hydrogel was synthesized with outstanding mechanical features and antibacterial capability. The hydrogel was developed through the combination of the eco-friendly and enduring antibacterial agent, lignin silver nanoparticles (Lig-Ag NPs), with polyvinyl alcohol (PVA) and sodium alginate (SA), in varying proportions. The successful synthesis of the hydrogel and the dispersed distribution of Lig-Ag NPs within the hydrogel were confirmed by various analytical techniques, including field emission scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), mercury intrusion porosimetry (MIP), Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). The formation of multiple hydrogen bonds between Lig-Ag NPs and the composites contributed to a more stable and dense network structure of the hydrogel, consequently enhancing its mechanical properties. Rheological tests revealed that the hydrogel exhibited an elastic response and demonstrated outstanding self-recovery properties. Significantly, the antibacterial hydrogel demonstrated effectiveness against Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus), achieving a <5% survival of bacteria within 12 h. This study presented a green and straightforward synthetic strategy for the application of antibacterial composite hydrogels in various fields.

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

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

The Rise of Boron-Containing Compounds: Advancements in Synthesis, Medicinal Chemistry, and Emerging Pharmacology

Boron-containing compounds (BCC) have emerged as important pharmacophores. To date, five BCC drugs (including boronic acids and boroles) have been approved by the FDA for the treatment of cancer, infections, and atopic dermatitis, while some natural BCC are included in dietary supplements. Boron's Lewis acidity facilitates a mechanism of action via formation of reversible covalent bonds within the active site of target proteins. Boron has also been employed in the development of fluorophores, such as BODIPY for imaging, and in carboranes that are potential neutron capture therapy agents as well as novel agents in diagnostics and therapy. The utility of natural and synthetic BCC has become multifaceted, and the breadth of their applications continues to expand. This review covers the many uses and targets of boron in medicinal chemistry.

Acid and Glutathione Dual-Responsive, Injectable and Self-Healing Hydrogels for Controlled Drug Delivery

Considering the complexity of physiological microenvironments and the risks of surgical infection, there still remains critical demand to develop a hydrogel as a drug release platform with multifunctional properties, including good neutral stability and sensitive multiple stimuli-responsive behaviors, as well as injectable and self-healing properties. Herein, we present a facile preparation of injectable, self-healing hydrogels with acid and glutathione (GSH) dual-responsiveness for controlled drug delivery. Initially, the anticancer drug camptothecin (CPT) was premodified with disulfide bonds and attached to poly(ethylenimine) (PEI) via the Schiff base reaction, resulting in PEI-CPT. Subsequently, OSA-IR780 was synthesized through the Schiff base reaction involving IR780 with amine groups (IR780-NH2) and oxidized sodium alginate with aldehyde groups (OSA). The formation of PEI-CPT/OSA-IR780 hydrogels with various solid contents occurred rapidly within 40 s through a simple mixing process of the aqueous solution of PEI-CPT and OSA-IR780. These hydrogels exhibited remarkable stability under neutral conditions and controlled release of CPT upon exposure to simulated tumor environments characterized by acidic conditions and elevated GSH concentrations. Furthermore, they had significant injectable and self-healing properties due to the dynamically imine-cross-linked networks. In addition, the prepared hydrogels exhibited long-term biodegradability and biocompatibility. Collectively, these features indicate the great potential of PEI-CPT/OSA-IR780 hydrogels as therapeutic delivery vehicles.

Advances in antimicrobial hydrogels for dental tissue engineering: regenerative strategies for endodontics and periodontics

Dental tissue infections have been affecting millions of patients globally leading to pain, severe tissue damage, or even tooth loss. Commercial sterilizers may not be adequate to prevent frequent dental infections. Antimicrobial hydrogels have been introduced as an effective therapeutic strategy for endodontics and periodontics since they have the capability of imitating the native extracellular matrix of soft tissues. Hydrogel networks are considered excellent drug delivery platforms due to their high-water retention capacity. In this regard, drugs or nanoparticles can be incorporated into the hydrogels to endow antimicrobial properties as well as to improve their regenerative potential, once biocompatibility criteria are met avoiding high dosages. Herein, novel antimicrobial hydrogel formulations were discussed for the first time in the scope of endodontics and periodontics. Such hydrogels seem outstanding candidates especially when designed not only as simple volume fillers but also as smart biomaterials with condition-specific adaptability within the dynamic microenvironment of the defect site. Multifunctional hydrogels play a pivotal role against infections, inflammation, oxidative stress, etc. along the way of dental regeneration. Modern techniques (e.g., 3D and 4D-printing) hold promise to develop the next generation of antimicrobial hydrogels together with their limitations such as infeasibility of implantation.

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.

Fabrication of triple-crosslinked gelatin/alginate hydrogels for controlled release applications

Hydrogels have been demonstrated as smart drug carriers to recognize the tumor microenvironment for cancer treatment, where the dynamic crosslinks in the hydrogel network contribute to the stimuli-responsive features but also result in poor stability and weak mechanical property of the hydrogels. Here, phenylboronic acid-grafted polyethyleneimine (PBA-PEI)-modified gelatin (PPG) was synthesized to crosslink alginate dialdehyde (ADA) through imine bonds and boronate ester bonds, and then calcium ions (Ca2+) were added to introduce the third calcium-carboxylate crosslinking in the network to form the triple-crosslinked PPG/ADA-Ca2+ hydrogels. Given the three types of dynamic bonds in the network, PPG/ADA-Ca2+ hydrogels possessed a self-healing manner, stimuli-responsiveness, and better mechanical properties compared to single- or double-crosslinked hydrogels. The controlled release capability of PPG/ADA-Ca2+ hydrogels was also demonstrated, showing the encapsulated molecules can be rapidly released from the hydrogel network in the presence of hydrogen peroxide while the release rate can be slowed down at acidic pH. Furthermore, PPG/ADA-Ca2+ hydrogels presented selected cytotoxicity and drug delivery to cancer cells due to the regulated degradation by the cellular microenvironment. Taken together, PPG/ADA-Ca2+ hydrogels have been demonstrated as promising biomaterials with multiple desirable properties and dynamic features to perform controlled molecule release for biomedical applications.

Alginate/polyacrylamide host-guest supramolecular hydrogels with enhanced adhesion

Although injectable hydrogels with minimally invasive delivery have garnered significant interest, their potential applications have been restricted by a singular property. In this study, a supramolecular hydrogel system with improved adhesion was constructed through host-guest interactions between alginate and polyacrylamide. The maximum tensile adhesion strength between the β-cyclodextrin and dopamine-grafted alginate/adamantane-grafted polyacrylamide (Alg-βCD-DA/PAAm-Ad, namely AβCDPA) hydrogels and pigskin reached 19.2 kPa, which was 76 % stronger than the non-catechol-based control hydrogel (β-cyclodextrin-grafted alginate/adamantane-grafted polyacrylamide, Alg-βCD/PAAm-Ad). Moreover, the hydrogels demonstrated excellent self-healing, shear-thinning, and injectable properties. The required pressure to extrude the AβCDPA2 hydrogel from a 16G needle at a rate of 2.0 mL/min was 67.4 N. As the polymer concentration and adamantane substitution degree increased, the hydrogels exhibited higher modulus, stronger network structure, and lower swelling ratio and degradation rate. Encapsulating and culturing cells within these hydrogels demonstrated good cytocompatibility. Therefore, this hydrogel can serve as a viscosity extender or bioadhesive, and as a carrier material to deliver encapsulated therapeutic substances into the body through minimally invasive injection methods.

Nano-crosslinked dynamic hydrogels for biomedical applications

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

Recent advances in self-healing polyurethane based on dynamic covalent bonds combined with other self-healing methods

To meet more application requirements, improving mechanical properties and self-healing efficiency has become the focus of current research on self-healing PU. The competitive relationship between self-healing ability and mechanical properties cannot be avoided by a single self-healing method. To address this problem, a growing number of studies have combined dynamic covalent bonding with other self-healing methods to construct the PU structure. This review summarizes recent studies on PU materials that combine typical dynamic covalent bonds with other self-healing methods. It mainly includes four parts: hydrogen bonding, metal coordination bonding, nanofillers combined with dynamic covalent bonding and multiple dynamic covalent bond bonding. The advantages and disadvantages of different self-healing methods and their significant role in improving self-healing ability and mechanical properties in PU networks are analyzed. At the same time, the possible challenges and research directions of self-healing PU materials in the future are discussed.

A new boronate ester-based crosslinking strategy allows the design of nonswelling and long-term stable dynamic covalent hydrogels

Dynamic hydrogels are viscoelastic materials that can be designed to be self-healing, malleable, and injectable, making them particularly interesting for a variety of biomedical applications. To design dynamic hydrogels, dynamic covalent crosslinking reactions are attracting increasing attention. However, dynamic covalent hydrogels tend to swell, and often lack stability. Boronate ester-based hydrogels, which result from the dynamic covalent reaction between a phenylboronic acid (PBA) derivative and a diol, are based on stable precursors, and can therefore address these limitations. Yet, boronate ester formation hardly occurs at physiological pH. To produce dynamic covalent hydrogels at physiological pH, we performed a molecular screening of PBA derivatives in association with a variety of diols, using hyaluronic acid as a polymer of interest. The combination of Wulff-type PBA (wPBA) and glucamine stood out as a unique couple to obtain the desired hydrogels. We showed that optimized wPBA/glucamine hydrogels are minimally- to non-swelling, stable long term (over months), tunable in terms of mechanical properties, and cytocompatible. We further characterized their viscoelastic and self-healing properties, highlighting their potential for biomedical applications.

Phosphorus-Containing Polymers as Sensitive Biocompatible Probes for 31P Magnetic Resonance

The visualization of organs and tissues using ³¹P magnetic resonance (MR) imaging represents an immense challenge. This is largely due to the lack of sensitive biocompatible probes required to deliver a high-intensity MR signal that can be distinguished from the natural biological background. Synthetic water-soluble phosphorus-containing polymers appear to be suitable materials for this purpose due to their adjustable chain architecture, low toxicity, and favorable pharmacokinetics. In this work, we carried out a controlled synthesis, and compared the MR properties, of several probes consisting of highly hydrophilic phosphopolymers differing in composition, structure, and molecular weight. Based on our phantom experiments, all probes with a molecular weight of ~3-400 kg·mol^-1, including linear polymers based on poly(2-methacryloyloxyethyl phosphorylcholine) (PMPC), poly(ethyl ethylenephosphate) (PEEP), and poly[bis(2-(2-(2-methoxyethoxy)ethoxy)ethoxy)]phosphazene (PMEEEP) as well as star-shaped copolymers composed of PMPC arms grafted onto poly(amidoamine) dendrimer (PAMAM-g-PMPC) or cyclotriphosphazene-derived cores (CTP-g-PMPC), were readily detected using a 4.7 T MR scanner. The highest signal-to-noise ratio was achieved by the linear polymers PMPC (210) and PMEEEP (62) followed by the star polymers CTP-g-PMPC (56) and PAMAM-g-PMPC (44). The ³¹P T₁ and T₂ relaxation times for these phosphopolymers were also favorable, ranging between 1078 and 2368 and 30 and 171 ms, respectively. We contend that select phosphopolymers are suitable for use as sensitive ³¹P MR probes for biomedical applications.

Advances in the Design of Phenylboronic Acid-Based Glucose-Sensitive Hydrogels

Diabetes, characterized by an uncontrolled blood glucose level, is the main cause of blindness, heart attack, stroke, and lower limb amputation. Glucose-sensitive hydrogels able to release hypoglycemic drugs (such as insulin) as a response to the increase of the glucose level are of interest for researchers, considering the large number of diabetes patients in the world (537 million in 2021, reported by the International Diabetes Federation). Considering the current growth, it is estimated that, up to 2045, the number of people with diabetes will increase to 783 million. The present work reviews the recent developments on the hydrogels based on phenylboronic acid and its derivatives, with sensitivity to glucose, which can be suitable candidates for the design of insulin delivery systems. After a brief presentation of the dynamic covalent bonds, the design of glucose-responsive hydrogels, the mechanism by which the hypoglycemic drug release is achieved, and their self-healing capacity are presented and discussed. Finally, the conclusions and the main aspects that should be addressed in future research are shown.

Self- and Cross-Fusing of Furan-Based Polyurea Gels Dynamically Cross-Linked with Maleimides

Bio-based polyureas (PUs) with main-chain furan rings were synthesized by the polyaddition of 2,5-bis(aminomethyl)furan with various diisocyanates, such as methylene diphenyl diisocyanate. Several PU's were soluble in polar organic solvents, and were cast to form thermomechanically stable films with softening temperatures of over 100 °C. The furan rings of the PU main chains underwent a dynamic Diels-Alder (DA) reaction with bismaleimide (BMI) cross-linkers. While the mixed solution of PU and BMI did not show any apparent signs of reaction at room temperature, the DA reaction proceeded to form gels upon heating to 60 °C, which became a solution again by further heating to 80 °C (retro-DA reaction). The solution phase was maintained by rapid quenching from 80 °C to room temperature, while the gel was reformed upon slow cooling. The recovered gels exhibited self-healing properties. A scratch made by a hot knife at temperatures above 80 °C disappeared spontaneously. When two different gels were cut using a knife at room temperature, placed in contact with each other, and heated to 60 °C, they fused. The ability to control the DA/retro-DA reaction allowed gels of varying composition to heal.

Self-Healing Alginate Hydrogel Formed by Dynamic Benzoxaborolate Chemistry Protects Retinal Pigment Epithelium Cells against Oxidative Damage

Oxidative stress is considered as a major factor causing retinal pigment epithelium (RPE) dysfunction and finally leading to retinal diseases such as age-related macular degeneration (AMD). Developing hydrogels for RPE cell delivery, especially those with antioxidant feature, is emerging as a promising approach for AMD treatment. Herein, a readily prepared antioxidant alginate-based hydrogel was developed to serve as a cytoprotective agent for RPE cells against oxidative damage. Alg-BOB was synthesized via conjugation of benzoxaborole (BOB) to the polysaccharide backbone. Hydrogels were formed through self-crosslinking of Alg-BOB based on benzoxaborole-diol complexation. The resulting hydrogel showed porous micro-structure, pH dependent mechanical strength and excellent self-healing, remolding, and injectable properties. Moreover, the hydrogel exhibited excellent cytocompatibility and could efficiently scavenge reactive oxygen species (ROS) to achieve an enhanced viability of ARPE-19 cells under oxidative condition. Altogether, our study reveals that the antioxidant Alg-BOB hydrogel represents an eligible candidate for RPE delivery and AMD treatment.

Indocyanine Green Performance Enhanced System for Potent Photothermal Treatment of Bacterial Infection

The instability in solution and aggregation-induced self-quenching of indocyanine green (ICG) have weakened its fluorescence and photothermal properties, thus inhibiting its application in practice. In this study, the cationic and anionic liposomes containing ICG were prepared based on 1,2-dioleoyl-3-trimethylammonium-propane (DOTAP) and 1,2-dipalmitoyl-sn-glycero-3-phospho-rac-glycerol (DPPG), respectively. Molecular dynamics (MD) simulations demonstrate that ICG molecules are better distributed in the membranes of cationic DOTAP-based liposomes, leading to a superior fluorescence and photothermal performance. The liposomal ICG also shows the physical and photothermal stability during irradiation and long-term storage. On this basis, the prepared DOTAP-based liposomal ICG was encapsulated in the self-healing hydrogel formed by guar gum through the borate/diol interaction. The proposed liposomal ICG-loaded hydrogel can not only convert near-infrared (NIR) light into heat effectively but also repair itself without external assistance, which will realize potent photothermal therapy (PTT) against bacterial infection and provide the possibility for meeting the rapidly growing needs of modern medicine.

Rheological Characterization and Theoretical Modeling Establish Molecular Design Rules for Tailored Dynamically Associating Polymers

Dynamically associating polymers have long been of interest due to their highly tunable viscoelastic behavior. Many applications leverage this tunability to create materials that have specific rheological properties, but designing such materials is an arduous, iterative process. Current models for dynamically associating polymers are phenomenological, assuming a structure for the relationship between association kinetics and network relaxation. We present the Brachiation model, a molecular-level theory of a polymer network with dynamic associations that is rooted in experimentally controllable design parameters, replacing the iterative experimental process with a predictive model for how experimental modifications to the polymer will impact rheological behavior. We synthesize hyaluronic acid chains modified with supramolecular host-guest motifs to serve as a prototypical dynamic network exhibiting tunable physical properties through control of polymer concentration and association rates. We use dynamic light scattering microrheology to measure the linear viscoelasticity of these polymers across six decades in frequency and fit our theory parameters to the measured data. The parameters are then altered by a magnitude corresponding to changes made to the experimental parameters and used to obtain new rheological predictions that match the experimental results well, demonstrating the ability for this theory to inform the design process of dynamically associating polymeric materials.

Recent Advances in Mechanical Reinforcement of Zwitterionic Hydrogels

As a nonspecific protein adsorption material, a strong hydration layer provides zwitterionic hydrogels with excellent application potential while weakening the interaction between zwitterionic units, leading to poor mechanical properties. The unique anti-polyelectrolyte effect in ionic solution further restricts the application value due to the worsening mechanical strength. To overcome the limitations of zwitterionic hydrogels that can only be used in scenarios that do not require mechanical properties, several methods for strengthening mechanical properties based on enhancing intermolecular interaction forces and polymer network structure design have been extensively studied. Here, we review the works on preparing tough zwitterionic hydrogel. Based on the spatial and molecular structure design, tough zwitterionic hydrogels have been considered as an important candidate for advanced biomedical and soft ionotronic devices.

Polydopamine Biomaterials for Skin Regeneration

Designing biomaterials capable of biomimicking wound healing and skin regeneration has been receiving increasing attention recently. Some biopolymers behave similarly to the extracellular matrix (ECM), supporting biointerfacial adhesion and intrinsic cellular interactions. Polydopamine (PDA) is a natural bioadhesive and bioactive polymer that endows high chemical versatility, making it an exciting candidate for a wide range of biomedical applications. Moreover, biomaterials based on PDA and its derivatives have near-infrared (NIR) absorption, excellent biocompatibility, intrinsic antioxidative activity, antibacterial activity, and cell affinity. PDA can regulate cell behavior by controlling signal transduction pathways. It governs the focal adhesion behavior of cells at the biomaterials interface. These features make melanin-like PDA a fascinating biomaterial for wound healing and skin regeneration. This paper overviews PDA-based biomaterials' synthesis, properties, and interactions with biological entities. Furthermore, the utilization of PDA nano- and microstructures as a constituent of wound-dressing formulations is highlighted.

Engineering Multifunctional Hydrogel-Integrated 3D Printed Bioactive Prosthetic Interfaces for Osteoporotic Osseointegration

3D printed porous titanium alloy implants is an advanced orthopedic material for joint replacement. However, the high risk of aseptic loosening and periprosthetic infection is difficult to avoid, and the declined autophagy of osteoporosis-derived bone marrow mesenchymal stem cells (OP-BMSCs) further severely impairs the osseointegration under the osteoporotic circumstance. It is thus becoming urgently significant to develop orthopedic materials with autophagy regulation and antibacterial bioactivity. In this regard, a novel class of multifunctional hydrogel-integrated 3D printed bioactive prosthetic interfaces is engineered for in situ osseointegration in osteoporosis. The hydrogel is fabricated from the dynamic crosslinking of synthetic polymers, natural polymers, and silver nanowires to deliver autophagy-regulated rapamycin. Therefore, the resultant soft material exhibits antibacterial ability, biocompatibility, degradability, conductive, self-healing, and stimuli-responsive abilities. In vitro experiments demonstrate that the hydrogel-integrated 3D printed bioactive prosthetic interfaces can restore the declined cellular activities of OP-BMSCs by upregulating the autophagy level and show excellent antibacterial activity against S. aureus and MRSA. More remarkably, the multifunctional 3D printed bioactive prosthetic interfaces significantly improve osseointegration and inhibit infection in osteoporotic environment in vivo. This study provides an efficient strategy to develop novel prosthetic interfaces to reduce complications after arthroplasty for patients with osteoporosis.

Metal Ion-Directed Functional Metal-Phenolic Materials

Metal ions are ubiquitous in nature and play significant roles in assembling functional materials in fields spanning chemistry, biology, and materials science. Metal-phenolic materials are assembled from phenolic components in the presence of metal ions through the formation of metal-organic complexes. Alkali, alkali-earth, transition, and noble metal ions as well as metalloids interacting with phenolic building blocks have been widely exploited to generate diverse hybrid materials. Despite extensive studies on the synthesis of metal-phenolic materials, a comprehensive summary of how metal ions guide the assembly of phenolic compounds is lacking. A fundamental understanding of the roles of metal ions in metal-phenolic materials engineering will facilitate the assembly of materials with specific and functional properties. In this review, we focus on the diversity and function of metal ions in metal-phenolic material engineering and emerging applications. Specifically, we discuss the range of underlying interactions, including (i) cation-π, (ii) coordination, (iii) redox, and (iv) dynamic covalent interactions, and highlight the wide range of material properties resulting from these interactions. Applications (e.g., biological, catalytic, and environmental) and perspectives of metal-phenolic materials are also highlighted.

An Injectable Dual-Function Hydrogel Protects Against Myocardial Ischemia/Reperfusion Injury by Modulating ROS/NO Disequilibrium

Acute myocardial infarction (MI) is the leading cause of death worldwide. Exogenous delivery of nitric oxide (NO) to the infarcted myocardium has proven to be an effective strategy for treating MI due to the multiple physiological functions of NO. However, reperfusion of blood flow to the ischemic tissues is accompanied by the overproduction of toxic reactive oxygen species (ROS), which can further exacerbate tissue damage and compromise the therapeutic efficacy. Here, an injectable hydrogel is synthesized from the chitosan modified by boronate-protected diazeniumdiolate (CS-B-NO) that can release NO in response to ROS stimulation and thereby modulate ROS/NO disequilibrium after ischemia/reperfusion (I/R) injury. Furthermore, administration of CS-B-NO efficiently attenuated cardiac damage and adverse cardiac remodeling, promoted repair of the heart, and ameliorated cardiac function, unlike a hydrogel that only released NO, in a mouse model of myocardial I/R injury. Mechanistically, regulation of the ROS/NO balance activated the antioxidant defense system and protected against oxidative stress induced by I/R injury via adaptive regulation of the Nrf2-Keap1 pathway. Inflammation is then reduced by inhibition of the activation of NF-κB signaling. Collectively, these results show that this dual-function hydrogel may be a promising candidate for the protection of tissues and organs after I/R injury.

Dopamine Assisted Self-Cleaning, Antifouling, and Antibacterial Coating via Dynamic Covalent Interactions

The rapid accumulation of dead bacteria or protein on a bactericidal surface can reduce the effectiveness of the modified surface and alter its biocidal activity by shielding the surface biocide functional groups, promoting microbial attachment and subsequent biofilm formation. Thus, the alteration of biocidal activity due to biofilm formation can cause serious trouble including severe infection or implant or medical device failure leading to death. Therefore, developing a smart self-cleaning surface is of great interest. Ideally, such a surface can not only kill the attached microbials but also release the dead cells and foulants from the surface under a particular incitement on demand. In this project, a sugar-responsive self-cleaning coating has been developed by forming covalent boronic ester bonds between catechol groups from polydopamine and a benzoxaborole pendant from zwitterionic and cationic polymers. To incorporate antifouling properties and enhance the biocompatibility of the coating, bioinspired zwitterionic compound 2-methacryloyloxyethyl phosphorylcholine (MPC) was chosen and benzoxaborole pendant containing zwitterionic polymer poly(MPC-st-MAABO) (MAABO: 5-methacrylamido-1,2-benzoxaborole) was synthesized. Additionally to impart antibacterial properties to the surface, a quaternary ammonium containing cationic polymer poly(2-(methacryloyloxy)ethyl trimethylammonium (META)-st-MAABO)) was synthesized. These synthesized polymers were covalently grafted to a polydopamine (PDA) coated surface by forming a strong cyclic boronic ester complex with a catechol group of the PDA layer endowing the surface with bacteria contact-killing properties and capturing specific protein. After the addition of cis-diol containing competitive molecules, i.e., saccharides/sugars, this boronic ester complex with a catechol group of PDA was replaced and the attached polymer layer was cleaved from the surface, resulting in the release of both absorbed protein and live/killed bacteria electrostatically attached to the polymer layer. This dynamic self-cleaning surface can be a promising material for biomedical applications avoiding the gathering of dead cells and debris that are typically encountered on a traditional biocidal surface.

Connecting the Dynamics and Reactivity of Arylboronic Acids to Emergent and Stimuli-Responsive Material Properties

Over the past two decades, arylboronic acid-functionalized biomaterials have been used in a variety of sensing and stimuli-responsive scaffolds. Their diverse applications result from the diverse reactivity of arylboronic acids,...

Injectable Micromotor@hydrogel system for antibacterial therapy

The drug delivery system based on nano/micromotors has become a research hot spot in recent years. However, naked micromotors may be ruptured or passivated under the complex biological environment, which will result in the leakage of drugs in advance or limited self-propulsion performance. Here, we propose an injectable micromotor@hydrogel drug delivery system to protect micromotors from the external environment. The micromotors were prepared through layer-by-layer assembly technology. The asymmetric decomposition of hydrogen peroxide catalyzed by the locally distributed platinum nanoparticles enabled efficient propulsion of the micromotors in low concentration of hydrogen peroxide. In order to protect micromotors, they were loaded into the Schiff base hydrogel. The micromotor@hydrogel system can be injected directly into the lesion to release micromotors in response to the environment, reducing external influence on micromotors and improving the sustained-release effect. Erythromycin (Ery) was loaded into the micromotors and the micromotor@hydrogel system demonstrated excellent antibacterial effect. Micromotors released from the hydrogel underwent enhanced diffusion in the surroundings of bacteria without addition of exogenous hydrogen peroxide, which was manifested by their appearance in edge of the inhibition zone. The proposed micromotor@hydrogel drug delivery system offers a new strategy for the treatment of bacterial infections.

Induction of mesenchymal stem cell differentiation by co-culturing with mature cells in double-layered 2-methacryloyloxyethyl phosphorylcholine polymer hydrogel matrices

The effects of differentiated cells on stem cell differentiation were analyzed via co-culturing using a cell-encapsulated double-layered hydrogel system. As a polymer hydrogel matrix, a water-soluble zwitterionic polymer having both a 2-methacryloyloxyethyl phosphorylcholine unit and a p-vinylphenylboronic acid unit (PMBV), was complexed spontaneously with poly(vinyl alcohol) (PVA) under mild cell culture conditions. The creep modulus of the hydrogel was controlled by changing the composition of the polymer in the solution. Mouse mesenchymal stem cells (MSCs), C3H10T1/2 cells, were encapsulated into PMBV/PVA hydrogels and cultured. In the PMBV/PVA hydrogel with a lower creep modulus (0.40 kPa), proliferation of C3H10T1/2 cells occurred, and the formation of cell aggregates was observed. On the other hand, a higher creep modulus (1.7 kPa) of the hydrogel matrix prevented cell proliferation. Culturing C3H10T1/2 cells encapsulated in the PMBV/PVA hydrogel in the presence of bone morphogenetic protein-2 increased the activity of intracellular alkaline phosphatase (ALP). This indicated that C3H10T1/2 cells differentiated into mature osteoblasts. When the C3H10T1/2 cells encapsulated in the PMBV/PVA hydrogel were cultured in combination with the mature osteoblasts in the hydrogel by a close contacting double-layered hydrogel structure, higher ALP activity was observed compared with the cells cultured separately. It was considered that the differentiation of C3H10T1/2 cells in the hydrogel layer was induced by cytokines diffused from mature osteoblasts encapsulated in another hydrogel layer. It could be concluded that the PMBV/PVA hydrogel system provides a good way to observe the effects of the surrounding cells on cell function in three-dimensional culture.

Fabrication of Cationic Poly(vinyl alcohol) Films Cross-Linked Using Copolymers Containing Quaternary Ammonium Cations, Benzoxaborole, and Carboxy Groups

Water-insoluble cationic poly(vinyl alcohol) (PVA) films were fabricated using a mixed aqueous solution of PVA and poly([2-(methacryloyloxy)ethyl]trimethylammonium chloride (METAC)-co-methacrylic acid (MAAc)-co-5-methacrylamido-1,2-benzoxaborole (MAAmBO)) copolymer (3D). The surface of the PVA film is typically negatively charged, and simple fabrication methods for water-insoluble PVA films with cationic surface charges are required to expand their application fields. METAC, which has a permanent positive charge owing to the presence of a quaternary ammonium cation, was selected as the cationic unit. The MAAc and MAAmBO units were used as two types of cross-linking structures for the thermal cross-linking of the hydroxy and carboxy groups of the MAAc unit (covalent bonding) as well as the diol and benzoxaborole groups of the MAAmBO unit (dynamic covalent bonding). The films were thermally cross-linked at 135 °C for 4 h without the addition of materials. After immersion in surplus water at 80 °C for 3 h, the cross-linked PVA/3D films retained almost 100% of their weights. The ζ-potential of the water-insoluble PVA/3D film was 9.4 ± 0.8 mV. The PVA/3D film was strongly dyed using anionic acid red 1 (AR1) because of its positively charged surface. Interestingly, it could also be slightly dyed using cationic methylene blue (MB) and became transparent (original state) after immersion in water for 2 days. These results suggested that positive and negative charges coexisted in the PVA/3D film, and the surface properties were positively inclined. Moreover, the degree of hemolysis of the PVA/3D films was similar to that of the negative control, which showed high blood compatibility. To our knowledge, this is the first report on the fabrication of water-insoluble cationic PVA films using two types of cross-linking structures containing carboxy and benzoxaborole groups. The cross-linked PVA films were analyzed using Fourier transform infrared (FT-IR) spectroscopy, differential scanning calorimetry (DSC), and contact angle (CA) and ζ-potential measurement, as well as by determining the mechanical properties, adsorption of charged molecules, and biocompatibility. These readily fabricated water-insoluble PVA films with positive charges can show potential applications in sensors, adsorption systems, and antimicrobial materials.

Designing Hydrogels for 3D Cell Culture Using Dynamic Covalent Crosslinking

Designing simple biomaterials to replicate the biochemical and mechanical properties of tissues is an ongoing challenge in tissue engineering. For several decades, new biomaterials have been engineered using cytocompatible chemical reactions and spontaneous ligations via click chemistries to generate scaffolds and water swollen polymer networks, known as hydrogels, with tunable properties. However, most of these materials are static in nature, providing only macroscopic tunability of the scaffold mechanics, and do not reflect the dynamic environment of natural extracellular microenvironment. For more complex applications such as organoids or co-culture systems, there remain opportunities to investigate cells that locally remodel and change the physicochemical properties within the matrices. In this review, advanced biomaterials where dynamic covalent chemistry is used to produce stable 3D cell culture models and high-resolution constructs for both in vitro and in vivo applications, are discussed. The implications of dynamic covalent chemistry on viscoelastic properties of in vitro models are summarized, case studies in 3D cell culture are critically analyzed, and opportunities to further improve the performance of biomaterials for 3D tissue engineering are discussed.

Double network laminarin-boronic/alginate dynamic bioink for 3D bioprinting cell-laden constructs

The design of dynamically crosslinked hydrogel bioinks for three-dimensional (3D) bioprinting is emerging as a valuable strategy to advance the fabrication of mechanically tuneable cell-laden constructs for 3Din vitrodisease modelling and tissue engineering applications. Herein, a dynamic bioink comprising boronic acid-functionalised laminarin and alginate is explored for bioprinting 3D constructs under physiologically relevant conditions. The formulated bioink takes advantage of a double crosslinked network that combines covalent but reversible boronate ester bonds and ionic gelation via divalent cations. Moreover, it exhibits suitable rheological properties and improved mechanical features owing to its modular crosslinking chemistry, yielding stable constructs with user-programmable architecture. We explored such dynamic bioink as a supporting matrix for different cell classes, namely osteoblast precursors, fibroblasts and breast cancer cells. The resulting cell-laden bioprinted hydrogels display a homogeneous cell distribution post-printing and exceptional cell viability (>90%) that can be maintained for prolonged time periods in culture (14 days) for all cell lines. This simple and chemically versatile approach is envisaged to accelerate the development of multifunctional bioinks and contribute towards the fabrication of biomimetic 3D scaffolds with applicability in a wide range of predictive or exploratory biomedical platforms.

In situ formation of nanocomposite double-network hydrogels with shear-thinning and self-healing properties

Nanocomposite double-network hydrogels (ncDN hydrogels) are recently introduced to address the limitations of traditional DN hydrogels, such as the lack of diversity in the network structure and the restricted functionalities. However, two challenges remain, including the time-consuming preparation and the lack of shear-thinning and self-healing properties. Here, our approach to developing versatile ncDN hydrogels is through the use of multiple interfacial crosslinking chemistries (i.e., noncovalent interactions of electrostatic interaction and hydrogen bonds as well as dynamic covalent interactions of imine bonds and boronate ester bonds) and surface functionalized nanomaterials (i.e. phenylboronic acid modified reduced graphene oxide (PBA-rGO)). PBA-rGO was used as a multivalent gelator to further crosslink the two polymer chains (i.e. triethylene glycol-grafted chitosan (TEG-CS) and polydextran aldehyde (PDA)) in DN hydrogels, forming the TEG-CS/PDA/PBA-rGO ncDN hydrogels in seconds. The microstructures (i.e. pore size) and properties (i.e. rheological, mechanical, and swelling properties) of the ncDN hydrogels can be simply modulated by changing the amount of PBA-rGO. The dynamic bonds in the polymeric network provided the shear-thinning and self-healing properties to the ncDN hydrogels, allowing the hydrogels to be injected and molded into varied shapes as well as self-repair the damaged structure. Besides, the designed TEG-CS/PDA/PBA-rGO ncDN hydrogels were cytocompatible and also exhibited antibacterial activity. Taken together, we hereby provide a nanomaterial approach to fabricate a new class of ncDN hydrogels with tailorable networks and favorite properties for specific applications.

Structure-property relations in linear viscoelasticity of supramolecular hydrogels

Extraordinary mechanical properties of supramolecular gels (fracture toughness, fatigue resistance, injectability and self-healing ability) are strongly affected by their viscoelastic response driven by rearrangement (association and dissociation) of physical bonds. The kinetics of rearrangement is traditionally studied in small-amplitude shear oscillatory tests by analyzing the effect of the frequency of oscillations ω on the storage G' and loss G'' moduli. Conventional Maxwell-type models describe observations rather poorly when the gels reveal a pronounced flattening of the graphs G''(ω) at high frequencies. A simple model is derived in linear viscoelasticity of supramolecular gels. Its advantage is that the model reproduces experimental data correctly, on the one hand, and involves only four material constants, on the other. Based on the analysis of experimental data on gels cross-linked by coiled-coil complexes, covalent and ionic bonds, phenylboronic acid-diol complexes and metal-ligand coordination bonds, the model is applied to develop structure-property relations that describe the influence of chemical structure of supramolecular gels (concentration of polymer chains and type and molar fraction of temporary bonds) and environmental conditions (temperature, pH and ionic strength of buffer solutions) on their viscoelastic response.

Dual Cross-Linked Hydrogels with Injectable, Self-Healing, and Antibacterial Properties Based on the Chemical and Physical Cross-Linking

Injectable hydrogels have become a promising material for biomedical engineering applications, but microbial infection remains a common challenge in their application. In this study, we presented an injectable antibacterial hydrogel with self-healing property based on a dual cross-linking network structure of dynamic benzoxaborole-sugar and quadruple hydrogen bonds of the 2-ureido-4-pyrimidone (UPy) moieties at physiological pH. Dynamic rheological experiments demonstrated the gelatinous behavior of the double cross-linking network (storage modulus G' > loss modulus G″), and the modulus showed frequency-dependent behavior. The noncovalent interactions of UPy units in the polymer segment endowed the injectable hydrogels with good mechanical strength. By varying the solid contents, UPy units, as well as the pH, the mechanical properties of hydrogels could be controlled. Additionally, the hydrogels exhibited not only excellent self-healing and injectable properties but also pH and sugar dual-responsiveness. Moreover, the hydrogels could effectively inhibit the growth of both Escherichia coli and Staphylococcus aureus while exhibiting low toxicity. 3D cell encapsulation experiment results also demonstrated the potential use of these hydrogels as cell culture scaffolds. Taken together, the injectability, self-healing, and antimicrobial properties of the prepared hydrogels showed great promise for translational medicine, such as cell and tissue engineering applications.

Antimicrobial/Biocompatible Hydrogels Dual-Reinforced by Cellulose as Ultrastretchable and Rapid Self-Healing Wound Dressing

Hydrogels as a wound dressing, integrated with ultrastretchability, rapid self-healing, and excellent antimicrobial activity, are in high demand, particularly for joint skin wound healing. Herein, a multifunctional and ductile composite hydrogel was developed using poly(vinyl alcohol) (PVA)-borax gel as a matrix that was synergized or dual-reinforced with dopamine-grafted oxidized carboxymethyl cellulose (OCMC-DA) and cellulose nanofibers (CNF). Moreover, neomycin (NEO), an aminoglycoside antibiotic with multifunctional groups, was incorporated into the hydrogel network as both an antibacterial agent and a cross-linker. The dynamic reversible borate ester linkages and hydrogen bonds between OCMC-DA, PVA, and CNF, along with dynamic cross-linking imine linkages between NEO and OCMC-DA, endowed the hydrogel with excellent self-healing ability and stretchability (3300%). The as-reinforced networks enhanced the mechanical properties of hydrogels significantly. More remarkably, the composite hydrogel with improved biodegradability and biocompatibility is pH-responsive and effective against a broad spectrum of bacteria, which is attributed to the controllable release of NEO for steady availability of the antibiotic on the wound location. Overall, the antimicrobial hydrogel with rapid self-healing and reliable mechanical properties holds significant promise as dressing material for wound healing.

Multi-responsive, injectable, and self-healing hydrogels based on benzoxaborole–tannic acid complexation

A bio-inspired, multi-responsive, injectable, and self-healing hydrogel was developed via the interaction of tannic acid (TA) and benzoxaborole-based linear copolymers.

Dual-faced borax mediated synthesis of self-healable hydrogels merging dynamic covalent bonding and micellization

A facile route towards multifunctional hydrogels was proposed via borax catalyzed thiol-acrylate and borax–diol chemistry. The hydrogels exhibited self-healable ability and anti-fatigue property by merging dynamic boronate ester bonds and micellization in one system.

An injectable self-healing hydrogel “Trojan Horse” for adjunctive therapy of colon cancer recurrence

The preparation and bio-applications of an injectable hydrogel prepared using a novel multi-aldehyde crosslinker.