Self-Healing Hydrogels of Low Molecular Weight Poly(vinyl alcohol) Assembled by Host–Guest Recognition

Self-Healing Hydrogel Bioelectronics

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

Prospects of charged cyclodextrins in biomedical applications

Cyclodextrins (CDs), recognized for their unique ability to form inclusion complexes, have seen broad utilization across various scientific fields. Recently, there has been a surge of interest in the use of charged cyclodextrins for biomedical applications, owing to their enhanced properties, such as superior solubility and improved molecular recognition compared to neutral CDs. Despite the growing literature, a comprehensive review of the biomedical utilisations of multi-charged cyclodextrins is scarce. This review provides a comprehensive exploration of the emerging prospects of charged cyclodextrin-based assemblies in the field of biomedical applications. Focusing on drug delivery systems, the review details how charged CDs enhance drug solubility and stability, reduce toxicity, and enable targeted and controlled drug release. Furthermore, the review highlights the role of charged CDs in gene therapy, notably their potential for DNA/RNA binding, cellular uptake, degradation protection, and targeted gene delivery. The promising potential of charged CDs in antibacterial and antiviral therapies, including photodynamic therapies, biofilm control, and viral replication inhibition, is discussed. Concluding with a future outlook, this review highlights the potential challenges and advancements that could propel charged CDs to the forefront of biomedicine.

Supramolecular nanomedicines based on host-guest interactions of cyclodextrins

In the biomedical and pharmaceutical fields, cyclodextrin (CD) is undoubtedly one of the most frequently used macrocyclic compounds as the host molecule because it has good biocompatibility and can increase the solubility, bioavailability, and stability of hydrophobic drug guests. In this review, we generalized the unique properties of CDs, CD-related supramolecular nanocarriers, supramolecular controlled release systems, and targeting systems based on CDs, and introduced the paradigms of these nanomedicines. In addition, we also discussed the prospects and challenges of CD-based supramolecular nanomedicines to facilitate the development and clinical translation of these nanomedicines.

Self-healing, antibacterial, and conductive double network hydrogel for strain sensors

Multifunctional hydrogels have great potential in smart wearable technology, flexible electronic devices, and biomedical research. However, it is highly challenging to prepare unique conductive hydrogels with combined properties such as self-healing, self-adhesive, and antibacterial activity. In this regard, herein, a conductive double network hydrogel (ACBt-PAA/CMCs) was fabricated using carboxymethyl chitosan (CMCs), acrylic acid (AA), and alkaline calcium bentonite (ACBt) via a convenient approach. Owing to the hydrogen bond interaction between PAA and CMCs, the ACBt-PAA/CMCs double network structured hydrogels exhibited excellent self-healing (the tensile strength recovered to 74.3 % after 1 h) and adjustable mechanical properties, in which the fracture stress and strain can be easily adjusted in the range of 0.039 to 0.93 MPa and 564 to 2900 %, respectively. In addition, the ACBt-PAA/CMCs hydrogels exhibited the remarkable antibacterial activities against Escherichia coli (bacterial inhibition efficiency of ~99.99 %) and Staphylococcus aureus (bacterial inhibition efficiency of ~99.98 %). Furthermore, the ACBt-PAA/CMCs hydrogel based wearable skin exhibited an excellent real-time sensing performance for monitoring various motions, signifying outstanding sensing and self-adhesion properties. Considering the unique features such as self-healing, excellent adhesion, highly active strain sensing, and antibacterial activities making the ACBt-PAA/CMCs hydrogel is an excellent multifunctional conductive hydrogel. Hence, we believe that this proposed design method for the fabrication of smart and multifunctional conductive hydrogels, and this ACBt-PAA/CMCs hydrogel could be a promising candidate for flexible wearable materials, health monitoring, and beyond.

High Mechanical Strength and Multifunctional Microphase-Separated Supramolecular Hydrogels Fabricated by Liquid-Crystalline Block Copolymer

The development of multifunctional supramolecular hydrogels with high mechanical strength and multifunction is in high demand. In this work, the diblock copolymer poly(acrylamide-co-1-benzyl-3-vinylimidazolium bromide)-block-polyAzobenzene is synthesized through reversible addition-fragmentation chain transfer polymerization. The dynamic host-guest interactions between the host molecule cucurbit[8] uril and guest units are used to fabricate a 3D network of supramolecular hydrogels. Investigations on the properties of the supramolecular hydrogels show that the tensile stress of the sample is 1.46 MPa, eight times higher than that of hydrogel without liquid-crystalline block copolymer, and the self-healing efficiency of the supramolecular hydrogels at room temperature is 88.3% (fracture stress) and 100% (fracture strain) after 24 h. Results show that microphase-separated structure plays a key role in the high-strength hydrogel, whereas the host-guest interaction endows the hydrogel with self-healing properties. The supramolecular hydrogels with high mechanical strength, photo-responsivity, injectability, and biocompatibility can be used in various potential applications.

A facile strategy to construct biocompatible poly(vinyl alcohol)-based self-healing hydrogels

Endowing polymer hydrogels with good self-healing ability can autonomously repair damage with improved reliability. In this work, the benzaldehyde group was first grafted onto a biocompatible poly(vinyl alcohol) (PVA) molecular chain by esterification to obtain aldehyde-functionalized PVA (APVA), and the reversible imine bonds were further formed by reacting with amine groups on a quaternized chitosan (HTCC) chain. And thus, the self-healing APVA/HTCC hydrogel was fabricated with such imine bonds as crosslinking points together with hydrogen bonds. Many more imine bonds of hydrogels formed with increasing aldehyde content, resulting in increasing crosslinking density, decreasing average pore diameter and formation of a compact dynamic network, imparting certain mechanical strength and toughness with hydrogels. Furthermore, the healing efficiency of the hydrogel reached as high as 91.7% by self-healing without any external stimulus and its microstructure could be reconstructed after damage, exhibiting rapid recovery and dynamic features. Biocompatible self-healing PVA-based hydrogels exhibited great potential application in biomedical fields, like smart infill biomaterials, tissue engineering scaffolds, etc.

Injectable Adhesive Hydrogels for Soft tissue Reconstruction: A Materials Chemistry Perspective

Injectable bioadhesives offer several advantages over conventional staples and sutures in surgery to seal and close incisions or wounds. Despite the growing research in recent years few injectable bioadhesives are available for clinical use. This review summarizes the key chemical features that enable the development and improvements in the use of polymeric injectable hydrogels as bioadhesives or sealants, their design requirements, the gelation mechanism, synthesis routes, and the role of adhesion mechanisms and strategies in different biomedical applications. It is envisaged that developing a deep understanding of the underlying materials chemistry principles will enable researchers to effectively translate bioadhesive technologies into clinically-relevant products.

Multicharged cyclodextrin supramolecular assemblies

Multicharged cyclodextrin (CD) supramolecular assemblies, including those based on positively/negatively charged modified mono-6-deoxy-CDs, per-6-deoxy-CDs, and random 2,3,6-deoxy-CDs, as well as parent CDs binding positively/negatively charged guests, have been extensively applied in chemistry, materials science, medicine, biological science, catalysis, and other fields. In this review, we primarily focus on summarizing the recent advances in positively/negatively charged CDs and parent CDs encapsulating positively/negatively charged guests, especially the construction process of supramolecular assemblies and their applications. Compared with uncharged CDs, multicharged CDs display remarkably high antiviral and antibacterial activity as well as efficient protein fibrosis inhibition. Meanwhile, charged CDs can interact with oppositely charged dyes, drugs, polymers, and biomacromolecules to achieve effective encapsulation and aggregation. Consequently, multicharged CD supramolecular assemblies show great advantages in improving drug-delivery efficiency, the luminescence properties of materials, molecular recognition and imaging, and the toughness of supramolecular hydrogels, in addition to enabling the construction of multistimuli-responsive assemblies. These features are anticipated to not only promote the development of CD-based supramolecular chemistry but also contribute to the rapid exploitation of these assemblies in diverse interdisciplinary applications.

Multifunctional Arabinoxylan-functionalized-Graphene Oxide Based Composite Hydrogel for Skin Tissue Engineering

Wound healing is an important physiological process involving a series of cellular and molecular developments. A multifunctional hydrogel that prevents infection and promotes wound healing has great significance for wound healing applications in biomedical engineering. We have functionalized arabinoxylan and graphene oxide (GO) using the hydrothermal method, through cross-linking GO-arabinoxylan and polyvinyl alcohol (PVA) with tetraethyl orthosilicate (TEOS) to get multifunctional composite hydrogels. These composite hydrogels were characterized by FTIR, SEM, water contact angle, and mechanical testing to determine structural, morphological, wetting, and mechanical behavior, respectively. Swelling and biodegradation were also conducted in different media. The enhanced antibacterial activities were observed against different bacterial strains (E. coli, S. aureus, and P. aeruginosa); anticancer activities and biocompatibility assays were found effective against U-87 and MC3T3-E1 cell lines due to the synergic effect of hydrogels. In vivo activities were conducted using a mouse full-thickness skin model, and accelerated wound healing was found without any major inflammation within 7 days with improved vascularization. From the results, these composite hydrogels might be potential wound dressing materials for biomedical applications.

Recent Advances of Self-Healing Polymer Materials via Supramolecular Forces for Biomedical Applications

Noncovalent interactions can maintain the three-dimensional structures of biomacromolecules (e.g., polysaccharides and proteins) and control specific recognition in biological systems. Supramolecular chemistry was gradually developed as a result, and this led to design and application of self-healing materials. Self-healing materials have attracted attention in many fields, such as coatings, bionic materials, elastomers, and flexible electronic devices. Nevertheless, self-healing materials for biomedical applications have not been comprehensively summarized, even though many reports have been focused on specific areas. In this Review, we first introduce the different categories of supramolecular forces used in preparing self-healing materials and then describe biological applications developed in the last 5 years, including antibiofouling, smart drug/protein delivery, wound healing, electronic skin, cartilage lubrication protection, and tissue engineering scaffolds. Finally, the limitations of current biomedical applications are indicated, key design points are offered for new biological self-healing materials, and potential directions for biological applications are highlighted.

Facile fabrication of a polyvinyl alcohol-based hydrophobic fluorescent film via the Hantzsch reaction for broadband UV protection

Excessive exposure to ultraviolet (UV) light is harmful to human health. However, the traditional preparation of anti-UV films through the doping of UV absorbers leads to unstable products. Chemical modification of polyvinyl alcohol (PVA) to fabricate functional derivatives expand the application of these materials. Herein, a 1,4-dihydropyridine (DHP) fluorescent ring with a conjugated structure as a strong UV-absorber group was introduced onto a polyvinyl alcohol acetoacetate (PVAA) film to improve its UV-blocking performance. Firstly, PVAA was prepared via transesterification using tert-butyl acetoacetate (t-BAA). Then, the Hantzsch reaction was carried out on the surface of the PVAA film at room temperature. The resulting film showed high transparency, bright fluorescence emission, good mechanical properties, and outstanding stability. The introduction of the hydrophobic carbon chain reduced the hydrophilicity and swelling capacity of the PVAA film. In addition, the conjugated structure endowed the fluorescent film with excellent UV-blocking performance, where almost 100% UVA and UVB spectra could be shielded. The UV-blocking properties of the prepared films were persistent when they were exposed to UV irradiation, solvents, and subjected to thermal treatment. This work presents a facile and environmentally-friendly strategy by which to fabricate a multifunctional PVA-based film, which holds great potential for application in the anti-counterfeiting and UV-blocking fields.

Aggregation-induced emission luminogen based self-healing hydrogels fluorescent sensors for α-amylase

A self-healing hydrogel with a dual network was prepared through the host–guest recognition of acrylate γ-cyclodextrins with tetraphenylethylenes, and the fluorescence of hydrogel was enhanced in the presence of α-amylase.

Advanced Hydrogels for Cartilage Tissue Engineering: Recent Progress and Future Directions

Cartilage is a tension- and load-bearing tissue and has a limited capacity for intrinsic self-healing. While microfracture and arthroplasty are the conventional methods for cartilage repair, these methods are unable to completely heal the damaged tissue. The need to overcome the restrictions of these therapies for cartilage regeneration has expanded the field of cartilage tissue engineering (CTE), in which novel engineering and biological approaches are introduced to accelerate the development of new biomimetic cartilage to replace the injured tissue. Until now, a wide range of hydrogels and cell sources have been employed for CTE to either recapitulate microenvironmental cues during a new tissue growth or to compel the recovery of cartilaginous structures via manipulating biochemical and biomechanical properties of the original tissue. Towards modifying current cartilage treatments, advanced hydrogels have been designed and synthesized in recent years to improve network crosslinking and self-recovery of implanted scaffolds after damage in vivo. This review focused on the recent advances in CTE, especially self-healing hydrogels. The article firstly presents the cartilage tissue, its defects, and treatments. Subsequently, introduces CTE and summarizes the polymeric hydrogels and their advances. Furthermore, characterizations, the advantages, and disadvantages of advanced hydrogels such as multi-materials, IPNs, nanomaterials, and supramolecular are discussed. Afterward, the self-healing hydrogels in CTE, mechanisms, and the physical and chemical methods for the synthesis of such hydrogels for improving the reformation of CTE are introduced. The article then briefly describes the fabrication methods in CTE. Finally, this review presents a conclusion of prevalent challenges and future outlooks for self-healing hydrogels in CTE applications.

Fabrication of self-healing pectin/chitosan hybrid hydrogel via Diels-Alder reactions for drug delivery with high swelling property, pH-responsiveness, and cytocompatibility

Self-healing hydrogels with pH-responsiveness could protect loaded drugs from being destroyed till it arrives to the target. The pectin-based hydrogel is a candidate due to the health benefit, anti-inflammation, antineoplastic activity, nontoxicity, and biospecific degradation, et al. However, the abundant existence of water-soluble branched heteropolysaccharide chains influenced its performance resulting in limitation of the potential. In the present study, we prepared a series of self-healing pectin/chitosan hydrogels via the Diels-Alder reaction. Moreover, pectin/chitosan composite hydrogel was prepared as a contrast. By comparison, it can be seen that the Diels-Alder reaction greatly improved the cross-linking density of hydrogels. The self-healing experiments showed excellent self-healing performance. In different swelling mediums, significant transformation in the swelling ratio was shown, indicating well-swelling property, pH- and thermo-responsiveness. The drug loading and release studies presented high loading efficiency and sustained release performance. The cytotoxicity assay that showed a high cell proliferation ratio manifested great cytocompatibility.

The macroscopic shape of assemblies formed from microparticles based on host-guest interaction dependent on the guest content

Biological macroscopic assemblies have inspired researchers to utilize molecular recognition to develop smart materials in these decades. Recently, macroscopic self-assemblies based on molecular recognition have been realized using millimeter-scale hydrogel pieces possessing molecular recognition moieties. During the study on macroscopic self-assembly based on molecular recognition, we noticed that the shape of assemblies might be dependent on the host-guest pair. In this study, we were thus motivated to study the macroscopic shape of assemblies formed through host-guest interaction. We modified crosslinked poly(sodium acrylate) microparticles, i.e., superabsorbent polymer (SAP) microparticles, with β-cyclodextrin (βCD) and adamantyl (Ad) residues (βCD(x)-SAP and Ad(y)-SAP microparticles, respectively, where x and y denote the mol% contents of βCD and Ad residues). Then, we studied the self-assembly behavior of βCD(x)-SAP and Ad(y)-SAP microparticles through the complexation of βCD with Ad residues. There was a threshold of the βCD content in βCD(x)-SAP microparticles for assembly formation between x = 22.3 and 26.7. On the other hand, the shape of assemblies was dependent on the Ad content, y; More elongated assemblies were formed at a higher y. This may be because, at a higher y, small clusters formed in an early stage can stick together even upon collisions at a single contact point to form elongated aggregates, whereas, at a smaller y, small clusters stick together only upon collisions at multiple contact points to give rather circular assemblies. On the basis of these observations, the shape of assembly formed from microparticles can be controlled by varying y.

An in situ and rapid self-healing strategy enabling a stretchable nanocomposite with extremely durable and highly sensitive sensing features

Progress toward the development of wearable electromechanical sensors with durable and reliable sensing performance is critical for emerging wearable integrated electronic applications. However, it remains a long-standing challenge to realize mechanically stretchable sensing materials with extremely durable and high-performing sensing ability due to the fundamental dilemma lying in the sensing mechanism. In this work, we proposed an in situ and rapid self-healing strategy through nano-confining a dynamic host-guest supramolecular polymer network in a graphene-based multilevel nanocomposite matrix to fabricate a mechanically stretchable and structurally healable sensing nanocomposite which is provided with intriguing sensing durability and sensitivity simultaneously. When repeatedly stretching and releasing the nanocomposite sensing film, the fast association kinetics of cyclodextrin and adamantane host-guest inclusion complexes and good polymer chain dynamics in the supramolecular polymer network endowed by the nanoconfinement effect enable autonomous and rapid repair of the micro-cracks in situ generated in the sensing material. As a result, our strain sensing devices can achieve an extremely high durability and retain stable sensing performance even after over 100 000 stretching-releasing cycles at large strain of 50%. Moreover, the brittle nature originated from the inorganically dominated structure in conjunction with the thermodynamically stable host-guest interactions and dynamic hydrogen bonds inside the multilevel nanocomposite allow the sensing material to exhibit an ultrahigh gauge factor over 1500 with a large working strain of 58%. This work presents a reliable approach for the construction of ultradurable and high-performing wearable electronics.

DAP derived fatty acid amide organogelators as novel carrier for drug incorporation and pH-responsive release

Low-molecular mass fatty acid amide gelators were synthesized using 2,6-diaminopyridine as a linker and alkyl chains of varying lengths. The prepared organogel-elusions are able to trap and release ibuprofen molecule without changing its structure and activity.

Fabrication of an antibacterial hydrogel nanocomposite with self-healing properties using ZnO/β-cyclodextrin dimer/modified alginate

The development of self-healing materials with the ability to repair damage has received considerable attention.

Self-healing supramolecular hydrogels through host-guest interaction between cyclodextrin and carborane

New self-healing hydrogels based on the strong host-guest interaction of carborane (CB) and β-cylcodextrin (CD) were constructed through CB-grafted dextran and β-CD-grafted poly(acrylic acid). The storage modulus of the hydrogels could reach as high as 10 kPa, and the hydrogels exhibited an outstanding self-healing rate in minutes.

Polypeptide-based self-healing hydrogels: Design and biomedical applications

Self-healing hydrogels can heal themselves on the damaged sites, which opens up a fascinating way for enhancing lifetimes of materials. Polypeptide/poly(amino acid) is a class of polymers in which natural amino acid monomers or derivatives are linked by amide bonds with a stable and similar secondary structure as natural proteins (α-helix or β-fold). They have the advantages of nontoxicity, biodegradability, and low immunogenicity as well as easy modification. All these properties make polypeptides extremely suitable for the preparation of self-healing hydrogels for biomedical applications. In this review, we mainly focus on the progress in the fabrication strategies of polypeptide-based self-healing hydrogels and their biomedical applications in the recent 5 years. Various crosslinking methods for the preparation of polypeptide-based self-healing hydrogels are first introduced, including host-guest interactions, hydrogen bonding, electrostatic interactions, supramolecular self-assembly of β-sheets, and reversible covalent bonds of imine and hydrazone as well as molecular multi-interactions. Some representative biomedical applications of these self-healing hydrogels such as delivery system, tissue engineering, 3D-bioprinting, antibacterial and wound healing as well as bioadhesion and hemostasis are also summarized. Current challenges and perspectives in future for these "smart" hydrogels are proposed at the end . STATEMENT OF SIGNIFICANCE: Polypeptides with the advantages of nontoxicity, biodegradability, hydrophilicity and low immunogenicity, are extremely suitable for the preparation of self-healing hydrogels in biomedical applications. Recently, the researches of polypeptide-based self-healing hydrogel have drawn the great attentions for scientists and engineers. A review to summarize the recent progress in design and biomedical applications of these polypeptide-based self-healing hydrogels is highly needed. In this review, we mainly focus on the progress in fabrication strategies of polypeptide-based self-healing hydrogels and biomedical applications in recent five years and aim to draw the increased attention to the importance of these "smart" hydrogels, facilitating the advances in biomedical applications. We believe this work would draw interest from readers of Acta Biomaterialia.

Mechanically Strong, Heat-Resistant, Water-Induced Shape Memory Poly(vinyl alcohol)/Regenerated Cellulose Biocomposites via a Facile Co-precipitation Method

In this work, poly(vinyl alcohol) (PVA) biocomposites with regenerated cellulose-softwood pulp (RC-SP) as a green reinforcement were prepared via co-precipitation method. Simultaneous precipitation of the two components promotes uniform dispersion of the RC-SP and constructs strong molecular chain entanglements and hydrogen bonding network inside the composites. This physical cross-linking network reduces the water absorption and improves the water resistance of the composites. The incorporation of RC-SP not only improves the thermal decomposition properties of the composites, but also enhances the mechanical properties and dynamic mechanical properties, attributed to the strong interaction between the filler and the matrix. Moreover, the fabricated PVA/RC-SP composites exhibit good water-induced shape memory effect, and shape recovery rate of 10% RC-SP reinforced composite reaches 95.3% after immersing for 35 min. This work provides useful information for the implementation of co-precipitation method and the application of renewable cellulose resources.

Stretchable self-healing hydrogels capable of heavy metal ion scavenging

Self-healing hydrogels were prepared by simply mixing phytic acid (PA) and chitosan (CS) in water. Determined by scanning electron microscopy (SEM), the hydrogels were found to be a three-dimensional (3D) porous network structure. The formation of the network structure was considered to be mainly driven by electrostatic interactions and hydrogen bonding, cooperating with the subtle balance of multiple noncovalent interactions. The rheological data indicated that the hydrogels presented excellent mechanical properties with an elastic modulus of 20 000 Pa and a yield stress exceeding 7000 Pa. The dynamic dissociation and recombination of hydrogen bonding and electrostatic interaction in fractured regions of the gels initiated the self-healable property of PA/CS hydrogels. Since PA had high coordination ability to metal ions, PA/CS hydrogels were shown to exhibit excellent capability for capturing heavy metal ions, for example, Pb2+ and Cd2+. The PA/CS hydrogels provided a simple, green, and high efficiency strategic approach to scavenging heavy-metal ions from industrial sewage.

Progress in self-healing hydrogels assembled by host–guest interactions: preparation and biomedical applications

Preparation and biomedical applications of self-healing hydrogels assembled from hosts of cyclodextrins and cucurbit[n]urils with various guests were reviewed.

Supramolecular and dynamic covalent hydrogel scaffolds: from gelation chemistry to enhanced cell retention and cartilage regeneration

This review highlights the most recent progress in gelation strategies of biomedical supramolecular and dynamic covalent crosslinking hydrogels and their applications for enhancing cell retention and cartilage regeneration.

A synergistic antibacterial effect between terbium ions and reduced graphene oxide in a poly(vinyl alcohol)–alginate hydrogel for treating infected chronic wounds

A graphene and rare earth ion-containing hydrogel was developed to effectively promote the healing of infected chronic wounds without using antibiotics.

Supramolecular phenoxy-alkyl maleate-based hydrogels and their enzyme/pH-responsive curcumin release

Low molecular-weight hydrogelators as stimuli-responsive drug carrier agents in the pharmaceutical field.

Cyclodextrin-based host–guest supramolecular hydrogel and its application in biomedical fields

CD-based host–guest supramolecular hydrogels and their potential biomedical application.