A self‐healing hydrogel is prepared by crosslinking acrylamide with a host–guest macro‐crosslinker assembled from poly(β‐cyclodextrin) nanogel and azobenzeneacrylamide. The photoisomerizable azobenzene moiety can change its binding affinity with β‐cyclodextrin, therefore the crosslinking density and rheology property of the hydrogel can be tuned with light stimulus. The hydrogel can repair its wound autonomously through the dynamic host–guest interaction. In addition, the wounded hydrogel will lose its ability of self‐healing when exposed to ultraviolet light, and the self‐healing behavior can be recovered upon the irradiation of visible light. The utilizing of host–guest macro‐crosslinking approach manifests the as‐prepared hydrogel reversible and light‐switchable self‐healing property, which would broaden the potential applications of self‐healing polymers.
Self‐healing hydrogels have been studied by many researchers via multiple cross‐linking approaches including physical and chemical interactions. It is an interesting project in multifunctional hydrogel exploration that a water soluble polymer matrix is cross‐linked by combining the ionic coordination and the multiple hydrogen bonds to fabricate self‐healing hydrogels with injectable property. This study introduces a general procedure of preparing the hydrogels (termed gelatin‐UPy‐Fe) cross‐linked by both ionic coordination of Fe3+ and carboxyl group from the gelatin and the quadruple hydrogen bonding interaction from the ureido‐pyrimidinone (UPy) dimers. The gelatin‐UPy‐Fe hydrogels possess an excellent self‐healing property. The effects of the ionic coordination of Fe3+ and quadruple hydrogen bonding of UPy on the formation and mechanical behavior of the prepared hydrogels are investigated. In vitro drug release of the gelatin‐UPy‐Fe hydrogels is also observed, giving an intriguing glimpse into possible biological applications.
A dextran‐based self‐healing hydrogel is prepared by reversible Diels–Alder reaction under physiological conditions. Cytocompatible fulvene‐modified dextran as main polymer chains and dichloromaleic‐acid‐modified poly(ethylene glycol) as cross‐linkers are used. Both macro‐ and microscopic observation as well as the rheological recovery test confirm the self‐healing property of the dextran‐l‐poly(ethylene glycol) hydrogels (“l” means “linked‐by”). In addition, scanning electrochemical microscopy is used to qualitatively and quantitatively in situ track the self‐healing process of the hydrogel for the first time. It is found that the longitudinal depth of scratch on hydrogel surface almost completely healed at 37 °C after 7 h. This work represents a facile approach for fabrication of polysaccharide self‐healing hydrogel, which can be potentially used in several biomedical fields.
Development of self‐healing polymers with spontaneous self‐healing capability and good mechanical performance is highly desired and remains a great challenge. Here, mechanical robust and self‐healable supramolecular hydrogels have been fabricated by using poly(2‐dimethylaminoethyl methacrylate) brushes modified silica nanoparticles (SiO2@PDMAEMA) as multifunctional macrocrosslinkers in a poly(acrylic acid) (PAA) network structure. The SiO2 nanoparticles serve as noncovalent crosslinkers, dissipating energy, whereas the electrostatic interactions between cationic PDMAEMA and anionic PAA render the hydrogel self‐healing property. This process provides a simple and broadly applicable strategy to produce mechanical strong and self‐healable materials.
High‐porosity interconnected, thermoresponsive macroporous hydrogels are prepared from oil‐in‐water high internal phase emulsions (HIPEs) stabilized by gelatin‐graft‐poly(N‐isopropylacrylamide). PolyHIPEs are obtained by gelling HIPEs utilizing the thermoresponsiveness of the copolymer components. PolyHIPEs properties can be controlled by varying the aqueous phase composition, internal phase volume ratio, and gelation temperature. PolyHIPEs respond to temperature changes experienced during cell seeding, allowing fibroblasts to spread, proliferate, and penetrate into the scaffold. Encapsulated cells survive ejection of cell‐laden hydrogels through a hypodermic needle. This system provides a new strategy for the fabrication of safe injectable biocompatible tissue engineering scaffolds.
Hypoxia plays a critical role in the development and wound healing process, as well as a number of pathological conditions. Here, dextran‐based hypoxia‐inducible (Dex‐HI) hydrogels formed with in situ oxygen consumption via a laccase−medicated reaction are reported. Oxygen levels and gradients were accurately predicted by mathematical simulation. It is demonstrated that Dex‐HI hydrogels provide prolonged hypoxic conditions up to 12 h. The Dex‐HI hydrogel offers an innovative approach to delineate not only the mechanism by which hypoxia regulates cellular responses, but may facilitate the discovery of new pathways involved in the generation of hypoxic and oxygen gradient environments.
The application of cyclodextrin (CD)‐based host–guest interactions towards the fabrication of functional supramolecular assemblies and hydrogels is of particular interest in the field of biomedicine. However, as of late they have found new applications as advanced functional materials (e.g., actuators and self‐healing materials), which have renewed interest across a wide range of fields. Advanced supramolecular materials synthesized using this noncovalent interaction, exhibit specificity and reversibility, which can be used to impart reversible cross‐linking, specific binding sites, and functionality. In this review, various functional CD‐based supramolecular assemblies and hydrogels will be outlined with the focus on recent advances. In addition, an outlook will be provided on the direction of this rapidly developing field.
The synthesis of a novel photoreactive poly(ethylene glycol) (PEG)‐based polymer with caged carbonyl groups is reported. We further demonstrate its use for the on‐demand fabrication of hydrogels. For rapid gelation, a hydrazide‐functionalized PEG is used as the second component for the hydrogel preparation. The photoreactive PEG‐based polymer is designed for controlled cleavage of the protecting groups upon exposure to UV light releases free aldehyde moieties, which readily react with hydrazide groups in situ. This hydrogel system may find applications in controlled release drug delivery applications, when combined with in situ gelation. Furthermore, the possibility of forming gels specifically upon UV irradiation gives an opportunity for 3D fabrication of degradable scaffolds.
A new multiblock copolymer self‐healing strategy is reported that centers on the synthesis of block copolymers designed with different self‐healing motifs incorporated into individual blocks. As a proof of concept, a novel pentablock copolymer (ABCBA) consisting of a poly(ethylene glycol) middle block and self‐healable symmetric blocks of a polymethacrylate with pendant disulfide linkages and carboxylic acids is synthesized by a combination of consecutive controlled radical polymerization with hydrolytic cleavage. Disulfide exchange reactions of pendant disulfide linkages and metal–ligand interactions of pendant carboxylic acids with ferric ions allow for the formation of dual crosslinked networks with dynamic disulfide and supramolecular crosslinkages. The resultant networks possessing self‐healing viscoelasticity enable self‐healing on macroscale damages through supramolecular metal–ligand interactions and disulfide exchange reactions at room or moderate temperatures. These preliminary results suggest that the strategy can offer the versatility in the development of multifunctional self‐healable materials in dual or multiple self‐healable mechanisms.
Furfuryl glycidyl ether (FGE) represents a highly versatile monomer for the preparation of reversibly cross‐linkable nanostructured materials via Diels–Alder reactions. Here, the use of FGE for the mid‐chain functionalization of a P2VP‐b‐PEO diblock copolymer is reported. The material features one furan moiety at the block junction, P2VP68‐FGE‐b‐PEO390, which can be subsequently addressed in Diels–Alder reactions using maleimide‐functionalized counterparts. The presence of the FGE moiety enables the introduction of dyes as model labels or the formation of hetero‐grafted brushes as shell on hybrid Au@Polymer nanoparticles. This renders P2VP68‐FGE‐b‐PEO390, a powerful tool for selective functionalization reactions, including the modification of surfaces.
Stimuli responsive surfaces that show reversible fluorescence switching behavior in response to temperature changes were fabricated. Oligo(ethylene glycol) methacrylate thermoresponsive polymers with amine end‐groups were prepared by atom transfer radical polymerization (ATRP). The polymers were patterned on silicon surfaces by electron beam (e‐beam) lithography, followed by conjugation of self‐quenching fluorophores. Fluorophore conjugated hydrogel thin films were bright when the gels were swollen; upon temperature‐induced collapse of the gels, self‐quenching of the fluorophores led to significant attenuation of fluorescence. Importantly, the fluorescence was regained when the temperature was cooled. The fluorescence switching behavior of the hydrogels for up to ten cycles was investigated and the swelling‐collapse was verified by atomic force microscopy. Morphing surfaces that change shape several times upon increase in temperature were obtained by patterning multiple stimuli responsive polymers.
Metal‐containing polymer hydrogels have attracted increasing interest in recent years due to their outstanding properties such as biocompatibility, recoverability, self‐healing, and/or redox activity. In this short review, methods for the preparation of metal‐containing polymer hydrogels are introduced and an overview of these hydrogels with various functionalities is given. It is hoped that this short update can stimulate innovative ideas to promote the research of metal‐containing hydrogels in the communities.
A photocleavable terpolymer hydrogel cross‐linked with o‐nitrobenzyl derivative cross‐linker is shown to be capable of self‐shaping without losing its physical integrity and robustness due to spontaneous asymmetric swelling of network caused by UV‐light‐induced gradient cleavage of chemical cross‐linkages. The continuum model and finite element method are used to elucidate the curling mechanism underlying. Remarkably, based on the self‐changing principle, the photosensitive hydrogels can be developed as photoprinting soft and wet platforms onto which specific 3D characters and images are faithfully duplicated in macro/microscale without contact by UV light irradiation under the cover of customized photomasks. Importantly, a quick response (QR) code is accurately printed on the photoactive hydrogel for the first time. Scanning QR code with a smartphone can quickly connect to a web page. This photoactive hydrogel is promising to be a new printing or recording material.
Hierarchical self‐assembly of transient composite hydrogels is demonstrated through a two‐step, orthogonal strategy using nanoparticle tectons interconnected through metal–ligand coordination complexes. The resulting materials are highly tunable with moduli and viscosities spanning many orders of magnitude, and show promising self‐healing properties, while maintaining complete optical transparency.
The dynamic covalent characteristics of oxime and boronate ester bonds have been explored. A small excess of a competing aldehyde under acidic conditions resulted in oxime polymer degradation from high molecular weights (30 kDa) to low molecular weight oligomers (2.2 kDa). The dynamic nature of oxime bonds imparts oxime cross‐linked hydrogels with self‐healing properties and the incorporation of phenyl boronic acid groups into the hydrogel network provides a platform for hydrogel functionalization. The addition of a polyphenol (tannic acid) proves a facile means to incorporate a second, dynamic covalent cross‐linking network through boronate ester formation which, owing to the increase in the degree of cross‐linking, is found to be nearly double the hydrogel strength (storage modulus increased from 4.6 to 8.5 kPa). Finally, the tannic acid cross‐linking network is selectively degraded returning the hydrogel storage modulus to its initial value and providing a means for the synthesis of materials with tunable mechanical properties.
In this study, mechanically strong hydrogels are synthesized by photopolymerization of 2‐vinyl‐4,6‐diamino‐1,3,5‐triazine, poly(ethylene glycol) methacrylate, and disulfide‐containing cross‐linker, N′N‐bis(acryloyl)cystamine. The bilayer hydrogel with distinct cross‐linking density is shown to self‐roll into a 3D tube, which could still be well reinforced by hydrogen bondings, upon exposing reductants such as 1,4‐dithio‐DL‐threitol (DTT) or L‐glutathione (GSH), because the redox‐induced cleavage of disulfide bonds results in the imbalanced internal shrinking stress between two layers. At an intracellular level of GSH, model L929 cells‐seeded bilayer gel sheet could curl up into a 3D tubular scaffold where the cells maintained good viability.
This work demonstrates a new reactive and functional hybrid (S‐MMA‐POSS) of polyhedral oligomeric silsesquioxane (POSS) and sulfur prepared with a direct reaction between a multifunctional methacrylated POSS compound (MMA‐POSS) and elemental sulfur (S8) through the “inverse vulcanization” process. S‐MMA‐POSS is an effective building block for imparting self‐healing ability to the corresponding thermally crosslinked POSS‐containing nanocomposites through a self‐curing reaction and co‐curing reaction with conventional thermosetting resins. Moreover, S‐MMA‐POSS is also a useful precursor for preparation of materials with high transparency in mid‐infrared region.
The term hydrogel describes a type of soft and wet material formed by cross‐linked hydrophilic polymers. The distinct feature of hydrogels is their ability to absorb a large amount of water and swell. The properties of a hydrogel are usually determined by the chemical properties of their constituent polymer(s). However, a group of hydrogels, called “smart hydrogels,” changes properties in response to environmental changes or external stimuli. Recently, DNA or DNA‐inspired responsive hydrogels have attracted considerable attention in construction of smart hydrogels because of the intrinsic advantages of DNA. As a biological polymer, DNA is hydrophilic, biocompatible, and highly programmable by Watson‐Crick base pairing. DNA can form a hydrogel by itself under certain conditions, and it can also be incorporated into synthetic polymers to form DNA‐hybrid hydrogels. Functional DNAs, such as aptamers and DNAzymes, provide additional molecular recognition capabilities and versatility. In this Review, DNA‐based hydrogels are discussed in terms of their stimulus response, as well as their applications.
A simple polymerization of trichlorophosphoranimine (Cl3P = N−SiMe3) mediated by functionalized triphenylphosphines is presented. In situ initiator formation and the subsequent polymerization progress are investigated by 31P NMR spectroscopy, demonstrating a living cationic polymerization mechanism. The polymer chain lengths and molecular weights of the resulting substituted poly(organo)phosphazenes are further studied by 1H NMR spectroscopy and size exclusion chromatography. This strategy facilitates the preparation of polyphosphazenes with controlled molecular weights and specific functional groups at the α‐chain end. Such well‐defined, mono‐end‐functionalized polymers have great potential use in bioconjugation, surface modification, and as building blocks for complex macromolecular constructs.
Poly (N‐isopropylacrylamide) (pNIPAm)‐based hydrogels and hydrogel particles (microgels) have been extensively studied since their discovery and “popularization” a few decades ago. While their uses seem to have no bounds, this Feature Article is focused on their development and application for sensing small molecules, macromolecules, and biomolecules. Hydrogel/microgel‐based photonic materials with order in one, two, or three dimensions are highlighted, which exhibit optical properties that depend on the presence and concentration of various analytes.
