Dialing in the Ratio of Covalent and Coordination Cross-links in Self-healing Hydrogels
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Hydrogels containing sugar and oxaborole residues with remarkable self-healing properties were synthesized by free-radical polymerization in a facile and one pot process. The strong covalent interactions between the oxaborole residues and free adjacent hydroxyl groups of the pendent sugar residues of the glycopolymer allowed the in situ formation of hydrogels achievable under either neutral or alkaline conditions. These hydrogels showed excellent self-healing and injectability behaviors in aqueous conditions and were found to be responsive to both pH and the presence of free sugars (such as glucose) in solution. Furthermore, these hydrogels can easily be reconstructed from their lyophilized powder into any desired three-dimensional scaffold. Additionally, the hydrogels can be designed to have very low cytotoxicity and hence can be used as a scaffold for cell encapsulation. With these unique properties, these biocompatible, biodegradable, rebuildable, and self-healable hydrogels offer great potential in many biomedical applications.
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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.
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Abstract A new biomaterial, a hydrogel, with dual‐crosslinked design, has been created with enhanced mechanical performance. The hydrogels are fabricated based on water‐soluble chitosan, with dual‐crosslinking of imine linkages and host–guest interactions. Phenolphthalein‐grafted N ‐carboxyethyl chitosan (CECS‐g‐PHP), as a guest polymer, is synthesized and structurally characterized and complexed with hexamethylenediamine modified β‐cyclodextrin (β‐CD‐HDA), as a host molecule. Oxidized sodium alginate (OSA) is added to form crosslinking networks via imine linkages with the existing amino groups. The hydrogels show significantly shorter gelation times and higher compressive stresses, compared with single‐crosslinked hydrogels. The phenolphthalein units in the hydrogel change color with pH and other added chemicals. Moreover, the hydrogels can be injected and are self‐healing with >80% recovery within 4 h. Thus, these dual‐crosslinked hydrogels, which respond to pH and other stimuli, are promising designs for new multifunctional biomaterials.
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Diethylenetriamine
Polyacrylic acid
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Hydrophobic effect
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Network covalent bonding
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Hydrogels that are injectable, self-healing, and multiresponsive are becoming increasingly relevant for a wide range of applications. In this work, we have successfully developed a novel approach in the fabrication of smart hydrogels with all the above properties. A symmetrical ABA triblock copolymer was first synthesized via atom transfer radical polymerization with a temperature responsive middle block and two permanently hydrophilic glycopolymer chains on both ends. Hydrogels were subsequently constructed by mixing the triblock copolymer with another linear hydrophilic copolymer bearing benzoxaborole groups. The interactions of the benzoxaborole groups with the sugar hydroxyl groups allowed the formation of dynamic covalent bonds. The resulting hydrogels exhibited temperature, pH, and sugar responsiveness. Rheological studies confirmed that the mechanical property can be tuned by changing the pH as well as the galactose/benzoxaborole molar ratio. Furthermore, the hydrogels showed excellent self-healing ability and shear-thinning performance due to the inherent well-known dynamic covalent bonds of boronic esters. Therefore, these types of hydrogels can have excellent biomedical applications.
Glycopolymer
Atom-transfer radical-polymerization
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UPy is used as a reversible and dynamic crosslinker to prepare hydrogels that are injectable and undergo rapid self-healing in response to damage.
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Mussel-inspired hydrogels held together by reversible catecholato–metal coordination bonds have recently drawn great attention owing to their attractive self-healing, viscoelastic and adhesive properties together with their pH-responsive nature. A major challenge in these systems is to orchestrate the degree of catechol oxidation that occurs under alkaline conditions in air and has a great impact on the aforementioned properties because it introduces irreversible covalent cross-links to the system, which stiffens the hydrogels but consume catechols needed for self-healing. Herein, we present a catechol-based hydrogel design that allows for the degree of oxidative covalent cross-linking to be controlled. Double cross-linked hydrogels with tunable stiffness are constructed by adding the oxidizable catechol analogue, tannic acid, to an oxidation-resistant hydrogel construct held together by coordination of the dihydroxy functionality of 1-(2′-carboxyethyl)-2-methyl-3-hydroxy-4-pyridinone to trivalent metal ions. By varying the amount of tannic acid, the hydrogel stiffness can be customized to a given application while retaining the self-healing capabilities of the hydrogel's coordination chemical component.
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Non-covalent interactions
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Abstract A dual cross‐linking design principle enables access to hydrogels with high strength, toughness, fast self‐recovery, and robust fatigue resistant properties. Imidazole (IMZ) containing random poly(acrylamide‐ co ‐vinylimidazole) based hydrogels are synthesized in the presence of Ni 2+ ions with low density of chemical cross‐linking. The IMZ‐Ni 2+ metal–ligand cross‐links act as sacrificial motifs to effectively dissipate energy during mechanical loading of the hydrogel. The hydrogel mechanical properties can be tuned by varying the mol% of vinylimidazole (VIMZ) in the copolymer and by changing the VIMZ/Ni 2+ ratio. The resultant metallogels under optimal conditions (15 mol% VIMZ and VIMZ/Ni 2+ = 2:1) show the best mechanical properties such as high tensile strength (750 kPa) and elastic modulus (190 kPa), combined with high fracture energy (1580 J m −2 ) and stretchability (800–900% strain). The hydrogels are pH responsive and the extent of energy dissipation can be drastically reduced by exposure to acidic pH. These hydrogels also exhibit excellent anti‐fatigue properties (complete recovery of dissipated energy within 10 min after ten successive loading–unloading cycles at 400% strain), high compressive strength without fracture (17 MPa at 96% strain), and self‐healing capability due to the reversible dissociation and re‐association of the metal ion mediated cross‐links.
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