We demonstrate a facile and universal strategy in the fabrication of dual-cross-linked (DC) single network hydrogels with high toughness, “nonswellability”, rapid self-healing, and versatile adhesiveness based on polymer–tannic acid (TA) multiple hydrogen bonds. Two widely used hydrogels, physically cross-linked poly(vinyl alcohol) and chemically cross-linked polyacrylamide, have been transformed to TA-based DC hydrogels by dipping the corresponding aerogels into TA solution. The second cross-link via multiple polymer–TA hydrogen bonds effectively suppresses the crack propagation, resulting in both DC gels with high mechanical strength. But these two TA-based DC hydrogels go through different deformation mechanisms during the stretching based on analyzing their stress–strain curves using the Mooney–Rivlin equation. Moreover, these DC hydrogels are swelling-resistant, with strong toughness, good self-recoverability, rapid self-healing, and versatile adhesiveness. This work provides a simple route to fabricate multifunctional DC hydrogels, hopefully promoting their applications as biomedical materials.
Few-layered graphene sheets, synthesized by direct current arc-discharge method using NH(3) as one of the buffer gases, were dispersed in chitosan/acetic acid solutions. FTIR and X-ray photoelectron spectroscopy showed the presence of oxygen-containing functional groups on the surface of graphene sheets that may assist the good dispersion of graphene in chitosan solution. Graphene/chitosan films were produced by solution casting method. The mechanical properties of composite films were tested by nanoindentation method. With the addition of a small amount of graphene in chitosan (0.1-0.3 wt %), the elastic modulus of chitosan increased over ∼ 200%. The biocompatibility of graphene/chitosan composite films was checked by tetrazolium-based colorimetric assays in vitro. The cell adhesion result showed that the L929 cell can adhere to and develop on the graphene/chitosan composite films as well as on pure chitosan film, indicating that graphene/chitosan composites have good biocompatibility. Because there is no metallic impurity in graphene raw materials, the time-consuming purification process for removing metal nanoparticles entrapped in carbon nanotubes is thus avoided when graphene is used to prepare biomedical materials. Graphene/chitosan composites are potential candidates as scaffold materials in tissue engineering.
The development of facile and versatile strategies with low-cost for hydrogel construction is of tremendous scientific interest. Herein, we demonstrate that naturally derived, cost-effective tannic acid (TA) can be an efficient gelation binder for the hydrogel formation with a series of commercially available water-soluble polymers. With a five-polyphenol-arm structure, TA molecules are able to grasp polymer chains through either hydrogen or ionic bonds and cross-link them together by coordinate bonds in the presence of Fe(III) ions. These two interactions can be elegantly balanced by tuning the weight ratios of polymer/TA and TA/ Fe 3+ , which is the key point for the construction of supramolecular hydrogels. The supramolecular hydrogels exhibit multiple functionalities including mechanical tenability, rapid self-healing, pH-stimuli responsiveness, and free radical scavenging abilities. TA as a dynamic and versatile catechol group modifier provides a simple path to the construction of multifunctional hydrogels, which shows obvious advantages such as easy and green processing, low cost, and large-scale preparation.
Electrostatic interaction is strong but usually diminishes in high ionic-strength environments. Biosystems can use this interaction through adjacent cationic–aromatic amino acids sequence of proteins even in a saline medium. Application of such specific sequence to the development of cationic polymer materials adhesive to negatively charged surfaces in saline environments is challenging due to the difficulty in controlling the copolymer sequences. Here, we discover that copolymers with adjacent cation–aromatic sequences can be synthesized through cation–π complex-aided free-radical polymerization. Sequence controlled hydrogels from diverse cation/aromatic monomers exhibit fast, strong but reversible adhesion to negatively charged surfaces in seawater. Aromatics on copolymers are found to enhance the electrostatic interactions of their adjacent cationic residues to the counter surfaces, even in a high ionic-strength medium that screens the electrostatic interaction for common polyelectrolytes. This work opens a pathway to develop adhesives using saline water.
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