Tough responsive hydrogels and applications as smart devices

摘要

Tough responsive hydrogels are promising materials for applications in bioinspired smart devices that mimic the soft actuations of animals or implants. Soft actuators are ubiquitous in nature, appearing in various plants and animals including the octopus, Venus flytrap, jellyfish, certain flowers, and so on. The actuators' sensitive response to external stimuli is critical to survive or proliferate and has inspired enormous interest in researching manmade smart devices including soft actuators, artificial muscles, and soft robotics. Many soft materials have been developed for the fabrication of devices that undergo shape transformations or actuations. Among these soft materials, polymer hydrogels are advantageous for their very high water content and biocompatibility, particularly for applications in bio-related smart devices. Hydrogels are flexible in both chemical modification and functionalization, allowing for unlimited manipulation of the chemical, biological, physical, and responsive properties. Numerous multiresponsive and multifunctional hydrogels have been developed and fabricated into devices that execute programmed performances. Good strength and toughness of hydrogels are needed for applications in smart devices. Since the 2000s, many tough hydrogels have been created, including double network hydrogels, slide-ring hydrogels, nanocomposite hydrogels, nanoparticle-reinforced hydrogels, and micelle-crosslinked hydrogels, to name a few. Sacrificial bonds/interactions are used to dissipate energy and thus toughen the hydrogels. Noncovalent bonds such as hydrogen bonds, supramolecular recognition, coordination, electrostatic interactions, and hydrophobic association have all been utilized to prepare tough hydrogels. Such reversible noncovalent bonds usually impart recoverability and self-healing properties to the hydrogels. Responsive hydrogels change their volume or shape when exposed to external stimuli such as temperature, pH, light/irradiation, magnetic fields, redox, and even biomolecules. By integrating responsive hydrogels into well-defined structures, or generating well-controlled heterogeneous structures or properties in hydrogels, the property contrast could drive a hydrogel-containing device to undergo shape deformation or actuation upon exposure to external stimuli. As such, smart hydrogel devices have been developed. This special issue provides an overview of the latest progress in the research on tough responsive hydrogels and their applications as smart devices. Some representative strategies to develop tough hydrogels are reviewed, with the toughening mechanisms comprehensively discussed. Chen et al. review novel double network hydrogels based on reversible noncovalent crosslinking. The noncovalent crosslinking not only provides outstanding mechanical properties, but also serves as a temporary shape locking mechanism for controllable shape transformations that are activated by external stimuli. Wu et al. establish hydrogen bonding between the two networks of PAAc/PNIPAm double network hydrogels and obtain very high modulus and strength. The dense hydrogen bonding is manipulated by temperature or pH to achieve shape memory. Spinks et al. develop a novel physical model to predict the threshold fracture energies of hydrogels physically crosslinked via hydrogen bonds and describe a procedure of consecutive and sequential dissociation of hydrogen bonded crosslinks during crack propagation. Fu highlights the recent developments in strong and tough hydrogels crosslinked by polymer colloids. Polymer colloids, including species from nanoparticles, microparticles, microgels, and micelles, are functionalized with reactive groups and utilized as macrocrosslinkers for the synthesis of hydrogels. In contrast to the rigid inorganic nanoparticles, polymer colloids are soft and able to deform upon loadings, particularly the microgels and hydrophobically associated micelles. The dislocation or deformation of polymer collo