Research progress on transient hydrogels

Abstract

Hydrogels are a kind of soft and wet materials, which have three-dimensional network structures formed by polymers or small molecules in water through self-assembly or chemical cross-linking. Hydrogels can be divided into two categories of chemical and physical hydrogels according to the ways of hydrogel formation. The chemical hydrogels containing covalent bonds have high chemical resistance and thermodynamic stability. However, they are usually nondegradable and unrecyclable due to the irreversibility of covalent bonds, which limits their practical application. By contrast, various stimuli-responsive hydrogels based on supramolecular interactions or dynamic covalent bonds have become the research focus in recent years. These studies expand the application of hydrogels in drug delivery, biosensing, shape memory, etc. However, the classical responsiveness of the hydrogels is relatively passive, which always requires an external stimulus to induce the assembly or disassembly of the hydrogel networks and lacks the ability of self-regulation. Self-regulating systems are very common in living organisms, such as the reversible formation of intracellular actin. The non-equilibrium dynamic processes generate transient assemblies, which inspires the researchers to design artificial non-equilibrium systems. In 2010, a transient supramolecular hydrogel mediated by chemical fuels was presented for the first time. In the following decade, the researchers have been exploring how to mimic the living systems by simply mixing chemical reagents to build artificial non-equilibrium systems. The emergence of transient hydrogels is a significant breakthrough in the field of smart hydrogel materials. In the system, a switch that can induce sol-gel transitions autonomously is set up, which endows the soft materials with autonomy or adaptability. The scientists have realized the versatile regulation of the structure, function and lifespan of the hydrogels by imitating the complex living systems and combining the existing biocatalytic and/or chemical reactions. For example, for injectable biomaterials, the precise control on the lifetime of the transient hydrogels contributes to the timed release of drugs. Moreover, the transient hydrogels can also be used as temporary seals or adhesives for biomedical applications. At the same time, the remote-controlled time programmability is of great significance for the in situ study of self-assembly in closed systems. Although the research on transient hydrogels is still in its infancy, the advanced characteristics of the transient hydrogels, as well as the spatiotemporal regulation mode for the precise control of material properties, suggest that the transient hydrogels will be an important class of intelligent soft materials useful in many fields such as drug delivery, data encryption, material repairing and reprocessing in the future. In short, the transient hydrogel systems regulated by the chemical reaction networks are one of the important results inspired from the dissipative assembly in living systems. They significantly broaden the application range of hydrogels. In general, the current research in the field of transient hydrogels mainly focuses on the expansion of transient hydrogel systems and the design of transition modes, which requires a deeper understanding of the hydrogel formation mechanisms. Therefore, it is necessary to make a systematic summary for this rapidly developing field at this point. In this review, we give a clear definition of transient hydrogels: transient hydrogels refer to water-based temporary gels formed in non-equilibrium states, which can spontaneously convert to sol states mediated by internal reactions. According to the difference in hydrogel formation mechanisms, the classification of hydrogels is presented. Furthermore, the existing transient hydrogel systems are reviewed in detail. The application and limitation of transient hydrogels are summarized, and the future development direction is discussed and prospected lastly in this review.