Self-healing hydrogels: Preparation, properties, and applications

Hydrogels are three-dimensional (3D) network-like hydrophilic polymers architectures, which can hold a large amount of water and swell without dissolving while upholding the structure due to physical or chemical crosslinking of discrete polymer chains. Owing to inherent morphological and physicochemical attributes, such as softness, elasticity, and high water content, these highly dense polymeric structures resemble biological tissues, which are of specific interest in diverse application prospects in biomedicine. Nonetheless, the traditional hydrogels are heterogeneous and vulnerable to external stress, significantly limiting their applicability. Therefore, various hydrogels with self-healing properties have been designed and synthesized, which can prolong the life of hydrogels and improve the durability, reliability, and safety of materials. Broadly speaking, hydrogels can be divided into physical and chemical hydrogels based on the interactions between the individual polymer chains. On the one hand, physical hydrogels are often based on non-covalent interactions, such as electrostatic, hydrophobic, hydrogen bonding, and host-guest interactions. These non-covalent interactions are generally weaker than covalent cross-linkages, resulting in the poor mechanical properties of hydrogels. On the other hand, chemical hydrogels are based on reversible covalent interactions, such as acyl hydrazone, imine, disulfide, imine, and borate bonds. These chemical linkages are relatively stronger over physical interactions, providing a stable cross-linked network while maintaining its dynamics within the hydrogel. In addition, the non-covalent effects of physical hydrogels usually require to be reversible within a certain range of environmental conditions, such as a suitable pH or temperature range, to achieve self-repair performance. Contrarily, if it exceeds this expedient range, the hydrogel structure may be destroyed, losing its self-healing ability. Therefore, self-healing hydrogels based on non-covalent interactions often possess responsiveness, such as temperature, pH, and light. The dynamic and stable cross-linked network within the chemical hydrogel is an ideal method for designing a self-healing hydrogel. Although the covalent bond is reversible, certain external conditions are required for chemical hydrogels, such as pH, temperature, chemical stimulation, or light, among others, to form a dynamic equilibrium. Thus, the composition of the molecules can be exchanged and recombined, and the self-repairing ability of chemical hydrogels can be realized. Due to their excellent properties, self-healing hydrogels have broadened the application prospects in various aspects of human life. In this framework, these hydrogels have garnered enormous interest for their utilization in various fields of biomedicine, such as tissue engineering, drug delivery, and wound dressing. Despite the success, the fabrication of these innovative materials remains highly challenging in terms of poor mechanical properties, incomplete biodegradability, and biocompatibility evaluations in various models. In addition, high research and development costs, as well as demanding expertise, have significantly hampered the utilization of hydrogels. Therefore, future research direction of self-healing hydrogel materials should be focused on improving the mechanical properties, reducing production costs, concentrating on their advantages over various materials, and performing related clinical investigations, enabling them towards clinical translation. Moreover, it is required to promote eco-friendly fabrication technologies towards developing degradable materials. This review presents the recent progress of self-healing hydrogel materials based on physical and chemical hydrogels. The principles and characteristics of self-healing hydrogels designed through non-covalent and covalent interactions are explained. Finally, we summarize the article with the challenges in developing self-healing hydrogels along with exciting prospects.