Supramolecular redox-responsive ferrocene hydrogels and microgels

Stimuli-responsive hydrogels have lately attracted a lot of attention in the chemistry and material fields because of the “smart” change of their properties under outside stimuli including light, temperature, electric or magnetic field, pH, chemicals, shear stress, and redox reagents. Ferrocenyl (Fc) is often employed as a redox-responsive building unit due to its properties of chemical and electrochemical redox reversibility. This property involves reversible change between hydrophobicity and hydrophilicity, which endows hydrogels with unexpected features. Also, Fc derivatives are used as guest molecules featuring host–guest interactions with macrocyclic host molecules, mainly including cyclodextrins and pillararenes, commonly leading to the formation of supramolecular hydrogels with shape-memory, self-healing and sol–gel transition performances. This review focuses on the fabrication of various kinds of Fc-containing hydrogels and describes their gelling mechanisms, characteristic structures and properties, as well as functional applications. The review is divided into covalently cross-linked hydrogels and supramolecular cross-linked hydrogels. Furthermore, Fc-containing microgels constructed by chemically cross-linked three-dimensional polymer networks that are related to traditional hydrogels are also discussed. Fc-containing hydrogels and microgels are becoming more and more important as advanced functional materials, especially biomedical, shape-memory and self-healing materials.