biological fluids via surface tension and capillary forces. [1-3] This unique behavior and the ability to tune their properties and viscoelasticity through compositional and structural control is enabling them to play a crucial role in the fabrication of next-generation sensors, [4] biomedical devices, [5] soft electronics, [6] and actuators [1,7] However, hydrogels fabricated using irreversible and static crosslinks are usually brittle, opaque, with limited structural complexity and exhibit large equilibrium volume swelling. [1,2,8] Moreover, they are not able to imitate the hierarchical complexity of the biological tissues well. [1] In this regard, there has been increasing attention to developing dynamic, reversible hydrogels as potential substitutes for their irreversible, static counterparts. [3] Dynamic hydrogels are able to respond to damage inherently at the molecular level and regain their initial mechanical strengths after damage. To impart such an interesting feature to hydrogels, it is a prerequisite that polymer chains have reactive groups either as pendant or terminal chain ends because such groups enable molecular mobility in or around cracks and damaged regions. [9] Thus far, different dynamic covalent bonds, for example, imines, [10] oximes, [11] Diels-Alder reaction products, [12] disulfides, [13] and non-covalent bonds, for example, metal-ligand coordination, host-guest recognition, hydrogen bonding, hydrophobic interactions, and/or π-π interactions, [14,15] have been proposed to synthesize such hydrogels. Herein, we focus on the rational design of mussel-inspired hydrogels reversibly cross-linked through catecholato−metal coordination chemistry. Coordination bonds have an important role in different biological systems being utilized in metalloproteins to provide structural integrity, storage, and/or catalytic properties, [16] while the dynamic feature of these bonds induces autonomous self-healing and self-recovery properties to materials. [17,18] A major challenge during the fabrication of such hydrogels is the catechol oxidation under slightly alkaline conditions in air, which has a profound impact on the properties of mussel-inspired hydrogels. It generates irreversible, static bonds in the network of hydrogels, that stiffens the hydrogel while reducing the number of catechol groups Nature has often been the main source of inspiration for designing smart functional materials. As an example, mussels can attach to almost any wet surfaces, for example, wood, rocks, metal, etc., due to the presence of catechols containing amino acid 3,4-dihydroxyphenyl-l-alanine (DOPA). Fabrication of mussel-inspired hydrogels using dynamic catecholato−metal coordination bonds has recently been in the limelight because of the hydrogels' ease of gelation, interesting self-healing, self-recovery, adhesiveness, and pH-responsiveness, as well as shear-thinning and mechanical properties. Mussel inspired hydrogels take advantage of catechols, for example, DOPA in the blue mussel, to undergo catecholatometal gelation ...
