In vivo gelation, where hydrogels form spontaneously in situ under physiological conditions (e.g., at the site of injection), is of broad interest for biomedical applications due to its minimal invasiveness and high translational potential. Examples include thermosensitive poloxamer-based gels used for antibiotic delivery to the inner ear, hydrophobically modified poly(ethylene glycol) gels for ophthalmic applications, and cell sheet transplantation using thermosensitive gelatin-based hydrogels. Many biomedical applications also use gel systems such as calcium-alginate gels which are based on ionic crosslinking of the biomolecule alginate with Ca 2+ . Calcium-alginate gels typically form rapidly upon mixing. While strategies to modulate the gelation kinetics exist (e.g., using calcium-releasing liposomes to trigger gelation or phosphate salts to slow gelation time ), preformed alginate gels such as implantable beads remain the most commonly used in both pre-clinical and clinical research. In contrast, gelation times for recently introduced metal-phenolic gelsprepared through coordination of the biomolecule tannic acid (Figure in Supporting Information) with Ti IVcan be readily tuned from less than 1 minute to more than 1 day in a robust gelation process that is insensitive to a large range of conditions. We thus hypothesize that these Ti IV -tannic acid gel systems ("TiTannic gels") are suitable candidates for in vivo gelation and future biomedical applications.