For Pluronic F127 gels, >75% of drug was released within 3 hours. In contrast, for TiTannic gels >75% of drug was released over 6 days (a >40-fold difference compared to Pluronic F127 gels). Drug release was still at a detectable level (around 1.5-2%) from the TiTannic gel even after 28 days, while for Pluronic F127 no drug release was observed after 2 days. This difference in release rate may be due to the ability of phenols to interact strongly with diverse biomolecules and pharmaceuticals, which may retard the release and enable sustained drug delivery. Conclusions. In this study, we demonstrate that metal-phenolic supramolecular gelation can occur successfully in vivo, and provide essential in vivo characterization for this emerging class of hydrogels. Following characterization with optical and electron microscopy, Raman spectroscopy, and rheological methods-as well as study of hydrogel porosity and permeability-we assess biocompatibility and immunogenicity following subcutaneous injection in immunocompetent mice.