Electron microscopy, Raman spectroscopy and rheology of the hydrogels. Metal-phenolic supramolecular gelation occurs spontaneously upon mixing of polyphenols (e.g. tannic acid, TA) with group IV metal ions (e.g. Ti IV ). This class of material was recently introduced and has been characterized with rheological methods, optical microscopy, UV-vis spectroscopy, X-ray photoelectron spectroscopy and X-ray diffraction among other techniques. To further explore the nanostructure of the gel, we performed a freeze-fracture procedure using liquid nitrogen, followed by freeze substitution in acetone and lyophilization before scanning electron microscopy (SEM) imaging (Figure ). The results show a structure with porosity on the micrometer and nanometer length scales (Figure ). It should be noted, however, that the freeze-substitution and lyophilization process may affect the pore structure compared to the pristine hydrated material. Raman spectroscopy has been extensively used to investigate materials prepared using coordination-driven assembly and coordination compounds. We applied a Raman spectroscopy probe with continuous spectral acquisition (with 1 second integration time) to obtain Raman spectra in situ during gelation (Figure ), the results are shown in Figure . When comparing Raman spectra of the individual components (TA solution and Ti IV solution) with that of the formed gel (TA + Ti IV ), two additional peaks, at 1361 cm -1 and at 1490 cm -1 , are readily observed (Figure ). These peaks increased in intensity during the first 30 seconds after mixing, and then remained largely stable during the whole experiment (1 hour), indicating that they arise due to the coordination reactions that drive gelation in this system. The TiTannic hydrogels were measured using rheology (Figure ), displaying a storage modulus (G' = 12.6 kPa) significantly larger than the loss modulus (G" = 0.595 kPa). An amplitude sweep was conducted to determine the linear viscoelastic region and crossover points of the hydrogels, whereas the frequency sweep showed frequency independent behavior of the hydrogels.