Physicochemical Properties of Egg-Box-Mediated Hydrogels with Transiently Decreased pH Employing Carbonated Water.

Anionic polysaccharides, including low-methoxy (LM) pectin, are extensively used in biomaterial applications owing to their safety, biocompatibility, and feasibility in constructing supramolecular assemblies by forming egg-box structures with divalent cations. Mixing an LM pectin solution with CaCO 3 spontaneously forms a hydrogel. The gelation behavior can be controlled by adding an acidic compound to change the solubility of CaCO 3 . CO 2 is used as the acidic agent and can be easily removed after gelation, thereby reducing the acidity of the final hydrogel. However, CO 2 addition has been controlled under varied thermodynamical conditions; therefore, specific CO 2 effects on gelation are not necessarily visualized. To evaluate the CO 2 impact on the final hydrogel, which would be extended to control hydrogel properties further, we utilized carbonated water to supply CO 2 into the gelation mixture without changing its thermodynamic conditions. The addition of the carbonated water accelerated gelation and significantly increased the mechanical strength, promoting cross-linking. However, the CO 2 volatilized into the atmosphere, and the final hydrogel became more alkaline than that without the carbonated water, probably because a considerable amount of the carboxy group was consumed for cross-linking. Moreover, when aerogels were prepared from the hydrogels with carbonated water, they exhibited highly ordered networks of elongated porosity in scanning electron microscopy, proposing an intrinsic structural change by CO 2 in the carbonated water. We also controlled the pH and strength of the final hydrogels by changing the CO 2 amounts in the carbonated water added, thereby validating the significant effect of CO 2 on hydrogel properties and the feasibility of using carbonated water.