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 CaCO₃ spontaneously forms a hydrogel. The gelation behavior can be controlled by adding an acidic compound to change the solubility of CaCO₃. CO₂ is used as the acidic agent and can be easily removed after gelation, thereby reducing the acidity of the final hydrogel. However, CO₂ addition has been controlled under varied thermodynamical conditions; therefore, specific CO₂ effects on gelation are not necessarily visualized. To evaluate the CO₂ impact on the final hydrogel, which would be extended to control hydrogel properties further, we utilized carbonated water to supply CO₂ 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 CO₂ 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 CO₂ in the carbonated water. We also controlled the pH and strength of the final hydrogels by changing the CO₂ amounts in the carbonated water added, thereby validating the significant effect of CO₂ on hydrogel properties and the feasibility of using carbonated water.