Efficient Control of Swelling and Strength of Polyacrylamide/Polyacrylic Acid/Chitosan Hydrogels for NaF Release

Topical fluoride administration has been widely recognized as an effective strategy for caries prevention. Hydrogels, owing to their unique hydrated networks and tunable physicochemical properties, have emerged as promising candidates for localized fluoride delivery. However, the controlled sustained release of fluoride ions (F–) from hydrogels in moist environments continues to present significant technical challenges. In this study, we constructed a fluoride-loaded hydrogel system through a copolymerization strategy of chitosan (CS), acrylic acid (AA), and acrylamide (AM). The mesh size of the hydrogel was significantly reduced by the synergistic cross-linking effect of the hydrogen bond network and Zr4+-carboxyl coordination, thereby achieving the sustained release performance of F–. Benefiting from the ion cross-linking sites provided by hydrated zirconium ions and the hydrogen bond interactions, the hydrogel shows remarkable property enhancements: tensile stress increases by 159% (from 0.37 to 0.96 MPa) while swelling ratio decreases by 97.4% (from 4600 to <120%). The in vitro release experiment results showed that the cumulative release rate at 24 h decreased from 90 to 24%, effectively avoiding the initial burst release phenomenon and significantly prolonging the release time. The release mechanism was further investigated by fitting the in vitro release data to the zero-order model, the first-order model, and the Korsmeyer–Peppas model. The kinetic model fitting results indicated that the Peppas model had the best fitting effect, with a correlation coefficient (R2) of 0.9875 and a diffusion exponent n = 0.4060, suggesting that it conformed to the Fickian diffusion mechanism.