Catechol–Phosphonate–Augmented Universal Adhesive for Hydrolysis-Resistant Dentin Bonds: A µTBS and Spectroscopic Study

Hydrolytic degradation at the resin-dentin interface remains a major challenge for the long-term success of adhesive restorations in dentistry. This study evaluated whether augmenting a 10-methacryloyloxydecyl dihydrogen phosphate (10-MDP)-based universal adhesive with a catechol-phosphonate (CatP) co-monomer could enhance resin-dentin bond durability and interfacial stability after aggressive hydrolytic aging. Fifty extracted human third molars were randomly assigned to receive either the original universal adhesive or the same adhesive modified by 3 wt% CatP. After standardized bonding to mid-coronal dentin and composite build-up, specimens underwent a combined aging regimen of 10,000 thermocycles and 6 months of water storage. The CatP-modified adhesive yielded significantly higher aged microtensile bond strength (34.10 ± 2.99 MPa) than control (27.01 ± 2.87 MPa; p < 0.001, g = 2.38). Interfacial hydraulic conductance was reduced by half in the CatP group (0.1923 ± 0.0416 vs. 0.3836 ± 0.0584 µL·min−1·cm−2·cm H2O−1; p = 0.002, g = −3.28). CatP modification led to significantly lower water sorption and solubility. Incorporation of CatP into a universal adhesive significantly improved the hydrolytic stability, bond strength, and sealing of resin–dentin interfaces, without compromising polymerization