Dendrimer-based extrafibrillar demineralization for optimizing resin-dentin bond stability

Objective: A selectively extrafibrillar dentin demineralization concept has recently been developed that preserves intrafibrillar minerals within collagen fibrils to prevent exposure and activation of endogenous dentin proteases that are important contributor to collagen degradation within hybrid layers. The objective of the present study was to investigate the potential of using PEGylated PAMAM dendrimer (PEG-P) with high MWs (﹥40 kDa) as an antimicrobial extrafibrillar dentin demineralization conditioner to improve durability of resin-dentin bonds. Methods: The chelation capacities of different concentrations of PEGylated PAMAM on calcium ion from dentin were measured using ICP-AES. The optimal degree and concentration of PEGylation for dentin bonding were determined by microtensile bond strength testing to make sure that bond strengths with the wet-bonding and dry-bonding technique before and after thermocycling was comparable to those of H₃PO₄ etching. Morphological observation of PEG-P-based extrafibrillar demineralization were examined by FE-SEM and AFM. In-situ zymography was performed to directly evaluate gelatinolytic activity within hybrid layers. For antibacterial testing, dead/live bacterial staining was performed to evaluate 20 % PEG-P potent antibacterial effects on oral bacteria. Results: The result of ICP-AES indicated that PEG-P had chelation capacities for Ca^{2 +}. Wet- and dry-bonding with the 20 % PEG-P-based demineralization produced tensile bond strengths equivalent to H₃PO₄-based wet-bonding technique, and those bond strengths were maintained after thermocycling. Morphological observation of PEG-P-based extrafibrillar demineralization by FE-SEM showed partially demineralized dentin with mineralized collagen fibrils separated by detectable extrafibrillar spaces in both wet- and dry- bonding modes, whereas the H₃PO₄-ethched dentin produced completely demineralized collagen fibrils that collapsed and formed a smooth interface without porous spaces in the air-dried modes. Representative AFM images confirmed that the surface of dentin conditioned with 20 % PEG-P was significantly rougher than that of 37 % H₃PO₄ group. In-situ zymography of gelatinolytic activity within hybrid layers created with PEG-P conditioning showed negligible fluorescence before and after thermocycling, as compared with the H₃PO₄ group for both wet- and dry-bonding. Antibacterial testing showed that 20 % PEG-P possesses potent antibacterial effects on oral bacteria. Significance: The PEG-P-based selectively extrafibrillar dentin demineralization preserves intrafibrillar minerals, prevents exposure and activation of endogenous dentin proteases, reduces collagen degradation by endogenous protease, and inhibits bacteria retained on dentin, which are important contributor to improve durability of resin-dentin bonds.