Effect of nanogel-modified adhesives on dentin bonding

Reactive nanogel additives have been shown to enhance bulk wet strength of dental adhesive resins. Objective: Investigate the effects of composition, molecular weight (MW) and intrinsic viscosity (IV) of nanogels as organic nanofillers on dentin shear bond strengths (SBS) in etch-and-rinse dentin adhesives. Methods: Five nanogels with co-polymerizable functional groups were prepared with either isobornyl methacrylate (IBMA)/urethane dimethacrylate (UDMA), IBMA/ethoxylated bisphenol-A-dimethacrylate (BisEMA), or di-tert-butylphenol urethane-methacrylate (DTBUMA)/bisphenol-A-glycidyl methacrylate (BisGMA) using light (BAPO) or thermal (AIBN) polymerization (Table). Five experimental adhesives were composed of 70wt% BisGMA/HEMA (70:30 mass ratio), 15wt% acetone, 14wt% nanogel, 1wt% BAPO. Two controls are Scotchbond multi-purpose-plus (SB) and nanogel-free adhesive (Ctrl-A: 84wt% BisGMA/HEMA, 15wt% acetone, 1wt% BAPO). Forty-two caries-free molars (n=6 per group) were prepared to expose mid-crown dentin, acid-etched for 15s, rinsed, primed with Scotchbond primer, followed by the experimental or control adhesives and light-cured for 10s. Z100 composite was then placed and light-cured for 1min. SBS (MPa) was tested after 24 h water storage. Results: One-way-ANOVA found no significant differences among the nanogel-containing and the control groups. IBMA/Bis-EMA showed significantly higher SBS than IBMA/UDMA/BAPO with low MW (p<0.05). Increasing IV in BAPO-cured nanogels resulted in higher SBS (R²=0.76). SEM of debonded dentin surface showed mostly cohesive failure in adhesive or composite resin. Confocal laser scanning microscopy proved the infiltration of a fluorescent-labeled IBMA/UDMA nanogel along with resin monomer into demineralized collagen to form a 5-µm-thick hybrid layer. Conclusions:  Nanogel-modified adhesives did not reduce SBS, but may improve long-term durability based on the potential for better retention of initial bulk adhesive properties. Support: NIH/NIDCR 1RC1-DE020480-01