Innovative self-etching dental adhesives formulated using reactive nanogel additives

Abstract

Purpose: Enzymatic degradation of poorly resin-infiltrated collagen and hydrolysis of the resin-based polymeric matrix are interfacial degradation mechanisms. However, since the depth of demineralization and resin infiltration in self-etching adhesives (SEAs) occurs simultaneously, the hydrolysis of the hydrophilicmonomers requires further investigation. The aim of this study was to evaluate the microtensile bond strength (mTBS) and the confocal interfacial ultra-morphology of innovative nanogel-modified two-step SEAs bonded to dentin. Methods and materials: Nanogel copolymers were synthesized using isobornyl-methacrylate, urethane-dimethacrylate (UDMA, Esstech, USA)in presence of 2-Mercaptoethanol as a chain-transfer agent and photo-polymerized (2h) in toluene solution with bis(2,4,6-trimethylbenzoyl)phenylphosphine oxide (320-500nm, 2500 mW/cm(2)). A two-step self-etching adhesive was prepared using UDMA, 2-hydroxyethyl methacrylate and 10-methacryloxydecyldihydrogen-phosphate (40/25/15wt%). Camphorquinone and N,N-ethyl-dimethylaminobenzoate were added (0.3 and 0.7 wt%, respectively). Ethanol (14wt%) and water (5wt%) were also included to solvate the resin blend to create a control primer, while non-solvated resin was used as the adhesive. The nanogels were added to create an experimental primer (15wt%) and adhesive (25wt%). Clearfil SE-Bond (CSE; Kuraray, Japan) was also used as control SEA. Several bonding primer/adhesive combinations were applied on middle dentin (5 specimens each group), light cured (20s) and resin composite (N’Durance, Septodont, USA) build-ups (5mm) were created and light-cured (40s each 1-mm layer). The resin-bonded specimens beams (0.9mm(2)) were submitted to mTBS after 24h or 6 months of water aging. The results were statistically analysed by two-way ANOVA and Student–Newman–Keuls test used (p 0.05) after 24h. However, only the control nanogel-free primer/adhesive and the primer/adhesive both containing nanogels showed significant mTBS reduction (p<0.05) after 6 months. The greatest resistance to water aging was attained when the nanogels where present only in the adhesive. The CLSM analysis showed nanogels infiltration within the hybrid layer and low micropermeability both after 24h and 3 months of water aging. Conclusion: The use of nanogels within the composition of two-step SEAs improved their long-termdurability due to infiltration within the hybrid layer, so making the resin-dentine interface hydrophobic andmore resistant to hydrolytic degradation.