Multiparameter evaluation of acrylamide HEMA alternative monomers in 2-step adhesives

As frequently added to adhesives, the mono-functional monomer 2-hydroxyethyl methacrylate (HEMA) acts as co-solvent and improves surface wetting. Nevertheless, HEMA promotes watersorption and hydrolysis at adhesive interfaces, affecting bond durability to dentin. This study investigated if two acrylamide co-monomer alternatives could replace HEMA in experimental adhesive-resin formulations as part of 3/2-step universal adhesives applied, respectively, in etch-and-rinse (E&R) and self-etch (SE) bonding modes. Upon priming dentin with the 10-MDP-based Clearfil SE Bond 2’ primer (‘C-SE2p’; Kuraray Noritake), three experimental adhesive resins, consisting of 50 wt.% Bis-GMA, 15 wt.% TEGDMA, and either 35 wt.% diethyl acrylamide (‘DEAA’), hydroxyethyl acrylamide (‘HEAA’) or HEMA (‘HEMA+’), were applied. The control HEMA-free adhesive resin contained 60 wt.% Bis-GMA and 40 wt.% TEGDMA (‘HEMA−’). All adhesives were evaluated for ‘immediate’ and ‘aged’ micro-tensile bond strength (μTBS) to dentin upon, respectively, 1-week (1w) and 6-month (6m) water storage, TEM adhesive-dentin interfacial interaction, 24-h and 6m three-point bending, contact-angle wetting, viscosity and watersorption. Linear mixed-effects model statistics revealed significantly better bonding performance of the adhesives applied in E&R than SE mode, except for DEAA_1w, with the highest μTBSs recorded for DEAA and HEMA− applied in SE mode. In E&R mode, aging did not significantly reduce DEAA’s μTBS. Best wetting on primed dentin was recorded for HEMA+, significantly better than DEAA, further HEAA and HEMA−, these directly related to their viscosity. HEAA absorbed significantly more water than all other adhesive-resin formulations. HEMA−>DEAA>HEAA>HEMA+ was the significant order for 6m bending strength. The acrylamide co-monomer DEAA could replace HEMA, while HEAA not.