Multi-parameter Evaluation of Acrylamide HEMA-alternative Monomers in 2-Step Adhesives.

Objectives: 2-hydroxyethyl methacrylate (HEMA) is frequently added to adhesives as co-solvent and to improve surface wetting. Nevertheless, HEMA promotes water sorption and thus hydrolysis at adhesive interfaces, thereby affecting bond durability to dentin. This study investigated if two acrylamide co-monomer alternatives could replace HEMA when 2-step adhesives were applied in etch-and-rinse (E&R) and self-etch (SE) bonding modes.
Methods: Bur-cut dentin was primed with the 10-MDP-based Clearfil SE Bond 2’ primer (‘C-SE2p’; Kuraray Noritake) prior to the application of three experimental adhesives, consisting of 50wt% BisGMA, 15wt% TEGDMA, and either 35wt% diethyl acrylamide (‘DEAA’), hydroxyethyl acrylamide (‘HEAA’) or HEMA (‘HEMA⁺’). A control HEMA-free bonding agent consisted of 60wt% BisGMA and 40wt% TEGDMA (‘HEMA^-’). The split-tooth design involved application in E&R mode on one tooth half versus SE mode on the corresponding half (8 teeth/group). Micro-tensile bond strength (μTBS) of half of the micro-specimens was immediately measured upon 1-week (1w) distilled-water storage (‘immediate μTBS’), with the other half was measured after additional 6-month (6m) water storage (‘aged μTBS’). Statistics involved a linear mixed-effects model with specific contrasts (p<.05). Furthermore, contact angle (adhesive drop on primed dentin), viscosity, water sorption and 3-point bending (24h, 6m) were measured.
Results: Overall, LME values of the adhesives applied in E&R were significantly higher than when applied in SE mode, except for DEAA_1w, with the lowest μTBSs recorded for HEAA and HEMA⁺ applied in SE mode. In E&R mode, ‘aged’ HEMA⁺ and HEMA^- revealed significantly lower LME values than their ‘immediate’ counterparts. Best wetting of the adhesive on primed dentin was recorded for HEMA^-, significantly better than DEAA, and then HEAA and HEMA⁺, these inversely related to their viscosity. HEAA absorbed significantly more water than all other adhesive formulations. HEMA⁺>DEAA>HEAA>HEMA^- is the significant order for bending strength.
Conclusions: The acrylamide co-monomer DEAA could replace HEMA, while HEAA not.