Multiparameter 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 comonomer 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 (‘CSE2p’; 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 directly 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 after 6m storage.
Conclusions: The acrylamide comonomer DEAA could replace HEMA, while HEAA not.