Triacrylamide-Based Adhesives Stabilize Bonds in Physiologic Conditions

In this study, an acrylamide-based adhesive was combined with a thiourethane-based composite to improve bond stability and reduce polymerization stress, respectively, of simulated composite restorations. The stability testing was conducted under physiologic conditions, combining mechanical and bacterial challenges. Urethane dimethacrylate was combined with a newly synthesized triacrylamide (TMAAEA) or HEMA (2-hydroxyethyl-methacrylate; control) to produce a 2-step total-etch adhesive system. Methacrylate-based composites (70 wt% silanized filler) were formulated, containing thiourethane oligomers at 0 (control) or 20 wt%. Standardized preparations in human third molars were restored; then, epoxy replicas were obtained from the occlusal surfaces before and after 7-d storage in water or with Streptococcus mutans biofilm, which was tested after storage in an incubator (static) or the bioreactor (mechanical challenge). Images were obtained from the replicas (scanning electron microscopy) and cross sections of the samples (confocal laser scanning microscopy) and then analyzed to obtain measurements of gap, bacterial infiltration, and demineralization. Microtensile bond strength of specimens stored in water or biofilm was assessed in 1-mm2 stick specimens. Data were analyzed with analysis of variance and Tukey's test (α = 0.05). HEMA-based materials had greater initial gap measurements, indicating more efficient bonding for the acrylamide materials. When tested in water, the triacrylamide-based adhesive had smaller gaps in the incubator or bioreactor. In the presence of biofilm, there was less difference among materials, but the acrylamide/thiourethane combination led to statistically lower gap formation in the bioreactor. HEMA and TMAAEA-based adhesives produced statistically similar microtensile bond strengths after being stored in water for 7 d, but after the same period with biofilm-challenged specimens, the TMAAEA-based adhesives were the only ones to retain the initial bond strength values. The use of a stable multiacrylamide-based adhesive led to the preservation of the resin-dentin bonded interface after a physiologically relevant challenge. Future studies will include a multispecies biofilm model.

In vitro performance of 2-step, total etch adhesives modified by thiourethane additives

OBJECTIVES

Thio-urethane oligomeric additives have been shown to improve the mechanical properties of dental composites and resin cements. To try to harness those same properties in dental adhesives, in this study, these oligomers (TU) were added to the matrix and/or as filler functionalization of experimental adhesives, and the effects on conversion and mechanical properties were analyzed.

METHODS

BisGMA and HEMA (60/40 wt%) were used as the monomer matrix, made polymerizable by the addition of 0.2 wt% 2,2-dimethoxy-2- phenylacetophenone. 2,6-di-tert-butyl-4-methylphenol was added at 0.5 wt% as the inhibitor. This material was used as the unfilled control (BH). TU oligomers were added at 20 wt % to the matrix (BH+20%TU, unfilled) and/or used as filler functionalization (TF, 10 wt%). Fillers functionalized with methacrylate (MF, 10 wt%) served as the control. The experimental adhesives groups containing fillers were: BH+10%MF; BH+10%TF; BH+20%TU+10%MF; BH+20%TU+10%TF. Flexural properties were tested in three-point bending (wet and dry). Polymerization kinetics was followed in real-time in near-IR. Water Sorption/Solubility (WS/SL, ISO 4049) and Viscosity (rotational rheometry) were also evaluated. For Microtensile bond strength 40 vol% ethanol was added to adhesives, which was applied onto sound dentin from third human molars. The data were analyzed with one-way ANOVA and Tukey post-hoc test, and test t for the comparison between storage time of the microtensile bond strength test (alpha = 0.05).

RESULTS

There was no significant difference between groups when yield strength (YS) and flexural modulus (FM) were evaluated in dry conditions. After water storage, all the groups containing TU in the matrix showed statistically lower YS/FM values. This was true in spite of the statistically higher conversion for those same groups. The maximum rate of polymerization (Rpmax) was higher for BH+10%TF and no significant difference was found for the groups BH and BH+10% MF. The lowest Rpmax values were found for BH+20%TU+10%TF and BH+20%TU. BH+20%TU+10%TF showed the highest viscosity values followed by BH+20%TU+10%MF and BH+20%TU, with statistically significant difference between them. For the microtensile bond strength test at 24h (p = 0.13) and 6 months (p = 0.11) and WS/SL (p > 0.05), no significant difference was found among groups. The storage time (24 h and 6 months) did not affect the microtensile bond strength results.

CONCLUSION

In spite of improving the conversion, the addition of TU in the matrix reduced the mechanical properties of the adhesives tested after water storage. This did not affect the bond strength at 24 h or 6 months.