Triacrylamide-Based Adhesives Stabilize Bonds in Physiologic Conditions

Abstract

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.

Keywords: acrylamides; biofilm; dental adhesives; dental restoration failures; mechanical testing; tooth demineralization.

Figure 1.

Experimental design used in this study. The total number of specimens was 60. HEMA, control; TMAAEA, triacrylamide; TU, thiourethane.

Figure 2.

Measurements of perimeter gap length and occlusal gap width for all groups tested in the presence of Streptococcus mutans (biofilm groups) or water. Measurements were made with the scanning electron microscopy images obtained for the initial and posttest replicas, as shown in Appendix Figures 5, 8, and 9. Data were analyzed with 2-way analysis of variance (material and mechanical challenge) for each storage condition. Initial condition results with similar uppercase letters and incubator/bioreactor results with similar lowercase letters are statistically similar (α = 5%). P values for the different comparisons are shown in Appendix Table 5. Values are presented as mean ± SD. HEMA, control; TMAAEA, triacrylamide; TU, thiourethane.

Figure 3.

Measurements of gap depth and axial gap width and percentage of bacterial infiltration and demineralization for all groups tested in the incubator or bioreactor in the presence of Streptococcus mutans. Measurements were made through the confocal laser scanning microscopy images (5× and 40× magnification) obtained for the samples after testing. Data were analyzed with 2-way analysis of variance (material and mechanical challenge). Similar lowercase letters or connection by a horizontal bar within the same graph indicates statistically similar results (α = 5%). Values are presented as mean ± SD. HEMA, control; TMAAEA, triacrylamide; TU, thiourethane.

Figure 4.

Maximum intensity projection confocal images (40× magnification) of the adhesive interface of the cross-sectioned samples. The gap width in these images represents only 1 point of measurement in relation to the whole marginal interface, while the scanning electron microscopy images provide a survey of the entire margin. The axial gap depth is calculated as a percentage of the total preparation depth (at the resolution provided by the confocal laser scanning microscopy method), as shown in Appendix Figure 7. In those calculations, the composite is used as a reference for the initial depth, since the demineralization reduces the height of the dentin wall. HEMA, control; TMAAEA, triacrylamide; TU, thiourethane.

Figure 5.

Dentin microtensile bond strength (left) and polymerization kinetics results (right). The bar graph displays microtensile bond strength values for all materials at 24 h after being stored for 7 d in water or biofilm. The UDMA-HEMA adhesive was the only group with pretest failures: 9% of the sticks did not survive the storage period. Different lowercase letters indicate significant differences among all groups (2-way analysis of variance), and an asterisk indicates statistical difference between the initial and 7-d microtensile bond strength values within the same material (1-way analysis of variance; single bond, P = 0.001; HEMA, P = 0.010; TMAAEA, P = 0.456). The line graph displays the rate of polymerization as a function of degree of conversion for 1 representative specimen. Average values (n = 3) for maximum rate of polymerization (Rp) with its corresponding degree of conversion (DC) values, as well as final DC, are also indicated in the graph. Different letters within the same variable indicate significant differences among the adhesive systems. Values are presented as mean ± SD. HEMA, control; TMAAEA, triacrylamide; UDMA, urethane dimethacrylate.