Structure of the mono-GDMAP and poly-GDMAP functional monomer contained in NEB1 and NEB2. Please note the number of polymerisation groups on each chain.
- Atomic Absorption Spectroscopy (AAS)
Each monomer (0.1 mM) was diluted in ethanol (50/50 vol%), while a calcium-containing solution was prepared by the dissolution of calcium chloride in de-ionized water (0.1 mM). The monomers were mixed (30 sec) with the calcium solution in equimolar ratios equivalent to 4 ppm of calcium to form monomer-calcium salts. Thereafter, the solutions were analysed by atomic absorption spectrophotometry (AA240FS, Varian, Palo Alto, CA, USA) for detection of the concentrations of the unbound calcium ions; the final concentration of calcium was inversely related to the formation of monomer-calcium salts. Pilot studies were undertaken to ensure that only the free-calcium ions (Ca++) were detected by AAS. All solutions were prepared in triplicate (n = 3). Data were analysed with one-way analysis of variance (ANOVA) and Tukey’s test (p < .05).
The results are shown in the graph below, which depicts no significant difference in the concentration of the unbound calcium ions between 10-MDP and GDMA-P
- Infrared Spectroscopy (ATR-FTIR)
Extracted human third molars were sectioned to expose a flat mid-coronal dentine surface by means of a slow-speed water-cooled diamond saw (Isomet, Buehler, Lake Bluff, IL, USA) and ground with wet 600-grit SiC papers (30 sec) to create a standard smear layer. Each monomer (15 mol%) was first dissolved in ethanol/water solution (50/50 vol%) and applied to the dentin surfaces for 30 sec, followed by air-drying (5 sec).
Fourier transform infrared spectroscopy (ATR-FTIR) was performed with the FTIR Nicolet 5700 (Thermo Fisher Scientific, Loughborough, UK) equipped with an ATR crystal. The spectra of untreated dentine and each pure monomer were also examined, as well the spectra of the dentine specimens that were treated with the monomer/ethanol/water solutions, and subsequently rinsed for 1 min in distilled water and absolute ethanol (30 sec), as described by Yoshihara et al. (2011). All spectra were obtained in a range of 1,800 to 800 cm-1, with 8 scans at 4 cm-1 resolution in transmission mode and 2.8 mm/sec mirror speed. The FTIR spectra were obtained in triplicate to evaluate the chemical interaction of GDMA-P and 10-MDP to the dentine substrate.
The FTIR spectra below show that both GDMA-P and 10-MDP remained bonded to the dentine surface after challenge (1 min in distilled water and 30 sec in absolute ethanol) due to their strong chemical affinity (bonding performance) to calcium in dentine.
- Microtensile Bond Strength
Further extracted human molars teeth were sectioned, prepared for microtensile bond strength testing as described by Feitosa et al., (2012), and bonded with the experimental self-etching adhesives containing GDMA-P or the gold standard 10MDP as functional monomer; UDMA, Bis-EMA and water/ethanol solvents as well as photo-initiators were also incorporated in the tested experimental self-etching adhesives. They were applied for 20 sec, gently air-dried, and light-activated for 20 sec with the halogen-lamp XL-2500 (>600 mW/cm², 3M-ESPE, St. Paul, MN, USA). The resin composite Filtek Z250 (3M-ESPE) was used for build-ups. Bonded teeth (n = 5) were immersed in distilled water for 24 hrs at 37ºC and subsequently sectioned into 0.8 ± 0.01 mm² sticks. The sticks were fixed to jigs with cyanoacrylate cement and tested to failure in a universal testing machine EZ-test (Shimadzu, Kyoto, Japan) with a crosshead speed of 1.0 mm/min. The bond strengths of sticks from the same bonded tooth were averaged, and the mean was used as a statistical unit. Sticks that failed prematurely were included as 0 MPa. Data were statistically analysed by one-way ANOVA and Tukey’s test (p < .05). In the graph above it is possible to see that both 10-MDP and GDMA-P present high bond strength (>35 MPa); this high bond strength remains stable over time with no significant reduction (p > .05). after 1 year of water storage.
- SEM and Confocal Images
Both SEM and Confocal microscopy analysis showed that NEB1 and NEB2 were able, when applied in self-etching mode (following the instructions of use), to create a thin hybrid layer (interdiffussion layer) 1-2 um. This is the results of the active pH of such adhesives that is ~3 when applied on the dental tissue and then drop to 1.8-2.0 when activated on the dentine surface via active application of the adhesive. At the end of the application step, the pH gets to 6-7; this is the ideal pH for polymerisation and to avoid further demineralisation underneath the hybrid layer.