## 4 Discussion  
Universal adhesive systems have very similar chemical compositions; the difference in the proportion and quantity of the elements influence the penetration of the adhesive into demineralized dentine [20]. The adhesives used in this study share some components: a hydrophilic monomer (HEMA, hydroxyethyl methacrylate) that facilitates the diffusion of resin in wet collagen and increases susceptibility to hydrolysis [21], and a hydrophobic monomer (Bis-GMA, bisphenol A-glycidyl methacrylate). They include solvents such as water and ethanol (which are essential for the composition of adhesives that need to be bonded to dentine), silane, dimethacrylate resins, initiators, and stabilizers. The main compound is the 10-MDP monomer, which interacts with the residual hydroxyapatite surrounding the collagen fibers, thus improving bond strength. Furthermore, the bond of 10-MDP to calcium creates a salt (MDP-Ca) that protects against hydrolysis as it is a hydrolytically stable salt [12]. Regarding the differences in chemical composition, MCAP, urethane dimethacrylate, and the Vitrebond copolymer stand out. Using a pH meter (Crison Instruments, SA, Barcelona, Spain), we measured the pH of the adhesives and found that there were differences with the data provided by the manufacturers (Table 1). Due to the rapid growth of adhesive systems and their diverse characteristics, each of them should be analyzed to identify the best mechanical properties and strategy of use. The in vitro mechanical micro push-out test results showed similar adhesion strength between the self-etching FUT and SCO with prior acid etching and a greater, although not significant, difference between SCO and ADH. ADH, with an ultra-high pH of 2.5-3, would have a lower capacity to dissolve the smear layer and demineralize the underlying dentine surfaces when applied as a self-etch [16; 17]. Therefore, this statement could explain the lower fracture resistance compared with the other adhesive systems used in the study. The control SCO has a lower acidic pH, similar to ADH, but its pre-conditioning with orthophosphoric acid would have improved the penetration of the resin into the dentine. Such a pre-etching strategy is more sensitive to the operator, but the presence of MDP, the polyalkenoic acid copolymer (Vitrobond), and silane in its composition allows the hydration of collagen, creating the hybrid layer even in dry demineralized dentine [3]. Vitrebond is the exclusive component in this system, causing a chemical and spontaneous bond to the hydroxyapatite, showing greater adhesion strength than if it were not present [6]. In this study, the two adhesives used as self-etchers present different results: ADH 13.66 \(\leftarrow\) FUT 14.48 MPa, in accordance with their results with the ultra-high pH of the ADH group (2.7-3), and more acidic pH of the FUT group (2.3). The FUT adhesive, with a mild pH degree, obtained similar mechanical results when compared to the control. That result could be due to traces of hydroxyapatite remaining for chemical interaction with a functional monomer [3]. A higher pH does not guarantee a higher bond strength, and the use of different solvents will not influence the results [9]. There could be a lack of knowledge by manufacturers in the application of self-etching agents, which improve properties by increasing the number of coats when applied [22]. The FUT adhesive, in turn, presented in a single dose, would avoid the deterioration of the compound's properties and the evaporation of solvents, besides being a faster and easier application. Regarding the hydrogen ion's potential and bond strength, we conclude that a more basic pH adhesive could mean a lower resistance to fracture. It could improve mechanical properties by acidifying the adhesive by lowering the pH or by applying previous conditioning, taking against the latter option the need for greater dexterity by the clinician and the possible appearance of post-operative tooth sensitivity.