# A Literature Review of Adhesive Systems in Density: Key Components and Their Clinical Applications
## 6 Resin Adhesive Systems
### Self-Etch Adhesive Strategy  
All SE systems contain a water component [33]. Water is needed to activate the ionization potential of their acidic functional monomers [9]. Thus, water is usually used as a co-solvent in SE adhesives, either alone or in combination with ethanol [14]. As they participate in the polymerization, the rinsing step is not required after their application, so they differ technically from ER adhesive systems [272]. Their acidic monomers may be derivatives of carboxylic acid groups (4-META) or P acid groups (Phenyl-P, 10-MDP, PENTA) [9; 48]. They demineralize and simultaneously infiltrate both the enamel and the dentin substrates [33; 273]. At the dentin level, the mineral phase of the smear layer is dissolved before superficially attacking the 5 \(\upmu\)m of underlying mineralized dentin [8]. The Ca and P ions pass into the solution in the liquid adhesive. The carboxyl groups or phosphates of certain functional monomers can form chemical bonds with the dissolved HAp phases, contributing to an enhanced cohesion of the infiltrated resin after polymerization and probably to better resistance to hydrolysis of this zone [266]. The smear layer is not completely eliminated but infiltrated. After the polymerization process, the organic constituents of this layer are impregnated with the resin of the adhesive, as well as the collagen fibers of the treated dentinal surface [9]. The hybrid zone, therefore, contains both the proteins from the smear layer and those from the dentin [33]. Since the pH of the monomers is in the range of 0.8-2.5, the HL is thinner (0.5-1.5 \(\upmu\)m for mild or moderate SE adhesives) than the one formed after H\({}_{3}\)PO\({}_{4}\) etching (pH \(<\) 1), which is more acidic (5 \(\upmu\)m for ER adhesives) [75]. Nevertheless, it is clear that adhesion to dentin does not depend on the thickness of the HL [274], nor the length of RTs [275]. The acidity of SE primers can be significant in terms of enamel efficiency and the durability of the bond they form [15; 33]. In 2003, [PERSON] et al. [263] classified SE adhesives according to their acidity: primer and/or strong acid adhesives (pH \(<\) 1), intermediate (pH = 1.5), and those with weaker acids (pH \(>\) 2). Thus, there are self-etching systems that require the successive application of two different products (SE II) and others that require only one application (SE I) [9]. Usually, SE adhesive systems do not provide a selective demineralization of the enamel, similar to that with 35% H\({}_{3}\)PO\({}_{4}\)[9]. Thus, SEE in a separate step with 35% H\({}_{3}\)PO\({}_{4}\) has been recommended [190; 276] (Figure 17). SE II systems utilize a two-bottle approach, with the first bottle containing a self-etching primer and the second containing the adhesive resin [15]. The self-etching primer modifies the smear layer on the surface of the dentin and incorporates the products into the coating layer [266]. These systems were introduced in the latter part of the 1990s and early 2000s. They dissolve the smear layer when applied and do not require rinsing; thus, they sought to eliminate the etching step, or chemically include it in one of the other steps: acidic primer applied to the tooth first, followed by an adhesive (self-etching primer + adhesive) [9], or (self-etching adhesive): two bottles or one dose containing an acidic primer and adhesive; a drop of each liquid is mixed and applied to the dental substrate [277]. Obviously, the biggest advantage of this system is that their effectiveness seems less dependent on the hydration state of dental surface [9; 278]. Unfortunately, early evaluation of these systems showed sufficient bonding to the etched dentin (41 MPa at 24 h), whereas bonding to enamel was less effective (25%) [9]. This could be related to the fact that they are constituted of an acidic solution with insufficient pH to etch the enamel properly, which is difficult to keep in place and must be continually refreshed [279].