# A Literature Review of Adhesive Systems in Density: Key Components and Their Clinical Applications
## 6 Resin Adhesive Systems
### Composition of the Adhesive Systems
#### 6.1.1 Acidic Components  
In the ER strategy, the etching agent applied is 35-37% H3 PO4, with a pH fluctuating between 0.5 and 1 (Figure 6). This pH becomes superior than 1 for the SE strategy using acidic monomers resultant from the esterification of H3 PO4. This acidic pH prepares both the enamel and the dentin substrates to receive the bonding agents [147]. On enamel, the etching phase cleans the enamel, creates a complex 3D microtopography at the surface of enamel, rises the enamel surface area accessible for bonding, creates micropores into which the resin mechanically interlocks (Figure 7), and increases wettability by exposing more reactive surface layer [104; 148]. Three types of enamel etching patterns may be detected under a microscope [148; 149; 150] (Figure 8):  
* Type I of etched enamel: privileged demineralization of the enamel prism core while keeping the prism periphery intact. The matching tags in this case are cone-shaped. * Type II of etched enamel: the inter prismatic enamel is removed preferentially, leaving the prism cores intact. The matching enamel tags are cup-shaped. * Type III of etched enamel: the pattern is less apparent, with portions that match type I and II patterns and others that bore no similarity to enamel prism [148; 150; 151]. Figure 8: Types of etched enamel [79]. Figure 6: The formation of bubbles resulting from the chemical reaction between the acid and a mineral structure (positive sign of a chemical reaction). (Courtesy of Prof [PERSON]). Figure 7: Difference in appearance of etched and unetched enamel rods. (**A**) A view of both etched and non-etched enamel. (**B**) Etched enamel rods. (**C**) Unetched enamel rods [148]. by micro-mechanical interlocking of the resin and enamel. While H3 PO4 cleans the enamel surface, it triggers salts precipitation on the etched surface; these salts might be eliminated more advantageously by means of rubbing the acid and rinsing it, delivering an ideal interface for the bonding [151]. Among the features that affect the acid etching on enamel substrate: the acid form (gel or liquid), the acid concentration and time of etched enamel, the acid type, the chemical type of enamel, the type of dentition (primary or permanent), and whether the enamel is demineralized or fluoridated [148; 152]. On dentin, the etching stage removes or modifies the smear layer, and demineralizes 5-8 um of the ITD matrix to create nanoscale porosities in the underlying collagen fibrillar matrix. Numerous acids or/and Ca chelators are used [153]. For acids: in operative dentistry, the most commonly used acid is 37% H3 PO4 (Figure 9) in both liquid and gel forms. Although H3 PO4 liquid has a lower pH, H3 PO4 gel has a superior etching impact on dentin than liquid acid. This is due to the fact that gel acids stay on the tooth surface longer than liquid acids, and this contact time has a higher influence on dentin depth than pH. Acids not only eliminate the smear layer, but also open the dental tubules, expose the collagen network into which resin monomers infiltrate, and modify the dentin's wetness and permeability [148; 154]. There are many other acids used for conditioning the dentin like maleic acid, nitric acid, oxalic acid, citric acid, and hydrochloric acid [148]. For Ca chelators: these are used to remove and/or alter the smear layer while leaving the surface of dentinal layer undamaged. Ethylenediaminetetraacetic acid (EDTA) is the common chelator used [4; 148]. Among the features that affect the acid etching on dentin substrate are the removal of cutting debris, the effective cavity preparation, the partial conditioning, and the formation of micropores [155]. As a result, the etching acid's form and pH are crucial. The degree and intensity of demineralization are induced by these variables. According to previous research, the primary variables influencing the dentin-conditioner interaction have been determined to be the acid content, osmolality, pH, and viscosity created by the thickening agents [156].