# A Literature Review of Adhesive Systems in Density: Key Components and Their Clinical Applications
## 6 Resin Adhesive Systems
### Functional monomers  
Functional monomers serve several purposes in dental adhesives. Firstly, they improve wetting owing to their hydrophilic nature. Additionally, they function as adhesion-promoting agents, effectively enhancing BS to demineralized dentin [48]. In ER systems, functional monomers are tailored to match the dental primer, mainly aimed at enhancing the wettability of collapsed collagen. On the other hand, in SE systems, functional monomers, often termed SE primers, are acidic in nature. These acids demineralize dental surfaces and facilitate the smooth infiltration of the adhesive resin. SE systems are broadly categorized into two major families: those containing phosphorus groups (like 10-MDP, dipentaerythritol penta-acrylate phosphate (PENTA), and HEMA-P) and those containing carboxylic acids (such as 4-MET and 4-methacryloxymethyl trimellitate anhydride (4-META)) [48; 86]. The acidity of these functional groups determines the etching aggressiveness, with sulfonic acid being the most aggressive, followed by phosphonic, phosphoric, carboxylic acid, and alcohol [168]. However, it is important to note that carboxyl and P groups are susceptible to water degradation [48]. This classification provides valuable insights into the diverse roles and properties of functional monomers in dental adhesive systems, aiding in the selection and optimization of adhesive formulations for clinical applications. There are a variety of functional monomers in dental adhesive systems:  
1. Pentamethacryloyloxyethylcyclohexanosphazene monofluoride (Dentsply company): Monomer with five methacrylate-alkyl chains and a fluoride as a functional group [48]. 2. N-phenylglycine glycolidyl methacrylate and N-tolylglycine glycidyl methacrylate: Monomer with tertiary aromatic amine group [48]. 3. Dimethylaminoethyl methacrylate (DMAEMA): Monomer with tertiary amine group [169]. 4. Methacryloyloxydodecylpyridinium bromide (MDPB): Monomer patented by Kuraray company with antibacterial agent and a methacrylate group. This molecule is hydrophobic when compared to other hydrophilic functional monomers [170]. 5. N-methacryloyl-5-aminoasicylic acid: Monomer patented by Kuraray company and presented in adhesive systems with a salicylic group (desensitizing outcome) [48]. 6. MA: Monomer rarely used in adhesives due to its risk of allergic reactions [48]. 7. Methyl methacrylate (MMA): Old monomer rarely used in adhesive formulation like MA monomer [48]. 8. 4-META: Monomer with hydrophobic aromatic group, functional hydrophilic carboxyl groups, and demineralization assets. The adhesion capacity of 4-META has been well defined in the literature and depends on the interaction between the substrate and the carboxyl groups that substitutes P ions with Ca ions in HAp [17; 33]. The resulting 4-META-Ca presents a low chemical stability [17; 33; 48]. This procedure is followed by the superficial dissolution of HAp due to the attack of hydronium ions resulting from the water protonation reaction with 4-META. After extracting Ca, P, and hydroxyl ions from the apatite surface, the solution turns out to be acidic, leading to the formation of dicalcium phosphate dihydrate precipitate [48]. Previous analysis showed that 4-META-Ca solubilized quicker than 10-MDP-Ca, which leads to decreased molecular stability [34]. This discovery supports the \"Adhesion-Decalcification concept\" (AD concept), which stipulates that the lower the solubility of Ca salt in the acidic molecule, the stronger and more stable the adhesion with the HAp substrate [17; 34]. 4-MET: 4-META + water = 4-MET: Two carboxylic groups attached to aromatic groups with methacrylate group as polymerizable group [48]. * 4-acryloyloxyethyl trimellitate anhydride: Patented by Shofu company (Kyoto, Japan), two carboxylic groups linked to aromatic groups similar to 4-MET except the presence of acrylate group instead of methacrylate group [48].