# A Literature Review of Adhesive Systems in Density: Key Components and Their Clinical Applications
## 6 Resin Adhesive Systems
### Universal Adhesives  
Multi-mode universal bonding agents were released a few years ago, continuing the trend of simplifying adhesive methods [25]. This eighth-generation system can be employed with either ER, SE, or SEE procedures, relying on the clinical circumstances and the operator's personal preferences [9; 41; 172] (Figure 19). SEE is frequently indicated, and UAs can be used on either dry or wet dentin [15; 267]. UAs have quickly gained popularity in clinical practice due to their ability to simplify and speed up the clinical bonding process by mixing the primer and adhesive resin in a single bottle [289]. Nonetheless, UAs are still viewed as adhesives that compromise efficiency for simplicity of use since:  
* UAs integrate the primer and the adhesive resin, each with a distinct purpose, in a single application step [15]. * UAs generally contain less resin and more solvent compared to systems with separate primer and adhesive components. This formulation can impact the adhesive layer's thickness and its properties. The increased solvent content aids in better wetting and penetration of the adhesive into the tooth structure, but it may also lead to thinner adhesive layers after evaporation and curing, which could be prone to suboptimal polymerization [9; 290]. UA's film thickness is usually less than 10 \(\upmu\)m, influenced by air blowing pressure and time. This thin layer can result in suboptimal polymerization due to polymerization-inhibition by oxygen, suboptimal adhesive interface stabilization, and reduced ability to absorb stress from the restorative composite [33]. Both excessively thin and overly thick adhesive resin films can negatively impact BS, emphasizing the need for an optimal film thickness to ensure effective adhesion and durability [291]. - UAs are more hydrophilic and do not fully convert the adhesive surface to a hydrophobic situation [292; 293]. - UAs absorb much more water and have lower hydrolytic resistance [294]. UAs have a pH range of 1.5-3.2 and are classified as ultra-mild (pH > 2.5), mild (pH < 2), or intermediately strong (pH between 1 and 2) [41]. To create suitable UAs, extremely precise functional and cross-linking monomers with multifunctional properties are necessary [276; 295]. They must be able to react with a variety of substrates, co-polymerize with resin-based restorations and cements, and have a hydrophobic property to prevent water sorption after polymerization. Furthermore, they must be hydrophilic in order to \"wet\" the dental substrate, which has a high water content [172]. Also, they must be acidic enough to be effective in a SE technique while not being highly acidic for successful polymerization [287]. The initiators of dual- and self-cure resin cement are interrupted in the case of UAs with high acidity. Some manufacturers also claim that UAs can be used as adhesive primers for zirconia, alloys, ceramics, and composites in indirect restoration applications [15; 295]. Additionally, a direct relationship occurs between the pH and the compatibility of UAs with cements and composites made from self- or dual-cure resin. The company says that some UAs are compatible with dual-cure and self-cure composite materials. As a simplification, the more acidic the adhesive, the less compatible it is with the self-cure mode of the dual-cure resin-based materials [287; 296]. This is mainly due to the acid deactivation of aromatic tertiary amines which play a fundamental role in the chemical curing mechanisms of these materials [297]. To alleviate this difficulty, numerous UAs, when used in conjunction with this mode, require the inclusion of a separate \"activator\" (typically arylsulfinate salts) that should be mixed with the UAs if using an amine-containing cement [172]. The thickness of the polymerized adhesive film must be minimal to avoid interfering with the seating of indirect restorations.