# A Literature Review of Adhesive Systems in Density: Key Components and Their Clinical Applications
## 5 Resin-Dentin Bonding
### Factors Influencing Adhesion  
#### 5.3.1 Wetting  
The adhesive, often a viscous fluid, is made of a material or film that joins two substrates and solidifies them [108].  
The adherent is the initial support, material or substrate on which the adhesive is applied. The types of adherents are enamel, dentin, metal alloy, and ceramic material [79]. Indeed, the contact of the adhesive with the adherent depends on the wettability of the substrate and its surface free energy [108].  
Wettability is the ability of a liquid to come into intimate contact with a solid. It is the phenomenon that controls the spread of a drop of liquid on the surface of a solid, when a drop of liquid is deposited on the surface of a solid; either the liquid spreads(on the so-called \"wet\" surface), or it forms a drop, making an angle of contact with the surface of the solid [109]. The wettability and the spreading velocity are also reliant on the chemical composition of adhesive agents [110]. As the viscosity of an adhesive increases, it becomes more challenging to achieve proper wetting of a substrate [111]. Wettability studies are generally based on the measurement of contact angles (CAs) as main data, which designates the degree of wetting when a solid and a liquid interact [112; 113]. Small CAs lower than 90\({}^{\circ}\) (<90\({}^{\circ}\)) correspond to high wettability, while large CAs (>90\({}^{\circ}\)) correspond to low wettability [109]. The contact angle (CA) is defined as the angle formed by the intersection of the liquid-solid interface. Surface energy is also called surface tension or interface energy [109; 112; 114]. Surface tension is a force that exists at any interface between two different media (between a solid or a liquid and a gas). The tension between identical media--whether it is two solids, two liquids, or even between a liquid and a solid--is typically referred to as interfacial tension. This force is what enables a drop of water to resist spreading on paper or allows certain insects to walk on water, often associated with the concept of capillarity [109].  
When dentin and enamel are demineralized by acid-etching, different changes in the surface energy of substrates could be observed. The surface free energy of enamel is improved, simplifying wettability [115]. Nonetheless, in dentin, the opposite will occur; while mineral is removed and collagen fibrils are exposed, surface free energy decreases [116]. To enhance the surface energy of dentin, it is essential to use an appropriate adhesive system that promotes good wettability [117].  
Finally, for optimal adhesion, the adhesive must completely \"wet\" the surface to be bonded [79; 108; 109]. \"Wetting\" means that the adhesive flows and covers a surface to maximize the contact surface and the forces of attraction between the adhesive and the bonding surface. Complete wetting occurs at a CA of 0\({}^{\circ}\) and no wetting occurs at an angle of 180\({}^{\circ}\). In order, for a liquid adhesive, to effectively wet a surface, the surface tension of the adhesive must be lower than the energy of the surface of the substrate must be increased [108; 109].  
Micro-tensile bond strength (\(\upmu\)TBS) of composite to dentin cavity floor has been shown to be affected by several factors, including denitinal tubule density, diameter, and sclerosis degree, as well as the presence of sclerotic dentin. Other influential factors encompass cavity preparation design, such as cavity shape and size, as well as the presence of bevels or chamfers. Additionally, the adhesive type and application protocol, the presence of contaminants like saliva or blood, and the method of polymerization can significantly impact \(\upmu\)TBS values. Furthermore, factors related to substrate characteristics, such as dentin age, composition, and moisture content, play a crucial role in determining BS outcomes. Additional considerations include the presence of secondary caries, thermal and mechanical stresses, and the use of desensitizing agents or cavity liners [118; 33; 119].