# A Literature Review of Adhesive Systems in Density: Key Components and Their Clinical Applications
## 6 Resin Adhesive Systems
### Functional monomers
#### 6.1.4 Other Components
Actually, among the substrates of this bonded area, the adhesive layer has the lowest modulus of elasticity. When subjected to masticatory forces, the adhesive layer experiences higher level of stress among the constituents. In general, stress that surpasses the intrinsic strength of an adhesive layer causes cracks, defects, or failure in the resin-dentinal bond [244]. Usually, adhesive agents do not include filler particles in their formulations [245]. However, from a theoretical viewpoint and in line with composite resin, adding fillers increases the mechanical assets of an adhesive layer [48]. This idea is known as the elastic cavity wall concept [244]. Previously, manufacturers incorporated variable quantities of glass filler particles (1-5 \(\upmu\)m in diameter) in the hydrophobic bonding bottle of ER III adhesive systems [48; 242; 246]. These filled adhesives were loaded up to 40-50 wt%, such as OBFL (Kerr Co, Orange, CA, USA) and PernQuick (Ultradent, South Jordan, UT, USA) adhesives [247]. Due to the fact that these densely filled adhesives performed exceptionally well in previous studies [248; 249], the same method was applied to ER II adhesive systems and SE adhesive systems [250]. Hydrophobic resins are paired with priming and/or acidic monomers in simplified adhesive systems, preventing the inclusion of significantly large amounts of filler. For example, ER II adhesive systems comprise approximately 8.5-15 wt% of fillers (OptiBond Solo (Kerr Co, Orange, CA, USA); One-Step Plus (Bisco, Inc., Schaumburg, IL, USA)) [251; 252], which is less than half of the quantity applied in ER III adhesive systems [49]. Instead of microfillers, nanofillers have been used in the adhesive agents [253]. Nanofilers have the ability to penetrate dentin tubules and the collagen network, thereby enhancing the strength of the adhesive layer [251]. Nanometer-sized silica (pure silicon dioxide) less than 20 nm is typically added as nanofiller [230]. Some ER II adhesive systems (Prime & Bond NT and XP Bond (Dentsply Sirona, York, PA, USA), Adper Scotchbond 2 XT (3M ESPE, St. Paul, MN, USA)) and one-step SE adhesive systems (SE I) (Clearfil S3 Bond (Kuraray, Tokyo, Japan), G-Bond (GC Corp, Tokyo, Japan)) include nanofillers in an amount typically ranging between 5 wt% and 10 wt% [48]. Investigations have denoted that simpler adhesives containing nanofillers might have superior mechanical features than unfilled bonding agents; nevertheless, such variations are considered material-dependent [254; 255]. Moreover, studies have shown that adding nanofillers does not strengthen bonding to the dentin substrate [256; 257]. A detailed examination yielded inconclusive outcomes when filled and unfilled adhesives were compared [258; 259]. All in all, the purpose of fillers is to improve the mechanical and physical properties of adhesives. The nanoparticles are intended to increase the viscosity of the adhesive, so that it can be used in a single layer. They make it possible to obtain a sufficiently thick resin layer above the HL, since even if they do not penetrate the collagen network, the film they generate stabilizes the HL. This charged film creates an elastic buffer zone which decreases the stress generated by polymerization shrinkage. In addition, the presence of fillers would increase the wettability of the adhesive and facilitate the infiltration of monomers into the dentin [260; 261]. However, adhesives become more viscous as filler levels increase (high filler amounts), compromising the wettability of dental substrates [242]. Though, a potential drawback of fillers not penetrating the collagen network is that the adhesive bond might rely more heavily on the mechanical interlocking and less on chemical interactions with the collagen, which could affect the overall durability and integrity of the bond over time. Furthermore, if the fillers are not sufficiently integrated with the collagen matrix, there is a risk of microleakage or weaker BS in the long-term [262].