# A Literature Review of Adhesive Systems in Density: Key Components and Their Clinical Applications
## 6 Resin Adhesive Systems
### Universal Adhesives
Those silanes contain one or more atoms of silicone. However, silane used for the adhesion of silica-based ceramic in dentistry, known as 3-methacryloxyppoltrimethoxysilane, is primary applied after etching of substrate with hydrofluoric acid. Before incorporating silane directly into the universal bonding agent, the treatment mentioned was the optimal choice. This advancement in UAs allowed for its use without the need for a separate application of a silane solution [172]. In application, UAs including silane simplified the adhesion procedure, but this has presented an unforeseen issue. The chemicals found in the adhesive might compromise the stability of the silane and hinder its performance. This problem, along with the manufacturers' belief that the chemical interaction of silane when used with silica-based ceramics is drastically inhibited when merged with all other monomers present in UAs, has prevented some manufacturers from adding silane to their formulations [172; 310]. Preceding investigations [298; 311] suggested that the incorporation of resin monomerssuch as MDP or Bis-GMA (used in UAs) in silane solutions considerably reduces the effectiveness of adhesion and chemical interaction of silane to silica-based lithium disilicate when compared to pure silane groups. Thus, it was previously advised that a bond to the silicate-based ceramic can be achieved by etching the porcelain with hydrofluoric acid, then applying the pre-hydrolyzed silane (e.g., RelyXTM Ceramic Primer (3M ESPE, St. Paul, MN, USA), Porcelain Primer (Bisco, Inc., Schaumburg, IL, USA), Ultradent Silane (Ultradent Products, Inc., South Jordan, UT, USA)), which is free of added monomers or resins (pure silane) [172]. Another study revealed the instability of silane mixed with the acidic species in UAs. Thus, the application of silane coat to acid etched glass ceramics was proposed [312]. Most UAs are commercialized in a single bottle [9]. Nonetheless, there are two-bottle UAs designed to improve performance by separating specific components. For example, the G2-Bond Universal (GC Corporation, Tokyo, Japan) is a two-bottle universal bonding agent in which the primer and bonding ingredient are in different bottles and applied independently from standard SE II adhesives [299; 313; 314; 315]. The G2-Bond Universal (GC Corporation, Tokyo, Japan) features Dual-H technology, allowing the adhesive to transition smoothly from hydrophilic to hydrophobic properties, optimizing adhesion to both tooth and composite. Unlike traditional adhesives, G2-Bond Universal (GC Corporation, Tokyo, Japan) is HEMA-free, enhancing the bonding layer's durability by making it extremely hydrophobic and less prone to water sorption and deterioration. This composition reduces the risk of gap formation and microleakage, leading to superior long-term performance [299]. Similarly, Tokuyama Universal Bond (Tokuyama Dental Corporation, Tokyo, Japan) and LuxaBond Universal (DMG America LLC, Englewood, NJ, USA) are two-bottle systems, but the contents are meant to be blended before use. The separation of acidic monomers and ceramic primers in these two-bottle systems prevents the deterioration of silane coupling agents, thereby preserving their adhesive properties and extending their shelf-life [15; 70; 202]. HEMA is the main hydrophilic monomer in most UAs. HEMA can disrupt the interaction between 10-MDP and Ca, thereby impeding the establishment of a sufficient bond in 10-MDP-containing adhesives [15]. The presence of HEMA in the same bottle with both hydrophilic and hydrophobic components may result in water sorption and hydrolysis of the adhesive layer, reducing universal bonding adhesion durability [276]. According to previous studies, UAs are material-dependent, and long-term research is still needed to assess the stability of resin-dentin interfaces generated by modern UAs [3; 316]. UAs' clinical performance is heavily influenced by the adhesive strategy, calling into doubt their claimed adaptability in terms of application method in a clinical situation [15].