# A Literature Review of Adhesive Systems in Density: Key Components and Their Clinical Applications
## 1 Introduction  
Throughout the years, adhesive systems have undergone significant advancements in modern restorative dentistry. Historical reviews highlight the evolution of these materials and their impact on dental practice. For instance, early developments in adhesive technology laid the groundwork for current innovations, as detailed in works such as this review and another historical perspective [1; 2; 3]. Despite these advancements, challenges persist regarding the long-term durability of the resin-dentin bond interface, as noted in recent studies [1; 3]. Bonding to enamel structure has become a predictable and well-established procedure [2], whereas bonding to dentin, due to its heterogenous structure and histology, is considered challenging [3; 4]. Indeed, the attainment of optimal interdiffusion of the adhesive system within collagen fibrils and maintaining stability at the resin-dentin interface are critically significant [5]. As a result, it is fundamental to understand the mechanism of dentin hybridization during which an interdiffusion zone, also known as a hybrid layer (HL), is formed, leading to the development of micro-mechanical retention of the dental restoration [6]. Thereby, the HL is made up of residual hydroxyapatite (HAp), solvents, collagen, and resin monomers; its level of strength is ultimately influenced by each component's resistance to degrading occurrences [7; 8]. During adhesion procedure, the mineral components are partially or totally removed by acidic monomers in self-etch (SE) or etch-and-rinse (ER) adhesive systems [9]. After the dentin and the enamel substrates have been totally etched with phosphoric acid (H\({}_{3}\)PO\({}_{4}\)), ER adhesives are applied, and thus the smear layer is removed [10]. On the contrary, the acid etching phase is removed for dentin with SE adhesives since they include monomers with acidic functional groups that concurrently etch and prepare the dental substrate [11]. Thus, the smear layer is modified and incorporated in the hybridized complex [12]. ER adhesive systems have two or three application step variations, whereas SE adhesive systems have one or two [9]. With regard to the ER adhesives, OptiBond FL (OBFL, Kerr Co., Orange, CA, USA), a three-step ER (ER III) adhesive, is recognized as one of the gold-standards of multiple bottle systems [10]. The adhesive used (OBFL) contains a highly hydrophobic bonding ingredient called glycerol-phosphate dimethacrylate (GPDM), which can chemically react with HAp in the etched and prepared enamel and dentin substrate to improve bond strength (BS) [13]. SE systems were then implemented, which eliminated on dentin the use of H\({}_{3}\)PO\({}_{4}\) and the accompanying rinsing phase [9]. Clearfil SE Bond (CSE, Kuraray Noritake Dental, Tokyo, Japan) has been documented as the gold-standard in this class; the primer and the adhesive of this system consist of 10-methacryloyloxydelyd dihydrogen phosphate (10-MDP), which interacts with calcium (Ca) to establish a strong and stable bond, hence forming nano-layers of 10-MDP-Ca [14]. Most recently, manufacturers have developed broader-ranging adhesive systems that can be utilized in both bonding procedures (ER and SE), in addition to the selective etching of enamel margins (SEE). The manufacturers propose that practitioners adopt bonding procedures based on their preferences and the type of tooth structure. This innovative set of dental adhesives, known as \"multi-mode\" or \"universal adhesives\" (UAs), is the newest version of dental adhesives [3; 9; 15]. Recent advancements also include the incorporation of nanoparticles in adhesive systems, which can significantly enhance the properties of adhesives. Nanoparticles improve the mechanical strength and durability of the bond by interacting at a microscopic level with the dental substrate [16]. Presently, these dentin bonding agents consist of monomers containing both hydrophobic and hydrophilic groups, along with polymerization modulators and relatively high levels of organic solvents [17].