# A Literature Review of Adhesive Systems in Density: Key Components and Their Clinical Applications
## 2 Enamel and Dentin--An Overview
The enamel epithelium and dental mesenchyme form the contact between dentin and enamel in the growing tooth. Coronal dentin is covered by enamel, while radicular dentin is covered by cementum [58; 59]. Dentin is the major structure of the teeth, so understanding this complicated tissue in connection to resin-dentin bonding necessitates an outline of the dental organic matrix that is embedded in a collagen fiber mesh [60]. The extracellular organic dentinal matrix (ECM) is constituted generally of a three-dimensional (3D) network of fibrillar collagens, specifically type I collagen (90%) with traces of type V and III collagens [61]. The remaining ECM components are non-collagenous proteins, mostly proteoglycans. Dentin is found in the following substances: sialoproteins, phosphoproteins, bone morphogenic proteins, enzymes, and growth factors [62]. Collagen is thin (diameter = 1.5 nm) and lengthy (300 nm). The bordering collagens are shifted from one another by one-quarter of their length = 67 nm [63]. Fibrillar collagens (types I, II, III, V, XI, XXIV, and XXVII) predominate [64]. The three \(\alpha\) chains possess N-terminal and C-terminal ends (Figure 2). The latter is associated with globular structure formation: the C-terminal propeptide. The Glycine-X-Y triplets of the chains then align to the N-terminus where the globular N-terminal propeptide generates a triple helix. The structure formed by the N- and C-terminal propeptides is called procollagen, which is subsequently converted into type I collagen [60; 61; 64] (Figure 3). Figure 3: (**a**) Structure of the three chains \(\alpha\). (**b**) Initiation of the formation of the triple helix at the level of the C-terminal propeptide. (**c**) Representation of the structure of the procollagen triple helix. ER = Endoplasmic reticulum [61]. Figure 2: Orientation of type I collagen molecules in hard-tissues. N- and C-terminal ends of successive type I collagen fragments as specific biological markers of degradation. (Courtesy of Prof [PERSON]). Bound water molecules occupied the voids inside and between the collagen fibers during the mineralization process of the dentin. Consequently, water is gradually substituted by minerals [64]. There was no evidence of resin infiltration in the mineralized dentin [60;64] (Figure 4). Interest in tooth adhesion is driven by the chemical interaction between the mineral platelets in hard tissues and the acidic monomers of SE and ER adhesive systems [33]. This process builds a channel for the resin to enter and enclose the collagen fibrils, enabling hybridization [3; 9]. In the HL, the interfibrillar gaps between the collagen are adequately large (\(30\pm 11\) nm) [8] which could simplify the penetration of small hydrophilic monomers such as bisphenol A-glycidyl methacrylate (Bis-GMA) (512 Da) and 2-hydroxy ethyl methacrylate (HEMA) (100 Da) [60]. A previous study proposes that molecules greater than 40 kDa were excluded from collagen water infiltration, while molecules smaller than 6 kDa in size diffused easily. While an obstacle occurs in the intrafibrillar space (1.26-1.33 nm), which is made of up to hundreds packed collagen molecules [65], other aspects should be taken into consideration such as the concentration (the ability of adhesive monomers to dislodge water from the collagen is restricted by their relative concentrations: water has a [55 mol/L] concentration, while co-monomers have a [3; 4 mol/L] concentration) and viscosity (lower viscosity of monomeric bonding is obligatory to exhibit collagen water) [60;66] (Figure 5). Figure 4: Representation of the progressive hydration of the collagen glycine–alanine peptide. Top row presents the perpendicular and the bottom row parallel view to the molecular axis at the same hydration level. (**a**) A view of the non-hydrated collagen, with the three peptide chains shown in different colors. (**b**) The first shell of water molecules (blue spheres), directly hydrogen-bonded to carbonyl, hydroxyl, or amide groups on the peptide surface.