# A Literature Review of Adhesive Systems in Density: Key Components and Their Clinical Applications
## 6 Resin Adhesive Systems
### Functional monomers
* 11-methacryloyloxy-1,1'-undecanedicarboxylic acid: Monomer patented by Tokuyama company (Tokyo, Japan); information regarding this monomer in the literature is quite sparse, 10 carbon atoms comparable to 10-MDP, with a hydrophobic spacer [48]. * 12-Phenyl-P: Monomer with monohydrogen phosphate group [48]. * 13-HEMA-P: Monomer with methacrylated H\({}_{3}\)PO\({}_{4}\)-HEMA esters group [48]. * 14-HEMA: Monomer with low molecular weight (small size) and often utilized in several adhesive systems [171; 172]. This hydrophilic monomer is frequently included in the formulation of adhesives because of its solvent like nature. It consists of a mixture of hydrophilic and hydrophobic polymers. Hydrophilic monomers are usually transported in a water-soluble solvent (acetone, ethanol, water) to encourage an adequate flow and a penetration in the hydrophilic dentin (to influence the strength of the resulting bonding), while hydrophobic monomers promote the chemical bond with resin layer [172; 173; 17]. This increases stability and helps in preserving hydrophobic and hydrophilic monomers in the mixture by reducing phase separation in the presence of water [174]. Adhesives lacking HEMA monomer might have issues related to phase separation [175]. Although HEMA has numerous positive attributes, it also has drawbacks. HEMA, both in the unpolymerized and polymerized state, certainly absorbs water from the underlying dentin through osmosis and from the outer oral environment [174]. Once polymerized, it can swell, discolor, and contribute to the hydrolysis of the adhesive interface (water blisters become entrapped in the adhesive layer) [48]. Finally, HEMA has been correlated with biocompatibility concerns, as it has been assigned rather substantial allergic potential, and even contributes to probable genotoxicity [176]. All in all, the most notable disadvantages of HEMA are as follow: low polymerization capacity, low mechanical strength, high water sorption, and critical biocompatibility in terms of allergic reaction. When the HEMA concentration declines beneath a critical level, phase separation will happen between the adhesive monomers and water, and a strong air-stream is needed to eliminate the water-containing droplets from the adhesives [177]. High amounts of HEMA might lessen the mechanical characteristics of the resulting polymer [178]. This will result in flexible polymers with inferior qualities. PolyHEMA is basically a flexible porous polymer ('gel'). As such, high concentrations of HEMA in an adhesive may have deteriorating effects on the mechanical properties of the resulting polymer. HEMA also reduces the vapor pressure of water, and probably also of alcohol [48; 178]. High amounts may therefore hinder good solvent evaporation from adhesive solutions. Like all methacrylates, HEMA is vulnerable to hydrolysis, especially at basic pH, but also at acidic pH [178]. Uncured HEMA also has the ability to lower the vapor pressure of water and can make evaporation more difficult through the air-drying stage [48]. The ideal HEMA concentration to obtain higher BSs in primer/adhesive is between 30-40% [179], and 5-25 wt% in single-bottle adhesives [17], though this can depend on the specific adhesive formulation. Despite its effectiveness, the use of HEMA remains controversial. Some studies have shown no significant differences between the clinical performances of HEMA-based and HEMA-free adhesive systems [180; 181]. While other findings revealed clinically significant difference between HEMA and HEMA-free adhesive systems [182; 183]. In current adhesives, manufacturers strive to significantly lower the HEMA contents or even substitute HEMA with other monomers such as methacrylamide monomer [33]. To conclude, it may be stated that in very favorable conditions, the presence or absence of HEMA monomer might not influence the clinical behavior of adhesive systems.