# A Literature Review of Adhesive Systems in Density: Key Components and Their Clinical Applications
## 6 Resin Adhesive Systems
### Functional monomers
For that reason, a HEMA-free adhesive system (like Prime & Bond Universal (PBU); Dentsply DeTrey GmbH, Konstanz, Germany) hypothetically has a stronger ionic binding potential compared to a HEMA-rich system. MDP is durable in an oral environment due to its hydrolysis stability and more favorable for chemical bonding than 4-MET and Phenyl-P [17; 192]. In addition, previous research demonstrated that zinc can be a competitor to MDP-Ca salts, not only HEMA or other components [193]. In 2017, [PERSON] et al. [194] suggested that UAs with less concentration of 10-MDP generate weak adhesion. Similarly, [PERSON] et al. [195] in 2018 indicated that the rate of 10-MDP-Ca salt formed by the demineralization of enamel and dentin relies on the components that are found in the commercial adhesive, rather than on the MDP and water concentrations. In 2018, [PERSON] et al. [196] proposed that higher bonding effectiveness of 10-MDP must not only be attributed to their more intense chemical bonding to tooth substrate, but to its higher etching potential as well (by measuring the HAp-dissolving capacity). Other functional monomers (GPDM, Phenyl-P, 4-META) lacked this combination. In 2018, [PERSON] et al. [197] pointed out that the minimal toxic concentration of the functional monomer 10-MDP can stimulate an inflammatory response and suppress an odontoblastic differentiation. In 2019, [PERSON] et al. [192] proved that HEMA-free adhesives are preferred since HEMA prevents the MDP-collagen protection. In 2021, [PERSON] et al. [198] showed that adhesives including 10-MDP presented higher bonding capacity than materials formulated with other acidic ingredients, although this outcome was dependent on the type of mechanical testing, the nature of the substrate, the acidic content of the adhesive, and the application category of the adhesive. In 2022, [PERSON] et al. [199] demonstrated that adhesives based on 10-MDP preserve the collagen inside the HL by concurrently enhancing collagen's resistance to exogenous enzymes and hindering matrix metalloproteinases (MMPs) activity, both of which provide the durability of resin-dentin bonding. In 2023, [PERSON] et al. [200] demonstrated that the combination of HEMA and 10-MDP monomers in self-etching adhesives had no effect on the clinical performance of restorations in terms of retention, postoperative sensitivity, and secondary caries incidence. However, this combination favorably affected the marginal adaption and marginal staining during the two-year follow-up. 16. GPDM: In 1949, [PERSON] invented the first adhesive technique by combining GPDM in a liquid cavity sealer with a chemically cured resin-based restorative substance (Sevriton(r)) [201]. GPDM is a monomer with two methacrylate groups linked by a short carbon spacer to one P acidic functional group. Due to the presence of two polymerizable methacrylate groups, GPDM can promote better polymer formation than 4-META, 10-MDP, and HEMA [17]. The hydrophilicity of GPDM aids in adhesive infiltration into the demineralized dentin due to having similar characteristics to HEMA monomer [13]. GPDM also enhances the interaction between the bonding agents and HAp, comparable in function to 4-META which forms an unstable GPDM-Ca salt. GPDM behaves in a decalification route, but unlike 10-MDP, it does not expose collagen, but rather promotes the creation of a thick HL with exposed collagen. However, the variances in chemical bonding do not eliminate the actual bonding effectiveness of adhesive systems comprising GPDM [13; 17]. The interaction between GPDM and the co-monomers should be further assessed to gain a better understanding of the positive outcomes with regards to bonding durability of adhesive systems containing this monomer [17; 33]. * PENTA-P: This monomer is characterized by P ester monomers containing carbon-carbon double bonds and a P group [-OP(=O)(OH)\({}_{2}\)]. When compared to 10-MDP functional monomer, the chain of PENTA is shorter.