# A Literature Review of Adhesive Systems in Density: Key Components and Their Clinical Applications
## 6 Resin Adhesive Systems
### Universal Adhesives
Several systematic reviews examined which application mode (SE vs. ER) was most effective for adhering to dentin and enamel. When utilizing mild adhesives, resin-dentin bonds were similar, but when using ultra-mild adhesives, they were significantly different (favoring the ER strategy) [276; 317; 318]. Furthermore, data suggests that SEE with H3 PO4 prior to the application of UAs is an appropriate method for the amelioration of adhesion [15]. To enhance the performance of UAs, several factors must be considered. Firstly, ensuring complete solvent evaporation is crucial, as all adhesive systems use solvents like acetone, ethanol, water, or a combination of them. Inadequate evaporation can lead to incomplete polymerization of the resin, NL, and reduced BS. Extending air-drying times beyond the manufacturer's recommendation or using a warm air stream can help mitigate these issues [27; 40; 319]. Secondly, proper clinical technique is essential. UAs typically suggest scrubbing the adhesive for at least 20 s, followed by adequate drying and curing with a high-quality light. Adherence to these guidelines is vital for optimal results [172; 33]. Additionally, checking the expiry date is important as bonding agents can degrade over time, particularly when exposed to high temperatures. Refrigeration can help maintain their efficacy, but adhesives should be brought to room temperature for at least 30 min before use [320]. Frequent bottle openings can also lead to solvent evaporation, so storing UAs in a refrigerator and securely recapping them after each use is recommended [70]. Studies have shown that the bonding performance of some adhesives decreases towards the end of their shelf-life, while others remain stable, emphasizing the need for careful shelf-life management. Furthermore, HEMA-free formulations using methacrylamides can contribute to an extended shelf-life [70,202]. Reading and following the specific instructions for each adhesive system is also critical, as different systems have unique placement and handling requirements [278]. Pre-etching with mildly acidic UAs can improve enamel BS but not dentin BS [321]. Lastly, the viscosity of the adhesive matters; applying two coats of very thin UAs can achieve the highest BS, while a single coat of more viscous adhesive is sufficient [322]. The advantages and disadvantages of UAs are summarized as follows:  
Advantages:  
* Suitable for ER and SE adhesives, as well as SEE, making them highly versatile [9]. * In SE mode, there is a potential for chemical interaction with HAp, thus, dentin margins are properly sealed [323,324]. * Application of this class of adhesives in SE mode with a scrubbing technique enhances the BS to enamel substrate [325]. * Keeping the dentin moist when employed in ER mode is not essential [9,324,326]. * Suitable for a wider range of restorative procedures [172]. * Used as zirconia primers [172]. * The Product Research and Evaluation by Practitioners Panel, a research group, reviewed the handling characteristics of UAs and determined that potentially all UAs scored highly for simplicity of use [327,328]. * Patient and operator factors may have a greater impact on restoration longevity than the adhesive used. When using UAs in clinical settings, isolation with a rubber dam is recommended, and moisture control is crucial. However, the reduced number of steps and the associated time savings may also be considered important advantages [124,324,329]. Disadvantages:  
* UAs do not propose etching dentin, necessitating a separate enamel acid-etching process. This increases clinical application time [172]. Clinical investigations have shown that the SE method has a lower retention rate than ER and SEE on enamel [330]. * UAs must be mixed with a dual-curve activator when used with self- or dual-curve composite materials, including build-up materials and resin cements with aromatic tertiary amines in the initiator system [331,332]. * When H\({}_{3}\)PO\({}_{4}\) is used to etch dentin, they do not effectively seal the margins [333].