The golden standard (3 steps)

To not limit the relevance of the information presented to one or another material, a discussion will be held on the conceptual level - how to do, why to do.

The application of acid  35-37.5% for 10-15 seconds (STEP 1) has the primary goal of removing the smear layer accompanied by the dissolution of the mineral surface of dentin and exposure of the collagen fibers, which result in increased tubule openings, allowing for the output of dentinal fluid. With this, the post-conditioning dentin surface becomes extremely moist and with considerable organic content which causes dentin to have low surface energy representing a major challenge to the establishment of successful adhesive bonding. Following the conditioning time, the cavity should be rinsed for 15 to 30 seconds with the aid of an air/water spray.

This step is critical and should be conducted very carefully to ensure the removal of any acid, and debris generated by conditioning.
Then, excesses of moisture must be removed carefully by gentle air-drying, so that the components of the adhesive system are not diluted. Then, the primer is applied to the dentin surface (STEP 2) for which an ethanol/water-based solvent is used as a carrier for the monomer. The primer consists of bifunctional monomers - with hydrophilic molecules (affinity to water) to penetrate the moist collagen network and a hydrophobic part (without affinity to water but a high chemical affinity for copolymerization with adhesive monomers) and acts as a link between the moist surface of the etched dentin and the adhesive agent. Typical amphiphilic monomers are hydroxyethyl methacrylate (HEMA) or TEGDMA. These molecules impregnate the collagen parts by penetration and amphiphilic action.

By penetrating the demineralized surface and filling the spaces once occupied by hydroxyapatite crystals, the components of the primer stabilize the collagen network and promote the evaporation of excess water. The result is the increase of surface free energy of dentin, making it capable to interact with the bonding agent. To ensure good results the primer should be applied to the full extent of dentin with the aid of a disposable applicator in a scrubbing motion for approximately 30 seconds - the optimal time required for infiltration of the hydrophilic and functional monomers. Gentle air-drying is necessary to promote evaporation of the solvent until the surface looks shiny without liquid film. Deficient primer penetration means that unfilled areas remain around some deeper areas of the demineralized matrix. This is referred to as “nanoleakage”. The incidence of postoperative hypersensitivities is of clinically higher relevance and is also a sign of incomplete resin penetration, allowing for fluid movement inside the dentinal tubules. 

The next step is the application of the adhesive resin in the conditioned primed dentin (STEP 3), the adhesive fills the interspaces of the exposed collagen fiber network, penetrating the dentinal tubules, forming extensions or resin tags. Light scrubbing motion for 15 seconds is required. Gentle air blow or a dry brush is used to remove excesses and then is light-curing is done. Light curing of the adhesive resin is mandatory for the protection of the hybrid layer. This results in a resin-infiltrated collagen network, originally called the hybrid layer by Nakabayashi et al in 1982. This region between the adhesive and dentin components extends from the area of dentin unaffected by etching until the surface of the exposed collagen fibers. This fourth-generation system guarantees simultaneous bonding and sealing of the dentin, and thus theoretically prevents bacterial leakage which is presumed to be the cause of most biological failures encountered with adhesive restorations. The fourth-generation adhesives are the first ones able to fully develop the fundamentals mechanisms of adhesion (surface wetting, microretention, and chemical interaction).

To be continued...
References are going to be collected and mentioned... All the related topics are coming one after the other... Stay tuned