Acid conditioning of enamel was recognized and considered an established procedure. The most usual classification of DBA divided them into generations. It is important to point out that it is not yet understood that the achieved adhesion is promoted by micro-mechanical retention, and not by chemical means. The dentin bonding system is applied on the dentinal surface and it is expected to form a hybrid layer. This hybrid layer was first described by Nakabayashi and was defined as formed by the definitive interlocking of collagen fibers and the dentin bonding agent.
Smear plugs have the same composition as the smear layer and are located inside dentinal tubules.
To form a hybrid layer it is necessary to remove superficial smear layer and also smear plugs. Analyzing dentin conditioning agents, acid solutions have been considered chemical agents. It is known that any acid solution shows all ideal characteristics of an etching solution, but it is important to know these characteristics in order to choose a solution which is closest to the ideal one. Another factor that must be considered is the diffusion of acid solution through the dentin surface and, consequently, its etching capacity. This factor depends on the viscosity of the acid solution and the molecular weight of the acid.