The larger portion of adhesive solvents is removed by air drying after adhesive application, but residual water still persists due to lowering of the vapor pressure of water by HEMA. It is known that monomers can infiltrate deeper than the hybrid layer, and that water inhibits polymerization of the adhesives. So it is possible to ABT-888 speculate that a weak area beneath the hybrid layer and ABRZ may be created,

which is partially demineralized, while the penetrated monomers are not completely polymerized [41], due to the phase separation and water existence. In contrast to the ABRZ, it is reasonable to assume that this area is probably more vulnerable to acid challenge, resulting in the formation of typical erosion areas beneath the ABRZ in some adhesive systems [42] and [43]. The ABRZ was discovered using a self-etching primer system. It was initially thought that the ABRZ may be specifically formed below the hybrid layer of adhesives that do not require acid etching of dentin. In order to probe this speculation and further clarify the attributes of this zone, Takagaki et al. evaluated the ultrastructural change of the adhesive–dentin interface after acid–base challenge using an acid etching adhesive

system, 4-META/MMA-TBB resin with three different conditions [44]. Super Bond C&B is methylmethacrylate (MMA)-based, and contains a chemical initiator of a tri-n-butyl borane (TBB) derivative and a functional monomer of 4-methacryloxyethyl AZD9291 nmr trimellitate CHIR-99021 nmr anhydride (4-META), giving an excellent adhesion to dentin, when dentin surface is pretreated with citric acid solution

containing ferric chloride [45], [46] and [47]. The dentin surfaces received one of the following pretreatments: no treatment (NT), 65% phosphoric acid for 10 s (PA) or 10% citric acid–3% ferric chloride for 10 s (10-3). After application of PA or 10-3, the dentin surfaces were rinsed with water and gently air-dried. The mixture of liquid and powder of Superbond C&B was applied on dentin surface with a brush-on technique according to the manufacturer’s instructions to bond a PMMA rod. The bonded specimens were left at room temperature for 30 min to secure the initial polymerization, and then stored in distilled water at 37 °C for 24 h. The SEM photographs of the adhesive–dentin interface after acid–base challenge are revealed in Fig. 6. In the NT group, the hybrid layer was not created at the interface, however, wall lesion (WL) was observed along the interface. Formation of the hybrid layer was observed in both the 10-3 and PA groups; however, an ABRZ was not detected in any of the groups. Nevertheless, without surface conditioning (NT), 4-META/MMA-TBB resin could not bond to dentin, because smear layer on the ground dentin surface prevented monomer penetration into underlying dentin. In the SEM observation after acid–base challenge, no hybrid layer formation was observed.