Single-step, self-etch adhesives behave as permeable membranes after polymerization. Part II. Silver tracer penetration evidence

Purpose: To investigate the existence of potential water-filled channels in resin-dentin interfaces of some single-step, self-etch adhesives. Methods: In Experiment I, Prompt L-Pop (3M ESPE) or an experimental adhesive RZ II (Sun Medical) were bonded to hydrated dentin and then coupled with a light-cured composite that was light-activated immediately upon application. In Experiment II, RZ II and FuturaBond (Voco) were similarly bonded to hydrated dentin. Unfilled resins were coupled in the dark to die bonded dentin using the chemical-cured mode, or light-cured mode with a 20-minute delay before light-activation. Bonded assemblies were challenged with ammoniacal silver nitrate as a tracer and examined with transmission electron microscopy. Results: In Experiment I, two types of silver patterns were observed within adhesive layers and hybrid layers: interconnecting reticular, finger-like and tree-like patterns that were collectively named "water trees", and discrete silver grains. In Experiment II, apart from the patterns described above, silver-filled water blisters were also observed between the adhesive and the unfilled coupling resin in RZ n. They were linked to the pre-existing "water trees" in the adhesive layers via multiple, short water bridges. In FuturaBond, similar blisters were located between the first adhesive layer and the second adhesive layer that was light-activated after placement of the coupling resin.