As already mentioned, the mechanism of bonding of resin-based DBAs is via a hybrid layer. This is a micromechanical interlocking of resin around dentinal collagen fibrils that have been exposed by demineralization. The interlocking occurs by the diffusion of the resins in the primer and bonding resin. The formation and structure of the hybrid layer has been extensively studied, and has also been referred to as the resin-impregnated layer, the resin-dentine interdiffusion zone. The thickness of the hybrid layer ranges from less than 1^m for the all-in-one systems to up to 5^m for the conventional systems. The strength of the bond is not dependent on the thickness of the hybrid layer, as the self-etching priming materials have shown bond strengths greater than many other systems but exhibit a thin hybrid layer. Sugizaki  showed that the etching, washing and drying process caused the dentine to collapse due to the loss of the supporting hydroxyapatite. Further work showed that this collapse of the collagen was an impediment to the successful diffusion of the resin to the base of the region of demineralization. To overcome this problem, Kanca  introduced the 'wet bonding technique' which left the demineralized collagen fibres supported by residual water after washing. This allowed the priming solution to diffuse throughout the collagen fibre network more successfully. However, when it comes to clinical practice, it is very difficult to find the correct balance of residual moisture. Sano et al.  showed in their work on nanoleakage that most resin-based DBAs allowed the ingress of silver nitrate along the base of the hybrid layer. However, the clinical significance of this is unclear. It may be a pathway for fluid to affect collagen not coated by resin, and the outcome may be degradation of the bond over time. However, the degree of nanoleakage is very much material dependent rather than system dependent, meaning that there are conventional systems and self-etching priming systems that show small amounts of nanoleakage whereas others show more. For the self-etching systems, these are able to solubilize the smear layer and demineralize the underlying dentine, forming a quite thin hybrid layer [25, 35, 36, 79].
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