The concept of bonding a restorative material to the dentine surface is by no means a new idea. Even at the time of Buonocore using phosphoric acid to bond to enamel, the idea of bonding to dentine was considered. However, due to limitations of materials and knowledge of the structure and nature of dentine the dream remained just that until the late '70s. In fact Buonocore did try to introduce a dentine adhesive but was unsuccessful . The earliest bonding agent which showed some success was introduced by Fusayama . At the same time Bowen  in the USA started investigating new formulations of resins that were more water tolerant as well as methods of treating the dentine with oxalates to gain adhesion. The concern of many clinicians at that time was the potential damage phosphoric acid was going to cause the dental pulp if dentine was etched . The first work to investigate the mechanism of bonding to the dentine was by Nakabayashi . His paper of 1982 has now become one of the classic papers to first identify a layer between the resin and dentine substrate referred to as 'hybrid' dentine, in that it was the organic components of the dentine that had been permeated by resin (Fig. 2). The term 'hybrid layer' has now become synonymous with bonding of resins to etched dentine. There has been a tremendous amount of research done on the hybrid layer, its structure, formation and how it can be improved. Without a hybrid layer a bond will not be formed to the dentine. Therefore, it is essential for some modification to be made to the dentine surface so a mechanical interlocking of resin around dentinal collagen can occur. This layer has also been referred to as the 'resin-dentine interdiffusion zone' .
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