One of the important factors in determining the dentinal bonding is collagen. When dentin is etched, smear layer and minerals from dentinal structure will be removed, hence exposing the collagen fibres. The areas where the minerals are removed are filled with water which functions as plasticizer for collagen and keeps it at expanded soft state. This means that the spaces for resin-dentin bonding are preserved. However, these collagen fibres can collapse in dry condition and if the organic layer of matrix is denatured, this will obstruct the resin to bond with dentin and form a hybrid layer.
Because of this, the presence of moist or wet dentin is required to achieve successful dentin bonding. This is due to presence of water miscible organic solvents like ethanol or acetone in the primers. The acetone trails water and hence improves the penetration of the monomers into the dentin for better micromechanical bonding. Also, water will prevent collagen fibres from collapsing, thus making better penetration and bonding between resin and dentin.
In order to get a moist dentin, it is advisable to not dry dentin with compressed air after rinsing away the etchant. Instead, high volume evacuation suction can be used to remove excess water and then blot the remaining water present on dentin using gauze or cotton. The dentin surface should appear glistening.
If the dentin surface is too wet, water will dilute the resin primer and compete for the sites in collagen network, which will prevent hybrid layer formation.
If the dentin surface is too dry, collapse of collagen fibres and demineralized dentin can occur, leading to low bond strength.