When dentin is etched with phosphoric acid, the mineral content of the dentin (hydroxyapatite) is removed, and demineralized collagen is left behind. To bond to dentin, an adhesive is applied, and it infiltrates into the demineralized collagen to form a hybrid layer. If dentin is over-etched, more mineral is removed, and the depth of demineralized collagen increases. Now, the mostly hydrophobic adhesive does not want to fully infiltrate the somewhat wet collagen. Therefore, there are voids at the bottom of the hybrid layer. These voids are more susceptible to hydrolytic or enzymatic degradation.
