Representative scanning electron microscopy images of permanent enamel after treatment with 37% orthophosphoric acid for 20 s (a); appearances of permanent enamel after treatment with Nova Combo Plus; passive application (b), manual active application (c), and rotary-brush active application (d). Phosphoric acid etched enamel showed no sign of smear layer and more irregular and rough surface properties (a) in comparison to enamel treated with Nova Combo Plus self-etch adhesive alone (b) or with both active application techniques (c, d).