A tapping mode AFM study of collapse and denaturation in dentinal collagen

Objectives: Tapping mode atomic force microscopy (AFM) was used to investigate the surface changes of collagen exposed to phosphoric acid treatment. We focus on denaturation and collapse following demineralization and exposure to air. Methods: Unpolished dentin disks, obtained from freshly extracted human molars, were etched in 37% phosphoric acid for 15 s, rinsed ultrasonically and gently blotted with soft paper; the specimens were then continuously observed using tapping mode AFM. Results: Immediately after the removal of bulk water, the surface consisted of a porous network of banded collagen fibrils, having periodicities of 67 nm. After approximately 8 min of subsequent air-drying, the spacing between fibrils was lost, and the surface was observed to consist of a dense array of closely spaced fibrils. The banding periodicity was still observable. Significance: The air drying of etched dentin results in the collapse of the collagen network, but not in the denaturation of the collagen fibrils. This study indicates that collapse and denaturation are separate phenomena. It further shows that water loss occurs rapidly, and disrupts the native conformation of the collagen network. This would have adverse effects on adhesion.