Superhydrophobic Modification of Hydroxyapatite Decreased Bacterial Adhesion and Calcium Releasing

Methods: Hydroxyapatite (HA) disc specimens were immersed in ethanol solution of fatty acids (FAs) with different carbon chain length (n=1, 2,..., 18) for 12 h, to form the self-assembly film of FAs on the HA native surface. The contact angle (CA) was measured to obtain hydroxyapatite disc specimens with different wettability. Superhydrophobic modified hydroxyapatite pellet specimens (n = 7), after coated with human saliva, were incubated in Streptococcus mutans suspension. The amount of bacteria bounded to the HA surface were counted as colony-forming units. HA specimens (n = 10) modified with ethanol solution of FAs with carbon chain length (n = 12) were immersed in an artificial caries demineralization solution. The calcium resolved in the acid solution was measured by atomic absorption spectoroscopy at 1 h, 8 hrs, 24 hrs and 48 hrs.

Results: The wettability of the modified surface exhibited distinct dependence on the chain length of the FA molecules. While the CA increased significantly with the increase of the chain length and the chain length was long (n ≥12) enough, the surface was superhydrophobic with CA larger than 150 degree. Comparing with the control group (1.698 ± 0.635 CFUx10³/ml), FA modified HA resulted in about 50% reduction (p < 0.05) of the bacteria adherent to the HA surface (0.845 ± 0.169 CFUx10³/ml). Calcium was significantly less released in the modified group at each time point as comparing to the control (p < 0.01). There was even a trend that the difference of calcium releasing in the two groups would be enlarged as time goes.

Conclusion: Superhydrophobic modification could efficiently reduce both the oral bacterial adhesion on HA surface and the releasing of calcium from HA.