AFM analysis of anisotropic dissolution in dense hydroxyapatite

Dense, polycrystalline hydroxyapatite (HA) ceramics prepared by microwave sintering process were immersed for 1-60s in an acidic solution of pH 3, similar to the circumstance of osteoclastic resorption. Dissolution behavior on the surface of HA immersed in the solution was investigated. The surface topography was evaluated by using atomic force microscopy (AFM) with tapping mode. The erosion of grain boundaries in the dense HA was observed and the grain boundary dissolution occurred to a great extent forming a wide gap (50-100 nm) between grains. Notably, anisotropic dissolution was observed on the HA surface showing two topographically different regions; grains (Grain 1) with relatively intact surface, and grains (Grain 2) involving formation of subgrain boundaries and aligned crystallites in a certain direction. The difference in RMS surface roughness evidenced the distinct dissolution feature of ceramic grains. The surface roughness of Grain 1 increased from 1.45+/-0.30 to 3.11+/-0.63 nm, whereas the roughness of Grain 2 increased from 2.36+/-0.11 to 5.27+/-1.34 nm.