Rate of Bonelike Apatite Formation Accelerated on Polarized Porous Hydroxyapatite

To estimate the polarization effect in porous hydroxyapatite (HA), bonelike apatite formations on the unpolarized and polarized pore walls in simulated body fluid (SBF) were compared by emphasizing the relationship between the deposition rate and the depth position of the pores. Two kinds of HA ceramics, which were commercial porous HA for a bone substitute with a porosity of 75% ( 11 mm x 2 mm; D-11/2) and a dense HA block with one drilled through-hole (5 mm x 5 mm x 20 mm, hole diameter of 2.5 mm; Th-20), were used for the investigation. During the immersion in SBF for 1-9 d, the weight of unpolarized and polarized D-11/2 increased almost linearly with time. The rates of increment were calculated to be 0.3 and 0.5 mass% d(-1) for unpolarized and polarized D-11/2, showing that the bonelike apatite formation on the polarized D-11/2 was accelerated. The linear increase in polarized D-11/2 revealed that the polarization effect was active at least for 9 d, even after the new deposits covered the original surfaces. According to SEM observations, the thickness of the bonelike apatite layer on the pore walls located at the negatively charged side of polarized D-11/2 was thicker than that on the unpolarized one. In contrast, the layer located at the positively charged side was thinner than that on the unpolarized one when the pore position was shallow from the topmost surface. The thinner layer near the positively charged surface, however, turned into a thicker layer with increasing depth, and finally coincided with the thickness on the negatively charged surface at a position deeper than 0.6 mm from the surface. This indicated the formation of complex mixing of the negatively and positively charged surfaces on the internal pore walls. In the case of Th-20, the crystal growth of the bonelike apatite was promoted on the hole walls of polarized Th-20 compared with that on the unpolarized hole wall regardless of the wall position. It also shows that the bonelike apatite formation can be enhanced on the polarized walls, where the negatively and positively charged surfaces are mixed in the same plane.