Hydroxyapatite pattern formation in PVA gels

Sodium hydroxide solution was allowed to diffuse from the edge of an acidic poly(vinyl alcohol) gel sheet containing dissolved calcium and phosphate ions, and calcium phosphate was observed to precipitate as the pH rose. The precipitation pattern changed depending on the solute concentration near the reaction front; precipitate “walls” were formed in areas in which the calcium phosphate concentration was higher or the sodium hydroxide concentration was lower than the conditions for homogeneous precipitation, and within a very limited concentration combination of calcium phosphate and sodium hydroxide, a regular stripe pattern with a pitch of about 100 μm was formed. The calcium phosphate precipitate obtained was a single phase of hydroxyapatite. It was also found that apatite ceramic sheets with periodic porous structures or with undulate patterns could be manufactured by sintering gels with stripe patterns.