Growth of fibrous hydroxyapatite in the gel system

Hydroxyapatite (HAP) was prepared in the agar gel system, where Ca²⁺ ions were incorporated in the gel and PO ₄ ^3– solution was layered over the gel. When the concentration of Ca²⁺ was lower than 1.0 M and the initial solution Ca/P molar ratio was lower than about unity, fibrous HAP several centimetres to several tens of centimetres in length was grown upwards in the PO ₄ ^3– solution from the gel phase, while at the larger Ca²⁺ concentration and initial Ca/P molar ratio CaHPO₄ · 2H₂O (DCPD) in the form of gelatinous precipitate, particulate precipitates or needle-like crystals were preferred to HAP. The fibrous HAP was calcium deficient and composed of small elongated hollow ovals linked in a zigzag row. This was considered to be formed in the following manner. First, Ca²⁺ ions supplied through pores in the gel reacted with PO ₄ ^3– ions to form a small spherulite at the pore exit, then the spherulite was bloated and elongated by the osmotic pressure or capillary force until a part of the oval was broken for Ca²⁺ ions to be gushed out into the PO ₄ ^3– solution. The above two processes were repeated to form elongated hollow ovals linked to the preceding ones.