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.
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Kamiya, K., Yoko, T., Tanaka, K. et al. Growth of fibrous hydroxyapatite in the gel system. J Mater Sci 24, 827–832 (1989). https://doi.org/10.1007/BF01148764
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