Synthesis of calcium phosphate crystals in a silica hydrogel containing phosphate ions
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Masakazu Kawashita Department of Biomedical Engineering, Graduate School of Biomedical Engineering, Tohoku University, Aramaki-Aoba, Aoba-ku, Sendai 980-8579, Japan
Giichiro Kawachi, Koichi Kikuta, and Chikara Ohtsuki Department of Crystalline Materials Science, Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan (Received 21 April 2008; accepted 17 March 2009)
Calcium phosphate crystals were synthesized by diffusing calcium ions into silica hydrogels containing phosphate ions. Hydroxyapatite [HAp, Ca10(PO4)6(OH)2] and octacalcium phosphate [OCP, Ca8(HPO4)2(PO4)45H2O] with different types of crystal morphology were formed in the gel. The HAp had an irregular or rod shape, a few micrometers in length, while the OCP had an irregular, spherulite, rod- or ribbonlike shape, ranging in size from a few micrometers to several tens of micrometers, depending on the amount of phosphoric acid added and the reaction temperature. The morphology of the OCP changed from an irregular shape to a ribbonlike or rod shape, via a spherulite shape, depending on the amount of phosphoric acid added and the reaction temperature. The changes in crystal morphology of the HAp and OCP were ascribed to the degree of supersaturation of the reaction environment and the rate-determining step in the HAp and OCP crystal growth mechanism. I. INTRODUCTION
Controlled crystal growth in a hydrogel environment is an environmentally friendly process, which takes place under standard temperature and pressure conditions. Sparingly soluble compounds, such as calcium carbonate,1,2 calcium tartrate,3 and calcium oxalate,4 have been synthesized via crystal growth in gels. Gel-mediated growth frequently occurs in nature during the formation of inorganic materials in living organisms, such as bones, teeth, and shells.5 Calcium phosphate is one of the most important inorganic compounds that forms in organisms, and the main inorganic component of bones.6 Hydroxyapatite [HAp, Ca10(PO4)6(OH)2] is an inorganic component of bone, and octacalcium phosphate [OCP, Ca8(HPO4)2(PO4)45H2O] acts as a precursor of hydroxyapatite.7 Therefore, it is considered meaningful to investigate the effects of using hydrogel as a reaction environment for the formation of calcium phosphates to develop a novel synthesis method for inorganic crystals at low temperatures and to obtain a fundamental understanding of bone mineralization. Reports show that various types of calcium phosphates can be synthesized by a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2009.0242
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http://journals.cambridge.org
J. Mater. Res., Vol. 24, No. 6, Jun 2009 Downloaded: 30 Mar 2015
gel-mediated growth. For example, HAp,8–10 OCP,9 dicalcium phosphate dihydrate (DCPD, CaHPO42H2O),10,11 and dicalcium phosphate anhydrate (DCPA, CaHPO4)8,11 were synthesized in a silica hydrogel, poly (acrylic acid) gel, or gelatine. Although crystalline formation via a gel-mediated process has received much attention, for
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