Transformation of monetite to hydroxyapatite in bioactive coatings on titanium

Calcium phosphates have a wide range of pH stability, depending on their Ca /P ratio. Under physiological conditions (pH approximate to 7), the most st able calcium phosphate is hydroxyapatite, Ca-10(PO4)(6)(OH)(2). Acidic calc ium phosphates, like dicalcium phosphate, CaHPO4 (monetite) and dicalcium p hosphate dihydrate, CaHPO4. 2H(2)O (brushite), are thermodynamically unstab le under pH values greater than 6-7 and undergo transformation into more st able calcium phosphates. It means that, when placed in vivo (pH approximate to 7), acidic calcium phosphates convert to hydroxyapatite. In the present study, a coating of crystalline monetite oriented along the [112] axis was electrochemically deposited on titanium substrates. This monetite coating was subsequently converted to hydroxyapatite by immersion in alkaline solut ions. The result was a crystalline hydroxyapatite coating oriented along th e [002] axis. Different alkaline solutions produced the same result. Studyi ng the effect of immersion time on the transformation indicated that 4 h we re required to complete the conversion from monetite to hydroxyapatite. The transformation occurred by a dissolution-reprecipitation mechanism, i.e. t he monetite coating was continuously dissolved and reprecipitated as hydrox yapatite. This combined electrochemical deposition and chemical conversion process produced hydroxyapatite coatings with satisfactory adhesion to the substrate and a thickness between 10 and 39 mum (C) 2001 Elsevier Science B .V; All rights reserved.