A new simplified calcifying solution to synthesize calcium phosphate coatings

In this work we set up new experimental conditions to deposit biomimetic coatings of different calcium phosphates onto titanium (Ti) substrates using over-simplified, economically convenient, slightly supersaturated solutions. The new supersaturated solutions, CaPPs, contain just calcium chloride and phosphate buffer, without addition of further salts and organic buffers usually employed for biomimetic coatings. The product precipitated at pH 7.2 and 37 °C was constituted of spherical aggregates of poorly crystalline hydroxyapatite (HA), similar to that obtained using HEPES as buffer system. Reduction of starting pH, which was varied from 7.2 to 6.6, promoted the precipitation of the kinetically favored phase, octacalcium phosphate (OCP), together with HA. Furthermore, OCP could be obtained as a single phase by increasing the Ca/P molar ratio of the calcifying solution from 1/1 to 2/1. Temperature reduction from 37 °C to 25 °C promoted the co-precipitation of calcium monohydrogen phosphate dihydrate (DCPD) together with HA and OCP, in agreement with the solubility isotherms of the different calcium phosphates. Variation of the experimental conditions was utilized to synthesize the coatings of spherical aggregates of poorly crystalline HA, petal-like crystalline OCP, as well as coatings containing both HA and OCP, onto titanium substrates in a few hours.