In-vitro forming of calcium phosphate layer on sol-gel hydroxyapatite-coated metal substrates

In-vitro deposition of calcium phosphate layer (CPL) on metallic substrate requires special surface preparation in order to provide an interfacial bond. In this work 316 stainless steel surface is modified through deposition of a thin film ( approximately 0.5 microm) of sol-gel hydroxyapatite (SG-HA). This well-bonded film acts as an intermediary and nucleation surface of the CPL film. The SG-HA films were annealed at 375 degrees C (samples coded 375-ACS) and 400 degrees C (400-ACS) to achieve different crystallinity of the films, and thus to affect and study the CPL nucleation process. The CPL growth was investigated in terms of deposition kinetics and microstructural development. A deposition rate of dense CPL of about 0.43 microm/day was achieved on the crystallized film of 400-ACS, and 0.22 microm/day of porous CPL on amorphous 375-ACS. A compositional variation of Ca/P ratio across the CPL film thickness (400-ACS) was observed. Lower Ca/P ratio of 1.2 was detected near the substrate-CPL interface and about 1.5 near the solution-CPL interface. Infrared analysis showed the CPL to be of apatitic calcium-deficient structure. Kinetic model explaining the advancement of the CPL upon the in-vitro immersion is proposed.