PRECONDITIONING AND DUAL CONSTANT COMPOSITION DISSOLUTION KINETICS OFPULSED-LASER DEPOSITED HYDROXYAPATITE THIN-FILMS ON SILICON SUBSTRATES

The kinetics of dissolution of pulsed laser deposited crystalline and amorphous thin films of hydroxyapatite on silicon substrates were meas ured at 37 degrees C and a pH value of 6.5 using the dual constant com position method. Solutions in which the pulsed laser deposited films w ere preconditioned (0.15 M NaCl) remained undersaturated or slightly s upersaturated with respect to hydroxyapatite after equilibrium was rea ched, indicating only a very small coating release and the absence of re-precipitation on the surfaces. The amorphous films released more ca lcium and phosphate during pre-conditioning than the more crystalline films. Dual constant composition dissolution rates decreased as film c rystallinity increased. The film with the lowest dissolution rate (app roximately one sixth that of a crystalline film deposited using a hydr oxyapatite powder target) was fabricated using a human tooth as the la ser target. During pre-conditioning of plasma-sprayed coatings, more c alcium and phosphate were released than for pulsed laser deposited fil ms, and dual constant composition dissolution rates were much higher.