Supercritical assisted atomization (SAA) has been applied to the production of coprecipitates formed by hydroxyapatite nanometric particles and chitosan, used as binding agent. Precipitation temperatures between 90 degrees C and 110 degrees C have been used and a chitosan concentration lower than 20 mg/mL has been required to avoid microparticle coalescence. Furthermore, to obtain a good compromise between particle size and hydroxyapatite loading, a gas to liquid ratio of 1.8 was adopted. The best operating conditions for particle diameter and coprecipitate stability were: precipitation temperature 110 degrees C, chitosan concentration in the starting solution 10 mg/mL, that produced spherical composite microparticles with a mean diameter of 0.6 mu m and a loading efficiency of about 91%. The nanostructure and particle surface roughness has been modulated varying hydroxyapatite percentages in the starting suspension. The results obtained have been explained in terms of suspension stabilization and atomization efficiency. Possible biomedical applications for these powders are described. (C) 2013 Elsevier B.V. All rights reserved.