Strength of hollow hydroxyapatite microspheres prepared by a glass conversion process

Hollow hydroxyapatite (HA) microspheres (diameter = 100-800 microm) were prepared by reacting solid Li(2)O-CaO-B(2)O(3) glass spheres in 0.25 M K(2)HPO(4) solution at 37 degrees C. The influence of subsequent heating on the microstructure, surface area, and compressive strength of the HA microspheres was evaluated using scanning electron microscopy, the BET method, and nano-mechanical testing. The surface area and rupture strength of the as-prepared microspheres were 135 m(2)/g and 1.6 +/- 0.6 MPa, respectively. On heating for 8 h at 600 degrees C, the surface area decreased to 27 m(2)/g, but there was no increase in the compressive strength (1.7 +/- 0.4 MPa). Heating to 800 degrees C (8 h) resulted in a marked decrease in the surface area (to 2.6 m(2)/g) and a sharp increase in the compressive strength (to >35 +/- 8 MPa). These hollow HA microspheres may be useful as devices for drug or protein growth factor delivery or as scaffolds for engineered tissues.