Microwave-processed nanocrystalline hydroxyapatite: simultaneous enhancement of mechanical and biological properties

Despite the excellent bioactivity of hydroxyapatite (HA) ceramics, poor mechanical strength has limited the applications of these materials primarily to coatings and other non-load-bearing areas as bone grafts. Using synthesized HA nanopowder, dense compacts with grain sizes in the nanometer to micrometer range were processed via microwave sintering between 1000 and 1150 degrees C for 20 min. Here we demonstrate that the mechanical properties, such as compressive strength, hardness and indentation fracture toughness, of HA compacts increased with a decrease in grain size. HA with 168 +/- 86 nm grain size showed the highest compressive strength of 395 +/- 42 MPa, hardness of 8.4+/-0.4 GPa and indentation fracture toughness of 1.9 +/- 0.2 MPa m(1/2). To study the in vitro biological properties, HA compacts with grain size between 168 nm and 1.16 microm were assessed for in vitro bone cell-material interactions with human osteoblast cell line. Vinculin protein expression for cell attachment and bone cell proliferation using MTT assay showed that surfaces with finer grains provided better bone cell-material interactions than coarse-grained samples. Our results indicate simultaneous improvements in mechanical and biological properties in microwave sintered HA compacts with nanoscale grain size.