Abstract

Dense, microporous and macroporous hydroxyapatite spherical granules for medical applications were prepared by a new manufacturing procedure. This process is related to a lost wax method and yields beads with a fully controlled porosity, in terms of shape and pore size.

A CaCO₃ scaffold exhibiting controlled size spherical cavities is impregnated with an aqueous hydroxyapatite suspension. After drying in a plaster mould, a thermal treatment destroys the carbonate scaffold and makes it possible to release the HA beads. By this process, HA beads with diameters in the range of 300 μm to 3 mm are obtained. A post-thermal treatment of the beads allows the ceramic to be densified at various densification levels.

Porous ceramics make it possible to functionalise the hydroxyapatite granules by loading the structure with biologically active substances. Drug releasing kinetics in continuous distilled water flow were analysed as a function of time and ceramic porosity. Spheres continue to release the biological molecules for more than 5 days.

Abstract

Some ceramics, such as Bioglass^®, sintered hydroxyapatite, and glass-ceramic A-W, spontaneously bond to living bone. They are called bioactive materials and are already clinically used as important bone substitutes. However, compared with human cortical bone, they have lower fracture toughness and higher elastic moduli. Therefore, it is desirable to develop bioactive materials with improved mechanical properties. All the bioactive materials mentioned above form a bone-like apatite layer on their surfaces in the living body, and bond to bone through this apatite layer. The formation of bone-like apatite on artificial material is induced by functional groups, such as Si–OH, Ti–OH, Zr–OH, Nb–OH, Ta–OH, –COOH, and PO₄H₂. These groups have specific structures revealing negatively charge, and induce apatite formation via formations of an amorphous calcium compound, e.g., calcium silicate, calcium titanate, and amorphous calcium phosphate. These fundamental findings provide methods for preparing new bioactive materials with different mechanical properties. Tough bioactive materials can be prepared by the chemical treatment of metals and ceramics that have high fracture toughness, e.g., by the NaOH and heat treatments of titanium metal, titanium alloys, and tantalum metal, and by H₃PO₄ treatment of tetragonal zirconia. Soft bioactive materials can be synthesized by the sol–gel process, in which the bioactive silica or titania is polymerized with a flexible polymer, such as polydimethylsiloxane or polytetramethyloxide, at the molecular level to form an inorganic–organic nano-hybrid. The biomimetic process has been used to deposit nano-sized bone-like apatite on fine polymer fibers, which were textured into a three-dimensional knit framework. This strategy is expected to ultimately lead to bioactive composites that have a bone-like structure and, hence, bone-like mechanical properties.