Nucleation and growth mechanism of apatite on a bioactive and degradable ceramic/polymer composite with a thick polymer layer

Nucleation and growth mechanism of apatite on a bioactive and degradable PLLA/SiO₂-CaO composite with a thick PLLA surface layer were investigated compared to that on a bioactive but non-degradable polyurethane (PU)/SiO₂-CaO composite with a thick PU surface layer. The bioactive SiO₂-CaO particles were made by a sol-gel method from tetraethyl orthosilicate and calcium nitrate tetrahydrate under acidic condition followed by heat treatment at 600°C for 2 h. The PLLA/SiO₂-CaO and PU/SiO₂-CaO composites were then prepared by a solvent casting method which resulted in thick PLLA and PU surface layers, respectively, due to precipitation of SiO₂-CaO particles during the casting process. Two composites were exposed to SBF for 1 week and this exposure led to form uniform and complete apatite coating layer on the PLLA/SiO₂-CaO composite but not on the PU/SiO₂-CaO composite. These results were interpreted in terms of the degradability of the polymers. A practical implication of the results is that a post-surface grinding or cutting processes to expose bioactive ceramics to the surface of a composite with a thick biodegradable polymer layer is not required for providing apatite forming ability, which has been considered as one of the pragmatic obstacles for the application as a bone grafting material.