Surface modification of P(EMA-co-HEA)/SiO2 nanohybrids for faster hydroxyapatite deposition in simulated body fluid?

P(EMA-co-HEA)/SiO2 nanocomposites with 0, 15 and 30 wt% of silica were obtained by copolymerization of ethyl methacrylate, EMA, and hydroxyethyl acrylate, HEA, during the simultaneous acid-catalyzed sol-gel polymerization of tetraethoxysilane, TEOS. A surface modification treatment was applied in order to reduce the induction time for hydroxyapatite (HAp) nucleation, combining a previous NaOH attack to increase the number of surface nucleating sites, and an alternate soaking process in Ca and P solutions to form apatite precursors, prior to the immersion in a simulated body fluid (SBF). The NaOH treatment was not effective by itself in shortening the HAp induction time. It introduced sodium carboxylates in the copolymer but hydrolyzed the silica network excessively, thus reducing the surface nucleating potential of its boundary silanols. Therefore, bioactivity was only due to the surface carboxylate groups of the organic phase. Maybe a controlled dissolution extent of the silica network so as to improve bioactivity could be attained by reducing the duration of the NaOH-treatment. This would be interesting in the hybrid with 30 wt% of silica, because its dense silica network is not able to hydrolyze in SBF without any previous treatment, whereas the silica network in the hybrid with 15 wt% of silica hydrolyzes at the surface promoting the deposition of HAp. The CaP treatment was able to coat the surfaces of the samples with a calcium phosphate layer within minutes. This amorphous calcium phosphate acted as HAp precursor, skipping the induction period in SBF.