Spark Plasma Sintered Reactive Calcium Phosphates : Toward highly bioresorbable materials for biomedical applications

Bone mineral is composed of non-stoichiometric polysubstituted apatite nanocrystals, consisting in an apatitic core and a non-apatitic surface hydrated layer with labile ions. It is possible to prepare biomimetic analogs by soft chemistry, e.g. coprecipitation [1]. The presence of this hydrated ionic layer was found for non-carbonated biomimetic apatites to allow effective low-temperature consolidation by SPS, typically at 150°C and 100 MPa [2]. Soft, non-conventional consolidation approaches are indeed required to prevent the alteration of the metastable nanocrystals. In order to approach further the composition of bone mineral, close to Ca8.3(PO4)4.3(CO3,HPO4)1.7(CO3,OH)0.3 [3], and to increase the bioresorbability of the consolidated scaffolds, the effect of carbonate substitution into biomimetic apatites was recently investigated [4]. Results pointed out, however, decreased SPS consolidation efficiency. The use of carbonated amorphous calcium phosphates (cACP) was envisioned; ACP being another promising bioresorbable material [5] used for example in coatings of metallic endoprostheses, cements and fillers in dentistry. However, all attempts to preserve the amorphous character of the starting materials failed, as all cACP tested in that study transformed into well crystallized apatite after SPS. In this light, the goal of this work was to investigate the influence of ion substitutions on the processing ability of different nanocrystalline apatites and amorphous compounds, with the general aim to increase their bioresorbability. In biomimetic apatites, increasing carbonation may allow favoring their solubility and limiting post-maturation in solution. For ACP, ion substitution may also appear as a possible approach to stabilize the amorphous structure. In this contribution, nanocrystalline apatites with different levels of carbonation and ACP doped with different amounts of carbonate and magnesium, two crystallization inhibitors, were prepared by soft chemistry and sintered by SPS at 150°C. The structure, chemical composition and mechanical resistance of the processed ceramics were investigated.