Synthetic calcium phosphate (CaP) materials are of interest as bone filling materials because their composition is close to the one of the native mineral phase of bone. Low crystallized and amorphous phase are metastable CaP materials. They offer good biological responses and provide ions for bone remodeling. To obtain bulk materials from such metastable CaP is challenging. Indeed, they evolve toward crystalized phases especially when the shaping process requires applying temperature, which is common to manufacture structural ceramics. Recently numerous researches intended to produce bulk CaP materials through various strategies using compression (SPS, direct uniaxial press with and without heating) relying on cold-sintering mechanism. This mechanism infers the use of a transient liquid or the use of hydrated out-layer to enhance the mass transfer of labile ions to bridge two grains. It allows improving the densification and therefore the mechanical properties of the CaP material. At high densification, the mechanical properties of CaP cold-sintered materials are in the range of natural trabecular bone. However, the very low porosity can be a drawback for biological application (cell colonization, vascularization…).
In this study, we propose a route to obtain a bulk part of amorphous calcium phosphate (ACP) using direct low-loading compression at room temperature and short dwell time (0.2 s). The objective is to produce a material with sufficient mechanical properties to be handle and with a porosity and tensile strength of the order of magnitude of the trabecular bone (30-90 % and 1-5 MPa respectively). The influence of tableting pressure [25; 200 MPa] was evaluated on the mechanical and chemical properties. A spray-dried ACP powder recently described as the aggregation of solid cluster cores (1nm) covered with a hydrated layer was used. Therefore, no transient liquid is needed in the compression process to achieve the cold sintering. ACP are 100 nm spherical units and provide high particle interactions that enable the consolidation of the tablet even at low pressure. Interestingly, goals porosity and tensile strength have been reached from pressure of 75 MPa (60 % and 1.75 MPa respectively) and beyond with preservation of the amorphous character of the initial powder. These properties were demonstrated by XRD, Raman, FTIR, picnometer analyzes and Brazilian test.
Furthermore, such a consolidated ACP retains its propensity to evolve spontaneously in water. Indeed, the aqueous evolution of the ACP tablet thus obtained was investigated until 72 h. ACP crystallization was observed on the surface of the tablet from the first minutes while the core seems to remain amorphous and the dimensions of the tablet (6 mm diameter and 1 mm thickness) are not affected. An increase of the porosity size in the 200 µm surface layer of the tablet with time was measured with XRay tomography.
These results can inspire the development of consolidated ACP for filling materials with appropriate mechanical properties, porosity evolution and biocompatibility of the amorphous that suit bone-healing process.