Bone substitute: Transforming β-tricalcium phosphate porous scaffolds into monetite

The goal of the present study was to assess the possibility to change the composition of a calcium phosphate scaffold from a high-temperature phase to a phase only stable at or close to room temperature without macrostructural changes. For that purpose, macroporous β-TCP scaffolds were converted into α-TCP by high-temperature thermal treatment and then dipped into a phosphoric acid solution to obtain a more acidic calcium phosphate phase called monetite or dicalcium phosphate (DCP; CaHPO4). Two different solid-to-liquid ratios (SLR: 0.067 and 0.200 g/mL) and three different temperatures (T: 37, 60 and 80 °C) were used. The reaction was followed by measuring the change of sample size and weight, by determining the compositional changes by X-ray diffraction (Rietveld analysis), and by looking at the micro- and macrostructural changes by scanning electron microscopy and micro-computed tomography. The results revealed that the transformation proceeded faster at a higher temperature and a higher SLR value but was achieved within a few days in all cases. Morphologically, the porosity decreased by 10%, the pore size distribution became wider and the mean macro pore size was reduced from 0.28 to 0.19 mm. The fastest conversion and the highest compressive strength (9 MPa) were measured using an incubation temperature of 80 °C and an SLR value of 0.2 g/mL.