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Calcite particles (Source; CaCO3, ∼0.4 μm in size) were 3D-printed into cube honeycombs (honeycombs) with ∼0.7 mm thick struts and ∼0.6 mm cellular window opening. The sintered honeycombs, regardless of the presence of a sintering aid in the printing slurry, gave ∼3 MPa or more in the compressive stress, which exceeded that of trabecular bone. The conversion of calcite to hydroxyapatite (HAp) was assessed in 0.1 M K2HPO4 (pH: 7.0 or 7.4) at 20 °C–80 °C due to the X-ray diffraction intensity of the calcite and HAp peaks, surface microstructure, and P(V) accumulation on the honeycomb strut surface. Although a premature HAp layer yielded on all samples within 1 h soaking, further conversion depended on the samples. The conversion continued on Source until the volume fraction of the HAp shell layer reached ∼35% (37 °C). It was hardly detected on the grains of sintered honeycombs (from the slurry with the sintering aid) within the whole soaking period (≤24 h). In contrast, the conversion on the sintered honeycombs (from the slurry without the aid) became vigorous when soaked for 5 h, and, it reached at 24 h the highest level that the as-printed honeycombs achieved. The critical factors controlling the conversion were calcite dissolution, the equilibria among the carbonate and phosphate ions, and the rates of migration of Ca(II) and P(V) through the inter-granular channels of pores within the struts. A particle stacking model was proposed for the most plausible interpretation of the present results.

Calcium carbonate-based bone substitutes derived from natural coral exoskeleton (aragonite) are resorbed and remodeled faster than calcium phosphate-based substitutes. However, coral species with structures appropriate for use as bone substitutes are very limited. Therefore, it is important to evaluate potential of artificial calcium carbonate ceramics as a bone substitute. In this study, calcium carbonate granules with various porosities and pore sizes were prepared by sintering a highly pure (>99.98%) calcium carbonate powder (calcite), and their resorption properties and bone formation abilities were examined in vivo for the first time. The sintered calcium carbonate was resorbed faster than β-tricalcium phosphate, which has a similar structure. However, sintered calcium carbonate did not promote new bone formation during long-term implantation. Furthermore, both resorption and new bone formation were affected by the pore structure. The optimal structures of the artificially sintered calcium carbonate bone substitute were also discussed. [ABSTRACT FROM AUTHOR]

In addition to calcium phosphate-based ceramics, glass-based materials have been utilized as bone substitutes, and silicate in these materials has been suggested to contribute to their ability to stimulate bone repair. In this study, a silicate-containing α-tricalcium phosphate (α-TCP) ceramic was prepared using a wet chemical process. Porous granules composed of silicate-containing α-TCP, for which the starting composition had a molar ratio of 0.05 for Si/(P ? Si), and silicate-free α-TCP were prepared and evaluated in vivo. When implanted into bone defects that were created in rat femurs, α-TCP ceramics either with or without silicate were biodegraded, generating a hybrid tissue composed of residual ceramic granules and newly formed bone, which had a tissue architecture similar to physiological trabecular structures, and aided regeneration of the bone defects. Supplementation with silicate significantly promoted osteogenesis and delayed biodegradation of α-TCP. These results suggest that silicate- containing α-TCP is advantageous for initial skeletal fixation and wound regeneration in bone repair. [ABSTRACT FROM AUTHOR]