ИСТИНА

Hydroxyapatite Са10(РО4)6(ОН)2 (HA) ceramics has long been regarded as a reference material for bone implants. Recently, research activity in this field is shifted toward calcium phosphates with a ratio of Ca/P less than that of hydroxyapatite, since they are more resorbable. We have proposed a route to fabricate biphasic ceramics Ca3(PO4)2/Ca2P2O7 from octacalcium phosphate (OCP, Ca/P ratio=1,33) as a precursor. A need for the precursor is dictated by the following reasons: (i) homogeneous distribution of the phases in the biphasic composite, (ii) low sinterability of the phosphate phases, especially calcium pyrophosphate (Ca2P2O7, CPP). The OCP powder was prepared by hydrolysis of CaHPO4•2H2O in a buffered solution. Prior to sintering, the powder of OCP was dehydrated at 4500С, giving “OCP-450” precursor. Sintering of the “OCP-450” compacted samples was carried out at temperature 11000 C for 3 hours. Ceramic composites were evaluated for cytotoxicity and cell viability in fibroblast culture in vitro, and for bending strength (three-point bending test). The as-received OCP powder consists of plate-like crystals having the so-called “apatitic” and of “hydrated” (“brushite”) layers in their crystal structure. The structure of “OCP-450” precursor is somewhat similar to that of OCP, but looks less perfect and contains P2O74--ions. HA and CPP as individual phases can be detected only during further dehydration at elevated temperatures. Thus, the structure of “OCP-450” represents HA-like layers and partially dehydrated “brushite” layers containing pyrophosphate anions. Thorough study of phase homogeneity of the ceramics fabricated from the “OCP-450” revealed rather uniform composition at the length scale of 50-100 nm (low-voltage EDX, BEI) apart from that one produced by mechanical mixing of CPP and tricalcium phosphate powders. The ceramics fabricated from “OCP-450” reached 85% density and demonstrated high bending strength (up to 127 MPa). The reason underlying such a behavior is a very tight and ordered arrangement of lamellar particle during compaction (pressing) of the powder leading to rather dense and homogeneous ceramic structure. In vitro tests gave an evidence that the biphasic ceramics Ca3(PO4)2/Ca2P2O7 do not show any toxic effects with respect to fibroblast cells, and, therefore, has a good potential as a material for tissue engineering. The work was supported by RFBR, grants # 10-03-00866, 11-03-12179-ofi-m-2011, 12-03-01025.