ИСТИНА

A variety of novel calcium phosphate-based materials for medical applications (CPs) have been developed recently. These materials are biocompatible due to their similarity to chemical composition of inorganic component of the mammalian bones. According to known phase diagrams and experience of different research groups the use of custom scheme of ceramic production permits to produce ceramic materials or composites for a just restricted number of crystalline phases. These are hydroxyapatite Са10(РО4)6(ОН)2 with Ca/P=1,67 (HAP), tricalcium phosphate Са3(РО4)2 with Ca/P=1,5 (TCP), pyrophosphate Са2Р2О7 with Ca/P=1 (CPP) and polyphosphate or metaphosphate Ca(PO3)2 with Ca/P=0,5. Sometimes it is necessary to use bioresistive materials such as HAP. The materials with Ca/P ratio below 1,67 are bioresorbable and dissolve in body fluids when implanted. In particular, upon construction of drug delivery systems or tissue engineering constructions for bone regeneration, it is necessary to use bioresorbable materials. Modern powder technologies, including technology of ceramic composites, need fine powders of high quality. It is preferably to use powders made via chemical methods, which exploit the well-known “bottom up” approach. The method of solution precipitation is the most convenient and popular if reactive powders are targeted. HAP is the only substance from the above list of CPs which can be synthesized via precipitation from aqua solutions. Other CPs can form only upon thermal conversion of the of the following wet-synthesized CPs: Ca-deficient HAP Са10-x(HРО4)x(РО4)6-x(ОН)2-2x; amorphous CPs like 3Са3(РО4)2*xH2O and Са2Р2О7*xH2O, brushite CaHPO4*2H2O, monetite CaHPO4, monocalcium phosphate Ca(H2PO4)2*2H2O or Ca(H2PO4)2. TCP can form as a result of thermal conversion of Ca-deficient HAP or amorphous CP 3Са3(РО4)2*xH2O which were synthesized at Ca/P=1,5 and 6≤pH≤8. Pyrophosphate can form due to thermal conversion of brushite, monetite or amorphous CP Са2Р2О7*xH2O. Metaphosphate or polyphosphate can form from monocalcium phosphate during thermal conversion. TCP and CPP can form due to a solid-state reaction of two calcium phosphates with Ca/P ratio different (above and below) from 1,5 for TCP or from 1 for CPP. It is possible to use the following pairs of powder reagents, e.g. “hydroxyapatite/pyrophosphate” for TCP or “Ca(PO3)2/TCP” for CPP. All phase transformations in compacts made from active calcium phosphate powders can affect the microstructure formation of CPs ceramic materials in CaO-P2O5 system. Acknowledgement: RFBR grants 07-08-00576, 10-03-00866, 09-03-01078 and RF State Contract № 14.740.11.0277.