ИСТИНА

Due to their properties, hydroxyapatite (HAp) and tricalcium phosphate (TCP) are the two calcium phosphates most commonly used in clinical medicine. HAp and other complex calcium phosphate salts are the end-products of the biological mineralization process. Calcium pyrophosphate Ca2P2O7 (β-CPP) is one of the intermediate products in this process. The biological response with respect to new bone formation is quite similar for CPP and HAp. Sintered CPP has better biological responses, and, thus, a great potential as a biodegradable bone substitute [ ]. Biphasic ceramics consisting of HAp and TCP are widely used as bioactive materials with regulated limit of biodegradation. The rate of biodegradation depends on: (1) material texture (porosity type and level), (2) quality of biodegradation phase (phase composition, grain size, properties of grain boundaries). Several sources for CPP phase in ceramics can be assumed. CPP phase may come from frit (CaO-P2O5, Ca/P=0.2-0.75) used as a sintering additive [ ]. Ceramics can be fabricated from powder of CPP with Na4P2O7 as sintering additive [ ]. In work [ ], phase of α-CPP forms due to interaction between H3PO4 or (NH4)2HPO4 and porous HAp at high temperature after soaking it in the former solutions. The aim of present work was focused on fabrication of multiphase ceramics with enhanced resorptivity due to presence of CPP phase and investigation of processes leading to formation of the multiphase ceramics based on HAp and CPP originated from CaHPO4. Ceramic materials have been made from mixtures of powders of stoihiometric HAp (Ca/P=1.67) and monetite (CaHPO4, Ca/P=1). Powders of HAp and monetite were synthesized by means of wet chemical precipitation from aqueous solutions of Ca(NO3)2*4H2O and (NH4)2HPO4 at 60 C and pH=9 for HAp and pH=4-5 for monetite. Synthesized dried powders were mixed and disaggregated in ethanol or acetone. Dried powders were passed through polyester sieve and pressed at 100 MPa. Component ratio HAp:monetite was varied from 0:100 to 100:0 % with a step of 20%. Powders of raw materials and the mixtures were tested by means of XRD, TG, DTG, SEM, dilatometry. Linear shrinkage, density and microstructure of samples of ceramic materials sintered at 900, 1000, 1100˚C with isothermal holding during 6 hours were tested. Complicated consequence of phase transformations took place during heating the the mixtures from 20 to 1200 C. The CPP (Ca/P=1, converted from CaHPO4 at 400-500˚C ) reacts with HAp (Ca/P=1.67) causing additional weight loss in the region of 600-1050˚C due to solid state reaction leading to TCP (Ca/P=1.55) formation. Linear shrinkage of HAp at 1100˚C after 6 hours was found to be about 21%; for Ca2P2O7 formed from monetite, and for the mixtures - less than 11%. Resulted ceramics with the phase composition of HAp, CPP and TCP, i.e. with a different content of degradable phase and different ratio of CPP/TCP, can be treated as a biocompatible bioactive material with a tunable rate and limit of biodegradation.