Possibilities for strengthening hydroxyapatite ceramics

Mechanical strength of extrusion freeformed calcium phosphate filaments

Hydroxyapatite-tricalcium phosphate mixtures of various compositions were extruded by a solid free-forming process to form lattice structures to serve as hard tissue scaffolds. The unwelded filaments, sintered at temperatures from 1100 to 1300 degrees C, had radii from 115 to 135 microm and were tested in three point flexural loading using a purpose-built fixture. Flexural strength ranged from 20 to 100 MPa depending on composition and sintering temperature. Weibull moduli up to 13 were obtained. Compositions with 50% or more tri-calcium phosphate did not develop strengths much above 40 MPa and the strength of most compositions fell when the sintering temperature exceeded 1250 degrees C. Multiple layer lattice structures were created and tested in compression.

Porous calcium phosphate ceramic granules and their behaviour in differently loaded areas of skeleton

Two kinds of calcium phosphate ceramic (CPC) granules of high porosity (50 +/- 5%) and improved (for such materials) compressive strength (10-25 MPa) consisted of hydroxyapatite (PHA) and a mixture of hydroxyapatite (HA) and beta-tricalcium phosphate (beta-TCP) in 60 HA/40 beta-TCP composition (PCPC) were developed. A comparative study of in vivo behavior of the materials implanted into an almost unloaded (greater trochanter of femur) and loaded (distal methaphysis of femur) zones in the skeleton of rabbits was performed. Significant activating influence of loading on the processes of new bone formation and reconstruction in macropores of both materials during all periods of implantation (up to 6 months) was observed. The role of relevant cells in the processes in the unsoluble PHA and the degradable PCPC (in which the processes was observed to intensify due to dissolution of the material) was studied and is discussed. Great disturbance in pore structure of the BCPC was revealed in more late periods of implantation. After 6 months, presence of large composite fragments located in intertrabecula spaces of greater trochanter was a characteristic feature of the PCPC crushing. The developed CPC materials seems to have good perspective for using in bone defect plasty in some loaded areas of the skeleton.

Radio frequency (rf) plasma spheroidized HA powders: powder characterization and spark plasma sintering behavior

The present study describes the synthesis of spheroidized hydroxyapatite (HA) powders using a radio frequency (rf) inductively coupled plasma (ICP) torch. The spheroidized powders were consolidated through a spark plasma sintering (SPS) system. The microstructure and crystallographic phases in the synthesized powders were characterized using scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffractometry (XRD) and Raman spectrometry. Results showed that the HA feedstock decomposed after rf plasma processing. Crystalline HA, alpha-tri-calcium phosphate (alpha-TCP), tetra-calcium phosphate (TTCP) and calcium oxide (CaO) were detected in the plasma-spheroidized powders. Raman spectra results indicated strong presence of amorphous calcium phosphate (ACP) in the spheroidized powders. The particle size distribution and specific surface area were influenced through the rf plasma working plate power levels. The sintering behavior of the rf plasma synthesized powders was analyzed through the SPS process and the results indicated that the spheroidized powders commence sintering at approximately 900 degrees C and through to 1150 degrees C. After sintering above 1100 degrees C for 3min, the relative densities of the SPS compacts reached 96% of the theoretical value. The SPS compacts were immersed in simulated body fluids (SBF) for different durations and the results confirmed their bioactivities.

Calcium phosphate apatites with variable Ca/P atomic ratio III. Mechanical properties and degradation in solution of hot pressed ceramics

Calcium deficient hydroxyapatite powders Ca(10-x)(PO4)(6-x)(HPO4)x(OH)(2-x) (0 < or = x < or = 1) were hot pressed to produce dense hydroxyapatite-tricalcium phosphate (HAP/TCP) biphasic materials. Ceramics hot pressed at 1000 degrees C were composed of an homogeneous distribution of the HAP and beta-TCP grains with an average size of 0.2 microm. Grain growth was observed for TCP loading > 70 wt%. The strength exhibited a maximum of sigma(f) = 150 MPa for 90/10 (w/w) HAP/TCP and it dropped for smaller and greater amounts of TCP. This value was twice that of pure HAP. When placed in Ringer's solution, only the surface of biphasic compounds was degraded after 60 days of immersion with a preferential etching of the TCP phase. After hot pressing at 1200 degrees C, grain growth was observed and the mechanical properties were decreased. This was explained by the allotropic transformation alpha/beta of TCP.

Sintering peculiarities for hydroxyapatite with different degrees of crystallinity

It has been shown that reduction of sintering temperature of hydroxyapatite is connected to the use of a powder, the particles of which consist of crystalline and amorphous-like constituents. Shrinkage of the pressings made of the powder starts at the middle temperatures (600-700 degrees C) and is realized by a mechanism of a mutual sliding of the particles. If the firing temperature of the pressings increases, the second stage, realized by a diffusion mechanism (over approximately 900 degrees C), begins. A shrinkage curve on the sliding stage has a stepped character. It is caused by a stopping effect on the shrinkage of the pressure of a gas, which is exuded as a result of crystallization of the amorphous-like constituent, and partly accumulates in closed cavities and pores. The contribution of the two mechanisms to the shrinkage process is determined by the degree of crystallinity of the powder (i.e. the ratio of the crystalline and the amorphous-like constituents). If a powder has a higher degree of crystallinity, higher temperatures result, and the stepped character of the shrinkage curve is smoothed. The sliding stage is practically lacking for pressings of crystalline powder, and a noticeable shrinkage starts at high temperatures (over approximately 900 degrees C) as diffusive processes develop.