Influence of synthesis and sintering parameters on the characteristics of carbonate apatite

Abstract

A new method to synthesise carbonate-substituted hydroxyapatite (CHA) powder has been set up introducing a CO₂ flux, as a source of carbonate, in the HA synthesis process based on the neutralisation reaction. The reactants are abundant and inexpensive. The yield is good compared to other CHA powder synthesis. The reaction may be performed at low temperature and without pH control and does not produce any by-products.

The influence of the synthesis parameters (temperature, H₃PO₄ solution dropping rate, i.e. reaction time, CO₂ flux, ageing time) has been tested to optimise the process conditions in order to obtain the highest carbonation degree and favour the B-type CHA precipitation with respect to A-type one.

The prepared powder (5.8 wt% of total carbonate with an A/B ratio of 0.78) was thermally treated at various temperatures in the range 500–1400°C in different atmospheres (air, wet and dry carbon dioxide).

The thermal treatments were performed with a double aim, to eliminate selectively the carbonate groups in A-position maintaining the B-type substitution, and to evaluate the thermal stability of the CHA and the total loss of carbonate as a function of temperature. The thermal treatment at 900°C in wet CO₂ gave the best result in terms of a high carbonate residue and a low A/B ratio.

We also investigate the use of different techniques (inductively coupled plasma, TGA, Fourier transformed infrared spectroscopy, X-ray diffraction) for characterising CHA and calculating sensitivity and accuracy in the quantification of carbonate ions for each molecular site.

Introduction

In the last decades, the target research was the production of highly crystalline and stoichiometric hydroxyapatite. More recently, the interest has focused on the preparation of apatites more similar to bone in terms of both crystallinity degree and deviation from stoichiometry.

Non-stoichiometric apatites are quite easy to synthesise. In fact, they can be obtained by any precipitation method over a wide range of pH, temperature and concentration. Their composition and crystal characteristics are, however, more difficult to control, mainly because of the ability of the lattice to accept substituents and vacancies [1]. The most interesting of the non-stoichiometric apatites are HPO₄²⁻ and CO₃²⁻ containing apatites which are close to bone mineral. Although bone mineral has a variable composition, diverse apatites mimicking the evolution of bone mineral in young and old animals can be prepared and their composition can be represented by a chemical formula analogous to that of bone mineral. The trivalent anionic phosphate sites may be occupied by both the bivalent ions hydrogen phosphate and carbonate, while the monovalent anionic site substitution of hydroxyl is possible only by carbonate. The phosphate site cannot accept vacancies, probably because the trivalent anions are quite large and vacancies would destabilise the lattice. Calcium sites can also be occupied by other cations and can also accept vacancies up to a maximum of two sites out of the 10 existing in stoichiometric apatites. At present, preparation of the carbonate apatite is a must. To study the effect of carbonate as a unique substituent, it is important to avoid the hydrogen phosphate and the cationic substitutions: otherwise, the identification of the stoichiometry of the apatitic phase via the chemical and physical characterisations become highly complicated, due to overlapping synergic or opposite effects. Up to now, the most common synthetic process involved calcium nitrate, hydrogen phosphate and (hydrogen)carbonate salts of Na⁺ and/or NH₄⁺. Among these, ammonium salts must be preferred, because even if it is incorporated in the precipitate, as Na⁺ does, it can be easily eliminated by heating above 500°C [1].

The choice of reactants, which do not contain counter ions that can substitute the apatitic sites, appears anyway better than the previous ones. In this work, the classical synthesis of HA was performed in CO₂ flux starting from calcium hydroxide and orthophosphoric acid and the influence of process parameters studied.

The reactants are abundant and inexpensive. The yield is good compared to other carbonate-substituted hydroxyapatite (CHA) powder synthesis, the reaction may be performed at low temperature and without pH control and does not produce any by-products that have to be disposed off. These are all things of great interest for an industrialisation of the process.

However problem remains, due to the possibility of the carbonate ion to partially substitute both the phosphate and the hydroxyl ion of the hydroxyapatite structure, giving rise to B- and A-type carbonated apatite, respectively [2].

The carbonate content in the bone mineral is about 4–8 wt% [3] and it has been shown to vary depending on the age of the individual [4], [5] with an increase of A-type in the old bone. On the other hand, type B carbonate apatite is the most abundant species in bone of young human beings [6].

The presence of B-carbonate in the apatite lattice was shown to cause a decrease in crystallinity and an increase in solubility both in vitro and in vivo tests [2]. With the aim of producing a synthetic material replicating the levels of carbonate found in biological apatites we studied a simple, high yield reaction to prepare a carbonate apatite with a prevalence of B-type. This is attributed to a decrease of the polar component of the surface of the biomaterials. Moreover, since the surface energy of the biomaterial greatly influences the initial cell attachment and affects the collagenous matrix deposition, it was found that the B-type substitution can enhance the solubility without changing the surface polar property of the material [7], [8], while the human trabecular osteoblastic cells have a lower affinity for the CHA-A surface compared to HA one [9], [10].