Mechanochemical–hydrothermal synthesis of carbonated apatite powders at room temperature

Abstract

Crystalline carbonate- and sodium-and-carbonate-substituted hydroxyapatite (CO₃HAp and NaCO₃HAp) powders were prepared at room temperature via a heterogeneous reaction between Ca(OH)₂/CaCO₃/Na₂CO₃ and (NH₄)₂HPO₄ aqueous solution using the mechanochemical–hydrothermal route. X-ray diffraction, infrared spectroscopy, thermogravimetry, and chemical analysis were performed. Room temperature products were phase-pure CO₃HAp and NaCO₃HAp containing 0.8–12 wt% of carbonate ions in the lattice. Dynamic light scattering revealed that the median agglomerate size of the room temperature CO₃HAp and NaCO₃HAp powders was in the range of 0.35–1.6 μm with a specific surface area between 82 and 121 m²/g. Scanning and transmission electron microscopy confirmed that the carbonated HAp powders consisted of mostly submicron aggregates of nanosized, ≈20 nm crystals. The synthesized carbonated apatite powders exhibit chemical compositions and crystallinities similar to those of mineral constituents of hard tissues and therefore are promising for fabrication of bone-resembling implants.

Introduction

Hydroxyapatite (HAp) with the chemical formula corresponding to Ca₁₀(PO₄)₆(OH)₂, has been extensively used in medicine for implant fabrication and is one of the most biocompatible materials owing to its similarity with mineral constituents found in hard tissue (i.e. teeth and bones) [1], [2]. It is commonly the material of choice for fabrication of dense and porous HAp bioceramics [3]. Its general uses include biocompatible phase/reinforcement in composites [4], coatings on metal implants [1], [2], [3], and granular fill for direct incorporation into human tissues [1]. Among the variety of HAp-based ceramics, carbonated HAp (CO₃HAp) appears to be an excellent material for bioresorbable bone substitutes [5]. Since the HAp phase present in natural bone, dentin, and enamel, respectively, contains approximately 7.4, 5.6, and 3.5 wt% of carbonate [6], CO₃HAp materials have excellent biocompatibility and properties, which can be favorably compared with those of hard tissue.

Sintered CO₃HAp ceramics can exhibit in vitro cell resorption rates similar to the resorption rate of natural bone minerals [5], which is higher than that of pure HAp ceramics [5], [7]. Cell appearance and proliferation does not differ on CO₃HAp and pure HAp [7], [8]. Recent in vivo study using rats indicated that the dissolution rate of dense sintered CO₃HAp ceramics implanted subcutaneously was intermediate between β-TCP and pure HAp [9]. In another in vivo study, it was found that the quantity of intermedullar bone formed around CO₃HAp implants increased with carbonate concentration [10]. CO₃HAp-forming injectable, moldable bone cement has been commercially available throughout Europe since 1997 and in the USA since 1998 [11]. The CO₃HAp bone cements can be used as in situ-hardened bone fillers [12] or screw fixations [13]. Nanosized CO₃HAp crystals were successfully applied to fabricate CO₃HAp/collagen biodegradable composites [14]. When implanted in rabbits, these materials underwent resorption and promoted new bone formation.

The above studies indicate that CO₃HAp ceramics should be superior to pure HAp for bioresorbable implants. Therefore, development of new synthesis techniques for CO₃HAp powders with controlled morphology and chemical composition is of a great importance. CO₃HAp powders with low crystallinity and nanometer particle size are suitable for the processing of bone-resembling materials [14]. In addition, controlling the level of carbonate substitution in HAp is a convenient way to change solubility [6] and morphology of the synthesized carbonated HAp crystals [6], which subsequently impact properties of the HAp-based biomaterials.

Multiple techniques have been used to prepare HAp and CO₃HAp powders, with wet chemical methods [1], [3], [6], [15], [16] and solid-state reactions [1], [17] as the most popular. Depending upon the technique, powders with different morphologies, stoichiometries and levels of crystallinity can be obtained. Recently, several papers regarding mechanochemical and mechanochemical–hydrothermal synthesis of HAp and CO₃HAp powders appeared in the literature [18], [19], [20], [21], [22], [23], [24], [25]. Mechanochemical powder synthesis is a solid-state synthesis method that takes advantage of the perturbation of surface-bonded species by pressure to enhance thermodynamic and kinetic reactions between solids [26]. In many cases, mechanochemistry can be used to prepare anhydrous multi-component oxides at room temperature in a single step. Pressure can be applied via conventional milling equipment ranging from low-energy ball mills to high-energy stirred mills (e.g. attrition, planetary, or vibratory mills). The main advantages of mechanochemical synthesis of ceramic powders are simplicity and low cost. A variety of chemical compounds have been prepared by this technique, such as CaSiO₃ [27], PbTiO₃ [28], and 0.9Pb(Mg_1/3Nb_2/3)O₃-0.1PbTiO₃ [29]. Since the mechanochemical synthesis involves only solid-state reaction, it should be clearly distinguished from the mechanochemical–hydrothermal synthesis (a.k.a., “wet” mechanochemical), which incorporates an aqueous phase in the system. An aqueous phase can actively participate in the mechanochemical reaction by accelerating kinetic processes that commonly rate limit a process, such as dissolution, diffusion, adsorption, reaction rate, and crystallization (nucleation and growth) [30]. The mechanochemical activation of slurries can generate local zones of high temperatures (up to 450–700°C) and high pressures due to friction effects and adiabatic heating of gas bubbles (if present in the slurry), while the overall temperature is close to the room temperature [31]. Thus, the mechanochemical–hydrothermal technique can be envisioned as being located at the intersection of hydrothermal [32] and mechanochemical [26] processing. The mechanochemical–hydrothermal route can produce large amounts of HAp powder. It also utilizes lower temperature, i.e. room temperature, as compared to 90–200°C for the hydrothermal process. Thus, for the mechanochemical–hydrothermal process, there is no need for a pressure vessel and external heating.

In an earlier study we have demonstrated that stoichiometric, crystalline HAp powder could be prepared at room temperature from heterogeneous reaction between Ca(OH)₂ powder and (NH₄)₂HPO₄ solution via the mechanochemical–hydrothermal route [25]. This process was advantageous when compared to prior reported mechanochemical HAp syntheses. Water actively participated in the reaction by both dissolving one of the reactant powders as well as serving as a reaction medium to produce a stoichiometric and highly crystalline product. In addition, the mechanochemical–hydrothermal route has been demonstrated to be applicable for reproducible and low-cost fabrication of high-quality HAp powders in large batch sizes [25]. In this paper, we will present a mechanochemical–hydrothermal method for preparation of nanocrystalline CO₃HAp and NaCO₃HAp powders with controlled carbonate substitution.