Fabrication and characterization of hydroxycarbonate apatite with mesoporous structure

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Abstract

Mesoporous hydroxycarbonate apatite (MHCAp) was converted directly from calcium carbonate particles by treatment with a phosphate buffer solution (PBS). After soaking in PBS, the calcium ions are released from CaCO3 particles and react with phosphate ions to form hydroxycarbonate apatite nanoparticles. The aggregation of the nanoparticles not only produces apatite plates, but also forms mesopores within the plates. The phases of CaCO3 play an important role in the conversion rate because of the different solubility products. If the main phase is vaterite, CaCO3 particles are converted completely to MHCAp after soaking in PBS for 24 h. However, if the main phase is calcite, the percentage of the unreacted CaCO3 particles is ∼19.7%. In addition, the conversion percentages of MHCAp increase with prolonging immersion time in PBS and increasing temperatures. The most of pore sizes of MHCAp are distributed around 4.1 ± 0.3 nm, which are not affected by the experimental conditions such as the phases of calcium carbonate, immersion time (6–24 h) and temperatures (25–50 °C). Simulated body fluid (SBF) immersion tests reveal that MHCAp exhibits a superior in vitro bone-forming bioactivity. It is hypothesized that the conversion mechanism of CaCO3 to MHCAp may be rationalized in terms of a dissolution–precipitation process.

Introduction

Hydroxycarbonate apatite is the major inorganic component of natural bones [1], and the corresponding synthetic material is effective in filling cavities and defects in bone owing to the excellent biocompatibility and bioactivity [2], [3]. The bone-forming ability of biomaterials is associated not only with their chemical composition and structure, but also with their textural properties, such as pore size, pore volume, pore structure [4]. The mesopores (2 nm < pore size < 50 nm) and micropores (<2 nm) are suggested to promote cell adhesion, adsorption of biological metabolites, and resorbability at a controlled rate to match that of tissue repair [5]. Moreover, the mesoporous biomaterials with large surface areas and large pore volumes make themselves become excellent candidates for local drug delivery systems [6], [7], such as for the delivery of anti-tumor agents and antibodies in the treatment of osteomyelitis [8]. The controlled drugs released from the porous biomaterials help to minimize the toxic side effects and bone infections.

Since the discovery of silica-based MCM-41 molecular sieve was first reported in 1992 [9], [10], a variety of mesoporous materials and related synthesis strategies have been developed [11], [12], [13], [14], [15]. Recently, mesoporous biomaterials, such as hydroxyapatite, bioactive glasses, and bioceramics, have been fabricated by template or surfactant synthesis. The common templates or surfactants include cetyltrimethylammonium bromide (CTAB) [16], mono-n-dodecyl phosphate (MDP) [17], block co-polymer EO106PO70EO106 (EO is poly(ethylene oxide) and PO is poly(propylene oxide)) [18], EO20PO70EO20 [4], [6], and EO39BO47EO39 (BO is poly(butylenes oxide)) [19]. Although template or surfactant synthesis is an available method of obtaining mesoporous biomaterials, such method has two disadvantages: (i) calcinations to remove the templates or surfactants impair partly the mesoporous structure and decrease the BET surface area; and (ii) the remained surfactants or templates will contaminate final samples and deteriorate the bioactivity of biomaterials.

In this work we present a new and simple synthesis strategy to fabricate mesoporous hydroxycarbonate apatite (MHCAp). Namely, after soaking in a phosphate buffer solution (PBS) at low temperatures, calcium carbonate is converted directly into MHCAp via a dissolution–precipitation reaction. This approach has four advantages: (i) MHCAp is chemically more similar to the biological apatite and has a larger surface area than sintered hydroxyapatite; (ii) the low temperature processing allows the introduction of proteins or drugs into MHCAp [20]; (iii) no templates or surfactants will contaminate final products and deteriorate the bioactivities; and (iv) the low temperature processing reduces the cost of material processing and prevents the second phase decomposition which often occurs at high temperatures.

Section snippets

Materials

The nacre of Corbicula fluminea was collected from Zhejiang province in China, composed of 98.1 wt% mineral phases and 1.9 wt% organic components. The nacre was obtained according to the literature [21]. The other materials for the synthesis of MHCAp were used as received.

Methods

The nacre was demineralized with a 1.0 mol/l HCl solution overnight, and diluted with deionized water till the concentration of calcium ions was 0.25 mol/l. After centrifugation and separation from the HCl-insoluble material, the

Structure and morphology of CaCO3 particles

Fig. 1 shows XRD patterns and FT-IR spectra of CaCO3 particles obtained in the Na2CO3 and CaCl2 solutions with HCl-soluble organic materials. CCMs and CCPs in the forms of both calcite (JCPDS Card No. 86-0174) and vaterite (JCPDS Card No. 72-0506) are obtained, as shown in Fig. 1a. FT-IR spectra are also used to determine the polymorphs of the CaCO3 particles. The absorption bands at 877 (v2), 744 (v4) cm−1 are attributed to vaterite, while the absorption bands at 877 (v2) and 712 (v4) cm−1 are

Conclusions

MHCAp was obtained from calcium carbonate by treatment with PBS at low temperatures without using any structure-directing agents. The formation mechanism of MHCAp is a dissolution–precipitation reaction. After soaking CaCO3 particles in PBS, the hydroxycarbonate apatite nanoparticles deposit on the surfaces and aggregate, resulting in the formation of slit-shaped mesopores without precise orientation ordering. The formation rate of MHCAp can be controlled by the experimental condition such as

Acknowledgments

This research was supported by Program for New Century Excellent Talents in University (NCET-04-0327) and Program of Excellent Team in Harbin Institute of Technology.

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