Production of ultra-fine bioresorbable carbonated hydroxyapatite

Abstract

Ionic-substituted hydroxyapatite (HAp) based materials may be a better choice than pure HAp owing to their similarity in chemical composition with biological apatite. The present study reports a process for the production of carbonated hydroxyapatite (CHAp) using microwaves. The CHAp was evaluated for its phase purity, chemical homogeneity, functionality, morphology, and solubility. The CHAp thus obtained was compared with a pure HAp and a biological apatite, which provides quite an interesting insight into the carbonate substitution. The in vitro ionic dissolution rates determined under physiological conditions clearly demonstrate the soluble nature of CHAp compared to HAp. The overall results indicate that the processed CHAp has increased resorption relative to pure HAp and has a chemical composition corresponding to some extent with that of biological apatite.

Introduction

Nature, always a paradigm, sets high standards for the researchers who design biomedical implants. This may be true, particularly in bone-related therapy, because there is no ideal material or implant available that mimics real bone. Despite the numerous attempts that have been made in the last few decades, there is still a substantial need for bone substitutes to stimulate research in a more dynamic way. Hydroxyapatite (HAp), Ca₁₀(PO₄)₆(OH)₂, has a long history of being used as a biomaterial in bone grafting, bone tissue engineering, and bone drug delivery, owing to its obvious properties of biocompatibility, bioactivity, osteoconductivity, non-toxicity, non-inflammatoriness and non-immunogenicity. Numerous techniques are also available for processing HAp at micro and nanoscale levels [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12]. Although it is considered as a good bone substitute, HAp slightly differs from the biological apatite in terms of structure, composition, crystallinity, solubility, and biological reactivity. Most of the biological apatites are non-stoichiometric, poorly crystalline, and contain several foreign ions, mainly carbonate $({\mathrm{CO}}_3^{2-})$ and traces of Na⁺, Mg²⁺, Fe²⁺, Cl⁻, ${\mathrm{HPO}}_4^{2-}$, F⁻ [13], [14]. Among them, ${\mathrm{CO}}_3^{2-}$ ions play a vital role in the bone metabolism; they occupy about 3–8 wt.% of the calcified tissue and may vary depending on the age factor. Therefore, ${\mathrm{CO}}_3^{2-}$ substituted HAp may have a tremendous potential in bone-related therapy. Further, HAp is the least soluble and the most stable material among the calcium phosphates, which is an undesirable characteristic because it may impede the rate of bone regeneration upon implantation [15]. By contrast, biological apatite has a higher solubility due to the presence of the above said trace elements.

It is always desirable that a bone substitute should be bioresorbable to some extent so that it can be replaced, over a period of time, with the regenerated bone. The resorbability of HAp can be improved with the help of some ceramic oxides, ionic doping agents, or by reducing its grain size to the nano level [16], [17], [18], [19], [20]. On the other hand, substitution of ${\mathrm{CO}}_3^{2-}$ ions into the apatite may lead to a higher solubility and it is expected that it may also increase mechanical strength. Therefore, it is important to pay a great deal of attention to the development of CHAp-related biomaterials; thereby this investigation presents an elegant process using microwaves, which allows the production of CHAp with improved characteristics.