Abstract
The aim of this study was to incorporate carbonate ions (CO3 2–) into the hydroxyapatite (HA) crystal structure followed by investigation on the effect of different carbonate to phosphate (CO3 2–/PO4 3–) ratios on the phase purity, crystal structure as well as CO3 2– content present in the apatite structure. CO3 2– substitution has been proposed to enhance the performance of HA-based material, particularly on the physico-chemical properties. Three different compositions of carbonated hydroxyapatite (CHA) powder with different CO3 2–/ PO4 3– ratios (namely, CHA 1:1, CHA 2:1 and CHA 4:1) were chemically synthesised by nanoemulsion method at 37°C and characterised for their physico-chemical properties. Results demonstrated that all as-synthesised powders formed single phase B-type CHA without any additional phases. Interestingly, an increasing amount of CO3 2– substituted into the apatite structure gives rise to the formation of CHA structure with a variation on their cell parameters and the degree of crystallinity. An increase in the CO3 2–/ PO4 3– ratio was also found to lead a higher amount of CO3 2– content present in the as-synthesised powder (in a range of 4 wt % to 10 wt %), which is comparable to the CO3 2– content found in the human bone mineral.
Highlights
Hydroxyapatite (HA), Ca10(PO4)6(OH)[2] is one of the calcium-phosphate-based materials, which had been widely used as bone and dental materials due to its affinity to the mineralised phase of natural human bone and tooth
HA has been reported to have significant drawbacks, where its resorption in vivo is too sluggish to induce the formation of new bone tissue and in terms of composition, HA is lacking of minor trace elements found in native bone, i.e. carbonate (CO32–), orthosilicate (SiO44–), magnesium ion (Mg2+), cobalt ion (Co2+), strontium ion (Sr2+) and zinc ion (Zn2+).[3]
It is noticed that the incorporation of CO32– into apatite structure had slightly altered the crystal lattice resulting in the contraction of a-axis and extension of c-axis relative to the standard reference of pure HA (a = 9.418 Å, c = 6.884 Å), indicating the formation of B-type carbonated hydroxyapatite (CHA) for all CHA produced powders.[17,18]
Summary
Hydroxyapatite (HA), Ca10(PO4)6(OH)[2] is one of the calcium-phosphate-based materials, which had been widely used as bone and dental materials due to its affinity to the mineralised phase of natural human bone and tooth. Researchers worldwide have been realised that the mineral phase of human bone is not solely calcium phosphate, and composed of a number of ions, where CO32– is the most abundant species (2 wt % to 10 wt %) found in native bone.[5,6] The incorporation of CO32– ions in HA was reported to increase the dissolution rate and solubility, which directly enhances its osteointegration rate.[7] carbonated hydroxyapatite (CHA) seems like a promising bone material since its chemical composition is more akin to the main inorganic mineralised part of the native bone mineral as compared to stoichiometric HA.[8,9]
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