Long and short range order structural analysis of In-situ formed biphasic calcium phosphates
© Kim et al. 2015
Received: 4 March 2015
Accepted: 9 May 2015
Published: 31 December 2015
Biphasic calcium phosphates (BCP) have attracted considerable attention as a bone graft substitute. In this study, BCP were prepared by aqueous co-precipitation and calcination method. The crystal phases of in-situ formed BCP consisting of hydroxyapatite (HAp) and β-tricalcium phosphate (β-TCP) were controlled by the degree of calcium deficiency of precursors. The long and short range order structures of biphasic mixtures was investigated using Rietveld refinement technique and high resolution Raman spectroscopy. The refined structural parameters of in-situ formed BCP confirmed that all the investigated structures have crystallized in the corresponding hexagonal (space group P63/m) and rhombohedral (space group R3c) structures.
The crystal phases, Ca/P molar ratio, and lattice parameters of in-situ formed BCP consisting of HAp and β-TCP were controlled by the degree of calcium deficiency of calcium phosphate precursors. The significant short range order structural change of BCP was determined by Raman analysis.
The long and short range order structural changes of in-situ formed BCP might be due to the coexistence of β-TCP and HAp crystal phases.
Biphasic calcium phosphates (BCP) have attracted considerable attention as a bone graft substitute [1–4]. Generally, BCP consisting of biocompatible hydroxyapatite (HAp) and biodegradable β-tricalcium phosphate (β-TCP) have better bio-resorbability and osseointegration than the individual HAp or β-TCP components because of their different dissolution behaviors under in vitro and in vivo biological conditions [5, 6].
In addition, the in-situ formation method of BCP can be also applied to various studies of ionic substitutions, biopolymer/calcium phosphate composites, local drug delivery system, and porous scaffolds [13–16]. Because the studies of various attempts for BCP still need to be focused in order to optimize the biological performances. Consequently, fundamental efforts to improve the biological response of in-situ formed BCP have recently based on studies of various biphasic controls of HAp/TCP ratios.
The research for crystal structure of in-situ formed BCP has been the subject of specific interest owing to its essential biological role in the comprehension for coexistence of two crystal phases. Despite having the similar elemental composition, HAp (Ca10(PO4)6(OH)2) and β-TCP (Ca3(PO4)2) differ considerably in their crystal system. For example, crystal system of β-TCP has generally a rhombohedral (space group R3c) or hexagonal structure (space group R3c). HAp has a hexagonal (space group P63/m) or monoclinic structure (space group P21/b). Such different crystallographic forms of HAp and β-TCP can be performed the different biological properties related to biodegradation and dissolution rate. Therefore, if a crystal system of BCP can be controlled and varied by in-situ formed bi-phases, it can suggest a new paradigm of bioceramic applications for improve biological properties. In this study, the ability to clearly identify an individual crystal structure of in-situ formed BCP demonstrate in the long and short range order structural analysis using the Rietveld refinement of X-ray diffraction (XRD) spectra and high resolution Raman spectroscopy.
β-TCP, HAp, and BCP powders were synthesized by the co-precipitation and calcination process. Firstly, an appropriate amount (Ca/P molar ratio 1.5 ~ 1.67) of calcium nitrate tetrahydrate (Ca(NO3)2°4H2O, Sigma-Aldrich) and diammonium hydrogen phosphate ((NH4)2∙HPO4, Sigma-Aldrich) was dissolved in distilled water by vigorously stirring at a rate of 1000 rpm. The pH of the mixed solution was maintained at 8 and 11 by the addition of ammonium hydroxide (NH4OH, Junsei) solution. The co-precipitated suspension was discharged from the reactor and allowed to settle for 24 h for the maturation of precipitate. After 24 h, the precipitates were separated through vacuum filtration technique and dried at 80 °C for 24 h in a drying oven. The as-dried precipitates were calcined at 1000 °C for 24 h in air.
X-ray diffraction analysis (X’Pert Pro, Philips), at 40 kV and 40 mA with a scanning speed of 1o/min, was performed to identify the phases of the as-calcined powders. A standard Bragg-Brentano geometry was applied with a Kα1 monochromatic beam from the Cu anode. Phase identification, quantitative analysis, determination of Ca/P ratio, and lattice parameters for the BCP powders were characterized using Phillips X’Pert HighScore Plus software with a full-pattern fit using Rietveld method [17, 18]. The Raman spectra were recorded on a Sentinel Raman spectrometer (Bruker Optics Ltd.) with a Unilab II probe (fiber optic) and a CCD detector was used in this study. A 532 nm Nd: YAG laser source was used for excitation with an incident laser power of 30 mW. The spectral range was 500 to 4400 cm−1 with a resolution of 4 to 6 cm−1.
Results and discussion
As shown in Fig. 1(b), BCP powders have indicated only the presence of HAp and β-TCP in its composition but their quantitative phase contents determined through Rietveld analysis were found to show significant variations, and their Ca/P ratio were totally dependent on the percentage of determined crystal phase.
HAp, β-TCP and in situ formed BCP powders were synthesized by the co-precipitation and calcination process. The refined structural parameters of in-situ formed BCP confirmed that all the investigated structures have crystallized in the corresponding hexagonal (space group P63/m) and rhombohedral (space group R3c) structures. The molar Ca/P ratio of in-situ formed BCP was also determined by Rietveld analysis. The crystal phases, Ca/P molar ratio, and lattice parameter of in-situ formed BCP consisting of HAp and β-TCP were controlled by the degree of calcium deficiency of calcium phosphate precursors. The significant short range order structural change of BCP was determined by Raman analysis. The short range order structures of in-situ formed BCP was considered to be affected by the coexistence of β-TCP and HAp phase.
Availability of supporting data
The data set supporting the results of this article is included within the article Tables 1, 2, 3, 4 are represented structure model of HAp and β-TCP crystal in XRD results of this study (Additional file 1).
This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (No. 2013R1A1A4A01009089). This work was also supported by Korea Institute of Industrial Technology.
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