Gas injection approach for synthesis of hydroxyapatite nanorods via hydrothermal method
Graphical abstract
Introduction
Bone and dental tissue is composed primarily of organic collagen and about 60% mineralized matrix. The mineral part consists of nanoscale Hydroxyapatite (HA) crystals [[1], [2], [3]]. HA is one of the most important and most valuable biomaterials on the market and has been widely used in medicine and dentistry for decades [4]. The chemical formula of HA in a stoichiometric state is Ca10(PO4)6(OH)2, in which the molar ratio of calcium to phosphorus is 1.67 [5]. HA crystals vary in size and morphology depending on the age and type of bones [6,7]. Synthetic HA has excellent biocompatibility due to its chemical structure being similar to hard bone tissue and HA implants have excellent integration with surrounding bone owing to its unique bioactivity [[8], [9], [10], [11]].
The applications of synthetic HA ceramics are very extensive including implant coatings [12], dentine tubule infiltration [13], hard tissue scaffolds [14], and as matrices for controlled drug release [15]. However, nanostructured HA has expanded the applications of these materials especially with regard to tissue engineering scaffolds due to the high surface to volume ratio with the degree of osteoconductivity directly related to the specific surface area [16,17]. Nanostructured HA particles can be used as building blocks for damaged enamel and osteoporosis [18]. The crystal size and crystallinity are important factors in implant applications because HA solubility decreases with increasing crystals size and crystallinity [19]. For these reasons, many studies have been carried out with various HA aspects, including rods [20,21], wires [22], ribbons [23], tubes [24,25].
Several methods have so far been reported for the synthesis of HA, including Solid-state reaction [26], sol-gel [27,28], combustion preparation [29], electrochemical deposition [30], precipitation [31,32], hydrolysis [33], multiple emulsion [34], biomimetic deposition [35], sputtering [36] and hydrothermal [[37], [38], [39]]. Common problems for these synthesis methods include agglomeration, long reaction time, uncontrolled particle size and non- stoichiometric products. Despite the plethora of synthesis methods, only a few of these methods provide sufficient morphological control. One of these is the hydrothermal technique, which uses high temperature and high pressure to achieve controlled crystal morphology [40]. So far, many studies have been done to synthesize one-dimensional HA such as ribbons, whiskers, platelets and tubes [[41], [42], [43], [44], [45], [46]] in a hydrothermal method and in some reports, organic modifiers have also been used [47,48]. Of course, this method has many other benefits [49]. In this method, precursors that are soluble in water, such as diammonium phosphate and calcium nitrate [50,51] are used, and the high pressure and temperature conditions lead to obtain the nano-sized hydroxyapatite powders with high crystallinity, which does not require post reaction calcination. The final product is HA nano-particles with accurate chemical stoichiometry and minimal agglomeration [[52], [53], [54]].
As mentioned, crystallinity of hydroxyapatite is one of the most important factors that affects biological properties and improves mechanical properties. Crystallinity increases with increasing time, pressure and hydrothermal temperature. One of the strategies for increasing pressure is the use of gas injection. Using hydrogen and argon mixed gas firstly raises pressure, and secondly, the presence of hydrogen produces hydrogen-rich water [55].
In this study, the mixture of argon and hydrogen gas was used to increase the hydrothermal pressure. This strategy has not been used before and the main purpose of this process is to increase the kinetics of hydrothermal reactions and crystallinity of synthesized powders. In this study, a traditional hydrothermal method and a new gas injection method were used to synthesize powders. First, the properties of synthesized powders were investigated and compared, and then the powders were consolidated and characterized to determine the effect of powder properties on the bulk properties.
Section snippets
Materials and methods
The chemicals used to synthesize powders, including Diammonium hydrogenphosphate ((NH4)2HPO4), Calcium nitrate tetrahydrate (Ca (NO3)2.4H2O), and ammonia solution (25%) were purchased from Sigma Aldrich.
The materials used in cell culture include 1) Human Mesenchymal Stem cells (Lonza, PT-2501), 2) DMEM, low glucose (Invitrogen, 11880-028), 3) FBS (Gibco, 10270, lot 41F1633K), 4) l-Glutamine (Gibco, 25030-081), 5) Penicillin-Streptomycin Solution (Sigma, P0781), 6) Tissue Culture Plate, 24 well,
Results and discussion
The analyses were done in two steps. First, the characterization of hydrothermal synthesized powders was performed and in the following, the characteristics of SPS sintered bulk samples were investigated.
Conclusions
The effect of hydrogen and argon mixed gas injection with a ratio of 15 to 85 on the synthesis of hydroxyapatite nanoparticles by hydrothermal method was studied in this research. The results of the XRD and Raman spectroscopy analysis showed that the crystallinity of the hydroxyapatite nanoparticles was increased. Microscopic images showed that the dimensions of the synthesized powders are in the nanometric range. It was also found that gas injection caused the growth rate to increase and
Declaration of competing interest
The authors report no relationships that could be construed as a conflict of interest.
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