Large scale synthesis of hydroxyapatite nanospheres by high gravity method

https://doi.org/10.1016/j.cej.2011.07.053Get rights and content

Abstract

Hydroxyapatite (HAp) nanospheres were synthesized in large scale using high gravity method. The effects of flow rates (300, 500 and 700 mL/min) and rotating speeds (1000, 2000 and 3000 rpm) on the size, distribution and morphology of nanoparticles were studied. The liquid flow rate and rotation speed was found to have a great influence on the formation of nanoparticles and uniform size distribution. The particle size decreased with an increase in the rotation speed of the RPB. The particle size was observed to be smaller at a lower liquid flow rate compared to that of the high liquid flow rate. TEM images of the granular HAp nanoparticles clearly displayed the spherical shape of nanoparticles. Densely assembled arranged nanospheres with diameters of 20–40 nm were observed at various rpm values. The obtained nanospheres appeared to be single crystalline from the image and SAED pattern analyses. In vitro cellular analysis showed very good improvement in the cell adhesion. For mechanical analysis, HAp/HMWPE composite was fabricated. The smaller size of HAp particles provided a relatively homogenous mixture of HAp and HMWPE which ensured the good mechanical strength.

Highlights

Hydroxyapatite nanospheres were prepared by high gravity method in large scale. ► Uniform nanosized distribution was achieved by controlling the reactor parameters. ► Nanospheres/polymer reinforcement has shown improved mechanical strength. ► In vitro cellular analysis exhibits good biocompatibility as the size of the nanospheres decreased.

Introduction

Nano dimensional particles have attracted active attention in recent years due to their potential applications in various fields because of their unique properties. Since the properties of a certain material are dependent on the size and morphology, nanoparticles are expected to have properties that are different from those of the bulk states. A colloid-based wet chemical process has been widely used for synthesizing nanoparticles. Morphological modification of nanoparticles in solution by various methods is one of the recent advancement in the field of chemical synthesis. In order to obtain and exploit novel properties and high performances of nanoparticles, not only the small size, but also the uniform size distribution of nanoparticles should be achieved [1]. It is imperative to develop a reliable large scale fabrication method of nanoparticles with a high product quality for more wide-spread and general applications of high performance nanoparticles for human benefits.

Hydroxyapatite with the chemical formula Ca10(PO4)6(OH)2 is the major inorganic element of teeth and bone. Synthetic HAp is biocompatible with the human body and is widely used as a biomaterial for bone tissue regeneration in medicine [2], [3]. It is also a promising material for reinforcing filler for bio-composites [4]. Despite their favorable biological properties, the poor mechanical properties of HAp bioceramics have hindered their clinical applications. Hence a number of studies have been focused on the improvement of the mechanical properties of HAp [5]. It is shown that, mechanical properties of the ceramic materials could be improved remarkably by the formation of one dimensional (1-D) nanoscale building blocks such as nanorods, nanofibers and nanotubes [6], [7], [8]. It has been reported that these nanostructured-HAp has unique properties such as enhanced biocompatibility, bioactivity and flexibility [9], [10], which are essential to improve their applicability in biomedicine and biomaterials.

Different morphologies of HAp particles like nanorods [11], nanoparticles [12], plate-like crystals [13] have been synthesized by various synthesis method such as wet chemical process [14], sol–gel process [15], emulsion process [15], chemical precipitation approach [16], hydrothermal reaction [17], sol–gel synthesis [18], [19], [20] and mechano-chemical synthesis [21]. Among these processes, wet chemical process has been extensively used since it is simple and the morphology of the nanoparticles can be modified by controlling reaction parameters [14], [22], [23].

Due to its tendency of agglomeration, synthesis of HAp nanoparticles in large scale is a challenging task. In this work we report the high gravity method to prepare HAp nanospheres in large scale with uniform size distribution with limited agglomeration. HAp particles with the desired stoichiometry and crystallinity were known to be produced through the optimization of parameters at relatively low temperatures in the high gravity method. The main idea behind the high gravity method is to intensify micromixing (mixing on the molecular scale) and mass transfer and therefore to enhance the reaction and control the process ideally [24]. It is reported that [25] sub-micron and even nanoparticles, including CaCO3, SrCO3, and Al(OH)3, were successfully produced by a high gravity rotating packed bed (RPB) reactor without adding any surfactants to the reacting solution. Similar method called spinning disc processors have been successfully modeled and applied for the synthesis of AgNO3 [26], BaSO4 [27], TiO2 [28]. But that method is a two step process where the reaction occurs at the surface of the spinning disc and completed in the continuous stir tank reactor (CSTR). But our high gravity method is a single step process unit where the complete reaction takes place in the RPB itself and the following process can be continuously arranged. Yang et al. reported the preparation of hydroxyapatite nanoparticles using high gravity precipitation combined with hydrothermal method which can be considered to be two step processes [29]. During the hydrothermal method various changes in crystallinity, morphology, crystal size etc., are likely to take place due to the possible solid–liquid reaction and mass transfer under the effects of pressure and temperature [11]. Hence it is very difficult to separate the specific effect of the processing parameters in high gravity method on the size, morphology and properties of nanoparticles after the high gravity method followed by the hydrothermal method. In hydrothermal synthesis, large scale production needs sophisticated reactor system to maintain the high pressure, temperature and the process is likely to be discontinuous. Herein, we report the successful large scale production of hydroxyapatite nanospheres by the single step high gravity method.

Section snippets

High gravity system

There are some detailed reports available about the high gravity method which is used for preparing sub micron-/nano-particles [24], [25]. A description about the system we used and the main principle is given here briefly. A schematic representation of high gravity set up used in this study is shown in Fig. 1. Rotating packed bed (RPB) set up is the key part of the high gravity system. The schematic depiction of RPB is given in Fig. 2. RPB mainly consists of a rotating packed rotator inside a

Characterization

The prepared samples were structurally characterized by X-ray diffraction (XRD) analysis using a Cu-Kα1 radiation (RIGAKU X-RAY DIFFRACTOMETER D/MAX-2200). The morphology, particle size and size distribution of particles were investigated by a field emission scanning electron microscope (FESEM JEOL JSM-6500) at 10 kV after sputter coated platinum for conduction. To gain further insight into the microstructures, transmission electron microscopic (TEM) investigations were performed using JEOL

Structural analysis

Fig. 3 shows the XRD patterns of the HAp particles prepared by chemical precipitation through the high gravity approach. All peaks for the HAp particles correspond to pure phase of HAp. The most intensive lines ((0 0 2), (2 1 0), (2 1 1), (3 0 0), and (2 0 2), (1 3 1), (2 2 2), (3 2 1) and (0 0 4)) were located between 2θ and 25–55 degrees. These results are also in good agreement with the standard JCPDS values (74-566). XRD peak broadening was observed by increasing the rpm of the RPB, which can be attributed

Conclusion

This work describes the large scale synthesis method of HAp nanoparticles by a high gravity method for the first time. Detailed characterization was carried out to examine the structure, morphology and particle size of HAp nanoparticles. The liquid flow rate and rotation speed have a great influence on the formation of nanoparticles and uniform size distribution. The particle size decreased with an increase in the rotation speed of the RPB. In a lower liquid flow rate, the particle size was

Acknowledgements

One of the authors AJN gratefully acknowledges the financial support from the Ministry of higher education, Taiwan, through Taiwan–India collaborative research project. Also this work was partially supported by Korea National Research Foundation (2009-0077110).

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