Microwave-assisted solution combustion synthesis of high aspect ratio calcium phosphate nanoparticles
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Joseph Lawrence Department of Bioengineering, College of Engineering, University of Toledo, Toledo, Ohio 43606
Sarit B. Bhaduri Department of Mechanical, Industrial, and Manufacturing Engineering, College of Engineering, University of Toledo, Toledo, Ohio 43606; and Division of Dentistry, College of Medicine, University of Toledo, Toledo, Ohio 43614 (Received 1 February 2013; accepted 4 October 2013)
Calcium phosphates (CaPs) are major chemical constituents of mammalian bone. Their osteoconductivity in vitro and in vivo has encouraged their use in biomaterial applications such as implant materials and drug delivery. High aspect ratio nanoparticles are attractive for many biomedical applications; however, precise control of the phase and morphology is challenging. The impact of fuel-to-oxidant ratio, pH, and cation chemistry on morphology and phase was studied for CaP-based compositions by microwave-assisted solution combustion synthesis (MASCS) in a urea–nitrate (fuel–oxidant) system. An initial calcium to phosphate ratio of 1.5 was used. Highly crystalline hydroxyapatite (HA) and biphasic CaP nanoparticle compositions were produced as confirmed by x-ray diffraction, scanning electron microscopy, and transmission electron microscopy. MASCS was capable of synthesizing high aspect ratio (;5 to 20) single and biphasic CaP nanoparticles with diameters ranging from 250 to 500 nm and lengths between 2 and 10 lm.
I. INTRODUCTION
Calcium phosphates (CaPs) are being investigated for a variety of medical applications, such as tissue engineering and drug and gene delivery due to their similarity with mammalian bone.1–5 Numerous synthesis routes have been identified for the synthesis of CaP nanoparticles such as hydrothermal processing, sol–gel, and molten salt synthesis.6–9 A variety of studies have optimized chemical processing routes to control CaP nanoparticle morphology and, in particular, to produce high aspect ratio nanoparticles of HA.6,8,10,11 High aspect ratio nanoparticles (nanowhiskers, nanorods, nanotubes, etc.) have several distinct advantages. They typically exhibit increased mechanical strength and high surface area to volume ratios as compared to spherical particles. 12 Improved mechanical strength is advantageous in load-bearing applications, such as in dental and orthopedic tissue engineering scaffolds.13 Gene and drug delivery applia)
Address all correspondence to this author. e-mail: [email protected] b) Present address: Vermont Lung Center, Department of Pulmonary and Critical Care, University of Vermont College of Medicine, Burlington, Vermont 05405 DOI: 10.1557/jmr.2013.314 J. Mater. Res., Vol. 28, No. 22, Nov 28, 2013
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cations benefit from increased surface area to volume ratios because higher drug concentrations can be delivered with lesser particulate matter.14 Templating agents and surfactants can be used to generate high aspect ratio CaPs, but they have been known to have cytotoxic effects, even at low residual concentrations.
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