Fabrication and Characterisation of Calcium Phosphate - Liposome Composites as an Implant Coating
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Fabrication and Characterisation of Calcium Phosphate - Liposome Composites as an Implant Coating
Tarinee Pongsaanutin and Jan T. Czernuszka Department of Materials, University of Oxford, Oxford, OX1 3PH, UK. ABSTRACT Calcium phosphate in the form of apatite has been successfully precipitated on the surface of liposomes. Liposome vesicles were prepared by sonication of phosphatidylcholine and this was introduced into an aqueous solution of calcium and phosphate ions supersaturated with respect to hydroxyapatite. Calcium phosphate was shown to precipitate solely on the outer layer surface of the liposome vesicles. These composite assemblies were then deposited onto a stainless steel cathode substrate using an electrophoretic method at physiological temperatures. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), powder xray diffraction (PXRD) and Fourier transform infrared spectroscopy (FTIR) were used to analyse the morphology, structure and chemical composition of the composite coating. The results from PXRD and FTIR show a mixture of amorphous and poor crystalline hydroxyapatite (HAp). This was verified by electron diffraction. Dark field images confirmed that the precipitated HAp deposited solely at the outer surface of the liposomes. SEM micrographs demonstrated a thin uniform coating at the microstructure level. These results suggest that these calcium phosphateliposome composites can be formed. They have tremendous potential for use as drug delivery devices in aiding the treatments of bone disorders.
INTRODUCTION Calcium phosphate ceramics, especially hydroxyapatite have been effectively used for dental and orthopaedic applications due to their excellent osteoconduction and biocompatibility [1, 2]. It is also known that liposomes can be used as effective drug delivery systems because they are biodegradable and they can be targeted to specific sites such as areas of inflammation and bone marrow [3]. It is well known that liposomes can be calcified internally [e.g.4]. However, we will show that it is possible to obtain calcification of these liposomes solely on their external surfaces. Thus, these assemblies can be then used to encapsulate drugs to treat infection, pain, osteoporosis, arthritis and tumours or to aid in bone formation.
EXPERIMENTAL PROCEDURE All reagents were analytical grade supplied from BDH Chemicals Ltd., Poole, England apart from the phosphatidylcholine (PC) which was obtained from Sigma Chemical Co., UK. All aqueous solutions were prepared in distilled water. Liposome vesicles were prepared by dispersing dried films of phosphatidylcholine in HEPES (N-2-hydroxy-ethylpiperazine-N -2LL4.7.1
ethane-sulfonic acid) - buffered solution (10mM, pH7.4). After 2 hours of ageing at room temperature the solution was sonicated for 40 minutes [5]. The calcification precipitation reactions were initiated by mixing the liposome suspensions with a buffered phosphate and calcium ion solution (10mM HEPES; 3.0 mM KH2PO4, 5.0 mM CaCl2.2H2O). All experiments were carried out at pH 7.
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