Electrospraying of a nano-hydroxyapatite suspension

Abstract

Over the past two decades it has been established that bone repair in the presence of implant materials can be optimized by control of their surface chemistry. Certain calcium phosphate materials are similar in composition to bone mineral and have been found to be osteoconductive [1–3]. These ceramic materials have therefore been developed as biomaterials for a range of different applications. However, although they promote direct bone apposition, their mechanical properties are relatively poor compared with other monolithic ceramics. Therefore, classes of applications have been developed to exploit the excellent biological properties of the materials while maintaining mechanically viable implants [4, 5]. There is now increasing evidence that surface topography both on the micro and nano-scale are important in determining the cell response to biomaterials [6– 11]. A number of studies have indicated that cell activity can be up-regulated through optimization of the surface properties of the substrate. Investigations originally concentrated on surface features of several tens of micrometers in scale, but more recently, evidence suggests that surface topography at a much finer scale may influence the cell response. Hydroxyapatite (HA) has been used in a number of different forms and several powder processing routes have been investigated in conjunction with HA [12– 18]. The application of jet-based suspension processing methods, such as ink-jet printing (IJP) [19, 20] and electrostatic atomization printing (EAP) [21, 22] allows solid freeforming of fine structures of advanced materials. In particular, EAP in the cone-jet mode enables the formation of <50 μm size relics of advanced materials, an order of magnitude finer than those produced using IJP, using needles having an internal diameter of ∼200 μm, ×4 coarser than those used in IJP. However, electrostatic atomization or electrospraying has been developed largely by the aerosol industry to process liquids and, only very recently, to process suspensions containing micrometer size particles [21, 22]. In this letter we describe the use of electrospraying to process a suspension contain nano-hydroxyapatite (nHA) particles to deposit droplets, which after spreading on a glass substrate, allows the preparation of relics <1 μm in size. nHA was synthesized by a precipitation reaction between calcium hydroxide (Ca(OH)2) and orthophosphoric acid (H3PO4) with a Ca/P ratio of 1.67. Both reagents were AnalaR grade, obtained from BDH, UK. 0.3 M H3PO4 solution was added drop wise to 0.5 M Ca(OH)2 solution under continuous stirring at room temperature, while the pH was kept above 10.5 by the addition of ammonia solution. The stirring was maintained for a further 16 h after complete addition of the reactants. The precipitate obtained was aged for a further week and then washed with boiling water. Transmission electron microscopy (TEM, a Jeol 200CX transmission electron microscope at accelerating voltage of 200 keV) of the precipitate revealed that rodlike nHA particles with the size of 50 to 80 nm were obtained and selected area diffraction (SAD) showed a spotted pattern, indicating a polycrystalline material (Fig. 1). The structure of nHA was studied by Xray diffraction (a Philips PW1730 diffractometer using CuKα radiation), and the product was found to be phase pure HA. The nHA particles were suspended in ethanol to give a slurry with a concentration of 3 vol.% nHA.