Electrohydrodynamic Deposition and Patterning of Nano-hydroxyapatite for Biomedical Applications

Electrohydrodynamic atomisation (EHDA) spraying is a promising materials deposition technique as it allows uniform and regular deposition, and offers a range of other advantages, such as low cost compared with other current techniques, easy set-up, high deposition rate, ambient temperature processing and the capability to generate specific surface topographies. This research is aimed at using EHDA spraying to produce hydroxyapatite (HA) deposition with desirable chemical, topographical and biological characteristics for bone implant. In principle, the EHDA process involves the flow of liquid/suspension from a needle under the influence of an electric field which results jetting and droplet formation. In this work, phase-pure nano-sized hydroxyapatite (nHA) was synthesised and taken up in ethanol to prepare a suitable suspension for electrohydrodynamic flow processing. A range of key EHDA process control parameters, such as needle size, needle to substrate distance, suspension flow rate, applied voltage and spraying time, were studied and optimised. A uniform nHA coating with nanometer scale topographical features was successfully prepared on a commercially pure titanium substrate. Furthermore, due to the significance of the surface structure to the cellular response, a novel technique, namely template-assisted electrohydrodynamic atomisation (TAEA) spraying, was innovated to prepare a well-defined surface topography for guiding cell attachment, spread and growth of osteoblasts. A range of precise micro-scale uniform nHA geometries with high resolution were prepared on implants materials. Finally, to systematically investigate the effect of needle geometry to the electrospraying process, which has not been documented in the research field of this technique, an in-depth study was carried out to uncover the relationship between the needle exit angle and the droplet relic size. The droplet relic size, which is crucial for the deposition properties, has been significantly reduced via engineering the needle geometry during the electrospraying process. The results of this work have demonstrated that EHDA deposition routes show great potential for the commercial preparation of nHA coatings and patterns for bone implants with enhanced bioactivity.