Electrophoretic deposition and characterization of nano-sized hydroxyapatite particles

Hydroxyapatite Ca10(PO4)6(OH)2 (HAP), a major inorganic component of bone, has been used extensively for biomedical implant applications and bone regeneration due to its bioactive, biodegradable and osteoconductive properties [1–3]. However, the pure HAP is very brittle and its poor mechanical properties have limited its clinical applications. For this reason, a great deal of research concentrates on the development of HAP coatings and/or composites. Deposition of HAP coatings onto the surface of metal implants is a relatively recent development in clinical orthopaedics and has been achieved by a number of methods, such as plasma spraying [4, 5], dip coating [6], sputter coating [7], biomimetic coating [8], and electrophoretic deposition [9,10]. Plasma spraying [4] is the most developed process and thermally sprayed implants have been used in clinical practice for some years by now. However, plasma spraying, for extreme high temperatures of the plasma flame acting on HAP, results in the formation of resoluble molten phases. Unfortunately, they degrade in a short time after implantation, decreasing the adhesion of the HAP layer to the surrounding bone, as well as to the metal substrate. Furthermore, thermal spraying is a costly procedure and line-of-sight process, so it is not ideal for coating implants of complex shape or morphology. Electrophoretic deposition (EPD) uses an applied electric field to deposit charged particles from suspension in a liquid onto a substrate, which acts as an electrode. The rate of deposition can be easily controlled by varying the applied current density or voltage and the composition of the deposited layer is determined by the composition of the particles in suspension. EPD is a low-cost, simple, non-line-of-sight and flexible ceramic coating process, and it can be used to deposit coatings on substrates of complex shape or surface morphology. Previous research demonstrates that EPD is an attractive method for formation of biomedical implants and a number of advantages and important possibilities of this method have been suggested [10]. Just like many other ceramic coating techniques, EPDcoated implants need a subsequent densification stage. The main problem associated with this process is the difficulty in sintering of the coatings. High sintering temperature is required for densification of the green coatings [11]. Lower sintering temperature can lead