Investigation of Plasma Electrolytic Oxidation Processes for Production of Porous Biocompatible Coatings on Ti alloys

Plasma electrolytic oxidation (PEO) attracts increasing interest in biomedical applications as it can form porous TiO2 coatings incorporated with Ca and P on Ti alloy substrates. However, the formation of PEO coatings containing crystalline apatite phases using a single stage PEO process is difficult. The aim of the present study was two-fold. Firstly, to expand the cyclic voltammetry (CV) approach into high anodic potentials to achieve a better understanding of the electrochemical behaviour of Ti alloy electrodes under the conditions of PEO. Secondly, to develop a range of novel electrolytic plasma processes to produce biocompatible coatings for Ti implants.
Initially, the CV method was employed to identify characteristic potential ranges corresponding to differential behaviour of Ti alloy electrodes in the sodium orthophosphate electrolyte containing additives of Ca salts of various carboxylic acids. The two Ca salts have then been identified to be suitable for PEO electrolytes and during this progressive study a method to produce uniformly porous CaP containing PEO coatings using a single stage PEO treatment has been developed. Finally, this mode has been adopted for PEO treatments of Ti in electrolytes containing particulate additives of hydroxyapatite (HA).
Coatings produced were examined using scanning electron microscopy (SEM), electron dispersive spectroscopy, X-ray diffraction analysis and glow discharge optical emission spectroscopy. Minkowski functionals were used to analyse the morphological features in the surface-plane SEM images, which allowed subjective interpretation of the coating morphology to be eliminated. The proliferation and viability of human osteosarcoma (MG-63s) and mouse osteoblast (MC3T3-E1) cells cultured on selected PEO coatings were studied and showed these characteristics to be comparable to those of the cells cultured on tissue culture plastic control.
This study has substantially expanded the understanding of electrochemical processes that occur during the PEO treatment of Ti and broadened the range of electrolytes for production of apatite containing PEO coatings for orthopaedic and orthodontic implant applications. These results provide a benchmark against which further work investigating different aspects of PEO processing of Ti and other materials could be undertaken.