Formation of strontium-substituted hydroxyapatite coatings on bulk Ti and TiN-coated substrates by plasma electrolytic oxidation

Crystalline strontium-substituted hydroxyapatite (Sr-HAp) coatings with porous structures were directly produced on bulk Ti and TiN-coated substrates by plasma electrolytic oxidation (PEO). PEO was conducted in the electrolytes consisting of 0.4 M Ca(CH3OOH)2·H2O and 0.2 M NaH2PO4·2H2O mixed with various concentrations of Sr(OH)2·8H2O ranging from 0-0.1 M at 350 V for 15 min on bulk Ti. The relative integrated peak intensity, hydrophilicity, and cell viability of obtained Sr-HAp coatings firstly increased and then decreased with increasing the Sr2+ content. This indicates that optimum Sr addition could enhance both the growth and cell viability of the PEO-produced Sr-HAp coatings. Subsequently, the optimum Sr addition [0.05 M Sr(OH)2·8H2O] was employed to make Sr-HAp coatings on TiN-coated substrates. The Sr-HAp coatings with fine porous morphology were obtained. Moreover, average growth rate of the coatings over TiN/Si was much higher than that on bulk Ti. The osteoblast-like cell cultivation tests revealed that the cell viability of the PEO-produced Sr-HAp coatings on TiN-coated substrates was significantly higher than that over bulk Ti. The Sr-HAp coatings with TiN seeding layers are very promising in biomaterials applications.