Electrochemical stability and bio-mineralization capability of zinc substituted and elemental zinc reinforced calcium phosphate composite coatings synthesized through pulsed electro-deposition

Abstract

Recent studies showed that zinc, as one of the essential element in the human body, is able to promote osteoblast proliferation and bone growth when present with calcium phosphate phases. In this study, zinc substituted and zinc reinforced calcium phosphate composites were prepared by electro chemical deposition from aqueous electrolyte bath. Nucleation rate and reaction time were varied to understand the relation between in-situ formation mechanism of various phases with the varying reaction kinematics and kinetics. The thin functional composite coatings were compared in terms of their surface morphology, lattice structure, surface elemental composition and related electrochemical corrosion resistance along with bio-mineralization capability under simulated body fluid (SBF). On a particular parameter of 10 mA/cm2 current density and 20% duty cycle, nano tubes enriched in elemental zinc; hopeite and calcium orthophosphate composite phases are formed. This unique microstructural morphology and composition make the coating surface 10 times more electrochemical corrosion resistant compared to all other coatings. A high amount of zinc substitution in calcium phosphate phases leads to the development of high compressive lattice strain, which in turn results in rapid dissolution of the coating under contact with SBF. During bio-mineralization, the zinc loading ratio is rapidly decreased for the initial 7 days through preferential dissolution and enrichment in calcium ions and subsequently stabilized at 60% from the initial 83% in nano tube structures. The effective adherence of osteoconduction product on the nano tube structure is achieved post 14 days. In summary, electrochemical deposition was found as an effective way to design various tailor-made/functionally graded zinc-calcium phosphate composite coatings based on specific requirements of biodegradability, biocompatibility and surface properties.