Structural optimization of Sr/Zn-phosphate conversion coatings triggered by ions preloading on micro/nanostructured titanium surfaces for bacterial infection control and enhanced osteogenesis

Phosphate conversion coatings on metallic implants can synergistically integrate functional components and structural regulation, offering excellent biocompatibility and osteogenic activity. However, the passive oxide layer on the titanium (Ti) surface impedes the following chemical reactivity, adversely affecting the microstructure and properties of phosphate coatings. This study proposes a strategy for achieving structural optimization and properties enhancement of strontium-zinc phosphate (SrZnP) conversion coatings on Ti via regulating interface chemical reaction between coatings and Ti substrates. The results indicated that Sr²⁺ and Zn²⁺ ions-preloading (IPL) treatment enhanced the interfacial reactivity, which can further achieve crystal refinement and uniform crystal size in nucleation. In contrast, microstructural modifications on Ti substrates induced by acid etching, sandblasting, and alkali etching had minimal effects on the phase composition and crystal morphology (irregular cubic) of the SrZnP coatings. The coatings on IPL-Ti exhibited better mechanical properties and corrosion resistance. Besides, the coatings with optimized structures and surface characteristics elicited bacterial growth inhibition rates of 91.09% and 84.04% against Staphylococcus aureus (S. aureus) and Escherichia coli (E. coli), respectively. Meanwhile, the crystal-refined coatings further significantly enhanced the adhesion, proliferation, and differentiation of bone marrow mesenchymal stem cells (BMSCs), proving anticipated osteogenic activity. Overall, the ions preloading strategy on variable micro/nanostructured Ti substrates facilitates the potential application of Sr/Zn-phosphate conversion coatings for repairing infected bone defects.