Microstructural evolution of strontium‑zinc-phosphate coating on titanium via changing Zn2+ concentration in phosphate solution for enhanced osteogenic activity

The microstructure of the phosphate coating on implant surface is critical for successful osseointegration. Based on the morphological variability of phosphate crystals, the microstructure of the coating can be regulated by designing the composition of the phosphate solution. In this study, a strontium‑zinc-phosphate (SrZn2(PO4)2, SZP) coating was prepared on titanium (Ti) surface by phosphate chemical conversion (PCC) method. The effect of Zn2+ concentration in phosphate solution, which was regulated by increasing the addition amount of Zn(H2PO4)2·2H2O, on the microstructure and properties of the SZP coating was investigated. The characterization of the coating was carried out by scanning electron microscope (SEM), X-ray diffractometer (XRD), Fourier transform infrared spectrometer (FTIR), X-ray photoelectron spectrometer (XPS), laser scanning confocal microscope (LSCM), contact angle goniometer, and electrochemical workstation system. The results revealed that the increase of Zn2+ concentration in phosphate solution could induce the crystal morphology of the SZP coating to evolve from irregular granules (Zn(H2PO4)2·2H2O: 1 g/L) to regular cubic blocks (Zn(H2PO4)2·2H2O: 8 g/L). This morphological evolution process was accompanied by an increase in thickness (from 4.8 ± 1.2 to 20.6 ± 1.9 μm), an improvement in wettability and corrosion resistance, as well as a decrease in the release amount of Sr2+ (from 3.97 ± 0.26 to 0.86 ± 0.12 ppm) and Zn2+ (from 0.31 ± 0.01 to 0.18 ± 0.01 ppm) after 21 days of immersion. In vitro evaluations proved that the adhesion, proliferation, and differentiation of MC3T3-E1 pre-osteoblasts on SZP coatings, especially for the coating with tiny cubic-block crystals, were significantly enhanced compared with pure Ti. This study clarified the rule of morphological evolution of the SZP crystal that is caused by changing the Zn2+ concentration in the phosphate solution, which contributes to the structural optimization of the SZP coating to obtain high osteogenic activity.