Superhydrophobic fluoride conversion coating on bioresorbable magnesium alloy – fabrication, characterization, degradation and cytocompatibility with BMSCs

A micro-nano structure CaF2 chemical conversion layer was prepared on fluoride-treated AZ31 alloy, then the composite fluoride conversion film (CaF2/MgF2) was modified by stearic acid (SA) and fabricated a superhydrophobic surface. The fluoride-treated magnesium, fluoride conversion film and superhydrophobic coating were characterized by SEM, EDS, XRD and FTIR. The properties of coatings’ adhesion and corrosion resistance were evaluated via tape test and electrochemical measurement. The cytocompatibility of the MgF2, CaF2 and super-hydrophobic CaF2/SA surface was investigated with bone marrow-derived mesenchymal stem cells (BMSCs) by direct culture for 24 h. The results showed that the superhydrophobic fluoride conversion coating composed of inner MgF2 layer and the outer CaF2/SA composite layer had an average water contact angle of 152°. SA infiltrated into the micro-nano structure CaF2 layer and formed a strong adhesion with CaF2 layer. Furthermore, the super-hydrophobic coating showed higher barrier properties and corrosion resistance compared with the fluoride conversion film and fluoride-treated AZ31 alloy. The BMSC adhesion test results demonstrated MgF2 CaF2 and CaF2/SA coatings were all nontoxic to BMSC. At the condition of in direct contact with cells, MgF2 showed higher cell density and enhanced the BMSCs proliferation, while CaF2 and CaF2/SA coating showed no statistically difference in cell density compared with glass reference but the CaF2 and CaF2/SA coating were not conducive to BMSCs adhesion.