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Biomimetic bone-active coatings composed of inorganic nano-hydroxyapatite (i.e., nHA) and organic silk fibroin (i.e., SF) are layer-by-layer deposited on Mg-Zn-Ca alloy by a controllable ultrasonic spray method. Meanwhile, plasma activation is developed as a promising strategy to pretreat magnesium surfaces, which facilitates the direct adhesion of coatings with enhanced bonding interfaces. In this work, we engineer the nHA/SF composite coatings with excellent mechanical properties and adhesion force. The optimized parameters of ultrasonic spray bring significant influence on the surface morphologies of coatings. Assisted by hybrid plasma of oxygen and nitrogen (i.e., O2/N2 plasma), the activated Mg-Zn-Ca surfaces are uniformly covered by a robust and compact nHA/SF composite coating, establishing a multifunctional system with superior corrosion resistance and biological performance. Interestingly, secondary oxygen plasma treatment of nHA/SF coatings (A-nHA/SF) promotes the hydrophilicity, leading to a rapid self-repair effect from surface damage. The improvement of anti-corrosion and self-repair provides a dependable platform for better cell adhesion, proliferation, spreading and differentiation. These favorable factors contribute to the preferable in vivo biocompatibility and the promotion of newly formed bones for the A-nHA/SF-coated Mg implants. This study lays important foundations for coating strategy on biomedical magnesium alloy as multifunctional osteogenic system in bioactive implantable applications.

To expand the future clinic applications of biodegradable magnesium alloy, polymer coatings with excellent biocompatibility are the keys to solve the local alkalinity and rapid hydrogen release. Natural−organic silk fibroin provides an approach to fabricate a protective coating on biomedical Mg−Zn−Ca alloy, however, the adhesion force and mechanical properties of the coating on substrates are ought to be further improved without any chemical conversion/intermediate layer. Hereby, based on VUV/O3 surface activation, a hybrid of silk fibroin and sodium alginate is proposed to enhance the adhesion force and mechanical properties of the composite coatings on hydrophilic Mg−Zn−Ca alloy surfaces. Various mass ratios of sodium alginate addition were investigated to achieve the optimum coating strategy. The nanoscratch test and nanoindentation test confirmed that the adhesion force was tripled and mechanical properties index was significantly improved when the mass ratio of silk fibroin/sodium alginate was 70/30 compared to pure silk fibroin or sodium alginate coatings. Meanwhile, the corrosion rate of the coated Mg−Zn−Ca alloy was significantly delayed with the addition of sodium alginate, resulting in a reaction layer during corrosion process. Furthermore, the mechanisms for both adhesion and corrosion processes were discussed in detail. Our findings offer more possibilities for the controllable surface performance of degradable metals.