A zinc phosphate layered biodegradable Zn-0.8Mg-0.2Sr alloy: Characterization and mechanism of hopeite formation

Abstract

The objective of this study is to comprehensively investigate the formation mechanism of hopeite on a promising biodegradable Zn-0.8Mg-0.2Sr alloy during its self-corrosion process and its subsequent influence on the interaction of the material with cells and bacteria. Notably, the self-corrosion process of Zn-based alloys inherently generates Zn2+ ions, eliminating the necessity for additional zinc compounds in the treatment. The investigation focused on the chemical composition of the applied baths, exposure time, and temperature, in order to analyze the structure, quality, distribution, phase composition, thickness, and roughness of the resulting layers bringing range of key pros and cons for suggested materials. While the thin layers are considered more beneficial in a wide range of applications, the resulting thinnest layer was observed after exposure to the bath containing additives at 50 °C for 24 h, measuring approximately 192 ± 21 pm in thickness. In addition, the layer possessed the most homogeneous distribution under these conditions. Generally, the layers formed in the bath without additives exhibited higher thickness and rougher surfaces compared to those in the bath containing additives. Such results had a direct impact on dissolution rate, cytotoxicity, and antibacterial properties. The highest dissolution rate (40.7 mg·cm−2·day−1) indicating a limited lifetime of the layer was obtained for material processed under the following coating condition: 50 °C, 24 h, and bath without additives. The general presence of surface layers manifested in increased cell viability under direct cytotoxicity test, while smoother surface conditions also supported the antibacterial properties, both making suggested treatments as interesting variations for biodegradable zinc-based materials.