Biological Response of ZX50 Mg Alloy: The Role of Surface Microstructure

Abstract

The surface mechanical attrition treatment (SMAT) and subsequent controlled annealing were performed on a ZX50 (Mg5Zn0.2Ca) alloy to optimize its near-surface microstructure for biodegradable applications. Graded ultrafine grain structures were produced by systematically varying the annealing temperature (100 to 400 °C), and the resulting specimens were tested in DMEM-FBS to study L929 cell cytocompatibility and DMEM media to study their corrosion behavior, the formation of a protective salt layer, and BSA (bovine serum albumin) solution tested for protein adsorption. The SMAT process resulted in a microstructural gradient, with finer twins near the surface and coarser twins in deeper regions. The grain size of the surface region of the samples annealed after the SMAT was much smaller (∼14.85 μm at 250 °C) than that of the non-SMATed sample (∼86.63 μm). The synergistic effect of SMAT-induced microstructural refinement and annealing at 100 and 250 °C resulted in a thicker, more uniform, and chemically more stable Ca and P-rich protective layer in DMEM-FBS media. In a cytocompatibility study, the non-SMATed samples showed a considerably lower cell viability of ∼16% in the 50% extract solution after 24 h; however, the SMATed samples showed significant protection, with ∼50% viability. The SMATed samples annealed at 100 and 250°C exhibited statistically better cell viability of ∼77% and ∼82%, respectively. Moreover, these samples exhibited the highest BSA protein adsorption (0.071 μg/cm2 at 250 °C). The improved behavior of the alloy decreased as the annealing temperature increased to 400 °C, due to an adverse effect of grain coarsening, which formed an inferior protective layer in biological media. In general, the integrated SMAT-annealing pathway outlines a viable processing regime that yields a stable, biologically favourable surface on the ZX50 alloy, enabling controlled resorption and enhancing the biological response of future biodegradable osteosynthesis devices.