Fabrication and characterization of novel sodium zirconate layers produced via chemical conversion of (Zr-Nb) thin films on biodegradable Mg for bone-fixation applications

Biodegradable Mg alloys have considerable promise for use in bone-fixation devices. Nonetheless, their clinical application is limited due to the high degradation rate of Mg, leading to deterioration of the mechanical properties, and drastic change in pH of the surrounding cellular environment of the implant during the healing process. Recently, the use of Na-containing biocompatible coatings on the Mg surface has created an innovative approach to overcome the above problems. In this study, a newly developed sodium–containing zirconate hydrogel layer with an amorphous structure has been designed and prepared through modification of the Nb-containing (Zr-Nb) thin films sputtered directly on pure Mg using wet chemical conversion in aqueous NaOH solution. The results of the Grazing incidence angle X-ray diffraction (GI-XRD), X-ray photoelectron spectroscopy (XPS), and Energy dispersive Spectroscopy (EDS) analyses have certified the formation of the desired layer on top of the thin films, indicating that ion exchange and chemical conversion between the surface layers and solution has been limited to less than 200 nm from the (Zr-Nb) thin film thickness, but without the morphological changes in the thin film's surface layers. In terms of reducing Mg degradation, the NaOH-treated sample has been demonstrated to be superior, compared to the other samples tested, representing a decrease in the corrosion rate (CR) from 3.49 mm.y⁻¹ to 0.75 mm.y⁻¹, increase in the corrosion resistance (Rp) from 181.32 Ω cm² to 1911 Ω cm². This increase in corrosion resistance can be ascribed to the Na ⁺ ions released from the modified layers, resulting in a change in the pH and composition of the surrounding solution environment. Mechanically, the indentation and nano-scratch testing results have explained a slight increase in Young's modulus of the NaOH-treated thin film compared to the untreated one and maintained the thin film's scratch properties after and before the chemical surface treatment which can be attributed to the thickness (<200 nm) of the modified layer. Cell culture studies have demonstrated enhanced cell viability of MG-63 cells in the NaOH-modified surfaces compared to the un-modified ones. These results demonstrate that the zirconate sodium layers have created the protective effect, appropriate cellular response, and nano-mechanical properties illustrating their potential for use in biodegradable bone-fixation devices.