Synthesis, characterization, corrosion and bioactivity investigation of nano-HA coating deposited on biodegradable Mg-Zn-Mn alloy

Abstract Electric discharge machining (EDM) is widely used to cut and shape the biomedical device substrates, following often a separate coating process to deposit another barrier layer to improve the corrosion performance. This paper presents an innovative single in-situ surface modification technique to deposit nano-hydroxyapatite (nHA) coating on biodegradable Mg-Zn-Mn alloy while shaping the device substrate via electric discharge machining (EDM). The key benefit of the technique is to carry out the whole process in a single setup, hence saving time and cost. The aim of the coating is to control implant's degradation rate and to improve in-vitro bioactivity with human cells. Morphology, elemental, and chemical composition of the nHA coated Mg-Zn-Mn surface were characterized by FE-SEM, EDS, and XRD, respectively, while microhardness is measured by a Vickers hardness tester. Corrosion tests were performed via potentiodynamic polarization measurements in a SBF (simulated body fluid) to evaluate the degradation kinetics. In-vitro cell culture study was carried out to evaluate biocompatibility and cell attachment onto the modified surface. Surface characterization results revealed that a biomimetic nHA containing interconnected nano-porosities of size 5-10 μm had been yielded on the substrate surface, which is beneficial for the apatite growth and osseo-integration. The deposited coating layer has comprised of Mg, Zn, Mn, O, Ca and P elements and formed intermetallic oxide phases, such as CaMg, Mg-Zn, Mn-CaO, Mn-P, and Ca-Mn-O, which improved the in-vitro corrosion performance. The degradation rate of Mg-Zn-Mn alloy was reduced by 90.85% from 0.82 mm/year to 0.07 mm/year by the deposition of nHA-coating layer. The microhardness of the modified surface was measured as 234 HV, which was 1.5 fold higher than the untreated surface. The corroded surface analysis showed that the dense intermetallic phases of nHA coating acted as a stable barrier layer, thus prohibiting the surface from degradation, and hence, improving the corrosion resistance. The in-vitro bioactivity analysis revealed that the nHA containing layer exhibited the superior bioactivity and promotes adhesion, growth, and proliferation of human osteoblastic MG-63 cells.