Sol Gel Glass Coating on Magnesium Alloys to be Used As Temporary Implants: Electrochemical and Surface Study in Simulated Body Fluid
Authors: Omar Sheila A , Ballarre Josefina , Schreiner Wido H , Castro Yolanda , Durán Alicia , Cere Silvia M 
Published: February 2020

Surface modification of metallic materials is the approach for developing functional properties to assure good biocompatibility, enhance tissue acceptance, and/ or inhibit corrosion rate. It opens a wide spectrum of possibilities oriented to develop the functional properties needed for an implant acceptance when the bare material is not enough to assure good biocompatibility and tissue regeneration. Further, the interface between the implant and the physiological environment becomes the key factor which conditions the prosthesis success or failure. The aim of this work is to generate a chemically modified surface of two commercial magnesium alloys with a 58S and 68S sol-gel made glass coating, obtained from organic precursors such as silicon and phosphate alkoxides and calcium lactate. The produced bioactive layer is sintered at low temperatures in order to maintain the substrate integrity. The glassy coating composition belongs to the SiO2-P2O5-CaO system and has the double goal of promote bone healing and retard the hydrogen evolution coming from the anodic reaction. AZ31B (Al 3%, Zn 1%, Mn 0.2%) and AZ91D (Al 9%, Zn 1%, Fe 0.005%, Mn 0.33%, Ni 0.002%) were used as substrates. Plane samples were polished until 2500 grit SiC paper and cleaned prior to coat. 58S and 68S glasses (60/70 mol% SiO2, 36/26 mol% CaO, 4/4 mol% P2O5, respectively) was prepared by sol-gel method using tetramethyl orthosilicate, methyltriethoxysilane, triethyl phosphite and calcium L-lactate hydrate as precursors, HNO31N was used as catalyser, ethylene glycol to improve the alkoxide condensation degree and methanol dissolve the calcium lactate. Micro-Raman assays were performed to analyze deposits composition (Invia Reflex Confocal, Renishaw RM 2000, UK) with a 785 nm wavelength laser. X-ray photoelectron spectroscopy (XPS) was selected to perform an elemental analysis of surfaces prior and after immersion of 72 h in HBSS. Measurements were made using a commercial VG ESCA 3000 system, using Mg Kα (1253.6 eV) radiation and an overall energy resolution of 0.8 eV. High resolution scans with 0.1 eV steps were conducted over Mg 2p, Ca 2p, P 2p, Si 2p and O 1s. In the entire cases surface charging effects were compensated by referencing the binding energy (BE) to the C 1s line of residual carbon set at 284.5 eV. Coatings visual integrity and deposits produced during immersion tests were examined by scanning electronic microscopy (SEM, JEOL JSM-6460LV, Japan) prior to immersion and after 24 h.

Electrochemical assays were performed in a GAMRY Ref 600 electrochemical unit (Gamry, USA) with a conventional three electrode cell, using a saturated calomel electrode (SCE, Radiometer Copenhagen) as reference electrode, a platinum wire as a counter electrode and the coated and bare magnesium alloys as working electrodes. Electrochemical impedance spectroscopy (EIS) tests were registered also at the Ecorr with amplitude of 0.01V rms sweeping frequencies from 20000 to 0.005 Hz. Polarization curves for each sample were also recorded with an initial and final voltage of -1.8V (vs. reference) and 1 mV/s of scan rate. Hank Buffered Salt Solution (HBSS) at 37 ºC was used as electrolyte.

Electrochemical test show that the coatings are able to retard anodic reaction being more efficiently on AZ91 alloy. Regarding the compositions, the coating with less calcium content (68S) is able to form more continuous films that the 58S with higher corrosion resistance after immersion in HBSS (Figure 1). Both coatings show signs of deposit of hydroxyapatite related compounds after immersion, as a first sign of bioactivity in vitro.

Figure 1