Mechanical, Surface and Electrochemical Studies of Mg-Alloys as Bioabsorbable Implant Materials

Abstract

Magnesium alloys has gained significant attention as bioabsorbable implants, but the fast-electrochemical reactions produce hydrogen gas, which detach the implant with home bone and tissues. The focus of present study is to introduce a multi structured organic/inorganic composite layer on magnesium alloys to improve their surface properties such as biocompatible osseointegration with home tissues and increase their resistance for corrosion process. The organic/inorganic composites consist on biodegradable polymers (chitosan, gelatin), bioactive glass powders (BG), zinc oxide and cerium oxide. Alternating current electrophoretic deposition (AC-EPD) was used for coating on an experimental magnesium alloy (Mg-Si-Sr) and two commercially available alloys (AZ31, ZK60). In this research work chitosan (MW 75–85% deacetylation) CS and gelatin (type (B) powder) G, 45S5 bioactive glass powders (7-61 μm) BG (a, b, c), ZnO (<100nm) and CeO2 (<50nm) nano powders were used for the coating of three magnesium alloys. Eight suspensions (A-H) were finalized for AC-EPD coating of Mg-Si-Sr with compositions of CS0.8-1.6-G0.8-1.6-BG(a,b,c)1.0. The influence of key AC EPD parameters (voltage amplitude, frequency, distance and time) as well as suspension parameters (biopolymer type and concentration, BG particle size) on the coating homogeneity was investigated. Coating microstructure and thickness were observed by SEM and FT-IR used for characterization of coating on the substrate. The electrochemical corrosion behavior was evaluated by potentiodynamic polarization curves in Ringer’s solution at 37oC. FT-IR spectra confirmed that chitosan crosslinking had taken place during AC-EPD parameters which is known to increase the coating stability and adhesion of the coating to the substrate. SEM images showed that biopolymer/BG layer was more homogeneous with no bubbles and cracks. SEM based profilometry test revealed that the coating thickness was up to ca. 20 μm for suspension CS0.8-G0.8-BG(b)1.0 with AC-EPD parameters (Voltage = 90V, Frequency = 100Hz, DC offset = 2V, Time = 190s, Distance = 7.5mm) with low corrosion current density (Icorr.) value 3.41 μA.cm-2 (corrosion rate 0.08 mm/y), after 10 min. in Ringer’s solution. ix The surface of AZ31 was modified by CS-G-BG(a,b)-ZnO/CeO2 through hybrid coating by using AC-EPD. Twelve suspensions (A-L) were finalized for AC EPD coating with composition of CS0.8-1.6-G0.8-1.6-BG(a,b)1.0-3.2-ZnO0.4-0.6/CeO2(0.4-0.6). SEM images showed that coating morphology was almost same. Potentiodynamic polarization measurements (PDP) indicated that best parameters of AC-EPD for AZ31 are (Voltage = 80V, Frequency = 100Hz, DC offset = 2V, Time = 240s, Distance = 10mm) which gave lowest corrosion current densities (Icorr.) values of 0.18 μA.cm-2 (corrosion rate 0.41x10-2 mm/y) and 1.00 μA.cm-2 (corrosion rate 2.26x10-2 mm/y) respectively, after 10 min. immersion in Ringer’s solution with suspensions CS0.8- G0.8-BG(b)1.0 and CS1.6-G1.6-BG(b)3.2-CeO2(0.4). SEM-based profilometry test revealed that coating thickness with suspensions CS0.8-G0.8-BG(b)1.0 and CS1.6-G1.6-BG(b)3.2- CeO2(0.4) was approximately 20, 15μm respectively. Further corrosion behavior of ZK60 also investigated for suspensions (A-L) which are already investigated for AZ31. For ZK60, PDP measurements showed that best parameters of AC-EPD for ZK60 are (Voltage = 35V, Frequency = 300Hz, DC offset = 2V, Time = 480s, Distance = 10 mm) which gave lowest corrosion current densities values 1.39 μA.cm-2 (corrosion rate 0.30x10-1 mm/y) and 1.84 μA.cm-2 (corrosion rate 0.39x10-1 mm/y), after 10 min. in Ringer’s solution with suspensions CS0.8-G0.8-BG(b)1.0-ZnO0.6 and CS0.8-G0.8-BG(a)1-CeO2 (0.6) respectively.Thickness test revealed that coating thickness with suspension CS0.8-G0.8-BG(b)1.0-ZnO0.6 was approximately 11.5 μm