Electrolytic Plasma Processing of Magnesium for Biomedical Applications

Abstract

Magnesium and magnesium alloys suffer from poor corrosion resistance that limits their application towards biodegradable orthopaedic implants. Such challenge could be tackled by Plasma Electrolytic Oxidation (PEO) technique, which generates inert ceramic coatings that provide excellent corrosion protection for the metallic substrate underneath. This research optimised PEO coatings through modifying the parameters involved in the process, including electrical regimes and electrolyte composition, to produce highly corrosion resistant PEO coatings providing adequate protection for the magnesium substrate for the intended application. Additionally, antibiotic molecules were loaded in Halloysite nanotubes (HNT) to be deposited on the PEO surfaces towards achieving a controlled drug release. The modification of the electrical regimes was realised through designing novel sawtooth pulse current waveforms. Four waveforms, denoted as UP, UN, BP and BN representing Unipolar or Bipolar current mode with slow Positive or Negative anodic ramp, were designed and employed in the PEO processing. The results indicated that UN, UP, and BP sawtooth pulse currents produced PEO coating with uneven surface and thickness; whereas a BN sawtooth pulse current achieved a morphology refinement of the PEO coating thanks to the defect-healing effect facilitated by the descending discharge intensity at each anodic polarisation. Subsequent optimisation of the PEO processing was performed through selecting corrosion resistant substrate materials (WE43 and WE54 magnesium alloys) and exploring electrolyte constituents (NaF and Hydroxyapatite particles) at different concentrations. PEO coated WE54 alloys in the electrolyte containing NaF and a high concentration of Hydroxyapatite particles yielded the best corrosion performance. Loading Vancomycin molecules in HNT was achieved by placing a suspension of pristine HNT in Vancomycin water solution under vacuum. The procedure was non-destructive as evident by the maintained tubular structure of the HNT after loading. FTIR and further characterisation using TEM identified the presence of the Vancomycin molecules with HNT. The release kinetics obtained by analysing the accumulated release of the Vancomycin molecules suggested that 99% of the molecules were release by day 6 of monitoring, out of which the first 24 hours of release was mainly contributed by the quick release of the molecules adsorbed on the outer surface of the nanotubes (65%); and the slow release afterwards was contributed by the molecules loaded inside the nanotube lumen (35%).

Date of Award	1 Aug 2024
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Aleksey Yerokhin (Supervisor) & Allan Matthews (Supervisor)