Control of magnesium alloy corrosion by bioactive calcium phosphate coating: Implications for resorbable orthopaedic implants

Control of the corrosion that occurs in magnesium alloys in vivo is a significant challenge for their use as resorbable orthopaedic implants. In this work, we report on the provision of bioactive calcium phosphate (CaP) coatings on magnesium alloys that can delay substrate corrosion while offering an attendant physiochemical environment with properties known to promote an osteoinductive response in vivo. RF magnetron sputtering from hydroxyapatite (HA) powder targets has been employed to create CaP coatings on AZ31 magnesium alloy substrates. Coatings of ∼70 and 210 nm thickness were achieved via regulation of sputtering parameters, in particular deposition time. XPS and ToF-SIMS were used to investigate the chemistry of the coating alloy interface and also to confirm composition and thickness. The Ca/P atomic ratio of the coatings was determined by EDX to be 1.54. µCT analysis showed a substrate volume loss after 14 days exposure to SBF of 5.89 ± 3.15 mm3 for the un-coated alloy while the presence of the ∼70 nm CaP coating reduced this to 3.42 ± 0.48 mm3 and the ∼210 nm coating to 0.30 ± 0.28 mm3. The corrosion rates were calculated to be 1.74 ± 0.06 mmpy for the AZ31 control; 1.57 ± 0.09 mmpy for the ∼70 nm CaP coated alloy and 1.01 ± 0.07 mmpy for the ∼210 nm CaP coating. This data confirms that CaP coating thickness can control the rate of corrosion of magnesium alloys while offering the potential for improved bioactivity.