Effects of sodium hydroxide and polydopamine pretreatment and calcium phosphate coating on the properties of biodegradable magnesium

Abstract

Magnesium (Mg) has shown great promise as a potential biocompatible and biodegradable implant material. Some of its unique properties include high strength/weight ratio and closer elastic modulus to that of the human bone. However, in a pure state, its in-vivo corrosion is too rapid to be used for implants. Hydrogen evolution during degradation leads to the elevation of body fluid pH which causes infection or inflammation and delaying tissue healing process. Despite many studies to improve the corrosion resistance of pure Mg, reports on sodium hydroxide (NaOH) and polydopamine (PDA) pre-treated Mg followed by calcium-phosphate (Ca-P) coating using electrodeposition (ED) technique are hardly found in the literature especially fluoridated hydroxyapatite (FHA) coating. This research is aimed to improve the corrosion resistance of Mg by NaOH and PDA pre-treatment followed by different phases of Ca-P coatings. In the first stage, pure Mg was pre-treated with NaOH (1M, 30 minutes) and PDA (2 mg/ml in 10 mM Tris buffer, pH 8.5). In the second stage, different phases of Ca-P were coated on the pre-treated specimens using ED technique with two different electrolytes at current density equal to 1 mA/cm2 for 60 minutes at room temperature. The pre-treated and coated specimens were analysed using X-ray photoelectron spectroscopy, grazing incidence X-ray diffraction, attenuated total reflectance-Fourier transform infrared spectroscopy, scanning electron microscopy, energy dispersive X-ray spectroscopy and optical microscopy. The specimens were also evaluated on their surface roughness, water contact angle and scratch hardness. In addition, corrosion behaviours of specimens were analysed using potentiodynamic polarization and in-vitro immersion tests. Results in the first stage showed that both pre-treatments decreased the corrosion rate of Mg, in particular coated substrates with PDA by almost 27 folds. PDA pre-treatment also improved surface properties by reducing water contact angle and increasing surface roughness by 2.3 and 4.5 folds respectively as compared to pure Mg. In the second stage, the results showed that dicalcium-phosphate dihydrate (DCPD) and FHA were formed on NaOH pre-treated specimens. It was found that DCPD coatings required posttreatment to convert DCPD to hydroxyapatite (HA). However, HA and FHA were able to be deposited directly on the PDA pre-treated specimens without requiring any post-treatment. In terms of adhesion strength between the Ca-P coatings and the substrates, PDA pre-treatment specimens were superior than NaOH pre-treatment. Comparing between FHA and HA coatings on both pre-treatments, FHA coated specimens demonstrated higher corrosion resistance and surface roughness. It is believed that by introducing fluorine into the coating, it stabilizes and increases the crystalline structure of FHA. The corrosion resistance of FHA-PDA coated Mg improved significantly (approximately 62 folds) as compared to uncoated pure Mg.