Synthesis, characterisation and deposition of Nano hydroxyapatite coatings on bio-degradable magnesium for potential orthopaedic applications

Today, Magnesium (Mg) based alloys are receiving increasing attention as potential biodegradable implant materials for orthopaedic applications. Despite advantageous properties such as density and elastic modulus that are similar to bone, magnesium’s rapid degradation rate when immersed in the highly corrosive body fluid environment has severely limited its clinical application. The focus of this thesis research was to develop biocompatible calcium phosphate coatings with tuneable biodegradable properties that are capable of extending the operational life of an Mg based implant and allow sufficient time for tissue regeneration to take place. The research developed and examined three types of calcium phosphate coatings designed to reduce the degradation rate and prolong the life of Mg test substrates. Dicalcium phosphate dihydrate (DCPD) or Brushite coatings were formed on Mg substrates via a straightforward chemical immersion technique. While amorphous calcium phosphate (ACP) coatings were formed on Mg substrates using an electrochemical technique. Brushite coatings were characterised by widespread flowerlike surface structures and the ACP coating were granular in structure with a surface covering of tube-like structures. The third coating examined was formed by the subsequent transformation of Brushite and ACP coatings to hydroxyapatite (HAP) via a low-temperature hydrothermal process. Importantly, HAP is the mineral component found in bone and is known to promote bone cell adhesion, differentiation and osteointegration. Advanced characterisation techniques such as X-ray diffraction, field emission scanning electron microscopy, energy dispersive spectroscopy and Fourier transform infrared spectroscopy were used to investigate the size, morphology, crystalline structure, composition and topographical features of both uncoated and coated substrates. Degradation behaviour studies were carried on pure Mg substrates and the various substrate coatings using two simulated body fluid electrolytes at human body temperature (37 oC). The first was phosphate buffer saline (PBS) solution and the second was Ringer’s solution. Corrosion rate measurements revealed DCPD coated substrates had the lowest corrosion rate in both PBS (0.126 mm/yr) and Ringer’s solution (0.1 mm/yr) compared to HAP coated [PBS (0.279 mm/yr) and Ringer’s solution (0.264 mm/yr)] and uncoated Mg substrates [PBS (1.829 mm/yr) and Ringer’s solution (3.828 mm/yr)]. The results of the research indicate that both DCPD and HAP coatings have the potential to reduce the corrosive effects produced by both PBS and Ringer’s solution. The results also suggest that the coatings have the ability to reduce the degradation rate of Mg substrates in the physiological environment. Furthermore, because of HAPs complex hexagonal structure it offers an effective high capacity absorbent matrix and the present research has shown that like bone HAP can accumulate metallic materials such as cadmium, copper, iron and zinc. The significant improvement in corrosion resistance and the ability of HAP coatings in particular to behave as a temporary repository of metallic materials during degradation is an important step in the development of a biodegradable Mg implant for orthopaedic applications.