An investigation on the adsorption of calcium phosphate on magnesium surfaces.

摘要

Many traditional orthopedic biomaterials used in bone fixation procedures are made primarily of titanium alloys and stainless steels coated with a bioceramic such as hydroxyapatite [Ca5(PO4)3OH] to improve both biocompatibility and fixation between the material and bone. In many instances such as fractures, appliances used to stabilize the injured site (screws, pins, plates) need to be in-vivo for only a short period of time (∼12-18 weeks). However, because of their inherent corrosion resistance these materials are often left inside the patient, which carries the risk of leaching of the material and of allergic reactions. Furthermore, the mechanical properties of these materials differ quite drastically from those of cortical bone, leading to stress shielding where healing of the bone at the injured site is compromised. It is for these reasons that alternative materials such as magnesium alloys have been investigated since they are both biodegradable and have mechanical properties similar to those of bone. By combining the favorable mechanical properties of magnesium with the biocompatibility of hydroxyapatite coatings, magnesium orthopedics implants could one day become a reality in the medical industry.;The overall objective of this work was to successfully deposit a calcium phosphate film onto a magnesium alloy substrate, characterize the coating with respect to its crystallinity, composition and thickness and finally deduce the mechanism of adsorption.;Using the molecular-scale resolution In-Situ ATR-FTIR technique, it was found that the di-protonated H2PO4- species had the highest affinity for the magnesium surface but the initial monolayers bind as Mg-O-PO32- due to a localized high pH at the surface/solution interface. Furthermore, it was found that subsequent phosphate deposition was improved with successive adsorption of calcium.;Kinetic studies that characterized bulk deposition found that early deposition was preferential towards the aluminum/zinc-rich beta phase indicating that adsorption is catalyzed by anodic dissolution of the alloy. Furthermore, it seems that by 24 hours deposition the predominant phase is pure hydroxyapatite but by 5 days immersion we see a dissolution/re-precipitation of a magnesium-substituted apatite.