Magnesium alloys are being investigated for biomedical applications due their bioresorbable properties. However, use of magnesium as an implant material has been limited due to the rapid corrosion of the alloy in the presence of body fluid. This, in turn, results in the generation of a large amount of hydrogen gas and alkalisation of the local body fluid that can inhibit wound healing and ultimately lead to necrosis of the surrounding tissue. The application of coatings to the surface of magnesium is one potential solution to modulate the corrosion rate in vivo.
Hydroxyapatite (HA, Ca10(PO4)6(OH)2) is one potential coating candidate. It is already commonly used as a coating for orthopaedic applications. Thermal coating techniques, such as plasma spray, are the most widely used methods to deposit hydroxyapatite onto metal substrates. These high temperature techniques are unsuitable for magnesium alloys due to the lFsubsow melting point of magnesium (≈ 600 °C).
CoBlast offers a low temperature alternative. Hydroxyapatite coatings have previously been deposited onto bioresorbable magnesium alloys using the CoBlast process. Figure 1 shows a HA coating on the surface of a magnesium alloy deposited using the CoBlast process. Figure 2 shows an elemental map of phosphorous on the surface of an uncoated and hydroxyapatite coated magnesium alloy. The turquoise colour corresponds to phosphorous contained in the hydroxyapatite coating. In vitro corrosion testing on a CoBlast deposited hydroxyapatite layer on magnesium showed that the presence of the coating significantly reduced the corrosion rate of the alloy compared to an unmodified control. This indicates that CoBlasted hydroxyapatite coatings offer a potential solution to modulate the corrosion rate of bioresorbable magnesium alloys. The next step is to investigate the performance of a HA coated implant in vivo. This is currently being investigated.
CoBlast also results in high coating bond strengths (≈ 50 MPa) that are significantly greater than the 15 MPa required for HA coatings in vivo. The high bond strength observed is due to bond formation between the substrate and coating during deposition using the CoBlast process. The process utilises a co-incident stream of abrasive blast medium and a stream of ‘dopant’ particles to modify the substrate surface. The abrasive roughens the surface while simultaneously disrupting the passivating oxide layer of the substrate and exposing the reactive metal. The stream of dopant reacts with the surface and binds with the exposed reactive metal to form an intimate chemical bond. The adhesion of the coating to the substrate is due to a combination of tribo-chemical bond formation and mechanical interlock between the bioceramic and the metal substrate.
Another benefit of the CoBlast process is that it produces crystalline coatings. Crystalline coatings have slower resorption rates than amorphous coatings in vivo making them more suitable to modulate corrosion. Alternative low temperature coating technique, such as sputter deposition and pulsed laser deposition, produce amorphous coatings. These require further heat treatments to crystallise them. The crystallisation temperature required is greater than the melting point of magnesium (≈600 °C).
Figure 1: Cross-section of a hydroxyapatite coated magnesium alloy
Figure 2: Elemental map of phosphorous on the surface of a magnesium alloy: uncoated (left) and coated (right). The turquoise colour corresponds to phosphorous contained in the hydroxyapatite coating.