CO2 laser surface treatment of magnesia partially stabilised zirconia (MgO-PSZ) for improved biocompatibility

Abstract

Magnesia partially stabilised zirconia (MgO-PSZ) is a bioinert material that exhibits high mechanical strength, excellent corrosion resistance and good compatibility with the physiological environment. It is, therefore, frequently used in high-load bearing sites such as artificial knee and bone screws in orthopaedic applications and jaw and crown in dental applications. Yet, bioinert ceramics have often failed clinically due to a lack of direct bonding with bone, that is, insufficient biointegration. Bioactivity and biointegration are the two essential aspects of good biocompatibility. Bioactivity is usually attributed to the ability to induce an apatite layer on a material's surface in physiological conditions, whilst biointegration (often osseointegration) is the ability to promote anchorage, attachment, spreading, growth and differentiation of bone cells. These interactions occur at the interface between the implant and tissue and are controlled by the surface properties of the biomaterial. At present, the processes available to engineers for the modification of biomaterials in order to enhance biocompatibility are invariably complex and consequently somewhat difficult to control. Lasers, on the other hand, can offer not only a clean and innocuous processing technique, but one that is rapid, flexible and highly controllable. However, no work has been conducted hitherto on laser surface processing to transform the surface properties of bioinert ceramics so as to improve the biocompatibility of implants. This current research was carried out to explore the potential of laser treatment for changing the surface properties and thereof, improving the biocompatibility of the MgO-PSZ surface. The experiments were performed with a 3 kW continuous wave (CW) CO 2 laser and the general effects thereof on the surface of the MgO-PSZ were observed. Variations in the CO 2 laser operating parameters were seen to have a significant effect on the microstructure obtained within the laser treated areas on the MgO-PSZ. Moreover, it was seen that the obtained solidification microstructures on the surface of the MgO-PSZ differed not only with changes in laser parameters, but even across the same track. This was explained by the theories of constitutional supercooling and stability morphology. Contact angle, θ, measurements revealed that CO 2 laser surface treatment of the MgO-PSZ brought about a reduction in θ, indicating that the wettability characteristics of the MgO-PSZ had been enhanced. It was subsequently deduced that the factors active in causing the observed modification in the wettability characteristics of the MgO-PSZ were the surface roughness, surface oxygen content and surface energy. By employing a novel technique the predominant mechanisms active in the wettability modification and effects of the microstructure features on the wettability characteristics was identified.