Buffer-regulated biocorrosion of pure magnesium

Abstract

Magnesium (Mg) alloys are being actively investigated as potential load-bearing orthopaedic implant materials due to their biodegradability in vivo. With Mg biomaterials at an early stage in their development, the screening of alloy compositions for their biodegradation rate, and hence biocompatibility, is reliant on cost-effective in vitro methods. The use of a buffer to control pH during in vitro biodegradation is recognised as critically important as this seeks to mimic pH control as it occurs naturally in vivo. The two different types of in vitro buffer system available are based on either (i) zwitterionic organic compounds or (ii) carbonate buffers within a partial-CO(2) atmosphere. This study investigated the influence of the buffering system itself on the in vitro corrosion of Mg. It was found that the less realistic zwitterion-based buffer did not form the same corrosion layers as the carbonate buffer, and was potentially affecting the behaviour of the hydrated oxide layer that forms on Mg in all aqueous environments. Consequently it was recommended that Mg in vitro experiments use the more biorealistic carbonate buffering system when possible.

Building towards a standardised approach to biocorrosion studies: a review of factors influencing Mg corrosion in vitro pertinent to in vivo corrosion

“…In fact, the comparison was actually substantially worse in the solution which contained proteins compared to the simpler SBF used (EBSS). A substantial increase in corrosion was also observed in a study by Kirkland et al [72] when 40 g L −1 of BSA was added to MEM, as presented in Fig. 3.…”

“…This result was supported by the study of Kirkland et al [72] where a minimal to slight difference was found between HBSS and EBSS when an active CO 2 buffer was used, as presented in Fig. 3.…”

“…40 g L −1 of BSA was added to include proteins in the MEM + BSA solution. Note: these corrosion rates were calculated from the % mass loss presented in Kirkland et al [72]. KMBKirkland Biocorrosion Medium, a modified SBF.…”

“…In the initial study by Liu et al [83] proteins were added to a maximum of 1 g L −1 , again a small fraction of the levels present in blood plasma. In stark comparison, Walker et al [48] and Kirkland et al [72] incorporated 40 g L −1 of proteins, achieving close to physiological levels, and much higher corrosion rates than in vivo. Additionally, the pH was controlled in this study, although there is concern over the volume of solution used.…”

“…This method is discussed in more detail in Section "Ratio of solution volume to sample area", and may be worthy of critical analysis. Kirkland et al [72] used a ratio of 20 mL of solution per 1 cm 2 of sample surface area, which is below the desired level, also discussed in Section "Ratio of solution volume to sample area" and in another work by the same group [58].…”