Influence of particle size on metal release from a cobalt- and nickel-containing alloy powder in an in vitro surrogate gastric fluid

When matrix effects are present in alloys, the content of the metal ingredients is no longer reliable to predict the metal ion release and hence toxicity. Thus, the metals' industry proposed an alternative concept for self-classification based on the comparison of the metal releases between alloys and their ingredients; this approach better reflects the toxicity of the alloys when matrix effects are present. Samples of alloy and ingredients (reference) often differ in particle size. This study assessed the impact of particle sizes on cobalt (Co) and nickel (Ni) releases from different particle size fractions of an alloy powder, following a bioelution gastric fluid protocol. Results were considered together with those from reference pure Co and Ni powders. Co-release per gram of alloy increased (3- to 4-fold) when median particle diameter decreased (17-fold). For Ni, the releases were much lower, but the relative increases with decreasing particle diameter were greater (38-fold). Results from the < 100 µm fraction were lower than those for the fraction matching the particle size of the reference samples (by 2.1-fold for Co and 7.6-fold for Ni) but were higher (by 1.2-fold for Co and 2.0-fold for Ni) than those from the sample on the market. For all fractions, an alloy matrix effect lowering metal releases was evident. Based on the above results, we made particle size recommendations when testing alloy powders in a surrogate gastric fluid protocol relevant to the oral route of exposure. Our work provides guidance for testing alloy powders to generate data relevant to hazard self-classification.