Influence of artificial gastric juice composition on bioaccessibility of cobalt- and tungsten-containing powders

The dissolution of metal-containing particles in the gastric compartment is poorly understood. The purpose of this study was to elucidate the influence of artificial gastric juice chemical composition on bioaccessibility of metals associated with ingestion-based health concerns. Dissolution rates were evaluated for well-characterized feedstock cobalt, tungsten metal, and tungsten carbide powders, chemically bonded pre-sintered (spray dryer material) and post-sintered (chamfer grinder) cemented tungsten carbide materials, and an admixture of pure cobalt and pure tungsten carbide, prepared by mechanically blending the two feedstock powders. Dissolution of each study material was evaluated in three different formulations of artificial gastric juice (from simplest to most chemically complex): American Society of Testing Materials (ASTM), U.S. Pharmacopoeia (USP), and National Institute for Occupational Safety and Health (NIOSH). Approximately 20% of cobalt dissolved in the first dissolution phase (t1/2 = 0.02 days) and the remaining 80% was released in the second long-term dissolution phase (t1/2 = 0.5 to 1 days). Artificial gastric juice chemical composition did not influence dissolution rate constant values (k, g/cm2 day) of cobalt powder, either alone or as an admixture. Approximately 100% of the tungsten and tungsten carbide that dissolved was released in a single dissolution phase; k-values of each material differed significantly in the solvents: NIOSH > ASTM > USP (p<0.05). The k-values of cobalt and tungsten carbide in pre- and post-sintered cemented tungsten carbide powders were significantly different from values for the pure feedstock powders. Solvent composition had little influence on oral bioaccessibility of highly soluble cobalt and our data support consideration of the oral exposure route as a contributing pathway to total-body exposure. Solvent composition appeared to influence bioaccessibility of the low soluble tungsten compounds, though differences may be due to variability in the data associated with the small masses of materials that dissolved. Nonetheless, ingestion exposure may not contribute appreciably to total body burden given the short residence time of material in the stomach and relatively long dissolution half-times of these materials (t1/2 = 60 to 380 days).