Unraveling physicochemical control of stone wool dissolution through bridging bulk and single-fiber in vitro acellular methods

Stone wool fiber dissolution in physiological environments is a key process underlying their long-term behavior and safety, yet robust and reproducible in vitro methodologies remain underdeveloped. In this work, we report on results from the standardized USP apparatus 4 (USP-4) compared with a custom-made, single fiber (SF) dissolution cell, for measuring in vitro dissolution rates of stone wool fibers (produced with and without binder and where binder was removed) under continuous flow-through conditions. Biosolubility of the stone wool fibers derived from dissolution rate in acellular in vitro tests can be used as a potential endpoint for correlation with fiber biopersistence in vivo in a future protocol development that will replace animal studies. However, developing a robust method requires thorough exploration of the physicochemical factors that affect dissolution rate. We have demonstrated that stone wool dissolution rates can be affected by solution feedback (i.e., dissolved species presence) when fluid flow-to-sample surface area (F/A) ratio is low and by poor fluid wetting of the fibers. Increasing the F/A ratio and introducing a surfactant into the USP-4 experiments brought the USP-4 results closer to those obtained with the SF method, although complete quantitative agreement was not achieved. Dissolution rates were obtained in the F/A region where dependence is solely on fluid and fiber composition, not on fluid transport or released species accumulation. By adjusting the amount of surfactant or/and applying plasma ashing for fiber treatment, it was possible to reliably measure dissolution rates for the fibers produced with binder, i.e., as they are in the market, and demonstrate no effect of the binder on dissolution, except altered wettability. Our findings establish a materials-driven basis to further support future in vitro acellular protocol development using USP-4 for stone wool fibers, in line with animal tests replacement according to New Approach Methodologies (NAMs) strategy.