Release of tritium activity in cell culture medium (percentage of total initial activity) as a function of time for two tested steel particle concentrations. The quantification of tritium activity has been performed by liquid scintillation counting for mineralized samples (total activity) and for the supernatant, after separation of the particulate fraction (tritium released). Data (Mean ± SEM, experiment in triplicates) are taken from (13 ). The fit is obtained with GraphPad Prism (GraphPad Prism s.d.) using non-linear regression with a three-parameter function: Tritium release (%) = B + (S – B)/(1 + exp(T₅₀ - t)), where B is a basal value (immediate release), S a saturation value and T₅₀ is the time point at which the release is half-way between the basal and the maximal.