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Abstract Background It is an established toxicological principle that the inhalation pathogenicity of respirable and biodurable fibres is caused by excessive fibre length as alveolar macrophages fail to uptake and remove such fibres. However, studies on carbon nanotubes showed that this principle needs revision, as thin, flexible variants showed reduced fibre-specific toxicity. One potential explanation is that the low flexural rigidity of thin fibres enables macrophages to bend and internalize even those that are long relative to the cell size. To evaluate this proposed “rigidity hypothesis,” the mechanisms governing the uptake of flexible long fibres that determine a critical threshold value for flexural rigidity require clarification. Methods We exposed NR8383 rat alveolar macrophages to three silver nanowire variants differing in diameter and length. Time-lapse microscopy captured fibre uptake processes. Successful internalization of long fibres was found to require fibre bending during uptake. A mechanical model was developed by combining established cytoskeletal biophysics with the observed fibre deformation dynamics. As flexural rigidity describes fibre behaviour under load, our model estimated rigidity by reproducing the observed bent fibre shape. By defining limit cases for physically ‘weak’ and ‘strong’ NR8383 macrophages, i.e., assuming upper bounds on the forces generated by their cytoskeletal nanomachinery, our model enabled us to derive a range for the critical fibre rigidity threshold. Results and conclusion A macrophage was observed bending an exceptionally long fibre (~ 140 μm) first into an arc and then a spiral for full internalization, initiated by a pseudopod extending along the fibre and buckling the internalized segment. Our model can reproduce such behaviour. It yielded a flexural rigidity of 20 mN·nm² for this fibre. Predicted critical rigidity limits for fibres that just fit into NR8383 macrophages range from 3 to 62 mN·nm². Using the conservative lower bound, long and biodurable fibres with a rigidity lower than 3 mN·nm² are expected to be readily phagocytized by this cell line. Although this rigidity scale may not be directly translatable to human alveolar macrophages, our experimental findings and their modeling emphasize the key role of rigidity in fibre–cell interactions. Fibre rigidity is therefore central for material safety aspects and sustainable product design.