In-vitro sensing of biomechanical forces in live cells by a whispering gallery mode biosensor

Direct measurement of the biomechanical stress induced by a live cell during endocytosis is reported. Fluorescent dye-doped polystyrene microspheres were used as microscopic remote optical sensors applying whispering gallery modes (WGMs) as transducer mechanism. Monitoring of the WGMs throughout the incorporation of the microsphere into the cell enabled the determination of the deformation experienced by the microsphere, characterized by both a broadening and a blue shift of the resonances, and consequently the stress induced by the cell. The results reveal an unexpectedly high stress with a magnitude of up to five times that of the passive cortical tension, which can be only explained by a so far undetermined active stress component induced by the cytoskeletal machinery during particle incorporation. The method is adaptable to the study of any other kind of phagocyte and thus provides a novel research tool of high interest for cell biology.