Optical tweezers for force measurements and rheological studies on biological samples

Light allows the observation of the microscopic world but also, by means of optical tweezers, the non-invasive manipulation of micro-objects and the measurement of the forces exerted on them. However, measuring forces inside living cells is still a challenge due the characteristics of the trapped organelles (non-spherical, unknown size and index of refraction) and the cell cytoplasm surrounding them (heterogeneous and dynamic, non-purely viscous). Here, we show how two very recent methods overcome these limitations: on the one hand, forces can be measured in such environment by the direct detection of changes in the light momentum; on the other hand, an active-passive calibration technique provides both the stiffness of the optical trap as well as the local viscoelastic properties of the cell cytoplasm.