Optical tweezers use a highly focused laser to trap micrometric transparent objects and manipulate them.

This technique allows to micromanipulate beads, parasite, cells and put them in contact with a partner substrate, monitoring in transmission or fluorescence microscopy the outcomes of the interaction. Using a micrometric trapped bead as a probe and the laser as a very soft spring, we also measure forces with a pN force resolution.

Upper image sequence : a tube of membrane is pulled from a T cell with a fluorescently labelled membrane. The cell is moved away from the trapped bead which position is kept constant. Lower part : typical force vs. time profile of pulling such a nanotube.

We used our setup to (a) present toxoplasma oocytes to macrophages and follow the phagocytosis (b) measure molecular forces between artificial substrates (c) press and pull on cellular surfaces to measure cell mechanics, in particular membrane cell tension. We developped methodologies for experiments, in particular regarding decorations of beads with given molecules, and open-source softwares for data processing.