Fig. 1. Tether force measurements. (a) DIC image of a tether force measurement using optical tweezers. Polystyrene beads (1 µm diameter) coated with IgG were attached to the plasma membrane of NIH-3T3 fibroblasts using laser optical tweezers (left panel) and a membrane tether was formed by pulling out the bead (right panel). At a constant length, tether force (F_tether) is pulling the bead back towards the cell opposing the force of the trap (F_trap). (b) Schematic view of the optical tweezers force measurement that defines the local adhesion energy term. At a constant length, the force on the bead was measured by its displacement (r) from the center of the laser trap. (c) Typical displacement trace (r), showing how far the center of the bead has been moved away from the center of the optical tweezers. This tether force is a measure of the apparent membrane tension or the energy required to move membrane from the plasma membrane into the tether.