Fig. 1.

Traction force measurements. (A) Principle of force measurements and specific geometrical relations. A single cell spreading between two parallel glass microplates generates traction forces. One plate is rigid and maintained at a fixed position, the other is flexible, calibrated in stiffness, and used as a nano-Newton force sensor. Thus, force, stiffness, and deflection are related: F = kδ. Moreover, the flexible plate deflection δ is equal to the cell shortening (L ₀ − L). Hence, the rate of force increase dF/dt is proportional to the speed V of cell shortening. (B) A single C2.7 cell spreading between the two parallel microplates. The upper plate is flexible, the other rigid. Cell-shape evolution during spreading and traction force generation were related (See also supplementary movies M1 and M2 in the SI Appendix). The plateau force F _p as well as the rate of force increase $\frac{dF}{dt}$ were increasing functions of the stiffness k of the flexible plate. The two sets of data are representative of force evolution at low (open circles, k = 21 nN/μm) and high stiffness values (filled circles, infinite k).