Traction forces and rigidity sensing regulate cell functions

Increasing evidence suggests that mechanical cues inherent to the extracellular matrix may be as important as its chemical nature in regulating cell behavior. Here, the response of cells to the mechanical properties of the substrate is examined by culturing 3T3 fibroblastic cells and epithelial cells on surfaces composed of a dense array of flexible microfabricated pillars. We focus on the influence of substrate rigidity on the traction forces exerted by cells, and on cell adhesion and migration. We first measure these forces by monitoring the deflection of the pillars. Then, by varying their geometric parameters, we control the substrate stiffness over a large range from 1 to 200 nN μm−1. We show that the force–rigidity relationship exhibits a similar behavior for both cell types. Two distinct regimes are evidenced: first, a linear increase of the force with the rigidity and then a saturation plateau for the largest rigidities. We observe that the cell spreading area increases with increasing rigidity, as well as the size of focal adhesions. Substrates with an anisotropic rigidity allow us to determine that the migration paths of 3T3 cells are oriented in the stiffest direction in correlation with maximal traction forces. Finally, to compare the force measurements on micro-textured surfaces and continuous flexible gels, we propose an elastic model that estimates the equivalent Young's modulus of a micropillar substrate. This qualitative model gives comparable results for both experimental approaches.