The traction forces developed by cells depend strongly on the substrate rigidity. In this letter, we characterize quantitatively this effect on MDCK epithelial cells by using a microfabricated force sensor consisting in a high-density array of soft pillars whose stiffness can be tailored by changing their height and radius to obtain a rigidity range from 2 nN/mu m up to 130 nN/mu m. We find that the forces exerted by the cells are proportional to the spring constant of the pillars meaning that, on average, the cells deform the pillars by the same amount whatever their rigidity. The relevant parameter may thus be a deformation rather than a force. These dynamic observations are correlated with the reinforcement of focal adhesions that increases with the substrate rigidity.

Mechanical forces play an important role in various cellular functions, such as tumor metastasis, embryonic development or tissue formation. Cell migration involves dynamics of adhesive processes and cytoskeleton remodelling, leading to traction forces between the cells and their surrounding extracellular medium. To study these mechanical forces, a number of methods have been developed to calculate tractions at the interface between the cell and the substrate by tracking the displacements of beads or microfabricated markers embedded in continuous deformable gels, These studies have provided the first reliable estimation of the traction forces under individual migrating cells. We have developed a new force sensor made of a dense array of soft micron-size pillars microfabricated using microelectronics techniques. This approach uses elastomeric substrates that are micropatterned by using a combination of hard and soft lithography. Traction forces are determined in real time by analyzing the deflections of each micropillar with an optical microscope. Indeed, the deflection is directly proportional to the force in the linear regime of small deformations. Epithelial cells are cultured on our substrates coated with extracellular matrix protein. First, we have characterized temporal and spatial distributions of traction forces of a cellular assembly. Forces are found to depend on their relative position in the monolayer : the strongest deformations are always localized at the edge of the islands of cells in the active areas of cell protrusions. Consequently, these forces are quantified and correlated with the adhesion/scattering processes of the cells.

We measure dynamic traction forces exerted by epithelial cells on a substrate. The force sensor is a high-density array of elastomeric microfabricated pillars that support the cells. Traction forces induced by cell migration are deduced from the measurement of the bending of these pillars and are correlated with actin localization by fluorescence microscopy. We use a multi pie-particle tracking method to estimate the mechanical activity of cells in real time with a high-spatial resolution (down to 2 mum) imposed by the periodicity of the post array. For these experiments, we use differentiated Madin-Darby canine kidney (MDCK) epithelial cells. Our data provide definite information on mechanical forces exerted by a cellular assembly. The maximum intensity of the forces is localized on the edge of the epithelia. Hepatocyte growth factor promotes cell motility and induces strong scattering activity of MDCK cells. Thus, we compare forces generated by MDCK cells in subconfluent epithelia versus isolated cells after hepatocyte growth factor treatment. Maximal-traction stresses at the edge of a monolayer correspond to higher values than those measured for a single cell and may be due to a collective behavior.