Measuring cell forces by a photoelastic method

A new method for measuring the mechanical forces exerted by cells on the substratum and through the substratum to act on other cells is described. This method depends upon the growth of cells on a photoelastic substratum, polydimethylsiloxane coated with a near monolayer of fibronectin. Changes in the forces applied by the cells to the substratum lead to changes in birefringence, which can be measured and recorded by the Polscope computer-controlled polarizing microscope. The changes in azimuth and retardance can be measured. A method for calibrating the stress is described. The method is sensitive down to forces of 1 pN per square microns. Fairly rapid changes with time can be recorded with a time resolution of approximately 1 s. The observations show that both isolated adhering, spread cells and also cells close to contact exert stresses on the substratum and that the stresses are those that would be produced by forces of 10-1000 pN per cell. The forces are almost certainly exerted on nearby cells since movement of one cell causes strains to appear around other nearby cells. The method has the defect that strains under the cells, though detectable in principle, are unclear due to birefringence of the components of the cytoplasm and nucleus. It is of special interest that the strains on the substratum can change in the time course of a few seconds and appear to be concentrated near the base of the lamellopodium of the cell as though they originated there. As well as exerting forces on the substratum in the direction of the long axis of the cell, appreciable forces are exerted from the lateral sides of the cell. The observations and measurements tend to argue that microtopography and embedded beads can concentrate the forces.