Traction Force Microscopy

Traction force microscopy was used to measure the contractile forces that cells exerted upon their substrate as previously described (Jiu et al., 2017). Briefly, cells were cultured for 3–8 h on custom-made 35-mm dishes (Matrigen Life Technologies, CA, United States) with fibronectin-coated PAA gel with either 25 or 0.5 kPa stiffness. The diameter of 200 nm yellow-green fluorescent (505/515) microspheres was immobilized to the surface of the gel. Images of the cells and the fluorescent microspheres directly underneath the cells were acquired during the experiments and after cell detachment with trypsin. By comparing the reference image with the experimental image, we computed the cell-exerted displacement field. From the displacement fields, and manual traces of the cell contours, together with knowledge of substrate stiffness, we computed the traction force fields using the approach of constrained Fourier-transform traction cytometry. From the traction fields, we calculated the strain energy by equation

It was the total deformation energy produced by the cells through applying the traction on the surface of the substrate, which suggested an integrated measure of cell traction.