Fig. 8.

a For low collagen concentrations, there is a considerable likelihood that—as a statistical artifact—some cells are located in an environment where they encounter many more binding spots in the direction of decreasing stiffness thus inducing migration against the stiffness gradient. For high collagen densities, the likelihood of such statistical artifacts substantially decreases. b The setup of the purple curve in Fig. 7a was reconfigured by inverting the stiffness gradient so that it pointed in negative x-direction. This caused a continued migration in that direction rather than a limited motion in the beginning followed by stagnation