Cells in the human body can display remarkable differences in their behaviour depending on the mechanical properties of the tissue surrounding them. This is especially true for immune cells, which migrate through the body and are thus constantly exposed to tissues with different properties and must respond appropriately. To enable investigation of how cells respond to dynamic changes in the stiffness of the extracellular environment, researchers from the Freiburg Signalling Research Clusters of Excellence BIOSS and CIBSS and the Cluster of Excellence livMatS joined forces to develop a hydrogel matrix whose stiffness can be reversibly tuned using light. The study represents the first joint project between CIBSS and livMatS and is featured on the cover of the latest issue of Advanced Materials.