To determine the responses of different regions of migrating cells to changes in substrate rigidity, we have developed a modulatable hydrogel by reacting linear polyacrylamide with a photosensitive crosslinker. This material allows UV-mediated softening by 20–30% upon irradiation at a dose tolerated by live cells. We found that localized softening of the posterior substrate of polarized cells causes no apparent effect, while softening of the anterior substratum elicits pronounced retraction, indicating that rigidity sensing is localized to the frontal region. |