Stretch tests -4 mbar to -600 mbar

Experiments were done to determine the pressure-strain dependency of a silicone membrane in a custom made stretching device. 1 µm Ø Dragon Green fluorescent polystyrene microbeads (Bangs Laboratories, Inc., Fishers, IN, USA) were adsorbed to the surface of the silicone membrane. ZEISS Axio Scope microscope with A-Plan 10x/0.25 Ph 1 objective was used to take image sequences of membrane deformation. The membrane was deformed by decreasing the pressure from -4 mbar to -350 mbar 75 mbar steps. The microscope was manually refocused after each step. Within one strain measurement, three locations near the edge of the cell culture well (imaging locations locations 1-3) and one location in the center (imaging location4) were measured. Three to five repeated image sequences were taken at each location.For analysis, Particle Tracker 2D/3D plugin (Mosaic) [29] in Fiji [30] was used to track the trajectories of the beads (data not shown).