This CytoSoft product comes with 10 standard T-25 flasks with substrates of a chosen stiffness. CytoSoft products provide a tool to culture cells on PDMS substrates with various rigidity covering a broad physiological range. The rigidity of the substrate to which cells adhere can have a profound effect on cell morphology and gene expression. Each flask has a defined elastic modulus bottom layer. The thickness of the silicone gel is uniform with a ~0.5 mm thick layer of silicone that is fully compatible with mammalian cell cultures. The silicone gels are activated and ready to bind to a purified ECM, such as PureCol type I collagen (#5005) prior to cell addition. The silicone substrates of CytoSoft products are optically clear and have a low auto-florescence. Silicone gels are not susceptible to hydrolysis, do not dry nor swell, are resilient and resistant to tearing or cracking, and their elastic moduli (rigidities) remain nearly unchanged during extended storage periods.