Before revision: For example, generating oxygen gradients was achieved by single-thin-layer PDMS construction within channels (thicknesses less than 50 μm, diffusion coefficient of oxygen in native PDMS at 25 °C, D= 3.55x10−5 cm2 s−1) without utilizing gas cylinders or oxygen scavenging agents; thus the microfluidic cell culture device can be placed in incubators and be operated easily.
After revision: For example, generating oxygen gradients was achieved by single-thin-layer PDMS construction within channels (thicknesses less than 50 μm, diffusion coefficient of oxygen in native PDMS at 25 °C, D= 3.55x10−5 cm2 s−1) without using gas cylinders or oxygen scavenging agents; thus the microfluidic cell culture device can be placed in incubators and be operated easily.