(a) Simulation of 2D cross-section of the device. Oxygen concentration boundary conditions are imposed at the gas channel positions. Snapshot shows t = 110 min, ~1.5 min after the swap. White line indicates evaluation location at the observation chamber. (b) Evolution of oxygen concentration at z = 100 µm. (c) Simulation at y = −250 µm. Oxygen percentage in red (left axis), and spatial gradient in purple normalized to fit in [−1, 1] (right axis), response function in green (right axis).