Simulation results of the concentration distribution at the top of the microfluidic network. (a, b) without the balance zone, the microfluidic platform will fail to generate satisfatory gradients if there is a small pressure difference (5%) between the two inlets. (c, d) with a balance zone, a good concentration difference can be maintained even if there is a pressure difference as large as 25%. (c) In device I, the flow resistance of counter-pressure-difference channels is 2.57 times that of the compensation channel. (d) In device II, the flow resistance of counter-pressure-difference channels is 9.82 times that of the compensation channel.