What are the chemical gradients directing the migration of the cells inside the devices?

There are specific chemicals that attract each cell. For Dicty, the chemical attractant is cAMP at 100nM concentration. For HL-60, the chemical attractant is fMLP at 100nM concentration. The gradients are established by diffusion between large reservoirs (to the right side in the movies) and the buffer solution in which the moving cells are (to the left in the movies). The steepest gradient is across the network of channels in the maze. The gradients are relatively stable for up to 4 hours, enough time for even the slowest cells to complete the race.