Visualization of the chemical gradients along the side channels.

a. Linear gradients, formed along the side channels, were imaged using fluorescent dyes of molecular weight comparable with that of the chemoattractant. Starting immediately after replacing the solution in the main channel with buffer, quantitative fluorescence was measured along the array of side channels at 10, 30 and 60 minutes. These measurements demonstrate the relative stability of the linear chemoattractant gradient over time. b. At 30 minutes after introducing the buffer, a linear gradient of fluorescent dye can be visualized along the side channels. The height of the main channel (to the left) is larger that that of the side channels and accounts for the apparent higher fluorescence of the main channel compared to the side channels.