Fig. 1. The microfluidic device. (A) Microfluidic device and portion of connected tubing, shown with a dime for scale (17.91 mm in diameter). (B) Drawing of microchannels in the device. (C) Micrograph of test region, showing flow channels and barrier. Solid arrows indicate direction of flow in the device during its normal operation. Dashed arrow shows flow in channel 5 during the removal of cells from the barrier. Dashed lines mark the boundaries between streams from channels 2, 1 and 3. (D) Schematic three-dimensional drawing of barrier and cells (not to scale). Curved dashed lines are flow lines with arrows indicating flow direction. B and C are rotated 90 deg. counterclockwise with respect to A and D, as indicated by the orientation of the x- and y-axis shown in the panels.