Photomicrograph of a microdialysis probe. Probes are constructed in the lab. The tubing in the middle is fused silica tubing with an inner diameter of 40 microns. This delivers the perfusate to the probe tip. The membrane is cellulose. The perfusate flows out of the inner tubing and is contained within the membrane. The exit port (not shown) is another piece of fused silica tubing.