In vivo microdialysis is a technique used to monitor the chemistry of the extracellular space in living tissue. This chemical biosensor thus gives you a preview of chemical changes in intact tissue - before those events are reflected as chemical changes in the blood. The microdialysis probe is designed to mimic a blood capillary. When a physiological salt solution - the perfusion medium - (e.g.,Ringer solution) is slowly perfused through the microdialysis probe it draws chemical substances from the extracellular space into the probe as it equilibrates with the extracellular fluid such that it will eventually contain a representative proportion of molecules found in the extracellular space.
The probe is based on a concentric tube whereby the perfusion medium enters through a fine steel inner cannula (hollow tube) and is pumped slowly to the distal end where it flows into the tip of the probe which is surrounded by the dialysis membrane. This is the site of dialysis i.e.,diffusion of substances across and back from the extracellualr fluid. The perfusion medium exits the probe by flowing through a larger outer steel cannula and the resulting dialysate is collected at the proximal end.
It is important to realise that dialysis is bi-directional i.e., there is exchange of molecules in both directions across the membrane and that the difference in the concentration of a specific molecule across the dialysis membrane will govern the direction of the diffusion gradient. Thus, one can collect an endogenous compound (e.g., a neurotransmitter) and at the same time introduce an exogenous compound (e.g., receptor agonist or antagonist) into the extracellular space.