text: The OFM probes are perfused with a physiological solution (the perfusate) which equilibrates with the ISF of the surrounding tissue. Operating flow rates range from 0.1 to 10 μL/min. OFM allows unrestricted exchange of compounds via an open structure across the open exchange area of the probe. This exchange of compounds between the probe’s perfusate and the surrounding ISF is driven by convection and diffusion, and occurs non-selectively in either direction (Figure 1).
  The direct liquid pathway between the probe’s perfusate and the surrounding fluid results in collection of ISF samples. These samples can be collected frequently and are then subjected to bioanalytical analysis to enable monitoring of substance concentrations with temporal resolution during the whole sampling period.
  The concentric OFM probe (Figure 2) works according to the same principle. The perfusate is pumped to the tip of the OFM probe through the inner, thin lumen and exits beyond the Open Exchange Area, where it then mixes with exogenous substances present in the ISF before being withdrawn through the outer, thick lumen.