Aqueous Boundary Layer (ABL)
The ABL constitutes a sticky layer of water adhering to the surface of membranes. Since little mixing or molecular motion occurs within this region, it is not the membrane, but the ABL that acts as the primary barrier to the permeation of highly lipophilic molecules. In unstirred systems this barrier is typically 1200 µm and in the human jejunum it is only 40-100 µm thick. The permeability in poorly or completely unstirred membrane systems depends on the thickness of the ABL, and not the membrane itself. Stirring control is therefore essential.
Precise Stirring Control for 96 wells
Precise Stirring Control for 96 wells Each well of the donor plate contains a small magnetic stir disc that flips in response to the precise rotation of a high magnetic field inside the Gut-Box. The front panel control is factory calibrated to deliver the exact ABL thickness setting.
True Membrane Permeability
True Membrane Permeability The permeability pH plot for chlorpromazine under a variety of stirring conditions is shown below. Under ideal conditions with no ABL, the permeability of this basic molecule would increase with a slope of 1 up until it is neutral and no further pH dependence is observed as indicated by the double-dashed line. The intersection between both straight line tangents crosses at the pKa of 9.24.