The rate at which a given molecule permeates across a membrane depends on the energy barrier represented by the tightly packed phospholipid bilayer. The structure of the lipid bilayer can be influenced by the presence of other, non-permeating molecules. This phenomenon is called permeability enhancement and has been studied extensively with regard to skin or intestinal permeability. Examples of simple permeation enhancers include ethanol, oleic acid, or dimethyl sulfoxide, but new enhancers and enhancement mechanisms are being actively investigated .