Human skin is a very impermeable membrane that protects the body from ingress of harmful substances and prevents water loss from underlying organs.
In some cases, penetration enhancers may disrupt the packing motif of the intercellular lipid matrix or keratin domains.
Cyclodextrins, chitosan, some surfactants, bile acids and salts, sodium tauro-24,25-dihydro-fusidate, and phospholipids were reported as penetration enhancers in nasal drug delivery both for humans and equines.
[9] Penetration enhancers have been applied to improve the absorption of poorly permeable, hydrophilic drugs or macromolecules.
[12] The above-mentioned permeation/penetration enhancers have a surfactant-like activity where they perturb the intestinal epithelium, promoting transcellular or paracellular absorption.