This paper addresses the modification of poly(dimethylsiloxane), i.e. PDMS, using plasma surface treatment and a novel application of the membrane created. A set of model compounds were analysed to determine their permeation through PDMS, both with and without plasma treatment. It was found that plasma treatment reduced permeation for the majority of compounds but had little effect on some compounds, such as caffeine, with results indicating that polarity plays an important role in permeation, as is seen in human skin. Most importantly, a direct correlation was observed between plasma-modified permeation data and literature data through calculation of membrane permeability (Kp) values suggesting plasma-modified silicone membrane (PMSM) could be considered as a suitable in vivo replacement to predict clinical skin permeation.

8Silicone membrane is frequently used as an in vitro skin mimic whereby experiments 9 incorporate a range of buffered media which may vary in pH. As a consequence of such 10 variability in pH there is a corresponding variability in the degree of ionisation which in turn, 11 could influence permeation through the mainly hydrophobic-rich membrane structure. This 12 study reports the effect of pH on the permeation of five model compounds (benzoic acid, 13 benzotriazole, ibuprofen, ketoprofen and lidocaine). For the five compounds analysed, each 14 at three distinct percentages of ionisation, it was found that the greater extent of permeation 15 was always for the more 'neutral', i.e. more greatly unionised, species rather than the anionic 16 or cationic species. These findings fit with the theory that the hydrophobic membrane 17 encourages permeation of 'lipid-like' structures, i.e. the more unionised form of compounds.

This study reports the effect of surfactant charge and concentration on the permeation of four model compounds (benzocaine, benzotriazole, ibuprofen and lidocaine). Surfactant charge was systematically varied using a range of surfactants that are known to possess specific head group charges, namely an anionic, a cationic, a zwitterionic and a neutral form over a series of surfactant concentrations, i.e. where possible, both above, and below, the critical micellar concentration for each surfactant. It was found that there was almost always a systematic reduction in permeation as the concentration of surfactant increased despite the wide range of physicochemical properties exhibited by the four model compounds studied. Overall, it was concluded that the presence of surfactant does generally seem to reduce permeation, regardless of the compound in question, and that the effect is surfactant concentration, as well as charge, dependent.

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