Direct estimation of the permeation of topical excipients through artificial membranes and human skin with non-invasive Terahertz time-domain techniques.

Background: Drug permeation through skin, or a synthetic membrane, from locally acting pharmaceutical products can be influenced by the permeation behaviour of pharmaceutical excipients.

Objective: Terahertz time-domain technology is investigated as a non-invasive method for a direct and accurate measurement of excipients permeation through synthetic membranes or human skin.

Methods: A series of in-vitro release and skin permeation experiments of liquid excipients (e.g. propylene glycol and polyethylene glycol 400) has been conducted with vertical diffusion cells. The permeation profiles of excipients through different synthetic membranes or skin were obtained using Terahertz pulses providing a direct measurement. Corresponding permeation flux and permeability coefficient values were calculated based on temporal changes of the terahertz pulses.

Results: The influence of different experimental conditions, such as the polarity of the membrane and the viscosity of the permeant, was assessed in release experiments. Specific transmembrane flux values of those excipients were directly calculated with statistical differences between cases. Finally, an attempt to estimate the skin permeation of propylene glycol with this technique was also achieved. All these permeation results were likely comparable to those obtained by other authors with usual analytical techniques.

Conclusion: Terahertz time-domain technology is shown to be a suitable technique for an accurate and non-destructive measurement of the permeation of liquid substances through different synthetic membranes or even human skin.