In vitro evaluation of topical microemulsion of capsaicin free of surfactant.

Topical microemulsion of capsaicin without surfactant was developed in this study. In these systems, the oil phase was benzyl alcohol, and the cosurfactant was propylene glycol and ethanol. The drop-size of the systems was measured by dynamic light scattering method in order to distinguish true solution from microemulsion. The transdermal performance of the microemulsions was evaluated in vitro by Franz diffusion cells fitted with rat skins. The results showed the drop-size of the microemulsions without surfactant was smaller than that of the systems with Tween 80 and the permeation rate of capsaicin decreased as the content of Tween 80 increased. In the system composed of water, benzyl alcohol and propylene glycol, the permeation rate increased with the enhancement of benzyl alcohol and water. But water content had little effect on the permeation rate in the microemulsions with ethanol as cosurfactant.