The effect of rheological behavior and microstructure of the emulgels on the release and permeation profiles of Terpinen-4-ol

Terpinen-4-ol, the main component of Australian tea tree oil, which was widely used in pharmaceutical and cosmetic fields, was chosen as the active component in this study because of its excellent antimicrobial properties and good permeability. The objective of this study was to investigate the relationships between rheological characters, in vitro release and permeation profiles of the emulgels. Particle size distribution, optical microscopy tests were performed to characterize the status of the emulgels. Scanning electron microscopy and rheology tests were carried out to assess the strength of the structures. In vitro release and percutaneous permeation study of Terpinen-4-ol through Cellophane® membrane and excised rabbit skin were conducted by vertical Franz diffusion cell, respectively. All of the emulgel samples showed a non-uniform bimodal distribution and the microstructure represented a matrix type which could inhibit the diffusion of oil droplets in formulation in some extent. Rheological data showed a good fit to the Herschel–Bulkley model in viscosimetric studies regardless of the polymers used. Moreover, 10% Sepiplus 400 had obtained the highest zero-shear viscosity, G′, G″ value and lowest τ95%G′ value corresponding to the strongest structure. The results of in vitro release tests revealed that an increase in viscosity may affect the release profiles inversely, irrespective of the polymers used. In vitro permeation of Terpinen-4-ol tests indicated that when the drug amount released could satisfy the essential driving force, permeation processes was independent of release. In contrast, the drug amount released could not satisfy the essential driving force, the permeation amount was inversely proportional to the viscosity as the release amount did.

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