This study investigated the effects of cosurfactants on the transdermal delivery of hydrocortisone (model drug) from eucalyptus oil microemulsion.
Eucalyptus oil which was successfully employed for steroidal drugs was used as the oil. Tween 80 which was readily miscible with eucalyptus
oil was used as surfactant. Ethanol, isopropanol and propylene glycol which are relatively tolerable by the skin were employed as cosurfactants.
Pseudo-ternary phase diagrams were constructed in the presence and absence of cosurfactants. Microemulsion formulations containing 20%
oil, 20% water and 60% of either Tween 80 or 1:1 surfactant/cosurfactant mixture were compared. Incorporation of cosurfactants expanded the
microemulsion zone. The cosurfactant free microemulsion was viscous showing pseudo-plastic flow. The cosurfactant containing preparations were
less viscous with Newtonian flow. The drug loading and release rate were increased in the presence of cosurfactants with the release depending
on the viscosity. Incorporation of hydrocortisone in microemulsion increased the transdermal flux compared to saturated aqueous solution. The
presence of cosurfactants increased the transdermal drug flux compared to the cosurfactant free formulation. Ethanol produced the greatest effect
followed by propylene glycol and isopropanol. The presence of cosurfactant and its type can thus affect both the phase behavior and the transdermal
delivery potential of microemulsion.