Formulation of electrically conducting microemulsion-based organogels

Gelatin-containing, electrically conducting, rigid water-in-oil (w/o) microemulsion-based organogels (MBG), both with and without the presence of a model drug, have been prepared using pharmaceutically acceptable oils and surfactants. As a precursor to MBG formation, preliminary formulation work was carried out investigating the factors affecting the preparation of w/o microemulsions containing large amounts of dispersed aqueous phase. From these studies isopropyl myristate (IPM) was favoured as oil due to its ability to support w/o microemulsion formation over a wide range of compositions. The single most effective surfactant for stabilising the w/o microemulsions was found to be Aerosol-OT (AOT), although synergistic effects on the extent of w/o microemulsion formation were observed upon its combination with a variety of non-ionic surfactants. Upon addition of gelatin to the w/o microemulsion, MBG could be formed when using AOT as stabiliser with most of the oils investigated (with the exception of the medium and large triglyceride oils, Miglyol 812 and soybean oil, respectively) and with a number of AOT/non-ionic surfactant/IPM combinations (both in the presence and absence of model drugs such as sodium salicylate). MBG could not however be formed with non-ionic, surfactants alone, or when used in combination with another non-ionic surfactant (regardless of the oil used). This latter observation was found to be not only a result of the inadequate level of water available for hydration of the surfactant head group and any gelatin present but also a consequence of the inability of these systems to form, upon heating, the percolated microstructures necessary to facilitate the supramolecular assembly of gelatin at the macroscopic level, a pre-requisite for MBG formation. (C) 2002 Elsevier Science B.V. All rights reserved.