429 JPP 2005, 57: 429–434 ß 2005 The Authors Received September 21, 2004 Accepted December 20, 2004 DOI 10.1211/0022357055795 ISSN 0022-3573 Department of Pharmaceutical Technology, Faculty of Pharmaceutical Sciences, Khon Kaen University, Khon Kaen 40002, Thailand Thaned Pongjanyakul, Aroonsri Priprem Department of Manufacturing Pharmacy, Faculty of Pharmacy, Mahidol University, Bangkok 10400, Thailand Satit Puttipipatkhachorn Correspondence: Thaned Pongjanyakul, Department of Pharmaceutical Technology, Faculty of Pharmaceutical Sciences, Khon Kaen University, Khon Kaen 40002, Thailand. E-mail: thaned@kku.ac.th Acknowledgements and funding: The authors wish to thank the Faculty of Pharmaceutical Sciences, Khon Kaen University (Khon Kaen, Thailand) for technical support and the Thai Red Cross (Bangkok, Thailand) for the provision of cobra skin. We thank the Commission on Higher Education, Ministry of Education (Bangkok, Thailand) and the Thailand Research Fund (Bangkok, Thailand) for financial support (grant no. MRG4680074). Influence of magnesium aluminium silicate on rheological, release and permeation characteristics of diclofenac sodium aqueous gels in-vitro Thaned Pongjanyakul, Aroonsri Priprem and Satit Puttipipatkhachorn Abstract The effect of magnesium aluminium silicate (MAS) on rheological, release and permeation charac- teristics of diclofenac sodium (DS) aqueous gels was investigated. DS aqueous gels were prepared using various gelling agents, such as 15% w/w poloxamer 407 (PM407), 1% w/w hydroxypropyl- methylcellulose (HPMC), and 1% w/w high and low viscosity grades of sodium alginate (HV-SA and LV-SA, respectively). Different amounts of MAS (0.5, 1.0 and 1.5% w/w) were incorporated into the DS gels. Incorporation of MAS into the DS gels prepared using SA or PM407 caused a statistical increase in viscosity (P < 0.05) and a shift from Newtonian flow to pseudoplastic flow with thixotropic property. The DS release rates of these composite gels were significantly decreased (P < 0.05) when compared with the control gels. This was due to an interaction between MAS and PM407 or SA, and adsorption of DS onto MAS particles. Moreover, a longer lag time and no change in DS permeation flux were found when MAS was added to the gels. The findings suggest that the rheological characteristics of gels prepared using PM407 or SA could be improved by incorporating MAS. However, the use of MAS could retard the DS release and extend the lag time of DS permeation. Introduction Gelling agents for pharmaceuticals and cosmetics can be classified into inorganic and organic substances on the basis of the nature of the colloidal phase. Clay, an example of an inorganic gelling agent, possesses a lamellar structure that can be extensively hydrated. The flat surfaces of particles are negatively charged, while the edges are positively charged. The attraction of face to edge of these colloidal lamellae creates a three-dimensional network of particles throughout the gel. Organic gelling agents are obtained from natural and synthetic polymers. The long chains of polymer are extended in water due to hydrogen bond formation between water and hydroxyl groups of the polymers, leading to high viscosity and gel formation (Zatz & Kushla 1989). Mixtures of polymers are commonly used to improve the rheological characteristics of food products. The nature of the synergy can be due to association of the different molecules of polymers or to non-association (Draget 2000). When two polymers associate, precipitation or gelation can occur. The viscosity synergism of some poly- mers has been reported by which an ionic and a non-ionic cellulose, such as sodium carboxymethylcellulose and hydroxypropylmethylcellulose (HPMC), produced an increase viscosity due to cross-linking. This led to the use of cellulose at reduced concentrations in formulated products (Walker & Wells 1982). Magnesium aluminium silicate (MAS) has been used in oral and topical formulations as a suspending and stabilizing agent either alone or in combination with other suspend- ing agents. The combination of MAS and anionic polymers, such as xanthan gum (Ciullo 1981) and carbomer (Ciullo & Braun 1991), resulted in viscosity synergism. The advan- tages of combining MAS with polymers in cosmetic products are: (i) the combination may be more economical than the use of either component alone; (ii) MAS can compen- sate for the loss of viscosity at elevated temperatures common to many polymers; and (iii) MAS can reduce the tacky, gummy or stringy nature of polymer dispersions (Veegum/ Van Gel; R. T. Vanderbilt Company, Inc. Technical literature, 1999). However, the