120 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS ever, an understanding of its mode of action could lead to the discovery of other methods to enhance percutaneous absorption. To achieve this goal, the in vitro I•ereutaneous Iressage of pierate ion in the presence of dimethyl sulfoxide has been examined in detail. MATERIALS AND METHODS White male guinea pigs ranging in size from 600-1200 g were obtained from Dublin Laboratory Animals, Inc., Dublin, Va. Reagent grade picric acid was t urchased from Allied Chemical and Dye Corp., New York, N. Y., "certified" reagent grade dimethyl sulfoxide from Fisher Scientific Company, Fairlawn, N. J., ethylene glycol monomethyl ether from Mallinckrodt Chem. Works, New York, N.Y. dimethyl-C TM sulfoxide and Liquifluor (25 X concentrate liquid scintillator) were ob- tained from New England Nuclear Corporation, Boston, Mass. All other chemicals were obtained as pure as possible. Intact abdominal guinea pig skin was utilized as a membrane between a set of glass diffusion chambers. Hair was removed from the skin ei- ther by wax epilation several days prior to sacrificing the animal or with an electric clipper. Animals were usually sacrificed by a lethal injection of MgSO4, and the skin was rapidly excised and frozen. Attempts were made to utilize the skin within 48 hours after procurement. Ainsworth has reported that the chemical permeability of excised skin is almost un- changed for at least two days if bacterial decomposition is prevented by cold storage (8). Since large variations in skin permeability were often seen from ani- mal to animal and have even been reported from different skin sites of a single animal (9, 10), any one series of experiments whose results are re- ported here in a single figure or graph was performed using skin from a relatively small area of the abdomen of a single guinea pig. It has also been observed in this laboratory that differences in the preparation of the animals could result in large variations in permeability characteristics from animal to animal. Skin from wax epilated animals often showed greater permeability characteristics than skin from animals which had not been so treated. This was probably due to an incomplete recovery of the barrier layer. On occasion, erratic results within a given series from the same animal were obtained which were attributed to in- herent or mechanically induced variations. Prior to beginning an experiment, portions of the frozen skin were cut and positioned as the membranes between L-shaped glass diffusion chambers. A pinch clamp which locked and sealed the chamber-mem- brane-chamber assembly in place was then applied. Any overlapping

DIMETHYL SULFOXIDE 121 Figure 1. Unassembled diffusion chambers Assembled diffusion chambers mounted in thermostated water bath Figure 2. skin was cut away leaving a circular membrane area of approximately 3 cm •. The entire assembly was then mounted on a Burrell wrist-action shaker which positioned the diffusion assembly in a thermostated water bath (Figs. 1 and 2). In a typical diffusion experiment, picric acid was dissolved in an 80% (v/v) DMSO/pH 7.0 buffer solution (monopotassium phosphate-so- dium hydroxide 0.025 M). Fifteen milliliters of this solution was then placed in the epidermal chamber and 15 ml. of pH 7.0 buffer was placed in the derreal chamber. Glass beads were added to ensure proper mix- ing and a genfie shaking motion was then initiated. All diffusion experi- ments reported in this paper were performed at pH 7.0 and 30 ø q- 0.05 øC.