DIMETHYL SULFOXIDE 123 the derreal chamber at various times, adding this to 15 ml of counting fluid (6 parts of ethylene glycol monomethyl ether to 10 parts of 1:25 Liquiflor scintillation fluid) and determining the amount of dimethyl- C TM sulfoxide using a Packard Tri-Carb Liquid Scintillation Spec- trometer. RESULTS AND DISCUSSION While the in vitro skin penetration of several ions and covalent mole- cules has been shown to obey Fick's First Diffusion Law (11), it was necessary to show that the penetration of pierate ion in the presence of DMSO is also a passive diffusion process. The results of a study demonstrating the applicability of Fick's Law are presented in Table I. The DMSO concentration was maintained at 80% (v/v) while the pierate concentration was changed over a 13 fold range. With the exception of one experiment, the data do exhibit ad- herence to equation 2. Thus, the system approximates a passive diffu- sion process. In order to get some estimate as to the reproducibility of the data ob- tainable with this system, five identical experiments were set up with skin membranes obtained from a small abdominal area of a single guinea pig. The results are shown in Table II. With evidence thus available on the kinetics of the diffusion process and on the reproducibility of the system, experiments were designed to study in detail the effect of DMSO on this system. The penetration rates of pierate ion as a function of dimethyl sulf- oxide concentration were studied, and the results are presented in Fig. 3. Table I Fick's Law Study a % DMSO (v/v) Initial Concentration of Absolute Rate Pierate Ion (M) Constant (em hr -•) 80 1.56 X 10 -2 13.7 X 10-4 80 4.14 X 10 -• 14.2 X 10 -• 80 6.68 X 10 -• 30.7 X 10 -• 80 9.75 X 10 -2 17.5 X 10-• 80 19.8 X 10 -• 13.8 X 10 -• a (A young male guinea pig was sacrificed by a lethal injection of MgSO4. The abdominal hair was clipped and the abdominal skin was immediately excised and frozen. The length of time between procurement and utilization of the skin was approximately 24 hours. The skin had been kept frozen until use.)

124 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS It can be concluded from these results, in agreement with others (3, 7), that large concentrations of DMSO are required before appreciable en- hancement of penetration can occur. Although Stoughton (1) has re- ported enhanced penetration with relatively low concentrations of DMSO, it is perhaps possible that his experimental procedures allowed for evaporation of water from the treatment solutions thus effectively changing the DMSO concentrations. It was also observed in these experiments that those skin membranes which had been in contact with the greater concentrations of dimethyl sulfoxide appeared more wrinkled and stretched and swollen, yet much less pliable than those skin membranes which had been in contact with buffer or lower concentrations of DMSO. A literature search revealed Table II Reproducibility Study • Initial Concentration Absolute Rate % DMSO (v/v) of Pierate Ion (M) Constant (cm hr -1) 70 3.75 X 10 -2 14.7 X 10 -4 70 3.75 X 10 -2 15.5 X 10 -4 70 3.75 X 10 -• 15.5 X 10 -4 70 3.75 X 10 -• 12.1 X 10 -4 70 3.75 X 10-• 17.6 X 10 -4 Average and standard deviation = 15.1 X 10 -4 4- 2.0 X 10 -4 • (The abdomen of a male guinea pig was wax epilated, and the animal was sacrificed 3 days later by a lethal injection of MgSO4. The abdominal skin was immediately excised and frozen. The length of time between procurement and utilization of the skin was 24-48 hours. The skin was kept frozen until use. ) Table III Effect of DMSO Concentration on Percutaneous Absorption of Pierate Ion s Initial Cone. Approximate % DMSO (v/v) of Pierate (M) Lag Time (hr) 0 4.81 X 10 -• 5.2 20 3.24 X 10-• 5.2 40 5.61 X 10-• 4.9 60 5.15 X 10-2 5.0 72 3.73 X 10-• 4.0 80 6.70 X 10 92 3.53 X 10 • (The abdomen of a male guinea pig was wax epilated, and the animal was sacrificed 3 days later with a lethal injection of MgSO4. The abdominal skin was immediately excised and frozen. The length of time between procurement and utilization of the skin was about 3 days. The skin was kept frozen until use.)