326 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Figure 5. 't has also been undertaken by means of radiotracer methods. In one particular technique developed (13, 14), the astringent properties of antiperspirants were studied by first treating live frogs with the astringent. The animals were pitbed and the permeability of the frog skin was measured by determining the rate and amount of iodine-131 which migrated through the membrane. Figure 7 illustrates the experimental method used for determining the amount of radio-iodine migration. Radioassay has been undertaken by measurement of the beta particle emission from iodine-131, or by deter- mining the gamma activity of a portion of the water in the tube using a gamma scintillation spectrometer. The rate of iodine migration through the skin is subsequently correlated with the effectiveness of the astringent. There is no other satisfactory method at the present time available to industry that will effectively evaluate astringents in respect to antiper- spirant properties. With slight modifications this technique has been used to study other classes of compounds. To exemplify: by use of tritium in the form of tritiated water in a manner very similar to the preceding procedure, the effect of such materials as "barrier creams" may be evaluated in respect to their properties of preventing penetration of water through a skin mem- brane. Again these are examples where radiotracer techniques have been

RADIOISOTOPES IN DETERGENT AND COSMETIC RESEARCH 327 Figure 6. devised to furnish results that are either very difficult to obtain or are unattainable by other means. Many studies are reported in the literature for determining retentiveness in the oral cavity and on human teeth of various agents from dentifrice formulas and from solution. Among the radiotracers used have been calcium-45, iodine-131, zinc-65, phosphorus-32, chlorine-38, flourine-18, sulfur-35, rubidium-86, potassium-42, sodium-24 and carbon-14 labeled compounds such as urea, nicotinanide, thiourea and acetamide. The length of retention, rate and depth of penetration into tooth structure as well as localization in defects in tooth surface have been studied. The increased use of fluorides in dentistry has prompted many investi- gations with radiotracers to determine conditions necessary for optimum adsorption and retention of this material on teeth. With the advent of a procedure for reactor production (15) of fluorine-18, many studies have been