CHARACTERIZATION OF HUMAN SKIN CHEMISTRY 285 humidity. Volunteer human subjects whose skin was to be characterized were, after their skin cleansing by a standardized technique described below, allowed at least 30 min equilibration with the clean room's atmospheric conditions prior to the initiation of any measurements on their skin. Contact angles with diagnostic fluids of known high purity and representing a range of surface tensions, molecular sizes and relative polar- to-dispersion-force interaction potentials, were measured and plotted according to the widely accepted techniques of Zisman (8). The resulting contact angle data plots yielded values of "critical surface tension" known to correlate well with the true outer- most atomic constitution of organic surfaces, with their coefficients of friction, their qualities of adhesion and degrees of roughness (9)., Similar studies of almost a decade ago, which were the pioneering efforts using contact angle techniques applied to cos- metically modified human skin, were mostly limited to measurements with droplets of water and acetone/water mixtures. These early studies showed striking relative dif- ferences in skin quality after various cosmetic treatments or cleaning procedures, but probably did not yield accurate data for the actual surface energetics of "clean" and modified skin surfaces (10). Surface-specific infrared spectra of the natural, cosmetically treated, wounded and sub- sequently healed skin of volunteer human subjects were acquired by the internal reflection spectroscopic technique described by Harrick (11). A special mirror assembly, which was made to order for our purposes (12), allowed the mounting of internal reflection prisms in a horizontal plane rather than in the vertical plane common to all other internal reflection spectroscopic accessories. The necessary internal reflec- tion prisms, constructed of the thallium bromide salt, KRS-5, or of pure germanium, were acquired from the same source (12). Infrared spectra were recorded on both Perkin-Elmer Models 457 and 700 spectrophotometers, but only traces produced on the latter instrument are included here to allow ready intercomparison among the many figures provided. The cosmetics that were spectrally characterized both before and after their application to human skin were of internationally distributed brands and were applied according to the written instructions provided by the suppliers. They are representative of products with similar generic functions widely distributed by the cos- metic industry. Epidermal skin profiling in depth, after the induction of mild epidermal wounds, was accomplished by the "Scotch tape stripping" technique commonly used in the study of epidermal wound healing (5). Prior to the initiation of any experimental series, the skin areas to be tested were gently washed with a standard liquid hand soap, thoroughly rinsed, towel dried and allowed to equilibrate with the clean room at- mosphere. Figure 1 illustrates the quite comfortable posture taken up by the volunteers during the recording of infrared spectroscopic signatures of their natural and treated skin zones. Contact angle measurements were obtained on the lateral regions of the same subjects' fingers, or the ventral surfaces of their forearms, supported--as in the spectroscopic device--on a horizontal stage of a contact angle goniometer. Figure 2 provides schematic drawings of the position of the horizontal prism mount in a record- ing spectrophotometer, together with sketches of the ventral surface of a subject's forearm upon the prism face during analysis, and an indication of the material trans- ferred (insensible perspiration, cosmetic residues, cellular debris and/or serous exu- dates from wounded tissue) on the prism face after the approximately 10-min period of contact required for recording of the entire infrared spectral trace. Figure 3 illustrates,

286 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Figure 1. Illustration of the comfortable posture allowed for volunteer subjects during the rapid, noninva- sive, analysis of their skin using a horizontal mount for multiple-attenuated-internal-reflection prisms. A similarly comfortable pose allows contact angle measurements on living skin which reveals the true outer- most skin chemistry, before and after cosmetic application