124 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Figure 12. Four images of skin replicas taken and day 8. ?om the same SLES treatment site at pretest, day 3, day 5, Figure 13. Four images of skin replicas taken from the same PEG-20 glyceryl monotallowate treatment site at pretest, day 3, day 5, and day 8. percentage change in TEWL caused by PEG-20 glyceryl monotallowate was 10% to 30% below zero, the possible moisturizing effect of PEG-20 glyceryl monotallowate was not statistically significant (27). High-frequency electrical conductance measurements best indicated the hydration con-

CHANGES IN STRATUM CORNEUM 125 * LEGEND t.3- i i N e i • Dayi 8 o.g n 0.8 0.7 P 8 0.6 t i 0.4 o 0.3 o. 2 • Day5 o.i 0 • ELE ELEE Ta i lo Nate Figure 14. Analysis of skin replicas (area/perimeter--cycle 1) during one week of daily treatments, October-November 1989. dition of the intact layers of the SC. For the SLS-patched sites, the mean values decreased during the first three days (test cycle 1) or two days (test cycle 2) and then dramatically increased. This probably reflects an induced drying effect of SLS on the outermost layers of the SC, followed by marked damage or destruction of the SC and measurement of the highly hydrated epidermal layers beneath the SC. After a weekend rest period, the mean electrical conductance was dramatically reduced, indicating regeneration of an intact SC. The findings are similar to those reported by Tagami et al. (22). For the SLES-patched sites, the lower mean electrical conductance values indicate a temporary drying effect on the outermost layers of the stratum corneum. By using both TEWL and electrical conductance measurements, it may be possible to differentiate the mechanisms underlying the SC changes. Drying effects appeared to be better assessed by electrical conductance measurements, while SC integrity is probably better demonstrated by TEWL measurements. The conditions for TEWL and electrical conductance measurements must be controlled to avoid background variability. Preliminary studies showed fairly large intra- and inter-individual variations when both measurements were carried out at ambient room conditions. In order to obtain reproducible results from these instruments, it was important to have the subjects adhere to instructions and to perform these measurements under regulated environmental conditions. The chamber that we used provided the necessary stable environmental conditions for measurement after a 30-minute acclima- tion period (see Methods section).