DIMETHYL SULFOXIDE AND SKIN 135 The increase in work index is reversible and this was revealed by the observations that the mechanical responses of the strips returned to the original values obtained before the treatment in DMSO. Since DMSO is capable of dissolving the lipids of the stratum comeurn, and since dur- ing the subsequent immersion in water soluble proteins may also be dis- solved (18), it appears that DMSO-insoluble keratin elements and not lipiris or soluble proteins bear the mechanical stress of the stratmn cor- neum under these conditions. The reversibility of these DMSO strengthening studies on stratum corneum at first does not seem to agree with the nonreversibility experienced with the DMSO skin permeability studies reported earlier (4). However, whereas a lipid role has been es- tablished for skin barrier function, the present physical studies with guinea pig stratum comeurn exclude it completely. Thus, reversibility might be expected in one case and not in the other. It was also noticed that there was a visible change in the refractive in- dices ol/the stratum corneum strips on immersion in DMSO. The strips became completely transparent. However, after rewetting with water they became opaque again. This change in appearance is most probably correlated with the dehydration-solvation process of the samples which occurs during the initial submerging in DMSO. El•ect ol • Other Chemical Reagents on the Work, Index o[ Stratum Corneitm Since the above data suggest that the DMSO effect upon the work index is dependent not upon lipids or soluble proteins but upon the in- soluble keratin components of the stratum corneum, studies were con- ducted to characterize the insoluble keratinaceous portions of the stratum corneum which bear the mechanical stress reported here. Thus, the effect of various chemical reagents on the work index was examined. Phenol, Trichloroacetic Acid, and Formaldehyde Solutions of phenol (2% and 5%), 5% trichloroacetic acid (TCA), and 10% formaldehyde were examined, and it was found that all of these materials increased the work index ol/stratum comeurn strips. A series of experiments was undertaken to establish the dependence of the work index on the phenol concentration. The results are pre- sented in Table IV. When 1% phenol was used, there was no significant change in the work index. With 2% and 5% solutions of phenol, the work index increased, although there was no significant change between the 2% and 5% concentrations. At first this effect was rather unexpected

136 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Table IV Effect of Phenol Concentration on Work Index of Guinea Pig Stratum Comeurn a Phenol Con- cen- tration (%) Work Index No. of Work Index No. of 8olvent• Experi- ( Final Water ) Experi- W•er / ments •Initial Water ments Comments 1 1.10 -4- 0.05 o 9 1.05 q- 0.06 9 2 1.65 q- 0.32 6 1.21 -4- 0.16 6 5 1.39 4- 0.12 9 1.11 4- 0.16 6c No significant change in properties Partly reversible Partly reversible All experiments performed on the skin from a single animal. Mean and 95% confidence level (17). Reduction in number of experiments resulted from mechanical breakage of strips. and contrary to that reported on wool. When wool fibers are treated with phenol they swell considerably and become much weaker This weaken- ing effect of phenol in the case of wool is reported to be reversible (19). However, there is literature evidence based on dilatometric and thermo- ,dynamic considerations that when phenol absorbs onto hair keratin, ex- tensive aggregation between adjoining phenol molecules occurs (20). 'This behavior could explain the ability of phenol to precipitate soluble proteins. Stratum corneum has several protein components of varying .solubility (18) and phenol may precipitate one or more of these fractions within the skin matrix. A combination of phenol-induced aggregation and protein precipitation could result in a strengthening effect providing ,other bond-breaking mechanisms are overcome. The composition of wool does not include the variety of protein [Tactions found in stratum corneum. With phenol, the increase in work index was only partly reversible. 'The strips soaked in phenol became icy-white and stiff. Apparently some permanent denaturation of the stratum corneum had taken place. Trichloroacetic acid (TGA) also strengthened the stratum corneum but the effect appeared to be reversible (Table V). Binding studies have ,suggested that TGA interacts with protein amide groups to form insolu- ble precipitates (21). Indeed, TGA is widely used as a means of depro- ,teinizing biological fluids. The combining ability ot• TCA with peptide