DIMETHYL SULFOXIDE AND SKiN 133 Table I Changes in Work Index of Human and Guinea Pig Stratum Corneum Strips Stretched in Water at Various Extensions Exten- Work Index Stratum sion (Final Water • Number of Corneum (%) \Initial Water•/ Experiments Comments Human 10 1.01 2 Reversible system Human 20 0.82 1 Irreversible system Guinea pig 5 1.05 4- 0.08a 10 Reversible system Guinea pig 10 1.02 2 Reversible system Mean and 95% confidence level (17). Table II Changes in Work Index of Guinea Pig Stratum Comeurn Strips Stretched Three Times in Water (5% Extension) Work Index Second Stretch in Water• Initial Stretch in Water•/ Work Index Third Stretch in Water• In• Stretch in Water] O.98 1.15 1.14 1.16 1.28 1.10 1.03 1.08 1 .O9 1.13 1.05 1.04 1.094-0.12• 1.11 4-0.05• Mean and 95% confidence level (17). cyclings were possible with no apparent differences occurring in the load- extension curves. To allow for a greater safety margin, 5% extension was adopted as the standard in further experiments. Effect of Dimethyl Sulfoxide on the Worh Index oJ • Stratum Comeurn The effect of a range of aqueous DMSO concentrations on stratum corneum within 24 hours was examined. The results are presented in Table III and Fig. 1. There was relatively little change in the work in- dex at the lower concentrations (20-40%). The work index increased as the concentration of DMSO was changed from 40% to 80%. The critical concentration seemed to be about t50% DMSO although the exact concentration point may differ from animal to animal. However, at an 80% concentration there was a marked increase in the work index.

134 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Table III Effect of DMSO on Work Index of Guinea Pig Stratum Corneum" DMSO Con- centra- tion (%) (v/v) Work Index No. of Work Index No. of Solvent• Experi- ( Final Water • Experi- •eater / ments •,Initial Water.,] ments Comments 20 0.90 q- 0.09• 6 1.10 -4- 0.22 4c 4o 1.04 -4- O. 08 9 1.05 q- O. 09 9 60 I .38 q- 0.14 17 1.11 q- 0.19 4 80 1.92 q- 0.22 16 1.13 q- 0.17 12c No significant change No significant change Reversible system Reversible system Mean values from different animals. Mean and 95% confidence level (17). Reduction in number of experiments resulted from mechanical breakage of strips. O strips in solvent ß strips back in water -• 1.4 1.0 _ 10 20 30 40 50 60 70' 80 D•qSO Concentration (%) Figure 1. The reversible effect of DMSO concentration on work index o[ guinea pig slratum corneum This dependence on a large concentration of the solvent has been ob- served in detail in other studies on hair swelling and skin permeability (l, 4), where it was suggested that DMSO has the ability to swell or un- fold protein chains. In hair, the swelling effect is accompanied by a l•ardening or loss of elasticity (6). Qualitative observations on skin have also suggested that the DMSO effect is accompanied by a wrinkling or hardening of the skin and the physical studies reported here correlate witl• those observations.