SKIN PLASTICISATION BY 2-HYDROXYOCTANOIC ACID 401 each disc weighed dry. One disc served as a control exposed to water only for subtrac- tion of background. The remainder were separately exposed to 1 ml aliquots of donor solution, radiolabelled as for penetration, for four hours at 30øC. This time was suffi- cient for equilibrium to be achieved. Each sample was then removed, briefly rinsed in water, blotted dry, and solubilised prior to liquid scintillation counting. Specific sorp- tion of donor solute was calculated for each group of seven as a mean ___ two standard errors. RESULTS AND DISCUSSION EXTENSIBILITY AS A FUNCTION OF RELATIVE HUMIDITY The influence of the relative humidity of the environment upon the water content and extensibility of stratum corneum is well documented (13). We wished to investigate the plasticisation efficacy of HCA at a range of relative humidities including those low values at which conventional moisturisers cannot function (6). This study was carried out using one sample of human corneum only. Its extensibility (% stretch per 100 g load) was measured before treatment, as the relative humidity (RH) of the cabinet containing the apparatus was changed in steps. Sufficient time was al- lowed for equilibration of the corneum with each new RH. Temperature was main- tained at 20øC. The sample was immersed in water for 3 hours at 20øC, blotted, and allowed to equilibrate with the cabinet environment before extensibility was measured again as a function of RH. Finally, the process was repeated after immersion of the sample for 3 hours in 0.15 mol 1-• HCA solution at 20øC. The results are presented in Figure 1. It is evident that the effectiveness of plasticisation by HCA was maintained throughout the range of humidity values used. This observation has important implications. The incidence of dry skin is higher at low RH values (1) and the effectiveness of tradi- tional (humectant type) moisturisers will be minimal with little available atmospheric water. It is, therefore, encouraging that the HCA plasticisation effect persists even at low RH, indicating that the effect is direct plasticisation of corneum rather than plasti- cisation via moisturisation. This is borne out by investigations which indicate that HCA does not substantially alter the water-binding capacity of corneum (7). Ideally we would have preferred to study further aspects of HCA plasticisation at low RH, where skin dryness problems are most evident. However, isolated corneum samples are brittle and difficult to handle and measure at this value. The above-de- scribed persistence of HCA plasticisation efficacy over a wide range of RH values was sufficiently encouraging that we chose a compromise RH of 65% (at 20øC) for all following measurements. It is of interest to note that when, at this RH, we treated our corneum samples with a solution of the commonly quoted moisturiser (4) sodium lac- tate at 0.2 mol 1- •, 20øC for 3 hours, we obtained no significant increase in extensibil- ity under our experimental conditions. Similarly, treatment under the same conditions with 10% solutions of either urea, sodium pyrrolidone carboxylate, or sodium chloride failed to produce any substantial extensibility increases. So at an RH of 65% we would expect no interfering effects of moisturisation from the additives to be reported below.

402 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS 0.5 -0.5 -1 -1.5 i I [ [ ] [ I i •o to 50 •o 70 no 9o • R.H. el 20 C Legend ß BEFORE [3 AFTER WATER ß AFTER HCA Figure 1. Dependence of human stratum corneum extensibility on ambient relative humidity. EFFECT OF pH The dependence of skin extensibility, penetration, and sorption on pH of the HCA solution is shown in Table I. The decrease in extensibility with increasing pH agrees with the previously reported observation that significant increases in extensibility are only obtained at low pH (7). Table I Effects of 0.2 mol D-• HCA Solution pH on Epidermis Extensibility and HCA Penetration Through and Sorption by Stratum Corneum Mean Mean Mean Sorption x 102 Extensibility Permeability constant, (moles/g stratum pH (adjusted Ratio (+_2 X S.E.) kp X 10 7 (ms -1, +-half corneum, +2 X S.E.) with TEA) at 20øC/65% RH range) at 30øC at 30øC --2.4 5 (+2) 1.7 (_+0.1) 2.0 (+--0.1) 3.0 5 (+- 1) 0.5 (-+0.3) 0.99 (+ 0.1) 3.5 3.9 (+0.7) -- -- 4.0 2.6 (+0.3) 1.34 (+0.02) 0.49 (+0.02) 5.0 1.3 (+-0.2) 0.3 (+0.3) 0.10 (+ 0.01) 6.0 1.0 (+0.2) Negligible • 0.08 (+-0.02) 6.8 1.2 (+ 0.2) Negligible a 0.034 (+ 0.005) b Steady-state penetration not achieved after 30 hours. Different skin donor.