Citation: IFSCC Magazine, 11 (2008) (2) 115–119
Abstract: Hydroxy acids enjoy tremendous interest in cosmetology thanks to their skin anti-ageing and water barrier enhancing activities. One of their actions is to promote the natural stratum corneum (stratum corneum) desquamatory process. However, their use is limited due to an inherent pH-related irritancy potential which is even more exacerbated on sensitive skin. Clearly there is an opportunity for improvement. In this research we evaluated in humans the efficacy of salicylic acid, and its salts, as a corneodesmolytic agent using the dihydroxyacetone method and measuring the reduction in skin staining with treatment over time using a chromameter. Salicylic acid at 2% in a preparation of pH 3.12 significantly increased exfoliation by 10.9% compared with placebo (P < 0.05), confirming its desquamatory enhancing properties. Then the effect of vehicle cream pH on salicylic acid activity was studied. Salicylic acid at close to neutral pH (mostly in its neutralized form as salicylate, pH 6.50) exerted a corneodesmolytic activity as good as that of salicylic acid in an acidic vehicle (pH 3.12) after only two days of application. Furthermore, the performance of glycolic acid and salicylic acid salts as exfoliants were compared at pH 6.50. When these two hydroxyl acids were formulated at the same molar level in a cosmetic base (14.47mmol L-1), the salicylic acid preparation gave an 8.2% increase in stratum corneum desquamation compared with the glycolic acid preparation (P < 0.05). The corneodesmolytic effects were confirmed using a tape-stripping assay combined with a quantitative protein assay. Neutralized salicylic acid was found to enhance the removal of stratum corneum proteins significantly more than the vehicle after 25 sequential tape strippings (14%; P < 0.05). Finally, salicylic acid had no significant influence on skin water barrier properties after 22 days of treatment. In the second phase of this research we assessed the suitability of neutralized salicylic acid as an ingredient for sensitive skin. A stinging test was performed according to the Frosch & Kligman method to evaluate the influence of the formulation base-pH on stinging potential. Salicylic acid formulated at pH 6.50 induced no stinging sensation (score 0) in contrast to salicylic acid at pH 3.12 (score 19; P < 0.05). In addition, a clinical study was conducted to assess the erythema induced on volunteers’ cheeks after a single application of a neutralized salicylic acid (1%) formulation compared with placebo. Visual redness was assessed by a dermatologist and then measured with a Mexameter. No significant differences were observed. Moreover, half of the panel had sensitive skin and no correlation could be established between redness and/or abnormal sensation and sensitive skin. In conclusion, neutralized salicylic acid at a 1% concentration is a suitable exfoliant agent for subjects with sensitive skin.
Keywords: exfoliation, hydroxy acids, Salicylic acid, stratum corneum
Paper presented at the IFSCC Conference 2007, Amsterdam, The Netherlands.