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J. Cosmet. Sci., 68, 55–58 ( January/February 2017) 55 An in vivo confocal Raman spectroscopic investigation of salicylic acid penetration: Variation with formulation parameters MARK A. DAVIES, Consumer Science, Ashland Specialty Ingredients. Abstract Salicylic acid (SA) is widely used in leave-on antiacne formulations, typically at a 2% level. As a β-hydroxy acid, it is a milder active ingredient than either α-hydroxy acids or benzoyl peroxide. SA is a keratolytic agent, a bacteriocide, and a comedolytic agent. For these reasons, improving the effi ciency of SA delivery is of interest. The objective of this work is to measure in vivo SA penetration from topically applied 2% SA leave-on products and to understand the penetration in terms of formulation parameters. Penetration of SA was measured in three depth zones—0–3 μm, 3–6 μm, and 6–9 μm below the surface—using in vivo confocal Raman spectroscopy. The delivery of SA from an emulsion, pH 4.0, and a hydrogel, pH 3.75, was compared and contrasted. A comparison of depth profi les reveals, e.g., signifi cant differences in SA distribution between-treatment profi les at various time points after treatment, particularly 3–6 μm below the surface. The hydrogel exhibited the higher normalized level of SA in the 3–6 μm depth zone. Confocal in vivo Raman spectroscopy is proving to be a valuable tool in determination of details of penetration of products into the skin. The penetration of various 2% SA anti-acne product forms will be compared and contrasted in this presentation. Delivery of SA will be discussed in terms of formulation parameters such as phase, pH, and specifi c ingredients and molecular-level interactions. INTRODUCTION Salicylic acid (SA) is a widely used active ingredient in antiacne products. It serves several functions—as a keratolytic agent, as a comedolytic agent, and as a bacteriocide. It is a β-hydroxy acid and is milder than α-hydroxy acids or benzoyl peroxide. Nevertheless, it can be irritating at a pH of 3–4, the pH range at which SA is most effi cacious in treat- ment of skin disease (1). One tactic for decreasing the irritation potential of SA is to control its delivery into the skin. The delivery of SA into the skin can be affected by formulation parameters (2). A major difference in the formulae tested here is the amount of ethanol and the matrix containing SA. Ethanol is known to be a skin penetration enhancer and is used in many transdermal formulae and patches. It can accomplish this function by several means. Ethanol may act as a solvent for the active of interest. In addition, it can aid in optimizing the solubility Address all correspondence to Mark Davies at MADavies@ashland.com.