j. Soc. Cosmet. Chem., 43, 13-20 (January/February 1992) Instrumental and sensory evaluation of the frictional response of the skin following a single application of five moisturizing creams MARIE LODI•N, H•KAN OLSSON, LIZBET SKARE, and TONY AXI•LL, ACO HUD AB, Research and Development, S-112 87 Stockholm, Sweden (M.L., L.S.), and Department of Oral Surgery and Oral Medicine, School of Dentistry, Carl Gustaft viig 34, S-214 21 Malmb', Sweden (H. 0., T.A. ). Received April 20, 1991. Synopsis Application of a cream to the skin induces tactile changes of the surface. The frictional characteristics of products are important in consumer acceptance. The frictional resistance following application of five moisturizing creams was studied using sensory analysis and a newly developed sliding friction instrument. The methods gave comparable results. During appli- cation of the products, an immediate reduction in friction was obtained, followed by a gradual increase. A urea-containing cream gave rise to high frictional values, whereas a cream containing a high concentration of oils gave low frictional values. Separate consumer tests showed different preferences due to the different behavior of the creams. Successful correlation between consumer tests and instrumental data would be a key to introducing new quantitative measures of product performance. INTRODUCTION In the cosmetic and pharmaceutical industries it is important to develop topical for- mulations that have maximum consumer and patient acceptability. Otherwise, the formulations will not be used, even if they have suitable biological effects. Application of products to the skin induces tactile and visual changes of the skin surface. Volatile compounds, such as water, evaporate, and the remaining constituents are mixed with other substances on the skin. For product attributes of creams, not only is the ratio between oil and water important, but also the type of oil as well as the amount and type of other ingredients (emulsifiers, humectants, etc.). The combination of substances influences the initial feel, how the formulation spreads on the skin, whether and how fast it is absorbed, and how the skin feels after use. To evaluate such product differences a variety of test methods are needed. Tactile perception involves a contact and a movement of a fingertip across the skin 13

14 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS surface. The relative movement of the two surfaces is restricted by friction. Thus friction of skin plays an important role in the objective evaluation of consumer-perceptible skin attributes. Quantitative measurements of skin friction can generate valuable guidelines in the course of product development, aimed at producing desirable tactile feel. The aim of the present study was to develop a set of techniques to measure the frictional response of skin following application of moisturizing creams. The friction was mea- sured objectively with a newly developed sliding friction instrument, as well as sub- jectively using sensory analysis. A consumer test was also undertaken in which we evaluated the subjective attitudes to the perceived skin feel. MATERIALS AND METHODS PREPARATIONS USED Five moisturizing creams (A-E) with variations in type and amount of oils, humectants, and emulsifiers were tested. The composition and viscosity of the creams are given below. The viscosity (mPas) was determined with Haake Viscosimeter RV 12 (Haake Mess-Technik GmbH u. Co., West Germany). The measurements were performed under the same conditions at 20øC. A. 5000 mPas urea, lactic acid, betaine, diethanolamine, cetylphosphate, cholesterol, lanolin, glyceryl monostearate, sodium chloride, water. B. 7000 mPas pyrollidone carboxylic acid, isopropyl myristate, peanut oil, mineral oil, alcohol, PEG-2 stearate/stearic acid, cetylphosphate/DEA cetylphosphate, diethanoi- amine, butylhydroxitoluene, parabens, water. C. 8000 mPas pyrollidone carboxylic acid, glycerine, cetearyl octanoate, dimethicone, mineral oil, isopropyl myristate, myristyl myristate, cetyl phosphate/DEA cetyl phos- phate, PEG-2 stearate/stearic acid, diethanolamine, parabens, butylhydroxitoluene, al- cohol, water. D. 10,000 mPas glycerine, peanut oil, mineral oil, glyceryl stearate, stearic acid, triethanolamine, parabens, water. E. 45,000 mPas mineral oil, cetearyl alcohol, ceteth-20, sodium citrate, citric acid, methylparaben, water. INSTRUMENTAL ASSESSMENT OF FRICTION Seven healthy volunteers were included in the study, two males and five females, with a mean age of 39 years (range 19-57). The creams (A-E) were tested once on the volunteers. The creams were also tested seven times on one volunteer (male, 28 years) during a time period of six months, with at least one day passing between each eval- uation. This was done in order to study the inter- and intraindividual variation in the skin response. Before application of the creams, a basal value of the skin friction was registered. The products were dispensed by a volumetric syringe to the inner forearm of the volunteers. The application rate was about 5 mg/cm 2. The friction was measured immediately after application of the product and then every fifth minute up to 30 minutes. Between each measurement the probe was cleaned.