J. Cosmet. Sci., 67, 37–44 ( January/February 2016) 37 Improving the accuracy of skin elasticity measurement by using Q-parameters in Cutometer DI QU and G. PAUL SEEHRA, Amway R&D, Ada, MI. Accepted for publication January 20, 2016. Synopsis The skin elasticity parameters (Ue, Uv, Uf, Ur, Ua, and R0 through R9) in the Cutometer are widely used for in vivo measurement of skin elasticity. Their accuracy, however, is impaired by the inadequacy of the defi nition of a key parameter, the time point of 0.1 s, which separates the elastic and viscoelastic responses of human skin. This study shows why an infl ection point (tIP) should be calculated from each individual response curve to defi ne skin elasticity, and how the Q-parameters are defi ned in the Cutometer. By analyzing the strain versus time curves of some pure elastic standards and of a population of 746 human volunteers, a method of determining the tIP from each mode 1 response curve was established. The results showed a wide distribution of this parameter ranging from 0.11 to 0.19 s, demonstrating that the current single-valued empirical parameter of 0.1 s was not adequate to represent this property of skin. A set of area-based skin viscoelastic parameters were also defi ned. The biological elasticity thus obtained correlated well with the study volunteers’ chronological age which was statistically signifi cant. We conclude that the Q-parameters are more accurate than the U and R parameters and should be used to improve measurement accuracy of human skin elasticity. INTRODUCTION Skin elasticity is an important biomechanical property of human skin, and the Cutome- ter® (Courage + Khazaka Electronic GmbH, Cologne, Germany) is the most widely used instrument for noninvasive measurement of skin elasticity. Extensive studies have been reported using the Cutometer to understand skin elastic properties in relation to age, gender, and race (1–6), to correlate elasticity with skin hydration state (7–8), and to detect changes in diseased skin and quantify treatment effects (9–13). Of the elasticity studies using the Cutometer, the vast majority used the U parameters (Ue, Uv, Uf, Ur, and Ua) under mode 1 conditions as described by Barel et al. (14). In a typical elasticity measurement, a constant negative pressure is applied to the skin and a response curve is generated showing the deformation or elongation of the skin versus time in both the suction and relaxation phases. Based on the curve, the U parameters are deter- mined and used to calculate various skin elasticity parameters (R0 through R9) (15). Among the R parameters, R7 (Ur/Uf ), which is often referred to as the “biological elasticity” since it measures the skin’s ability to return to its initial position following deformation (3), has been found to decrease with age in several studies (2,4,5). Part of this study was originally presented at the 2006 U.S. Symposium of ISBS, Atlanta, Georgia. Address all correspondence to Di Qu at di.qu@amway.com.

JOURNAL OF COSMETIC SCIENCE 38 Despite the numerous statistically signifi cant correlations reported in the literature, re- searchers feel that the Cutometer is not accurate enough to measure relatively small mag- nitude changes in skin elasticity. Murray and Wickett compared the elasticity of dry and moisturized skin, but observed no signifi cant changes in the elastic parameters Ur/Ue and Ur/Uf (16). Neto et al. noted the complex and inconsistent use of the U parameters re- ported in the literature, and argue that the current analysis of multiple U parameters does not seem to add relevant data for the study of skin elasticity (17), pointing to the confu- sion and limitations of this method. In an attempt to improve accuracy, we reviewed the defi nition of the current U-parameters and identifi ed one critical parameter that might have contributed to the perceived inac- curacy in Cutometer skin elasticity measurement. The parameter was the time during which the immediate elastic deformation in the suction phase, or the elastic return in the relaxation phase, took place. Currently, this time is defi ned by an empirical value of 0.1 s in the Cutometer mode 1 computational algorithms. It is then used to calculate the vis- coelastic parameters of Ue, Uv, Ur, and the R parameters R5, R6, and R7, through the following defi nitions (15): Ue = e(0.1), Uv = e(a) – e(0.1), Ur = e(a) – e(a + 0.1) R5 = Ur/Ue, R6 = Uv/Ue, R7 = Ur/Uf where, e is the elongation or deformation measured at a given time point, the number 0.1 is the time in seconds after the start of the suction or relaxation phase, and a is the mea- suring time of the suction phase. When we examined the properties of this critical parameter, we asked a basic question: how closely would the use of this empirical, single-value parameter represent individual skin properties knowing that there exists a wide variation in skin properties among the general population? Therefore, the aim of this study was to show why and how this parameter should be redefi ned in order to improve the accuracy of skin elasticity measure- ments. Since 2009, the algorithms and the new skin elasticity parameters thus defi ned have been adopted in the Cutometer analysis software as the Q-parameters. The technical details of these parameters, however, have not been reported until now. MATERIAL AND METHODS CUTOMETER SETTINGS The elasticity of skin and other materials were evaluated using a Cutometer MPA 580 (Courage + Khazaka Electronic GmbH, Cologne, Germany) with a 2-mm aperture probe. The negative pressure (vacuum) was set at 450 mbar and a single cycle mode 1 measure- ment performed. The duration of the suction and relaxation phases was 2 s each. PURE ELASTIC MATERIAL Cured super-soft silicone rubber pieces, Ecofl ex 5 and Ecofl ex 0–10 (Smooth-On, Inc., Easton, PA), were used as pure elastic standards for the elasticity measurements.