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.

IMPROVING THE ACCURACY OF SKIN ELASTICITY MEASUREMENT 39 The pieces had a fl at and smooth surface with hardness scores of Shore 5A and Shore 0–10, respectively. IN VIVO HUMAN SKIN ELASTICITY MEASUREMENT The strain versus time curves of mode 1 measurement were collected from the database of our laboratory, in which numerous previously conducted clinical studies were archived. The studies were carried out in West Michigan of the United States and followed the guidelines of human clinical study on skin property testing. Volunteers of both genders in the age range of 18–82 years old and of various ethnicities (Caucasian, Asian, Hispanic, and African American) living in the United States participated those studies. In each study, the participants were asked to remove their facial makeup and cleanse the skin sites. Following acclimation to room temperature for 15 min, skin elasticity was measured in triplicate on each test site (the cheek or the inner forearm). The individual response curves of strain versus time were saved for calculation of the infl ection points (tIPs) and other skin elasticity parameters. DETERMINATION OF INFLECTION POINT A computational algorithm was developed to calculate the tIP from each of the mode 1 response curves. Since the elastic property of a material is represented by the linear region of the response curve, the tIP in a mode 1 response curve is defi ned as the point at which the curve begins to deviate from the linear region to a nonlinear viscoelastic region, as shown by tIP on the time scale in Figure 1. The algorithm was written in Visual Basic and the calculations carried out using Microsoft Excel (Microsoft Corporation, Redmond, WA) to process multiple response curves in a batch mode. Figure 1. An illustration to identify the infl ection point (tIP) and to defi ne viscoelastic parameters from a typical mode 1 strain versus time response curve of human skin. In the diagram, e(t) is the mathematical function of the response curve. The tIP is defi ned as the time in the relaxation phase when e(t) begins to devi- ate from the linear regression line of the elastic recovery region. The right-angled trapezoid bordered by e(ta), e(tIP), and the regression line is hence the elastic recovery area, and the remaining area bordered by e(tIP) and e(t) is that of the viscoelastic recovery.