JOURNAL OF COSMETIC SCIENCE 40 RESULTS AND DISCUSSION RESPONSE CURVE OF PURE ELASTIC MATERIAL Under the current theory, the Cutometer elasticity measurement for an ideal elastic mate- rial should produce a linear response in the strain versus time curve under the mode 1 testing conditions. In our study, we fi rst examined the linearity of the response curves using silicone rubber slabs as pure elastic standard. That test produced near perfect linear response curves (the elastic deformation region before reaching the plateau) in the relax- ation phase when tested under various negative pressure settings (Figure 2). However, the response was not as linear in the suction phase. The coeffi cient of determination values (r2) for the relaxation and suction phases were 0.991 ± 0.006 and 0.956 ± 0.029, respectively. These results suggest that the relaxation phase refl ects the ideal elastic property more accurately than the suction phase, and should be the region used for skin elasticity measurements. INFLECTION POINT For the combined study populations in the database, we calculated the individual tIPs in the relaxation phase from the 4234 response curves of the cheek and forearm sites. The tIP values thus obtained had a distribution ranging from 0.11 to 0.19 s, with a mean value of 0.14 s, as shown by the histogram in Figure 3. Comparing these actual values to the single empirical value of 0.1 s, we can see how much error in elasticity measurements could have been introduced to the calculation of U and R parameters. Calculating the values of parameter R7 (Ur/Uf) using their corresponding tIPs and compare them with the R7 values based on the single empirical value of 0.1 s, we found that the new method produced higher values of Ur/Uf by an average of 0.184 unit, which is a 46.3% increase in skin biological elasticity. Since the tIP was calculated from each individual response curve, we believe the skin elasticity thus obtained is more accurate than the previous empirical approximation method. Figure 2. The response curves of pure elastic standards (silicone rubber, Ecofl ex 5, and Ecofl ex 0–10) at various negative pressure settings. Ideal elastic recovery (ER) is seen in the relaxation phase on the right side of the chart. The ER of Ecofl ex 5 at 500 mbar is represented by the right-angled trapezoid bordered by lines of e(ta) and e(t).

IMPROVING THE ACCURACY OF SKIN ELASTICITY MEASUREMENT 41 DEFINING NEW VISCOELASTIC PARAMETERS OF SKIN Using the individually determined tIPs, we could calculate the elastic and viscoelastic properties from each individual response curve. For the pure elastic standard material, its deformation completely recovers in the relaxation phase. Using the areas above the curve to represent this recovery, we defi ned the ideal elastic recovery (ER) as the area bordered by the relaxation curve and the maximum amplitude, Uf. As illustrated in Figure 2, the area for the response curve of Ecofl ex 5 at 500 mbar is the right-angled trapezoid in the relaxation phase, and its corresponding mathematical representation (RE) is given in equation (1). Comparing RE to the total area of the relaxation phase, R0 [equation (2)], we obtained the elastic recovery of the pure elastic material, ER, as the ratio of RE and R0 [equation (3)]. ( )dt 0 ( ) b E f R U e t = - ෾ (1) f R0 U b = (2) / 0 R E E R R = (3) For the human skin, we could calculate the ER using the above equations from the cor- responding response curves. Since the tIP separates the elastic and viscoelastic responses, the ER of skin is graphically represented by the trapezoid bordered by e(ta) and e(tIP) in Figure 1. Then, the viscoelastic recovery (VR) can be quantifi ed by the area within the boundaries of e(tIP) and e(t). Their corresponding mathematical representations are shown as: ER area: 0 ( ) ( ( ) ) dt tIP E f IP IP f R U e t b t U e t ෾ (4) VR area: ( ) ( ) dt b V IP tIP R e t e t ෾ (5) Figure 3. Distribution of the infl ection points (tIP) determined from 4234 response curves of people of dif- ferent age, gender, and ethnicity. The parameter was determined in the relaxation phase of response curves of cheek and forearm skin sites. Grey bars = frequency distribution of tIP values Dashed line = the single-valued empirical parameter of 0.1 s.