IN VIVO ELASTICITY 219 45 Amplitude of the 1st rebound 35 25 15 o o • n o '"'-.....•'g Ci Ci o ,• O O O O [3 .-.c! D ci Ci ci no ci o o•o oocici or] [] rl 10 20 30 40 50 60 AGE Figure 6. Effect of age on skin elasticity (temple). 70 80 Table IV Effect of Vitamin A Palmitate Product on Skin Elasticity Active Weeks of % p treatment Amplitude ñ SD Change Value Placebo % p Amplitude - S.D. Change Value Baseline 20.75 3.65 2 Weeks 23.75 3.35 14.47 (0.0001 6 Weeks 25.45 2.36 22.66 (0.0001 2 Weeks regression 23.24 2.28 12.00 (0.0001 21.52 3.31 20.48 2.46 -4.8 N/S 21.84 2.40 1.49 N/S 21.66 2.36 0.67 N/S an eight-week treatment regimen. The results shown in Table VI indicate that mois- turization alone is not able to increase significantly the elasticity of the skin, as only a 3% increase in this parameter was observed after eight weeks of daily application of the product. These results agree with the data observed in the previous experiment, where we observed less than a 2% increase of skin elasticity after six weeks of daily application of the placebo product. DISCUSSION The results presented herein clearly demonstrate the ability of the ballistometer to measure some of the viscoelastic properties of the skin. Evidently, the data presented above do not suggest that the superficial layers of the skin are involved in this mea-

220 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Table V Effect of Vitamin A Palmitate Product on Skin Thickness via Ultrasound A-Scan Active Control Weeks of % p treatment MM + S.D. Change Value MM % p + S.D. Change Value Baseline 1.03 0.11 1.03 0.11 2 Weeks 1.11 0.06 8.32 N/S 1.02 0.11 4 Weeks 1.19 0.06 15.89 0.003 1.03 0.11 1 Week regression 1.15 0.05 11.78 0.006 1.04 0.09 - 1.51 N/S - 0.11 N/S 1.08 N/S Table VI Effect of Simple Moisturizer on Skin Elasticity Weeks of treatment Amplitude -+S.D. % Change p Value Baseline 21.42 2.98 4 Weeks 22.57 2.93 5.37 N/S 8 Weeks 22.11 2.56 3.22 N/S surement, but rather the deeper layers such as the living fraction of the epidermis and the dermis as well. The reason for this assertion is the fact that we were never able to measure any short-term effects, such as the moisturization of the stratum corneum by a moisturizer, using this technique. In this study, treatments based on the topical application of vitamin A palmitate led to a significant change in the skin's viscoelastic properties, resulting in an increase in the amplitude as well as the number of rebounds after two weeks of application. Similarly, we were able to distinguish between the skin's viscoelastic properties as a function of the subjects' age groups as well as of different body sites. These results indicate that the information provided by the data obtained in these experiments is related to the con- dition of the living layers of the epidermis and the whole dermis. These results provide support for the hypothesis that the significant increase in the amplitude of the first rebound of the ballistometer probe, which we have related to the elasticity of the skin, is due to the long-term improvement of the skin's structure caused by the topical application of the product containing 0.1% vitamin A palmitate (8,9). As we have clearly established, this effect is not short-term, and it does not happen shortly after application of the product. In fact, our data indicate that it takes at least two weeks of daily application of a product containing vitamin A palmitate to be able to see some measurable benefits using the ballistometer. In addition, our data suggest that after a six-week treatment, the increase of skin elasticity measured with this technique persists for at least two weeks without treatment of any kind. This observation was further supported by the fact that a moisturizer had no effect on the elastic prop- erties of the skin even after eight weeks of daily application. In such conditions, only an in-depth modification of the skin structure, such as recon- figuration of the elastin network or an increase of the thickness of the epidermis, can result in a change of the skin's elasticity that can be evaluated by the ballistometer. The fact that this instrument is not able to measure any of the superficial benefits provided