Skin moisturisation and elasticity originate from at least two different mechanisms

Skin moisturisation and elasticity are clearly linked in people's mind. For a large series of neat personal care ingredients we measured both their skin moisturising and plasticising performance on human skin in-vivo, and were able to rank the products according to their effectiveness. Efficacy was expressed relative to benchmark products: glycerine for moisturisation and water for elasticity. A good spread in skin moisturisation efficacy was found: 15 of our products had a relative performance of less than 30%, 23 products scored between 30 and 70% and 7 ingredients marked above 70%. For skin elasticity, the number of products in the corresponding groups was 36, 4 and 2. This discrepancy could be due to the choice of benchmark products, but when the values for relative elasticity performance of the various ingredients were plotted against those of relative moisturisation effectiveness, it became obvious that skin moisturisation and elasticity are not linearly correlated. This is at least remarkable as water was used as the benchmark product for skin elasticity. The common belief that increased skin moisturisation will automatically lead to enhanced skin elasticity is therefore unlikely to be true. The only logical conclusion seems to be that different molecular and/or biochemical mechanisms are contributing to these two skin functionalities. By means of Principal Component Analysis, the correlation between molecular descriptors such as skin penetration and humectancy on the one hand and substantivity and occlusivity on the other was evaluated to explain the observed skin moisturisation. It became obvious that none of these purely molecule-based molecular descriptors could explain the measured skin effects. Skin itself plays an important role in a yet unidentified manner. If both activities are desired in one cosmetic formulation, at least two active ingredients will need to be incorporated, each contributing to one of the two effects. Examples of multifunctional mixtures and an assessment of the feasibility of obtaining multifunctional molecules are presented.