Skin viscoelasticity displays site- and age-dependent angular anisotropy.

One of the dominant characteristics of skin aging is loss of elasticity. Although the changes in the mechanical properties of the skin over several decades of life are substantial, objective measurements have failed to capture their magnitude thus far. Moreover, the mechanical properties of the skin are not uniform in all directions, and there is a need to understand this angular anisotropy. In this work we present a methodology of documenting the angular anisotropy of skin elasticity with high sensitivity and dynamic range using the Reviscometer RVM 600 (Courage & Khazaka Electronic GmbH, Cologne, Germany). The method is based on determining the directional dependence of the speed of an acoustic shear wave on the skin surface at intervals of 3 degrees . Based on the angular distribution of the resonance running time, we define two parameters: the anisotropy and the angular dispersion width. We find that with increasing age the anisotropy increases, while the angular dispersion width decreases. The ratio of these values provides a sensitive parameter for the assessment of the directional behavior of the skin mechanical properties. This parameter provides a large effective dynamic range capable of demonstrating close to an order of magnitude differences in skin viscoelasticity from infants up to adults 75 years of age. Furthermore, we show that the direction of the angular anisotropy relates to the direction of the dermal cleavage lines as defined by Langer, indicating that the anisotropy of the mechanical properties of skin stems from structural parameters. Based on these results, we conclude that the proposed methodology is able to capture accurately the age-dependent changes of the mechanical properties of the skin and to demonstrate a structure-function relationship.