Assessment of Mechanical Properties of Skin by Shearwave Propagation and Acoustic Dispersion

Abstract

In this chapter, we present a methodology of documenting the angular anisotropy of skin elasticity with high sensitivity and dynamic range using the Reviscometer® RVM 600 and its use to document changes in keratinocyte morphology elicited by topical application of a reversible antagonist to keratinocyte nicotinic acetylcholine receptors (nAChRs). 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°. Based on the angular distribution of the resonance running time, we define two parameters: the anisotropy and the width of the angular dispersion. The mechanical properties of the skin are not isotropic (uniform in all directions), and there is a need to assess this angular anisotropy. During development, from infancy to adolescence, skin is expected to respond to tension isotropically to accommodate for growth, while in adulthood this isotropic behavior regresses due to site-specific habituation to tension. We find that with increasing age the anisotropy increases while the angular dispersion width decreases. Changes in viable epidermis keratinocyte morphology and stratum corneum moisturization may be assessed and documented by these parameters. 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 difference in skin viscoelasticity from infants up to 75 years of age and documenting efficacy of topical treatments. 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 age-related and morphological changes in the skin of the mechanical properties and demonstrate a structure-function relationship.