Methods for characterizing the anisotropic behavior of the human skin’s relief and its mechanical properties in vivo linked to age effects

Abstract

Human skin has a complex multilayer structure consisting of non-homogeneous, non-linear, viscoelastic, anisotropic materials subjected to in vivo pre-tension. This natural tension is due to the networks of collagen and elastin fibers in the skin. The three-dimensional (3D) organization of the collagen and elastin fibers initiates the multidirectional natural tensions in the skin volume. The state of these networks influences the surface topography of the skin which depends on the area of the body and on the age of the person. This work presents two experimental methods for characterizing the anisotropic behavior of human skin and its natural tension in vivo by measuring the Rayleigh wave propagation speed in the skin and analyzing the relief of the latter. All the tests were performed on the forearm of 11 female volunteers representing two age groups: [23–28] and [39–54] years old. Non-contact impact tests were conducted, and skin replicas taken using new experimental devices. The non-contact impact test generated a Rayleigh wave that spread in the skin. The speed of this wave was measured in 7 directions and permitted quantifying the main directions of propagation. The image of the skin relief was reconstructed by optical confocal microscopy which allows analyzing the micro-relief of the skin’s surface. This enabled quantifying the main directions of the skin lines. The comparison of the two methods highlights the link existing between the surface topography and the 3D organization of the fiber networks. Moreover, these methods showed the remarkable effect of age on the mechanical behavior of human skin in vivo. These mechanical properties and the natural tension of skin decrease with age. Anisotropic behavior becomes more marked with age. The main directions of skin tension were oriented in preferred directions that corresponded to the direction of Langer’s lines. These directions were [20°–60°] and [120°–140°] for the young group, and [20°–60°] for the older group.