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. Noncontact 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 microrelief 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.

Influence of the Biochemical Properties of the Human Skin Fibers on its Mechanical Properties in Vivo According to Age for Two Body Areas: the Forearm and the Thigh

“…These observations denote an anisotropic behavior of the skin. This result is coherent with the literature [1,2,10,19].…”

“…The Young's modulus remains high in these directions. In the previous study [10], the authors show that theses directions correspond with two families of lines that are perpendicular one to the other. The first directions presented are the directions of skin tension (direction of Langer lines) and the second directions presented are the directions perpendicular to the directions of skin tension.…”

“…The influence of the biochemical properties on the mechanical properties of skin depends also on body areas and measurement directions. For the forearm, this influence is more important in the directions 0 °, 30 °, 120 °, 150 ° and 180 ° (which are the directions perpendicular to the directions of the skin tension of the forearm) than the directions 60 ° and 90° (which are directions of skin tension of the forearm) [10]. For the thigh, the influence of biochemical properties is more important in the directions from 60 ° to 120 ° (which are the directions perpendicular to the directions of the skin tension of the thigh) than the directions 0°, 30 °, 150 ° and 180 ° (which are the directions of skin tension of the thigh) [10,19].…”

“…The mechanical properties of skin were determined by the non-contact impact test. This test was performed using the WaveSkin ® device developed in the Laboratory of Tribology and Dynamics of Systems (LTDS, Lyon, France) and described in Ayadh et al [10]. Its principle is to mechanically generate an air flow and apply it onto the skin surface, then to measure the resulting displacement of the skin using a laser profilometer (l = 7 mm, 800 sensors, wavelength = 405nm).…”

“…The pressure applied generates the propagation of a Rayleigh wave in the skin. The propagation speed of the wave is calculated by the method described in Ayadh et al [10] and synthetized here. The minima of the displacement curve for each sensor and the time at which they are reached are identified.…”