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The wave phenomenon in free surface media stems from the propagation of mode grouping. Due to the nature of propagation in a given medium, this phenomenon expresses different types of dependence on the medium's properties and represents its mechanical admittance. In contrast with body wave propagation, dependencies related to surface propagation in a medium can be described by spatial-temporal characteristics. These characteristics can be obtained by performing appropriate experiments and do not require prior knowledge of the physical properties of the medium. In this study, we propose an original surface wave investigation and a phenomenological analysis approach adapted to the mechano-bio-structural states evaluation of in vivo human skin. Two objectives are sought with the method proposed: the first concerns the development of a non-invasive device for generating and tracking surface waves in human skin called Free-Skin-Surface-Wave (FSSW); the second concerns the adaptation of the Multi-Chanel Analysis of Surface Waves (MASW) method to evaluate the mechano-bio-structural states of human cutaneous tissue in vivo on the basis of the propagating phenomena observed. As an illustration of the proposed method application, we have done an in vivo evaluation, on intern-forearm of female volunteers population. In addition, we proposed a study of the aging effect and a comparison with ultrasound B-Mode technique, to validate the method sensitivity to follow the mechano-morphological properties of the in vivo human skin. In this study, our medium of application was human skin in vivo, but it is conceivable to extend this application to other soft biological media.

Chronological skin ageing is a phenomenon which imposes structural and functional changes on the cutaneous tissue. Mechanically, these changes can be related to structural rearrangements of the cutaneous tissue on surface and in volume (layers thickness). At the micro-structural level, the constitutional elements of the skin, collagen and elastin fibres, undergo also this rearrangement. The evolution of skin's mechanical properties at this level is the origin of a primordial in-vivo mechanical characteristic known as the natural pretension. In the context of understanding the in-vivo skin mechanical behaviour, related to the natural pretension, a lot of instrumentations have been demonstrated in the literature. They are mainly based on the interaction between dynamic adapted solicitation and the observed reaction on the skin. In this study, we evaluate the mechanical behaviour of human skin, following an impact which induces wave propagation. The use of impact solicitation allows the direct correlation between the dynamic induced reaction (vibration, and wave propagation) of the cutaneous tissue and its mechanical property. In our development, impact solicitation is contactless, with an air blast as generator of local deformation. The estimation of the speed of wave propagation enables the characterization of the mechanical behaviour of the skin. In order to validate the developed approaches, to understand the chronological ageing, gender and anisotropy effects on the skin properties, measurements have been realized on 77 healthy volunteers separated in five age groups. The obtained results are consistent with earlier works and confirm the efficiency of the developed instrumentation to estimate the changes of mechanical behaviour of the skin under age and gender effects.

The human finger plays an extremely important role in tactile perception, but little is known about how age and gender affect its biophysical properties and their role in tactile perception. We combined studies on contact characteristics, mechanical properties and surface topography to understand age and gender effects on the human finger. The values obtained regarding contact characteristics (i.e. adhesive force) were significantly higher for women than for men. As for mechanical properties (i.e. Young's modulus E ), a significant and positive correlation with age was observed and found to be higher for women. A positive correlation was observed between age and the arithmetic mean of surface roughness for men. However, an inverse age effect was highlighted for women. The age and gender effects obtained have never been reported previously in the literature. These results open new perspectives for understanding the weakening of tactile perception across ages and how it differs between men and women.

Les donnees concernant les modifications quantitatives et qualitatives des principaux composants lipidiques cellulaires liees aux processus de differenciation et de maturation de la muqueuse intestinale sont passees en revue