R. Vargiolu

A new approach to describe the skin surface physical properties in vivo

In the present paper, we describe a new mechanical method characterising the physico-chemical properties of human skin and their variations along with liquid exposure scenario to the skin surface. A specific bio-tribometer has been developed to study the physical properties of the skin in vivo by measuring the maximum adhesion force between the skin and the bio-tribometer. We showed that the lipidic film present on skin surface was responsible for skin adhesion due to capillary phenomena. The measure of pull-off force between skin and bio-tribometer has permitted to estimate the liquid/vapour surface tension of the lipidic film (gamma(LV) approximately 6.3mJ/m(2) in 30-year-old volunteer). The kinetic of sorption/desorption (sorption means indifferently adsorption and absorption process) of distilled water from the skin has been observed through the variation of the indenter/skin pull-off force versus time after distilled water application to the skin surface. This permits to follow in real time the variation of the skin physico-chemical properties after liquid application onto the skin surface. Finally, the increasing of skin friction coefficient after distilled water application onto skin surface was explained by the capillary adhesion force between the probe and the skin.

The morphological tree transform of surface motifs. Incidence in tribology

In this paper we show two new developments in the three-dimensional characterisation of rough surfaces. The basic idea is to decompose the three-dimensional roughness as a combination of 3D motifs defined by amplitude, wave length, local and overall direction of various components. The multi-scale approach developed in this work is very useful in tribology, the summit of the morphological tree permits the characterisation of the waviness of the summits envelope, each summit can be characterised by the height and the radius of curvature in x and y direction necessary for the computing of Hertzian pressure. The local slopes of the summits envelope play an important role in Coulombic friction, adherence and leakage. The fluid reservoir can be characterised by the multi-scale aspect of the 3D motifs size and their orientation.

Effect of lateral resolution on topographical images and three-dimensional functional parameters

The influence of the analysis scale on the characterisation of surface roughness is set out in this work. First the effect of lateral resolution on the local morphology of a random engineered surface is shown. The effect of the geometry of a tactile profilometer tip is studied by the variation of the tip radius of curvature. The incidence of the scale observation is analysed from micro- to nano-scale with three microscopes: a tactile, laser and atomic force microscope. The lateral resolution influences not only the roughness amplitude parameters, but also spectral range, slopes, local radius of curvature of the summits, bearing area, developed surface, the void or material volume. In the second half of this work the contribution of fractal geometry is examined. This is because, through its roughness index, called the Holder index, it may be independent of the measurement. By using the Weierstrass-Mandelbrot function as a three-dimensional multi-scale model of the topography, the evolution of the bearing area, the developed surface and the volume as a function of the Holder roughness index can be shown.

In vivo skin biophysical behaviour and surface topography as a function of ageing.

Normal skin ageing is characterised by an alteration of the underlying connective tissue with measurable consequences on global skin biophysical properties. The cutis laxa syndrome, a rare genetic disorder, is considered as an accelerated ageing process since patients appear prematurely aged due to alterations of dermal elastic fibres. In the present study, we compared the topography and the biomechanical parameters of normal aged skin with an 17 year old cutis laxa patient. Skin topography analyses were conducted on normal skin at different ages. The results indicate that the skin relief highly changes as a function of ageing. The cutaneous lines change from a relatively isotropic orientation to a highly anisotropic orientation. This reorganisation of the skin relief during the ageing process might be due to a modification of the skin mechanical properties, and particularly to a modification of the dermis mechanical properties. A specific bio-tribometer, based on the indentationtechnique under light load, has been developed to study the biophysical properties of the human skin in vivo through two main parameters: the physico-chemical properties of the skin surface, by measuring the maximum adhesion force between the skin and the bio-tribometer; and the bulk mechanical properties. Our results show that the pull-off force between the skin and the biotribometer as well as the skin Youngs modulus decrease with age. In the case of the young cutis laxa patient, the results obtained were similar to those observed for aged individuals. These results are very interesting and encouraging since they would allow the monitoring of the cutis laxa skin in a standardised and non-invasive way to better characterize either the evolution of the disease or the benefit of a treatment.

Preliminary investigation of the frictional response of reptilian shed skin

Abstract Developing deterministic surfaces relies on controlling the structure of the rubbing interface so that not only the surface is of optimized topography, but also is able to self-adjust its tribological behavior according to the evolution of sliding conditions. In seeking inspirations for such designs, many engineers are turning toward the biological world to correlate surface structure to functional behavior of bio-analogs. From a tribological point of view, squamate reptiles offer diverse examples where surface texturing, submicron and nano-scale features, achieve frictional regulation. In this paper, we study the frictional response of shed skin obtained from a snake (Python regius). The study employed a specially designed tribo-acoustic probe capable of measuring the coefficient of friction and detecting the acoustical behavior of the skin in vivo. The results confirm the anisotropy of the frictional response of snakes. The coefficient of friction depends on the direction of sliding: the value in forward motion is lower than that in the backward direction. In addition it is shown that the anisotropy of the frictional response may stem from profile asymmetry of the individual fibril structures present within the ventral scales of the reptile.

A study on the frictional response of reptilian shed skin

Deterministic surfaces are constructs of which profile, topography and textures are integral to the function of the system they enclose. They are designed to yield a predetermined tribological response. Developing such entities relies on controlling the structure of the rubbing interface so that, not only the surface is of optimized topography, but also is able to self-adjust its tribological behaviour according to the evolution of sliding conditions. In seeking inspirations for such designs, many engineers are turning toward the biological world to study the construction and behaviour of bio-analogues, and to probe the role surface topography assumes in conditioning of frictional response. That is how a bio-analogue can self-adjust its tribological response to adapt to habitat constraints. From a tribological point of view, Squamate Reptiles, offer diverse examples where surface texturing, submicron and nano-scale features, achieves frictional regulation. In this paper, we study the frictional response of shed skin obtained from a snake (Python regius). The study employed a specially designed tribo-acoustic probe capable of measuring the coefficient of friction and detecting the acoustical behavior of the skin in vivo. The results confirm the anisotropy of the frictional response of snakes. The coefficient of friction depends on the direction of sliding: the value in forward motion is lower than that in the backward direction. Diagonal and side winding motion induces a different value of the friction coefficient. We discuss the origin of such a phenomenon in relation to surface texturing and study the energy constraints, implied by anisotropic friction, on the motion of the reptile.

Acoustic Tribology of Human Skin

From the point of view of the tribology of the touch quality, when one speaks about softness of an object, one very easily associates it his roughness, the friction and the mechanical properties of materials [1,2,3]. With regard to the human skin, the stratum corneum (SC), or cornea layer, is the part of skin surface, which is permanently requested during a friction test. In this work, we present a new approach developed in our laboratory, in order to understand the friction mechanisms of the human skin, and to identify the acoustic, signatures which can be correlated with the touch quality. The results show that the acoustic emission is correlated the stiffness and roughness of the stratum coreneum. The application to the analysis of ageing, show clearly the increase of acoustic emission during ageing of human skin.Copyright