Gwendal Josse

A nonlinear elastic behavior to identify the mechanical parameters of human skin in vivo

Background/purpose: Various analyses have been performed to identify the mechanical properties of the human skin tissue in vivo. They generally use different approaches and hypotheses (behavior laws as well as mechanical tests) and the obtained results are consequently difficult to analyze and compare. In this paper, an inverse method that can be adapted to any kind of mechanical tests and behavior laws is presented.

A new experimental method for measuring skin's natural tension

Purpose: The precise determination of skins mechanical properties is still an open question. When performing an in vivo test, the piece of skin tested is not as well defined as it is in material testing. Moreover, the body zone and the body posture imply an initial stress on the skin. Consequently, a precise mechanical analysis needs a precise measurement of the natural skin tension.

MEASURING INTERFACIAL ADHESION BETWEEN A SOFT VISCOELASTIC LAYER AND A RIGID SURFACE USING A PROBE METHOD

A reliable measure of the adhesion between a very deformable material and a solid surface is rather difficult, since the interface boundary conditions and the bulk deformation of the layer are closely and very nonlinearly coupled. In this article, a new methodology to assess the adhesion of a soft viscoelastic layer on a solid surface is proposed, where we have used a specific experimental geometry minimizing the bulk deformation of the layer. A flat-ended probe is first put in contact with a thin layer of soft material and removed at a constant velocity. The probe is then stopped at a preset level of tensile force and the time for complete debonding of the layer from the probe is measured. For our model system, comprised of a soft acrylic removable adhesive and a silicone-coated surface, the higher the applied force the faster the interfacial fracture occurs, leading to an experimental curve of the adhesion energy as a function of average crack velocity. We find that the methodology is relatively simple to implement and should be widely applicable for weakly adhering soft layers of arbitrary viscoelastic properties. The assumptions involved in such an analysis and their inherent limitations are also illustrated experimentally and critically discussed.

Measurement of the mechanical properties of the skin using the suction test

Introduction: The suction test is commonly used to study the mechanical properties of human skin in vivo. The unevenness of the stress fields complicates obtaining the intrinsic mechanical parameters of the skin in vivo because the values of the local stresses and deformations cannot be calculated directly from the displacements and forces applied by the test apparatus. In general, users only take into account the negative pressure applied and the elevation of the dome of skin drawn up in order to deduce the properties of the skin. This method has the major disadvantage of being dependent on the experimental conditions used: in particular, the size of the suction cup and the negative pressure applied. Here, we propose a full mechanical study of the test to provide rigorous results. We compare the frequently used geometric method (making the thin plate hypothesis), Timoshenkos method (which can take greater plate thicknesses into account) and finally various results obtained by the finite elements (FE) technique.

Association between collagen production and mechanical stretching in dermal extracellular matrix: In vivo effect of cross-linked hyaluronic acid filler. A randomised, placebo-controlled study

BACKGROUND The effects of hyaluronic acid (HA) injection on tissue collagen anabolism are suggested to be related to the induction of mechanical stress, causing biochemical changes in skin physiology. OBJECTIVES To ascertain the association between dermal mechanics modulated by a hyaluronic acid-based filler effect and metabolism. METHODS Sixty females were randomised to receive a 0.5mL injection of HA gel or isotonic sodium chloride (control) in the arm. Skin biopsies were taken at baseline and after 1, 3 and 6 months. Protein and gene expression of procollagen, matrix metalloproteinases (MMP) and MMP tissue inhibitors (TIMP1) were measured blind by ELISA and qPCR, respectively. Injected volumes were measured by high-frequency ultrasound and radiofrequency analysis. Skin layer effects of injections were analysed by finite element digital modelling. RESULTS One month after injection, the filler induced an increase in procollagen (p=0.0016) and TIMP-1 (p=0.0485) levels and relative gene expression of procollagen III and I isoforms compared with the controls. After 3 months, procollagen levels remained greater than in the controls (p=0.0005), whereas procollagen expression and TIMP-1 and MMP content were no longer different. Forty-three percent of the injected filler volume was found at 1 month, 26% after 3 months and 20% after 6 months. LIMITATIONS The ultrasound imaging technique limited the scope of the investigation and precluded an evaluation of the action of the filler at the hypodermic level. CONCLUSIONS Integrating both mechanical and biological aspects, our results suggest that mechanical stress generated by cross-linked HA plays a role in dermal cell biochemical response.

Prevalence of dermatoporosis in elderly French hospital in‐patients: a cross‐sectional study

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In vivo visualization of hyaluronic acid injection by high spatial resolution T2 parametric magnetic resonance images

Background/purpose: In recent years, increasing use of injectable resorbable fillings has been reported for facial wrinkle treatment. However, the physiological processes involved such as the localization and subsequent diffusion of the injected product in skin tissues are poorly documented. This may be noninvasively achieved using quantitative magnetic resonance imaging (MRI), which is duly presented in this pilot study.

In-vivo imaging of skin under stress: potential of high-frequency (20 MHz) static 2-D elastography

The aim of this study was to evaluate the potential of high-frequency static two-dimensional (2-D) elastography for in vivo exploration of the mechanical behavior of skin. Our device was based on the combination of a 20 MHz sonographer and a patented extensiometer device able to apply calibrated uniaxial stretching of the skin. We used a new algorithm to compute elastograms that improve elastographic signal-to-noise ratio (SNRe) without sacrificing resolution. Mechanical behavior was described according to the axial strain and lateral displacements induced in the tissue. The efficacy of the strain anpolyvinyl alcohol first evaluated in poly vinyl alcohol (PVA)-cryogel phantoms. Several in vivo experiments then were conducted, mainly with the multistretching averaging method, and demonstrated the potential of this technique in the evaluation of mechanical behavior of the dermis and the hypodermis under stress

Automatic measurement of epidermal thickness from optical coherence tomography images using a new algorithm.

Background/purpose: Optical coherence tomography (OCT) is an imaging system that enables in vivo epidermal thickness (ET) measurement. In order to use OCT in large‐scale clinical studies, automatic algorithm detection of the dermo‐epidermal junction (DEJ) is needed. This may be difficult due to image noise from optical speckle, which requires specific image treatment procedures to reduce this. In the present work, a description of the position of the DEJ is given, and an algorithm for boundary detection is presented.

4-Hydroxynonenal impairs transforming growth factor-β1-induced elastin synthesis via epidermal growth factor receptor activation in human and murine fibroblasts

Elastin is a long-lived protein and a key component of connective tissues. The tissular elastin content decreases during chronological aging, and the mechanisms underlying its slow repair are not known. Lipid oxidation products that accumulate in aged tissues may generate protein dysfunction. We hypothesized that 4-hydroxynonenal (4-HNE), a highly reactive α,β-aldehydic product generated from polyunsaturated fatty acid peroxidation, could contribute to inhibiting elastin repair by antagonizing the elastogenic signaling of transforming growth factor-β1 (TGF-β1) in skin fibroblasts. We report that a low 4-HNE concentration (2µmol/L) inhibits the upregulation of tropoelastin expression stimulated by TGF-β1 in human and murine fibroblasts. The study of signaling pathways potentially involved in the regulation of elastin expression showed that 4-HNE did not block the phosphorylation of Smad3, an early step of TGF-β1 signaling, but inhibited the nuclear translocation of Smad2. Concomitantly, 4-HNE modified and stimulated the phosphorylation of the epidermal growth factor receptor (EGFR) and subsequently ERK1/2 activation, leading to the phosphorylation/stabilization of the Smad transcriptional corepressor TGIF, which antagonizes TGF-β1 signaling. Inhibitors of EGFR (AG1478) and MEK/ERK (PD98059), and EGFR-specific siRNAs, reversed the inhibitory effect of 4-HNE on TGF-β1-induced nuclear translocation of Smad2 and tropoelastin synthesis. In vivo studies on aortas from aged C57BL/6 mice showed that EGFR is modified by 4-HNE, in correlation with an increased 4-HNE-adduct accumulation and decreased elastin content. Altogether, these data suggest that 4-HNE inhibits the elastogenic activity of TGF-β1, by modifying and activating the EGFR/ERK/TGIF pathway, which may contribute to altering elastin repair in chronological aging and oxidative stress-associated aging processes.