Alexandre Delalleau

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.

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.

A multitechnique evaluation of topical corticosteroid treatment.

Background/purpose: Cortiscosteroids are widely prescribed for systemic or local treatment of inflammatory autoimmune disorders. Long‐term therapy is associated with side effects and causes cutaneous atrophy of the epidermis and the dermis. The present study aims to evaluate with several noninvasive techniques, the skin modifications observed during cortiscosteroids treatment. The potential of skin mechanical measurement and ultrasound radio frequency (RF) signal analysis are proposed as new measures more closely related to the functional impairments.

Mechanical skin thinning-to-thickening transition observed in vivo through 2D high frequency elastography.

This study was based on two dimensional (2D) high frequency elastography to describe quantitatively the mechanical behavior of the human dermis in vivo. The study was conducted on the forearm skin and elastographic tests were performed using a combination of two devices: an extensiometer developed for the in vivo study of the mechanical behavior of the skin using uniaxial stretching stress, and a 20MHz real time sonographer (Dermcup 2020™) for ultrasound skin imaging. The staggered strain estimation algorithm (SSE) was used to produce elastograms. A temporal cumulative technique was applied to improve elastogram quality and to monitor variations in skin strain during stretching. The influence of the natural skin tension controlled by arm bending was studied and distinctive mechanical behavior was observed for low and high mechanical stress levels. In a preliminary analysis, the reproducibility of measurements was assessed by means of coefficient of variation (CV) in 5 selected healthy volunteers.Finally, two hypotheses linked to the geometrical and structural properties of the dermis are proposed to account for the new findings described in this study.

A human skin ultrasonic imaging to analyse its mechanical properties

The analysis of the skin mechanical behaviour is a key-point for different field of investigation. As the skin is a complex structure, studies are usually based on inverse methods that compare experimental and finite element numerical results. Besides the considered behaviour law, one of the most important question concerns the geometrical aspects of the skin tissue. In this paper, it is shown how high frequency ultrasound imaging helps the calculation of skin mechanical parameters. The hypodermis influence is firstly discussed through elastographic analyses. A specific procedure to measure the dermis thickness is then proposed to highlight that such a measurement must be considered to draw reliable conclusions. The obtained results are finally discussed to point out the interest of such simplifications for the study of more complex behaviour laws.

Use of the Kalman filters for the analysis of the mechanical properties of human skin in vivo

In this article, we present an inverse method for the identification of the mechanical properties of human skin, a complex multi-layered organ which has been studied in vivo using a suction deformation technique. To identify the required properties, experimental results were compared to finite element solutions of the test, with the assumption that skin behaves as a single isotropic hyperelastic layer. The inverse method used in this article is based on that of the extended Kalman filters principle, with two modifications of this standard formulation for use in skin analysis. The modified formulation was then tested using Finite Element Method (FEM) simulated mechanical data and also with a study of linear and nonlinear theoretical problems. The results of the new formulation were also compared with the Gauss–Newton, recursive least square and Kalman smoother approaches. Finally, the reliability of the method was tested on a case study.

Dual-parameter optimisation of the elastic properties of skin.

This paper presents a procedure for characterising the mechanical properties of skin using stochastic inverse identification. It is based on the minimisation of a cost function relative to the comparison between experimental suction experiments and their corresponding finite element models. Two different models are compared: a classical single-layer approach and a dual-layer medium which account for both the dermis and the hypodermis. Finite element results are used to construct the pre-optimisation database which is required for the inverse analysis. To compare the calculations, the entire identification is based on a dual-parameter optimisation procedure: for the single-layer approach a quadratic hyperelastic constitutive equation is used, whereas for the dual-layer medium a simple neo-Hookean potential is used. Theoretical conclusions, which are developed first, are then compared with actual case studies.

An a priori Shading Correction Technique for Contact Imaging Devices

Digital imaging devices are increasingly used for color measurements in fields such as forensics, the food industry, and ecological and medical research. Especially concerning the skin, in the follow-up of benign or malignant lesions, it is crucial to ensure that the measured colors are accurate and reproducible. Several color calibration methods having been presented over the last few years. However, the choice of illuminant used remains a major source of color misinterpretation, thus, much effort is being spent in trying to evaluate this a posteriori. The device presented overcomes this problem by integrating its own light source, although corrections in lighting heterogeneity are still required. In this paper, we present a lighting modelling technique used for shading correction which improves color consistency (as assessed by ΔE evaluation versus colorimeter), noise filtering, computation time, and memory consumption for this type of device.

A new stochastic inverse identification of the mechanical properties of human skin

The study of the mechanical properties of human skin is a key point to better understand surgery, skin ageing and pathologies. As the skin is a living tissue, it must be studied in vivo, hence analytical solutions are really difficult to obtain. In this study, a new stochastic inverse method for the identification of its mechanical properties is proposed. The developed optimization method is first presented. It is based on an iterative stochastic approach which ensures the identification of a global extremum. The suction actual case study is then analysed through comparisons between experimental data and finite element models of this test. Only the elastic components of the skin are considered here. The solutions for the recursive least squares and Gauss-Newtons problems are finally compared with the proposed approach to conclude this study and to briefly present our future works.

A method to individually consider the dermis thickness for skin mechanical analyses

The skin clinical studies usually require a large number of measurements to statistically analyse the effects of a medical treatment on its mechanical properties. Hence, rapid and relevant numerical methods need to be used. Owing to its multi-layered structure, which is composed of three main entities, namely the epidermis, the dermis and the hypodermis (Agache et al. 1992), and due to its complex behaviour law (Delalleau et al. 2007a), the skin mechanical properties analysis is notoriously a difficult problem. Numerical studies thus generally propose to use simplifying assumptions that are rarely discussed (Delalleau et al. 2007b). Most of them are related to the skin behaviour law, which is usually considered as isotropic and hyperelastic. However, besides the considered behaviour law, one of the most important questions is related to the geometrical aspects of the skin tissue which is generally modelled as a single layer (Hendricks et al. 2003; Delalleau et al. 2007a). This paper first aims at comparing elastographic images and finite element calculations to state for an order of magnitude of the hypodermis elastic modulus. Owing to its considered low value, the skin model can be reduced to a single layer which accounts for the epidermis and the dermis thicknesses. To individually consider such a thickness, a specific procedure, based on active contour imaging techniques, is used to process high-frequency ultrasound images of the skin. This measurement finally stands for an input of the inverse identification computation. A stochastic optimisation technique is used to compare the experimental measurements with a thicknessinterpolated numerical data basis. An experimental study is finally presented to illustrate the theoretical aspects of this paper and to conclude on our current prospects.