Jean-Michel Lagarde

Model of the viscoelastic behaviour of skin in vivo and study of anisotropy

Background: The single‐axis extension test is relatively little used to study the mechanical properties of human skin in vivo. A campaign of tests was carried out with an original, modern machine developed in our laboratory. It can perform extension or compression tests using servo‐controlled position or force in different directions. The load can either be of the extension or monotonous compression type, creep or relaxation. The results obtained were used to develop a viscoelastic model. The elastic modulus calculated helps us to determine the main directions of anisotropy on the forearm.

Skin topography measurement by interference fringe projection: a technical validation.

Background/aims: The quantitative analysis of skin topography is frequently used in cosmetology to evaluate the efficacy of hydrating or anti‐wrinkle creams (micro‐topography studies) or creams for slimming or to alleviate stretch marks (macro‐topography studies). Numerous methods involving the three‐dimensional (3‐D) reconstruction of the topography from silicone replicas have been developed. Some of the main techniques applied include optical profilometry based on the reflection of oblique lighting or on transparency, and profilometry by laser focusing or triangulation.

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.

Topography and anisotropy of the skin surface with ageing

Background/purpose: Most studies of how the relief of the skin surface changes on ageing are based solely on topography, and a few have taken into account anisotropy parameters. However, the calculation of these parameters is often carried out manually and as the techniques of data acquisition have progressed rapidly, a certain degree of obsolescence has been reached. It is for this reason that we have undertaken a similar study but with more advanced equipment.

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.

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 quantification of epidermis pigmentation and dermis papilla density with reflectance confocal microscopy: variations with age and skin phototype

Abstract:  Reflectance confocal microscopy (RCM) may help to quantify variations of skin pigmentation induced by different stimuli such as UV radiation or therapeutic intervention. The objective of our work was to identify RCM parameters able to quantify in vivo dermis papilla density and epidermis pigmentation potentially applicable in clinical studies. The study included 111 healthy female volunteers with phototypes I–VI. Photo‐exposed and photo‐protected anatomical sites were imaged. The effect of age was also assessed. Four epidermis components were specifically investigated: stratum corneum, stratum spinosum, basal epidermal layer and dermo‐epidermal junction. Laser power, diameter of corneocytes and upper spinous keratinocytes, brightness of upper spinous and interpapillary spinous keratinocytes, number of dermal papillae and papillary contrast were systematically assessed. Papillary contrast measured at the dermo‐epidermal junction appeared to be a reliable marker of epidermis pigmentation and showed a strong correlation with skin pigmentation assessed clinically using the Fitzpatrick’s classification. Brightness of upper spinous and interpapillary spinous keratinocytes was not influenced by the skin phototype. The number of dermal papillae was significantly lower in subjects with phototypes I–II as compared with darker skin subjects. A dramatic reduction in the number of dermal papillae was noticed with age, particularly in subjects with fair skin. The method presented here provides a new in vivo investigation tool for quantification of dermis papilla density and epidermal pigmentation. Papillary contrast measured at the dermo‐epidermal junction may be selected as a marker of skin pigmentation for evaluation in clinical studies.

Automatic measurement of dermal thickness from B-scan ultrasound images using active contours

Background/purpose: Measurement of dermal thickness is useful in the evaluation of dermo‐cosmetics for assessing not only morphological changes but also mechanical properties of this layer. Our aim was first to standardise the manual dermal thickness measurement procedure on B‐scan ultrasound images, then to develop an automatic operator independent method to detect the boundaries of the dermis.

Validation of a non-contact technique for local skin temperature measurements.

Here we propose to quantify local temperature variations using thermal imaging to assess the effect of dermatological lasers.

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.