The assessment of natural human skin tension orientation and its variation according to age for two body areas: Forearm and thigh.

Human skin has a complex multilayer structure consisting of non-homogeneous, non-linear, viscoelastic and anisotropic materials subjected to in vivo natural pre-tension. This natural tension stems from networks of collagen and elastin fibers. The 3D organization of the collagen and elastin fibers underpins the multidirectional natural tensions in the skin volume while the state of the networks formed influences the surface topography of the skin. This topography depends on the area of the body and on the age of the person. Experiments reported in the literature have been performed ex vivo or on cadavers. By contrast, this work proposes the characterization of the anisotropic natural tension of the human skin in vivo. Experimental tests were performed on the forearms and thighs of 42 female volunteers representing two age groups [20 - 30] and [45-55] years old. Non-contact impact tests and skin-folding tests were conducted using devices developed at the LTDS (Lyon, France). The impact test generated a Rayleigh wave that spread in the skin. The speed of this wave was measured in 7 directions to study the anisotropy of the skin tension. The image of the skin relief at rest and during the skin folding test was reconstructed by optical confocal microscopy and provided the density of the skin lines printed on the outer surface of the skin. Skin folding test enables the clinician's manual procedure to be instrumented to identify tension lines i.e., Langer lines, for better healing during a surgical procedure. The main directions of natural skin tension deduced from the measured wave speed and the densities of skin lines were [40°-60°] for the forearm and [0°-20°] for the thigh, considering that the longitudinal axis of the body is situated at 90° and the transversal axis at 0°. This method shows the remarkable effect of age and body area on the mechanical behavior of human skin in vivo. The elastic properties and natural tension of the skin decrease with age. This decrease is greater in the directions orthogonal to the skin's tension lines, leading to the accentuation of the anisotropic behavior of the cutaneous tissue. The main direction of skin tension is highly dependent on the area of the body and is directed towards a preferred direction which corresponds to the main direction of skin tension.

Viscoelasticity assessment of tumoral skin with the use of a novel contact-free palpation methodology based upon surface waves.

The ensuing pilot investigation sheds new light on characterizing tumoral and non-tumoral human skin mechanical properties that will not only assist the dermatologist's diagnosis but also could constitute the creation of an Artificial Intelligence database for upcoming research. A modern, non-invasive, and contact-free methodology-UNDERSKIN-was developed, and hinges upon Fourier transform computations that permit the analysis of surface wave dispersion with a specific skin inversion model and viscoelastic model. It yields a detailed look at how particle movements of the medium propagate throughout its near sub-surface, hence a novel knowledge of the mechanical responses of skin tumors. The research results display the tumors' viscoelastic responses alongside their respective healthy skin outcomes for each skin layer as well as the dermatologist's touch analysis. Although dermatologists are capable of sensing and having a fair overall assessment of what they are palpating, they are unable heretofore to quantify it and inform where the firmness or softness derives from, which it is necessary to be acquainted with so as to perform an accurate diagnosis, prognosis, treatment, future surgery, and teledermatology.

Mechanical and topographical anisotropy for human skin: Ageing effect.

Skin ageing is a complex process which strongly impacts the three skin layers (epidermis, dermis, hypodermis) both functionally and structurally. Of particular interest are the effects of ageing on the dermis biomechanics and how this evolution can impact the reorganization of the cutaneous lines which compose the skin relief. It has been argued that the skin relief could reflect the underlying mechanical condition of the skin. Nevertheless, there is not yet conclusive evidence of the existence of such a link. This work aims at experimentally studying, in vivo, the correlation between the anisotropy of human skin biomechanics and skin topography as a function of ageing. The study was conducted on a panel of 20 men divided into 4 groups according to age (from 23 to 64 years old). The measurements were performed on the right volar forearm of each volunteer. For the biomechanical measurements, an innovative contactless bio-rheometer was developed. It allows access to the mechanical behaviour of the skin in several directions. This device generates an air blast without any contact with the skin area and measures its dynamic response (evaluation of speed of wave propagation) with a linear laser. Moreover, a turntable enables measurements to be made in different angular directions. To analyse the topography of skin relief, we proposed a new method, based on watershed and linear radon transformations. First, an optical analysis of a replica of the skin relief is performed. Then, from the skin image obtained, the density of the cutaneous lines is calculated in different directions using watersheld transformation. The orientation of the detected lines is then estimated with an algorithm based on linear radon transformation. The results observed show a good correlation between the skin relief and the mechanical properties of the skin all along the ageing process. For both topography and mechanical properties, there is a transition from an almost isotropic mechanical behaviour to an anisotropic one as a function of ageing process. Thus, we might conclude that the skin relief reflects the underlying mechanical conditions of the skin.

Static and active tactile perception and touch anisotropy: aging and gender effect.

Although the human finger is the interface used for the touch process, very few studies have used its properties to provide a description of tactile perception regarding age and gender effects. Age and gender effects on the biophysical properties of the human finger were the main topics of our previous study. Correlating tactile perception with each parameter proved very complex. We expand on that work to assess the static and dynamic touch in addition to the touch gestures. We also investigate the age and gender effects on tactile perception by studying the finger size and the real contact area (static and dynamic) of forty human fingers of different ages and gender. The size of the finger and the real contact area (static and dynamic) define the density of the mechanoreceptors. This density is an image of the number of mechanoreceptors solicited and therefore of tactile perception (static and dynamic). In addition, the touch gestures used to perceive an object's properties differ among people. Therefore, we seek to comprehend the tactile perception of different touch gestures due to the anisotropy of mechanical properties, and we study two different directions (top to bottom and left to right).

Impact of finger biophysical properties on touch gestures and tactile perception: Aging and gender effects.

The human finger plays an extremely important role in tactile perception, but little is known about the role of its biophysical properties (mechanical properties, contact properties and surface topography) in tactile perception. In addition, the touch gestures used to perceive an object's properties differ among people. We combined studies on the biophysical properties and the vibrations measured from the human finger to understand the age and gender effects on the tactile perception and the difference between the touch gestures. In addition, a new algorithm, Mel-frequency cepstral coefficients (MFCCs), was used to analyze the vibratory signal obtained from the physical contact of the finger, and a surface is proposed and validated. The values obtained regarding the correlation between the tribohaptic system results and the biophysical properties show that the Young's modulus and the surface topography are the most important. An inverse correlation was observed between the MFCC and the tactile perception. This last observation explained the results of better tactile perception with left to right touch gestures. It also demonstrated a better tactile perception for women.

Biophysical properties of the human finger for touch comprehension: influences of ageing and gender.

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.

Study of the tactile perception of bathroom tissues: Comparison between the sensory evaluation by a handfeel panel and a tribo-acoustic artificial finger.

Tactile perception is one of the sensorial modes most stimulated by our daily environment. In particular, perceived softness is an important parameter for judging the sensory quality of surfaces and fabrics. Unfortunately, its assessment greatly depends on the tactile sense of each person, which in turn depends on many factors. Currently, the predominant method for evaluating the tactile perception of fabrics is the human handfeel panel. This qualitative approach does not permit the quantitative measure of touch feel perception. In this study, we present a new artificial finger device to investigate the tactile sensing of ten bathroom tissues. It enables simultaneously measuring the friction and vibrations caused when sliding an artificial finger on the surface of the tissue. The comparison between the results obtained with the artificial finger and the tactile perception evaluated using a handfeel panel showed that the artificial finger is able to separate the two parts of the tactile perception of bathroom tissues: softness and surface texture (velvetiness). The statistical analysis suggests that there is a good correlation between the vibrations measured with the artificial finger and the softness evaluated by the panel. It then shows that the friction measured by the artificial finger is related to the surface texture of a bathroom tissue. The ability of the artificial finger to mimic human touch is demonstrated. Finally, a Principal Component Analysis orders the signatures of the tactile perception of the bathroom tissues in four different groups.

Multiscale Approach to Characterize Mechanical Properties of Tissue Engineered Skin.

Tissue engineered skin usually consist of a multi-layered visco-elastic material composed of a fibrillar matrix and cells. The complete mechanical characterization of these tissues has not yet been accomplished. The purpose of this study was to develop a multiscale approach to perform this characterization in order to link the development process of a cultured skin to the mechanical properties. As a proof-of-concept, tissue engineered skin samples were characterized at different stages of manufacturing (acellular matrix, reconstructed dermis and reconstructed skin) for two different aging models (using cells from an 18- and a 61-year-old man). To assess structural variations, bi-photonic confocal microscopy was used. To characterize mechanical properties at a macroscopic scale, a light-load micro-mechanical device that performs indentation and relaxation tests was designed. Finally, images of the internal network of the samples under stretching were acquired by combining confocal microscopy with a tensile device. Mechanical properties at microscopic scale were assessed. Results revealed that adding cells during manufacturing induced structural changes, which provided higher elastic modulus and viscosity. Moreover, senescence models exhibited lower elastic modulus and viscosity. This multiscale approach was efficient to characterize and compare skin equivalent samples and permitted the first experimental assessment of the Poisson's ratio for such tissues.

Elastic wave induced by friction as a signature of human skin ageing and gender effect.

BACKGROUND: In this work, we propose an innovative approach based on a rotary tribometer coupled with laser velocimetry for measuring the elastic wave propagation on the skin. METHODS: The method is based on a dynamic contact with the control of the normal force (Fn ), the contact length and speed. During the test a quantification of the friction force is produced. The elastic wave generated by friction is measured at the surface of the skin 35 mm from the source of friction exciter. In order to quantify the spectral range and the energy property of the wave generated, we have used laser velocimetry whose spot laser diameter is 120 µm, which samples the elastic wave propagation at a frequency which may reach 100 kHz. In this configuration, the speaker is the friction exciter and the listener the laser velocimetry. In order to perform non-invasive friction tests, the normal stress has been set to 0.3 N and the rotary velocity to 3 revolutions per second, which involves a sliding velocity of 63 mm/s. RESULTS AND CONCLUSIONS: This newly developed innovative tribometer has been used for the analysis of the elastic wave propagation induced by friction on human skin during chronological ageing and gender effect. Measurements in vivo have been made on 60 healthy men and women volunteers, aged from 25 to 70. The results concerning the energy of the elastic wave signature induced by friction show a clear difference between the younger and older groups in the range of a low band of frequencies (0-200 Hz). The gender effect was marked by a 20% decrease in the energy of elastic wave propagation in the female group.

Effect of water desorption on the rheology and dynamic response of human hair to a non-contact impact.

Human hair is a non-homogeneous complex material made of keratin fibers oriented along the longitudinal axis which offer anisotropic mechanical properties. Nowadays, it is possible to measure the mechanical properties of hairs with the classical tests, but most often, these tests are destructive and make hard to measure the influence of some external factors or treatments on the behavior of a same hair fiber. In the current paper, vibrations induced by a non-contact impact have been utilized as a representative response of the mechanical behavior of hair. The characteristics of the vibratory response allow measuring the variation in the mechanical properties and the instantaneous effect of an external factor on the properties of a same sample. First, load relaxation tests have been performed on hair samples after moisturization and for different times of an air-drying process in order to characterize the change in the visco-elastic behavior of hair during the water desorption. Other hair samples have been tested with our non-contact impact and vibration technique in order to observe the change in the vibratory response during the water desorption. The vibratory response has then been correlated to the mechanical properties of the hair fiber.

Rheological behaviour of reconstructed skin.

Reconstructed skins have been developed to replace skin when the integrity of tissue has been compromised following severe injury, and to provide alternative methods validating the innocuousness and effectiveness of dermatological and cosmetic products. However the functional properties of tissue substitutes have not been well characterised, mainly since mechanical measurement devices have not been designed to test cell culture materials in vitro. From the mechanical standpoint, reconstructed skin is a heterogeneous multi-layer viscoelastic material. To characterise the time-dependent behaviour of reconstructed skin, spherical indentation load-relaxation tests were performed with a specific original device adapted to measure small soft tissue samples. Load-relaxation indentation tests were performed on a standard reconstructed skin model and on sub-components of the reconstructed skin (3D-scaffold alone and dermal equivalent). Generalised Maxwell and Kelvin-Voigt rheological models are proposed for analysing the mechanical behaviour of each biological tissue. The results indicated a modification of the rheological behaviour of the samples tested as a function of their biological structure. The 3D-scaffold was modelled using the one-branch Maxwell model, while the dermis equivalent and the reconstructed skin were modeled using a one-branch and a two-branch Kelvin-Voigt model, respectively. Finally, we demonstrated that skin cells contribute to global mechanical behaviour through an increase of the instantaneous relaxation function, while the 3D-scaffold alone influences the mechanical response of long relaxation times.

Assessment of the in-plane biomechanical properties of human skin using a finite element model updating approach combined with an optical full-field measurement on a new tensile device.

Human skin is one of the most important organ of the body. The assessment and knowledge of its properties are very useful for clinical or cosmetic research. Many techniques are used to measure the mechanical properties of this organ, like suction, indentation, torsion or tension tests. The aim of this paper is to present a new device based on tension technique and combining mechanical and optical measurements. The whole procedure used to assess the displacement field as described, and first results of tests performed in vivo are 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 Young's 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.

Non contact method for in vivo assessment of skin mechanical properties for assessing effect of ageing.

The assessment of human tissue properties by objective and quantitative devices is very important to improve the understanding of its mechanical behaviour. The aim of this paper is to present a non contact method to measure the mechanical properties of human skin in vivo. A complete non contact device using an air flow system has been developed. Validation and assessment of the method have been performed on inert visco-elastic material. An in vivo study on the forearm of two groups of healthy women aged of 23.2±1.6 and 60.4±2.4 has been performed. Main parameters assessed are presented and a first interpretation to evaluate the reduced Young's modulus is proposed. Significant differences between the main parameters of the curve are shown with ageing. As tests were performed with different loads, the influence of the stress is also observed. We found a reduced Young's modulus with an air flow force of 10 mN of 14.38±3.61 kPa for the youngest group and 6.20±1.45 kPa for the oldest group. These values agree with other studies using classical or dynamic indentation. Non contact test using the developed device gives convincing results.

Mucoadhesion evaluation of polysaccharide gels for vaginal application by using rheological and indentation measurements.

The influence of hyaluronic acid (HA) and fructooligosaccharides (FOS) addition on low methyl pectin (LMP) gelation has been investigated in order to produce adhesive gel-based microparticles suitable for the development of a vaginal delivery system of pro- and prebiotics. First, dynamic rheological measurements were performed on LMP/Ca(2+) gels with or without FOS and HA in presence or not of porcine stomach mucins. This rheological method is known to translate the interactions between polymer and mucins and then simulate the polymer bioadhesion potential. Nevertheless, as this method is disputed, in vitro and ex vivo indentation test measurements were also achieved in order to correlate the results obtained. Despite some different results, the overall tendency indicates that addition of HA and FOS enhanced the mucoadhesive properties of LMP gels. Moreover, gel-based microparticles obtained according to an emulsification/gelation method and composed by LMP 3% (w/v), FOS 5% (w/v) and HA 0.5% (w/v) displayed a mucoadhesive potential adapted to vaginal delivery system.

Interpretation of the human skin biotribological behaviour after tape stripping.

The present study deals with the modification of the human skin biotribological behaviour after tape stripping. The tape-stripping procedure consists in the sequential application and removal of adhesive tapes on the skin surface in order to remove stratum corneum (SC) layers, which electrically charges the skin surface. The skin electric charges generated by tape stripping highly change the skin friction behaviour by increasing the adhesion component of the skin friction coefficient. It has been proposed to rewrite the friction adhesion component as the sum of two terms: the first classical adhesion term depending on the intrinsic shear strength, τ(0), and the second term depending on the electric shear strength, τ(elec). The experimental results allowed to estimate a numerical value of the electric shear strength τ(elec). Moreover, a plan capacitor model with a dielectric material inside was used to modelize the experimental system. This physical model permitted to evaluate the friction electric force and the electric shear strength values to calculate the skin friction coefficient after the tape stripping. The comparison between the experimental and the theoretical value of the skin friction coefficient after the tape stripping has shown the importance of the electric charges on skin biotribological behaviour. The static electric charges produced by tape stripping on the skin surface are probably able to highly modify the interaction of formulations with the skin surface and their spreading properties. This phenomenon, generally overlooked, should be taken into consideration as it could be involved in alteration of drug absorption.

Evaluation of the efficacy of a dill extract in vitro and in vivo.

Lysyl oxidase-like (LOXL) is an extracellular enzyme that catalyses the cross-linking between microfibrils and tropoelastin (TE), thereby ensuring elastic fibre functionality. With ageing, LOXL expression decreases, thus participating in the loss of skin elasticity. In a previous study, we showed that a dill seed extract [INCI name: Peucedanum graveolens (Dill) extract] could increase LOXL expression in cultured dermal fibroblasts. Besides, we showed a good correlation between the measurements of skin elasticity obtained in vitro and in vivo using a fully automated bio-tribometer designed to measure the biomechanical properties of soft and complex materials like skin. The aim of this study was to evaluate the ability of the dill extract to improve skin elasticity in vitro and in vivo using different models. Using the bio-tribometer, we first showed that the lateral elasticity of dermis equivalents (DEs) treated with the dill extract at 1% was significantly increased by +29% (P < 0.01) when compared to untreated DEs. In vivo, skin firmness and elastic recovery measured using cutometry methods were also significantly improved compared to placebo in volunteers treated for 56 days with a formula containing 1% of dill extract. Moreover, the clinical evaluation evidenced significant improvements in 'skin elasticity' compared to placebo. A majority of subjects treated with the dill extract also noted significant improvements in skin elasticity, firmness and slackness of the jaw line. Finally, mean wrinkle area and length were also significantly reduced compared to placebo after 84 days as measured using silicone replicas taken from the crow's feet. In summary, this study showed that the dill extract could improve elasticity of DEs in vitro as well as skin biomechanical properties and appearance in vivo. It also highlights the relevance of using the bio-tribometer as an exploratory tool for the measurement of skin elasticity in vitro.

Simultaneous characterization of oxygen transport into and through porcine skin exposed to oxygen-saturated water.

Oxygen delivery to the skin is a promising approach for treatment of dermatological diseases (e.g. ischemic wound healing). However, characterization of oxygen transport into and through the skin exposed to oxygen carrier formulations has not been reported. In the present study, we developed an original lab-made static diffusion cell mounted with porcine skin enabling the assessment of oxygen uptake into the skin (i.e., oxygen penetration) and passage through the skin (i.e., oxygen permeation). Oxygen penetration and permeation were recorded by using an optical probe implanted into the skin tissue and a Clark-type electrode plunged into the receptor solution of the diffusion cells. Permeability parameters (i.e., maximal and steady-state flux; permeability coefficient) of oxygen were determined after a 2-hour application of oxygen-saturated water to either the skin surface (exogenous delivery) or the dermis (endogenous delivery). Similar experiments were performed by using intact or stripped skin in order to appreciate the role of the stratum corneum as oxygen barrier. Exogenous delivery of oxygen to skin tissue was found more effective than endogenous delivery through intact and stripped skin. However, exogenous oxygen permeation was found smaller than that determined from endogenous delivery. The upper layers of the skin would constitute a potential oxygen reservoir created by the high solubility of oxygen in epidermal lipids. Therefore, oxygen carrier formulations might significantly improve the oxygen status in the skin for further biological effects.