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The skin, the outermost organ of the human body, is vital for sensing and responding to stimuli through mechanotransduction. It is constantly exposed to mechanical stress. Consequently, various mechanical therapies, including compression, massage, and microneedling, have become routine practices for skin healing and regeneration. However, these traditional methods require direct skin contact, restricting their applicability. To address this constraint, we developed shear wave stimulation (SWS), a contactless mechanical stimulation technique. The effectiveness of SWS was compared with that of a commercial compression bioreactor used on reconstructed skin at various stages of maturity. Despite the distinct stimulus conditions applied by the two methods, SWS yielded remarkable outcomes, similar to the effects of the compression bioreactor. It significantly increased the shear modulus of tissue-engineered skin, heightened the density of collagen and elastin fibers, and resulted in an augmentation of fibroblasts in terms of their number and length. Notably, SWS exhibited diverse effects in the low- and high-frequency modes, highlighting the importance of fine-tuning the stimulus intensity. These results unequivocally demonstrated the capability of SWS to enhance the mechanical functions of the skin in vitro, making it a promising option for addressing wound healing and stretch mark recovery., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 The Authors. Published by Elsevier Ltd.. All rights reserved.)

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., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2023 Elsevier Ltd. All rights reserved.)

Skin irritation is a common phenomenon that becomes a real concern when caused by the use of medical devices. Because the materials used for the design of these devices are usually carefully selected for chemical compatibility with the skin, it is reasonable to assume that the irritations result from the mechanical interaction between the devices and the skin. The aim of this work was to develop a new device to study both the shear strains in the layers of the skin, using Digital Image Correlation (DIC), and the friction behaviour of ex vivo skin interacting with objects. Pig skin samples with various surface preparations were tested in friction experiments involving different contacting materials encountered in the conception of medical devices. The measure of the static and dynamic coefficients of friction as well as the length of adhesion has highlighted the great influence of skin surface conditioning on friction properties. Strain maps obtained through DIC provided insights into the impact of friction and adhesion effects on shear strain distribution in the skin as a function of depth beneath its surface., (Copyright © 2022 Elsevier Ltd. All rights reserved.)

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., (Copyright © 2019 Elsevier Ltd. All rights reserved.)

Objective: Aneurysm wall biomechanics are not yet an integral part of aneurysm rupture risk evaluation. We aimed to develop a new technique describing the biomechanical properties of aneurysm wall and correlating them to rupture status., Methods: Aneurysm wall samples collected during surgery were submitted before and after freezing to tensile tests or as fresh samples to indentation tests. The lateral stiffness or the Young's modulus of the different samples was determined as a function of the mechanical test used. The impact of freezing on biomechanical properties was evaluated. The correlation of clinical and radiologic data with the biomechanical profile of the aneurysm samples was investigated. Two-photon microscopy was used to study collagen fiber organization., Results: Sixteen aneurysm samples (11 unruptured and 5 ruptured) were included. Freezing decreased tissue stiffness. No significant difference was found between ruptured and unruptured aneurysm wall samples regarding demographic characteristics, ethnicity, smoking status, arterial hypertension, site, size and shape of the aneurysm, PHASES score, mechanical profile, or overall Young's modulus. Indentation tests found that the rupture occurred in a restricted area of increased elastic capacity and unruptured areas had increased stiffness. Two-photon microscopy found disruption of the collagen fiber network in rupture zones., Conclusions: The indentation test of fresh aneurysm wall samples described the heterogeneity of biomechanical properties of the tissue and found increased elastic capacity in the rupture zone and increased stiffness in the remainder of the aneurysm. This study could be a basis for further research aimed at building a biomechanical-based model of aneurysm rupture risk., (Copyright © 2018 Elsevier Inc. All rights reserved.)

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., (Copyright © 2016. Published by Elsevier B.V.)

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., (Copyright © 2015 Elsevier Ltd. All rights reserved.)

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., (Copyright © 2014 Elsevier Ltd. All rights reserved.)

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., (Copyright © 2013 Elsevier Ltd. All rights reserved.)

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., (© 2013 Elsevier Ltd. All rights reserved.)

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., (Copyright © 2011 Elsevier B.V. All rights reserved.)

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.