Contactless mechanical stimulation of the skin using shear waves.

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.

Development of a facile method to compute collagen network pathological anisotropy using AFM imaging.

Type I collagen, a fundamental extracellular matrix (ECM) component, is pivotal in maintaining tissue integrity and strength. It is also the most prevalent fibrous biopolymer within the ECM, ubiquitous in mammalian organisms. This structural protein provides essential mechanical stability and resilience to various tissues, including tendons, ligaments, skin, bone, and dentin. Collagen has been structurally investigated for several decades, and variation to its ultrastructure by histology has been associated with several pathological conditions. The current study addresses a critical challenge in the field of collagen research by providing a novel method for studying collagen fibril morphology at the nanoscale. It offers a computational approach to quantifying collagen properties, enabling a deeper understanding of how collagen type I can be affected by pathological conditions. The application of Fast Fourier Transform (FFT) coupled with Atomic Force Microscope (AFM) imaging distinguishes not only healthy and diseased skin but also holds potential for automated diagnosis of connective tissue disorders (CTDs), contributing to both clinical diagnostics and fundamental research in this area. Here we studied the changes in the structural parameters of collagen fibrils in Ehlers Danlos Syndrome (EDS). We have used skin extracted from genetically mutant mice that exhibit EDS phenotype as our model system (Col1a1Jrt/+ mice). The collagen fibrils were analyzed by AFM based descriptive-structural parameters, coupled with a 2D Fast Fourier Transform(2D-FFT) approach that automated the analysis of AFM images. In addition, each sample was characterized based on its FFT and power spectral density. Our qualitative data showed morphological differences in collagen fibril clarity (clearness of the collagen fibril edge with their neighbouring fibri), D-banding, orientation, and linearity. We have also demonstrated that FFT could be a new tool for distinguishing healthy from tissues with CTDs by measuring the disorganization of fibrils in the matrix. We have also employed FFT to reveal the orientations of the collagen fibrils, providing clinically relevant phenotypic information on their organization and anisotropy. The result of this study can be used to develop a new automated tool for better diagnosis of CTDs.

Characterization of the anisotropy of the natural human cheek skin tension in vivo.

Natural skin tension plays an important role during surgical procedures and during the healing process especially for the face. The study of skin tension can be a means of assessing the aging effect, or the application of a medical or cosmetic product. In this work we propose a characterization of the natural human cheek skin tension in vivo and its variability with age using three characterization methods. These methods consist of facial photography to assess the ptosis of the lower face and the nasolabial fold, suction test to estimate mechanical parameters using the cutometer, and topographic analysis of the skin at rest and during folding test to study the skin relief. The study was carried out on 41 volunteers representing two age groups: 18 young volunteers [20-30] years-old and 23 elderly volunteers [50-65] years-old. The results show that the ptosis of the lower face and the nasolabial fold increase with age. The sagging of the skin observed on the facial photos is related to the loss of elasticity and the increase in the skin viscoelasticity with age. The analysis of the cheek skin relief shows that it has a very fine and flexible lines network. This analysis of the skin relief at rest and during the folding test allowed to determine the main directions of skin tension for the different age groups: [20°-40°] for the young group and [20°-60°] for the elderly group. The natural skin tension decreases with age, wrinkles appear and the skin becomes more anisotropic.

A new device for the combined measurement of friction and through-thickness deformation on ex vivo skin samples.

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.

Perspectives on Astringency Sensation: An Alternative Hypothesis on the Molecular Origin of Astringency.

Flavor is one of the main drivers of food consumption and acceptability. It is associated with pleasure feels during eating. Flavor is a multimodal perception corresponding to the functional integration of information from the chemical senses: olfaction, gustation, and nasal and oral somatosensory inputs. As a result, astringency, as a sensation mediated by the trigeminal nerves, influences food flavor. Despite the importance of astringency in food consumer acceptance, the exact chemosensory mechanism of its detection and the nature of the receptors activated remain unknown. Herein, after reviewing the current hypotheses on the molecular origin of astringency, we proposed a ground-breaking hypothesis on the molecular mechanisms underpinning this sensation as a perspective for future research.

Early-stage knee OA induced by MIA and MMT compared in the murine model via histological and topographical approaches.

Osteoarthritis (OA) is a common degenerative disease whose early management includes promising mechanical treatments. New treatments are initially validated using an animal model in which OA is induced. The MMT (mechanical induction) and MIA (chemical induction) models of OA induction are widespread, but their use to generate early OA is poorly documented. We analyzed and compared early-stage knee OA-induction via these two methods in 16 rats divided into two groups. After 4 weeks of induction, the knees were sampled and studied using both histology (Toluidine Blue and Sirius Red) and surface topology, an innovative technique for characterizing osteoarthritic cartilage. The Mankin-modified score confirms that the two OA-induction models evolved at the same speed. At this early stage, the two models can be differentiated morphologically, although no significant differences were revealed by either cellularity or birefringence analysis. However, the topological analysis generated two forms of quantitative data, the deformation ratio and the cohesion index, that differentiated between the two groups. Thus, the early-stage OA induced by these two models is revealed to differ. The patterns of cartilage damage induced point to MMT as the better choice to assess mechanical approaches to clinical OA treatment.

Biomechanical Characterization of Intracranial Aneurysm Wall: A Multiscale Study.

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.

Tactile Perception and Friction-Induced Vibrations: Discrimination of Similarly Patterned Wood-Like Surfaces.

The tactile perception of a surface texture is mediated by factors such as material, topography, and vibrations induced by the sliding contact. In this paper, sensory characterizations are developed together with topographical and tribo-tactile characterizations to relate perceived features with objective measurements of tribological and dynamic signals. Two sets of surface samples are used in this study: the first set is made of a commercial floor covering tiles that aim at counter-typing natural wood flooring, with both a visual and a tactile texture mimicking wood. A second set is custom-made by replicating the first set using a plain purple polyurethane resin. The comparison between tribo-tactile signals and sensory analysis allowed the identification of objective indices for textures with slight topographical differences. Even though the topography of the replicated samples is the same as their corresponding commercial products, the fact that the material is different, induces differences in the contact and vibrational parameters. This in turn modifies the discrimination performances during the sensory experiment. Tactile characteristics collected during sensory procedures are found to be in agreement with objective indices such as friction coefficients and induced vibrations.

Femtosecond laser nano/micro patterning of titanium influences mesenchymal stem cell adhesion and commitment.

Surface improvement of implants is essential for achieving a fast osseo-integration. Technically, the creation of a precise pattern on a titanium alloy surface is challenging. Here, the femtosecond laser was chosen as an innovative technology for texturing with accuracy a nano-micro topography. By adjusting the laser parameters, three biomimetic textures were fabricated on the titanium surface: micropits with nano-ripples in the pits, micropits with nano-ripples around the pits, and a texture with only nano-ripples. Mesenchymal stem cells (MSCs, C3H10T1/2) grown on these surfaces displayed altered morphometric parameters, and modified their focal adhesions in term of number, size, and distribution depending on surface type. These results indicate that the MSCs perceived subtle differences in topography. Dynamic analyses of early cellular events showed a higher speed of spreading on all the textured surfaces as opposed to the polished titanium. Concerning commitment, all the laser-treated surfaces strongly inhibited the expression of adipogenic-related genes (PPARϒ2, C/EBPα) and up-regulated the expression of osteoblastic-related genes (RUNX2, osteocalcin). Interestingly, the combination of micropits to nano-ripples enhanced their osteogenic potential as seen by a twofold increase in osteocalcin mRNA. Alkaline phosphatase activity was increased on all the textured surfaces, and lipid production was down-regulated. The functionalization of metallic surfaces by this high-resolution process will help us understand the MSCs' interactions with substrates for the development of textured implants with predictable tissue integrative properties.

Contribution to the determination of in vivo mechanical characteristics of human skin by indentation test.

This paper proposes a triphasic model of intact skin in vivo based on a general phenomenological thermohydromechanical and physicochemical (THMPC) approach of heterogeneous media. The skin is seen here as a deforming stratified medium composed of four layers and made out of different fluid-saturated materials which contain also an ionic component. All the layers are treated as linear, isotropic materials described by their own behaviour law. The numerical simulations of in vivo indentation test performed on human skin are given. The numerical results correlate reasonably well with the typical observations of indented human skin. The discussion shows the versatility of this approach to obtain a better understanding on the mechanical behaviour of human skin layers separately.

Temporal evolution of skeletal regenerated tissue: what can mechanical investigation add to biological?

The objective here was to experimentally characterize the temporal evolution of the structural and mechanical properties of large volume immature regenerated tissues. We studied these evolving tissues from their genesis in controlled mechanical conditions. We developed an animal model based on the periosteal properties leading to unloaded regenerated skeletal tissue. To characterize the temporal evolution of mechanical properties, we carried out indentation tests coupled with macroscopic examinations and histological studies. This combined methodology yielded a range of information on osteogenesis at different scales: macroscopic by simple observation, mesoscopic by indentation test and microscopic by histological study. Results allowed us to identify different periods, providing a link between biological changes and material property evolution in bone tissue regeneration. The regenerated tissue evolves from a viscous, homogeneous, soft material to a heterogeneous stiffer material endowed with a lower viscosity. From a biological point of view, cell organization progresses from a proliferated cell clot to a mature structure closer to that of the bone. During the first 7 days, mechanical and biological results revealed the same evolution: first, the regenerated tissue grew, then, differentiated into an osteochondral tissue and finally calcification began. While our biological results confirm those of other studies, our mechanical results provide the first experimental mechanical characterization by reduced Young's modulus of such tissue.

Effects of physical and chemical treatments upon biophysical properties and micro-relief of human skin.

The aim of the present study was to determine the attendant effects of physical (tape-stripping) and chemical (three commercial hydrating formulations) treatments upon biophysical and micro-relief properties of human skin. In the first set of experiment, the effects of tape-stripping onto human stratum corneum (SC) biophysical and micro-relief properties were assessed in nine volunteers. Transepidermal water loss (TEWL), skin hydration and micro-relief parameters (including total length of the lines in mm per mm(2); total surface in %; roughness of the skin measured in gray level (Ra); maximum profile valley (Rv) depth; maximum profile peak height (Rp); maximum height (Rt), peak density (Pc) and coefficient of anisotropy) were determined by using SkinEvidence Pro after subsequent tape-stripping of SC. The relevance of roughness determination as gray level by SkinEvidence Pro was confirmed by using surface roughness standards. In the second set of experiment, the effectiveness of three commercial hydrating formulations onto human SC biophysical parameters and micro-relief properties were assessed in six volunteers. TEWL, hydration and micro-relief parameters were assessed onto pre-treated acetone skin and then treated by three commercial hydrating formulations after 2, 4 and 6 h skin exposure. A linear relation between hydration and cutaneous parameters (total length of the lines, Ra and Rp) as function of SC removed was shown. Skin barrier properties evaluated by TEWL measurements, were not modified by topical formulations. However, skin treated by topical formulations showed slightly higher hydration than the one determined in control group, while micro-relief parameters were not modified. In this study was showed that biophysical and micro-relief parameters were closely related in tape-stripping experiment. Efficiency of topical formulations was suggested upon skin hydration but not onto skin micro-relief and barrier function recovering. From both experiments, it appears that different mechanisms relating to skin hydration and potential modification of cutaneous micro-relief were suggested.

Characterization of the mechanical properties of skin by inverse analysis combined with the indentation test.

This study proposes a new method to determine the mechanical properties of human skin by the use of the indentation test [Pailler-Mattei, 2004. Caractérisation mécanique et tribologique de la peau humaine in vivo, Ph.D. Thesis, ECL-no. 2004-31; Pailler-Mattei, Zahouani, 2004. Journal of Adhesion Science and Technology 18, 1739-1758]. The principle of the measurements consists in applying an in vivo compressive stress [Zhang et al., 1994. Proceedings of the Institution of Mechanical Engineers 208, 217-222; Bosboom et al., 2001. Journal of Biomechanics 34, 1365-1368; Oomens et al., 1984. Selected Proceedings of Meetings of European Society of Biomechanics, pp. 227-232; Oomens et al., 1987. Journal of Biomechanics 20(9), 877-885] on the skin tissue of an individual's forearm. These measurements show an increase in the normal contact force as a function of the indentation depth. The interpretation of such results usually requires a long and tedious phenomenological study. We propose a new method to determine the mechanical parameters which control the response of skin tissue. This method is threefold: experimental, numerical, and comparative. It consists combining experimental results with a numerical finite elements model in order to find out the required parameters. This process uses a scheme of extended Kalman filters (EKF) [Gu et al., 2003. Materials Science and Engineering A345, 223-233; Nakamura et al., 2000. Acta Mater 48, 4293-4306; Leustean and Rosu, 2003. Certifying Kalman filters. RIACS Technical Report 03.02, 27pp. http://gureni.cs.uiuc.edu/~grosu/download/luta + leo.pdf; Welch and Bishop, An introduction to Kalman filter, University of North Carolina at Chapel Hill, 16p. http://www.cs.unc.edu/~welch/kalman/]. The first results presented in this study correspond to a simplified numerical modeling of the global system. The skin is assumed to be a semi-infinite layer with an isotropic linear elastic mechanical behavior [Zhang et al., 1994. Proceedings of the Institution of Mechanical Engineers 208, 217-222] This analysis will be extended to more realistic models in further works.

Topical ascorbic acid on photoaged skin. Clinical, topographical and ultrastructural evaluation: double-blind study vs. placebo.

Vitamin C is known for its antioxidant potential and activity in the collagen biosynthetic pathway. Photoprotective properties of topically applied vitamin C have also been demonstrated, placing this molecule as a potential candidate for use in the prevention and treatment of skin ageing. A topically applied cream containing 5% vitamin C and its excipient were tested on healthy female volunteers presenting with photoaged skin on their low-neck and arms in view to evaluate efficacy and safety of such treatment. A double-blind, randomized trial was performed over a 6-month period, comparing the action of the vitamin C cream vs. excipient on photoaged skin. Clinical assessments included evaluation at the beginning and after 3 and 6 months of daily treatment. They were performed by the investigator and compared with the volunteer self assessment. Skin relief parameters were determined on silicone rubber replicas performed at the same time-points. Cutaneous biopsies were obtained at the end of the trial and investigated using immunohistochemistry and electron microscopy. Clinical examination by a dermatologist as well as self-assessment by the volunteers disclosed a significant improvement, in terms of the 'global score', on the vitamin C-treated side compared with the control. A highly significant increase in the density of skin microrelief and a decrease of the deep furrows were demonstrated. Ultrastructural evidence of the elastic tissue repair was also obtained and well corroborated the favorable results of the clinical and skin surface examinations. Topical application of 5% vitamin C cream was an effective and well-tolerated treatment. It led to a clinically apparent improvement of the photodamaged skin and induced modifications of skin relief and ultrastructure, suggesting a positive influence of topical vitamin C on parameters characteristic for sun-induced skin ageing.