Coupling tensile test with LC-OCT and ultrasound imaging: investigation of the skin sublayers mechanical behaviour.

The skin is an envelope that covers the entire body. Nowadays, understanding and studying the mechanical, biological and sensory properties of the skin is essential, especially in dermatology and cosmetology. The in-depth study of the skin's mechanical behaviour is a highly intriguing challenge, enabling the differentiation of the behaviour of each layer. An extension device was developed to perform relaxation and extension tests to characterize the skin. The device has also been coupled with imaging tools (LC-OCT and ultrasound), allowing us to observe layer-by-layer deformations during the tests. Relaxation tests revealed significant skin anisotropy, as well as an influence of age and gender on skin viscoelastic parameters calculated from relaxation curves and a skin viscoelastic model. These tests also unveiled their ability to distinguish certain characteristic pathologies that alter the mechanical properties of the skin, such as scleroderma or heliodermatitis. Furthermore, the optical-mechanical coupling and deformation calculation through image analysis demonstrated that the skin layers exhibit distinct mechanical behaviours owing to their different structures. Finally, Poisson's ratio of the skin was obtained by calculating the deformation in two directions for each layer.

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.