Anisotropic mechanical characterization of human skin by in vivo multi-axial ring suction test.

Human skin is a soft tissue behaving as an anisotropic material. The anisotropy emerges from the alignment of collagen fibers in the dermis, which causes the skin to exhibit greater stiffness in a certain direction, known as Langer's line. The importance of determining this anisotropy axis lies in assisting surgeons in making incisions that do not produce undesirable scars. In this paper, we introduce an open-source numerical framework, MARSAC (Multi-Axial Ring Suction for Anisotropy Characterization: https://github.com/aflahelouneg/MARSAC), adapted to a commercial device CutiScan CS 100® that applies a suction load on an annular section, causing a multi-axial stretch in the central zone, where in-plane displacements are captured by a camera. The presented framework receives inputs from a video file and converts them into displacement fields through Digital Image Correlation (DIC) technique. From the latter and based on an analytical model, the method assesses the anisotropic material parameters of human skin: Langer's line ϕ, and the elastic moduli E1 and E2 along the principal axes, providing that the Poisson's ratio is fixed. The pipeline was applied to a public data repository, https://search-data.ubfc.fr/femto/FR-18008901306731-2021-08-25_In-vivo-skin-anisotropy-dataset-for-a-young-man.html, containing 30 test series performed on a forearm of a Caucasian subject. As a result, the identified parameter averages, ϕˆ=40.9±8.2∘ and the anisotropy ratio E1ˆ/E2ˆ=3.14±1.60, were in accordance with the literature. The intra-subject analysis showed a reliable assessment of ϕ and E2. As skin anisotropy varies from site to site and from subject to subject, the novelty of the method consists in (i) an optimal utilization of CutiScan CS 100® probe to measure the Langer's line accurately and rapidly on small areas with a minimum diameter of 14mm, (ii) validation of an analytical model based on deformation ellipticity.

An open source pipeline for design of experiments for hyperelastic models of the skin with applications to keloids.

The aim of this work is to characterize the mechanical parameters governing the in-plane behavior of human skin and, in particular, of a keloid-scar. We consider 2D hyperelastic bi-material model of a keloid and the surrounding healthy skin. The problem of finding the optimal model parameters that minimize the misfit between the model observations and the in vivo experimental measurements is solved using our in-house developed inverse solver that is based on the FEniCS finite element computational platform. The paper focuses on the model parameter sensitivity quantification with respect to the experimental measurements, such as the displacement field and reaction force measurements. The developed tools quantify the significance of different measurements on different model parameters and, in turn, give insight into a given model's ability to capture experimental measurements. Finally, an a priori estimate for the model parameter sensitivity is proposed that is independent of the actual measurements and that is defined in the whole computational domain. This estimate is primarily useful for the design of experiments, specifically, in localizing the optimal displacement field measurement sites for the maximum impact on model parameter inference.

Intra- and inter-individual variability in the mechanical properties of the human skin from in vivo measurements on 20 volunteers.

BACKGROUND/PURPOSE: The mechanical properties and behavior of the human skin in vivo are of medical importance, particularly to surgeons who have to consider the skin extension capabilities in the preparation of surgical acts. Variable data can be found in literature that result from diverse kinds of tests (in vivo, ex vivo, and postmortem) performed with different instruments. METHODS: This paper presents the results of in vivo measurements performed on a cohort of 20 healthy volunteers with an ultralight homemade uniaxial extensometer. Different anatomical zones were explored under different directions of solicitation in order to document inter- and intra-individual variability as well as skin anisotropy. RESULTS: The experimental data obtained are fitted with a phenomenological exponential model allowing the identification of three parameters characteristic of the tested skin behavior. These parameters can be related to the concept of skin extensibility used by surgeons. CONCLUSION: The inter- and intra-variability observed on that cohort confirms the need for a patient-specific approach based on the in vivo measurement of the mechanical behavior of the human skin of interest. Even the direction of higher skin stiffness is found to be individual-dependent. The capability of the extensometer used in this study to fulfill such measurement needs is also demonstrated.

Ultra-light extensometer for the assessment of the mechanical properties of the human skin in vivo.

BACKGROUND/PURPOSE: This paper aims to present an ultra-light extensometer device dedicated to the mechanical characterization of the human skin in vivo. METHODS: The device developed was conceived to be non-invasive, to work without any stand and to perform various uniaxial tensile tests with either effort or displacement control. We also use specific guarding tabs to make in vivo extension tests analogous to traction tests. RESULTS: Force-displacement curves are derived from the data provided by the device's sensors. The latter are converted into stress-strain curves thanks to complementary measurements of the skin thickness. We present typical experimental data and results that demonstrate the device ability to built stress-strain curves characteristic of the human skin behavior. An additional imaging unit records a sequence of images of the solicited skin area for further calculations of the displacement fields by digital image correlation. CONCLUSION: The analysis of the displacement and deformation fields validates the guarding tab efficiency and the capacity of the device to characterize the mechanical behavior of the human skin in vivo.

Numerical analysis of the V-Y shaped advancement flap.

The V-Y advancement flap is a usual technique for the closure of skin defects. A triangular flap is incised adjacent to a skin defect of rectangular shape. As the flap is advanced to close the initial defect, two smaller defects in the shape of a parallelogram are formed with respect to a reflection symmetry. The height of the defects depends on the apex angle of the flap and the closure efforts are related to the defects height. Andrades et al. 2005 have performed a geometrical analysis of the V-Y flap technique in order to reach a compromise between the flap size and the defects width. However, the geometrical approach does not consider the mechanical properties of the skin. The present analysis based on the finite element method is proposed as a complement to the geometrical one. This analysis aims to highlight the major role of the skin elasticity for a full analysis of the V-Y advancement flap. Furthermore, the study of this technique shows that closing at the flap apex seems mechanically the most interesting step. Thus different strategies of defect closure at the flap apex stemming from surgeon's know-how have been tested by numerical simulations.

Geometrical analysis of the V-Y advancement flap applied to a keystone flap.

BACKGROUND: The V-Y advancement flap and, more recently, the keystone flap are commonly used to cover skin defects. Both flaps allow for primary closure after advancement by substituting the initial defect for a narrower defect distributed over a greater length. The first objective of this study was to develop a geometrical analysis of the V-Y advancement flap. The second objective was to explain the benefit of using the keystone flap compared to a single V-Y advancement flap. MATERIAL AND METHOD: A geometrical analysis is proposed using a two-dimensional analysis in which the flaps are assumed to have a rigid-body behaviour. First, in the case of the V-Y advancement flap, a trigonometric relationship is defined between the distance of closure before and after advancement, thus implying the value of the flap's apex angle. Second, by considering the keystone flap as the association of three V-Y advancement flaps, the trigonometric relationship is applied to the keystone flap. RESULTS: In the case of the V-Y advancement flap, the optimal apex angles are between 20° and 60°. At less than 20°, the length of the flap increases in an exaggerated manner. At greater than 60°, the distance of closure, particularly at the apex of the flap where a corner stitch is performed, is greater than the distance of closure of the initial defect. In the case of the keystone flap, the width of the final defect around the flap is clearly smaller and more regular compared to the final defect around a single V-Y advancement flap. CONCLUSION: The geometrical analysis of the V-Y advancement flap in our description illustrates the major benefit of the keystone flap over a single V-Y advancement flap.