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

Background The single-axis extension test is relatively little used to study the mechanical properties of human skin in vivo. A campaign of tests was carried out with an original, modern machine developed in our laboratory. It can perform extension or compression tests using servo-controlled position or force in different directions. The load can either be of the extension or monotonous compression type, creep or relaxation. The results obtained were used to develop a viscoelastic model. The elastic modulus calculated helps us to determine the main directions of anisotropy on the forearm. Methods We use a new in vivo single-axis extension machine (patent no. FR03/09220 application in progress). With it, we can carry out monotonous, creep and relaxation tests on the forearm. An associated finite elements model enables conversion to the intrinsic parameters of the skin under stress and strain from external stress applied in force and displacement. From the tests, we can propose a viscoelastic model and the identification of his parameters. We carried out tests in four directions with respect to the axis of the forearm of 63 people of different ages. The present report is limited to a brief presentation of the experimental set-up used, and a more complete presentation of the viscoelastic model and how it is defined and also the work on the anisotropy in the elastic domain. Results The viscoelastic model proposed has only four intrinsic parameters: elasticity parameters E(e) and E(ve) and viscosity parameters epsilon(ve) and A. Skin being considered as orthotropic, we were able to determine the average main direction of 63 people, which is of 5.33+/-5.78 around the longitudinal axis of the arm. An average modulus E(1) (ave)=6.57E(5) (Pa) can be found in the direction close to the axis of the arm and E(2) (ave)=1.30E(5) (Pa) in the perpendicular direction and a G(12)=1.32E(5) (Pa) shear modulus. Conclusions The parameters obtained with the viscoelastic model are independent of the type of load, the same coefficients enable a correct representation in creep and relaxation tests. The main directions vary from one person to another, Young's modulus in these directions could be an indicator for dermatologists and cosmeticians.

Model of the viscoelastic behaviour of skin in vivo and study of anisotropy.

Backgrounds/aims Human skin is a complex tissue consisting of different layers To gain better insight into the mechanical behaviour of different skin layers, the mechanical response was studied with experiments of various length scales Also, the influence of (superficial) hydration on the mechanical response is studied The work is based on the hypothesis that experiments with different length scales represent the mechanical behaviour of different skin layers For suction, this means that a large aperture diameter reflects the behaviour of mainly dermis, whereas a very small diameter reflects the behaviour of only the top layer of the skin Methods Suction measurements at varying pressures and aperture sizes were performed on the volar forearm of 13 subjects aged 29-47 years The deformation of the skin was visualized using ultrasound (US) (dermis) and optical coherence tomography (OCT) (epidermis and dermis) US measurements were performed on hydrated skin, OCT measurements on dry and hydrated skin The experiment was simulated by a finite element model (FEM) exhibiting extended Mooney material behaviour An identification method was used to compare the experimental and numerical results to identify the parameters of the material Results The material parameters C(10) and C(11) were calculated for four subjects: C(10)=296+/-211 kPa and C(11)=493+/-613 kPa for 6 mm aperture diameter, C(10)=115+/-87 kPa and C(11)=183+/-126 kPa for 2 mm aperture diameter and C(10)=108+/-95 kPa and C(11)=93+/-77 kPa for 1 mm aperture diameter Skin hydration caused ambiguous effects on the mechanical response Conclusions US and OCT, combined with suction, using varying apertures sizes, proved to be a valuable tool to study the mechanical behaviour of different skin layers With increasing experimental length scale, increasing values for the parameters of the material model were found This indicates the need of a multi-layered material layer FEM, which can be used to identify mechanical behaviour of epidermis and dermis

Influence of hydration and experimental length scale on the mechanical response of human skin in vivo, using optical coherence tomography.

Critics of minimally invasive methods sometimes argue that the summed lengths of all trocar sites have a morbidity similar to that for an open incision of equal length. This argument assumes correctly that pain and scarring are proportional to the total tension normal to a linear incision. But the argument also assumes that total tension sums linearly with incision length. This report demonstrates why that premise is not valid. Wounds of various sizes are compared using a simple mathematical model. The closing tension perpendicular to any linear incision is a function of the incision’s length, varying symmetrically together with a maximum at the midpoint of length. If tension rises linearly across an incision, integration of the tension relationship demonstrates that the total wound tension actually is proportional to the square of the length. In this report, incisions of various lengths are modeled, and plausible alternative incision scenarios for various procedures (e.g., Nissen, appendectomy) are compared. Total tension rises nonlinearly with increasing wound length. Thus, total tension across multiple incisions is always less than the total tension for an incision of the same total length. For example, an open appendectomy creates 2.7-fold more wound tension than a laparoscopic appendectomy. Similarly, two 3-mm trocars create less total tension than a single 5-mm trocar. Conventional incisions are subject to more total tension than any combination of trocar incisions of equal total length. This inequality yields three clinically relevant corollaries. First, it supports the practice of using the smallest effective trocars (or even no-trocar methods) to minimize pain and scar. Second, addition of a trocar in difficult cases adds relatively little morbidity. Finally, using two small trocars is better than using a single larger trocar.

Incisions do not simply sum.

Determining the mechanical properties of an individual’s skin is important in the fields of pathology, biomedical device design, and plastic surgery. To address this need, we present a finite element model that simulates the skin of the anterior forearm and posterior upper arm under a rich set of three-dimensional deformations. We investigated the suitability of the Ogden and Tong and Fung strain energy functions along with a quasi-linear viscoelastic law. Using non-linear optimization techniques, we found material parameters and in vivo pre-stresses for different volunteers. The model simulated the experiments with errors-of-fit ranging from 13.7 to 21.5%. Pre-stresses ranging from 28 to 92 kPa were estimated. We show that using only in-plane experimental data in the parameter optimization results in a poor prediction of the out-of-plane response. The identifiability of the model parameters, which are evaluated using different determinability criteria, improves by increasing the number of deformation orientations in the experiments.

Modeling the Mechanical Response of In Vivo Human Skin Under a Rich Set of Deformations

Background/aims: A problem commonly encountered in plastic and reconstructive surgery is the prediction of the stress put on the skin when various types of skin flaps are used.

Methods/results: We developed a mathematical model based on the finite element method, in order to determine the stress field, by simulating the mechanical behaviour of human skin during wound closure. We chose to take into account the low compressive strength by modifying the mechanical parameters of the model at each step of the calculation. The model has been developed and tested on a diamond-shaped incision and applied on a Limberg incision.

Conclusion: The results presented are the Von Mises stress in the area of skin around the scar and, when possible, the resultant closure force. They show the relevance of the method.

A. Burtheret

Papers

Nonlinear model of skin mechanical behaviour analysis with finite element method.

Propriétés mécaniques de la peau humaine. Mise au point d'une méthodologie pour l'expérimentation ex vivo