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

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 Ee and Eve and viscosity parameters ɛve 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 E1 (ave)=6.57E5 (Pa) can be found in the direction close to the axis of the arm and E2 (ave)=1.30E5 (Pa) in the perpendicular direction and a G12=1.32E5 (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.