Experimental Characterisation: Rich Deformations

Human skin is a complex material that exhibits a non-linear stress-strain response, anisotropy, and viscoelasticity. In addition, skin in vivo is under an anisotropic pre-stress, which varies according to location and person. While several methods have been developed to measure the in vivo mechanical response of skin, many of these are incapable of characterising the anisotropy. Few also attempt to measure the in vivo stress. To quantify the anisotropy, it is necessary to apply deformations to the skin in a number of directions. This chapter provides an overview of a method where a rich set of deformations are applied to the surface of the skin and the nonlinear, anisotropic, and viscoelastic response is characterised using finite element analyses and nonlinear optimisation. The in vivo stress is also estimated. Different constitutive models were tested as to their suitability to represent skin. Material parameters and pre-stresses were identified for points on the anterior forearm, upper arm, and the face.