Mechanical Behaviour of Human Skin in Vivo

The mechanical properties of human skin are of great importance to predict skin doming in a shaver. Brokken [1] showed that among others the role of the different skin layers is important. The goal of this joint PhD-project by Eindhoven University of Technology and PCI is to determine the mechanical properties of different skin layers. Current knowledge suggests that this goal can be achieved by combining (existing) skin imaging methods with mechanical experiments. This literature review describes the available knowledge on the structure of the skin, some imaging methods that can be used to visualize the different skin layers, and a variety of mechanical experiments that have been performed on the skin. A detailed description is given of ultrasound (US), confocal microscopy (CM), optical coherence tomography (OCT) and nuclear magnetic resonance (NMR), all applied to the skin. Some features of the overall mechanical behaviour of the skin and some skin components are described. An extended review is given on mechanical experiments applied to in vivo human skin (tensile tests, torsion tests, suction and indentation tests) and on mechanical experiments applied to the stratum corneum (both in vivo and in vitro). Conclusions: The structure of the skin is very complex. In this project 4 layers will be considered: the stratum corneum, the epidermis, the dermis (fibres and matrix) and the subcutaneous fat. A large variety of mechanical experiments have been performed on the skin in the past. However, in most cases little effort has been made to describe the observed material behaviour with appropriate constitutive models. In all but one publication (Diridollou et al. [2]) only superficial skin behaviour was measured. Usually, no difference is made in the contribution of the different layers, though it has been known for almost 3 decades that the c ©Koninklijke Philips Electronics N.V. 2001 iii 2001/820 Unclassified Report mechanical behaviour of the stratum corneum is strongly influenced by environmental conditions. The goal of the current project is to develop a numerical model that is capable to describe the non-linear and time-dependent behaviour of the skin, taking its stratified structure into account. To achieve this goal, a combination of mechanical experiments of different kinds (e.g. parallel and perpendicular to the skin surface) and on different length scales will be developed. Based on the results of numerical simulations of indentation and suction experiments on a 4 layer skin model, it is chosen to measure subsurface deformation during suction. Experiments will be performed for various aperture sizes to excite the different skin layers. Subsurface deformation will be measured with US, OCT and CM. iv c ©Koninklijke Philips Electronics N.V. 2001 Unclassified Report 2001/820