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

Background/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=29.6+/-21.1 kPa and C-11=493+/-613 kPa for 6 mm aperture diameter, C-10=11.5+/-8.7 kPa and C-11=18.3+/-12.6 kPa for 2 mm aperture diameter and C-10=10.8+/-9.5 kPa and C-11=9.3+/-7.7 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.