Fig 1.
Human skin anatomy and topography.
a) Schematic of human skin layers, b) 30–40 years-old topography, c) 50–60 years-old topography,d) 70–80 years-old topography. Three-dimensional surface geometries are reconstructed based on the work by Zahonuani et al [42]. e) Anatomical site information of the reconstructed three-dimensional surfaces.
Fig 2.
Finite element model creation.
a) Indentation step. The domain is more than three times larger than the indenter radius to minimize boundary effects. The spherical indenter interacts with a portion of the domain that has a detailed microrelief geometry, shown in the zoomed region (i). The mesh consists of Hex8 elements, with 12 elements through the thickness to capture the four layers of skin. Additional detail is shown in the subpanel (ii). b) Indenter directions of movement in displacement step are decided based on the anatomical axis (lateral U1, proximal-distal U2) as well as perpendicular to the primary skin lines (U3, U4).
Table 1.
Material parameters for the different human skin layers [33, 46, 76].
Fig 3.
Normal strain contours due to indentation.
Component E33 of the Green Lagrange strain tensor for different SC condition (wet or dry) for the three different microrelief geometries and the flat control. The strain resembles Hertzian contours. However, even in the flat control, the strain contours are distorted by the changing mechanical properties across skin layers as well as boundary conditions associated with the finite thickness of the skin. The wetness of the SC has a noticeable effect. As the SC dries, its stiffness increases, and the corresponding contours extend over a larger region in the underlying skin layers. The microrelief further contributes to the distortion of the strain contours compared to the flat geometry. In particular, the contours become asymmetric and localized around the features of the surface topography.
Fig 4.
Strain through the skin thickness following indentation.
Components of the Green-Lagrange strain plotted across the skin thickness right underneath the indenter for the different age topographies and different SC condition (wet or dry): a) E13, b) E33. The coordinate system is shown in c.
Fig 5.
Effect of dermal thickness and stiffness on the mechanical response to indentation.
Contours of the Green Lagrange strain components E33 and E13 for the two oldest microrelief geometries for the reduced thickness model with dry SC condition, and for three values of the shear modulus for the dermis (a). Components E33 and E13 of the Green Lagrange strain plotted across the skin thickness in three different positions underneath the indenter (b).
Fig 6.
Stress due to indenter displacement along anatomical axes.
Stress contours at the skin surface for the flat control and the three different skin topographies, for wet and dry SC conditions. The contours correspond to the final position of the indenter after it has moved in the direction U1 for 1mm at an indentation depth of 500 μm: a) Maximum principal stress and b) Maximum shear stress.
Table 2.
Average contact area [mm2] for the simulation with the flat skin surface and the three different skin topographies when the indenter moves in the U1 direction.
Fig 7.
Stress due to indenter movement orthogonal to primary lines.
Maximum shear stress contours for three different skin age topographies, for the displacement directions U3 and U4 given by the primary skin lines. The results correspond to: a)wet SC and b)dry SC material properties.
Fig 8.
Normal and tangential forces during indenter movement.
Behavior of in-plane (Ft and Fb), and normal (Fn) reaction forces on the indenter for three different skin age topographies and four displacement directions (black: U1, red U2, blue U3, green U4). The forces corresponding to the flat control are shown in grey. All plots correspond to the properties of the dry SC.
Fig 9.
Strains through the skin thickness during indenter movement.
Normal strain E11 in the direction of the indenter movement at three time points during indenter displacement (t = 0, 0.5, 1s), for the two SC conditions, and for the four different geometries:a) Flat, b) 30–40 years-old, c) 50–60 years-old and d) 70–80 years-old.
Table 3.
Coefficient of friction for three different skin topographies, four displacement directions, and two conditions of the SC.