Hierarchical Poromechanical Approach to Investigate the Impact of Mechanical Loading on Human Skin Micro-Circulation.

Extensive research on human skin anatomy has revealed that the skin functions as a complex multi-scale and multi-phase system, containing up to 70% of bounded and free circulating water. The presence of moving fluids significantly influences the mechanical and biological responses of the skin, affecting its time-dependent behavior and the transport of essential nutrients and oxygen to cells. Poroelastic modeling emerges as a promising approach to investigate biologically relevant phenomena at finer scales while embedding crucial mechanisms at larger scales as it facilitates the integration of multi-scale and multi-physics processes. Despite extensive use of poromechanics in other tissues, no hierarchical multi-compartment porous model that incorporates blood supply has yet been experimentally evaluated to simulate the in vivo mechanical and micro-circulatory response of human skin. This paper introduces a hierarchical two-compartment model that accounts for fluid distribution within the interstitium and the micro-circulation of blood. A general theoretical framework, which includes a biphasic interstitium (comprising interstitial fluid and non-structural cells), is formulated and studied through a one-dimensional consolidation test of a 100 μm column. The inclusion of a biphasic interstitium allows the model to account separately for the motion of cells and interstitial fluid, recognising their differing characteristic times. An extension of the model to include biological exchanges such as oxygen transport is discussed in the appendix. The preliminary evaluation demonstrated that cell viscosity introduces a second characteristic time beyond that of interstitial fluid movement. However, at high cell viscosity values and short time scales, cells exhibit behavior akin to that of solid materials. Based on these observations, a simplified version of the model was used to replicate an experimental campaign carried out on short time scales. Local pressure (up to 31 kPa) was applied to the skin of the dorsal face of the middle finger through a laser Doppler probe PF801 (Perimed Sweden) attached to an apparatus as previously described (Fromy Brain Res 1998). The model demonstrated its qualitative ability to represent both ischaemia and post-occlusive reactive hyperaemia, aligning with experimental observations. All numerical simulations were performed using the open source software FEniCSx v0.9.0. To promote transparency and reproducibility, the anonymized experimental data and the corresponding finite element codes are publicly available on GitHub.