Integrating serial block-face SEM with voxel-based finite element analysis for high-fidelity micromechanical modelling of anisotropic soft tissues. Application to human dermis

Background: Collagen-rich biological soft tissues exhibit strongly directional mechanical properties. While current continuum fibre-reinforced constitutive models incorporate structural information via invariants based on structure tensors, most numerical models rely on spatially uniform fibre orientation distributions. This study investigated how spatially heterogeneous fibre orientations captured via high-resolution imaging and integrated into image-based finite element models could provide novel mechanistic insights into soft tissue micromechanics. Materials and Methods: Serial block face scanning electron microscopy (SBF-SEM) captured collagen fibre structural information from fresh human skin dermis. Image stacks were processed to extract three-dimensional local fibre orientation at voxel level using structure tensor analysis. Voxel-based hexahedral finite element meshes were generated with element-level assignment of local fibre orientations and material phases. High-resolution models were integrated into Abaqus/Standard® with custom UMAT subroutines featuring invariant-based constitutive models for transversely isotropic hyperelasticity. Regions of interest were analysed under various loading conditions and compared to classical models with spatially uniform fibre distributions. Results: Accounting for spatially-varying fibre orientation distributions within tissue representative volumes significantly affected micromechanical responses compared to classical spatially uniform distribution approaches. Notably, exponential strain stiffening (i.e. toe region) emerged from structural deformations of the fibre phase rather than from an exponential strain energy density for the fibres. Conclusion: The integrated methodology provides novel quantitative and mechanistic insights in the micromechanics of human skin dermis. It is generic and applicable to a wide range of soft tissues and organs from cornea and cartilage to arteries and lungs.