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

While continuum fibre-reinforced constitutive models of collagen-rich soft tissues incorporate microstructural information via structure tensor invariants, most of their numerical implementations assume spatially uniform fibre orientations. This study examined how spatially heterogeneous orientations, captured by high-resolution imaging and embedded in image-based finite element models, could provide novel mechanistic insights into tissue micromechanics. Serial block-face scanning electron microscopy (SBF-SEM) captured collagen architecture from fresh human skin dermis. Voxel-level 3D fibre orientations were extracted via structure tensor analysis. Voxel-based hexahedral meshes with element-level orientations were implemented in Abaqus/Standard with custom UMATs® user subroutines for invariant-based transversely isotropic hyperelasticity, and compared to classical models with spatially uniform fibre distributions under various loading conditions. Spatial heterogeneity significantly altered micromechanical responses. Under pseudo-homogeneous uniaxial extension aligned with the mean fibre orientation, Models 1A (uniform orientation, no dispersion), 1B (uniform orientation with dispersion), and 1C (spatially heterogeneous orientations) yielded nominal stresses at 44.6% Green-Lagrange strain differing by a factor of four (0.5, 0.8 and 2.1 MPa, respectively). Model 1C's maximum principal logarithmic strain showed a broader range with a secondary peak at 0.5 versus unimodal peaks at 0.3 for Models 1A/B. With a purely quadratic fibre energy, Model 1C recovered exponential-like macroscopic stiffening for fibre moduli of 25-500 MPa, confirming the J-shape arises from microstructural mechanism rather than through fibre material nonlinearity. The methodology delivers quantitative mechanistic insights into dermal micromechanics and generalises to a wide range of soft tissues from cornea and cartilage to arteries and lungs.