Soft Tissue Mechanics

A robust anisotropic hyperelastic formulation for the modelling of soft tissue

david nolanThe Holzapfel–Gasser–Ogden (HGO) model for anisotropic hyperelastic behaviour of collagen fibre reinforced materials was initially developed to describe the elastic properties of arterial tissue, but is now used extensively for modelling a variety of soft biological tissues. A compressible form (HGO-C model) is widely used in finite element simulations where by the isotropic part of Ψ is decoupled into volumetric and isochoric parts and the anisotropic part of Ψ is expressed in terms of isochoric invariants. Here, by using three simple deformations (pure dilatation, pure shear and uniaxial stretch), we demonstrate that the compressible HGO-C formulation does not correctly model compressible anisotropic material behaviour, because  the anisotropic component of the model is insensitive to volumetric deformation due to the use of isochoric anisotropic invariants. In order to correctly model compressible anisotropic behaviour we present a modified anisotropic (MA) model, where by the full anisotropic invariants are used, so that a volumetric anisotropic contribution is represented. The MA model correctly predicts an anisotropic response to hydrostatic tensile loading, whereby a sphere deforms into an ellipsoid. It also computes the correct anisotropic stress state for pure shear and uniaxial deformations.    Link to paper

A computational test-bed to assess coronary stent implantation mechanics using a population-specific approach

clare conwayIn-stent restenosis (ISR) is hypothesized to occur for reasons that include injury to the vessel wall caused by stent placement. This research aims to facilitate more comprehensive evaluation of stents in the design phase, by generating more realistic arterial environments and corresponding stress states than have been considered heretofore. A systematic geometric test-bed with varying levels of arterial curvature and stenosis severity is developed and used to evaluate the implantation behaviour of two stent designs using finite element analysis. Results are analysed using tissue damage estimates and lumen gain comparisons for each design. Results indicate that stent design does not have a major impact on lumen gain behaviour but may have an influence on the potential for tissue damage. The level of stenosis in the arterial segments is seen to have a strong impact on the results while the effects of arterial curvature appear to be design dependent. Link to paper