General framework to implement isotropic and anisotropic hyperelastic biomaterials into finite element method

Hyperelastic models are extensively employed in the simulation of biological tissues under large deformation. While classical hyperelastic models are incorporated into certain finite element packages, new hyperelastic models for both isotropic and anisotropic materials are emerging in recent years for various soft materials. Fortunately, most hyperelastic models are formulated based on strain invariants, which provides a feasible way to directly implement these newly developed models into the numerical simulation. In this paper, we present a general framework for employing strain-invariant-based hyperelastic models in finite element analysis. We derive the general formulation for the Cauchy stress and elasticity tensor of both isotropic and anisotropic materials. By substituting the strain-energy density into these general forms, we are able to directly implement various hyperelastic models, such as the Fung-Demiray model and the Lopez-Pamies model for isotropic materials, and the Gasser-Ogden-Holzapfel model, the Merodio-Ogden model, and the Horgan-Saccomandi model for anisotropic materials, within the ABAQUS user-defined material subroutine, offering a numerical approach to implement materials not available through the built-in material models. To demonstrate the feasibility of our approach, we utilize these subroutines to compute several classic examples related to both homogeneous and inhomogeneous problems. The good agreement between the obtained results and the analytical or experimental solutions confirms the validity of developing these models by the proposed framework. The general framework and results presented in this study are useful for fast implementing newly developed hyperelastic models and are helpful to the finite element simulation of biological tissues.