Material property characterisation of tissue engineered fibrous cap structures: An inverse Finite Element study

Introduction: Atherosclerosis is characterised by the buildup of plaque within the arterial wall and it is often the underlying cause effect of deaths related to cardiovascular diseases. Thin cap fibroatheromas are plaques with a high risk of causing clinical events due to rupture and espousal of thrombogenic components to the bloodstream. The rupture of the plaque is not yet fully understood and for this reason, tissue engineered plaques were created in a previous study to assess the rupture of the plaques based on the displacement field registered with Digital Image Correlation during a uniaxial tensile experiment. The knowledge of the material properties of the tissue-engineered fibrous cap structures makes it possible to link deformations to external loads and contributes to the understanding of plaque rupture. The study aims to create a pipeline for local mechanical property characterisation of tissue engineered fibrous plaque structures.Methods: In this novel method inverse Finite Element Method (iFEM) was combined with the Differential Evolution machine learning algorithm to assess global and local mechanical properties of tissue engineered fibrous plaque structures. The method required three main steps. Step one was the implementation of the uniaxial tensile test into a computational model using ABAQUS version 2016 Finite Element Method (FEM) software. To couple loads and deformations the hyperelastic reduced polynomial function of second order was implemented in the FEM. The characterisation of the c10 [kPa] and c_20 [kPa] parameters in the model is the main focus of this study. After the creation of the FEM, the computed displacement field and the previously registered DIC displacement field were implemented into the iFEM pipeline. Preliminary to the experimental data study, the pipeline was tested on a synthetically generated displacement field, in order to investigate the expected accuracy of the method. In step two the global mechanical properties of the fibrous plaque structures were investigated, using the assumption of homogeneous material property distribution in the samples. The resulting material properties after the global estimation served as an initial guess for the local estimation procedure. In step three the local material properties were investigated by creating sections with independently variable material properties, thus introducing heterogeneous distribution of material properties within the samples.Results: The global mechanical property assessment was carried out successfully and the resulting material properties are within the range of previously reported stiffness values of plaques with a similar composition. Local mechanical properties were characterised using up to twelve independently variable material parameters to investigate the heterogeneous mechanical behaviour of the constructs. Conclusion: During this project a new method was established to assess the local mechanical properties of tissue engineered fibrous cap structures. The pipeline shows high potential to be useful when investigating plaque rupture in a controlled environment using tissue engineered constructs. The knowledge of local material properties in combination with local deformations is a great addition to the understanding of plaque rupture.Biomedical Engineerin