Atherosclerosis is an inflammatory disease characterised by the formation of plaque in the intimal layer of the artery. The plaque is made of a lipid rich necrotic core which is covered by a collagen rich fibrous cap. If this fibrous cap ruptures, it can lead to sudden thrombotic occlusion of the artery. Rupture of the fibrous cap is linked to many factors. Recently, through high resolution imaging, microcalcifications have been found in the fibrous caps. The role of these microcalcifications in fibrous cap rupture mechanics is a debated theory. Calcifications, by virtue of their high stiffness, are predicted to increase local stresses in the less stiff surrounding collagen tissue by creating stress concentrations. This interaction between collagen and microcalcifications has not been studied extensively. Fibrous cap rupture mechanics can be studied through mechanical tests such as uniaxial tensile tests. Due to limitations in access to human plaque tissues and differences in the mechanisms of cap rupture in animal models, there is a need for an in-vitro platform to study fibrous cap rupture mechanics. In this study, a simplified model of a fibrous cap incorporating two components, collagen and calcifications, for the development of an in-vitromodel of an atherosclerotic fibrous cap was explored. High levels of calcium and phosphate have been found in microcalcifications in fibrous caps. Mesenchymal Stem Cells (MSCs) have been linked to vascular calcifications and have been found to deposit calcium phosphate in collagen scaffolds. Collagen type 1 scaffolds were seeded with MSCs to create calciumphosphate deposits with the aim of emulating an atherosclerotic fibrous cap from the collagen and calcifications aspect. The collagen scaffold constructs were mechanically tested to study the mechanical properties and effects of the calcium phosphate deposits on the mechanical behaviour. The structure and failure behaviour was studied through histology and scanning electron microscopy. Deposits of calcium phosphate were successfully formed inside the collagen scaffold leading to a calcified scaffold. The calcified collagen scaffoldswere mechanically and structurally characterised. The composition and size of the calcium phosphate deposits were in line with microcalcifications found in atherosclerotic fibrous caps. The failure was characterised by noticeable initial failures, multiple miniature failures and high stretch before the final complete failure. The calcified scaffolds can potentially serve as a baseline for the development of an in-vitro model of an atherosclerotic fibrous cap and for gaining useful insights into fibrous cap rupture mechanics.

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.

Background: The efficacy of a thrombectomy procedure for acute ischaemic stroke (AIS) is largely dependent on mechanical behaviour of thrombi and interactions with the thrombectomy device. Studies have examined clot analogues and evidence suggests that thrombus mechanical properties largely depend on composition, yet evidence based on thrombi retrieved from AIS patients is still lacking. Therefore, this study aims to characterize the mechanical properties of thrombi retrieved from AIS patients and to determine the relation to thrombus composition. Additionally, results were compared to literature in order to assess clot analogue representativeness.Methods: Directly following a thrombectomy procedure, unconfined compression tests were performed on thrombi retrieved from acute ischaemic stroke patients. For all tested samples, the material properties were characterized and related to the histologically determined composition. Identified histological components were 1) Fibrin & platelets, 2) Red blood cell and 3) Leukocytes. A subgroup analysis was performed to compare values with literature, where samples were stratified into four groups based on fibrin & platelet content (F&P -low, -moderate low, -moderate high and -high). Results: A total of 18 patients yielded 39 samples which were successfully tested and histologically analysed. Sample stiffness was found to be positively correlated to fibrin & platelet content (Rs=0.69, p<0.001). A good histological distribution was present within the data, as the fibrin & platelet content ranged from 7% to 99%. Subgroup analysis showed little difference in mechanical behaviour between the F&P moderate-low and F&P moderate-high subgroups, with the F&P low and F&P high groups respectively exhibiting a decreased and increased stiffness. Comparing to current literature, the results demonstrated that analogues most accurately resemble thrombi with a low fibrin & platelet content. Furthermore, all samples displayed viscoelastic and non-linear stress-strain behaviour.Conclusion: It was found that composition is a strong influencing factor of thrombus mechanical properties. Both at high and low fibrin & platelet contents, the relation between composition and stiffness was strongest, while it was least pronounced at moderate fibrin & platelet contents (approximately 25%-75%).

BACKGROUND - The success of the mechanical thrombectomy treatment in acute ischemic stroke is highly dependent on mechanical clot behavior and characteristics like stiffness. This mechanical treatment is tested in clot analogs. Realistic clot analogs could potentially be made in a realistic formation environment. The effects of blood pressure and flow were mimicked in this study. Factors affecting the mechanical behavior of thrombus analogs should be investigated to understand treatment possibilities better.METHODS - First, the effect of blood pressure was mimicked in a static pressure experiment by putting a weight on blood which generates a constant force. The clot analogs were mechanically assessed by unconfined compression testing. The effect of whole blood clot formation under pressure, the effect of different heights, and the effect of different hematocrit levels (a volumetric hematocrit of 1%H, 40%H, and 99%H) were investigated. Secondly, fibrin clots are formed by clotting platelet poor plasma on tissue factor under flow, with four different shear rates (shear=0/s, 50/s, 150/s, and 300/s). These clots were assessed by micro-indentation and confocal imaging. RESULTS - A total of 50 clot analogs under static conditions were successfully analyzed. Hyperelastic strain stiffening and visco-elastic behavior was seen in all clots, higher heights resulted in a higher stiffness in strain >60%, and 1%H clots were stiffer than 99%H clots. Furthermore, a total of 43 fibrin clots formed under flow were successfully investigated. It was seen that flow reduced the clot heights, fibers seemed to align with the flow direction, flow reduced the density in the top of the clots, and clots formed under shear had a higher stiffness than similar statically formed clots. CONCLUSION - A range of clot analogs under static conditions and flow were made and tested. The significant differences in mechanical properties and microstructure found can have new implications in thrombectomy research.

Background: Ischemic stroke is a major cause of death worldwide. Atherosclerosis in the carotid arteries is an established predictor of these events. Ideally, patient-specific prevention plans can be developed that target advanced plaque development prior to events. Morphometry of advanced calcified plaque phenotypes can be assessed through Computed Tomography Angiography (CTA) but predicting progression on morphometry alone is insufficient as atherosclerosis is nonlinear and heterogeneous. Therefore, it was hypothesized that biomechanical triggers stimulate advanced plaque growth. The biomechanical response can be observed through structural stress and strain in the vessel wall as a response to plaque composition, blood pressure and tethering. So, the research aim of this study is to develop a framework that allows for local and global assessment of structural biomechanical stimuli and morphometrical changes in atherosclerotic carotid arteries Methods Nine CTA scans paired at baseline and follow-up were selected from the Plaque At RISK study. The carotid bifurcation at the cervical spine was segmented by two independent observers using QAngioCT (Medis Medical Imaging, Leiden, The Netherlands). Contour data was reconstructed into a 3D geometry using a two-phase developed method in which 3D surface was computed first, and the second phase converted this to volumetric parts. Finite Element (FE) models were developed for baseline geometries where Neo-Hookean for calcified and Holzapfel-Gasser-Ogden for non-calcified materials were assigned. For both timepoints, local morphometry was defined in wall thickness (WT), principal curvatures and calcium localization. Contour maps allowed the local association analysis between biomechanics and morphometry. Global plaque progression was computed using the morphometrical parameters and overall plaque burden (PB). Results: This pipeline was successfully run for nine different carotid arteries, which proved overall robustness. The Dice similarity index computed an average segmentation observer similarity of 0.80 (St. Dev. 0.06) and 0.87 (St. Dev. 0.07) for surface reconstruction. Throughout reconstruction, the FE-modeling set the requirements for reconstruction outcome thus the focus was laid on connecting these phases. No patient data was made available during this study, so standardized systolic blood pressure resulted in an average maximum stress of 269.07 kPa and 0.14 strain. Morphometrical analysis detected diseased WT in seven out of nine cases at baseline and all at follow-up. Local principal curvatures uncovered a relation with diseased thickening, indicating its success in detection of irregularities on a surface. Calcified tissue was found in all cases but one at baseline. The average morphometrical change increased 2.71 mm (St. Dev. 7.86 mm) in maximum WT, 0.69 % (St. Dev. 5.75 %) for maximum PB and 12.91 mm3 (St. Dev. 12.50 mm3) for calcium. These preliminary results highlighted the importance of multicomponent morphometry analysis. Moreover, correlations between calcium growth and stress (R2 = 0.33) and WT increase (R2 = 0.67) indicate that future studies should focus on comprehending atherosclerotic pathways involved in calcified plaque formation. Conclusion: This developed method has laid the groundwork for future research and exposed important relations between analysis methods. The close dependency between reconstruction and FE-modeling, and anatomy and biomechanics emphasize that there is still a lot to discover.

Introduction: Congenital diaphragmatic hernia (CDH) is a rare developmental defect of the diaphragm characterised by herniation of abdominal organs into the thoracic cavity during prenatal development. This herniation is usually accompanied by pulmonary hypertension (PH) and cardiac dysfunction (CD). Although several parameters are known to predict the clinical outcomes in CDH, most of these parameters do not monitor the degree of PH and CD and cannot be continuously measured. This retrospective observational trial aimed to monitor the degree of PH and CD in CDH using the oxygen saturation index (OSI), peripheral oxygen saturation (SpO2), heart rate (HR), heart rate variability (HRV), arterial blood pressure (ABP) and derivatives of these parameters. Methods: the study population consisted of neonates admitted to the paediatric intensive care unit between 2019 and 2022 for treatment of CDH. The degree of PH and CD was determined for each cardiac ultrasound (CUS) performed. A 15-minute window of vital parameters, mechanical ventilator, and electrocardiogram data before each CUS was extracted to calculate the predictors. After preprocessing the data and meeting the statistical assumptions, both univariable and multivariable logistic mixed effects models were fitted and validated. Results: in total, 136 CUS of 57 patients were included in the study. Of the univariable linear mixed-effects models, the median values of HR, pulse pressure (PP), preductal SpO2, dSpO2, OSI and the interquartile range (IQR) of HR were statistically significant predictors of PH. For the prediction of CD, this was the case for the power of HRV in the very low frequency band (HRV-VLF) and for the median values of HR, mean arterial pressure (MAP) and OSI. The multivariable model for the prediction of PH contained the median values of of dSpO2, HR, PP and OSI and the standard deviation of the normal-to-normal beat intervals (SDNN). The multivariable model for the prediction of CD included the median of dSpO2, HRV-VLF, SDNN and the IQR of the systolic arterial pressure (SAP) as predictors. Conclusions: the most promising predictors are the median values of preductal SpO2, dSpO2 and OSI for the prediction of PH. For the prediction of CD, HRV-VLF and the median values of HR, OSI and dSpO2 were the most promising predictors. Despite limited predictive performance of the regression models, this study contributes to the improvement of monitoring of patients with CDH, which can lead to more timely interventions and eventually improved outcomes within this patient population.