Mechanical characterization of thrombi by studying shear and friction

Abstract

Stroke, caused by large vessel occlusions, is the second leading cause of death worldwide. Large vessel occlusions are the result of thrombosis, the undesired coagulation of blood within the vasculature. Due to this coagulation, a vessel can get blocked, prohibiting blood flow to areas distal from the occlusion, with all the serious consequences that may ensue. Since 2015, mechanical thrombectomy procedures have been widely accepted as a successful treatment technique to remove thrombi from the vasculature. However, complete reperfusion is only reached in 50% of acute ischemic stroke cases.

To successfully remove a thrombus from the vasculature, it is necessary to apply a specific retrieval force. This force must exceed the opposing forces, including the impaction force generated by the blood pressure gradient across the thrombus and the interaction forces between the thrombus and the vessel wall. Multiple studies have focused on tensile and compressive properties of thrombi. However, little is known about shear loading of the thrombus and about the interaction properties of the thrombus-vessel wall interface. Therefore, the aim of this study is to gain a better comprehension of thrombus biomechanics by studying the shear behavior of thrombi and the interaction of the thrombus-vessel wall interface in vitro.

In order to perform these in vitro studies, two custom-made test setups have been designed and developed. The friction test setup contains a plate of which the angle can be inclined slowly. By placing a thrombus-vessel wall sample on top of this plate, it is aimed to determine the static and kinetic coefficient of friction of the thrombus-vessel wall interface. Furthermore, it is aimed to determine the effect of time on this interaction. The thrombus-vessel wall interface was created by obtaining a piece of vein and blood from pigs. To study the shear behavior of thrombi a shear test setup has been developed. Two thrombi types have been utilized for this study, red blood cell- and fibrin-rich. Within the shear test setup it is possible to perform shear experiments under different normal loading conditions. A comprehensive analysis of the data acquired from experiments performed with both test setups has been conducted. Furthermore, a computational model has been developed to fit towards the experimental data obtained from the shear experiments.

The friction experiments suggest that time positively influences the bonds formed between a thrombus and the vessel wall as the coefficients of friction increase with an increased waiting time. Furthermore, a strong positive correlation was found between the static and kinetic coefficient of friction. This result was also found when doing an extensive analysis of the data obtained from the shear experiments. Additionally, the shear experiment showed that the thrombus composition influences its mechanical properties. Higher shear moduli and kinetic coefficients of friction were found for the fibrin samples, compared to the red blood cell samples.

The results obtained from the friction and shear experiments provide valuable insights into thrombus biomechanics. By extending the performed studies a better comprehension on thrombus mechanics and the thrombus-vessel wall interaction can be achieved.