Clot Machining in the Brain Vessels

Blood clots are clumps of coagulated blood and can adversely form, travel, and occlude a vessel in the brain, heart, lung, and extremities, causing stroke, myocardial infarction, pulmonary embolism, and deep vein thrombosis, respectively. Stroke is the fifth cause of death and leading cause of long-term disability in the US. To treat stroke, timely, completely, and minimally invasive removal of the clots to restore the blood flow is key. Mechanical thrombectomy is a catheter-based interventional procedural to remove clots and has become a standard of care for stroke since 2015. Two types of mechanical thrombectomy devices have been approved. One is the suction catheter which vacuums the clots out. Another is the stent retriever which uses an expanded wire mesh to grasp and pull the clots out. These two devices are also used in combination. The success rate to achieve complete reperfusion of the previously occluded vascular territories is around 50%. Achieving complete clot removal is currently a major challenge. This dissertation studies the clot removal mechanism of a cutting-based catheter. First, mechanical properties of 16 clots from stroke patients were analyzed by tensile test and their compositions were analyzed by histology. Second, 10 types of clot analogs (CAs) from human blood materials (red blood cells, plasma, and platelets) were fabricated to mimic the tensile and histological properties of the patient clots. Third, a cutting-based catheter was built with a miniature cutting tool and a smoothed particle Galerkin (SPG) model was developed to estimate the cutting forces. The SPG model was validated experimentally. Last, the fluid behavior near the catheter tip was observed by a high-speed camera and modelled by computational fluid dynamics. Clots from patients were histologically and mechanically highly variant among and within patients. Clot compositions ranged from 0.45% to 91% (43%±35%) red blood cells, 0 to 95% (40%±37%) fibrin, and 1.7% to 56% (18%±16%) platelets. Clot tensile properties range from 105% to 489% (241%±104%) ultimate strain and 63 to 2396 kPa (569±695 kPa) ultimate stress. The ultimate strain was strongly (|r| ≥ 0.6) and positively correlated with the platelet percentage and the ultimate tensile stress was strongly and positively correlated with the fibrin percentage. Similar correlations were also found in the CAs, which had an ultimate strain ranging from 0.78 to 1.36 and ultimate stress ranging from 16 to 949 kPa. A multi-variable regression model was built to guide the fabrication of CAs with desired histological compositions. For the difficult-to-remove clots, the cutting forces at rotational speeds of 60,000, 90,000, and 120,000 rpm were measured to be 0.74, 0.59, and 0.36 N, respectively and the predicted cutting forces from the SPG model matched the experiment measurements with errors less than 20%. During clot cutting, air bubbles could flow into the catheter due to air leakage. Clot chips could be pushed out of the catheter tip at about 2 m/s speed due to the vortex flow near the catheter tip and poor chip evacuation. To address those risks, a better seal is needed and the cavitation-assisted chip breaking can be introduced by the vortex-induced low pressure (< −99 kPa) zones. These findings lay the groundwork for developing effective and safe mechanical thrombectomy devices. The clot tensile properties, as well as the experiment and modelling methodologies, can be applied to mechanical thrombectomy device development for a better stroke treatmentPhDMechanical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttps://deepblue.lib.umich.edu/bitstream/2027.42/151757/1/yliume_1.pd