Platelet deposition estimation: A novel method for emulating the pump thrombosis potential of blood pumps.

Pump thrombosis potential exists in most blood pumps and limits their clinical use. To improve the pump thrombosis performance of blood pumps, a method for emulating the platelet deposition on the flow passage component surfaces inside blood pumps was presented and tested. The method emulates the blood platelet deposition, employing laser-induced fluorescence tracing technology. The blood pump was rotated in a mock circulation loop with deionized water filled with fluorescent particles. The component surfaces were then explored via laser. The fluorescent particles were induced by laser and imaged in a charge-coupled device (CCD) camera to show the distribution of fluorescent particles gathering on the blood pump component surfaces. The activated platelet deposition was emulated by fluorescent particle gathering. The experiment showed obvious particle gathering on the interface surfaces and cross-sectional surface (perpendicular to the flow). This platelet deposition estimation (PDE) method can be easily incorporated in the in vitro testing phase to analyze and decrease a pump's thrombosis potential before animal experimentation, thereby reducing the cost of blood pump development. This methodology of emulating blood platelet deposition indicates its potential for improving flow passage component structure and reducing device thrombosis of blood pumps.

Numerical investigation of the influence of a bearing/shaft structure in an axial blood pump on the potential for device thrombosis.

Adverse events caused by flow-induced thrombus formation around the bearing/shaft of an axial blood pump remain a serious problem for axial blood pumps. Moreover, excessive anticoagulation with thrombosis around the bearing potentially increases the risk of postoperative gastrointestinal bleeding. The purpose of this study is to analyze the influence of the bearing structure on the thrombosis potential of an axial blood pump. The bearing/shaft structure was embedded into an axial blood pump numerical model. The numerical simulation and analysis are focused on the low wall shear stresses, recirculation, and residence time close to the bearing region to evaluate the potential for thrombosis around the bearing. Then, the flow field near the blood pump bearing was tested via in vitro particle image velocimetry experiments to verify the numerical results. The simulation results showed that after embedding the bearing/shaft structure a recirculation zone appeared in the outlet guide vane bearing/shaft region, the residence time increased 11-fold in comparison to the pump without the bearing/shaft structure, the scalar shear stress in the shaft surface was less than 7.8 Pa, and the stress accumulation was less than 0.10 Pa s. The numerical results showed that platelets that flow through the bearing region are exposed to significantly lower wall shear stress and a longer residence time, leading to activated platelet adhesion. The reduced stress accumulation and increased time in the bearing region lead to increased platelet activation.

Numerical Investigation of the Influence of Blade Radial Gap Flow on Axial Blood Pump Performance.

The gaps between the blades and the shroud (or hub) of an axial blood pump affect the hydraulics, efficiency, and hemolytic performance. These gaps are critical parameters when a blood pump is manufactured. To evaluate the influence of blade gaps on axial blood pump performance, the flow characteristics inside an axial blood pump with different radial blade gaps were numerically simulated and analyzed with special attention paid to the hydraulic characteristics, gap flow, hydraulic efficiency, and hemolysis index (HI). In vitro hydraulic testing and particle image velocimetry testing were conducted to verify the numerical results. The simulation results showed that the efficiency and pressure rise decreased when the gap increased. The efficiency of the axial blood pump at design point decreased from 37.1% to 27.1% and the pressure rise decreased from 127.4 to 71.2 mm Hg when the gap increased from 0.1 to 0.3 mm. Return and vortex flows were present in the outlet guide vane channels when the gap was larger than 0.2 mm. The HI of the blood pump with a 0.1 mm gap was 1.5-fold greater than that with a 0.3 mm gap. The results illustrated poor hydraulic characteristics when the gap was larger than 0.15 mm and rapidly deteriorated hemolysis when the gap was larger than 0.1 mm. The numerical and experimental results demonstrated that the pressure rise, pump efficiency, and scalar shear stress decreased when the gap increased. The HI did not strictly decrease with gap increases. The preliminary results encourage the improvement of axial blood pump designs.

Design and numerical evaluation of an axial partial-assist blood pump for Chinese and other heart failure patients.

A fully implantable axial left ventricular assist device LAP31 was developed for Chinese or other heart failure patients who need partial support. Based on the 5-Lpm total cardiac blood output of Chinese without heart failure disease, the design point of LAP31 was set to a flow rate of 3 Lpm with 100-mmHg pressure head. To achieve the required pressure head and good hemolytic performance, a structure that includes a spindly rotor hub and a diffuser with splitter and cantilevered main blades was developed. Computational fluid dynamics (CFD) was used to analyze the hydraulic and hemodynamic performance of LAP31. Then in vitro hydraulics experiments were conducted. The numerical simulation results show that LAP31 could generate a 1 to 8 Lpm flow rate with a 60.9 to 182.7 mmHg pressure head when the pump was rotating between 9,000 and 12,000 rpm. The average scalar shear stress of the blood pump was 21.7 Pa, and the average exposure time was 71.0 milliseconds. The mean hemolysis index of LAP31 obtained using Heuser's hemolysis model and Giersiepen's model was 0.220% and 3.89 × 10-5% respectively. After adding the splitter blades, the flow separation at the suction surface of the diffuser was reduced. The cantilever structure reduced the tangential velocity from 6.1 to 4.7-1.4 m/s within the blade gap by changing the blade gap from shroud to hub. Subsequently, the blood damage caused by shear stress was reduced. In conclusion, the hydraulic and hemolytic characteristics of the LAP31 are acceptable for partial support.

Numerical and In Vitro Experimental Investigation of the Hemolytic Performance at the Off-Design Point of an Axial Ventricular Assist Pump.

The ventricular assist pumps do not always function at the design point; instead, these pumps may operate at unfavorable off-design points. For example, the axial ventricular assist pump FW-2, in which the design point is 5 L/min flow rate against 100 mm Hg pressure increase at 8,000 rpm, sometimes works at off-design flow rates of 1 to 4 L/min. The hemolytic performance of the FW-2 at both the design point and at off-design points was estimated numerically and tested in vitro. Flow characteristics in the pump were numerically simulated and analyzed with special attention paid to the scalar sheer stress and exposure time. An in vitro hemolysis test was conducted to verify the numerical results. The simulation results showed that the scalar shear stress in the rotor region at the 1 L/min off-design point was 70% greater than at the 5 L/min design point. The hemolysis index at the 1 L/min off-design point was 3.6 times greater than at the 5 L/min design point. The in vitro results showed that the normalized index of hemolysis increased from 0.017 g/100 L at the 5 L/min design point to 0.162 g/100 L at the 1 L/min off-design point. The hemolysis comparison between the different blood pump flow rates will be helpful for future pump design point selection and will guide the usage of ventricular assist pumps. The hemolytic performance of the blood pump at the working point in the clinic should receive more focus.

Computational fluid dynamics-based hydraulic and hemolytic analyses of a novel left ventricular assist blood pump.

The advent of various technologies has allowed mechanical blood pumps to become more reliable and versatile in recent decades. In our study group, a novel structure of axial flow blood pump was developed for assisting the left ventricle. The design point of the left ventricular assist blood pump 25 (LAP-25) was chosen at 4 Lpm with 100 mm Hg according to our clinical practice. Computational fluid dynamics was used to design and analyze the performance of the LAP-25. In order to obtain a required hydraulic performance and a satisfactory hemolytic property in the LAP-25 of a smaller size, a novel structure was developed including an integrated shroud impeller, a streamlined impeller hub, and main impeller blades with splitter blades; furthermore, tandem cascades were introduced in designing the diffuser. The results of numerical simulation show the LAP-25 can generate flow rates of 3-5 Lpm at rotational speeds of 8500-10,500 rpm, producing pressure rises of 27.5-148.3 mm Hg with hydraulic efficiency points ranging from 13.4 to 27.5%. Moreover, the fluid field and the hemolytic property of the LAP-25 were estimated, and the mean hemolysis index of the pump was 0.0895% with Heuser's estimated model. In conclusion, the design of the LAP-25 shows an acceptable result.

Design optimization of an axial blood pump with computational fluid dynamics.

A fully implantable, axial flow blood pump has been developed in our hospital. Both in vitro and in vivo tests showed that the hemolysis and thrombus characteristics of the pump were in an acceptable but not in an ideal range. Computational fluid dynamics (CFD) and in vitro test results showed that the pump worked at off-design point with a low hydraulic efficiency; CFD analysis also showed regions of reverse flow in the diffuser, which not only decreases the pump's hydrodynamic efficiency, but also increases its overall potential for blood trauma and thrombosis. To make a blood pump atraumatic and nonthrombogenic, several methods were taken to reach a final model of the optimized blood pump using CFD, which decreased the rotational speed from 9,000 to 8,000 rpm, and the design flow rate from 11 to 6 L/min. More significantly, the flow separation and recirculation in the diffuser region were eliminated, which mitigated the traumatic and thrombus effect on blood. The acceptable results of the numerical simulations encourage additional in vitro and in vivo studies.

A novel integrated rotor of axial blood flow pump designed with computational fluid dynamics.

Due to the smaller size, smaller artificial surface, and higher efficiency, axial blood pumps have been widely applied in clinic in recent years. However, because of its high rotor speed, axial flow pump always has a high risk for hemolysis, which the red blood cells devastated by the shearing of tip clearance flow. We reported a novel design with the integrated blade-shroud structure that was expected to solve this problem by abolishing the radial clearance between blade and casing designed with the techniques of computational fluid dynamics (CFD). However, the numerical simulation result of the newly designed structure showed an unexpected backflow (where flow velocity is reverse of the main flow direction) at the blade tip. In order to eliminate this backflow, four flow passes were attempted, and the expansion angles (which reflect the radial amplification of the flow pass, on the meridional section, and should be defined as the angle between the center line of the flow pass and the axial direction) of the blades of the integrated rotor are 0 degrees, 8 degrees, 15 degrees, and 20 degrees, respectively. In the CFD result, it could be easily found as the expansion angles increased, the backflow was restrained gradually, and was eliminated at last. After numerous "cut and try" circles, the pump model was finally optimized. The numerical simulation of this model also showed a stable hydraulic characteristic.