| Ventricular assist devices(VADs)can provide short-or long-term circulation support for the patients,which have been an effective treatment for end-stage heart failure.The newer-generation continuous-flow VADs use rotary blood pumps instead of the early volume displacement pumps.The high-speed rotating impeller in the blood pump may lead to strong non-physiological flow characteristics which are known to damage red blood cells,resulting in hemolysis.Reducing the flow-induced hemolysis to a desired level while meeting the specific hydraulic performance is the key problem in the design stage of VADs.Currently,most of the studies on flow analysis of the continuous-flow VADs are carried out under steady flow conditions,and devices are also optimized based on the hemolytic performance under steady flow conditions.However,in the clinical use,VADs normally operate in parallel with the failing natural heart,and the actual operating conditions are determined by the characteristics of both VAD and cardiovascular system.Influenced by the natural heart,the flow through the pump actually has a pulsatile component.As the research continues,study on the hemolysis performance of VADs should conform to reality in order to further improve their safety in clinical use.Therefore,the research content of this paper mainly centre on the device hemolytic performance and the pulsatile working condition,which are the key particularities of VADs when compared with traditional small industrial pumps.This study aims to improve the existing in vitro evaluation method of the hemolytic performance of VADs,and to provide guidance for the optimization design of impeller.This dissertation starts with investigating the mechanism of the flow-induced hemolysis,aiming to develop a quantitative model which describes the relationship between hemolysis,shear stress,and exposure time.Considering that the shear stress distribution in VADs is non-uniform,a new hemolysis model is proposed,which is sensitive to the loading history.Then,a computational fluid dynamics(CFD)model for predicting hemolysis in the rotary blood pump is built,with the proposed hemolysis model implemented and adapted for the three-dimensional complex flow.The model is validated by calculating hemolysis in a clinical blood pump,and comparing the predicated results with experimental measurements.On this basis,the hydraulic and hemolytic performance of a centrifugal blood pump,which developed by our group and intended for use as a LVAD,are investigated under the steady-state conditions and the optimization design of the impeller is also carried out.For the pulsatile working condition,interactions between VAD and cardiovascular system are studied,and the hemolysis and fluid dynamics influenced by the native heart are investigated.By analyzing the comparison results between the pulsatile and corresponding steady-state condition,some valuable information has been obtained.It suggests that traditional assessment for hemolytic performance under steady states may not fully represent that in the clinical use.The analysis of internal flow characteristics based on steady-state conditions may potentially ignore the adverse factors due to the flow deceleration,such as the occurrence of flow separation and high turbulent shear stress,and the pulsatile working condition may influence the optimization design.For the ventricular assist pumps at the design stage,taking into account the pulsatile condition in the optimization design and eliminating the concern about the extra hemolysis under the pulsatile condition will be helpful for the subsequent in vivo experiments.The contents of this dissertation are as follows:1.Research on the hemolysis model.Considering that the shear stress distribution in VADs is non-uniform,the effect of the past shear stress history acting on the blood corpuscle should be taken into account.Hemolysis estimation is regarded as a system identification problem,and the system function characterizes the resistance to hemolysis.On the basis of the linear system assumption,the system function is derived from a power-law equation,which is derived under the condition of uniform shear stress.Then the hemolysis can be calculated under time-varying shear stress conditions.Two state variables are introduced in the proposed model,and the influence of the different initial conditions of blood on the subsequent hemolysis is taken into account.Comparing the estimated results with the published experimental data,it shows that the proposed model has sensitivity of loading history,and the accuracy is notably improved compared with the previous power-law based models.2.Research on the hemolytic performance of VADs under steady-state conditions.CFD simulations of a commercial blood pump are carried out under steady-state conditions.Lagrangian approach is employed to combine the simulation results with the proposed hemolysis model and the hemolysis index is calculated.In vitro hydraulic performance and hemolysis tests of the commercial pump are carried out,and by comparing the predicted results with the experimentally measured data,the above CFD-based model for hemolysis estimation in a rotary blood pump is validated.On this basis,the hydraulic and hemolytic performance of the pump developed by our group are studied under steady-state conditions,and the impeller optimization is carried out.By combining an orthogonal experimental design and the CFD simulations,the effects of the number of blades,blade outlet angle,blade outlet height and blade inlet angle on the pressure head,hemolysis and efficiency of the pump are studied.Quantitative analysis is conducted to reveal the impact degree of different factors.Based on the results,a set of optimal impeller design parameters is given.Simulation results show that the hemolysis index and the internal flow characteristics of the optimized impeller improve remarkably compared with the initial model.Additionally,the in vitro hemolysis test shows that the hemolytic performance of the pump with the optimized impeller is satisfied.3.Research on the interactions between VADs and cardiovascular system.A lumped parameter based mathematical model of the cardiovascular system is built as a simulation platform for VADs.The model is validated by reproducing the representative cardiovascular response in normal and heart failure conditions.The mathematical model of the blood pump is established,taking into account the influence of the flow acceleration and deceleration,and the combined system simulation of the pump assisted circulation system is carried out.The pump assisted cardiovascular system response with left ventricular failure and the dynamic characteristics of the pump under the influence of cardiovascular system are investigated.4.Research on the hemolytic performance of VADs under pulsatile flow condition.By introducing an electric cylinder driven pulsation module into the hemolysis test loop,the in vitro hemolysis tests under pulsatile flow conditions are conducted.The testing results indicate a significant increase in the hemolysis level due to the pulsatile flow.Subsequently,transient CFD simulations are carried out.The differences of hemolysis index,flow separations,turbulence shear stress in the internal flow field between the pulsatile and non-pulsatile conditions are analyzed to reveal the flow characteristics responsible for the higher hemolysis and guide the design improvement.5.Research on the effect of impeller passage diffusivity on the performance of VADs.Considering that significant higher level of hemolysis was observed under the pulsatile flow condition,it is proposed to increase the blade outlet thickness,reducing the diffusion factor of the impeller channel,to suppress the occurrence of flow separation during the deceleration phase.Numerical investigations of the effect of the impeller diffusion factor on the hydraulic performance,hemolysis index and internal flow characteristics are carried out,under both steady-state and pulsatile conditions.Based on the results,an improving design for the blade outlet thickness is proposed. |
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