Simulation And Experimental Study On The Process Of Ultra-short Pulse Laser Induced Blood Plasma Formation

Laser angioplasty is a technique for intravascular interventional laser ablation of plaque.Ultra-short pulse laser ablation is considered to be one of the frontier technologies for safer ablation because the particles produced are in the sub-micron level,and the risk of blocking blood vessels is low.Laser-induced plasma formation and plasma-induced ablation of thrombus will occur in this process.This paper mainly focuses on the simulation and experiment of the laser-induced water and blood plasma formation and the plasma-induced ablation of thrombus in vitro.Some important issues were investigated including free electron density distribution,plasma profile and size,energy transmission,plasma absorption,plasma center position,blood absorption,thrombus ablation threshold and plasma-induced ablation rate.The content of the paper includes the following aspects:（1）Laser-induced plasma formation research progress was reviewed.The existing spatial and temporal distribution numerical models of plasma formation were compared and analyzed.It was concluded that there were still problems such as insufficient description of the nonlinear absorption of plasma,the value of the threshold free electron density,and the value of the energy band gap of water molecules.（2）A simplified time-domain model of the water ionization process was constructed,which laid the foundation for the computation of plasma formation spatial and temporal distribution.The threshold free electron density was determined to be 10²⁰/cm³ in comparison with classical experiments.It was calculated that the breakdown threshold will decrease from～10¹³W/cm² to～10¹⁰-10¹¹W/cm² as the pulse width increases from 10fs to 100ns,and almost unchanged in the range of 1-100ns.As the pulse width decreases,the effect of avalanche ionization gradually decreases.The multiphoton ionization dominates the femtosecond interval.（3）In order to analyze the spatial and temporal distribution of laser-induced blood plasma formation,a spatial and temporal distribution numerical model in water was first constructed.The plasma profile showed a crescent towards the laser incident direction.The plasma center position was 11.8-170.7μm away from the focus when the pulse energy was 7.01-62.7μJ.Once the breakdown condition was reached,energy transmission ratio would drop rapidly.Spatial and temporal distribution numerical model of laser-induced water plasma formation was constructed and similar simulation were performed.It was found that compared with water,the higher extinction coefficient of blood restricted the plasma area to the focus and optical axis,and the stronger absorption capacity of plasma made the plasma shift toward the laser incident direction.（4）The breakdown pulse energies of different liquid obtained through experiments,including distilled water,10g/L,50g/L,100g/L blood solution,were compared with computation results.The difference between experiment and simulation was because the laser had been attenuated when it reaches the initial breakdown position.It was found that the energy transmittance of the four liquids were similar to the simulated values.When the pulse energy was 27.7μJ,33.5μJ,41.5μJ,50.6μJ,the plasma center deviates from the focus by 150-200μm,which was consistent with the simulated value of 109.1-170.7μm..The ablation thresholds of swine thrombus,swine blood vessels,and calf bone were measured when the ablation time was20s,15s,10s,and 5s in the air.Based on the ablation threshold of thrombus in the air,a thrombus ablation experiment in water was conducted.It was found that when the pulse energy increased from 3.80μJ to 8.16μJ,the ablation mass of thrombus increased from 36mg to 67mg under two minutes of ablation time,while from 8.16μJ to 40.63μJ,it increased from 67mg to110mg.The reason was that the former surpassed the breakdown threshold.In conclusion,a numerical model of laser-induced liquid plasma formation was constructed,and the scope of its application was expanded from water to blood.The nonlinear absorption description of the plasma,the value of the threshold free electron density and the value of the energy band gap of water molecules in the model were determined,and rationality was verified through experiments.