Research On Properties Of Nitinol Alloy Wire Suitable For Peripheral Vascular Stent And Preparation Of Long Stent

With the development of the aging of the Chinese population and changes in dietary structure,the incidence of peripheral vascular disease,especially peripheral arterial disease(PAD),is increasing rapidly year by year.More and more PAD need operative treatment.Self-expanding stent intervention has become an effective and safe PAD treatment because of its unique advantages.In this paper,the long peripheral vascular stents(length>20 cm)have been investigated.Firstly,in order to obtain Ni Ti alloy wire with excellent properties,heat treatment process has been studied,which are suitable for the preparation of long peripheral vascular stents.Secondly,the long peripheral vascular stents have been fabricated by continuous suture technology.The results are as follows.(1)Two kinds of NiTi alloy wires,which are named as NTa and NTb alloy wire,have been heat-treated at 400,405,410,and 415?,30 min,60 min,respectively.The metallographic structure results show that the aforementioned four types of annealing treatments caused the transformation of martensite phase,and meanwhile the second phase precipitated from the parent phase(austenite).Then two kinds of Ni Ti alloy wires were annealed at 580?,560?,540?,10 min and 520?,500?,10 min respectively.It was found that the austenite transformation could be achieved when the annealing temperature was 580?and 520?.The transformation of austenite phase happened.The results of tensile tests indicate that the first-step heat treatment caused the decrease of the yield strength(?_s)of both alloy wire,where the yield strength(?_s)at 400?,30 min was the highest,The fractured dimples are large and deep,and the shaping is enhanced.After annealing at 400?for 60 min,compared with the original NTb alloy wire,the yield strength(?_s)of the NTb alloy wire was reduced,the cross-section dimples were large and deep,and the plasticity was enhanced.After the second-step heat treatment at 580?and 520?for NTa and NTb alloy wires,the superelasticity of the alloy wire was restored and the shape was weakened,which met the requirements of the vascular stent for the unit stent ring.(2)According to the microstructure,tensile properties and fracture morphologies,the optimized annealing treatment parameters are as follows.As far as NTa alloy wire is concerned,the first-step annealing treatment parameters are 400?,30 min,and the second annealing treatment parameters are 580?,10 min.As far as NTb alloy wire is concerned,the first annealing treatment parameters are 400?,60 min,the second step The annealing process parameters were 520°C for 10 min.Throug the optimized heat treatment process,the NTa and NTb alloy stent cell with good designed structure can be obtained.(3)The phase transition temperature of both kinds of Ni Ti alloy wires has been measured before and after the two-step annealing treatment.It is found that the first-step annealing treatment can cause the increase of the phase transition temperature of NTa and NTb alloy wires,which of Af are 50?and 53?.The second-step annealing treatment can cause the decrease of the phase transition temperature of NTa and NTb alloy wires,which of Af are 28?and 27?.The shape recovery temperature of Ni Ti alloy after two-step annealing treatment is 23?for NTa alloy wire and 21?for NTb alloy wire,which is beneficial to the complete recover of stents back to the design structure in the human body.(4)NiTi peripheral vascular stents with 3 support body diameters and 9 different stent structures were simulated by finite element modelling.The stent shows excellent mechanical properties when the support body diameters are 0.15 mm,0.2 mm,and 0.25 mm,the support body length is 6 mm,and the connecting arc radius is 0.4-0.5 mm,the included angle?=10°,15°,and the included angle?=10°,15°,20°.In this experiment,Ni Ti alloy wire with a wire diameter of 0.2 mm was selected,and the prepared geometric structure was L=6 mm,R=0.45mm,?=10°,15°,?=10°,20°unit scaffold ring.A straight tube with a length of 20 cm and a diameter of 6 mm was sewn by the continuous seam method,and then three kinds of stent with different structural unit stent rings were obtained by continuous suture technology.