The Research On Physical And Mechanical Properties Of Composite Braided Stents

Lower extremity arterial disease is one of the most common diseases among the aged.It is of high risks in cardiac cerebral stroke and cardiovascular incidents.Because of their lower rates of surgical risks and operative complications compared with the open surgery,stents and stent-grafts have been widely adopted in treating artery diseases.However,most studies ignored the complex mechanical environment of low extremity arteries.Some stent-grafts are then lack of biomechanical properties,especially the bending flexibility.And its deformation mechanism under bending fatigue is not clear.The mass transfer performance in the thickness direction of stents is also ignored.Besides,the reendothelialization was not established fully after stent implantation,leading to thrombus and restenosis.Therefore,to enhance the biomechanical performance and endothelialization ability of vascular scaffolds and increase the long-term patency rates through material selection,structure design and surface functional modification are urgent problems to be solved.In addition,the comprehensive evaluation system of low extremity stents needs to be improved furtherly.Some evaluation methods are used for reference from upper limb stents,while the pulsation and compression force are main forces in upper limb blood vessels.But lower limb arteries undergo more complex and combined forces,like compression,bending and torsion.Some other evaluation methods,such as using water permeability to indicate the blood permeability of vascular scaffolds,cannot reflect the service environment of the product accurately.In this research,we designed and prepared braided stents based on braiding technique using polyester multifilament and NiTi wires,for the low extremityartery applications.The mechanical,fatigue and mass transfer performance of fabricated stents were studied.Besides,surface modifications were applied to promote endothelial cell attachment,proliferation and functions.Hereby,the evaluation system for low extremity arterial stent-grafts was set up preliminarily.At the same time,the influence of stent structure on its mechanical properties,deformation mechanism and the mass transfer efficiency was also explored.First of all,we selected polyester(PET)multifilament with good biocompatibility and widely used NiTi wires with shape memory ability and super-elasticity as the raw materials.PET multifilament was braided with NiTi wires on 32 bobbins braiding machine because of excellent axial flexibility of braiding structure.Before that,NiTi wires were wrapped by PET multifilament on 4 bobbins braiding machine,in order to explore yarn friction influence on stent mechanical performance.And by changing the arrangement of NiTi wires on the braiding machine,two kinds of stents were obtained:type A stent-grafts with NiTi wires parallel on the surface and type B stents with NiTi wires crossed on the surface.The radial compression,bending and torsion properties of different stents were evaluated.It was successfully demonstrated that the proposed composite structure can introduce superiority in stent-graft radial force,axial flexibility and torsion resistance,by cooperating NiTi yarns with a PET multifilament.The success of this design lay in its suitable yarn friction that restricted stent elongation and increased stent compactness.Optimized cover yarns for NiTi wires and the stent-graft structure were very important to impart the design with suitable biomechanical properties.Specifically,the type A stent-graft was dominant in higher radial force,elastic recovery rate and lower bending rigidity.The type B stent-graft exhibited better performance in torsion.In addition,the bending fatigue performance of the composite braided stents was investigated by the in vitro bending fatigue tester.One extremity of stent-graft was fixed on the horizontal terminal while another was attached to the vertical one,transforming forward and afterward.Its cyclic motion caused repeated bending of stents from 40°to 130°.Bending durability evaluation was performed at 1 Hz in a cube glass chamber full of phosphate buffered saline(PBS,pH=7.2).The tests were set at 7 days(6.048×10~5 cycles)and 30 days(2.592×10~6 cycles).Morphology and thermodynamic changes before and after fatigue were observed.Quantitative evaluation and energy consumption during fatigue were also studied.Here,four possible yarn-crossover-based deformation modes were proposed for braided stents in bending:(a)accordion buckling of SG_A,(b)diamond-shaped buckling for internal surface of SG_B,(c)neck propagation for the external surface of SG_B;(d)microbuckling.Accordion buckling and diamond-shaped buckling are two competing deformation modes.The dominant one depends upon the structure of braided tubes.For the outer surface in the braided stents,a neck forms first at the bending point.It then propagates under constant load through the specimen.Microbuckling is a localized material instability involving the rotation of fibers.With regard to SG_A,NiTi yarns were spiral along the length and interlaced with PET multifilament.As the structural support,they undertook almost the whole loading when bent.NiTi yarns appeared parallel under compression and extension,like an accordion.For SG_B,NiTi yarns crossed both with themselves and PET yarn.It had radially symmetrical structure.Compressive deformation by a diamond shaped buckling in SG_B involved localized buckling in diagonal lines of the cylinders.It included buckle initiation and subsequently buckle propagation along the axis of stents.Neck propagation was the dominant deformation at outer surface of braids for SG_B.The diameter of the tube decreased with the increase of the length,as well as the diminution of braiding angles.Both diamond shaped buckling and neck propagation are elastic deformation.However,due to friction between yarns,stents can’t recover to their original shape.In order to study the mass transfer performance of stents,we prepared braided stents with different structures.Analytical models based on through-thickness pore parameters were established.The mass transfer performance of the stent-graft was then evaluated in vitro by using the proposed methodology.And the accuracy of the models was verified by vascular scaffolds water permeability tester in vitro.Firstly,the porosity,hydraulic radius,Reynolds coefficient and resistance coefficient along the thickness direction of braided stents were calculated.According to Darcy’s law,the fluid volume along the thickness direction under a certain pressure was predicted.Artificial blood was synthesized according to ASTM F1670-2008 to replace the water in ISO7198-2016 to evaluate the blood permeability of stents in vitro.The surface tension of artificial blood was close to human body fluid.According to the classical theory of fiber filtration,the microthrombus transport performance of stents was also studied.Interception and inertial impaction of single fiber filtration mechanism were involved.In order to simulate the thrombus particles in human body,duck blood was used to prepare the dried thrombus,and it was classified according to the particle size.The results show that the calculated blood transfer performance is close to the measured one.In addition,the calculated stent-graft microthrombus retention efficiency is consistent with the experiments,especially for microthrombi larger than 50.In-stent restenosis,caused by vascular smooth muscle cells proliferation,and thrombosis,induced by inadequate reendothelialization of stents after implantation,remains major complications.Drug-eluting stents have dramatically reduced in-stent stenosis by restraining smooth muscle cells proliferation.But results also show that the drug coatings inhibit endothelial cells adhesion,migration and proliferation,resulting in poor reendothelialization and late stent thrombosis.Many efforts have been attempted to accelerate stents/stent-grafts endothelialization via the cell seeding.Preculturing endothelial cells on the surface of vascular prostheses before implantation is efficient to achieve fast endothelialization.However,the whole process is time-consuming and costly.Besides,endothelial cells are easily to be rubbed or washed off during implantation.Researchers have also been trying to modify prostheses surface by metals or metallic oxides like titanium and titanium dioxide to vary the surface topology and hydrophily.But it may have some limitations during applications on polymer fabrics.In this study,we aimed at developing a modified surface that has biological functions to prevent blood clotting and promote endothelial cells proliferation,as well as not alter the structure and mechanical properties of scaffolds.Braided stent-grafts were immobilized by polydopamine nanoparticles.REDV was then covalently bonded with polydopamine to enhance endothelial cells adhesion,proliferation and function.Mechanical performance,hemocompatibility and endothelial cells proliferation property were investigated.Results show that there are no significant changes for stent-graft outer diameter,compression force and bending rigidity after modifications.And the hemolysis rates of all samples are well below 1%.Besides,REDV helps the proliferation of endothelialization greatly.To sum up,polyester/NiTi braided stents were prepared successfully in our research.The influence of manufacture,yarn property and stent structure on its mechanical performance was researched.The bending deformation mechanisms were studied.We also established predicting models for stent mass transfer performance based on parameters along with their thickness directions.Besides,surface modification was applied to help stent reendothelialization.The results of this study may inspire future development of stents as medical devices.